JP2004100465A - Air-fuel ratio controller changing limit value of target air-fuel ratio in reducing treatment by catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the exhaust of an injurious ingredient in exhaust gas after a catalyst reducing treatment. <P>SOLUTION: An air-fuel ratio controller for controlling an air-fuel ratio of an internal combustion engine is provided with a reducing means for reducing a catalyst in a catalyst reducing mode conducted after finishing an operation with a lean air-fuel ratio, and an air-fuel control means for controlling the air-fuel ratio to converge the output of an exhaust gas sensor arranged in an exhaust pipe at a predetermined target value in an air-fuel ratio control mode executed after finishing the catalyst reducing mode. The air-fuel ratio means calculates an control input for controlling the air-fuel ratio. The control input is processed for a limit with a limiter using an upper limit and a lower limit. The air-fuel ratio is controlled using the control input processed for the limit. When the internal combustion engine is in the catalyst reducing mode, the limiter shifts the upper limit to a rich direction to prevent the air-fuel ratio from changing to the lean direction over the predetermined amount when the internal combustion engine shifts from the catalyst reducing mode into the air-fuel ratio control mode. <P>COPYRIGHT: (C)2004,JPO

Description

【００４５】
Ｖｏ２は、式（１）に示されるように、Ｏ２センサ１７の出力値Ｖｏ２／ＯＵＴの目標値Ｖｏ２／ＴＡＲＧＥＴに対する偏差（以下、センサ出力偏差と呼ぶ）を示す。実空燃比偏差ｋａｃｔは、基準値ＦＬＡＦ／ＢＡＳＥに対するＬＡＦセンサの出力ＫＡＣＴの偏差を示す（ｋａｃｔ＝ＫＡＣＴ−ＦＬＡＦ／ＢＡＳＥ）。空燃比の基準値ＦＬＡＦ／ＢＡＳＥは、目標空燃比の中心的な値になるように設定され、たとえば理論空燃比を示す値（すなわち、１）に設定される。基準値ＦＬＡＦ／ＢＡＳＥは、一定値でもよいし、または運転状態に応じて決めるようにしてもよい。
【００４６】
ｄ１は、排気系１９が有するむだ時間を示す。むだ時間ｄ１は、ＬＡＦセンサ１６によって検出された空燃比がＯ２センサ１７の出力に反映されるのに要する時間を示す。ａ１、ａ２およびｂ１はモデルパラメータであり、後述する同定器によって生成される。
【００４７】
一方、エンジン１およびＥＣＵ５からなる空燃比を操作する系は、式（２）のようにモデル化されることができる。目標空燃比偏差ｋｃｍｄは、基準値ＦＬＡＦ／ＢＡＳＥに対する目標空燃比ＫＣＭＤの偏差を示す（ｋｃｍｄ＝ＫＣＭＤ−ＦＬＡＦ／ＢＡＳＥ）。ｄ２は、該空燃比操作系におけるむだ時間を示す。むだ時間ｄ２は、算出された目標空燃比ＫＣＭＤがＬＡＦセンサ１６の出力ＫＡＣＴに反映されるのに要する時間を示す。
【００４８】
【数２】

【００４９】
図６は、図５に示される制御器５１のさらに詳細なブロック図を示す。制御器５１は、同定器５２、推定器５３、スライディングモード制御器５４およびリミッタ５５を備える。
【００５０】
同定器５２は、式（１）におけるモデルパラメータａ１、ａ２およびｂ１を、モデル化誤差をなくすように同定する。同定器５２によって実施される同定方法を以下に示す。
【００５１】
前回の制御サイクルで算出されたモデルパラメータａ１（ｋ−１）、ａ２（ｋ−１）およびｂ１（ｋ−１）を用い（以下、これらのパラメータをａ１（ｋ−１）ハット、ａ２（ｋ−１）ハットおよびｂ１（ｋ−１）ハットと呼ぶ）、今回のサイクルのセンサ出力偏差Ｖｏ２（ｋ）（以下、これをセンサ出力偏差Ｖｏ２（ｋ）ハットと呼ぶ）を式（３）に従って求める。
【００５２】
【数３】

【００５３】
式（３）で算出されたセンサ出力偏差Ｖｏ２（ｋ）ハットと、今回の制御サイクルで実際に検出されたセンサ出力偏差Ｖｏ２（ｋ）との同定誤差ｉｄ／ｅ（ｋ）は、式（４）に従って求められる。
【００５４】
【数４】

【００５５】
同定器５２は、同定誤差ｉｄ／ｅ（ｋ）を最小にするように、今回のサイクルにおけるａ１（ｋ）ハット、ａ２（ｋ）ハットおよびｂ１（ｋ）ハットを算出する。ここで、式（５）に示されるようにベクトルΘを定義する。
【００５６】
【数５】

【００５７】
同定器５２は、式（６）に従い、ａ１（ｋ）ハット、ａ２（ｋ）ハットおよびｂ１（ｋ）ハットを求める。式（６）に示されるように、前回の制御サイクルで決定されたａ１（ｋ−１）ハット、ａ２（ｋ−１）ハットおよびｂ１（ｋ−１）ハットを、同定誤差ｉｄ／ｅ（ｋ）に比例する量だけ変化させることにより、今回の制御サイクルにおけるａ１（ｋ）ハット、ａ２（ｋ）ハットおよびｂ１（ｋ）ハットを求める。
【００５８】
【数６】

【００５９】
推定器５３は、排気系１９のむだ時間ｄ１および空燃比を操作する系のむだ時間ｄ２を補償するため、むだ時間ｄ（＝ｄ１＋ｄ２）後のセンサ出力偏差Ｖｏ２を推定する。
【００６０】
まず、排気系のモデル式（１）に、空燃比を操作する系のモデル式（２）を代入すると、式（７）が導かれる。
【００６１】
【数７】

【００６２】
式（７）で示されるモデル式は、排気系１９および上記の空燃比を操作する系を合わせた系を表現している。式（７）を用いることにより、むだ時間ｄ後のセンサ出力偏差Ｖｏ２（ｋ＋ｄ）の推定値Ｖｏ２（ｋ＋ｄ）バーが、式（８）のようにして求められる。係数α１、α２およびβｊは、同定器５２で算出されたモデルパラメータを用いて算出される。目標空燃比偏差の過去の時系列データｋｃｍｄ（ｋ−ｊ）（ただし、ｊ＝１、２、．．．３）は、むだ時間ｄの長さの間に取得された目標空燃比偏差を含む。
【００６３】
【数８】

【００６４】
むだ時間ｄ２以前の空燃比偏差ｋｃｍｄの過去の値ｋｃｍｄ（ｋ−ｄ２）、ｋｃｍｄ（ｋ−ｄ２−１）、．．．ｋｃｍｄ（ｋ−ｄ）の値を、上記の式（２）を用いてＬＡＦセンサ１６の偏差出力ｋａｃ（ｋ）、ｋａｃｔ（ｋ−１）、．．．ｋａｃｔ（ｋ−ｄ＋ｄ２）で置き換えることができる。その結果、式（９）が得られる。
【００６５】
【数９】

【００６６】
スライディングモード制御器５４は、スライディングモード制御を実行するため、切換関数σを式（１０）のように設定する。
【００６７】
【数１０】

【００６８】
ここで、Ｖｏ２（ｋ−１）は、前述したように前回のサイクルで検出されたセンサ出力偏差を示す。Ｖｏ２（ｋ）は、今回のサイクルで検出されたセンサ出力偏差を示す。ｓは、切換関数σの設定パラメータであり、−１＜ｓ＜１となるよう設定される。
【００６９】
切換関数σ（ｋ）＝０とした式は等価入力系と呼ばれ、制御量であるセンサ出力偏差Ｖｏ２の収束特性を規定する。σ（ｋ）＝０とすると、式（１０）は以下の式（１１）のように変形することができる。
【００７０】
【数１１】

【００７１】
ここで、図７および式（１１）を参照して、切換関数σの特性を説明する。図７は、縦軸がＶｏ２（ｋ）および横軸がＶｏ２（ｋ−１）の位相平面上に、式（１１）を線４１で表現したものである。この線４１を切換直線と呼ぶ。Ｖｏ２（ｋ−１）およびＶｏ２（ｋ）の組合せからなる状態量（Ｖｏ２（ｋ−１），　Ｖｏ２（ｋ））の初期値が、点４２で表されているとする。スライディングモード制御は、点４２で表される状態量を、切換直線４１上に載せて該直線４１上に拘束するよう動作する。スライディングモード制御によると、状態量を切換直線４１上に保持することにより、該状態量を、外乱等に影響されることなく、極めて安定的に位相平面上の原点０に収束させることができる。言い換えると、状態量（Ｖｏ２（ｋ−１），　Ｖｏ２（ｋ））を、式（１１）に示される入力の無い安定系に拘束することにより、外乱およびモデル化誤差に対してロバストにセンサ出力Ｖｏ２／ＯＵＴを目標値Ｖｏ２／ＴＡＲＧＥＴに収束させることができる。
【００７２】
切換関数設定パラメータｓは、可変に設定することができるパラメータである。設定パラメータｓを調整することにより、センサ出力偏差Ｖｏ２の減衰（収束）特性を指定することができる。
【００７３】
図８は、スライディングモード制御の応答指定特性の一例を示すグラフである。グラフ４３は、ｓの値が“−１”である場合を示し、グラフ４４はｓの値が“−０．８”である場合を示し、グラフ４５はｓの値が“−０．５”である場合を示す。グラフ４３〜４５から明らかなように、ｓの値に従って、センサ出力偏差Ｖｏ２の収束速度が変化する。ｓの絶対値を小さくするほど、収束速度が速くなる。
【００７４】
切換関数σの値をゼロにするよう、３つの制御入力が決定される。すなわち、状態量を切換直線上に拘束するための制御入力Ｕｅｑ、状態量を切換直線上に載せるための制御入力Ｕｒｃｈ、およびモデル化誤差および外乱を抑制しつつ、状態量を切換直線に載せるための制御入力Ｕａｄｐが算出される。これら３つの制御入力Ｕｅｑ、ＵｒｃｈおよびＵａｄｐの和を算出して、空燃比偏差ｋｃｍｄを算出するための要求偏差Ｕｓｌを求める。
【００７５】
等価制御入力Ｕｅｑは、状態量を切換直線上に拘束するための入力であるので、式（１２）を満たすことが条件となる。
【００７６】
【数１２】

【００７７】
したがって、σ（ｋ＋１）＝σ（ｋ）とするための等価制御入力Ｕｅｑは、式（７）および（１０）から、式（１３）のように算出される。
【００７８】
【数１３】

【００７９】
切換関数σの値に応じた値を持つ到達則入力Ｕｒｃｈを、式（１４）に従って算出する。この実施例では、到達則入力Ｕｒｃｈは切換関数σの値に比例した値を持つ。Ｋｒｃｈは到達則のフィードバックゲインを示し、これは、切換直線σ＝０への収束の安定性および速応性等を考慮して、シミュレーション等に基づいて予め定められる。
【００８０】
【数１４】

【００８１】
切換関数σの積算値に応じた値を持つ適応則入力Ｕａｄｐを、式（１５）に従って算出する。この実施例では、適応則入力Ｕａｄｐは切換関数σの積算値に比例した値を持つ。Ｋａｄｐは適応則のフィードバックゲインを示し、これは、切換直線σ＝０への収束の安定性および速応性等を考慮して、シミュレーション等に基づいて予め定められる。ΔＴは、制御サイクルの周期を示す。
【００８２】
【数１５】

【００８３】
センサ出力偏差Ｖｏ２（ｋ＋ｄ）およびＶｏ２（ｋ＋ｄ−１）と、切換関数の値σ（ｋ＋ｄ）は、むだ時間ｄが考慮された予測値であるので、これらを直接求めることはできない。そこで、推定器５３によって求められた推定偏差Ｖｏ２（ｋ＋ｄ）バーおよびＶｏ２（ｋ＋ｄ−１）バーを用い、等価制御入力Ｕｅｑを求める。
【００８４】
【数１６】

【００８５】
また、推定器５３によって算出された推定偏差を用いて、式（１７）に示されるように切換関数σバーが算出される。
【００８６】
【数１７】

【００８７】
切換関数σバーを用いて、到達則入力Ｕｒｃｈおよび適応則入力Ｕａｄｐを算出する。
【００８８】
【数１８】

【００８９】
【数１９】

【００９０】
式（２０）に示されるように、等価制御入力Ｕｅｑ、到達則入力Ｕｒｃｈおよび適応則入力Ｕａｄｐを加算し、要求偏差Ｕｓｌを求める。
【００９１】
【数２０】

【００９２】
リミッタ５５は、要求偏差Ｕｓｌに対してリミット処理を行い、空燃比偏差ｋｃｍｄを求める。具体的には、リミッタ５５は、要求偏差Ｕｓｌが許容範囲内にあれば、該要求偏差Ｕｓｌを空燃比偏差ｋｃｍｄとする。要求偏差Ｕｓｌが許容範囲から逸脱している場合は、該許容範囲の上限値または下限値を、空燃比偏差ｋｃｍｄに設定する。
【００９３】
リミッタ５５で使用される許容範囲は、図３の参照番号２９に示されるように、ウィンドウ２７を略中心として、これを含むさらに広い範囲に設定される。具体的には、以下の許容範囲が予め設定される。図９に、これらの許容範囲を示す。
【００９４】
（ｉ）安定判別によって不安定レベルが低いと判断されたときに用いられる許容範囲
（ｉｉ）安定判別によって不安定レベルが高いと判断されたときに用いられる許容範囲
（ｉｉｉ）燃料カットが終了してから所定時間が経過するまでの間、エンジンが始動してから所定時間が経過するまでの間、およびリーン運転が終了してから所定時間が経過するまでの間、に用いられる許容範囲
（ｉｖ）車両が発進してから（エンジンが駆動輪の駆動を開始してから）所定時間が経過するまでの間に用いられる許容範囲
（ｖ）エンジンがアイドリング状態にあるときに用いられる許容範囲
（ｖｉ）通常の運転状態において用いられる許容範囲（適応許容範囲と呼ぶ）
（ｖｉｉ）触媒還元モードから適応空燃比制御モードに切り換えられる時に用いられる許容範囲。
【００９５】
低不安定用許容範囲（ｉ）は、上限値Ｈおよび下限値Ｌによって規定される。高不安定用許容範囲（ｉｉ）は、上限値ＳＴＡＢＨおよび下限値ＳＴＡＢＬによって規定される。燃料カット／始動／リーン運転後用の許容範囲（ｉｉｉ）の上限値は、ＡＦＣＨである。下限値は、ＳＴＡＢＬからＬＬの間で可変に設定される。下限値は、許容範囲に対する要求偏差Ｕｓｌの逸脱状況に応じて、各サイクルで決定される。
【００９６】
負荷駆動後用の許容範囲（ｉｖ）の上限値は、ＳＴＡＢＨからＨＨの間で可変に設定される。上限値は、許容範囲に対する要求偏差Ｕｓｌの逸脱状況に応じて、各サイクルで決定される。下限値はＶＳＴＬである。アイドリング用の許容範囲（ｖ）は、上限値ＨＩおよび下限値ＬＩによって規定される。
【００９７】
適応許容範囲（ｖｉ）の上限値は、ＳＴＡＢＨからＨＨの間で可変に設定される。下限値は、ＳＴＡＢＬからＬＬの間で可変に設定される。上限値および下限値は、許容範囲に対する要求偏差Ｕｓｌの逸脱状況に応じて、各サイクルで決定される。
【００９８】
エンジンが触媒還元モードから適応空燃比制御モードに移行する時に使用される許容範囲（ｖｉｉ）の上限値は、適応許容範囲（ｖｉ）の上限値と同じであり、ＳＴＡＢＨからＨＨの間で可変に設定される。下限値は、ＬＩに固定される。こうして、エンジンが触媒還元モードから適応空燃比制御モードへ移行した時に、操作量Ｕｓｌがリーン方向へ大きく変化するのが防止される。
【００９９】
リミッタ５５によって求められた空燃比偏差ｋｃｍｄは基準値ＦＬＡＦ／ＢＡＳＥに加算され、目標空燃比ＫＣＭＤが求められる。該目標空燃比ＫＣＭＤを、制御対象である排気系１９に与えることにより、Ｏ２センサの出力Ｖｏ２／ＯＵＴを目標値Ｖｏ２／ＴＡＲＧＥＴに収束させることができる。
【０１００】
代替の実施形態においては、空燃比の基準値ＦＬＡＦ／ＢＡＳＥは、リミッタ５５によるリミット処理が終了した後、スライディングモード制御器５４によって算出される適応則入力Ｕａｄｐに応じて設定される。具体的には、基準値ＦＬＡＦ／ＢＡＳＥは、初期値として理論空燃比が設定される。適応則Ｕａｄｐが予め決められた上限値を超えているならば、基準値ＦＬＡＦ／ＢＡＳＥは所定量だけ増やされる。適応則Ｕａｄｐが予め決められた下限値を下回っているならば、基準値ＦＬＡＦ／ＢＡＳＥは所定量だけ減らされる。適応則Ｕａｄｐが上限値および下限値の間にあれば、基準値ＦＬＡＦ／ＢＡＳＥは維持される。設定されたＦＬＡＦ／ＢＡＳＥは、次回のサイクルにおいて用いられる。こうして、基準値ＦＬＡＦ／ＢＡＳＥは、目標空燃比ＫＣＭＤの中心的な値になるよう調整される。
【０１０１】
基準値ＦＬＡＦ／ＢＡＳＥの設定処理を上記のリミット処理と組み合わせることにより、要求偏差Ｕｓｌの許容範囲が正負にバランスされる。基準値ＦＬＡＦ／ＢＡＳＥの設定処理は、Ｏ２センサ出力Ｖｏ２／ＯＵＴが目標値Ｖｏ２／ＴＡＲＧＥＴにほぼ収束し、スライディングモード制御が安定状態にあると判断されたときに行われるのが好ましい。
【０１０２】
空燃比制御フロー　
図１０は、本発明の一実施形態に従う、空燃比の制御フローを示す。ステップＳ１０１において、同定器によるモデルパラメータの算出を許可するかどうかを判断する。同定器による算出が許可されたならば、許可フラグＦ＿ＩＤＣＡＬが１に設定される。触媒還元モードにあるとき、該フラグはゼロに設定される。
【０１０３】
ステップＳ１０２において、フラグＦ＿ＩＤＣＡＬの値を調べる。Ｆ＿ＩＤＣＡＬ＝１ならば、ステップＳ１０３に進み、モデルパラメータａ１、ａ２、およびｂ１を、前述した式（３）〜（６）に従って求める。Ｆ＿ＩＤＣＡＬ＝０ならば、ステップＳ１０３をスキップする。こうして、触媒還元モードにあるとき、同定器によるモデルパラメータ算出は停止される。この場合、触媒還元モードに入る前に最後に算出されたモデルパラメータがメモリに保存される。触媒還元モードが終了して適応空燃比制御を再び開始するとき、該保存されたモデルパラメータを用いてモデルパラメータの算出が開始される。
【０１０４】
ステップＳ１０４において、ステップＳ１０３で算出されたモデルパラメータを用い、推定器によって推定偏差Ｖｏ２バーを、前述した式（９）に従って求める。ステップＳ１０５において、切換関数σバー、等価制御入力Ｕｅｑ、適応側入力Ｕａｄｐ、および到達側入力Ｕｒｃｈを、前述した式（１６）〜（１９）に従って求める。制御入力Ｕｓｌを、式（２０）に従って求める。図には示されていないが、触媒還元モードにあるとき、適応則入力Ｕａｄｐの算出処理は行われない。触媒還元モードに入る前に最後に算出された適応則入力Ｕａｄｐがメモリに保存される。触媒還元モードが終了して適応空燃比制御を再び開始するとき、該保存された適応則入力Ｕａｄｐを使用して、適応則入力Ｕａｄｐの算出を開始する。
【０１０５】
ステップＳ１０６において、適応空燃比制御の安定判別が実施される（図１１）。ステップＳ１０７において、制御入力Ｕｓｌのリミット処理を実施し、目標空燃比偏差ｋｃｍｄを求める（図１２および図１３）。
【０１０６】
図１１は、図１０のステップＳ１０６で実施される、適応空燃比制御の安定判別を示すフローチャートである。
【０１０７】
ステップＳ１１１において、今回の制御サイクルで求められた切換関数σ（ｋ＋ｄ）バーと、該σ（ｋ＋ｄ）バーおよび前回の制御サイクルで求められた切換関数σ（ｋ＋ｄ−１）バーの偏差Δσと、に基づいて、安定判別基本パラメータＰｓｔｂ（＝σ（ｋ＋ｄ）バー・Δσバー）を算出する。Ｐｓｔｂは、切換関数σバーに関するリアプノフ関数σバー^２／２の時間微分値に相当する。
【０１０８】
ステップＳ１１２において、適応空燃比制御による空燃比操作が開始してから所定時間が経過したかどうかを判断する。所定時間が経過していなければ、安定判別を行うことなく、ステップＳ１１３の処理を介してこのルーチンを抜ける。所定時間が経過したならば、ステップＳ１１４に進む。ステップＳ１１３では、このルーチンで使用する各種パラメータを初期化する処理が実施される。
【０１０９】
ステップＳ１１４において、基本パラメータＰｓｔｂを所定値ε（＞０）と比較する。Ｐｓｔｂ≦εならば、空燃比制御が暫定的に安定であると判断される。すなわち、切換関数σの値がゼロに収束しているか、収束しつつある状態であると判断することができる。この場合、不安定と判断された頻度を示すカウンタｃｎｔ／ｊｕｄｓｔの値が維持される（Ｓ１１５）。Ｐｓｔｂ＞εならば、空燃比制御が暫定的に不安定であると判断される。すなわち、切換関数σの値がゼロから離間しつつある状態であると判断することができる。この場合、カウンタｃｎｔ／ｊｕｄｓｔの値が１だけインクリメントされる（Ｓ１１６）。
【０１１０】
ステップＳ１１７において、カウンタｃｎｔ／ｊｕｄｓｔの値を、第１のしきい値ＳＳＴＢ１と比較する。ｃｎｔ／ｊｕｄｓｔ≦ＳＳＴＢ１ならば、空燃比制御が安定であると判断され、安定判別フラグｆ／ｓｔｂ１をゼロに設定する（Ｓ１１８）。
【０１１１】
ステップ１１７において、ｃｎｔ／ｊｕｄｓｔ＞ＳＳＴＢ１ならば、ｃｎｔ／ｊｕｄｓｔを第２のしきい値ＳＳＴＢ２と比較する（Ｓ１１９）。ここで、ＳＳＴＢ１＜ＳＳＴＢ２である。ｃｎｔ／ｊｕｄｓｔ≦ＳＳＴＢ２ならば、空燃比制御が低レベルの不安定状態にあると判断され、ｆ／ｓｔｂ１の値を１に設定する（Ｓ１２０）。ｃｎｔ／ｊｕｄｓｔ＞ＳＳＴＢ２ならば、空燃比制御が高レベルの不安定状態にあると判断され、ｆ／ｓｔｂ１およびｆ／ｓｔｂ２の値をそれぞれ１に設定する（Ｓ１２１）。
【０１１２】
ステップＳ１２２において、カウンタｃｎｔ／ｊｕｄｓｔによる計測を開始してからの経過時間を測るタイマカウンタｔｍ／ｊｕｄｓｔを１だけデクリメントする。カウンタｃｎｔ／ｊｕｄｓｔは、タイマカウンタｔｍ／ｊｕｄｓｔがゼロになるたびに、リセットされる。
【０１１３】
ステップＳ１２３において、タイマカウンタｔｍ／ｊｕｄｓｔがゼロかどうかを判断する。ｔｍ／ｊｕｄｓｔ≠０ならば、このルーチンを抜ける。ｔｍ／ｊｕｄｓｔ＝０ならば、安定判別フラグｆ／ｓｔｂ１の値を調べる。ｆ／ｓｔｂ１＝１ならば、タイマカウンタｔｍ／ｊｕｄｓｔを初期値ＴＭＪＵＤＳＴにセットし、カウンタｃｎｔ／ｊｕｄｓｔをゼロにリセットする（Ｓ１２６）。ｆ／ｓｔｂ１＝０ならば、今回の制御サイクルで空燃比制御が安定であると判断されたことを示す。この場合、不安定フラグｆ／ｓｔｂ２をゼロにリセットした後に（Ｓ１２５）、ステップＳ１２６の処理を介してこのルーチンを抜ける。
【０１１４】
図１２および図１３を参照して、要求偏差Ｕｓｌのリミット処理について説明する。図９を参照して述べたように、運転状態に応じて、（ｉ）から（ｖｉｉ）の許容範囲のいずれかが選択される。選択された許容範囲について、エンジン１がアイドリング状態にある時とエンジン１がアイドリング状態にない時の上限値および下限値が異なる場合がある。以下の説明において、前者についての上限値および下限値を、それぞれＡＨＦＩおよびＡＬＦＩで表し、後者についての上限値および下限値を、それぞれＡＨＦおよびＡＬＦで表す。さらに、前述したように、許容範囲の中には、可変の上限値および／または可変の下限値を持つものがある。この可変に変化する適応上限値および適応下限値を、それぞれａｈおよびａｌで表す。
【０１１５】
図１２は、図１０のステップＳ１０７で実施される、要求偏差Ｕｓｌのリミット処理を示すフローチャートである。ステップＳ１２８において、エンジンが触媒還元モードにあるかどうか（還元処理が実施されているかどうか）を判断する。触媒還元モードにあるならば、空燃比偏差ｋｃｍｄを、所定値ＳＬＤＨＯＬＤ（＞０）に設定する。こうして、還元処理を促進するため、目標空燃比はリッチ化された空燃比に設定される。
【０１１６】
ステップＳ１３０において、前述した許容範囲（ｖｉｉ）を設定する。具体的には、適応下限値ａｌ（ｋ）を所定値ＬＩに設定する。こうして、エンジンが触媒還元モードにあるとき、許容範囲の下限値は値ＬＩに固定され、触媒還元モードから適応空燃比制御へ移行したときに目標空燃比が過度にリーン方向へ変化することを防止する。
【０１１７】
ステップＳ１２８において、エンジンが触媒還元モードになければ、ステップＳ１３１に進む。ステップＳ１３１において、今回の制御サイクルで用いる許容範囲を決定する。ここで、図１３を参照して、ステップＳ１３１で実行される許容範囲決定ルーチンを説明する。
【０１１８】
ステップＳ１６１において、フラグｆ／ｓｔｂ２の値を調べる。ｆ／ｓｔｂ２＝１ならば、空燃比制御が高レベルの不安定状態にあることを示す。ステップＳ１６２において、上限値ＡＨＦおよび下限値ＡＬＦを、許容範囲（ｉｉ）の上限値ＳＴＡＢＨおよび下限値ＳＴＡＢＬ（図９参照）にそれぞれ設定する。アイドリング状態用の下限値ＡＨＦＩおよび上限値ＡＬＦＩも、許容範囲（ｉｉ）の上限値ＳＴＡＢＨおよび下限値ＳＴＡＢＬに設定する。適応上限値ａｈおよび適応下限値ａｌ（正確には、ａｈ（ｋ−１）およびａｌ（ｋ−１）と表される）も、許容範囲（ｉｉ）の上限値ＳＴＡＢＨおよび下限値ＳＴＡＢＬに初期化される。こうして、空燃比制御が高レベルの不安定状態にあるときは、最も範囲の狭い許容範囲（ｉｉ）が選択される。この許容範囲（ｉｉ）は、アイドリング状態においても使用される。
【０１１９】
ステップＳ１６３において、フラグｆ／ｓｔｂ１の値を調べる。ｆ／ｓｔｂ１＝１ならば、空燃比制御が低レベルの不安定状態にあることを示す。ステップＳ１６４において、上限値ＡＨＦおよび下限値ＡＬＦは、許容範囲（ｉ）の上限値Ｈおよび下限値Ｌ（図９参照）にそれぞれ設定される。アイドリング状態用の上限値ＡＨＦＩおよび下限値ＡＬＦＩは、アイドリング許容範囲（ｖ）の上限値ＨＩおよび下限値ＬＩ（図９参照）にそれぞれ設定される。適応上限値ａｈおよび適応下限値ａｌは、許容範囲（ｉ）の上限値Ｈおよび下限値Ｌにそれぞれ設定される。こうして、空燃比制御が低レベルの不安定状態にある時は、許容範囲（ｉ）が選択される。空燃比制御が低レベルの不安定であり、かつエンジンがアイドリング状態にあるときは、低レベル不安定用の許容範囲（ｉ）よりも狭いアイドリング許容範囲（ｖ）が選択される。
【０１２０】
ステップＳ１６１およびＳ１６３の判断がＮｏならば、空燃比制御は安定状態にある。ステップＳ１６５において、燃料カットが終了してからの経過時間が所定時間より小さいかどうかを判断する。ステップＳ１６６において、エンジンが始動してからの経過時間が所定時間より小さいかどうかを判断する。ステップＳ１６７において、リーン運転が終了してからの経過時間が所定時間より小さいかどうかを判断する。ステップＳ１６５〜１６７のうちいずれかの判断がＹｅｓならば、ステップＳ１６８に進む。
【０１２１】
ステップＳ１６８において、上限値ＡＨＦを、許容範囲（ｉｉｉ）の上限値ＡＦＣＨ（図９参照）に設定する。許容範囲（ｉｉｉ）の下限値は可変である。したがって、下限値ＡＬＦは、前回の制御サイクルで決定された適応下限値ａｌ（ｋ−１）に設定される。アイドリング状態用の上限値ＡＨＦＩは、許容範囲（ｉｉｉ）の上限値ＡＦＣＨに設定される。下限値ＡＬＦＩは、許容範囲（ｖ）の下限値ＬＩに設定される。こうして、エンジンがアイドリング状態になければ、許容範囲（ｉｉｉ）が選択される。エンジンがアイドリング状態にあれば、下限値は、アイドリング許容範囲（ｖ）の下限値ＬＩに固定される。
【０１２２】
ステップＳ１６５〜Ｓ１６７の判断がすべてＮｏならば、ステップＳ１６９に進み、エンジンがアイドリング状態にあるかどうかを判断する。アイドリング状態にあるならば、ステップＳ１７０に進み、上限値ＡＨＦＩおよび下限値ＡＬＦＩが、アイドリング許容範囲（ｖ）の上限値ＨＩおよび下限値ＬＩに設定される。
【０１２３】
ステップＳ１６９の判断がＮｏのとき、ステップＳ１７１に進む。ステップＳ１７１〜Ｓ１７８の処理は、適応許容範囲（ｖｉ）に示される可変の適応上限値および適応下限値を制限する処理を示す。ステップＳ１７１において、適応上限値ａｈ（ｋ−１）をＳＴＡＢＨと比較する。ａｈ（ｋ−１）＜ＳＴＡＢＨならば、ａｈ（ｋ−１）の値をＳＴＡＢＨに設定する（Ｓ１７２）。ａｈ（ｋ−１）≧ＳＴＡＢＨならば、ステップＳ１７３に進み、適応上限値ａｈ（ｋ−１）をＨＨと比較する。ａｈ（ｋ−１）＞ＨＨならば、ａｈ（ｋ−１）の値をＨＨに設定する（Ｓ１７４）。ステップＳ１７１およびＳ１７３の判断がＮｏならば、ａｈ（ｋ−１）の値は維持される。こうして、適応上限値ａｈ（ｋ−１）は、ＳＴＡＢＨとＨＨの間に制限される。
【０１２４】
ステップＳ１７５において、適応下限値ａｌ（ｋ−１）をＬＬと比較する。ａｌ（ｋ−１）＜ＬＬならば、ａｌ（ｋ−１）の値をＬＬに設定する（Ｓ１７６）。ａｈ（ｋ−１）≧ＬＬならば、ステップＳ１７７に進み、適応下限値ａｌ（ｋ−１）をＳＴＡＢＬと比較する。ａｌ（ｋ−１）＞ＳＴＡＢＬならば、ａｌ（ｋ−１）の値をＳＴＡＢＬに設定する（Ｓ１７８）。ステップＳ１７５およびＳ１７７の判断がＮｏならば、ａｌ（ｋ−１）の値は維持される。こうして、適応下限値ａｌ（ｋ−１）は、ＬＬとＳＴＡＢＬの間に制限される。
【０１２５】
ステップＳ１７９において、上限値ＡＨＦおよび下限値ＡＬＦに、適応上限値ａｈ（ｋ−１）および適応下限値ａｌ（ｋ−１）をそれぞれ設定する。こうして、適応許容範囲（ｖｉ）が定められる。
【０１２６】
ステップＳ１８０において、車両が発進してから所定時間が経過していなければ、下限値ＡＬＦは、許容範囲（ｉｖ）の下限値ＶＳＴＬに設定される（Ｓ１８１）。上限値ＡＨＦは、適応許容範囲（ｖｉ）の上限値と同じ値が用いられる。
【０１２７】
図１２に戻り、ステップＳ１３２において、エンジンがアイドリング状態にあるかどうかを判断する。アイドリング状態にあるならば、図１３のステップＳ１６２、Ｓ１６４、Ｓ１６８およびＳ１７０のいずれかで設定された上限値ＡＨＦＩおよび下限値ＡＬＦＩによって規定される許容範囲を用いて、要求偏差Ｕｓｌがリミット処理される。具体的には、Ｕｓｌ＜ＡＬＦＩならば（Ｓ１３３）、目標空燃比偏差ｋｃｍｄは下限値ＡＬＦＩに設定される（Ｓ１３４）。Ｕｓｌ＞ＡＨＦＩならば（Ｓ１３５）、目標空燃比偏差ｋｃｍｄは上限値ＡＨＦＩに設定される（Ｓ１３６）。ステップＳ１３３およびＳ１３５の判断がＮＯならば（すなわち、ＡＬＦＩ≦Ｕｓｌ≦ＡＨＦＩならば）、現在の要求偏差Ｕｓｌが、目標空燃比偏差ｋｃｍｄに設定される（Ｓ１３７）。
【０１２８】
ステップＳ１３３またはＳ１３５がＹｅｓのとき、適応則入力Ｕａｄｐが大きくなるのを防止するため、今回のサイクルで算出された切換関数σバーの積算値に、前回のサイクルで算出された積算値の値を設定する（Ｓ１３８）。ステップＳ１３９において、現在の適応上限値ａｈ（ｋ−１）および下限値ａｌ（ｋ−１）が保持される。
【０１２９】
ステップＳ１３２においてエンジンがアイドリング状態にないと判断されたならば、図１２のステップＳ１６２、Ｓ１６４、Ｓ１６８、Ｓ１７９、およびＳ１８１のいずれかで設定された上限値ＡＨＦおよび下限値ＡＬＦによって規定される許容範囲を用いて、要求偏差Ｕｓｌをリミット処理する。
【０１３０】
具体的には、Ｕｓｌ＜ＡＬＦならば（Ｓ１４０）、目標空燃比偏差ｋｃｍｄは下限値ＡＬＦに設定される（Ｓ１４１）。Ｕｓｌ＞ＡＨＦならば（Ｓ１４２）、目標空燃比偏差ｋｃｍｄは上限値ＡＨＦに設定される（Ｓ１４３）。ＡＬＦ≦Ｕｓｌ≦ＡＨＦならば、現在の要求偏差Ｕｓｌが、目標空燃比偏差ｋｃｍｄに設定される（Ｓ１４４）。
【０１３１】
ＡＬＦ≦Ｕｓｌ≦ＡＨＦのとき、適応下限値ａｌ（ｋ−１）をΔＤＥＣ（＞０）だけ増やし、上限値ａｈ（ｋ−１）をΔＤＥＣだけ減らす（Ｓ１４５）。こうして、現在の要求偏差Ｕｓｌが許容範囲内に収まっているとき、適応許容範囲を狭くする。
【０１３２】
Ｕｓｌ＜ＡＬＦのとき、ステップＳ１４６において、車両が発進してからの経過時間が所定時間より小さいかどうかを判断する。この判断がＮｏならば、適応下限値ａｌ（ｋ−１）をΔＩＮＣ（＞０）だけ減らし、上限値ａｈ（ｋ−１）をΔＤＥＣだけ減らす（Ｓ１４７）。ここで、ΔＤＥＣ＜ΔＩＮＣである。こうして、現在の要求偏差Ｕｓｌが許容範囲の下限側で逸脱している場合には、上限値および下限値を小さくして、許容範囲を下方に移動させる。ステップＳ１４６の判断がＹｅｓならば、適応上限値ａｈ（ｋ−１）および下限値ａｌ（ｋ−１）の値は維持される（Ｓ１４８）。
【０１３３】
Ｕｓｌ＞ＡＨＦのとき、ステップＳ１４９において、燃料カットが終了してからの経過時間が所定時間内、またはエンジンが始動してからの経過時間が所定時間内、またはリーン運転を終了してからの経過時間が所定時間内かどうかが判断される。これらの状態のいずれにも該当しないとき、適応上限値ａｈ（ｋ−１）をΔＩＮＣだけ増やし、下限値ａｌ（ｋ−１）をΔＤＥＣだけ増やす（Ｓ１５０）。こうして、現在の要求偏差Ｕｓｌが許容範囲の上限側で逸脱している場合には、上限値および下限値を大きくして、許容範囲を上方に移動させる。ステップＳ１４９の判断においていずれかの状態がＹｅｓならば、適応上限値ａｈ（ｋ−１）および下限値ａｌ（ｋ−１）の値は維持される（Ｓ１５１）。
【０１３４】
ステップＳ１４０またはＳ１４２のいずれかの判断がＹｅｓの場合、ステップＳ１３８と同様に、今回のサイクルで算出された切換関数σバーの積算値に、前回のサイクルで算出された切換関数σバーの積算値を設定する。
【０１３５】
こうして、エンジンが触媒還元モードにあるとき、要求偏差Ｕｓｌの下限値ａｌ（ｋ）は、図１２のステップＳ１３０に示されるように、リッチ方向へシフトされた値（この例では、値ＬＩ）に固定される。適応空燃比制御が開始されたとき、該値ＬＩが設定された下限値ＡＬＦによって操作量Ｕｓｌはリミット処理されるので（Ｓ１４０）、目標空燃比ＫＣＭＤがリーン方向へ大きく変化することが防止される。
【０１３６】
図１４は、本発明の一実施形態に従う、リーン運転が終了した後のＯ２センサ出力Ｖｏ２／ＯＵＴ、目標空燃比ＫＣＭＤ、および排ガスの有害成分ＮＯｘの排出量の遷移を示す。
【０１３７】
リーン運転中は許容範囲の更新処理は実施されず、許容範囲の上限値および下限値は、該リーン運転に入る前に最後に設定された値に維持される。この例は、下限値が、たとえば図９に示されるＬＩ〜ＬＬの間の所定の値に維持されている形態を示している。
【０１３８】
リーン運転が終了した時間ｔ２において、触媒還元モードが開始される。触媒還元モードにおいて、触媒に吸着された酸素を除去するため、目標空燃比ＫＣＭＤは所定のリッチ空燃比に設定される。前述したように、触媒還元モードにおいて、目標空燃比ＫＣＭＤの下限値は所定値ＬＩに固定される。時間ｔ３において、上流触媒の還元処理が終了すると、Ｏ２センサ出力Ｖｏ２／ＯＵＴはリーンからリッチへ反転する。
【０１３９】
時間ｔ４において、下流触媒の還元処理が終了して適応空燃比制御が開始される。この時、Ｏ２センサ出力Ｖｏ２／ＯＵＴは、目標値Ｖｏ２／ＴＡＲＧＥＴよりもリッチ側に存在する。適応空燃比制御は、Ｏ２センサ出力Ｖｏ２／ＯＵＴを目標値に収束させるため、目標空燃比ＫＣＭＤをリーン方向へシフトしようとする。しかしながら、時間ｔ４において、下限値が、リッチ側にシフトされた値ＬＩにセットされているので（参照番号１９０）、目標空燃比ＫＣＭＤは、該下限値より小さい値に設定されることがない。したがって、目標空燃比ＫＣＭＤがリーン方向へ所定量以上変化することが防止され、ＮＯｘの排出量が抑制される。
【０１４０】
この明細書においては、スライディングモード制御を用いて適応空燃比制御を実施する例を説明した。しかしながら、他の応答指定型制御を用いて適応空燃比制御を実施する場合にも、本発明を適用することができる。
【０１４１】
本発明は、クランク軸を鉛直方向とした船外機などのような船舶推進機用エンジンにも適用が可能である。
【０１４２】
【発明の効果】
この発明によると、触媒を還元する処理後の排ガスの有害成分の排出を低減することができる。
【図面の簡単な説明】
【図１】この発明の一実施例に従う、内燃機関およびその制御装置を概略的に示す図。
【図２】この発明の一実施例に従う、（ａ）触媒装置および排ガスセンサの配置、および（ｂ）空燃比制御の概要を示す図。
【図３】この発明の一実施例に従う、空燃比制御装置の全体的な機能ブロック図。
【図４】この発明の一実施例に従う、制御対称である排気系を示すブロック図。
【図５】この発明の一実施例に従う、空燃比制御の制御ブロック図。
【図６】この発明の一実施例に従う、制御器の詳細な機能ブロック図。
【図７】この発明の一実施例に従う、応答指定型制御における切換直線を概略的に示す図。
【図８】この発明の一実施例に従う、応答指定型制御における応答特性を示す図。
【図９】この発明の一実施例に従う、許容範囲の例を示す図。
【図１０】この発明の一実施例に従う、空燃比制御フローを示す図。
【図１１】この発明の一実施例に従う、安定判別の処理を示すフローチャート。
【図１２】この発明の一実施例に従う、リミット処理を示すフローチャート。
【図１３】この発明の一実施例に従う、許容範囲を決定する処理を示すフローチャート。
【図１４】この発明の一実施例に従う、触媒還元処理が終了した後における、排ガスセンサ出力、目標空燃比、および排ガスの有害成分の排出量の遷移を示す図。
【図１５】従来の空燃比制御に従う、触媒還元処理が終了した後における、排ガスセンサ出力、目標空燃比、および排ガスの有害成分の排出量の遷移を示す図。
【符号の説明】
１　エンジン
５　ＥＣＵ
１４　排気管
１５　触媒装置
１６　ＬＡＦセンサ
１７　Ｏ２センサ
２５　上流触媒
２６　下流触媒[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a control device for an internal combustion engine that controls an air-fuel ratio based on an output of an exhaust gas sensor provided in an exhaust system of the internal combustion engine.
[0002]
[Prior art]
A catalyst device is provided in an exhaust system of an internal combustion engine. The catalyst device oxidizes HC and CO with excess oxygen present in the exhaust gas when the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is lean, and converts NOx by HC and CO when the air-fuel ratio is rich. To reduce. When the air-fuel ratio is in the stoichiometric air-fuel ratio range, HC, CO and Nox are simultaneously and effectively purified.
[0003]
An exhaust gas sensor is provided downstream of the catalyst device. The exhaust gas sensor detects the oxygen concentration in the gas exhausted to the exhaust system. Feedback control of the air-fuel ratio of the internal combustion engine is performed based on the output of the exhaust gas sensor.
[0004]
As an example of feedback control of the air-fuel ratio, Japanese Unexamined Patent Application Publication No. 2000-234550 proposes a response assignment control using a switching function. This control converges the output of the exhaust gas sensor to a predetermined target value by converging the value of the switching function to zero. Specifically, an operation amount for converging the output of the exhaust gas sensor to a predetermined target value is calculated using the switching function. The operation amount is subjected to a limit process based on an upper limit value and a lower limit value set in accordance with the operation state, and a target air-fuel ratio is obtained. The fuel supply amount to the internal combustion engine is controlled according to the target air-fuel ratio. Thus, the air-fuel ratio is stably controlled.
[0005]
[Problems to be solved by the invention]
Recently, the operating range (including fuel cut) based on a lean air-fuel ratio has tended to be expanded in order to improve fuel efficiency. When the desired operating state cannot be achieved by the lean air-fuel ratio, the operation based on the stoichiometric air-fuel ratio or the rich air-fuel ratio is performed. When the engine is operated at the stoichiometric air-fuel ratio, the air-fuel ratio control based on the above-described response assignment control is executed in order to suppress the emission of harmful components of the exhaust gas.
[0006]
During operation with the lean air-fuel ratio (including fuel cut), the supply of fuel is suppressed, so that a large amount of oxygen is supplied to the catalyst device. The supplied oxygen is adsorbed on the catalyst device. If the catalyst device adsorbs excessive oxygen, the performance of the catalyst, particularly the ability to reduce Nox, deteriorates. In order to remove oxygen adsorbed on the catalyst device, a control for enriching the air-fuel ratio when fuel supply is restarted has been proposed.
[0007]
FIG. 15 shows the transition of the exhaust gas sensor output Vo2 / OUT, the target air-fuel ratio KCMD, and the emission amount of the harmful component NOx of the exhaust gas in the catalyst reduction mode and the adaptive air-fuel ratio control mode after the operation with the lean air-fuel ratio. In this example, the catalyst device includes an upstream catalyst and a downstream catalyst, and the exhaust gas sensor is provided between the upstream catalyst and the downstream catalyst.
[0008]
At a time t2 when the operation based on the lean air-fuel ratio ends, the catalyst reduction mode is started. In the catalyst reduction mode, the target air-fuel ratio KCMD is set to a predetermined rich air-fuel ratio in order to remove oxygen adsorbed on the catalyst. At time t3, the reduction process of the upstream catalyst ends, and the exhaust gas sensor output Vo2 / OUT is inverted from lean to rich.
[0009]
At time t4 when the reduction process of the downstream catalyst ends, the adaptive air-fuel ratio control is started. At time t4, the exhaust gas sensor output Vo2 / OUT exists on the richer side than the target value Vo2 / TARGET. In the adaptive air-fuel ratio control, the target air-fuel ratio KCMD is changed in the lean direction so that the exhaust gas sensor output Vo2 / OUT converges to the target value (reference numeral 200). This change in the target air-fuel ratio KCMD is limited by a lower limit set according to the operating state. However, depending on the magnitude of the lower limit, the change in the lean direction of the target air-fuel ratio KCMD increases. If the change in the lean direction toward the target air-fuel ratio KCMD is large, the NOx reduction ability of the catalyst decreases, and as a result, the NOx emission increases (reference numeral 201).
[0010]
Therefore, there is a need for air-fuel ratio control capable of reducing the amount of emission of harmful components of exhaust gas after the completion of the catalyst reduction treatment.
[0011]
[Means for Solving the Problems]
According to one aspect of the present invention, the air-fuel ratio control device is implemented after the catalyst reduction mode is performed after the lean air-fuel ratio operation is completed, and after the catalyst reduction mode is completed. In the air-fuel ratio control mode, there is provided air-fuel ratio control means for controlling the air-fuel ratio so that the output of the exhaust gas sensor arranged in the exhaust pipe converges to a predetermined target value. The air-fuel ratio control unit further includes a controller that calculates an operation amount for operating the air-fuel ratio, a limiter that limits the operation amount using an upper limit value and a lower limit value, and a limiter that uses the operation amount subjected to the limit process. Operating means for operating the air-fuel ratio. When the internal combustion engine is in the catalyst reduction mode, the limiter shifts the lower limit in the rich direction, and when the internal combustion engine shifts from the catalyst reduction mode to the air-fuel ratio control mode, the air-fuel ratio changes in the lean direction by a predetermined amount or more. To prevent
[0012]
According to the present invention, when shifting from the catalyst reduction mode to the air-fuel ratio control mode, the operation amount is limited using the lower limit shifted in the rich direction, so that the air-fuel ratio changes in the lean direction by a predetermined amount or more. Can be prevented. Therefore, the amount of harmful components discharged after the completion of the reduction process can be suppressed.
[0013]
According to another aspect of the present invention, the catalyst reduction mode is performed after the operation in the fuel cut state in which the fuel is not supplied is completed. According to the present invention, after the operation in the fuel cut state ends, the catalyst reduction mode is performed. When shifting from the catalyst reduction mode to the air-fuel ratio control mode, the operation amount is limited using the lower limit value shifted in the rich direction, so that it is possible to prevent the air-fuel ratio from changing more than a predetermined amount in the lean direction. it can. Therefore, the amount of harmful components discharged after the completion of the reduction process can be suppressed.
[0014]
According to another aspect of the present invention, the upper limit value and the lower limit value are set according to the operation amount and the operating state. By limiting the operation amount using the upper limit value and the lower limit value set in this way, it is possible to optimally maintain the purification ability of the catalyst.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Configuration of internal combustion engine and control device
Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an overall system configuration diagram of an internal combustion engine (hereinafter, referred to as “engine”) and a control device thereof according to an embodiment of the present invention.
[0016]
An electronic control unit (hereinafter, referred to as an “ECU”) 5 includes an input interface 5a for receiving data transmitted from each section of the vehicle, a CPU 5b for performing an operation for controlling each section of the vehicle, and a read-only memory (ROM). ) And a memory 5c having a random access memory (RAM), and an output interface 5d for sending a control signal to each part of the vehicle. The ROM of the memory 5c stores a program for controlling each part of the vehicle and various data. A program for realizing air-fuel ratio control according to the present invention, and data and tables used when executing the program are stored in the ROM. The ROM may be a rewritable ROM such as an EEPROM. The RAM is provided with a work area for calculation by the CPU 5b. Data sent from each part of the vehicle and control signals sent to each part of the vehicle are temporarily stored in the RAM.
[0017]
The engine 1 is, for example, an engine having four cylinders. An intake pipe 2 is connected to the engine 1. A throttle valve 3 is provided upstream of the intake pipe 2. A throttle valve opening sensor (θTH) 4 connected to the throttle valve 3 supplies an electric signal corresponding to the opening of the throttle valve 3 to the ECU 5.
[0018]
A passage 21 that bypasses the throttle valve 3 is provided in the intake pipe 2. A bypass valve 22 for controlling the amount of air supplied to the engine 1 is provided in the bypass passage 21. The bypass valve 22 is driven according to a control signal from the ECU 5.
[0019]
The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of an intake valve (not shown) of the intake pipe 2. The fuel injection valve 6 is connected to a fuel pump (not shown), and receives supply of fuel from a fuel tank (not shown) via the fuel pump. The fuel injection valve 6 is driven according to a control signal from the ECU 5.
[0020]
The intake pipe pressure (Pb) sensor 8 and the intake temperature (Ta) sensor 9 are provided on the intake pipe 2 downstream of the throttle valve 3. The intake pipe pressure Pb and the intake air temperature Ta detected by the Pb sensor 8 and the Ta sensor 9 are sent to the ECU 5, respectively.
[0021]
The engine water temperature (Tw) sensor 10 is attached to a cylinder wall (not shown) of the cylinder block of the engine 1 which is filled with cooling water. The temperature Tw of the engine cooling water detected by the Tw sensor 10 is sent to the ECU 5.
[0022]
The rotation speed (Ne) sensor 13 is mounted around a camshaft or a crankshaft (both not shown) of the engine 1. The Ne sensor 13 outputs a CRK signal pulse at a cycle of a crank angle (for example, 30 degrees) shorter than a cycle of a TDC signal pulse output at a crank angle related to the TDC position of the piston, for example. The CRK signal pulse is counted by the ECU 5, and the engine speed Ne is detected.
[0023]
An exhaust pipe 14 is connected to the downstream side of the engine 1. The engine 1 exhausts through an exhaust pipe 14. A catalyst device 15 provided in the exhaust pipe 14 purifies harmful components such as HC, CO, and NOx in the exhaust gas passing through the exhaust pipe 14. The catalyst device 15 is provided with two catalysts. The catalyst provided on the upstream side is called an upstream catalyst, and the catalyst provided on the downstream side is called a downstream catalyst.
[0024]
The wide area air-fuel ratio sensor (LAF) sensor 16 is provided upstream of the catalyst device 15. The LAF sensor 16 detects a wide range of air-fuel ratio from lean to rich. The detected air-fuel ratio is sent to the ECU 5.
[0025]
The O2 (exhaust gas) sensor 17 is provided between the upstream catalyst and the downstream catalyst. The O2 sensor 17 is a binary exhaust gas concentration sensor. The O2 sensor outputs a high-level signal when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, and outputs a low-level signal when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The output electric signal is sent to the ECU 5.
[0026]
The signal sent to the ECU 5 is passed to the input interface 5a, and is subjected to analog-digital conversion. The CPU 5b processes the converted digital signal according to a program stored in the memory 5c, and generates a control signal to be sent to an actuator of the vehicle. The output interface 5d sends these control signals to the actuators of the bypass valve 22, the fuel injection valve 6, and other mechanical elements.
[0027]
FIG. 2A shows the structure of the catalyst device 15. The exhaust gas flowing into the exhaust pipe 14 passes through the upstream catalyst 25, and then passes through the downstream catalyst 26. The air-fuel ratio control based on the output of the O2 sensor provided between the upstream and downstream catalysts optimizes the NOx purification rate more than the air-fuel ratio control based on the output of the O2 sensor provided downstream of the downstream catalyst. I know it is easy to maintain. Therefore, in the embodiment according to the present invention, the O2 sensor 17 is provided between the upstream and downstream catalysts. The O2 sensor 17 detects the oxygen concentration of the exhaust gas after passing through the upstream catalyst 25.
[0028]
Alternatively, an O2 sensor may be provided downstream of the downstream catalyst 26. When the catalyst device 15 is realized by one catalyst, an O2 sensor is provided downstream of the catalyst device 15.
[0029]
FIG. 2B shows the purification behavior of the upstream catalyst and the downstream catalyst. Window 27 indicates an air-fuel ratio region where CO, HC and NOx are optimally purified. In the upstream catalyst 25, the oxygen in the exhaust gas is consumed for the purification action, so that the exhaust gas supplied to the downstream catalyst 26 has a reducing atmosphere (that is, a rich state) as indicated by the window 28. . In such a reducing atmosphere, a further amount of NOx is purified. Thus, the exhaust gas is exhausted in a clean state.
[0030]
In the adaptive control of the air-fuel ratio according to the present invention, in order to maintain the purification performance of the catalyst 15 optimally, the output of the O2 sensor 17 is made to converge to a target value so that the air-fuel ratio falls within the window 27.
[0031]
Reference numeral 29 exemplifies an allowable range for defining the limit of the operation amount of the air-fuel ratio in the adaptive air-fuel ratio control, which will be described later in detail.
[0032]
Overview of air-fuel ratio control
FIG. 3 shows an overall configuration of a control device for controlling an air-fuel ratio according to an embodiment of the present invention. The fuel cut unit 31 receives the throttle valve opening θTH and the engine speed NE detected by the throttle valve opening sensor 4 and the engine speed sensor 13 (FIG. 1). When the throttle valve is fully closed for a predetermined time or more and the engine speed is equal to or more than the predetermined speed, the fuel cut unit 31 sets the fuel cut flag to 1 and sends a control signal to the fuel injection valve to supply fuel. To stop.
[0033]
When the engine speed NE falls below the predetermined speed after entering the fuel cut state, or when the throttle valve is opened, the fuel cut release unit 32 resets the fuel cut flag to zero, and sets the fuel cut valve to The control signal is sent to restart fuel supply.
[0034]
The lean operating section 33 determines whether or not an operation based on a lean air-fuel ratio (hereinafter, referred to as a lean operation) has been requested. The request is issued when performing the lean operation after starting the engine or when performing the lean operation for improving the fuel efficiency. When the request is received, the lean operation unit 33 sets the lean flag F_LEAN to 1 and executes the lean operation. When the request for the lean operation is released, the lean operation canceling unit 34 resets the lean flag F_LEAN to zero and ends the lean operation.
[0035]
When the fuel cut ends or the lean operation ends, the catalyst reduction mode is started. The reduction processing unit 35 sets the air-fuel ratio to a predetermined rich air-fuel ratio and performs the reduction processing. The reduction processing unit 35 determines whether the reduction processing of the downstream catalyst 26 has been completed. When the reduction processing of the downstream catalyst 26 is completed, the catalyst reduction mode is ended.
[0036]
The determination as to whether the reduction treatment of the downstream catalyst 26 has been completed can be performed in any appropriate manner. In one embodiment, a virtual O2 sensor is provided downstream of the downstream catalyst 26, and the output of the virtual O2 sensor is estimated. The virtual O2 sensor does not physically exist. If the output of the virtual O2 sensor changes from lean to rich, it is determined that the reduction process of the downstream catalyst 26 has been completed.
[0037]
The inversion of the virtual O2 sensor output from lean to rich can be determined, for example, as follows. In each control cycle, the gas amount for the reduction process is estimated based on the operation state. The estimated gas amount of each control cycle is integrated. The integrated value obtained when the output of the O2 sensor provided between the upstream catalyst and the downstream catalyst is reversed from lean to rich indicates the amount of gas required to reduce the upstream catalyst. Based on the amount of gas required to reduce the upstream catalyst, the total amount of gas required to reduce the upstream and downstream catalysts is calculated. When the integrated value reaches the total gas amount, the output of the virtual O2 sensor is inverted from lean to rich.
[0038]
Alternatively, the oxygen storage amount during the fuel cut may be calculated, and the air-fuel ratio may be feedforward controlled so as to reduce the oxygen storage amount. Alternatively, the O2 sensor may be actually provided downstream of the downstream catalyst, and if the output of the O2 sensor changes from lean to rich, it may be determined that the reduction process of the downstream catalyst has been completed.
[0039]
When the reduction process of the downstream catalyst is completed, the air-fuel ratio control by the adaptive control unit 36 is started. The adaptive control unit 36 calculates the target air-fuel ratio KCMD so that the output Vo2 / OUT of the O2 sensor 17 converges to the target value.
[0040]
While the reduction processing is being performed by the reduction processing unit 35, the air-fuel ratio control by the adaptive control unit 36 is not performed. In order to prevent the air-fuel ratio control from becoming unstable when the adaptive air-fuel ratio control is restarted, a part of the calculation performed by the adaptive control unit 36 is prohibited. Specifically, 1) the calculation of the integral term included in the control input to the control target is prohibited, and 2) the identification processing of the model parameters is prohibited. These details will be described later.
[0041]
Adaptive air-fuel ratio control
FIG. 4 is a block diagram from the LAF sensor 16 to the O2 sensor 17 in FIG. The LAF sensor 16 detects an air-fuel ratio KACT of exhaust gas supplied to the upstream catalyst 25. The O2 sensor 17 outputs the oxygen concentration of the exhaust gas purified by the upstream catalyst 25 as a voltage Vo2 / OUT. An exhaust system 19 from the LAF sensor 16 to the O2 sensor 17 is a control target (plant) of the adaptive air-fuel ratio control according to the present invention.
[0042]
FIG. 5 shows a control block diagram of the adaptive air-fuel ratio control. The output Vo2 / OUT of the O2 sensor 17 of the exhaust system 19 to be controlled is compared with the target value Vo2 / TARGET. Based on the comparison result, the controller 51 obtains a target air-fuel ratio deviation kcmd. The target air-fuel ratio deviation kcmd is added to the reference value FLAF / BASE to determine the target air-fuel ratio KCMD. The fuel injection amount corrected by the target air-fuel ratio KCMD is supplied to the engine 1. Thereafter, the output Vo2 / OUT of the exhaust system O2 sensor 17 is detected again.
[0043]
As described above, the controller 51 performs the feedback control for obtaining the target air-fuel ratio KCMD so that the output Vo2 / OUT of the O2 sensor 17 converges to the target value Vo2 / TARGET. The exhaust system 19 to be controlled can be modeled as in equation (1), with the output as Vo2 / OUT and the input as the output KACT of the LAF sensor. The exhaust system 19 is modeled as a discrete time model. The discrete-time model makes the algorithm of air-fuel ratio control simple and suitable for computer processing. k is an identifier for identifying a cycle.
[0044]
(Equation 1)

[0045]
Vo2 indicates a deviation of the output value Vo2 / OUT of the O2 sensor 17 from the target value Vo2 / TARGET (hereinafter, referred to as sensor output deviation), as shown in Expression (1). The actual air-fuel ratio deviation kact indicates a deviation of the output KACT of the LAF sensor from the reference value FLAF / BASE (kact = KACT-FLAF / BASE). The air-fuel ratio reference value FLAF / BASE is set to be a central value of the target air-fuel ratio, and is set to a value indicating the stoichiometric air-fuel ratio (ie, 1). The reference value FLAF / BASE may be a constant value or may be determined according to the operating state.
[0046]
d1 indicates a dead time of the exhaust system 19. The dead time d1 indicates a time required for the air-fuel ratio detected by the LAF sensor 16 to be reflected on the output of the O2 sensor 17. a1, a2, and b1 are model parameters, which are generated by an identifier described later.
[0047]
On the other hand, a system for operating the air-fuel ratio, which is composed of the engine 1 and the ECU 5, can be modeled as in equation (2). The target air-fuel ratio deviation kcmd indicates a deviation of the target air-fuel ratio KCMD from the reference value FLAF / BASE (kcmd = KCMD-FLAF / BASE). d2 indicates a dead time in the air-fuel ratio operation system. The dead time d2 indicates a time required for the calculated target air-fuel ratio KCMD to be reflected on the output KACT of the LAF sensor 16.
[0048]
(Equation 2)

[0049]
FIG. 6 shows a more detailed block diagram of the controller 51 shown in FIG. The controller 51 includes an identifier 52, an estimator 53, a sliding mode controller 54, and a limiter 55.
[0050]
The identifier 52 identifies the model parameters a1, a2, and b1 in the equation (1) so as to eliminate a modeling error. The identification method performed by the identifier 52 will be described below.
[0051]
The model parameters a1 (k-1), a2 (k-1) and b1 (k-1) calculated in the previous control cycle are used (hereinafter these parameters are referred to as a1 (k-1) hat, a2 (k -1) hat and b1 (k-1) hat), and the sensor output deviation Vo2 (k) of the current cycle (hereinafter, referred to as sensor output deviation Vo2 (k) hat) are obtained according to equation (3). .
[0052]
(Equation 3)

[0053]
The identification error id / e (k) between the sensor output deviation Vo2 (k) hat calculated by Expression (3) and the sensor output deviation Vo2 (k) actually detected in the current control cycle is expressed by Expression (4). ).
[0054]
(Equation 4)

[0055]
The identifier 52 calculates a1 (k) hat, a2 (k) hat, and b1 (k) hat in the current cycle so as to minimize the identification error id / e (k). Here, a vector Θ is defined as shown in Expression (5).
[0056]
(Equation 5)

[0057]
The identifier 52 obtains a1 (k) hat, a2 (k) hat, and b1 (k) hat according to the equation (6). As shown in Expression (6), the a1 (k-1) hat, a2 (k-1) hat, and b1 (k-1) hat determined in the previous control cycle are identified by the identification error id / e (k ), The a1 (k) hat, a2 (k) hat and b1 (k) hat in the current control cycle are obtained.
[0058]
(Equation 6)

[0059]
The estimator 53 estimates the sensor output deviation Vo2 after the dead time d (= d1 + d2) to compensate for the dead time d1 of the exhaust system 19 and the dead time d2 of the system for operating the air-fuel ratio.
[0060]
First, when the model formula (2) of the system for operating the air-fuel ratio is substituted into the model formula (1) of the exhaust system, the formula (7) is derived.
[0061]
(Equation 7)

[0062]
The model equation represented by the equation (7) expresses a system combining the exhaust system 19 and the system for operating the air-fuel ratio. By using Expression (7), an estimated value Vo2 (k + d) bar of the sensor output deviation Vo2 (k + d) after the dead time d is obtained as in Expression (8). The coefficients α1, α2, and βj are calculated using the model parameters calculated by the identifier 52. The past time-series data kcmd (kj) (j = 1, 2,...) Of the target air-fuel ratio deviation includes the target air-fuel ratio deviation acquired during the length of the dead time d. .
[0063]
(Equation 8)

[0064]
Past values kcmd (k-d2), kcmd (k-d2-1),... Of the air-fuel ratio deviation kcmd before the dead time d2. . . The values of kcmd (k−d) are calculated by using the above equation (2) to calculate the deviation outputs kac (k), kact (k−1),. . . kact (k−d + d2). As a result, equation (9) is obtained.
[0065]
(Equation 9)

[0066]
The sliding mode controller 54 sets the switching function σ as in Expression (10) in order to execute the sliding mode control.
[0067]
(Equation 10)

[0068]
Here, Vo2 (k-1) indicates the sensor output deviation detected in the previous cycle as described above. Vo2 (k) indicates the sensor output deviation detected in the current cycle. s is a setting parameter of the switching function σ, and is set to satisfy −1 <s <1.
[0069]
The equation in which the switching function σ (k) = 0 is called an equivalent input system, and defines the convergence characteristic of the sensor output deviation Vo2 that is the control amount. Assuming that σ (k) = 0, equation (10) can be modified as equation (11) below.
[0070]
[Equation 11]

[0071]
Here, the characteristics of the switching function σ will be described with reference to FIG. 7 and Expression (11). FIG. 7 is a diagram in which Expression (11) is represented by a line 41 on a phase plane in which the vertical axis is Vo2 (k) and the horizontal axis is Vo2 (k−1). This line 41 is called a switching straight line. It is assumed that the initial value of the state quantity (Vo2 (k-1), Vo2 (k)) composed of a combination of Vo2 (k-1) and Vo2 (k) is represented by a point 42. The sliding mode control operates such that the state quantity represented by the point 42 is placed on the switching straight line 41 and constrained on the switching straight line 41. According to the sliding mode control, by keeping the state quantity on the switching straight line 41, the state quantity can be extremely stably converged to the origin 0 on the phase plane without being affected by disturbance or the like. In other words, by constraining the state quantities (Vo2 (k-1), Vo2 (k)) to a stable system having no input shown in Expression (11), the sensor output can be robust against disturbances and modeling errors. Vo2 / OUT can be made to converge to the target value Vo2 / TARGET.
[0072]
The switching function setting parameter s is a parameter that can be set variably. By adjusting the setting parameter s, an attenuation (convergence) characteristic of the sensor output deviation Vo2 can be designated.
[0073]
FIG. 8 is a graph illustrating an example of a response designation characteristic of the sliding mode control. The graph 43 shows a case where the value of s is “−1”, the graph 44 shows a case where the value of s is “−0.8”, and the graph 45 shows a case where the value of s is “−0.5”. Is shown. As is clear from the graphs 43 to 45, the convergence speed of the sensor output deviation Vo2 changes according to the value of s. The smaller the absolute value of s, the faster the convergence speed.
[0074]
Three control inputs are determined so that the value of the switching function σ becomes zero. That is, a control input Ueq for constraining the state quantity on the switching straight line, a control input Urch for placing the state quantity on the switching straight line, and a state input on the switching straight line while suppressing modeling errors and disturbance. Of the control input Uadp is calculated. The required deviation Usl for calculating the air-fuel ratio deviation kcmd is calculated by calculating the sum of these three control inputs Ueq, Urch and Uadp.
[0075]
Since the equivalent control input Ueq is an input for constraining the state quantity on the switching straight line, the condition is that the equation (12) must be satisfied.
[0076]
(Equation 12)

[0077]
Therefore, the equivalent control input Ueq for setting σ (k + 1) = σ (k) is calculated from Expressions (7) and (10) as in Expression (13).
[0078]
(Equation 13)

[0079]
A reaching law input Urch having a value corresponding to the value of the switching function σ is calculated according to equation (14). In this embodiment, the reaching law input Urch has a value proportional to the value of the switching function σ. Krch indicates a feedback gain of a reaching law, which is predetermined based on a simulation or the like in consideration of the stability and quick response of convergence to the switching line σ = 0.
[0080]
[Equation 14]

[0081]
An adaptive law input Uadp having a value corresponding to the integrated value of the switching function σ is calculated according to equation (15). In this embodiment, the adaptive law input Uadp has a value proportional to the integrated value of the switching function σ. Kadp indicates a feedback gain of the adaptive law, which is predetermined based on a simulation or the like in consideration of stability and quick response of convergence to the switching line σ = 0. ΔT indicates the cycle of the control cycle.
[0082]
[Equation 15]

[0083]
Since the sensor output deviations Vo2 (k + d) and Vo2 (k + d-1) and the value σ (k + d) of the switching function are predicted values in consideration of the dead time d, they cannot be directly obtained. Therefore, the equivalent control input Ueq is obtained by using the estimated deviations Vo2 (k + d) and Vo2 (k + d-1) obtained by the estimator 53.
[0084]
(Equation 16)

[0085]
Further, using the estimated deviation calculated by the estimator 53, the switching function σ bar is calculated as shown in Expression (17).
[0086]
[Equation 17]

[0087]
The reaching law input Urch and the adaptive law input Uadp are calculated using the switching function σ bar.
[0088]
(Equation 18)

[0089]
[Equation 19]

[0090]
As shown in Expression (20), the required deviation Usl is obtained by adding the equivalent control input Ueq, the reaching law input Urch, and the adaptive law input Uadp.
[0091]
(Equation 20)

[0092]
The limiter 55 performs a limit process on the required deviation Usl to obtain an air-fuel ratio deviation kcmd. Specifically, if the required deviation Usl is within the allowable range, the limiter 55 sets the required deviation Usl to the air-fuel ratio deviation kcmd. When the required deviation Usl is out of the allowable range, the upper limit or the lower limit of the allowable range is set to the air-fuel ratio deviation kcmd.
[0093]
As shown by reference numeral 29 in FIG. 3, the allowable range used by the limiter 55 is set to a wider range including the window 27 substantially at the center. Specifically, the following allowable ranges are set in advance. FIG. 9 shows these allowable ranges.
[0094]
(I) Tolerable range used when the stability determination determines that the unstable level is low.
(Ii) Tolerable range used when the stability level is determined to be high by the stability determination
(Iii) Until a predetermined time elapses after the fuel cut ends, until a predetermined time elapses after the engine starts, and until a predetermined time elapses after the lean operation ends. Tolerances used for
(Iv) The allowable range used after the vehicle starts (after the engine starts driving the driving wheels) until a predetermined time elapses.
(V) Tolerable range used when the engine is idling
(Vi) An allowable range used in a normal operation state (referred to as an adaptive allowable range)
(Vii) An allowable range used when switching from the catalyst reduction mode to the adaptive air-fuel ratio control mode.
[0095]
The allowable range for low instability (i) is defined by an upper limit value H and a lower limit value L. The allowable range for high instability (ii) is defined by an upper limit value STABH and a lower limit value STABL. The upper limit of the allowable range (iii) for the fuel cut / start / lean operation is AFCH. The lower limit is variably set between STABL and LL. The lower limit is determined in each cycle in accordance with the deviation of the required deviation Usl from the allowable range.
[0096]
The upper limit value of the allowable range (iv) for driving after the load is variably set between STABH and HH. The upper limit value is determined in each cycle according to the deviation of the required deviation Usl from the allowable range. The lower limit is VSTL. The allowable range (v) for idling is defined by an upper limit HI and a lower limit LI.
[0097]
The upper limit of the adaptive permissible range (vi) is variably set between STABH and HH. The lower limit is variably set between STABL and LL. The upper limit value and the lower limit value are determined in each cycle in accordance with the state of deviation of the required deviation Usl from the allowable range.
[0098]
The upper limit of the allowable range (vii) used when the engine shifts from the catalyst reduction mode to the adaptive air-fuel ratio control mode is the same as the upper limit of the adaptive allowable range (vi), and is variably set between STABH and HH. Is set. The lower limit is fixed at LI. Thus, when the engine shifts from the catalyst reduction mode to the adaptive air-fuel ratio control mode, the operation amount Usl is prevented from greatly changing in the lean direction.
[0099]
The air-fuel ratio deviation kcmd obtained by the limiter 55 is added to the reference value FLAF / BASE to obtain the target air-fuel ratio KCMD. By giving the target air-fuel ratio KCMD to the exhaust system 19 to be controlled, the output Vo2 / OUT of the O2 sensor can be made to converge to the target value Vo2 / TARGET.
[0100]
In an alternative embodiment, the reference value FLAF / BASE of the air-fuel ratio is set according to the adaptive law input Uadp calculated by the sliding mode controller 54 after the limit processing by the limiter 55 is completed. Specifically, the stoichiometric air-fuel ratio is set as the reference value FLAF / BASE as an initial value. If the adaptation law Uadp exceeds a predetermined upper limit, the reference value FLAF / BASE is increased by a predetermined amount. If the adaptive law Uadp is below a predetermined lower limit, the reference value FLAF / BASE is reduced by a predetermined amount. If the adaptive law Uadp is between the upper limit and the lower limit, the reference value FLAF / BASE is maintained. The set FLAF / BASE is used in the next cycle. In this way, the reference value FLAF / BASE is adjusted to be a central value of the target air-fuel ratio KCMD.
[0101]
By combining the setting process of the reference value FLAF / BASE with the above-described limit process, the allowable range of the required deviation Usl is balanced between positive and negative. The reference value FLAF / BASE setting process is preferably performed when the O2 sensor output Vo2 / OUT has almost converged to the target value Vo2 / TARGET, and it is determined that the sliding mode control is in a stable state.
[0102]
Air-fuel ratio control flow
FIG. 10 shows an air-fuel ratio control flow according to an embodiment of the present invention. In step S101, it is determined whether calculation of model parameters by the identifier is permitted. If the calculation by the identifier is permitted, the permission flag F_IDCAL is set to 1. When in the catalyst reduction mode, the flag is set to zero.
[0103]
In step S102, the value of the flag F_IDCAL is checked. If F_IDCAL = 1, the process proceeds to step S103, where model parameters a1, a2, and b1 are obtained according to the above-described equations (3) to (6). If F_IDCAL = 0, step S103 is skipped. Thus, when in the catalyst reduction mode, the calculation of the model parameters by the identifier is stopped. In this case, the model parameters calculated last before entering the catalyst reduction mode are stored in the memory. When the catalyst reduction mode ends and the adaptive air-fuel ratio control is restarted, calculation of model parameters is started using the stored model parameters.
[0104]
In step S104, using the model parameters calculated in step S103, the estimator calculates the estimated deviation Vo2 bar according to the above-described equation (9). In step S105, the switching function σ bar, the equivalent control input Ueq, the adaptive-side input Uadp, and the arrival-side input Urch are obtained according to the above-described equations (16) to (19). The control input Usl is obtained according to the equation (20). Although not shown in the figure, the calculation process of the adaptive law input Uadp is not performed in the catalyst reduction mode. Before entering the catalyst reduction mode, the adaptive law input Uadp calculated last is stored in the memory. When the catalyst reduction mode ends and the adaptive air-fuel ratio control is started again, the calculation of the adaptive law input Uadp is started using the stored adaptive law input Uadp.
[0105]
In step S106, the stability determination of the adaptive air-fuel ratio control is performed (FIG. 11). In step S107, a limit process for the control input Usl is performed to determine a target air-fuel ratio deviation kcmd (FIGS. 12 and 13).
[0106]
FIG. 11 is a flowchart showing the stability determination of the adaptive air-fuel ratio control performed in step S106 of FIG.
[0107]
In step S111, the switching function σ (k + d) bar obtained in the current control cycle, the deviation Δσ between the σ (k + d) bar and the switching function σ (k + d-1) bar obtained in the previous control cycle, , A stability determination basic parameter Pstb (= σ (k + d) bar · Δσ bar) is calculated. Pstb is the Lyapunov function σ bar for the switching function σ bar²/ 2 time differential value.
[0108]
In step S112, it is determined whether a predetermined time has elapsed since the start of the air-fuel ratio operation by the adaptive air-fuel ratio control. If the predetermined time has not elapsed, the routine exits from this routine via the process of step S113 without performing the stability determination. If the predetermined time has elapsed, the process proceeds to step S114. In step S113, processing for initializing various parameters used in this routine is performed.
[0109]
In step S114, the basic parameter Pstb is compared with a predetermined value ε (> 0). If Pstb ≦ ε, it is determined that the air-fuel ratio control is provisionally stable. That is, it can be determined that the value of the switching function σ has converged to zero or is in a state of converging. In this case, the value of the counter cnt / judst indicating the frequency determined to be unstable is maintained (S115). If Pstb> ε, it is determined that the air-fuel ratio control is provisionally unstable. That is, it can be determined that the value of the switching function σ is moving away from zero. In this case, the value of the counter cnt / judst is incremented by 1 (S116).
[0110]
In step S117, the value of the counter cnt / judst is compared with a first threshold value SSTB1. If cnt / judst ≦ SSTB1, it is determined that the air-fuel ratio control is stable, and the stability determination flag f / stb1 is set to zero (S118).
[0111]
If it is determined in step 117 that cnt / judst> SSTB1, cnt / judst is compared with a second threshold value SSTB2 (S119). Here, SSTB1 <SSTB2. If cnt / judst ≦ SSTB2, it is determined that the air-fuel ratio control is in a low-level unstable state, and the value of f / stb1 is set to 1 (S120). If cnt / judst> SSTB2, it is determined that the air-fuel ratio control is in a high-level unstable state, and the values of f / stb1 and f / stb2 are set to 1 (S121).
[0112]
In step S122, the timer counter tm / judst for measuring the elapsed time from the start of the measurement by the counter cnt / judst is decremented by one. The counter cnt / judst is reset each time the timer counter tm / judst becomes zero.
[0113]
In step S123, it is determined whether the timer counter tm / judst is zero. If tm / judst ≠ 0, exit this routine. If tm / judst = 0, the value of the stability determination flag f / stb1 is checked. If f / stb1 = 1, the timer counter tm / judst is set to the initial value TMJUDST, and the counter cnt / judst is reset to zero (S126). If f / stb1 = 0, it indicates that the air-fuel ratio control has been determined to be stable in the current control cycle. In this case, after resetting the unstable flag f / stb2 to zero (S125), the process exits this routine through the process of step S126.
[0114]
With reference to FIG. 12 and FIG. 13, a limit process of the required deviation Usl will be described. As described with reference to FIG. 9, any one of the allowable ranges (i) to (vii) is selected according to the driving state. Regarding the selected allowable range, the upper limit value and the lower limit value when the engine 1 is in the idling state and when the engine 1 is not in the idling state may be different. In the following description, the upper and lower limits for the former are represented by AHFI and ALFI, respectively, and the upper and lower limits for the latter are represented by AHF and ALF, respectively. Further, as described above, some allowable ranges have a variable upper limit and / or a variable lower limit. The adaptive upper limit value and the adaptive lower limit value that variably change are represented by ah and al, respectively.
[0115]
FIG. 12 is a flowchart showing the limit process of the required deviation Usl performed in step S107 of FIG. In step S128, it is determined whether or not the engine is in the catalyst reduction mode (whether or not reduction processing is being performed). If the mode is the catalyst reduction mode, the air-fuel ratio deviation kcmd is set to a predetermined value SLDHOLD (> 0). Thus, the target air-fuel ratio is set to the enriched air-fuel ratio in order to promote the reduction process.
[0116]
In step S130, the aforementioned allowable range (vii) is set. Specifically, the adaptive lower limit value al (k) is set to a predetermined value LI. Thus, when the engine is in the catalyst reduction mode, the lower limit of the allowable range is fixed to the value LI, and the target air-fuel ratio is prevented from excessively changing in the lean direction when shifting from the catalyst reduction mode to the adaptive air-fuel ratio control. I do.
[0117]
If the engine is not in the catalyst reduction mode in step S128, the process proceeds to step S131. In step S131, the allowable range used in the current control cycle is determined. Here, the allowable range determination routine executed in step S131 will be described with reference to FIG.
[0118]
In step S161, the value of the flag f / stb2 is checked. If f / stb2 = 1, it indicates that the air-fuel ratio control is in a high-level unstable state. In step S162, upper limit value AHF and lower limit value ALF are set to upper limit value STABH and lower limit value STABL (see FIG. 9) of allowable range (ii), respectively. The lower limit AHFI and the upper limit ALFI for the idling state are also set to the upper limit STABH and the lower limit STABL of the allowable range (ii). The adaptive upper limit ah and the adaptive lower limit al (accurately, ah (k-1) and al (k-1)) are also initialized to the upper limit STABH and the lower limit STABL of the allowable range (ii). Is done. Thus, when the air-fuel ratio control is in a high-level unstable state, the narrowest allowable range (ii) is selected. This allowable range (ii) is used even in the idling state.
[0119]
In step S163, the value of the flag f / stb1 is checked. If f / stb1 = 1, it indicates that the air-fuel ratio control is in a low-level unstable state. In step S164, upper limit value AHF and lower limit value ALF are set to upper limit value H and lower limit value L (see FIG. 9) of allowable range (i), respectively. The upper limit value AHFI and the lower limit value ALFI for the idling state are set to the upper limit value HI and the lower limit value LI (see FIG. 9) of the idling allowable range (v), respectively. The adaptive upper limit value ah and the adaptive lower limit value al are set to the upper limit value H and the lower limit value L of the allowable range (i), respectively. Thus, when the air-fuel ratio control is in a low-level unstable state, the allowable range (i) is selected. When the air-fuel ratio control is at a low level of instability and the engine is in an idling state, an idling allowable range (v) narrower than the allowable range for low level instability (i) is selected.
[0120]
If the determinations in steps S161 and S163 are No, the air-fuel ratio control is in a stable state. In step S165, it is determined whether the elapsed time from the end of the fuel cut is shorter than a predetermined time. In step S166, it is determined whether the elapsed time since the start of the engine is shorter than a predetermined time. In step S167, it is determined whether the elapsed time since the end of the lean operation is shorter than a predetermined time. If any of the determinations in steps S165 to S167 is Yes, the process proceeds to step S168.
[0121]
In step S168, the upper limit value AHF is set to the upper limit value AFCH (see FIG. 9) of the allowable range (iii). The lower limit of the allowable range (iii) is variable. Therefore, the lower limit ALF is set to the adaptive lower limit al (k-1) determined in the previous control cycle. The upper limit value AHFI for the idling state is set to the upper limit value AFCH of the allowable range (iii). Lower limit value ALFI is set to lower limit value LI of allowable range (v). Thus, if the engine is not idling, the permissible range (iii) is selected. When the engine is idling, the lower limit is fixed to the lower limit LI of the idling allowable range (v).
[0122]
If the determinations in steps S165 to S167 are all No, the process proceeds to step S169 to determine whether the engine is in an idling state. If the vehicle is idling, the process proceeds to step S170, where the upper limit value AHFI and the lower limit value ALFI are set to the upper limit value HI and the lower limit value LI of the idling allowable range (v).
[0123]
When the determination in step S169 is No, the process proceeds to step S171. The processing in steps S171 to S178 indicates processing for limiting the variable adaptive upper limit value and adaptive lower limit value indicated in the adaptive allowable range (vi). In step S171, the adaptive upper limit value ah (k-1) is compared with STABH. If ah (k-1) <STABH, the value of ah (k-1) is set to STABH (S172). If ah (k-1) ≧ STABH, the process proceeds to step S173 to compare the adaptive upper limit value ah (k-1) with HH. If ah (k-1)> HH, the value of ah (k-1) is set to HH (S174). If the determinations in steps S171 and S173 are No, the value of ah (k-1) is maintained. Thus, the adaptive upper limit value ah (k-1) is limited between STABH and HH.
[0124]
In step S175, the adaptive lower limit value al (k-1) is compared with LL. If al (k-1) <LL, the value of al (k-1) is set to LL (S176). If ah (k−1) ≧ LL, the process proceeds to step S177, where the adaptive lower limit al (k−1) is compared with STABL. If al (k-1)> STABL, the value of al (k-1) is set to STABL (S178). If the determinations in steps S175 and S177 are No, the value of al (k-1) is maintained. Thus, the adaptive lower limit al (k-1) is limited between LL and STABL.
[0125]
In step S179, the adaptive upper limit value ah (k-1) and the adaptive lower limit value al (k-1) are set for the upper limit value AHF and the lower limit value ALF, respectively. Thus, the adaptation allowable range (vi) is determined.
[0126]
In step S180, if the predetermined time has not elapsed since the vehicle started, the lower limit ALF is set to the lower limit VSTL of the allowable range (iv) (S181). As the upper limit value AHF, the same value as the upper limit value of the adaptive allowable range (vi) is used.
[0127]
Returning to FIG. 12, in step S132, it is determined whether the engine is in an idling state. If the vehicle is in the idling state, the required deviation Usl is limited using the allowable range defined by the upper limit value AHFI and the lower limit value ALFI set in any of steps S162, S164, S168, and S170 in FIG. . Specifically, if Usl <ALFI (S133), the target air-fuel ratio deviation kcmd is set to the lower limit ALFI (S134). If Usl> AHFI (S135), the target air-fuel ratio deviation kcmd is set to the upper limit value AHFI (S136). If the determinations in steps S133 and S135 are NO (ie, if ALFI ≦ Usl ≦ AHFI), the current required deviation Usl is set to the target air-fuel ratio deviation kcmd (S137).
[0128]
When step S133 or S135 is Yes, in order to prevent the adaptive law input Uadp from increasing, the integrated value of the switching function σ bar calculated in the current cycle is added to the integrated value calculated in the previous cycle. It is set (S138). In step S139, the current adaptive upper limit value ah (k-1) and lower limit value al (k-1) are held.
[0129]
If it is determined in step S132 that the engine is not idling, the allowable range defined by the upper limit value AHF and the lower limit value ALF set in any of steps S162, S164, S168, S179, and S181 in FIG. Is used to limit the required deviation Usl.
[0130]
Specifically, if Usl <ALF (S140), the target air-fuel ratio deviation kcmd is set to the lower limit value ALF (S141). If Usl> AHF (S142), the target air-fuel ratio deviation kcmd is set to the upper limit AHF (S143). If ALF ≦ Usl ≦ AHF, the current required deviation Usl is set to the target air-fuel ratio deviation kcmd (S144).
[0131]
When ALF ≦ Usl ≦ AHF, the adaptive lower limit al (k−1) is increased by ΔDEC (> 0), and the upper limit ah (k−1) is reduced by ΔDEC (S145). Thus, when the current required deviation Usl is within the allowable range, the adaptive allowable range is narrowed.
[0132]
When Usl <ALF, in step S146, it is determined whether the elapsed time since the vehicle started is smaller than a predetermined time. If this determination is No, the adaptive lower limit al (k-1) is reduced by ΔINC (> 0), and the upper limit ah (k-1) is reduced by ΔDEC (S147). Here, ΔDEC <ΔINC. Thus, when the current required deviation Usl deviates on the lower limit side of the allowable range, the upper limit value and the lower limit value are reduced, and the allowable range is moved downward. If the determination in step S146 is Yes, the values of the adaptive upper limit value ah (k-1) and the lower limit value al (k-1) are maintained (S148).
[0133]
When Usl> AHF, in step S149, the elapsed time after the fuel cut ends within a predetermined time, the elapsed time after the engine starts within the predetermined time, or the elapsed time after the lean operation ends It is determined whether the time is within a predetermined time. If none of these states applies, the adaptive upper limit ah (k-1) is increased by ΔINC and the lower limit al (k-1) is increased by ΔDEC (S150). Thus, when the current required deviation Usl deviates on the upper limit side of the allowable range, the upper limit value and the lower limit value are increased, and the allowable range is moved upward. If any of the states are determined to be Yes in the determination in step S149, the values of the adaptive upper limit value ah (k-1) and the lower limit value al (k-1) are maintained (S151).
[0134]
If the determination in either step S140 or S142 is Yes, as in step S138, the integrated value of the switching function σ bar calculated in the current cycle is added to the integrated value of the switching function σ bar calculated in the previous cycle. Set.
[0135]
Thus, when the engine is in the catalyst reduction mode, the lower limit al (k) of the required deviation Usl is changed to a value shifted in the rich direction (the value LI in this example) as shown in step S130 of FIG. Fixed. When the adaptive air-fuel ratio control is started, the operation amount Usl is limited by the lower limit ALF in which the value LI is set (S140), so that the target air-fuel ratio KCMD is prevented from largely changing in the lean direction. .
[0136]
FIG. 14 shows transition of the O2 sensor output Vo2 / OUT, the target air-fuel ratio KCMD, and the emission amount of the harmful component NOx of the exhaust gas after the lean operation is completed, according to an embodiment of the present invention.
[0137]
During the lean operation, the allowable range is not updated, and the upper limit and the lower limit of the allowable range are maintained at the last set values before the lean operation is started. This example shows a mode in which the lower limit is maintained at a predetermined value between LI and LL shown in FIG. 9, for example.
[0138]
At time t2 when the lean operation ends, the catalyst reduction mode is started. In the catalyst reduction mode, the target air-fuel ratio KCMD is set to a predetermined rich air-fuel ratio in order to remove oxygen adsorbed on the catalyst. As described above, in the catalyst reduction mode, the lower limit of the target air-fuel ratio KCMD is fixed to the predetermined value LI. At time t3, when the reduction process of the upstream catalyst ends, the O2 sensor output Vo2 / OUT is inverted from lean to rich.
[0139]
At time t4, the reduction process of the downstream catalyst ends, and the adaptive air-fuel ratio control starts. At this time, the O2 sensor output Vo2 / OUT exists on the richer side than the target value Vo2 / TARGET. The adaptive air-fuel ratio control attempts to shift the target air-fuel ratio KCMD in the lean direction so that the O2 sensor output Vo2 / OUT converges to the target value. However, at time t4, since the lower limit is set to the value LI shifted to the rich side (reference numeral 190), the target air-fuel ratio KCMD is not set to a value smaller than the lower limit. Therefore, the target air-fuel ratio KCMD is prevented from changing more than a predetermined amount in the lean direction, and the amount of NOx emission is suppressed.
[0140]
In this specification, an example has been described in which adaptive air-fuel ratio control is performed using sliding mode control. However, the present invention can also be applied to a case where the adaptive air-fuel ratio control is performed using another response assignment type control.
[0141]
The present invention can also be applied to a marine propulsion engine such as an outboard motor having a vertical crankshaft.
[0142]
【The invention's effect】
According to the present invention, it is possible to reduce the emission of harmful components of the exhaust gas after the treatment for reducing the catalyst.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing an internal combustion engine and a control device thereof according to one embodiment of the present invention.
FIG. 2 is a diagram showing (a) an arrangement of a catalyst device and an exhaust gas sensor, and (b) an outline of air-fuel ratio control, according to an embodiment of the present invention.
FIG. 3 is an overall functional block diagram of an air-fuel ratio control device according to an embodiment of the present invention.
FIG. 4 is a block diagram illustrating an exhaust system that is control-symmetric according to one embodiment of the present invention.
FIG. 5 is a control block diagram of air-fuel ratio control according to one embodiment of the present invention.
FIG. 6 is a detailed functional block diagram of a controller according to one embodiment of the present invention.
FIG. 7 is a diagram schematically showing a switching straight line in response assignment control according to an embodiment of the present invention.
FIG. 8 is a diagram showing response characteristics in response assignment control according to one embodiment of the present invention.
FIG. 9 is a diagram showing an example of an allowable range according to one embodiment of the present invention.
FIG. 10 is a diagram showing an air-fuel ratio control flow according to one embodiment of the present invention.
FIG. 11 is a flowchart showing stability determination processing according to an embodiment of the present invention.
FIG. 12 is a flowchart showing a limit process according to one embodiment of the present invention.
FIG. 13 is a flowchart showing a process for determining an allowable range according to one embodiment of the present invention.
FIG. 14 is a diagram showing transition of an exhaust gas sensor output, a target air-fuel ratio, and a discharge amount of harmful components of exhaust gas after a catalyst reduction process is completed according to an embodiment of the present invention.
FIG. 15 is a diagram showing transition of an exhaust gas sensor output, a target air-fuel ratio, and a discharge amount of harmful components of exhaust gas after a catalyst reduction process is completed according to conventional air-fuel ratio control.
[Explanation of symbols]
1 engine
5 ECU
14 exhaust pipe
15 catalyst device
16 LAF sensor
17 O2 sensor
25 upstream catalyst
26 downstream catalyst

Claims (3)

An air-fuel ratio control device that controls an air-fuel ratio of an internal combustion engine,
A reducing means for reducing the catalyst in a catalyst reduction mode that is performed after the operation based on the lean air-fuel ratio is completed;
Air-fuel ratio control means for controlling an air-fuel ratio so that an output of an exhaust gas sensor arranged in an exhaust pipe converges to a predetermined target value in an air-fuel ratio control mode performed after the catalyst reduction mode ends. ,
The air-fuel ratio control means further comprises:
A controller for calculating an operation amount for operating the air-fuel ratio,
Using an upper limit and a lower limit, a limiter that limits the operation amount,
Operating means for operating the air-fuel ratio using the operation amount subjected to the limit processing,
When the internal combustion engine is in the catalyst reduction mode, the limiter shifts the lower limit in a rich direction, and when the internal combustion engine shifts from the catalyst reduction mode to the air-fuel ratio control mode, the air-fuel ratio becomes a predetermined amount. An air-fuel ratio control device that prevents a change in the lean direction. The air-fuel ratio control device according to claim 1, wherein the operation based on the lean air-fuel ratio includes an operation in a fuel cut state in which fuel is not supplied. 3. The air-fuel ratio control device according to claim 1, wherein the upper limit and the lower limit are set according to the operation amount and an operating state of the internal combustion engine. 4.

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