JP4019861B2 - Engine air-fuel ratio control device - Google Patents

Description

ただし、Ｔｐ：基本噴射パルス幅［ｍｓｅｃ］、
Ｔｆｂｙａ：目標当量比［無名数］、
Ｋａｔｈｏｓ：過渡補正量［ｍｓｅｃ］、
ＦＨＯＳ：パージ分補正係数（遅れなまし処理後の値）、
α：空燃比フィードバック補正係数、
αｍ：空燃比学習値、
Ｔｓ：無効パルス幅［ｍｓｅｃ］、
の式により燃料噴射弁４に与える燃料噴射パルス幅Ｔｉ［ｍｓｅｃ］を演算する。この演算は図２中の基本噴射パルス幅演算部Ｂ７、空燃比フィードバック補正係数演算部８、燃料噴射パルス幅演算部Ｂ９における処理に対応する。
【００９３】
ここで、パージ分補正係数ＦＨＯＳで過渡補正量Ｋａｔｈｏｓを除いた部分（Ｔｐ×Ｔｆｂｙａ）を補正することにしているのは、パージにより供給される燃料は気体燃料であるため壁流補正を行う必要がないためである。
【００９４】
基本噴射パルス幅Ｔｐはほぼ理論空燃比の得られる噴射パルス幅で、吸入空気流量Ｑとエンジン回転速度Ｎから求めている。
【００９５】
目標当量比Ｔｆｂｙａは１．０を中心とする値で、冷却水温が低いときやエンジン始動直後には１．０より大きな値となる。エンジンの暖機完了後はＴｆｂｙａ＝１．０である。
【００９６】
空燃比フィードバック補正係数αは排気空燃比が理論空燃比を中心とするいわゆるウィンドウに収まるように空燃比センサ１５の出力に基づいて演算される値である。具体的には１００％を中心とする値で、定常偏差（ベース空燃比エラー）がないとした場合、検出された空燃比が理論空燃比よりもリッチ側にあるとき１００％よりも小さな値に、検出された空燃比が理論空燃比よりもリーン側にあるときに１００％よりも大きな値になる。
【００９７】
空燃比学習値αｍは空燃比フィードバック補正係数αに基づいて演算される値で、主に定常偏差を補償するために導入されている。
【００９８】
燃料噴射弁４から噴射された燃料の一部は吸気ポート壁や吸気弁傘裏部に壁流燃料を形成し、この壁流燃料は加速や減速といった過渡時に大きな応答遅れを有してシリンダに流入する。過渡補正量Ｋａｔｈｏｓはこの過渡時の壁流燃料の応答遅れ分を補償するための値である。
【００９９】
無効パルス幅Ｔｓは、燃料噴射弁４が所定のタイミングで噴射信号を受けてから実際に開弁するまでの作動遅れを補償するための値である。
【０１００】
また、（７）式はシーケンシャル噴射（４気筒ではエンジン２回転毎に１回、各気筒の点火順序に合わせて噴射）の場合の式であるため、数字の２が入っている。
【０１０１】
図１２、図１３は目標パージ率ＭＰＲを設定するためのもので、一定時間毎（例えば１０ｍｓｅｃ毎）に実行する。この演算は図２中の目標パージ率設定部Ｂ１における処理に対応する。
【０１０２】
ステップ６１では初回の較正処理後であるか否かを判定する。これは初回の較正処理の前後で目標パージ率の設定方法が異なるためである。初回の較正処理前であるときにはそのまま今回の処理を終了する。なお、初回の較正処理前の目標パージ率の設定については、後述するブートアップ制御のところで説明する。
【０１０３】
初回の較正処理後であるときにはステップ６２に進み、キャニスタモデルに基づき演算された脱離量Ｄｇ（図４ステップ６により演算されている）とパージ流量ＰＱとに基づきパージガスの空燃比（パージ空燃比）を演算する。ここで、パージ流量ＰＱは目標パージ率と吸入空気流量Ｑとの積であり、この場合の目標パージ率には前回値を用いる。
【０１０４】
このパージ空燃比に対しては次のような補正を施してもかまわない。すなわち、運転状態、例えばエンジン回転速度、エンジン負荷、吸入空気流量などのパラメータから、パージ空燃比の誤差を推定する。例えば、図１４を内容とするテーブルを検索することにより求める。図示のように吸入空気流量が少なくなるほど、またパージ率が小さくなるほどパージ空燃比誤差は大きくなる値である。あるいは、パージ空燃比誤差を、図１５に示すようにパージ空燃比とパージ空燃比誤差の関係を規定したテーブルを検索することにより求めるようにしてもよい。パージ空燃比誤差が求まったら、ステップ６２で求めたパージ空燃比をこの誤差に基づいて補正する。
【０１０５】
なお、パージ空燃比はＨＣセンサによって検出するようにしてもよいが、キャニスタモデルに基づき演算される脱離量等に基づき演算によって求めればパージ空燃比を安価かつ正確に演算することができる。
【０１０６】
ステップ６３ではこのようにして求めた（あるいは誤差補正後の）パージ空燃比に基づきパージ率変化量制限値を演算する。パージ率が変化すると排気空燃比が変化するが、このときの理論空燃比からの空燃比偏差が許容幅以内に収まるようにパージ率変化量制限値を演算する。理論空燃比からの空燃比偏差の許容幅は空燃比フィードバック制御により吸収可能な、排気エミッションを悪化させない幅に設定する。
【０１０７】
ステップ６４では、パージバルブ２７のサイズから規定されるパージ率上限値ＰＶＭＸを演算する。このようなパージ率上限値ＰＶＭＸを求めるのは、目標パージ率がパージバルブ２７を最大開度として得られるパージ率よりも大きな値に設定されてしまうと、パージ率と目標パージ率との不一致が生じ、ＦＨＯＳの演算の誤差が大きくなるため、理論空燃比からの空燃比偏差が増加する。これにより、排気エミッション悪化等の問題が生じるからである。具体的には、パージバルブ２７のサイズが一定の場合、パージバルブ２７の前後差圧が大きいほど流せるパージガスの流量も多くなることから、パージバルブ２７の前後差圧が大きいときにパージ率上限値ＰＶＭＸとして大きな値を演算する。
【０１０８】
ステップ６５では、燃料噴射弁４の性能に応じて決まる最小噴射パルス幅、目標パージ率の前回値、パージ分補正係数との関係から燃料噴射弁４の性能に基づくパージ率上限値ＴＩＭＮＭＸを演算する。パージ率が高くなるとパージによってエンジンに供給される燃料量が増加するので、燃料噴射弁４からの燃料噴射量がその分だけ減らされるように燃料噴射パルス幅を補正するが、燃料噴射弁４の噴射精度を確保するためには噴射パルス幅は所定の最小パルス幅よりも大きくなくてはならない。言い換えれば、燃料噴射パルス幅を最小パルス幅より大きくするためにはパージ率はある値よりも小さくしなくてはならない。このような理由から、燃料噴射弁４の噴射性能によってもパージ率の上限を規定する。
【０１０９】
また、ステップ６６では、現在の運転領域から想定しうる全ての運転領域を想定し、その中での最小パージ率を予測し、この最小パージ率とパージ率変化量制限値とからパージ率上限値ＰＲＭＮＭＸを演算する。例えば、アクセルペダルをあるだけ踏み込んでで加速した場合に目標パージ率はごく小さな値に設定するが、このアクセルペダルをあるだけ踏み込む直前に目標パージ率が大きな値に設定していると、パージ率の変化量が変化量制限値以下に制限されていることからパージ率を目標パージ率に追従させることができなくなる。この追従遅れは排気エミッション増大の原因等となることから、こうした追従遅れを生じないように想定しうる最小パージ率からもパージ率の上限を規定する必要がある。
【０１１０】
また、ステップ６７では、空燃比フィードバック補正係数αをモニターし、所定値以下であれば空燃比フィードバック補正係数αを所定値以上とするパージ率のうち最も大きな値をパージ率上限値ＡＬＰＭＸとして演算する。このような上限値ＡＬＰＭＸを設けるのは、空燃比フィードバック制御では空燃比フィードバック補正係数αは１００±２５％に収まるように制御されているが、空燃比フィードバック補正係数αが前記制限値近傍（例えば８０％）で制御されているような場合は、大量のパージを行っているとパージ以外の外乱を受けて前記制御範囲から外れやすくなるからである。
【０１１１】
ステップ６８では上記４つの上限値ＰＶＭＸ、ＴＩＭＮＭＸ、ＰＲＭＮＭＸ、ＡＬＰＭＸから最も小さい値を選択し、その値を最大パージ率ＰＲＭＡＸとして設定する。
【０１１２】
ステップ６９、７０では目標パージ率の前回値と最大パージ率ＰＲＭＡＸとを比較し、目標パージ率の前回値と最大パージ率ＰＲＭＡＸとが等しいときにはステップ６９よりステップ７３に進んで目標パージ率を前回値のままとし、目標パージ率の前回値が最大パージ率ＰＲＭＡＸよりも大きいときにはステップ６９、７０よりステップ７２に進んで目標パージ率をその前回値からパージ率変化量制限値を引いた値とし、目標パージ率の前回値が最大パージ率ＰＲＭＡＸよりも小さいときにはステップ６９、７０よりステップ７１に進んで目標パージ率をその前回値にパージ率変化量制限値を加えた値とする。
【０１１３】
図１３のステップ７４〜８１については本願発明により追加した部分であり、後述する。
【０１１４】
このようにして、目標パージ率は、最大パージ率ＰＲＭＡＸを目標としてパージ率変化量制限値の範囲内でこれに追従するように設定され、排気エミッションを悪化させずに大量のパージを行う上で最良のパージ率が設定される。また、最大パージ率ＰＲＭＡＸを設定する際に、物理的な制限、現在の運転領域等で決まる上限値ＰＶＭＸ、ＴＩＭＮＭＸ、ＡＬＰＭＸだけでなく、運転領域が変化した場合でも遅れなくその領域での最大パージ率に移行できるように決定される上限値ＰＲＭＮＭＸをも考慮しているので、運転条件が変化しても排気エミッションを悪化させずに大量パージを行う上で最良のパージ率を設定することができる。
【０１１５】
このようにして設定された目標パージ率はパージバルブへのデューティ比に変換され、パージバルブ２７に出力される。
【０１１６】
ところで、キャニスタモデルに基づくパージ処理（モデル規範パージ処理）は、エンジン始動後に較正処理が一度も行われておらずキャニスタモデルで用いる初期値（初期吸着量）が存在しない間は、実行することができない。しかし、エンジン始動後より大量パージを実現するためには、たとえ較正処理前であってもパージ処理を実行する必要がある。そこで初回の較正処理が実行されるまでは、キャニスタモデルに基づくパージ処理に代えて次に示す制御（図１６に示すブートアップ制御）により目標パージ率を設定し、設定された目標パージ率でパージ処理を実行する。なお、このブートアップ制御では、パージによる排気空燃比への影響は空燃比フィードバック補正係数αによって吸収し、燃料噴射パルス幅の補正は行わない（ＦＨＯＳ＝１．０）。
【０１１７】
図１６はブートアップ制御を行うためのもので、一定時間毎（例えば１０ｍｓｅｃ毎）に実行する。このブートアップ制御は図２中のブートアップ制御部Ｂ１４、切換スイッチＢ１５における処理に対応する。
【０１１８】
ステップ９１で吸入空気流量Ｑを読み込み、ステップ９２で初回の較正処理前であるか否かを判定する。初回の較正処理前である場合には、ステップ９３で積算パージ流量（パージを開始してからの総パージ流量）ＳＱＰとパージ配管容積（キャニスタ２４からパージバルブ２７までの配管の容積）を比較する。積算パージ流量ＳＱＰがパージ配管容積を超えているときにはステップ９４に進み、これに対して超えていないときにはステップ９７に進む。
【０１１９】
ステップ９７では目標パージ率ＭＰＲとして初期パージ率（１％以下の小さな値）を設定する。このような小さな値に設定するのは、積算パージ流量ＳＱＰがパージ配管容積に達してない場合はパージ開始前にパージ配管内のガスがエンジンに供給されることになるが、このパージ配管内のガスの空燃比が不明であり、このままステップ９４〜９６に示す目標パージ率設定処理を行うとエンジンの燃焼安定性悪化等の問題を生じるからである。
【０１２０】
つまり、パージ開始時にパージ配管中に存在する低濃度のパージガスが供給され、これによる理論空燃比からの空燃比偏差が小さいと、さらに大量のパージが可能であると判断されて大きな目標パージ率ＭＰＲが設定されるが、このようにして大きな目標パージ率ＭＰＲが設定されてしまうと、配管内の低濃度のガスが全て供給されて本来の高濃度のパージガスが供給されるときに大量の脱離燃料が突然供給されることになり、エンジンの燃焼安定性等を悪化させる原因となるからである。
【０１２１】
積算パージ流量ＳＱＰが配管容積を超えたらステップ９４に進み実空燃比フィードバック偏差と目標空燃比フィードバック偏差との差を演算する。
【０１２２】
ここで、目標空燃比フィードバック偏差とは、空燃比フィードバック補正係数の目標値ｔαと空燃比フィードバック補正係数の中心値（１００％）との偏差（＝｜ｔα−１００｜％）をいい、実空燃比フィードバック偏差とは実際の空燃比フィードバック補正係数αと空燃比フィードバック補正係数の中心値との偏差（＝｜α−１００｜％）をいう。例えば、パージによる理論空燃比からの空燃比偏差を空燃比フィードバック制御で十分吸収できる範囲内で大量のパージ流量を確保することを目的として空燃比フィードバック補正係数αの目標値ｔαが８０％に設定されると、目標空燃比フィードバック偏差は｜８０−１００｜＝２０％に設定される。
【０１２３】
ステップ９５では図１７を内容とするテーブルを検索することにより上記目標空燃比フィードバック偏差と実空燃比フィードバック偏差との差に応じたパージ率変化量を算出する。パージ率変化量は、目標空燃比フィードバック偏差と実空燃比フィードバック偏差の差の絶対値が大きくなるほど大きな値を設定して、目標値への収束性を高めるのであるが、目標空燃比フィードバック偏差と実空燃比フィードバック偏差の差の正負によって、偏差の絶対値が同じであっても異なる値を設定し、空燃比フィードバック偏差の差が負側にずれた場合の方が大きな値（絶対値）をパージ率変化量に設定する。
【０１２４】
このようにパージ率変化量を空燃比フィードバック偏差の正負で異なる特性とするのは、空燃比フィードバック偏差の差が負側にずれている場合には空燃比フィードバック補正係数αが目標とする８０％よりも小さな値になっており、逆の正側にずれている場合と比べてパージ以外の外乱によってエンジン安定性、排気エミッションの悪化を招きやすく、不利な状態あるといえるからである。つまり、パージ率変化量を空燃比フィードバック偏差の差の正負に応じて特性を変えるのは、エンジンの燃焼安定性及び排気エミッション悪化防止の観点から、制御点を速やかに安全側に復帰させるためである。
【０１２５】
以上のようにしてパージ率変化量を演算したらステップ９６に進み、本ルーチン前回実行時に求めた目標パージ率に対して、ステップ９５で演算したパージ率変化量を付加し、新たな目標パージ率ＭＰＲを演算する。
【０１２６】
ステップ９８ではこのようにして得た目標パージ率ＭＰＲと吸入空気流量Ｑからパージ流量ＰＱを求め、これをステップ９９において積算パージ流量の前回値であるＳＱＰｚに加算することにより積算パージ流量ＳＱＰを更新する。
【０１２７】
したがって、ステップ９４〜９６の処理によると、キャニスタ２４の吸着状態によらず、最適な目標パージ率ＭＰＲを設定することができ、また、想定以上の濃度のパージガスが供給された場合でも、それによる排気空燃比の変化を受けて目標パージ率ＭＰＲが適宜変更され、常に最適な目標パージ率ＭＰＲを設定することができる。
【０１２８】
一方、初回の較正処理後にはステップ９２よりステップ１００以降に進み、積算パージ流量ＳＱＰに基づいてパージ処理が終了したか否かを判定する。すなわち、ステップ１００でパージ終了判定フラグ（ゼロに初期設定）をみる。エンジン始動直後にはパージ終了判定フラグ＝０であるので、ステップ１０１に進み脱離量Ｄｇ（図４のステップ６で演算されている）と目標パージ率ＭＰＲ（図１２、図１３により演算されている）を読み込む。ステップ１０２では、この読み込んだ目標パージ率ＭＰＲと吸入空気流量Ｑとからパージ流量ＱＰを算出し、これをステップ１０３において積算パージ流量の前回値であるＳＱＰｚに加算することにより積算パージ流量ＳＱＰを更新する。
【０１２９】
ステップ１０４では初回の較正処理タイミングであるか否かを判定する。ここで、初回の較正処理タイミングはステップ９２で初回の較正処理前でなくなった初めてのタイミングであるので、このときだけステップ１０５に進み、ステップ１０２で演算されているパージ流量ＰＱとステップ１０１で読み込んでいる脱離量Ｄｇとに基づいてパージ空燃比を演算し、このパージ空燃比からステップ１０６において図１８を内容とするテーブルを検索することにより、パージ終了判定値ＳＬＳＱＰを演算する。
【０１３０】
ここで、パージ終了判定値ＳＬＳＱＰはキャニスタ２４からのパージガス中にほぼ脱離燃料がなくなったと思われる積算パージ流量である。図１８のように、この判定値ＳＬＳＱＰはパージ空燃比が大きくなるほど小さくなる。
【０１３１】
ステップ１０７ではこのパージ終了判定値ＳＬＳＱＰと積算パージ流量ＳＱＰとを比較する。積算パージ流量ＳＱＰがパージ終了判定値ＳＬＳＱＰ未満であればそのまま今回の処理を終了する。やがて積算パージ流量ＳＱＰがパージ終了判定値ＳＬＳＱＰ以上になればキャニスタ２４からのパージガス中にほぼ脱離燃料がなくなったと判断し、パージが終了したことを表すためステップ１０８に進んでパージ終了判定フラグ＝１とする。
【０１３２】
パージ終了判定フラグ＝１になると次回からはステップ９１、９２、１００と流れてもステップ１０１以降に進むことができないので、そのまま処理を終了する。
【０１３３】
次に、パージ処理を行うことによる本実施形態の全体的な作用について先に説明する。
【０１３４】
パージ処理時、目標パージ率ＭＰＲはエンジン燃焼安定性低下、排気エミッション増大を起こさない範囲でできる限り大きな値が設定され、この目標パージ率ＭＰＲが実現されるようにパージバルブ２７が開かれる（図１２ステップ６１〜７３）。
【０１３５】
パージ処理中はキャニスタ２４から脱離した燃料を含んだパージガスがエンジンに供給されることになるので、エンジンコントローラ１１ではキャニスタ２４から脱離してくる燃料量Ｄｇを推定し、この脱離量Ｄｇ、目標パージ率、吸入吸気質量Ｑｇに基づいて、パージ燃料によって生じる空燃比フィードバック補正係数αの変化を予測する値であるパージ分補正係数ＦＨＯＳを演算し（図４ステップ１、３、４、６、７、８またはステップ１、３、５、６、７、８）、このパージ分補正係数ＦＨＯＳにより基本噴射パルス幅Ｔｐを補正することで、パージによる理論空燃比からの空燃比偏差が小さなものに抑えられる。
【０１３６】
このときキャニスタ２４からの脱離量Ｄｇは、（１）式から（４）式で表されるキャニスタモデルを用いて推定され、脱離量Ｄｇは短時間でかつ正確に推定される。ここで、キャニスタモデルに基づき演算される脱離量Ｄｇ等は誤差を含んでおり、キャニスタモデルの動作時間が長くなるにつれこの誤差が積算され大きくなるので、エンジンコントローラ１１は空燃比フィードバック補正係数αの変化よりキャニスタ２４から脱離された燃料量を推定し、この推定した脱離量から逆演した吸着量Ｙｒでキャニスタモデルの内部変数である吸着量の値を較正する。この較正処理は、パージ以外の外乱が小さく空燃比フィードバック係数αの変化をほぼ全てパージによるものとみなすことができ、かつ、パージによる排気空燃比への影響が比較的大きいときに実行される。
【０１３７】
また、パージバルブ２７が開かれてから脱離燃料がエンジンのシリンダに到達するまでに遅れがあり、また到達するまでに燃料の拡散もあるので、この遅れとなまし作用を考慮して上記のパージ分補正係数ＦＨＯＳに対して遅れ補正が施されている。
【０１３８】
また、上記キャニスタモデルを用いたパージ処理（モデル規範パージ処理）は、較正処理によって吸着量の初期値が求まるまではその効果を発揮することができないが、キャニスタモデルの初期値が演算されるまでは目標空燃比フィードバック偏差と実空燃比フィードバック偏差との差に応じて目標パージ率が設定され、この目標パージ率が実現されるようにパージバルブ２７が開かれる。これにより、較正処理によって初期値が演算される前であってもパージ処理を行うことができ、全領域で効果的なパージを行うことができる。
【０１３９】
これで先願装置（特願２００１−７１５６２号参照）におけるパージ処理と同様の部分の説明を終了し、次には本願発明で追加した図１３、図１９、図２０、図２２、図２４、図２５、図２６に示すフローチャートを説明する。
【０１４０】
まず図１３においてステップ７４では目標パージ率を変数ＭＰＲ０に移す。
【０１４１】
ステップ７５ではパージ用リミッタ判定フラグをみる。このパージ用リミッタ判定フラグにはパージ用下限リミッタ判定フラグとパージ用上限リミッタ判定フラグの２つがあり、これら２つのフラグの設定については図１９、図２０のフローより説明する。
【０１４２】
図１９はパージ用下限リミッタ判定フラグを、また図２０はパージ用上限リミッタ判定フラグを設定するためのもので、いずれも一定時間毎（例えば１０ｍｓｅｃ毎）に実行する。
【０１４３】
空燃比フィードバック補正係数αには、従来よりαによる制御範囲を限定するため図２７に示したように、αの中心値（１００％）を中心として上下に第１リミッタ±ＬＭＴ１（例えば±２５％）を設けている。すなわち、αの第１上限リミッタは１２５％（＝１００＋ＬＭＴ１）、αの第１下限リミッタは７５％（＝１００−ＬＭＴ１）であり、演算されたαが、αの第１上限リミッタである１２５％を超えるときには、このαを１２５％に制限し、この逆に演算されたαが、αの第１下限リミッタである７５％を下回るときには、このαを７５％に制限している。
【０１４４】
本実施形態では、この第１リミッタ±ＬＭＴ１とは別にパージ用にこれより小さい第２リミッタ±ＬＭＴ２を新たに導入している。すなわち、第２下限リミッタは１００−ＬＭＴ２、第２上限リミッタは１００＋ＬＭＴ２であり、図２７において、αの中心値である１００％と第１下限リミッタである１００−ＬＭＴ１との間に、１００−ＬＭＴ１からある程度の余裕代を持たせた位置に第２下限リミッタである１００−ＬＭＴ２がくるようにしている。すなわち、１００−ＬＭ２（第２下限リミッタ）と１００−ＬＭＴ１（第１下限リミッタ）の間をαの補正代として残し、パージ以外の外乱があった場合に、この残したαの補正代により理論空燃比からの空燃比偏差を小さなものとする。
【０１４５】
そして、αが第２下限リミッタ以下の領域にあるか否かあるいは第２上限リミッタ以上の領域にあるか否かを示すため、フラグを設定する。すなわち、演算されたαが第２下限リミッタである１００−ＬＭＴ２以下になるとパージ用下限リミッタ設定フラグ＝１とし、この逆に演算されたαが第２上限リミッタである１００＋ＬＭＴ２以上となるとパージ用上限リミッタ設定フラグ＝１とする。
【０１４６】
ただし、第２下限リミッタ、第２上限リミッタにはそれぞれヒステリシスＨＹＳを設けている。これは、αが各リミッタ付近にいることにより各フラグがゼロとなったり１となったりを頻繁に繰り返すことを防止するためである。
【０１４７】
具体的に図１９から説明すると、ステップ１１１では空燃比フィードバック制御中であるか否かを判定する。空燃比フィードバック制御中でなければそのまま今回の処理を終了する。
【０１４８】
空燃比フィードバック制御中であるときには、ステップ１１２に進んで空燃比フィードバック補正係数αを読み込む。
【０１４９】
このαは図示しないフローにおいて空燃比センサ１５に基づいて演算している。ここでは、簡単のため比例動作の場合で説明すると、空燃比センサ１５により検出される実際の排気空燃比と理論空燃比との偏差を求め、この偏差に比例ゲインを乗算することにより求めればよい。このときαの波形は、実際の排気空燃比が理論空燃比よりリッチ側にあるあいだは、実際の排気空燃比を理論空燃比へと戻すため制御周期毎にαが階段状に小さくなってゆき（図２７参照）、この逆に実際の排気空燃比が理論空燃比よりリーン側にあるあいだは、実際の空燃比を理論空燃比へと戻すため制御周期毎にαが階段状に大きくなってゆく。
【０１５０】
ステップ１１３ではパージ用下限リミッタ判定フラグ（エンジン始動時にゼロに初期設定）をみる。始動直後であればパージ用下限リミッタ判定フラグ＝０であることよりステップ１１４に進みαとパージ用下限リミッタである１００−ＬＭ２とを比較する。αが１００−ＬＭＴ２を超えているときにはそのまま今回の処理を終了し、αが１００−ＬＭＴ２以下になると、ステップ１１５に進んでパージ用下限リミッタ判定フラグ＝１として今回の処理を終了する。
【０１５１】
このパージ用下限リミッタ判定フラグ＝１により次回にはステップ１１３よりステップ１１６に進みαと１００−ＬＭ２＋ＨＹＳ（ＨＹＳは正の一定値）を比較する。αが１００−ＬＭ２＋ＨＹＳ以下であればステップ１１５に進んでパージ用下限リミッタ判定フラグ＝１のままとし、αが１００−ＬＭ２＋ＨＹＳを超えるとステップ１１７に進んでパージ用下限リミッタ判定フラグ＝０とする。
【０１５２】
図２０に移ると、ステップ１２１、１２２は図１９のステップ１１１、１１２と同様である。
【０１５３】
ステップ１２３ではαのパージ用上限リミッタ判定フラグ（エンジン始動時にゼロに初期設定）をみる。始動直後であればパージ用上限リミッタ判定フラグ＝０であることよりステップ１２４に進みαとパージ用上限リミッタである１００＋ＬＭ２とを比較する。αが１００＋ＬＭＴ２未満であるときにはそのまま今回の処理を終了し、αが１００＋ＬＭＴ２以上になると、ステップ１２５に進んでパージ用上限リミッタ判定フラグ＝１として今回の処理を終了する。
【０１５４】
このパージ用上限リミッタ判定フラグ＝１により次回にはステップ１２３よりステップ１２６に進みαと１００＋ＬＭ２−ＨＹＳ（ＨＹＳは正の一定値）を比較する。αが１００＋ＬＭ２−ＨＹＳ以上であれステップ１２５に進んでパージ用上限リミッタ判定フラグ＝１のままとし、αが１００＋ＬＭ２−ＨＹＳ未満になるとステップ１２７に進んでパージ用上限リミッタ判定フラグ＝０とする。
【０１５５】
図１３のステップ７５に戻り、パージ用リミッタ判定フラグ（パージ用下限リミッタ判定フラグまたはパージ用上限リミッタ判定フラグ）＝０であるときにはステップ７６に進みＭＰＲ０の値をそのまま目標パージ率ＭＰＲとして設定する。
【０１５６】
これに対してパージ用リミッタ判定フラグ＝１であるときにはステップ７７に進みパージ率低下処理を行う。
【０１５７】
このパージ率低下処理は図２１に示したように、例えばパージ用下限リミッタ判定フラグが０から１へと切換わったタイミングでの目標パージ率（図１２のステップ６１〜７３で演算されている）を初期値として所定時間Δｔ当たり所定値Δずつ減らしてゆく制御である。ただし、目標パージ率がゼロ未満になったときにはゼロに制限する。
【０１５８】
図２２（図１３ステップ７７のサブルーチン）はこのパージ率低下処理を行うためのものである。
【０１５９】
図２２において、ステップ１３１、１３２では今回と前回のパージ用リミッタ判定フラグをみる。今回にパージ用リミッタ判定フラグ＝１かつ前回にパージ用リミッタ判定フラグ＝０（つまりパージ用リミッタ判定フラグが０より１へと切換わった直後）のときステップ１３３に進みそのときのＭＰＲ０の値を変数ＭＰＲ１に移す。これは、パージ用リミッタ判定フラグ０より１へと切換わったタイミングでの目標パージ率を初期値としてサンプリングするためである。
【０１６０】
ステップ１３４ではタイマ１をいったんゼロにクリアした後、ステップ１３５でインクリメントを開始する。
【０１６１】
今回、前回ともパージ用リミッタ判定フラグ＝１であるときにはステップ１３３、１３４を飛ばしてステップ１３５の操作を実行する。
【０１６２】
ステップ１３６ではタイマ１値と所定時間Δｔを比較する。タイマ１値が所定時間Δｔ以上になると、ステップ１３７、１３８に進み、タイマ１値をゼロにクリアすると共に、
ＭＰＲ１＝ＭＰＲ１ｚ−Δ…（８）
ただし、ＭＰＲ１ｚ：ＭＰＲ１の前回値、
の式により変数ＭＰＲ１を所定値Δ（正の値）だけ小さくする。タイマ１値が所定時間Δｔ未満のときにはステップ１３９に進み現在のＭＰＲ１を維持する。
【０１６３】
ステップ１３６〜１３９の操作により、ＭＰＲ１は所定時間Δｔが経過する毎に所定値Δずつ小さくなってゆく。
【０１６４】
ここで、パージ率を急激に低下させると排気空燃比が変化し運転性や排気エミッションに影響するので、そうならないように所定時間Δｔと所定値Δの値は目標パージ率を設定する際のパージ率変化量制限値を用いて、またはこれに基づいて定めればよい。
【０１６５】
ステップ１４０ではＭＰＲ１とゼロを比較する。ＭＰＲ１がゼロ以上であればステップ１４２に進みＭＰＲ１を目標パージ率ＭＰＲとして、これに対してＭＰＲ１が負の値になったときにはステップ１４１に進みゼロを目標パージ率ＭＰＲとして設定する。
【０１６６】
このようにしてパージ率低下処理を終了したら図１３に戻り、ステップ７８でパージ終了判定フラグ（図１６により設定されている）をみる。パージ終了判定フラグ＝１のときには、空燃比学習を開始するためステップ７９に進み空燃比学習用のパージ率を目標パージ率ＭＰＲに入れる。これによって空燃比学習を開始するときには図２３に示したように目標パージ率ＭＰＲが小さくなる。
【０１６７】
一方、パージ終了判定フラグ＝０のときにはステップ８０に進みベース空燃比エラー判定フラグをみる。このベース空燃比エラー判定フラグの設定については図２４のフローにより説明する。
【０１６８】
図２４はベース空燃比エラー判定フラグを設定するためのもので、一定時間毎（例えば１００ｍｓｅｃ毎）に実行する。
【０１６９】
ステップ１５１ではベース空燃比エラー判定フラグ（始動時にゼロに初期設定）をみる。始動直後にはベース空燃比エラー判定フラグ＝０であるので、ステップ１５２以降に進む。
【０１７０】
ステップ１５２〜１５４はベース空燃比エラー判定のための条件が成立しているか否かを判定する部分である。パージ実行中かつ吸入空気流量Ｑが大きい領域であるときかつ実パージ率＝０％ときにはベース空燃比エラー判定のための条件が成立したと判断しステップ１５５に進んでタイマ２（ゼロに初期設定）をインクリメントする。このタイマ２はパージ中のパージ率低下処理によりパージ率がゼロになったタイミングからの経過時間を計測するためのものである。
【０１７１】
吸入空気流量Ｑが大きい領域であることをベース空燃比エラー判定のための条件とするのは、Ｑが小さいときにはブローバイガスによる排気空燃比への影響が大きいので、これを避けるためである。
【０１７２】
ステップ１５４では実際のパージ率をパージバルブ２７に与えるデューティ比からみているが実パージ率に代えて目標パージ率を用いることもできる。
【０１７３】
ステップ１５６ではタイマ２値と所定値（一定値）を比較する。タイマ２値が所定値以上になるとステップ１５７に進みベース空燃比エラー判定フラグ＝１とする。
【０１７４】
なお、ステップ１５２〜１５４での条件が総て成立し、ステップ１５５でのタイマ２のインクリメントを開始していても、その後に例えば吸入空気流量Ｑが小さくなると、ステップ１５８に進みタイマ２をゼロにクリアする。すなわち、ベース空燃比エラーであると判定するためにはベース空燃比エラー判定のための条件が、途中でとぎれることなく連続して成立している必要がある。
【０１７５】
図１３のステップ８０に戻り、ベース空燃比エラー判定フラグ＝１であるときにはステップ８１に進み目標パージ率＝０とする。ベース空燃比エラーが生じているときには即座に空燃比学習を開始させるが、目標パージ率＝０とするのは、空燃比学習値の更新を精度良く行わせるにはパージカットの状態であることが必要だからである。
【０１７６】
図２５は空燃比学習開始フラグを設定するためのもので、一定時間毎（例えば１０ｍｓｅｃ毎）に実行する。
【０１７７】
ステップ１７１では空燃比学習開始フラグ（ゼロに初期設定）をみる。エンジン始動直後であれば空燃比学習開始フラグ＝０であることよりステップ１７２、１７３に進み、パージ終了判定フラグ（図１６により設定されている）、ベース空燃比エラー判定フラグ（図２４により設定されている）をみる。
【０１７８】
パージ終了判定フラグ＝１であるときと、パージ終了判定フラグ＝０であってもベース空燃比エラー判定フラグ＝１であるときとには空燃比学習を開始させるためステップ１７４に進み空燃比学習開始フラグ＝１とする。
【０１７９】
図２６は空燃比学習値の更新を行うためのもので、所定のタイミング毎（例えば所定のエンジン回転速度の間隔毎）に実行する。
【０１８０】
ステップ１８１では空燃比学習開始フラグをみる。空燃比学習開始フラグ＝１であるときにはステップ１８２に進み空燃比学習許可条件が成立しているか否かを判定する。空燃比学習許可条件は従来と同様である。例えば空燃比フィードバック制御中であることや空燃比フィードバック制御に関わる空燃比センサ１５等に故障がないことなどを総て満足するときに空燃比学習許可条件が成立していると判断し、ステップ１８３に進んで空燃比学習値を更新する。この空燃比学習値の更新方法には次式によって更新する方法がある。
【０１８１】
αｍ＝αｍｚ＋（α−１００）×Ｍ…（９）
ただし、αｍ ：更新後の空燃比学習値、
αｍｚ：更新前の空燃比学習値、
Ｍ ：学習割合（一定値）、
なお、実施形態は空燃比センサ１５に基づく空燃比フィードバック制御であるが、これに代えてＯ₂センサに基づく空燃比フィードバック制御であってもよく、この場合には、Ｏ₂センサの反転タイミングを空燃比学習値の更新タイミングとすればよい。
【０１８２】
ここで、本実施形態のパージ実行中の作用を図２７、図２８の波形図を参照しながら説明する。
【０１８３】
図２７はベース空燃比エラーがない状態でのエンジン始動後において、初回の較正処理が行われた後のｔ１でアクセルペダルを踏み込み、その踏み込み量をしばらく一定に保ち、ｔ２でアクセルペダルを戻した場合をモデル的に示している。ただし、空燃比学習は外した状態で考えている。
【０１８４】
ｔ１でのアクセルペダルの踏み込みにより最大パージ率がステップ的に大きくなり、この最大パージ率に対して目標パージ率がパージ率変化量制限値により規制された所定のスピードで近づいてゆく。目標パージ率がｔ１以前より大きくなると、それだけパージガス燃料が多くなり、排気空燃比が理論空燃比よりリッチ側に傾くので、これを見越してパージ分補正係数ＦＨＯＳがｔ１より階段状に小さくなってゆくはずであるが（細線参照）、実際にはＦＨＯＳはｔ１より階段状に大きくなっており（太線参照）、このことは初回の較正エラーに起因してＦＨＯＳが正しく演算されていないことを示している。
【０１８５】
この正しくないＦＨＯＳにより（７）式の燃料噴射パルス幅Ｔｉが増量されるので、排気空燃比はリッチ側に傾き、このリッチ側に傾いた排気空燃比を理論空燃比に戻そうと空燃比フィードバック補正係数αがｔ１より階段状に小さくなってゆく。
【０１８６】
そして、小さくなってゆくαが第２下限リミッタである１００−ＬＭＴ２の付近まで小さくなった後には、１００−ＬＭＴ２の付近で周期的に変化している。このことは排気空燃比が理論空燃比の付近に保たれていることを意味する。すなわち、ＦＨＯＳが正しく演算されていなくても、最終的にはαにより排気空燃比が理論空燃比付近へと制御されている。
【０１８７】
その理由を具体的に説明すると、αが１００−ＬＭＴ２を横切って小さくなったタイミングでパージ用下限リミッタ判定フラグが０から１に切換わる（図１９ステップ１１３、１１４、１１５）。パージ用下限リミッタ判定フラグ＝１になるとパージ率低下処理が働いて目標パージ率が減らされる（図１３ステップ７５、７７、図２２）と共にＦＨＯＳも小さくなる。排気空燃比がリーン側に向かうのでαが今度は１００−ＬＭＴ２を横切って大きくなる側に変化する。そして、１００−ＬＭＴ２を越えていても１００−ＬＭＴ２＋ＨＹＳ以下にある間は、パージ用下限リミッタ判定フラグ＝１のままであり（図１９ステップ１１３、１１６、１１５）、αが１００−ＬＭＴ２＋ＨＹＳより大きくなったタイミングでパージ用下限リミッタ判定フラグが１から０に切換わる（図１９ステップ１１３、１１６、１１７）。パージ用下限リミッタ判定フラグ＝０になるとパージ率低下処理が中止され、パージ率低下処理前の目標パージ率へと戻される（図１３ステップ７５、７６）。すると、排気空燃比がリッチ側に向かうのでαが再び１００−ＬＭＴ２を横切って小さくなり、パージ用下限リミッタ判定フラグが０から１に切換わる（図１９ステップ１１３、１１４、１１５）。パージ用下限リミッタ判定フラグ＝１になるとパージ率低下処理が働いて目標パージ率が減らされる図１３ステップ７５、７７、図２２）と共にＦＨＯＳも小さくなる。。
【０１８８】
このような繰り返しにより、αが第２下限リミッタである１００−ＬＭＴ２の付近で周期的に変化しているわけである。
【０１８９】
これにより次のメリットが得られている。
【０１９０】
（１）較正エラーに起因してパージ分補正係数ＦＨＯＳが正しく演算されていなくても、空燃比フィードバック補正係数αにより理論空燃比からの空燃比偏差を小さなものにできている。
【０１９１】
（２）この場合に、空燃比フィードバック補正係数αには第２下限リミッタでる１００−ＬＭＴ２と第１下限リミッタである１００−ＬＭＴ１までの間に補正代を残しており、この状態でパージ以外の外乱があっても、残っている補正代を用いて理論空燃比からの空燃比偏差を小さなものにすることが可能である。
【０１９２】
すなわち、本実施形態（請求項１に記載の発明）によれば、空燃比フィードバック補正係数αの中心値と、もともとある空燃比フィードバック補正係数αに対するリミッタ（第１リミッタ）±ＬＭＴ１との間にパージ用のリミッタとして第２リミッタ±ＬＭＴ２を設け、空燃比フィードバック補正係数αが、中心値の側からこの第２リミッタを横切って外れた場合に目標パージ率を徐々に低下させる処理を行うので、パージによる排気空燃比への影響が低下すると共にパージ分補正係数ＦＨＯＳも小さくなるため、空燃比フィードバック補正係数αが第２リミッタを横切って第１リミッタに移動するまで何もしない場合より空燃比フィードバック補正係数αの補正代が拡大することから、
キャニスタモデルの較正エラーに伴いパージ分補正係数ＦＨＯＳにエラーが生じていても、第２リミッタ±ＬＭＴ２から第１リミッタ±ＬＭＴ１までの間を空燃比フィードバック補正係数の補正代として残しつつ理論空燃比からの空燃比偏差を小さなものにすることができ、残した補正代でパージ以外の外乱に対するロバスト性を確保することができる。
【０１９３】
一方、図２８は吸入空気流量Ｑが大きな領域にあってベース空燃比エラーが判定される場合をモデル的に示している。ただし、この場合も空燃比学習は外した状態で考えている。
【０１９４】
ここで、ベース空燃比とはパージなど外乱のない場合の空燃比ことで、燃料噴射弁４の目詰まりやエアフローメータ１３の流量特性の異常などによりベース空燃比エラーが生じ得る。例えば、エアフローメータ１３が実際の流量を外れて多めの出力をするようになると、このエアフローメータ出力を用いて演算される基本噴射パルス幅Ｔｐは、実際の吸入空気流量に対するよりも大きな値になる。このため、排気空燃比が理論空燃比を外れてリッチ側に傾き、このリッチ側に傾いた排気空燃比を理論空燃比に戻そうと空燃比フィードバック補正係数αが小さくなり、図示のように、αが第２下限リミッタである１００−ＬＭＴ２と第１下限リミッタである１００−ＬＭＴ１との間で周期的に変化している。
【０１９５】
なお、図示しないが、αのこの周期的変化はｔ３以前においても継続しており、ｔ３以前での作用を説明すると、αが第２下限リミッタである１００−ＬＭＴ２を横切って小さくなったタイミングでパージ用下限リミッタ判定フラグが０から１へと切換わり（図１９ステップ１１３、１１４、１１５）、ベース空燃比エラーが大きいためにそれ以後、パージ用下限リミッタ判定フラグ＝１のままが継続している。パージ用下限リミッタ判定フラグ＝１であればパージ率低下処理が働いて目標パージ率が所定時間Δｔ当たり所定値Δずつ低下してゆき、やがてゼロになる（図１３ステップ７５、７７、図２２）。目標パージ率＝０になると、タイマ２のインクリメントによりタイマ２値が徐々に増えてゆく（図２４ステップ１５１〜１５５）。そして、タイマ２のインクリメントが開始されてすぐのタイミングがｔ３であり、このときタイマ２値はまだ所定値に達していない。
【０１９６】
図２８に示すｔ３以降に移ると、タイマ２値のインクリメントはｔ３からも続きｔ４を経過しても続く。そしてタイマ２値が所定値と一致するｔ５のタイミングでベース空燃比エラー判定フラグが０から１へと切換わる（図２４ステップ１５６、１５７）。
【０１９７】
すなわち、パージ実行中に大きなベース空燃比エラーの存在により排気空燃比が理論空燃比から外れてリッチ側になるので、これを理論空燃比に戻そうと空燃比フィードバック補正係数αを第１下限リミッタである７５％まで移動させたのでは、空燃比フィードバック補正係数αの補正代がなくなり、この状態におけるパージ以外の外乱に対するロバスト性が低下してしまうのであるが、本実施形態（請求項５に記載の発明）によれば、演算される空燃比フィードバック補正係数αが第２下限リミッタである１００−ＬＭＴ２を含んで中心値（１００％）の側に収まらないとき、パージ率低下処理により目標パージ率がゼロに向かって低下してゆき、同時にＦＨＯＳも小さくなり、これによってパージによる排気空燃比への影響が小さくなるので、空燃比フィードバック補正係数αに第１下限リミッタである７５％までの補正代が残される。
【０１９８】
そして、パージ率低下処理を続けて目標パージ率がゼロになると、パージによる排気空燃比への影響を完全に排除した状態となり、この状態ではベース空燃比エラーを精度良く検出することが可能となる。
【０１９９】
このように本実施形態（請求項５に記載の発明）によれば、第２リミッタから第１リミッタまでの間を空燃比フィードバック補正係数αの補正代として残しつつベース空燃比エラーの精度の良い検出を可能とし、残したαの補正代でパージ以外の外乱に対するロバスト性を確保することができる。また、ベース空燃比エラーとパージ処理とが重なった場合の排気エミッションの悪化を防止できてもいる。
【０２００】
また、大きなベース空燃比エラーがあると、空燃比フィードバック補正係数αの追従遅れや空燃比フィードバック補正係数αの第１リミッタへの貼り付き等により排気エミッションの悪化要因となるが、本実施形態（請求項６に記載の発明）によれば、ベース空燃比エラーがあると判定したとき、パージカットを行い（図１３ステップ８０、８１）、即座に空燃比学習を開始させるので（図２５ステップ１７３、１７４）、ベース空燃比エラー状態を速やかに解消できる。また、エンジン始動後にパージ処理が終了するまで空燃比学習を開始しないようにしている場合に比べて学習開始までの時間を短縮できる。
【０２０１】
また、実施形態では図示しなかったが、空燃比学習値を燃料系システムの故障診断に用いているものがある。このものでは、ベース空燃比エラーが生じている場合、その原因として燃料系システムに故障が生じている可能性がある。従ってエンジン始動後に空燃比学習の開始が遅れると燃料系システムの故障診断も遅れることとなる。このとき本実施形態（請求項７に記載の発明）を適用し、ベース空燃比エラーであると判定したとき、速やかに空燃比学習を開始させることで、、燃料系システムに故障があった場合でも故障診断を早期に行うことができる。
【０２０２】
また、吸入空気流量Ｑが小さい領域では、ブローバイガスの影響で理論空燃比からの空燃比偏差が大きくなることがあり、このとき誤まってベース空燃比エラーであると判定されてしまうのであるが、本実施形態（請求項９に記載の発明）によれば、ベース空燃比エラーであるか否かを判定する運転領域を、ブローバイガスの影響が小さくなる吸入空気流量の大きい領域に限定するので、ブローバイガスの影響を受けた場合に誤まってベース空燃比エラーであると判定することを防止できる。
【０２０３】
さらに、キャニスタ２４からの燃料の脱離特性は、キャニスタ内の燃料吸着剤（活性炭等）の温度の影響を受けるので、脱離量演算式は吸着剤温度と脱離量の関係を考慮した式とする（請求項１１に記載の発明）。脱離量が多い領域では脱離量の演算誤差が排気空燃比に及ぼす影響が大きく特に高い演算精度が要求され、しかも、キャニスタ２４からの燃料の脱離現象が吸熱反応であるため、大量の脱離が行われるときはキャニスタ内の吸着剤温度が低下し脱離特性も変化する。そのため、吸着剤温度と脱離量の関係を考慮しないと脱離量が多い領域では脱離量の演算精度が低下するが、吸着剤温度も考慮に入れて脱離量を演算するようにキャニスタモデルを構成すれば、脱離量の演算精度の悪化を抑え、演算精度の悪化に起因する排気エミッション悪化等を防止できる。
【０２０４】
なお、吸着燃料量とパージ率と吸着剤温度とに基づき脱離燃料量を演算する脱離量演算式としては、フロイントリッヒ（Freundlich）の式をベースにキャニスタ２４の脱離現象に応用した式を用いることができる。また、吸着剤温度はキャニスタ２４からの燃料の脱離量に基づき吸着剤温度の降下量を求めることによって求めることができる（請求項１２に記載の発明）。
【０２０５】
また、本実施形態（請求項１３に記載の発明）によれば、キャニスタモデルの内部変数であるキャニスタ吸着量の較正処理も行われる。キャニスタモデルは近似モデルであり、それによって演算される値は幾らかの誤差を含んだものとなり、また、キャニスタモデルは前回値を用いて新たな値を演算するため、その動作時間が長くなるにつれ誤差が積分され大きくなるが、このような較正処理を行うことで誤差を適宜修正し、高い演算精度を保つことができる。なお、この較正処理は、理論空燃比からの空燃比偏差から推定される脱離量から吸着量を逆算し、内部変数の吸着量をこの逆算された吸着量で較正するというものであり、特殊なシーケンスを踏む必要が無く、パージ率を落とす必要もない。なお、吸着量はキャニスタモデルにおける脱離量演算式の逆演算によって求めることができる（請求項１４に記載の発明）。
【０２０６】
上記較正処理はパージ以外の外乱による理論空燃比からの空燃比偏差が生じないときに行われ、理論空燃比からの空燃比偏差を全てパージによるものとみなしてキャニスタからの燃料の脱離量を推定し、この推定脱離量から逆算したキャニスタ２４の実吸着量に基づき較正が行われるので、較正処理の精度が確保できる（請求項１５に記載の発明）。
【０２０７】
また、本実施形態（請求項１６、１７に記載の発明）によれば、脱離量の演算精度を高めるべく、較正処理はパージによる理論空燃比からの空燃比偏差が比較的大きなとき（パージ率が高く、パージ空燃比が濃いとき）に行われる。運転領域によっては十分な較正処理の演算精度が得られない場合があるが、このように十分な精度が得られる領域のみで較正処理を実行することにより、較正処理の精度を向上させ、キャニスタモデルの精度をさらに向上させることができる。
【０２０８】
以上、本発明の実施の形態について説明したが、上記実施形態の構成は本発明が適用される構成の一例を示したものであり、本発明の範囲を上記構成に限定するものではない。上述した通り、上記実施形態においては、キャニスタモデルによるパージ処理が可能となるまでは、図１６に示したパージ処理が補助的に実行されるが、図１６に示したパージ処理を継続して用いるようにしてもよい。
【０２０９】
また、請求項５に記載の発明は、先願装置のパージ処理を行うものに限らず、始動直後より通常のパージ処理を行うものにも適用があることはいうまでもない。
【図面の簡単な説明】
【図１】本発明に係る蒸発燃料処理装置の全体構成図。
【図２】エンジンコントローラにおけるパージ処理の概要を示したブロック図。
【図３】キャニスタモデルの構成を示したブロック図。
【図４】パージ分補正係数の演算を説明するためのフローチャート。
【図５】較正処理可能条件を示したフローチャート。
【図６】較正処理を説明するためのフローチャート。
【図７】キャニスタモデルに基づく脱離量の演算を説明するためのフローチャート。
【図８】遅れ補正を説明するためのフローチャート。
【図９】吸入空気流量と無駄時間の関係を示す特性図。
【図１０】吸入空気流量となまし係数の関係を示す特性図。
【図１１】遅れ補正における無駄時間処理の概要を示した図。
【図１２】目標パージ率の設定を説明するためのフローチャート。
【図１３】目標パージ率の設定を説明するためのフローチャート。
【図１４】吸入空気流量及びパージ率に対するパージ空燃比誤差の関係を示す特性図。
【図１５】パージ空燃比に対するパージ空燃比誤差の関係を示す特性図。
【図１６】ブートアップ制御を説明するためのフローチャート。
【図１７】空燃比フィードバック偏差の差（＝目標空燃比フィードバック偏差−実空燃比フィードバック偏差）とパージ率変化量の関係を示す特性図。
【図１８】パージ空燃比に対するパージ終了判定値の関係を示す特性図。
【図１９】パージ用下限リミッタ判定フラグの設定を説明するためのフローチャート。
【図２０】パージ用上限リミッタ判定フラグの設定を説明するためのフローチャート。
【図２１】パージ率低下処理を説明するための波形図。
【図２２】パージ率低下処理を説明するためのフローチャート。
【図２３】パージ終了が判定されたときの目標パージ率の変化を示す波形図。
【図２４】ベース空燃比エラー判定フラグの設定を説明するためのフローチャート。
【図２５】空燃比学習開始フラグの設定を説明するためのフローチャート。
【図２６】空燃比学習値の更新を説明するためのフローチャート。
【図２７】空燃比学習は外した状態で初回の較正エラーに起因してパージ分補正係数が正しく演算されていない場合をモデル的に示した波形図。
【図２８】空燃比学習は外した状態でベース空燃比エラーが判定される場合をモデル的に示した波形図。
【符号の説明】
４ 燃料噴射弁
５ 排気通路
８ 吸気通路
１１ エンジンコントローラ
１３ エアフローメータ
１５ 空燃比センサ（空燃比検出手段）
２１ 燃料タンク
２２ 配管
２４ キャニスタ
２６ 配管
２７ パージバルブ[0001]
[Industrial application fields]
The present invention relates to an air-fuel ratio control device for an engine, and more particularly to a device provided with an evaporated fuel processing device.
[0002]
[Prior art]
The engine once adsorbs the evaporated fuel generated in the fuel tank while the engine is stopped to the activated carbon in the canister, and uses the intake pipe pressure (pressure lower than atmospheric pressure) under the predetermined operating conditions after engine startup. In addition, an evaporative fuel processing device is provided that desorbs the fuel adsorbed on the activated carbon and introduces it into the intake passage downstream of the intake throttle valve for combustion treatment (see Japanese Patent Application Laid-Open No. 7-166978).
[0003]
In the following, “purge” or “purge process” refers to desorbing the fuel adsorbed by the canister and supplying it to the engine, “purge fuel” as the fuel desorbed from the canister, and not only the purge fuel but also the purge fuel. The fresh air supplied to the engine is sometimes referred to as “purge gas”.
[0004]
In such a conventional apparatus, the influence of the purge on the exhaust air / fuel ratio is treated as a disturbance, and even if the exhaust air / fuel ratio deviates from the theoretical air / fuel ratio due to the disturbance, the oxygen concentration sensor provided in the exhaust passage is used. Air-fuel ratio feedback control is performed to return the exhaust air-fuel ratio to the stoichiometric air-fuel ratio, so that the conversion efficiency of the three-way catalyst provided in the exhaust passage is not lowered.
[0005]
[Problems to be solved by the invention]
By the way, since the change of the air-fuel ratio feedback correction coefficient α (air-fuel ratio feedback correction amount) due to the supply of purge fuel to the engine can be predicted, the predictable air-fuel ratio feedback correction coefficient α is set to the purge correction coefficient FHOS (purge And a basic value of the amount of fuel supplied to the engine with this purge correction coefficient FHOS so that the air-fuel ratio deviation from the theoretical air-fuel ratio due to the purge becomes small,
(1) an expression for calculating the amount of fuel adsorbed by the canister;
(2) Fuel desorbed from the canister based on the amount of adsorbed fuel and the target purge rate
Formula to calculate the amount
An apparatus that estimates the amount of fuel desorbed from the canister using a canister model configured as follows, and calculates the purge correction coefficient FHOS based on the desorbed fuel amount, the target purge rate, and the intake air mass Has been previously proposed by the same applicant as the present application (see Japanese Patent Application No. 2001-71562).
[0006]
According to the purge process (model reference purge process) by this prior application apparatus, the disturbance due to the purge is exclusively handled by the purge correction coefficient FHOS, and there is no base air-fuel ratio error and the FHOS is accurately calculated. For example, the exhaust air-fuel ratio is controlled to the stoichiometric air-fuel ratio without performing the air-fuel ratio feedback control. That is, when the air-fuel ratio feedback control condition is satisfied during the purge process and the air-fuel ratio feedback control is started, the air-fuel ratio feedback correction coefficient α moves as if there is no purge.
[0007]
For the air-fuel ratio feedback correction coefficient α, a first limiter that determines the limit of the correction allowance is provided in advance, for example, ± 25%, and α is increased from 100%, which is the central value of α, to 125% (upper limiter). Side full correction allowance, and 75% (lower limiter) from 100% is the full correction allowance for reducing α. According to the conventional apparatus, the influence of the purge on the exhaust air / fuel ratio is controlled by air / fuel ratio feedback. Since the correction coefficient α compensated, the correction amount of α with respect to the disturbance was reduced accordingly, but according to the prior application device, the air-fuel ratio feedback correction coefficient α becomes free for purging, and α Can be fully used for disturbances other than purge.
[0008]
In addition, by setting the maximum purge rate within a certain limit range depending on the flow rate of the purge valve as the target purge rate, a large amount of purge can be performed earlier than immediately after the engine is started.
[0009]
Now, there was room for improvement in these prior application devices. Specifically, since the canister model is an approximate model, values (desorption amount, adsorption amount, etc.) calculated using the canister model are slightly deviated from actual values. Since the canister model is a recurrence formula as will be described later, as the model operation time becomes longer, the error is integrated and the deviation between the calculated value and the actual value increases. For this reason, in order to calibrate this deviation and maintain high calculation accuracy of the canister model, the value of the adsorption amount of the canister, which is one of the internal variables of the canister model, is calibrated at the calibration processing timing.
[0010]
However, since the calibration process of the canister model involves an error, an error also occurs in the purge correction coefficient FHOS under the influence of the calibration error. Thus, even when an error occurs in the purge correction coefficient FHOS, the air-fuel ratio deviation from the stoichiometric air-fuel ratio due to the purge is eliminated by the air-fuel ratio feedback correction coefficient α during the air-fuel ratio feedback control.
[0011]
This is because, according to the prior application apparatus, the effect of the purge on the exhaust air / fuel ratio is compensated by the purge correction coefficient FHOS, so that the correction allowance of the air / fuel ratio feedback correction coefficient α should be fully usable. If an error occurs in the minute correction coefficient FHOS, it means that the correction margin of the air-fuel ratio feedback correction coefficient α becomes narrow again.
[0012]
In this case, the purge correction coefficient FHOS is proportional to the target purge rate (the ratio of the purge gas flow rate to the intake air flow rate). Therefore, if the target purge rate is reduced, the purge correction coefficient FHOS is also reduced, and the influence of the erroneous purge correction coefficient FHOS on the exhaust air / fuel ratio becomes small.
[0013]
Therefore, the present invention is based on the premise that the second limiter is used as a purge limiter between the central value of the air-fuel ratio feedback correction coefficient α and the limiter (first limiter) for the original air-fuel ratio feedback correction coefficient α. When the air-fuel ratio feedback correction coefficient α deviates from the center value side across the second limiter, the target purge rate is gradually decreased (by a predetermined value per predetermined time), and the purge rate is reduced. When the air-fuel ratio feedback correction coefficient α returns to the center value side across the second limiter by the process, the purge rate reduction process is canceled as unnecessary, and an error occurs in the purge correction coefficient due to the calibration error of the canister model. Even if it occurs, the correction range for the air-fuel ratio feedback correction coefficient is from the second limiter to the first limiter. To reduce the air-fuel ratio deviation from the stoichiometric air-fuel ratio due to the purge while leaving, and an object thereof is to ensure the robustness against disturbance other than the purge correction allowance left.
[0014]
On the other hand, the deviation of the base air-fuel ratio from the stoichiometric air-fuel ratio due to aging deterioration and variation of components such as the air flow meter and fuel injection valve, and the non-linearity of the injection pulse width-flow characteristic of the fuel injection valve is referred to as the base air-fuel ratio error. In order to eliminate this base air-fuel ratio error, air-fuel ratio learning control is performed during the stoichiometric operation (when the stoichiometric air-fuel ratio is the target air-fuel ratio).
[0015]
When such air-fuel ratio learning control is combined with a purging process in which a large amount of purging is performed immediately after the engine is started as in the prior application device, the following problem occurs. That is, even if there is a base air-fuel ratio error, if the air-fuel ratio learning control is operating normally, the base air-fuel ratio error is absorbed by the air-fuel ratio learning value, so that the base air-fuel ratio error does not exist, If there is a significant base air-fuel ratio error immediately after the engine is started for some reason, there may be a case where the correction allowance is not sufficient by the correction allowance by the air-fuel ratio feedback correction coefficient α and cannot be corrected. If correction is not possible, that is, if the air-fuel ratio deviation from the stoichiometric air-fuel ratio due to purging is large, exhaust emissions will deteriorate.
[0016]
On the other hand, since purging is required to flow in a large amount after starting for the purpose of reducing deterioration of exhaust emission due to purging, it is desirable to start the purging process early as in the prior application apparatus.
[0017]
Therefore, after starting a large amount of purge immediately after starting, it is possible to determine whether the influence that appears in the exhaust air / fuel ratio during the air / fuel ratio feedback control is due to the purge or the base air / fuel ratio error. difficult.
[0018]
In this case, it may be possible to detect the base air-fuel ratio error before the start of the purge process, but the base air-fuel ratio error is determined in a short time in a state where the engine has not been warmed up as before the purge start. This is difficult considering disturbances. Although it is possible to cut the purge process for a predetermined time during the purge execution and detect the base air-fuel ratio error during the cut, this method cannot continue the purge gas flow during the purge cut. This is not in line with the original requirement for the purging process to perform a large amount of purging process promptly.
[0019]
Therefore, the present invention presupposes that the air-fuel ratio learning correction control is combined with a purge process that performs a large amount of purge immediately after the engine is started, as in the prior application, and the center value of the air-fuel ratio feedback correction coefficient α. A second limiter is provided as a purge limiter between the limiter (first limiter) for the original air-fuel ratio feedback correction coefficient α, and the air-fuel ratio feedback correction coefficient α crosses the second limiter from the center value side. When the target purge rate falls, the target purge rate is gradually reduced (by a predetermined value per predetermined time), and even if the target purge rate becomes zero in this purge rate reduction processing, the state where the target purge rate has gone across the second limiter continues. For example, the second limit is measured by measuring the continuing time and determining that there is a base air-fuel ratio error when the time reaches a predetermined value. The until first limiter to allow detection of the base air-fuel ratio error while leaving a correction allowance of the air-fuel ratio feedback correction coefficient from an object to secure the robustness against disturbance other than the purge correction allowance left.
[0020]
[Means for solving problems]
According to the first aspect of the present invention, the air-fuel ratio detection means for detecting the air-fuel ratio in the exhaust gas and the air-fuel ratio feedback correction amount are set so that the exhaust air-fuel ratio detected by the air-fuel ratio detection means matches the stoichiometric air-fuel ratio. An air-fuel ratio feedback correction amount calculating means for calculating, a first limiter setting means for setting a first limiter having a predetermined width from the center value of the air-fuel ratio feedback correction amount, and the air-fuel ratio feedback correction amount including the first limiter. An air-fuel ratio feedback correction amount limiting means for limiting the calculated air-fuel ratio feedback correction amount so as to fall within the center value side, and an air-fuel ratio for correcting the amount of fuel supplied to the engine with the limited air-fuel ratio feedback correction amount An air-fuel ratio control apparatus for an engine comprising feedback correction means, and a canister for adsorbing evaporated fuel generated in a fuel tank; A purge valve for opening and closing a pipe that communicates the intake passage of the canister and the engine, a purge valve control means for controlling the purge valve so that the target purge ratio is obtained,
at least,
(A) an adsorbed fuel amount calculation formula for calculating the amount of fuel adsorbed to the canister by subtracting the previous value of the amount of fuel desorbed from the canister from the previous value of the amount of fuel adsorbed to the canister;
(B) Adsorbed fuel amount calculated by the adsorbed fuel amount calculation formula Desorption index with the value obtained by dividing the value by the desorption coefficient A desorbed fuel amount calculation formula for calculating the amount of fuel desorbed from the canister based on the exponential function of
A canister model consisting of
Purge amount correction that calculates the purge amount correction amount based on the desorbed fuel amount calculated using the canister model so that the air-fuel ratio deviation from the theoretical air-fuel ratio becomes smaller by performing the purge process at the target purge rate The amount calculating means, the purge amount correcting means for correcting the amount of fuel supplied to the engine with this purge amount correction amount, and the calibration process timing, the air-fuel ratio deviation from the theoretical air-fuel ratio is regarded as being caused by the purge process, and the canister The amount of fuel desorbed is estimated, and the amount of fuel adsorbed to the canister is calculated from the estimated amount of desorbed fuel using the desorbed fuel amount calculation formula, and the adsorbed fuel that is an internal variable in the canister model Means for calibrating the amount value with the back-calculated adsorbed fuel amount value, and a second limiter having a narrower range to the center value of the air-fuel ratio feedback correction amount than the first limiter. And means, when the air-fuel ratio feedback correction amount is calculated is not fit to the side of the central value include a second limiter, and a purge rate reduction processing means for performing processing of reducing the target purge rate.
[0021]
The invention according to claim 5 is an air-fuel ratio learning value storage means for storing an air-fuel ratio learning value, an air-fuel ratio learning value reading means for reading out the air-fuel ratio learning value according to operating conditions, and the read air-fuel ratio learning. An air-fuel ratio correcting means for correcting the amount of fuel supplied to the engine by the value, an air-fuel ratio detecting means for detecting the air-fuel ratio in the exhaust, and the exhaust air-fuel ratio detected by the air-fuel ratio detecting means matches the stoichiometric air-fuel ratio. Air-fuel ratio feedback correction amount calculating means for calculating the air-fuel ratio feedback correction amount, first limiter setting means for setting a first limiter having a predetermined width from the center value of the air-fuel ratio feedback correction amount, and air-fuel ratio feedback correction Air-fuel ratio feedback correction amount limiting means for limiting the calculated air-fuel ratio feedback correction amount so that the amount falls within the center value including the first limiter; An air-fuel ratio control device for an engine comprising an air-fuel ratio feedback correction means for correcting an amount of fuel supplied to an engine with a limited air-fuel ratio feedback correction amount. A purge valve that opens and closes a pipe that communicates with the intake passage, a purge valve control means that controls the purge valve so as to obtain a target purge rate, and a narrower range to the center value of the air-fuel ratio feedback correction amount than the first limiter. Means for setting two limiters, and purge rate lowering processing means for lowering the target purge rate toward zero when the calculated air-fuel ratio feedback correction amount does not fall within the center value including the second limiter. And a period during which the target purge rate continues to be zero due to this purge rate reduction process. And a base air-fuel ratio error determination means determines that the base air-fuel ratio error when a value.
[0022]
[Action and effect]
When the purge correction amount is calculated correctly, the air-fuel ratio feedback correction amount is the same as when the purge is not performed.However, since the calibration process of the canister model involves an error, the influence of this calibration error is In response, an error also occurs in the purge correction amount. Thus, even if an error occurs in the purge correction amount, the air-fuel ratio deviation from the stoichiometric air-fuel ratio is reduced by the air-fuel ratio feedback correction amount. This is because if the correction amount of the air-fuel ratio feedback correction amount should be fully usable by compensating the influence of the purge on the exhaust air-fuel ratio by the purge amount correction amount, but an error occurs in the purge amount correction amount. This again means that the correction amount of the air-fuel ratio feedback correction amount becomes narrow.
[0023]
In contrast, in the first aspect of the invention, the second limiter is used as a purge limiter between the center value of the air-fuel ratio feedback correction amount and the limiter (first limiter) for the original air-fuel ratio feedback correction amount. And when the air-fuel ratio feedback correction amount deviates from the center value side across the second limiter, the target purge rate is gradually reduced, so that the effect of the purge on the exhaust air-fuel ratio is reduced and purge is performed. Since the minute correction amount itself is also small, the correction amount of the air-fuel ratio feedback correction amount is larger than when nothing is done until the air-fuel ratio feedback correction amount moves across the second limiter to the first limiter.
[0024]
Thus, according to the first aspect of the present invention, even if an error occurs in the purge correction amount due to the calibration error of the canister model, the air-fuel ratio feedback correction is performed between the second limiter and the first limiter. The air-fuel ratio deviation from the stoichiometric air-fuel ratio can be reduced while leaving the amount as a correction allowance, and the remaining correction allowance can ensure robustness against disturbances other than purge.
[0025]
Next, since the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio during the purge execution, if the air-fuel ratio feedback correction amount is moved to the first limiter to return it to the stoichiometric air-fuel ratio, the air-fuel ratio feedback correction amount However, the robustness against disturbances other than purge during air-fuel ratio feedback control is reduced, but the calculated air-fuel ratio feedback correction amount is the second limiter. When the target value is not within the center value, the target purge rate is reduced to zero by the purge rate reduction process, so that the influence of the purge on the exhaust air / fuel ratio is reduced, and thereby the air / fuel ratio feedback correction amount. Will be left with a correction fee.
[0026]
Then, when the purge rate reduction process is continued and the target purge rate becomes zero, the influence of the purge on the air-fuel ratio is completely eliminated, and in this state, the base air-fuel ratio error can be detected.
[0027]
Thus, according to the fifth aspect of the present invention, it is possible to detect the base air-fuel ratio error while leaving the interval from the second limiter to the first limiter as the correction allowance for the air-fuel ratio feedback correction amount, and the remaining correction allowance. Thus, robustness against disturbances other than purge can be ensured. Further, it is possible to prevent the exhaust emission from deteriorating when the base air-fuel ratio error and a large amount of purge overlap.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0029]
FIG. 1 shows the overall configuration of an engine equipped with an evaporative fuel processing device.
[0030]
Reference numeral 1 denotes an engine body. A fuel injection valve 4 is provided for each cylinder in the intake passage 2 downstream of the intake throttle valve 3, and a predetermined air-fuel ratio is set according to an operating condition by an injection signal from the engine controller 11. Thus, fuel is injected and supplied during intake. More specifically, a signal from the crank angle sensor 12, a signal of the intake air flow rate from the air flow meter 13, a signal of the engine cooling water temperature from the water temperature sensor 14, etc. are input to the engine controller 11 mainly composed of a microcomputer, and the engine Based on these signals, the controller 11 calculates a basic injection pulse width Tp that provides a basic air-fuel ratio (theoretical air-fuel ratio), and performs various corrections such as an increase in water temperature to obtain the fuel injection pulse width Ti. The fuel injection valve 4 is opened for a period corresponding to this Ti at a predetermined timing.
[0031]
The exhaust passage 5 is provided with a three-way catalyst (not shown). The three-way catalyst reduces NOx in the exhaust gas and oxidizes HC and CO with maximum conversion efficiency when the air-fuel ratio in the exhaust gas is in a narrow range centered on the stoichiometric air-fuel ratio. Therefore, the engine controller 11 calculates the air-fuel ratio feedback correction coefficient α so that the air-fuel ratio in the exhaust detected by the air-fuel ratio sensor 15 matches the stoichiometric air-fuel ratio, and this air-fuel ratio feedback correction coefficient α (air-fuel ratio feedback) The air-fuel ratio feedback control is performed by correcting the basic injection pulse width Tp (basic value of the amount of fuel supplied to the engine) with the correction amount).
[0032]
In this case, with respect to the air-fuel ratio feedback correction coefficient α, a first limiter ± LMT1 (± 25%) having a predetermined width around the center value (100%) of α as a range in which α can move is provided in advance. When the calculated α becomes larger than the center value and exceeds the upper limit limit 100 + LMT1, this upper limit limiter is used. When the calculated α becomes smaller than the center value and falls below the lower limit limit 100-LMT1, this lower limit limiter is set. Limited.
[0033]
On the other hand, the evaporative fuel processing device is for processing evaporative fuel generated in the fuel tank 21. The evaporative fuel generated in the fuel tank 21 is a canister 24, a pipe 22 communicating the canister 24 and the fuel tank 21, and the canister 24 and the intake throttle valve 3 of the engine. A pipe 26 that communicates with the downstream intake passage 2 and a purge valve 27 that opens and closes the pipe 26 are provided.
[0034]
The evaporated fuel generated in the fuel tank 21 is guided to the canister 24 via the pipe 22, and only the fuel component is adsorbed by the fuel adsorbent (activated carbon) 24 a in the canister 24, and the remaining air is supplied from the atmosphere release port 25. Released to the outside. In order to combust the fuel adsorbed on the activated carbon 24a, the purge valve 27 is opened, and fresh air is introduced into the canister 24 from the atmosphere opening 25 using the intake pipe pressure (pressure lower than the atmosphere) developed downstream of the intake throttle valve 3. Is introduced. As a result, the fuel adsorbed on the activated carbon 24a by the fresh air is desorbed and introduced into the intake passage 2 of the engine 1 through the pipe 26 together with the fresh air.
[0035]
When performing processing (purge processing) in which the fuel adsorbed on the canister 24 is desorbed and supplied to the engine (purge processing), the engine controller 21 uses the target purge rate as high as possible without deteriorating the combustion stability and exhaust emissions of the engine. The ratio of the purge flow rate to the intake air flow rate is set, and the opening degree of the purge valve 27 is controlled so that the target purge rate is realized after the engine is started.
[0036]
Further, after the purge process is started, the purge becomes a disturbance and an air-fuel ratio deviation from the stoichiometric air-fuel ratio occurs. Since the exhaust air-fuel ratio deviates to the rich side from the stoichiometric air-fuel ratio at the beginning of the purge, the air-fuel ratio feedback correction coefficient α moves to a side smaller than the center value until the deviated air-fuel ratio returns to the stoichiometric air-fuel ratio. The target purge rate is set as high as possible after the start, and when a large amount of purging is performed, α moves close to 75%, which is the lower limiter of α.
[0037]
In this case, if a disturbance other than purge occurs and the exhaust air / fuel ratio deviates from the stoichiometric air / fuel ratio and becomes richer, α moves to a smaller side, but immediately reaches the lower limiter and decreases further. Don't be. In other words, since the α correction amount for reducing α to the disturbance caused by the purge is almost used up, there is little correction fee remaining for the disturbance other than the purge. Even if it is used, if the exhaust air-fuel ratio cannot be returned to the stoichiometric air-fuel ratio, the exhaust emission deteriorates.
[0038]
For this reason, the engine controller 11 estimates the amount of fuel desorbed from the canister using a canister model, which will be described later, and a purge correction coefficient based on the estimated amount of desorbed fuel Dg, target purge rate, and intake air mass By calculating FHOS and correcting the basic injection pulse width Tp with this purge correction coefficient FHOS, even if a large amount of purge is performed with a target purge rate set as high as possible after engine startup, the theoretical air-fuel ratio The air-fuel ratio deviation from That is, since the purge correction coefficient FHOS compensates for the influence of the purge on the exhaust air / fuel ratio, the air / fuel ratio feedback correction coefficient α behaves in the same manner as when there is no purge, and thus α is caused by the purge. It is not necessary to use a correction allowance for the disturbance, and the correction allowance remains full, and has strong robustness against disturbances other than purge.
[0039]
As will be described later, a change in the air-fuel ratio feedback correction coefficient α can be predicted by supplying the desorption amount Dg calculated by the canister model to the engine. This predicted α is a purge correction coefficient FHOS. is there. Therefore, the predicted α increases as the target purge rate increases, increases as the desorption amount Dg increases, and decreases as the intake air mass Qg increases. Therefore, the purge correction coefficient FHOS increases as the target purge rate increases, increases as the desorption amount Dg increases, and decreases as the intake air mass Qg increases.
[0040]
Further, it is necessary to perform a calibration process on the canister model, and if a calibration error occurs at that time, the purge correction factor FHOS is not correctly calculated. As a result, the erroneous purge correction factor FHOS causes the theoretical air-fuel ratio to be corrected. When the air-fuel ratio deviation of the air-fuel ratio occurs, the air-fuel ratio feedback correction coefficient α uses the correction margin so that the air-fuel ratio deviation from the theoretical air-fuel ratio becomes small. As a result, the correction allowance of α is reduced. Therefore, the engine controller 11 sets the second limiter ± LMT2 that is narrower than the first limiter ± LMT1 to the center value of the air-fuel ratio feedback correction amount, and is calculated as an air-fuel ratio. When the feedback correction coefficient α does not fall on the center value side including the second limiter, a process for reducing the target purge rate is performed.
[0041]
On the other hand, in order to eliminate the base air-fuel ratio error caused by the deterioration and variation of components such as the air flow meter 13 and the fuel injection valve 4 and the non-linearity of the injection pulse width-flow characteristic of the fuel injection valve 4, the engine controller 11 When the stoichiometric operation (when the stoichiometric air-fuel ratio is set to the target air-fuel ratio) is being performed, the air-fuel ratio learning control is performed. If for some reason there is a significant base air-fuel ratio error immediately after the engine is started, In this case, there may be a case where the correction amount by the air-fuel ratio feedback correction coefficient α is not sufficient and the correction amount is not sufficient. Moreover, if a large amount of purge is started at this time, it is difficult to determine whether the influence appearing in the exhaust air / fuel ratio is due to the large amount purge or due to the base air / fuel ratio error.
[0042]
For this reason, the engine controller 11 performs a process of gradually reducing the target purge rate when the air-fuel ratio feedback correction coefficient α deviates from the center value side across the second limiter. Even if the purge rate becomes zero, if the state where it has crossed the second limiter continues, the continuing time is measured, and when the time reaches a predetermined value, it is determined that there is a base air-fuel ratio error.
[0043]
A bypass valve 29 that bypasses the vacuum cut valve 23 that opens when the passage on the fuel tank 21 side becomes lower than atmospheric pressure is provided in the pipe 22, and a pressure sensor 30 that measures the pressure in the pipe is provided in the pipe 26. Further, a drain cut valve 28 is provided at the air release port 25 of the canister 24, but these are necessary for the diagnosis of the evaporation processing apparatus and are not directly related to the purge processing.
[0044]
Such control performed by the engine controller 11 will be first outlined with reference to the block diagrams shown in FIGS.
[0045]
FIG. 2 is a block diagram showing an outline of a portion related to purge processing and air-fuel ratio control. In FIG. 2, the target purge rate setting unit B1 calculates the maximum purge rate that can be set in the current operation region based on the performance limit of parts related to the purge process, and sets the target purge rate to follow this maximum purge rate. Set. However, sudden changes in the purge rate cause large fluctuations in the exhaust air / fuel ratio (increase in the air / fuel ratio deviation from the theoretical air / fuel ratio) and cause deterioration of exhaust emissions, etc., so the purge rate is not changed rapidly. Thus, the change amount of the purge rate is limited to a predetermined amount (purge rate change amount limit value) or less.
[0046]
Further, the duty ratio calculation unit B2 calculates the duty ratio of the purge valve 27 necessary for realizing the target purge rate, and the purge valve drive unit B3 drives the purge valve 27 with the duty ratio calculated by the duty ratio calculation unit B2.
[0047]
On the other hand, in the desorption amount calculation unit B4, the amount of fuel desorbed from the canister 24 when purging at the target purge rate using a physical model of the canister 24 (hereinafter referred to as “canister model”) which will be described later. Is calculated.
[0048]
In the purge correction coefficient calculation unit B5, the purge correction coefficient is calculated so that the deviation of the air-fuel ratio from the theoretical air-fuel ratio caused by the purge is reduced based on the estimated desorption amount (desorbed fuel amount), the target purge rate, and the intake air flow rate. Calculate FHOS. The delay correction unit B6 applies a delay correction including a dead time correction and a smoothing process in consideration of the intake air flow rate to the purge correction coefficient FHOS, and the correction coefficient after the delay correction is changed to a purge correction coefficient FHOS. .
[0049]
The basic injection pulse width calculation unit B7 calculates the basic injection pulse width Tp from the intake air flow rate and the engine speed, and the air-fuel ratio feedback correction coefficient calculation unit B8 calculates the air-fuel ratio feedback correction coefficient α based on the actual exhaust air-fuel ratio. In the fuel injection pulse width calculation unit B9, various corrections and corrections using the air-fuel ratio feedback correction coefficient α are performed on the basic injection pulse width Tp as described above, and the purge correction coefficient FHOS after delay correction is used. Correction is performed to calculate the final fuel injection pulse width Ti. The fuel injection valve drive unit B10 drives the fuel injection valve 4 with the fuel injection pulse width Ti calculated in this way.
[0050]
As shown in FIG. 3, the canister model includes an adsorption amount calculation unit B21, a reference desorption amount calculation unit B22, a purge flow rate equivalent desorption amount calculation unit B23, and an activated carbon temperature calculation unit B24. Each calculation is performed every time.
[0051]
The adsorption amount calculation unit B21 calculates the current value Y of the fuel amount adsorbed by the canister 24 by the following formula.
[0052]
[Adsorption amount calculation formula]
Y = Yz−Dgz (1)
Where Y: current value of adsorption amount,
Yz: previous value of adsorption amount,
Dgz: previous value of desorption amount,
This adsorption amount calculation formula calculates the current (current) adsorption amount Y [g] by subtracting Dgz which is the fuel amount desorbed last time from the previous value Yz of the adsorption amount (adsorbed fuel amount).
[0053]
The reference desorption amount calculation unit B22 calculates the desorption amount Dgk [g] at the reference purge flow rate by the following equation.
[0054]
[Desorption amount calculation formula at the reference purge flow rate]
Dgk = (Y / A) ^ n (T) (2)
Where Y: adsorption amount,
A: Desorption constant,
n (T): Desorption index,
T: Activated carbon temperature,
This formula applies the concept of adsorption / desorption phenomenon (Freundlich's formula) to the fuel desorption phenomenon from the canister 24, and thereby expresses the fuel desorption characteristics from the canister 24 almost accurately. The Freundlich formula is described in “Theory on the Surface II” (Maruzen, Tsukada), p.25-p.27, p.108-p.115.
[0055]
The purge flow equivalent desorption amount calculation unit B23 calculates the desorption amount Dg [g] from the following equation.
[0056]
[Desorption calculation formula according to purge flow rate]
Dg = K × PQ × Dgk (3)
Where K is a constant,
PQ: purge flow rate (= target purge rate × intake air flow rate),
Dgk: Desorption amount at the reference flow rate,
The desorption amount calculation formula corresponding to the purge flow rate PQ is to calculate the desorption amount by linear approximation since the purge flow rate PQ and the desorption amount are substantially proportional. Here, the desorption amount Dgk at the reference flow rate is obtained from the equation (2), and the desorption amount Dg is calculated by multiplying this by the purge flow rate PQ in the equation (3), but the equation (2) , (3) may be combined into one expression.
[0057]
The activated carbon temperature calculation unit B24 calculates the activated carbon temperature T [K] by the following equation.
[0058]
[Activated carbon temperature calculation formula]
T = Tz−Kt1 × (Yz2−Yz) + Kt2 × (Tz−Ta) (4)
Where T: current value of activated carbon temperature,
Tz: previous value of activated carbon temperature,
Kt1: endothermic coefficient,
Yz2: value before and after adsorption amount,
Yz: previous value of adsorption amount,
Kt2: heat transfer coefficient,
Ta: Canister ambient temperature,
This activated carbon temperature calculation formula includes the past temperature (first term on the right side), a temperature drop proportional to the desorption amount (Yz2-Yz) (second term on the right side), and a temperature rise due to heat transfer (third on the right side). Item). The activated carbon temperature T is calculated in this way because the desorption index n (T) in the equation (2) is affected by the activated carbon temperature T, and particularly when the desorption amount is large, the decrease in the activated carbon temperature T is large. This is because the influence on the fuel desorption characteristics in the canister 24 cannot be ignored.
[0059]
Therefore, the canister model is composed of the above four expressions (1) to (4), and when the expressions (2) and (3) are combined, the canister model is composed of three expressions.
[0060]
Returning to FIG. 2, the canister model expresses the desorption characteristics of the canister with high accuracy as described above. However, since the canister model is an approximate model, values calculated using the canister model (desorption amount, adsorption amount, etc.) ) Is slightly different from the actual value. Further, since the canister model is a recurrence formula using the previous calculation result as shown in the above equation (1), the error is integrated as the model operation time becomes longer, and the deviation between the calculated value and the actual value increases. To do.
[0061]
Therefore, in order to calibrate this deviation and maintain high calculation accuracy of the canister model, when the calibration timing determination unit B11 determines that the calibration processing is possible, the calibration processing unit B12 is one of the internal variables of the canister model. The adsorption amount value of the canister 4 is calibrated. Specifically, in the calibration timing determination unit B11, when a condition that allows almost all deviations from the center value (100%) of the air-fuel ratio feedback correction coefficient α during the air-fuel ratio feedback control to be considered to be purged is satisfied. It is determined that the calibration process can be executed at a timing, and the switch B13 is turned on. In the calibration processing unit B12, the deviation of the total air-fuel ratio correction coefficient (α × FHOS) from the center value of the air-fuel ratio feedback correction coefficient α at that timing The amount of fuel desorbed from the canister 24 is estimated based on the target purge rate and the intake air flow rate, and the amount of adsorption is calculated backward from the estimated amount of desorption. Then, the value of the adsorption amount possessed by the canister model is calibrated with this value.
[0062]
Hereinafter, specific contents of the purge process and the air-fuel ratio control performed by the engine controller 11 will be described with reference to FIGS. 4, 6, 7, 8, 12, 13, 16, 19, 20, 22, and 24. This will be described with reference to the flowcharts shown in FIGS.
[0063]
FIG. 4 is for calculating the purge correction coefficient, and is executed at regular intervals (for example, every 10 msec).
[0064]
In step 1, it is determined whether or not a purge permission condition is satisfied. If the purge permission condition is not satisfied, the process proceeds to step 2 where the purge correction coefficient FHOS = 1.0 and the current process is terminated.
[0065]
When the purge permission condition is satisfied, the process proceeds to step 3 to determine whether or not it is a timing at which the calibration process of the adsorption amount, which is an internal variable of the canister model, can be executed. When the disturbance due to factors other than purge is small and the influence of the purge on the air-fuel ratio feedback correction coefficient α is relatively large, that is, almost all deviations from the center value of the air-fuel ratio feedback correction coefficient α can be regarded as the influence of the purge. If it is possible, it is determined that the calibration processing can be executed.
[0066]
Specifically, if all of the “steady condition”, “purge valve accuracy condition”, and “purge influence condition” shown in FIG. If any one of them is not established, it is determined that the calibration process cannot be executed. This determination process corresponds to the process in the calibration timing determination unit B11 in FIG.
[0067]
As shown in FIG. 5, “steady-state conditions” include misfire conditions (the engine has not misfired), fuel cut conditions (no fuel cut), blow-by conditions (no blow-by gas), EGR condition (exhaust gas recirculation rate is constant), intake throttle valve opening area and engine rotation speed condition (intake throttle valve opening area, engine rotation speed are constant), purge rate condition (purge rate is constant) )) Is set. When all of these conditions are satisfied and it is determined that the disturbance other than the purge is small, it is determined that the steady condition is satisfied.
[0068]
Further, as the “purge valve accuracy condition”, a purge flow rate condition (the purge flow rate is a predetermined amount or more) is set. When the purge flow rate is small, the control accuracy of the purge flow rate is lowered, and the calculation accuracy in the calibration process described later is lowered. Therefore, when the purge flow rate is smaller than the predetermined amount, it is determined that the purge valve accuracy condition is not satisfied.
[0069]
The “purging influence condition” includes a purge establishment condition (purging is performed), a purge concentration condition (purge gas concentration is higher than a predetermined concentration, for example, an α change amount per purge rate of 1% is 1%. The purge rate condition (the purge rate is a predetermined value or more, for example, the purge rate is 30% or more) is set. When all of these conditions are satisfied and it is determined that the influence of the purge on the air-fuel ratio is relatively large, it is determined that the purge influence degree condition is satisfied.
[0070]
Thus, if it is determined in step 1 that the calibration process can be performed, the process proceeds to step 4 to execute the calibration process. This calibration process will be described with reference to the flow of FIG. 6. The calibration process estimates the amount of fuel desorbed from the canister 24 based on the change in the air-fuel ratio feedback correction coefficient α, and further returns the estimated desorption amount to the canister 24 by inverse calculation. The amount of fuel that has been adsorbed is calculated, and the value of the amount of adsorption, which is an internal variable of the canister model, is calibrated to the value of the amount of adsorption obtained by this inverse operation. This calibration process is performed by a calibration processing unit in FIG. This corresponds to the process in B12.
[0071]
In step 31, it is determined whether purge is being executed. The determination of whether or not the purge is being executed is based on the premise that the arithmetic processing in the subsequent steps 32 and 33 is being purged. Therefore, if these processes are performed when the purge is not executed, correct calibration is performed. It is because it becomes impossible. Therefore, when the purge is not being executed, this routine is ended and the calibration process is not performed.
[0072]
When it is determined that the purge is being executed, the routine proceeds to step 32 where the intake air mass Qg [g] obtained from the intake air flow rate Q and the intake air temperature, the target purge rate MPR, the purge correction factor FHOS, and the air-fuel ratio feedback correction factor α. From
Dg = K1 × (1−DLT + K2 × MPR) × Qg (5)
Where Dg: Desorption amount,
DLT: Total air-fuel ratio correction coefficient (= α × FHOS),
MPR: target purge rate,
K1: coefficient (a constant determined by the nature of the desorbed fuel),
K2: coefficient (a constant determined by the nature of the air),
Qg: weight of intake air,
The desorption amount Dg [g] is calculated by the following formula.
[0073]
This expression is based on the deviation of the total air-fuel ratio correction coefficient from 1.0 (center value) (first term and second term on the right side), the target purge rate MPR (third term on the right side) and the intake air mass Qg at that time. This is an equation for calculating the fuel amount Dg desorbed from the canister 4. That is, both the air-fuel ratio feedback correction coefficient α and the purge component correction coefficient FHOS are values having a center value of 1.0, and the center value of the total air-fuel ratio correction coefficient DLT, which is a value obtained by multiplying these α and FHOS. The amount of desorption is estimated by assuming that all deviations from (1.0) are caused by purging.
[0074]
In step 33, from the desorption amount Dg calculated in step 32,
Yr = KD × Dg ^ (1 / n (T)) (6)
Where n (T): desorption index,
KD: Desorption coefficient,
T: Activated carbon temperature,
The amount of adsorption Yr (mass) of the canister 24 is calculated by the following equation. Equation (6) is an inverse operation of Equation (2), which is one of the equations constituting the above-mentioned canister model.
[0075]
In step 34, the adsorption amount Y used when calculating the desorption amount Dg based on the canister model is replaced with the adsorption amount Yr calculated in step 33. Thereby, the value of the adsorption amount used in the canister model can be calibrated to a correct value, and the calculation accuracy of the subsequent desorption amount Dg can be improved.
[0076]
When the calibration process is completed in this manner, the process returns to FIG. 4, and in step 6, the amount of desorption Dg from the canister 4 is calculated using the canister model. The calculation of the desorption amount Dg will be described with reference to the flowchart of FIG. This process corresponds to the process in the desorption amount calculation unit B4 in FIG.
[0077]
In step 41, the current value Y of the amount of fuel adsorbed by the canister is calculated by the above equation (1). However, when the calibration process shown in FIG. 6 is executed, the calculation of the expression (1) is not performed, or the value calculated by the expression (1) is ignored, and in the subsequent calculations, the adsorption amount calculated by the calibration process is ignored. Yr is used as the adsorption amount Y.
[0078]
In step 42, the desorption amount Dgk at the reference purge flow rate is calculated from the adsorption amount Y by the above equation (2). In step 43, the desorption amount is calculated from the reference desorption amount Dgk and the purge flow rate PQ by the above equation (3). Dg is calculated. In step 44, the activated carbon temperature T is calculated from the desorption amount Dg by the above equation (4).
[0079]
When the calculation of the desorption amount Dg and the activated carbon temperature T is completed in this manner, the process returns to FIG. 4. In step 7, the purge correction coefficient FHOS is calculated based on the target purge rate in addition to the desorption amount Dg and the intake air mass Qg. To do. The calculated purge correction coefficient FHOS is sequentially stored in a predetermined data storage location (see FIG. 11) in the engine controller 11.
[0080]
A change in the air-fuel ratio feedback correction coefficient α can be predicted by supplying the desorption amount Dg calculated by the canister model to the engine. The predicted α is the purge correction coefficient FHOS. Therefore, FHOS is the same unit as α, and the center value of FHOS is 1.0 (= 100%), which is the same as α. For example, when the desorption amount Dg from the canister 24 increases or the target purge rate increases and the amount of fuel supplied to the engine increases, the exhaust air-fuel ratio shifts to a richer side than the stoichiometric air-fuel ratio. Since the air-fuel ratio feedback correction coefficient α is expected to change to be smaller than 100% in order to return to the value, the purge correction coefficient FHOS changes correspondingly to a value smaller than 100%. Further, even if the desorption amount Dg and the target purge rate are the same, if the intake air mass Qg increases, the air-fuel ratio of the exhaust gas shifts to the lean side from the stoichiometric air-fuel ratio, and the air-fuel ratio feedback correction coefficient α is Since it is expected to change to the larger side, the purge correction coefficient FHOS changes to the larger side accordingly.
[0081]
In step 8, the purge correction coefficient FHOS is subjected to delay correction composed of dead time correction and smoothing processing. The dead time correction is performed because there is a delay corresponding to the purge gas transition speed and the distance between the purge valve 27 and the engine cylinder until the purge gas reaches the engine cylinder after the purge valve 27 is opened. The reason why the annealing process is performed is that there is fuel diffusion until the fuel desorbed from the canister 24 reaches the cylinder of the engine.
[0082]
This delay correction will be described with reference to the flow of FIG. 8. The delay correction corresponds to the processing in the delay correction unit B6 in FIG.
[0083]
In step 51, the intake air flow rate is calculated from the output of the air flow meter 13, and in steps 52 and 53, the dead time and the smoothing coefficient are obtained by searching tables having the contents shown in FIGS. As the intake air flow rate increases, the intake air flow rate increases, so the value decreases as the intake air flow rate increases during the dead time.When the intake air flow rate increases and the intake air flow rate increases, the speed at which the detached fuel diffuses also increases. Since the speed increases, the value that increases as the intake air flow rate increases is set for the smoothing coefficient.
[0084]
In step 54, the purge gas moving speed equivalent value is calculated from the dead time. This purge gas movement speed equivalent value is calculated as the reciprocal of the dead time obtained in step 22.
[0085]
In step 55, the purge correction coefficient FHOS stored in the data storage location (see FIG. 11) in the engine controller 11 corresponding to the distance between the purge valve 27 and the cylinder of the engine is read. In step 56, the purge gas is moved. Data is shifted to the cylinder side by the speed equivalent value. In step 57, the average value of the data overflowed from the data storage location due to the data shift is obtained.
[0086]
In step 58, the average value of the overflow data obtained in step 57 is subjected to a smoothing process using the smoothing coefficient obtained in step 52, and the value after the smoothing process is again set as a purge correction coefficient FHOS. . Note that the annealing process is a general annealing process using a first-order lag system, and the degree of annealing increases as the annealing coefficient decreases.
[0087]
FIG. 11 is a diagram showing an outline of dead time correction in delay correction. In the drawing, black circles and white circles indicate data before the data shift and data after the data shift, respectively.
[0088]
As shown in this figure, the memory of the engine controller 11 is provided with a data storage location corresponding to the distance between the purge valve 27 and the cylinder of the engine, and the purge amount calculated according to the amount of fuel desorbed from the canister 24. Correction coefficients FHOS are sequentially stored in the storage location. In the dead time correction, these data are shifted to the cylinder side by the amount corresponding to the purge gas transition speed (reciprocal of the dead time), and the overflow gas from the data storage location by this data shift reaches the cylinder and is supplied to the purge gas. Is a correction coefficient corresponding to. A value obtained by subjecting the average value of the overflowed data to a smoothing process is used for correcting the fuel injection pulse width Ti described later. Thus, the arrival delay of the purge gas can be accurately corrected by combining the dead time correction and the annealing process.
[0089]
When the delay correction is completed in this way, the process returns to FIG. 4 to complete the process at the calibration executable timing.
[0090]
On the other hand, if it is determined in step 3 that the calibration process cannot be performed, the process proceeds to step 5 to determine whether the calibration process has been executed in the past. Such a determination is made because the initial value (initial adsorption amount) necessary for operating the canister model does not yet exist when the calibration process has never been performed. In such a case, the canister This is because the purge process based on the model is not performed. As a result of the determination, if the calibration process has been performed once in the past, the operations of steps 6 to 8 are executed. If the calibration process has not been performed for this, the routine is performed after the operation of step 2 is performed. Exit.
[0091]
The purge process is not performed when the calibration process has never been performed, and the purge process is executed by a purge process (FIG. 16, boot-up control) that does not use a canister model, which will be described later.
[0092]
In the calculation routine of the fuel injection pulse width Ti (not shown) using the purge correction coefficient FHOS calculated in this way,

Where Tp: basic injection pulse width [msec],
Tfbya: target equivalent ratio [anonymous number],
Kathos: transient correction amount [msec],
FHOS: purge correction coefficient (value after delay smoothing),
α: Air-fuel ratio feedback correction coefficient,
αm: air-fuel ratio learning value,
Ts: Invalid pulse width [msec],
The fuel injection pulse width Ti [msec] given to the fuel injection valve 4 is calculated by the following equation. This calculation corresponds to the processing in the basic injection pulse width calculation unit B7, the air-fuel ratio feedback correction coefficient calculation unit 8 and the fuel injection pulse width calculation unit B9 in FIG.
[0093]
Here, the part (Tp × Tfbya) excluding the transient correction amount Kathos is corrected by the purge correction coefficient FHOS because the fuel supplied by the purge is a gaseous fuel, and it is necessary to correct the wall flow. Because there is no.
[0094]
The basic injection pulse width Tp is an injection pulse width at which a theoretical air-fuel ratio can be obtained, and is obtained from the intake air flow rate Q and the engine speed N.
[0095]
The target equivalent ratio Tfbya is a value centering on 1.0, and becomes a value larger than 1.0 when the cooling water temperature is low or immediately after the engine is started. After the engine is warmed up, Tfbya = 1.0.
[0096]
The air-fuel ratio feedback correction coefficient α is a value calculated based on the output of the air-fuel ratio sensor 15 so that the exhaust air-fuel ratio falls within a so-called window centered on the stoichiometric air-fuel ratio. Specifically, when the value is centered on 100% and there is no steady deviation (base air-fuel ratio error), when the detected air-fuel ratio is on the rich side with respect to the theoretical air-fuel ratio, the value is smaller than 100%. When the detected air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the value becomes larger than 100%.
[0097]
The air-fuel ratio learning value αm is a value calculated based on the air-fuel ratio feedback correction coefficient α, and is introduced mainly to compensate for a steady deviation.
[0098]
Part of the fuel injected from the fuel injection valve 4 forms wall flow fuel on the intake port wall and the back of the intake valve umbrella, and this wall flow fuel has a large response delay in the transient state such as acceleration and deceleration, and enters the cylinder. Inflow. The transient correction amount Kathos is a value for compensating for the response delay of the wall flow fuel during the transient.
[0099]
The invalid pulse width Ts is a value for compensating for an operation delay from when the fuel injection valve 4 receives the injection signal at a predetermined timing until it actually opens.
[0100]
In addition, since the expression (7) is an expression in the case of sequential injection (in the case of four cylinders, injection is performed once every two engine revolutions in accordance with the ignition order of each cylinder), the number 2 is entered.
[0101]
12 and 13 are for setting the target purge rate MPR, and are executed at regular intervals (for example, every 10 msec). This calculation corresponds to the processing in the target purge rate setting unit B1 in FIG.
[0102]
In step 61, it is determined whether or not it is after the first calibration process. This is because the target purge rate setting method differs before and after the initial calibration process. If it is before the first calibration process, the current process is terminated. The setting of the target purge rate before the first calibration process will be described in the bootup control described later.
[0103]
When it is after the first calibration process, the routine proceeds to step 62 where the purge gas air-fuel ratio (purge air-fuel ratio) is calculated based on the desorption amount Dg calculated based on the canister model (calculated in step 6 in FIG. 4) and the purge flow rate PQ. ) Is calculated. Here, the purge flow rate PQ is the product of the target purge rate and the intake air flow rate Q, and the previous value is used as the target purge rate in this case.
[0104]
The purge air-fuel ratio may be corrected as follows. That is, the purge air-fuel ratio error is estimated from parameters such as operating conditions, such as engine speed, engine load, and intake air flow rate. For example, it is obtained by searching a table having the contents shown in FIG. As shown in the figure, the purge air-fuel ratio error increases as the intake air flow rate decreases and the purge rate decreases. Alternatively, the purge air-fuel ratio error may be obtained by searching a table that defines the relationship between the purge air-fuel ratio and the purge air-fuel ratio error as shown in FIG. When the purge air-fuel ratio error is obtained, the purge air-fuel ratio obtained in step 62 is corrected based on this error.
[0105]
Although the purge air-fuel ratio may be detected by an HC sensor, the purge air-fuel ratio can be calculated inexpensively and accurately if it is obtained by calculation based on the desorption amount calculated based on the canister model.
[0106]
In step 63, the purge rate change limit value is calculated based on the purge air-fuel ratio thus obtained (or after error correction). When the purge rate changes, the exhaust air-fuel ratio changes. The purge rate change amount limit value is calculated so that the air-fuel ratio deviation from the theoretical air-fuel ratio at this time is within the allowable range. The allowable width of the air-fuel ratio deviation from the stoichiometric air-fuel ratio is set to a width that can be absorbed by the air-fuel ratio feedback control and does not deteriorate the exhaust emission.
[0107]
In step 64, a purge rate upper limit value PVMX defined by the size of the purge valve 27 is calculated. Such a purge rate upper limit value PVMX is obtained when the target purge rate is set to a value larger than the purge rate obtained by setting the purge valve 27 to the maximum opening degree, and the mismatch between the purge rate and the target purge rate occurs. Since the error in calculating FHOS increases, the air-fuel ratio deviation from the stoichiometric air-fuel ratio increases. This is because problems such as deterioration of exhaust emission occur. Specifically, when the size of the purge valve 27 is constant, the larger the differential pressure across the purge valve 27, the larger the flow rate of purge gas that can flow. Therefore, when the differential pressure across the purge valve 27 is large, the purge rate upper limit PVMX is large. Calculate the value.
[0108]
In step 65, the purge rate upper limit TIMNMX based on the performance of the fuel injection valve 4 is calculated from the relationship between the minimum injection pulse width determined according to the performance of the fuel injection valve 4, the previous value of the target purge rate, and the purge correction coefficient. . As the purge rate increases, the amount of fuel supplied to the engine by the purge increases, so the fuel injection pulse width is corrected so that the fuel injection amount from the fuel injection valve 4 is reduced by that amount. In order to ensure the injection accuracy, the injection pulse width must be larger than the predetermined minimum pulse width. In other words, in order to make the fuel injection pulse width larger than the minimum pulse width, the purge rate must be smaller than a certain value. For this reason, the upper limit of the purge rate is also defined by the injection performance of the fuel injection valve 4.
[0109]
In step 66, all the operation regions that can be assumed from the current operation region are assumed, the minimum purge rate is predicted, and the purge rate upper limit value is calculated from the minimum purge rate and the purge rate change limit value. Calculate PRMNMX. For example, when the accelerator pedal is depressed and accelerated, the target purge rate is set to a very small value, but if the target purge rate is set to a large value immediately before the accelerator pedal is depressed as much as possible, the purge rate Since the amount of change of is limited to the change amount limit value or less, the purge rate cannot follow the target purge rate. Since this follow-up delay causes an increase in exhaust emission, it is necessary to define the upper limit of the purge rate from the minimum purge rate that can be assumed so as not to cause such follow-up delay.
[0110]
In step 67, the air-fuel ratio feedback correction coefficient α is monitored, and if it is equal to or less than a predetermined value, the largest value among the purge rates that make the air-fuel ratio feedback correction coefficient α equal to or greater than the predetermined value is calculated as the purge rate upper limit value ALPMX. . The upper limit value ALPMX is provided so that the air-fuel ratio feedback correction coefficient α is controlled to be within 100 ± 25% in the air-fuel ratio feedback control, but the air-fuel ratio feedback correction coefficient α is close to the limit value (for example, 80%), if a large amount of purging is performed, disturbances other than purging may occur and the control range tends to deviate.
[0111]
In step 68, the smallest value is selected from the above four upper limit values PVMX, TIMNMX, PRMNMX, and ALPMX, and that value is set as the maximum purge rate PRMAX.
[0112]
In steps 69 and 70, the previous value of the target purge rate is compared with the maximum purge rate PRMAX. When the previous value of the target purge rate and the maximum purge rate PRMAX are equal, the routine proceeds from step 69 to step 73, where the target purge rate is set to the previous value. When the previous value of the target purge rate is larger than the maximum purge rate PRMAX, the routine proceeds from step 69 and step 70 to step 72, where the target purge rate is set to a value obtained by subtracting the purge rate change limit value from the previous value. When the previous value of the purge rate is smaller than the maximum purge rate PRMAX, the routine proceeds from step 69, 70 to step 71, where the target purge rate is set to the previous value plus the purge rate change amount limiting value.
[0113]
Steps 74 to 81 in FIG. 13 are added by the present invention and will be described later.
[0114]
In this way, the target purge rate is set to follow the purge rate change amount limit value with the maximum purge rate PRMAX as a target, and when performing a large amount of purge without deteriorating exhaust emissions. The best purge rate is set. In addition, when setting the maximum purge rate PRMAX, not only the upper limit values PVMX, TIMNMX, and ALPMX determined by physical limitations and the current operation region, but also the maximum purge in that region without delay even if the operation region changes Since the upper limit value PRMNMX determined so as to be able to shift to the rate is also taken into consideration, the best purge rate can be set for performing a large purge without deteriorating the exhaust emission even if the operating condition changes. .
[0115]
The target purge rate set in this way is converted into a duty ratio for the purge valve and is output to the purge valve 27.
[0116]
By the way, the purge process based on the canister model (model reference purge process) can be executed while the calibration process has never been performed after the engine is started and there is no initial value (initial adsorption amount) used in the canister model. Can not. However, in order to realize a large purge after the engine is started, it is necessary to execute the purge process even before the calibration process. Therefore, until the first calibration process is executed, the target purge rate is set by the following control (boot-up control shown in FIG. 16) instead of the purge process based on the canister model, and the purge is performed at the set target purge rate. Execute the process. In this boot-up control, the influence of the purge on the exhaust air / fuel ratio is absorbed by the air / fuel ratio feedback correction coefficient α, and the fuel injection pulse width is not corrected (FHOS = 1.0).
[0117]
FIG. 16 is for performing boot-up control, and is executed at regular intervals (for example, every 10 msec). This boot-up control corresponds to the processing in the boot-up control unit B14 and changeover switch B15 in FIG.
[0118]
In step 91, the intake air flow rate Q is read, and in step 92, it is determined whether or not it is before the first calibration process. If it is before the first calibration process, in step 93, the integrated purge flow rate (total purge flow rate after starting the purge) SQP and the purge pipe volume (the volume of the pipe from the canister 24 to the purge valve 27) are compared. When the integrated purge flow rate SQP exceeds the purge pipe volume, the routine proceeds to step 94, and when it does not exceed this, the routine proceeds to step 97.
[0119]
In step 97, an initial purge rate (a small value of 1% or less) is set as the target purge rate MPR. This small value is set when the accumulated purge flow rate SQP does not reach the purge pipe volume, the gas in the purge pipe is supplied to the engine before the purge starts. This is because the air-fuel ratio of the gas is unknown, and if the target purge rate setting process shown in steps 94 to 96 is performed as it is, problems such as deterioration in engine combustion stability occur.
[0120]
That is, when the low concentration purge gas existing in the purge pipe is supplied at the start of the purge, and the air / fuel ratio deviation from the theoretical air / fuel ratio is small, it is determined that a larger amount of purge is possible and a larger target purge rate MPR. However, if a large target purge rate MPR is set in this way, a large amount of desorption occurs when all of the low concentration gas in the pipe is supplied and the original high concentration purge gas is supplied. This is because the fuel is suddenly supplied, which causes deterioration of combustion stability of the engine.
[0121]
When the integrated purge flow rate SQP exceeds the pipe volume, the routine proceeds to step 94 where the difference between the actual air-fuel ratio feedback deviation and the target air-fuel ratio feedback deviation is calculated.
[0122]
Here, the target air-fuel ratio feedback deviation means a deviation (= | tα-100 |%) between the target value tα of the air-fuel ratio feedback correction coefficient and the center value (100%) of the air-fuel ratio feedback correction coefficient. The fuel ratio feedback deviation is a deviation (= | α−100 |%) between the actual air fuel ratio feedback correction coefficient α and the center value of the air fuel ratio feedback correction coefficient. For example, the target value tα of the air-fuel ratio feedback correction coefficient α is set to 80% for the purpose of securing a large purge flow rate within a range where the air-fuel ratio deviation from the theoretical air-fuel ratio due to purge can be sufficiently absorbed by the air-fuel ratio feedback control. Then, the target air-fuel ratio feedback deviation is set to | 80-100 | = 20%.
[0123]
In step 95, a purge rate change amount corresponding to the difference between the target air-fuel ratio feedback deviation and the actual air-fuel ratio feedback deviation is calculated by searching a table having the contents shown in FIG. The purge rate change amount is set to a larger value as the absolute value of the difference between the target air-fuel ratio feedback deviation and the actual air-fuel ratio feedback deviation increases to improve the convergence to the target value. Even if the absolute value of the deviation is the same depending on whether the difference in the actual air-fuel ratio feedback deviation is positive or negative, a larger value (absolute value) is set if the difference in the air-fuel ratio feedback deviation is shifted to the negative side. Set the purge rate change amount.
[0124]
In this way, the purge rate change amount has different characteristics depending on whether the air-fuel ratio feedback deviation is positive or negative. When the air-fuel ratio feedback deviation difference is shifted to the negative side, the target air-fuel ratio feedback correction coefficient α is 80%. This is because a disturbance other than purging tends to cause deterioration of engine stability and exhaust emission, as compared to a case where the value is smaller than the opposite positive side. In other words, the reason for changing the characteristics of the purge rate change amount according to the difference in the difference in air-fuel ratio feedback deviation is to quickly return the control point to the safe side from the viewpoint of engine combustion stability and prevention of exhaust emission deterioration. is there.
[0125]
When the purge rate change amount is calculated as described above, the process proceeds to step 96, where the purge rate change amount calculated in step 95 is added to the target purge rate obtained at the previous execution of this routine, and a new target purge rate MPR is obtained. Is calculated.
[0126]
In step 98, the purge flow rate PQ is obtained from the target purge rate MPR thus obtained and the intake air flow rate Q, and is added to SQPz, which is the previous value of the cumulative purge flow rate, in step 99, thereby updating the integrated purge flow rate SQP. To do.
[0127]
Therefore, according to the processing of steps 94 to 96, the optimum target purge rate MPR can be set regardless of the adsorption state of the canister 24, and even when a purge gas having a concentration higher than expected is supplied. The target purge rate MPR is appropriately changed in response to the change in the exhaust air-fuel ratio, and the optimum target purge rate MPR can always be set.
[0128]
On the other hand, after the first calibration process, the process proceeds from step 92 to step 100 and the subsequent steps, and it is determined whether the purge process is completed based on the integrated purge flow rate SQP. That is, in step 100, the purge end determination flag (initially set to zero) is checked. Immediately after the engine is started, the purge end determination flag = 0, so that the process proceeds to step 101 and the desorption amount Dg (calculated in step 6 in FIG. 4) and the target purge rate MPR (calculated in FIGS. 12 and 13). Read). In step 102, the purge flow rate QP is calculated from the read target purge rate MPR and the intake air flow rate Q, and in step 103, this is added to SQPz which is the previous value of the cumulative purge flow rate, thereby updating the cumulative purge flow rate SQP. To do.
[0129]
In step 104, it is determined whether or not it is the first calibration processing timing. Here, since the first calibration process timing is the first timing that disappeared before the first calibration process in step 92, the process proceeds to step 105 only at this time, and the purge flow rate PQ calculated in step 102 is read in step 101. The purge air-fuel ratio is calculated based on the desorption amount Dg, and the purge end determination value SLSQP is calculated by searching the purge air-fuel ratio from the purge air-fuel ratio in step 106 with the table shown in FIG.
[0130]
Here, the purge end determination value SLSQP is an integrated purge flow rate at which almost no desorbed fuel appears in the purge gas from the canister 24. As shown in FIG. 18, the determination value SLSQP decreases as the purge air-fuel ratio increases.
[0131]
In step 107, the purge end determination value SLSQP is compared with the integrated purge flow rate SQP. If the integrated purge flow rate SQP is less than the purge end determination value SLSQP, the current process is terminated. Eventually, if the integrated purge flow rate SQP becomes equal to or higher than the purge end determination value SLSQP, it is determined that almost no desorbed fuel is present in the purge gas from the canister 24, and the process proceeds to step 108 to indicate that the purge has ended. Set to 1.
[0132]
When the purge end determination flag = 1, since it is not possible to proceed to step 101 and subsequent steps even if it flows through steps 91, 92, and 100 from the next time, the processing is ended as it is.
[0133]
Next, the overall operation of the present embodiment by performing the purge process will be described first.
[0134]
During the purge process, the target purge rate MPR is set as large as possible within a range in which the engine combustion stability is not lowered and the exhaust emission is not increased, and the purge valve 27 is opened so as to realize the target purge rate MPR (FIG. 12). Steps 61-73).
[0135]
During the purging process, purge gas containing fuel desorbed from the canister 24 is supplied to the engine. Therefore, the engine controller 11 estimates the amount of fuel Dg desorbed from the canister 24, and this desorption amount Dg, Based on the target purge rate and the intake air mass Qg, a purge correction coefficient FHOS, which is a value for predicting a change in the air-fuel ratio feedback correction coefficient α caused by the purge fuel, is calculated (steps 1, 3, 4, 6, and 6 in FIG. 4). 7, 8 or steps 1, 3, 5, 6, 7, 8), the basic injection pulse width Tp is corrected by the purge correction coefficient FHOS, so that the air-fuel ratio deviation from the theoretical air-fuel ratio due to the purge is small. It can be suppressed.
[0136]
At this time, the desorption amount Dg from the canister 24 is estimated using a canister model expressed by the equations (1) to (4), and the desorption amount Dg is estimated accurately in a short time. Here, the desorption amount Dg calculated based on the canister model includes an error, and this error is integrated and increased as the operation time of the canister model becomes longer. From this change, the amount of fuel desorbed from the canister 24 is estimated, and the value of the amount of adsorption, which is an internal variable of the canister model, is calibrated with the amount of adsorption Yr reversed from this estimated amount of desorption. This calibration process is executed when disturbances other than purge are small and almost all changes in the air-fuel ratio feedback coefficient α can be regarded as being caused by the purge, and when the influence of the purge on the exhaust air-fuel ratio is relatively large.
[0137]
In addition, there is a delay from when the purge valve 27 is opened until the desorbed fuel reaches the cylinder of the engine, and there is also fuel diffusion until it reaches the purge cylinder 27. Delay correction is applied to the minute correction coefficient FHOS.
[0138]
Further, the purge process using the canister model (model reference purge process) cannot exert its effect until the initial value of the adsorption amount is obtained by the calibration process, but until the initial value of the canister model is calculated. The target purge rate is set according to the difference between the target air-fuel ratio feedback deviation and the actual air-fuel ratio feedback deviation, and the purge valve 27 is opened so that this target purge rate is realized. Thus, the purge process can be performed even before the initial value is calculated by the calibration process, and the effective purge can be performed in the entire region.
[0139]
This completes the description of the same part as the purge process in the prior application device (see Japanese Patent Application No. 2001-71562), and next, FIG. 13, FIG. 19, FIG. 20, FIG. The flowcharts shown in FIGS. 25 and 26 will be described.
[0140]
First, at step 74 in FIG. 13, the target purge rate is shifted to the variable MPR0.
[0141]
In step 75, the purge limiter determination flag is checked. There are two purge limiter determination flags, a purge lower limiter determination flag and a purge upper limiter determination flag. The setting of these two flags will be described with reference to the flowcharts of FIGS.
[0142]
19 is for setting a purge lower limiter determination flag, and FIG. 20 is for setting a purge upper limiter determination flag, both of which are executed at regular intervals (for example, every 10 msec).
[0143]
As shown in FIG. 27, the air-fuel ratio feedback correction coefficient α has a first limiter ± LMT1 (for example, ± 25%) up and down around the center value of α (100%) as shown in FIG. ). That is, the first upper limiter of α is 125% (= 100 + LMT1), the first lower limiter of α is 75% (= 100−LMT1), and the calculated α is 125%, which is the first upper limiter of α. Is exceeded to 125%, and conversely, when α is less than 75%, which is the first lower limiter of α, this α is limited to 75%.
[0144]
In the present embodiment, in addition to the first limiter ± LMT1, a smaller second limiter ± LMT2 is newly introduced for purging. That is, the second lower limiter is 100-LMT2, and the second upper limiter is 100 + LMT2, and in FIG. Therefore, the second lower limiter 100-LMT2 comes to a position having a certain margin. In other words, when a disturbance other than purge is left between 100-LM2 (second lower limiter) and 100-LMT1 (first lower limiter), and the remaining α correction allow Let the air-fuel ratio deviation from the air-fuel ratio be small.
[0145]
Then, a flag is set in order to indicate whether α is in the region below the second lower limiter or whether it is in the region above the second upper limiter. That is, when the calculated α becomes 100−LMT2 or less which is the second lower limiter, the purge lower limit limiter setting flag = 1, and conversely, when the calculated α becomes 100 + LMT2 or more which is the second upper limiter, the purge upper limit Limiter setting flag = 1.
[0146]
However, a hysteresis HYS is provided for each of the second lower limiter and the second upper limiter. This is to prevent frequent repetition of each flag becoming zero or one due to α being in the vicinity of each limiter.
[0147]
Specifically, referring to FIG. 19, in step 111, it is determined whether air-fuel ratio feedback control is being performed. If the air-fuel ratio feedback control is not in progress, the current process is terminated.
[0148]
When the air-fuel ratio feedback control is being performed, the routine proceeds to step 112 where the air-fuel ratio feedback correction coefficient α is read.
[0149]
This α is calculated based on the air-fuel ratio sensor 15 in a flow (not shown). Here, for the sake of simplicity, the case of proportional operation will be described. The deviation between the actual exhaust air-fuel ratio detected by the air-fuel ratio sensor 15 and the theoretical air-fuel ratio is obtained, and this deviation is multiplied by a proportional gain. . At this time, while the actual exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, the waveform of α decreases stepwise at each control period in order to return the actual exhaust air-fuel ratio to the stoichiometric air-fuel ratio. On the contrary, while the actual exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, in order to return the actual air-fuel ratio to the stoichiometric air-fuel ratio, α increases stepwise at each control cycle. go.
[0150]
In step 113, a purge lower limiter determination flag (initially set to zero when the engine is started) is checked. If it is immediately after the start, the purge lower limiter determination flag = 0, so that the routine proceeds to step 114 where α is compared with the purge lower limiter 100-LM2. When α exceeds 100−LMT2, the current process is terminated as it is. When α becomes 100−LMT2 or less, the routine proceeds to step 115 where the purge lower limit limiter determination flag = 1 and the current process is terminated.
[0151]
Next, the purge lower limit limiter determination flag = 1 proceeds from step 113 to step 116, where α is compared with 100−LM2 + HYS (HYS is a positive constant value). If α is less than or equal to 100−LM2 + HYS, the routine proceeds to step 115 where the purge lower limit limiter determination flag = 1 is maintained, and when α exceeds 100−LM2 + HYS, the routine proceeds to step 117 where the purge lower limit limiter determination flag = 0.
[0152]
Moving to FIG. 20, steps 121 and 122 are the same as steps 111 and 112 of FIG.
[0153]
In step 123, an α purge upper limit limiter determination flag (initially set to zero when the engine is started) is checked. If it is immediately after the start, the purge upper limit limiter determination flag = 0, so that the routine proceeds to step 124 where α is compared with the purge upper limiter 100 + LM2. When α is less than 100 + LMT2, the current process is terminated as it is, and when α is equal to or greater than 100 + LMT2, the process proceeds to step 125, where the purge upper limit limiter determination flag = 1 is set and the current process is terminated.
[0154]
This purge upper limiter determination flag = 1 next time, the process proceeds from step 123 to step 126, where α is compared with 100 + LM2-HYS (HYS is a positive constant value). If α is equal to or greater than 100 + LM2-HYS, the routine proceeds to step 125 where the purge upper limit limiter determination flag = 1 is maintained, and when α is less than 100 + LM2-HYS, the routine proceeds to step 127, where the purge upper limit limiter determination flag = 0.
[0155]
Returning to step 75 of FIG. 13, when the purge limiter determination flag (purge lower limiter determination flag or purge upper limiter determination flag) = 0, the routine proceeds to step 76 where the value of MPR0 is set as the target purge rate MPR as it is.
[0156]
On the other hand, when the purge limiter determination flag = 1, the routine proceeds to step 77 where purge rate reduction processing is performed.
[0157]
As shown in FIG. 21, the purge rate lowering process is performed, for example, at the target purge rate at the timing when the purge lower limit limiter determination flag is switched from 0 to 1 (calculated in steps 61 to 73 in FIG. 12). Is an initial value that is decreased by a predetermined value Δ per predetermined time Δt. However, when the target purge rate becomes less than zero, it is limited to zero.
[0158]
FIG. 22 (subroutine of step 77 in FIG. 13) is for performing the purge rate lowering process.
[0159]
In FIG. 22, in steps 131 and 132, the current and previous purge limiter determination flags are viewed. If the purge limiter determination flag = 1 at this time and the purge limiter determination flag = 0 at the previous time (that is, immediately after the purge limiter determination flag is switched from 0 to 1), the routine proceeds to step 133, and the MPR0 value at that time is set. Move to variable MPR1. This is because the target purge rate at the timing when the purge limiter determination flag 0 is switched to 1 is sampled as an initial value.
[0160]
In step 134, the timer 1 is once cleared to zero, and then incrementing is started in step 135.
[0161]
At this time, when the purge limiter determination flag = 1 at this time, the steps 133 and 134 are skipped and the operation of the step 135 is executed.
[0162]
In step 136, the timer 1 value is compared with a predetermined time Δt. When the timer 1 value exceeds the predetermined time Δt, the process proceeds to steps 137 and 138 to clear the timer 1 value to zero,
MPR1 = MPR1z−Δ (8)
Where MPR1z: the previous value of MPR1,
The variable MPR1 is reduced by a predetermined value Δ (positive value) by the following equation. When the timer 1 value is less than the predetermined time Δt, the routine proceeds to step 139 and the current MPR1 is maintained.
[0163]
By the operations in steps 136 to 139, MPR1 decreases by a predetermined value Δ every time the predetermined time Δt elapses.
[0164]
Here, if the purge rate is drastically reduced, the exhaust air-fuel ratio changes and affects the operability and exhaust emission. Therefore, the predetermined time Δt and the predetermined value Δ are the values for setting the target purge rate so as not to occur. The rate change amount limit value may be used or determined based on this.
[0165]
In step 140, MPR1 is compared with zero. If MPR1 is greater than or equal to zero, the routine proceeds to step 142, where MPR1 is set as the target purge rate MPR, whereas when MPR1 becomes a negative value, the routine proceeds to step 141 where zero is set as the target purge rate MPR.
[0166]
When the purge rate lowering process is completed in this manner, the process returns to FIG. 13 and the purge end determination flag (set by FIG. 16) is viewed at step 78. When the purge end determination flag = 1, the routine proceeds to step 79 in order to start air-fuel ratio learning, and the purge ratio for air-fuel ratio learning is set to the target purge ratio MPR. Thus, when the air-fuel ratio learning is started, the target purge rate MPR becomes small as shown in FIG.
[0167]
On the other hand, when the purge end determination flag = 0, the routine proceeds to step 80 where the base air-fuel ratio error determination flag is viewed. The setting of the base air-fuel ratio error determination flag will be described with reference to the flowchart of FIG.
[0168]
FIG. 24 is for setting the base air-fuel ratio error determination flag, and is executed at regular intervals (for example, every 100 msec).
[0169]
In step 151, a base air-fuel ratio error determination flag (initially set to zero at start-up) is checked. Immediately after the start, the base air-fuel ratio error determination flag = 0, so the routine proceeds to step 152 and thereafter.
[0170]
Steps 152 to 154 are portions for determining whether or not a condition for determining the base air-fuel ratio error is satisfied. When purging is being performed and the intake air flow rate Q is in a large region and when the actual purge rate = 0%, it is determined that the condition for determining the base air-fuel ratio error is satisfied, and the routine proceeds to step 155 and timer 2 (initially set to zero) Is incremented. This timer 2 is for measuring the elapsed time from the timing when the purge rate becomes zero by the purge rate lowering process during the purge.
[0171]
The reason why the base air-fuel ratio error determination is that the intake air flow rate Q is in a large region is to avoid the influence of blow-by gas on the exhaust air-fuel ratio when Q is small.
[0172]
In step 154, the actual purge rate is viewed from the duty ratio applied to the purge valve 27, but the target purge rate can be used instead of the actual purge rate.
[0173]
In step 156, the timer 2 value is compared with a predetermined value (a constant value). When the timer 2 value exceeds the predetermined value, the routine proceeds to step 157, where the base air-fuel ratio error determination flag = 1 is set.
[0174]
Even if all the conditions in steps 152 to 154 are satisfied and the timer 2 starts to be incremented in step 155, if the intake air flow rate Q becomes small thereafter, the process proceeds to step 158 and the timer 2 is set to zero. clear. That is, in order to determine that there is a base air-fuel ratio error, the conditions for determining the base air-fuel ratio error need to be satisfied continuously without being interrupted.
[0175]
Returning to step 80 of FIG. 13, when the base air-fuel ratio error determination flag = 1, the routine proceeds to step 81 where the target purge rate = 0. When the base air-fuel ratio error has occurred, the air-fuel ratio learning is started immediately, but the target purge rate = 0 is a purge cut state in order to update the air-fuel ratio learning value with high accuracy. Because it is necessary.
[0176]
FIG. 25 is for setting the air-fuel ratio learning start flag, and is executed at regular intervals (for example, every 10 msec).
[0177]
In step 171, the air-fuel ratio learning start flag (initially set to zero) is checked. If the air-fuel ratio learning start flag is 0 immediately after the engine is started, the routine proceeds to steps 172 and 173, where the purge end determination flag (set by FIG. 16) and the base air-fuel ratio error determination flag (set by FIG. 24) are set. See).
[0178]
When the purge end determination flag = 1, and when the purge end determination flag = 0, the base air / fuel ratio error determination flag = 1, the routine proceeds to step 174 to start the air / fuel ratio learning, and the air / fuel ratio learning starts. Flag = 1.
[0179]
FIG. 26 is for updating the air-fuel ratio learning value, and is executed at predetermined timings (for example, at predetermined engine speed intervals).
[0180]
In step 181, the air-fuel ratio learning start flag is checked. When the air-fuel ratio learning start flag = 1, the routine proceeds to step 182 where it is determined whether the air-fuel ratio learning permission condition is satisfied. The air-fuel ratio learning permission condition is the same as the conventional one. For example, it is determined that the air-fuel ratio learning permission condition is satisfied when the air-fuel ratio feedback control is being performed and the air-fuel ratio sensor 15 and the like related to the air-fuel ratio feedback control are all satisfied. Proceed to, and the air-fuel ratio learning value is updated. There is a method of updating the air-fuel ratio learning value by the following equation.
[0181]
αm = αmz + (α−100) × M (9)
Where αm is the updated air-fuel ratio learning value,
αmz: Air-fuel ratio learning value before update,
M: learning ratio (constant value),
The embodiment is air-fuel ratio feedback control based on the air-fuel ratio sensor 15, but instead of this, O ₂ It may be air-fuel ratio feedback control based on a sensor. In this case, O ₂ The sensor inversion timing may be used as the update timing of the air-fuel ratio learning value.
[0182]
Here, the action during the purge execution of this embodiment will be described with reference to the waveform diagrams of FIGS.
[0183]
FIG. 27 shows that after starting the engine in a state where there is no base air-fuel ratio error, the accelerator pedal was depressed at t1 after the first calibration process was performed, the depression amount was kept constant for a while, and the accelerator pedal was returned at t2. The case is shown as a model. However, the air-fuel ratio learning is considered to be removed.
[0184]
As the accelerator pedal is depressed at t1, the maximum purge rate increases stepwise, and the target purge rate approaches the maximum purge rate at a predetermined speed regulated by the purge rate change limit value. When the target purge rate becomes larger than before t1, the amount of purge gas fuel increases accordingly, and the exhaust air / fuel ratio tilts to the richer side than the stoichiometric air / fuel ratio. Therefore, the purge correction coefficient FHOS decreases stepwise from t1 in anticipation of this. Although it should be (see thin line), FHOS is actually larger in steps than t1 (see thick line), which indicates that FHOS was not calculated correctly due to the first calibration error. Yes.
[0185]
The fuel injection pulse width Ti of the equation (7) is increased by the incorrect FHOS, so that the exhaust air-fuel ratio is inclined to the rich side, and the air-fuel ratio feedback is made to return the exhaust air-fuel ratio inclined to the rich side to the stoichiometric air-fuel ratio. The correction coefficient α becomes smaller stepwise than t1.
[0186]
And after (alpha) which becomes small becomes small to the vicinity of 100-LMT2 which is the 2nd lower limiter, it changes periodically near 100-LMT2. This means that the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio. That is, even if FHOS is not calculated correctly, the exhaust air-fuel ratio is finally controlled to be close to the theoretical air-fuel ratio by α.
[0187]
Specifically, the reason is that the purge lower limiter determination flag is switched from 0 to 1 at the timing when α becomes smaller than 100-LMT2 (steps 113, 114, and 115 in FIG. 19). When the purge lower limit limiter determination flag = 1, the purge rate lowering process works to reduce the target purge rate (steps 75, 77, and FIG. 22 in FIG. 13), and the FHOS also decreases. Since the exhaust air-fuel ratio goes to the lean side, α changes to the side that becomes larger across 100-LMT2. And even if it exceeds 100-LMT2, while it is below 100-LMT2 + HYS, the purge lower limit limiter determination flag remains 1 (FIG. 19, steps 113, 116, 115), and α is larger than 100-LMT2 + HYS. At the same time, the purge lower limit limiter determination flag is switched from 1 to 0 (steps 113, 116, and 117 in FIG. 19). When the purge lower limit limiter determination flag = 0, the purge rate reduction process is stopped, and the target purge rate before the purge rate reduction process is restored (steps 75 and 76 in FIG. 13). Then, since the exhaust air-fuel ratio goes to the rich side, α again decreases across 100-LMT2, and the purge lower limiter determination flag is switched from 0 to 1 (steps 113, 114, 115 in FIG. 19). When the purge lower limit limiter determination flag = 1, the purge rate lowering process works to reduce the target purge rate, and the FHOS also decreases with the steps 75, 77, and FIG. .
[0188]
By such repetition, α periodically changes in the vicinity of 100-LMT2, which is the second lower limiter.
[0189]
As a result, the following merits are obtained.
[0190]
(1) Even if the purge correction coefficient FHOS is not correctly calculated due to the calibration error, the air-fuel ratio deviation from the theoretical air-fuel ratio can be made small by the air-fuel ratio feedback correction coefficient α.
[0191]
(2) In this case, the air-fuel ratio feedback correction coefficient α has a correction margin between 100-LMT2 which is the second lower limiter and 100-LMT1 which is the first lower limiter. Even if there is a disturbance, it is possible to reduce the air-fuel ratio deviation from the theoretical air-fuel ratio by using the remaining correction allowance.
[0192]
That is, according to the present embodiment (the invention described in claim 1), between the center value of the air-fuel ratio feedback correction coefficient α and the limiter (first limiter) ± LMT1 for the original air-fuel ratio feedback correction coefficient α. Since the second limiter ± LMT2 is provided as a purge limiter and the target purge rate is gradually reduced when the air-fuel ratio feedback correction coefficient α deviates across the second limiter from the center value side, Since the effect of the purge on the exhaust air-fuel ratio is reduced and the purge correction coefficient FHOS is also reduced, the air-fuel ratio feedback is higher than when nothing is performed until the air-fuel ratio feedback correction coefficient α moves across the second limiter to the first limiter. Since the correction cost of the correction coefficient α is expanded,
Even if an error occurs in the purge correction coefficient FHOS due to the calibration error of the canister model, the range from the second limiter ± LMT2 to the first limiter ± LMT1 is left as a correction allowance for the air-fuel ratio feedback correction coefficient, and the theoretical air-fuel ratio is The air-fuel ratio deviation can be made small, and the remaining correction allowance can ensure robustness against disturbances other than purge.
[0193]
On the other hand, FIG. 28 schematically shows a case where the base air-fuel ratio error is determined in the region where the intake air flow rate Q is large. In this case, however, the air-fuel ratio learning is taken off.
[0194]
Here, the base air-fuel ratio is an air-fuel ratio when there is no disturbance such as purge, and a base air-fuel ratio error may occur due to clogging of the fuel injection valve 4, an abnormality in the flow characteristic of the air flow meter 13, or the like. For example, when the air flow meter 13 deviates from the actual flow rate and produces a larger output, the basic injection pulse width Tp calculated using the air flow meter output becomes a value larger than the actual intake air flow rate. . For this reason, the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio and leans to the rich side, and the air-fuel ratio feedback correction coefficient α decreases to return the exhaust air-fuel ratio leaning to the rich side to the stoichiometric air-fuel ratio. α periodically changes between 100-LMT2 which is the second lower limiter and 100-LMT1 which is the first lower limiter.
[0195]
Although not shown, this periodic change of α continues even before t3, and the operation before t3 will be described. At the timing when α decreases across 100-LMT2, which is the second lower limiter. The purge lower limiter determination flag is switched from 0 to 1 (steps 113, 114, and 115 in FIG. 19). Since the base air-fuel ratio error is large, the purge lower limiter determination flag = 1 continues thereafter. Yes. If the purge lower limiter determination flag = 1, the purge rate lowering process is performed, and the target purge rate is decreased by a predetermined value Δ per predetermined time Δt and eventually becomes zero (steps 75, 77, and FIG. 13 in FIG. 13). . When the target purge rate = 0, the timer 2 value gradually increases as the timer 2 is incremented (steps 151 to 155 in FIG. 24). The timing immediately after the start of the increment of the timer 2 is t3. At this time, the timer 2 value has not yet reached the predetermined value.
[0196]
When moving to t3 and after shown in FIG. 28, the increment of the timer 2 value continues from t3 and continues even after t4. Then, the base air-fuel ratio error determination flag is switched from 0 to 1 at timing t5 when the timer 2 value matches the predetermined value (steps 156 and 157 in FIG. 24).
[0197]
That is, since the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio due to the presence of a large base air-fuel ratio error during the purge execution, the air-fuel ratio feedback correction coefficient α is set to the first lower limit limiter to return it to the stoichiometric air-fuel ratio. If it is moved to 75%, the correction margin for the air-fuel ratio feedback correction coefficient α is eliminated, and the robustness against disturbances other than purge in this state is reduced. This embodiment (Claim 5) When the calculated air-fuel ratio feedback correction coefficient α does not fall within the center value (100%) including the second lower limiter 100-LMT2, the target purge is performed by the purge rate reduction process. The rate decreases toward zero and at the same time the FHOS also decreases, thereby reducing the effect of the purge on the exhaust air / fuel ratio. , The correction allowance of up to 75%, which is the first lower limiter on the air-fuel ratio feedback correction coefficient α is left.
[0198]
When the purge rate reduction process is continued and the target purge rate becomes zero, the influence of the purge on the exhaust air / fuel ratio is completely eliminated, and in this state, the base air / fuel ratio error can be accurately detected. .
[0199]
Thus, according to the present embodiment (the invention described in claim 5), the accuracy of the base air-fuel ratio error is high while leaving the space from the second limiter to the first limiter as the correction allowance for the air-fuel ratio feedback correction coefficient α. Detection can be performed, and robustness against disturbances other than purge can be ensured by the remaining α correction allowance. Further, the exhaust emission can be prevented from deteriorating when the base air-fuel ratio error and the purge process overlap.
[0200]
In addition, if there is a large base air-fuel ratio error, it causes exhaust emission deterioration due to a delay in following the air-fuel ratio feedback correction coefficient α, sticking the air-fuel ratio feedback correction coefficient α to the first limiter, etc. According to the sixth aspect of the invention, when it is determined that there is a base air-fuel ratio error, purge cut is performed (steps 80 and 81 in FIG. 13), and air-fuel ratio learning is immediately started (step 173 in FIG. 25). 174), the base air-fuel ratio error state can be quickly resolved. Further, the time until the start of learning can be shortened compared to the case where the air-fuel ratio learning is not started until the purge process is completed after the engine is started.
[0201]
In addition, although not shown in the embodiment, there is one that uses the air-fuel ratio learning value for failure diagnosis of the fuel system. In this case, when a base air-fuel ratio error has occurred, there is a possibility that a failure has occurred in the fuel system as the cause. Therefore, if the start of air-fuel ratio learning is delayed after engine startup, the failure diagnosis of the fuel system will also be delayed. At this time, when the present embodiment (the invention according to claim 7) is applied and it is determined that the base air-fuel ratio error has occurred, the air-fuel ratio learning is promptly started, so that there is a failure in the fuel system. But failure diagnosis can be done early.
[0202]
In the region where the intake air flow rate Q is small, the air-fuel ratio deviation from the stoichiometric air-fuel ratio may increase due to the influence of blow-by gas, and at this time, it is erroneously determined that the base air-fuel ratio error has occurred. According to the present embodiment (the invention described in claim 9), the operating region for determining whether or not the base air-fuel ratio error is present is limited to a region where the intake air flow rate is large where the influence of blow-by gas is small. Therefore, it is possible to prevent erroneous determination of a base air-fuel ratio error when it is affected by blow-by gas.
[0203]
Further, since the desorption characteristics of the fuel from the canister 24 are affected by the temperature of the fuel adsorbent (activated carbon or the like) in the canister, the desorption amount calculation formula is an equation considering the relationship between the adsorbent temperature and the desorption amount. (Invention of Claim 11) In the region where the desorption amount is large, the calculation error of the desorption amount greatly affects the exhaust air / fuel ratio, and particularly high calculation accuracy is required. Moreover, since the desorption phenomenon of the fuel from the canister 24 is an endothermic reaction, When desorption is performed, the temperature of the adsorbent in the canister decreases and the desorption characteristics also change. Therefore, if the relationship between the adsorbent temperature and the desorption amount is not taken into account, the calculation accuracy of the desorption amount is reduced in a region where the desorption amount is large, but the canister is calculated so that the desorption amount is also taken into account. If the model is configured, it is possible to suppress the deterioration in the calculation accuracy of the desorption amount and prevent the exhaust emission deterioration caused by the deterioration in the calculation accuracy.
[0204]
As a desorption amount calculation formula for calculating the desorbed fuel amount based on the adsorbed fuel amount, the purge rate, and the adsorbent temperature, an equation applied to the desorption phenomenon of the canister 24 based on the Freundlich equation. Can be used. Further, the adsorbent temperature can be determined by determining the amount of decrease in adsorbent temperature based on the amount of fuel desorbed from the canister 24 (the invention according to claim 12).
[0205]
Further, according to the present embodiment (the invention described in claim 13), the calibration process of the canister adsorption amount which is an internal variable of the canister model is also performed. The canister model is an approximate model, and the values calculated by it are some errors, and the canister model calculates a new value using the previous value, so that its operation time becomes longer. Although the error is integrated and becomes large, by performing such a calibration process, the error can be appropriately corrected and high calculation accuracy can be maintained. This calibration process is to calculate the adsorption amount from the desorption amount estimated from the air-fuel ratio deviation from the theoretical air-fuel ratio, and to calibrate the adsorption amount of the internal variable with this reverse calculation adsorption amount. There is no need to go through a simple sequence and there is no need to reduce the purge rate. The adsorption amount can be obtained by inverse calculation of the desorption amount calculation formula in the canister model (the invention according to claim 14).
[0206]
The calibration process is performed when there is no air-fuel ratio deviation from the stoichiometric air-fuel ratio due to disturbances other than purge, and all the air-fuel ratio deviation from the stoichiometric air-fuel ratio is considered to be due to purge, and the amount of fuel desorption from the canister is determined. Since the calibration is performed based on the actual adsorption amount of the canister 24 that is estimated and calculated backward from the estimated desorption amount, the accuracy of the calibration process can be ensured (the invention according to claim 15).
[0207]
Further, according to the present embodiment (the inventions described in claims 16 and 17), the calibration process is performed when the air-fuel ratio deviation from the theoretical air-fuel ratio due to the purge is relatively large (purge) in order to improve the calculation accuracy of the desorption amount. When the rate is high and the purge air-fuel ratio is high). Depending on the operation area, sufficient calculation accuracy of the calibration process may not be obtained, but by executing the calibration process only in the area where sufficient accuracy is obtained in this way, the accuracy of the calibration process is improved, and the canister model Accuracy can be further improved.
[0208]
Although the embodiment of the present invention has been described above, the configuration of the above embodiment shows an example of the configuration to which the present invention is applied, and the scope of the present invention is not limited to the above configuration. As described above, in the above-described embodiment, the purge process illustrated in FIG. 16 is performed supplementarily until the purge process using the canister model is enabled, but the purge process illustrated in FIG. 16 is continuously used. You may do it.
[0209]
Further, the invention described in claim 5 is not limited to the purging process of the prior application device, but is naturally applicable to the purging process performed immediately after starting.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of a fuel vapor processing apparatus according to the present invention.
FIG. 2 is a block diagram showing an outline of purge processing in an engine controller.
FIG. 3 is a block diagram showing the configuration of a canister model.
FIG. 4 is a flowchart for explaining calculation of a purge correction coefficient.
FIG. 5 is a flowchart showing conditions for enabling calibration processing.
FIG. 6 is a flowchart for explaining calibration processing;
FIG. 7 is a flowchart for explaining calculation of a desorption amount based on a canister model.
FIG. 8 is a flowchart for explaining delay correction;
FIG. 9 is a characteristic diagram showing the relationship between the intake air flow rate and the dead time.
FIG. 10 is a characteristic diagram showing a relationship between an intake air flow rate and a smoothing coefficient.
FIG. 11 is a diagram showing an outline of dead time processing in delay correction.
FIG. 12 is a flowchart for explaining setting of a target purge rate.
FIG. 13 is a flowchart for explaining setting of a target purge rate.
FIG. 14 is a characteristic diagram showing the relationship of the purge air-fuel ratio error with respect to the intake air flow rate and the purge rate.
FIG. 15 is a characteristic diagram showing a relationship between a purge air-fuel ratio error and a purge air-fuel ratio.
FIG. 16 is a flowchart for explaining boot-up control;
FIG. 17 is a characteristic diagram showing the relationship between the difference in air-fuel ratio feedback deviation (= target air-fuel ratio feedback deviation−actual air-fuel ratio feedback deviation) and the purge rate change amount;
FIG. 18 is a characteristic diagram showing the relationship between the purge end determination value and the purge air-fuel ratio.
FIG. 19 is a flowchart for explaining the setting of a purge lower limit limiter determination flag;
FIG. 20 is a flowchart for explaining the setting of a purge upper limit limiter determination flag;
FIG. 21 is a waveform diagram for explaining purge rate reduction processing;
FIG. 22 is a flowchart for explaining purge rate reduction processing;
FIG. 23 is a waveform diagram showing a change in the target purge rate when it is determined that the purge has been completed.
FIG. 24 is a flowchart for explaining the setting of a base air-fuel ratio error determination flag.
FIG. 25 is a flowchart for explaining setting of an air-fuel ratio learning start flag.
FIG. 26 is a flowchart for explaining the update of the air-fuel ratio learning value.
FIG. 27 is a waveform diagram schematically showing a case where the purge correction coefficient is not correctly calculated due to the initial calibration error with the air-fuel ratio learning removed.
FIG. 28 is a waveform diagram schematically showing a case where a base air-fuel ratio error is determined with air-fuel ratio learning removed.
[Explanation of symbols]
4 Fuel injection valve
5 Exhaust passage
8 Intake passage
11 Engine controller
13 Air flow meter
15 Air-fuel ratio sensor (air-fuel ratio detection means)
21 Fuel tank
22 Piping
24 Canister
26 Piping
27 Purge valve

Claims (17)

Air-fuel ratio detection means for detecting the air-fuel ratio in the exhaust;
Air-fuel ratio feedback correction amount calculating means for calculating the air-fuel ratio feedback correction amount so that the exhaust air-fuel ratio detected by the air-fuel ratio detection means matches the stoichiometric air-fuel ratio;
First limiter setting means for setting a first limiter having a predetermined width from the center value of the air-fuel ratio feedback correction amount;
Air-fuel ratio feedback correction amount limiting means for limiting the calculated air-fuel ratio feedback correction amount so that the air-fuel ratio feedback correction amount is within the center value side including the first limiter;
In an air-fuel ratio control apparatus for an engine, comprising:
A canister that adsorbs the evaporated fuel generated in the fuel tank;
A purge valve that opens and closes a pipe that communicates the canister and the intake passage of the engine;
Purge valve control means for controlling the purge valve so as to obtain a target purge rate;
at least,
(A) an adsorbed fuel amount calculation formula for calculating the amount of fuel adsorbed to the canister by subtracting the previous value of the amount of fuel desorbed from the canister from the previous value of the amount of fuel adsorbed to the canister;
(B) Calculate the amount of fuel desorbed from the canister based on the exponential function of the desorption index with the value obtained by dividing the amount of adsorbed fuel calculated by the equation for calculating the amount of adsorbed fuel divided by the desorption coefficient, and the target purge rate. A canister model composed of a desorbed fuel amount calculation formula,
Purging correction that calculates the purge correction amount based on the desorbed fuel amount calculated using the canister model so that the air-fuel ratio deviation from the stoichiometric air-fuel ratio is reduced by performing the purge process at the target purge rate. A quantity calculation means;
A purge correction means for correcting the amount of fuel supplied to the engine by the purge correction amount;
At the calibration processing timing, the amount of fuel desorbed from the canister is estimated by assuming that the air-fuel ratio deviation from the theoretical air-fuel ratio is due to the purge processing, and the amount of fuel adsorbed to the canister from this estimated desorbed fuel amount A means for calibrating the amount of adsorbed fuel that is an internal variable in the canister model with the calculated value of the adsorbed fuel amount,
Means for setting a second limiter having a narrower range to the center value of the air-fuel ratio feedback correction amount than the first limiter;
An air-fuel ratio of the engine, comprising: purge rate reduction processing means for reducing the target purge rate when the calculated air-fuel ratio feedback correction amount does not fall within the center value including the second limiter. Control device.   2. The engine according to claim 1, wherein the purge rate reduction process is stopped when the air-fuel ratio feedback correction amount falls within a center value including the second limiter by performing the purge rate reduction process. Air-fuel ratio control device.   The engine air-fuel ratio control apparatus according to claim 1 or 2, wherein a hysteresis is provided in the second limiter.   The engine air-fuel ratio control apparatus according to claim 1 or 2, wherein the target purge rate is decreased by a predetermined value at predetermined time intervals. Air-fuel ratio learning value storage means for storing the air-fuel ratio learning value;
An air-fuel ratio learned value reading means for reading out the air-fuel ratio learned value according to operating conditions;
Air-fuel ratio correction means for correcting the amount of fuel supplied to the engine with the read air-fuel ratio learning value;
Air-fuel ratio detection means for detecting the air-fuel ratio in the exhaust;
Air-fuel ratio feedback correction amount calculating means for calculating the air-fuel ratio feedback correction amount so that the exhaust air-fuel ratio detected by the air-fuel ratio detection means matches the stoichiometric air-fuel ratio;
First limiter setting means for setting a first limiter having a predetermined width from the center value of the air-fuel ratio feedback correction amount;
Air-fuel ratio feedback correction amount limiting means for limiting the calculated air-fuel ratio feedback correction amount so that the air-fuel ratio feedback correction amount is within the center value side including the first limiter;
In an air-fuel ratio control apparatus for an engine, comprising:
A canister that adsorbs the evaporated fuel generated in the fuel tank;
A purge valve that opens and closes a pipe that communicates the canister and the intake passage of the engine;
Purge valve control means for controlling the purge valve so as to obtain a target purge rate;
Means for setting a second limiter having a narrower range to the center value of the air-fuel ratio feedback correction amount than the first limiter;
A purge rate reduction processing means for performing a process of reducing the target purge rate toward zero when the calculated air-fuel ratio feedback correction amount does not fall on the center value side including the second limiter;
And a base air-fuel ratio error determining means for determining that a base air-fuel ratio error has occurred when a period during which the target purge rate continues to be zero by the purge rate lowering process reaches a predetermined value. Air-fuel ratio control device.   When the air-fuel ratio learning is started after the purge process is completed after the engine is started, the purge process is stopped and the air-fuel ratio learning is started immediately when it is determined that the base air-fuel ratio error has occurred. The engine air-fuel ratio control apparatus according to claim 5.   When the air-fuel ratio learning value is used for failure diagnosis of the fuel system, when it is determined that the base air-fuel ratio error has occurred, the purge process is stopped and the prohibition of updating the air-fuel ratio learning value is canceled to cancel the air-fuel ratio learning value. 6. The engine air-fuel ratio control apparatus according to claim 5, wherein update of the engine is permitted.   The case where the air-fuel ratio learning value is used for fuel system failure diagnosis is a case where a fuel system failure is diagnosed when the air-fuel ratio learning value is equal to or greater than a predetermined value. The air-fuel ratio control device for an engine according to claim 7.   The engine air-fuel ratio control apparatus according to any one of claims 5 to 8, wherein an operation region for determining whether or not a base air-fuel ratio error is present is a region where the intake air flow rate is large. at least,
(A) an adsorbed fuel amount calculation formula for calculating the amount of fuel adsorbed to the canister based on the previous value of the fuel amount adsorbed on the canister and the previous value of the amount of fuel desorbed from the canister;
(B) a canister model including an adsorbed fuel amount calculated by an adsorbed fuel amount calculating equation and a desorbed fuel amount calculating equation for calculating an amount of fuel desorbed from the canister based on a target purge rate;
Purging correction that calculates the purge correction amount based on the desorbed fuel amount calculated using the canister model so that the air-fuel ratio deviation from the stoichiometric air-fuel ratio is reduced by performing the purge process at the target purge rate. A quantity calculation means;
The engine air-fuel ratio control apparatus according to any one of claims 5 to 9, further comprising a purge correction unit that corrects the amount of fuel supplied to the engine by the purge correction amount.   The desorbed fuel amount calculation formula of the canister model calculates the amount of fuel desorbed from the canister based on the adsorbed fuel amount calculated by the adsorbed fuel amount calculation formula, the target purge rate, and the temperature of the fuel adsorbent in the canister The engine air-fuel ratio control apparatus according to any one of claims 1, 2, 3, 4, and 10, characterized by:   The canister model further includes an adsorbent temperature calculation formula that calculates the temperature drop of the fuel adsorbent based on the amount of fuel desorbed from the canister and calculates the temperature of the fuel adsorbent based on the temperature drop. The engine air-fuel ratio control apparatus according to claim 11.   Means for estimating the amount of fuel desorbed from the canister on the assumption that the air-fuel ratio deviation from the theoretical air-fuel ratio is due to purge processing, and means for back-calculating the amount of fuel adsorbed to the canister from the estimated amount of desorbed fuel The engine air-fuel ratio control according to claim 11 or 12, further comprising means for calibrating an adsorbed fuel amount value, which is an internal variable in the canister model, with the back-calculated adsorbed fuel amount value. apparatus.   The means for calculating back the amount of fuel adsorbed to the canister calculates the amount of fuel adsorbed to the canister from the estimated amount of desorbed fuel by inverse calculation of the desorbed fuel amount calculation formula of the canister model. The air-fuel ratio control apparatus for an engine according to claim 13.   The means for estimating the amount of desorbed fuel estimates the amount of desorbed fuel from the canister based on the air-fuel ratio deviation from the stoichiometric air-fuel ratio when there is no air-fuel ratio deviation from the stoichiometric air-fuel ratio due to disturbances other than purge. The engine air-fuel ratio control apparatus according to claim 13 or 14,   The means for estimating the amount of desorbed fuel is the desorption from the canister based on the air / fuel ratio deviation from the stoichiometric air / fuel ratio when the purge air / fuel ratio is deeper than a predetermined value and the air / fuel ratio deviation from the stoichiometric air / fuel ratio is sufficiently expressed by the purge. The engine air-fuel ratio control apparatus according to any one of claims 13 to 15, wherein the fuel amount is estimated.   The means for estimating the amount of desorbed fuel is that the purge rate is higher than a predetermined value, and the desorbed fuel from the canister is calculated from the air-fuel ratio deviation from the stoichiometric air-fuel ratio when the air-fuel ratio deviation from the stoichiometric air-fuel ratio due to the purge sufficiently appears. The engine air-fuel ratio control device according to any one of claims 13 to 16, wherein the amount is estimated.

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