JP4193334B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

ここで、Ｓ再生速度係数は、吸蔵型ＮＯｘ触媒２５の温度によりＳ再生速度が異なることを補正するためのもので、各触媒温度でのＳ再生時間を７００℃相当のＳ再生時間に換算するためのものである。Ｓ再生速度は吸蔵型ＮＯｘ触媒２５の温度が高くなるにつれて指数関数的に増加するので、吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat が５８０℃以下の低温時では０とし、それよりも高温時では、指数関数により近似した下式（３）をもって算出する。
Ｓ再生速度係数＝exp ｛−ｋｋ×（１／Ｔ₁ ）−（１／Ｔ₀ ）｝・・（３）
ここで、ｋｋは、吸蔵型ＮＯｘ触媒２５のＳ再生反応に応じて設定される所定の係数、Ｔ₁ は、吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat （Ｋ）、Ｔ₀ は、９７３（７００＋２７３）（Ｋ）である。
なお、本実施形態では、ＥＣＵ２８内では指数関数による計算は行わずに予め計算した値を記憶した触媒温度に対するＳ再生速度係数マップから求めるようにしている。また、前述したＡ／Ｆ係数は、Ａ／ＦによるＳ再生度合を示すもので、空燃比がリーンモードまたは燃料カット時では０とし、それ以外のモードでは１とする。なお、より細かく分類して、Ａ／Ｆ係数を空燃比がリーンモードまたは燃料カット時では０とし、ストイキＦ／Ｂモードでは２／３とし、リッチ（Ｏ／Ｌ）モードでは１としてよい。
【００３２】
なお、Ｓ再生頻度は、所定値Ｄ（例えば、１．５ｓ／km）以上でリセットされ、７００℃換算Ｓ再生時間を０ｓ、走行距離を０kmとする。つまり、Ｓ再生頻度が所定値Ｄ以上であれば、吸蔵型ＮＯｘ触媒２５は十分にパージされているとして再生を停止する。Ｓ再生頻度をリセットする際のしきい値Ｄの設定方法としては、次のようにすればよい。即ち、図４に示すように、Ｓ再生頻度が高いほどＮＯｘ排出値は小さくなることがわかっており、ＮＯｘの排出規制値以下となるようにＳ再生頻度を設定すればよい。このしきい値Ｄは、触媒特性やＮＯｘ量の目標値にも影響されるが、一般的に概ね１．５ｓ／km以上に設定すればよいことがわかっている。
【００３３】
このようにＳ再生頻度が求められたら、ステップＳ３にて、昇温設定温度ＺＳＴＥＭＰ（℃）を算出する。この場合、図５に示すように、Ｓ再生頻度に対するＺＳＴＥＭＰのマップが予め設定されている。このマップによれば、Ｓ再生頻度が低いときには、ＺＳＴＥＭＰは６００℃に設定され、Ｓ再生頻度の上昇に伴ってＺＳＴＥＭＰが上昇し、所定値Ｄで８００℃としている。即ち、昇温設定温度ＺＳＴＥＭＰはＳ再生頻度に基づいて変更されるようになっている。
【００３４】
そして、ステップＳ４において、吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat がＺＳＴＥＭＰ以上であるかどうかを判定し、触媒温度Ｔcat がＺＳＴＥＭＰ以上であれば、Ｓ再生頻度がある程度低いと同時に吸蔵型ＮＯｘ触媒２５がある程度昇温されており、Ｓパージしやすい状態にあると推定されるため、ステップＳ５に移行し、制御モードをＳパージモードに切り換える。これにより吸蔵型ＮＯｘ触媒２５に吸蔵されたＳＯｘの除去（Ｓパージ制御）が開始される。
【００３５】
即ち、ステップＳ６では、点火時期を制御、つまり、リタード（遅角）させることで吸蔵型ＮＯｘ触媒２５を昇温する。つまり、点火時期リタードにより吸蔵型ＮＯｘ触媒２５に流入する排気を十分に昇温させることにより、吸蔵型ＮＯｘ触媒２５はＳパージ制御に適した温度（例えば、６５０℃〜８００℃）まで昇温されることになる。この場合、Ｓパージ制御によって放出されるＳＯ₂ は、上述したように、還元雰囲気のもとで反応してＨ₂Ｓを発生するのであるが、この際にＳ再生度合い（Ｓ再生頻度）に応じて点火時期リタードのリタード量を小さくし、あるいは変化速度を遅くして吸蔵型ＮＯｘ触媒２５の昇温最高温度を低くし、あるいは昇温速度を遅くすると、このＳＯ₂ の放出量が一気に増加してＨ₂Ｓが多量に発生し易いＳパージ制御の開始時において、ＳＯ₂ の放出量を抑制してＨ₂Ｓの発生濃度を低く抑えることができる。一般に、Ｈ₂Ｓのピーク濃度が高いほど、Ｈ₂Ｓの臭気が問題となりやすいが、本制御は特にＨ₂Ｓのピーク濃度を低く抑えるのに有効である。
【００３６】
そして、この点火時期は下記式（４）によりリタードするように設定される。

ここで、ＺＳＳＡは、目標平均有効圧Ｐｅとエンジン回転速度情報Ｎｅとに応じて設定されたリタードマップに基づいて設定されるものであり、燃焼限界となるリタード量に余裕量を考慮した量として設定される。また、反映係数は、図６に示すように、触媒温度推定値、つまり、触媒温度Ｔcat に対応したマップに基づいて設定され、燃料のカット時やスロットル開度が所定値以上のときは０としている。更に、反映係数Ａは、０から１まで所定時間（所定行程数）かけて変化させるものであり、Ｓパージ制御の開始時のＳ再生頻度が小さいほど所定時間を長くし、触媒昇温速度を低下させて吸蔵型ＮＯｘ触媒２５をゆっくり昇温させる。また、反映係数Ｂは、Ｓパージ制御の開始時のＳ再生頻度のマップから求めるものであり、Ｓパージ制御の開始時のＳ再生頻度が小さいほど反映係数Ｂを小さくして最終的な点火時期リタード量を小さくし、触媒最高温度を低下させる。
【００３７】
つまり、図７に、ステップＳ１からステップＳ４の実施結果、ステップＳ５以降でのＳパージ制御の実施時における点火時期の変化（ａ）に基づいた触媒温度の時間変化（ｂ）とＨ₂Ｓの発生濃度の時間変化（ｃ）とがタイムチャートとして示してある。この場合、従来のように点火時期をＳパージ制御の開始から直ぐ（もしくは徐々に変化させている場合においても、数サイクル程度の速い変化）に大きな量でリタードした場合を破線で示し、本実施形態のようにＳ再生頻度に応じて点火時期を徐々に所定の小さなリタード量に向けて移行させた場合を実線で示してある。このタイムチャートからわかるように、従来の制御（破線）では、Ｓパージ制御の開始直後に吸蔵型ＮＯｘ触媒２５が昇温されて触媒温度が直ちに所定の高温度まで上昇することで、一時的に大量のＨ₂Ｓが発生していたが、本実施形態の制御（実線）では、Ｓパージ制御の開始直後から吸蔵型ＮＯｘ触媒２５が徐々に昇温されて触媒温度がゆっくりと従来より低い所定の温度まで上昇することで、大量のＨ₂Ｓの発生現象はなく、Ｓパージ開始直後のＨ₂Ｓの発生濃度を低くすると共にこのまま保持することができ、Ｈ₂Ｓ濃度を低く抑えて排気ガスが異臭を放つことを確実に防止することができる。
【００３８】
また、特に、点火時期を徐々にリタードさせて吸蔵型ＮＯｘ触媒２５を徐々に昇温すると共に触媒温度を抑制することにより、Ｈ₂Ｓの発生濃度を低い値のまま保持してＳＯ₂ の放出を効率よく促進することができる。これにより、Ｓパージ制御の長期化を極力防止し、燃費の悪化を好適に防止しながら、Ｈ₂Ｓ濃度を低く抑えることができる。
【００３９】
なお、上述の実施形態においては、反映係数Ａ及び反映係数Ｂは予めマップ等により定められた値を用いてオープンループ的な制御としたが、フィードバック的な制御としてもよい。即ち、Ｓ再生頻度に応じてまず目標とする触媒昇温速度及び触媒最高温度を設定し、それらから求まる設定目標温度と現時点での触媒水底温度（高温センサ２４の出力に基づいて推定）との偏差から、触媒推定温度が設定目標温度となるように点火時期リタード量をフィードバック制御により変化させるのである。このフィードバック制御方法としては、一般的な積分制御、比例制御、微分制御等を組み合わせて用いればよい。
【００４０】
なお、Ｓパージを行うときに点火時期リタードに応じてスロットル弁１７を作動してベーススロットル開度に対してスロットル開度θthを調整し、吸入空気量を操作しており、これによって点火時期リタードによるトルク低下分を補いトルクがほぼ一定となるように制御している。この場合、スロットル開度は下記式（５）により設定される。
スロットル開度＝ベーススロットル開度−ＺＳＥＴＶ×反映係数・・（５）
ここで、ＺＳＥＴＶは、目標平均有効圧Ｐｅとエンジン回転速度情報Ｎｅとに応じて設定されたスロットル開度補正マップに基づいて設定されるものであり、また、反映係数は、前述したように、図６に示すマップに基づいて設定される。なお、この場合、補正された点火時期（リタード後の点火時期）に対するスロットル開度θthのマップにより設定してもよい。
【００４１】
続いて、ステップＳ７では、空燃比が所定のリッチＡ／Ｆとなるように制御する。即ち、リーン運転を禁止し、ストイキＦ／Ｂ（リッチシフトを含む）もしくはオープンループのリッチ運転のみとし、空燃比がストイキもしくはリッチとなるようにする。
【００４２】
そして、図２に戻り、ステップＳ８では、触媒温度Ｔcat が所定値Ｋ（例えば、６５０℃）よりも低い状態が所定の設定時間Ｃ（例えば、４５sec ）経過したら、ステップＳ９にて、吸蔵型ＮＯｘ触媒２５の昇温をやめると共に空燃比をリーン化してこのルーチンを抜ける。つまり、Ｓパージモードにしても触媒温度Ｔcat がイオウ成分の脱離に最適な温度（例えば、６５０℃以上）へなかなか近づかない場合に、燃費悪化を抑制するためにＳパージモードを停止させるようにしている。一方、触媒温度Ｔcat が所定時間Ｃが経過するまでに所定値Ｋ以上になれば、吸蔵型ＮＯｘ触媒２５の昇温及び空燃比のリッチ化を継続する。
【００４３】
このようにしてＳパージモードでは、Ｓパージ制御のルーチンを繰り返すが、吸蔵型ＮＯｘ触媒２５に吸蔵されたＳＯｘが放出されると、吸蔵型ＮＯｘ触媒２５のＳ再生頻度が大きくなり、図５のマップに基づいてＺＳＴＥＭＰも上昇する。そして、ＺＳＴＥＭＰが触媒温度Ｔcat を越えると、ステップＳ４において、吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat がＺＳＴＥＭＰより低くなって制御のルーチンを抜けてステップＳ９に進み、Ｓパージ制御を停止する。
【００４４】
なお、三元触媒２６がセリア、ジルコニア等のＯ₂ ストレージ剤を含有している場合は内部に酸素を貯留する機能を有しているため、吸蔵型ＮＯｘ 触媒２５において発生したＨ₂Ｓは、この三元触媒２６の内部の酸素によって良好に酸化処理される。これによっても排気ガスが異臭を放つことが確実に防止される。
【００４５】
ところで、上述した実施形態では、Ｓパージ制御時において、点火時期を徐々にリタードさせて吸蔵型ＮＯｘ触媒２５を徐々に昇温すると共にリタード量を制限して触媒最高温度を抑制するようにしたが、例えば、上記ステップＳ６において、Ｓパージ制御が開始された時点で、Ｓパージ制御の点火時期を徐々にリタードさせて触媒最高温度については従来と同様の触媒温度まで昇温したり、また、Ｓパージ制御のリタード量を従来より低めの固定値として求め、この目標リタード量に応じてリタード量変化速度については一義に設定するようにしてもよい。この場合であっても、ＳＯ₂ の放出時間は長くなるものの、従来、Ｓパージ制御の開始直後にＨ₂Ｓが一時的に大量に発生していたのを抑えてＨ₂Ｓの発生濃度を十分に低くするようにでき、Ｈ₂Ｓ濃度を低く抑えて排ガスが異臭を放つことを確実に防止することができる。
【００４６】
なお、上述した実施形態では、Ｓパージ制御時にＳ再生頻度に応じてリタード量を所定値に向けて連続して滑らかに移行させたが、複数に分けて段階的（段階状）に移行させてもよい。また、吸蔵型ＮＯｘ触媒２５の昇温方法も、点火時期リタードに限定されるものではなく、例えば、主燃料噴射後に膨張行程で追加燃料噴射を行う２段噴射でもよく、この場合、Ｓ再生頻度が小さいほど追加燃料噴射量を徐々に増やすことにより触媒昇温温度を制御すればよい。また、Ｓ再生頻度が小さいほど最終的な追加燃料噴射量を少なくすれば触媒最高温度を低くすることができる。その他、吸蔵型ＮＯｘ触媒２５の上流の排気管２１内に直接燃料を噴射するようなものであってもよく、更に、電気加熱触媒やバーナーを利用する等の加熱手段を用いてもよい。
【００４７】
また、触媒温度の求め方としては、上述の実施形態のように、高温センサ２４から求める方法の他に、目標平均有効圧Ｐｅ（エンジン負荷）とエンジン回転速度Ｎｅのマップとして予め実験的に触媒温度をＥＣＵ２８にメモリさせた値から求めてもよい。また、より簡略化して車速に対するマップとして予め実験的に触媒温度をＥＣＵ２８にメモリさせた値から求めてもよい。この場合、精度は多少低下するが、高温センサ２４を用いる必要がないためにコストが低減されるメリットがある。
【００４８】
ここで、上述したＳパージモードへの切換制御を図３のタイムチャートに基づいて具体的に説明する。図３に示すように、ＳパージモードがＯＦＦで、Ｓ再生頻度が低いとき、昇温設定温度ＺＳＴＥＭＰは６００℃に設定されている。そして、ドライバの運転状態、例えば、加速時あるいは郊外道路、高速道路や山岳道路などを走行して触媒温度Ｔcat が上昇し、この触媒温度Ｔcat がＺＳＴＥＭＰを越えると、ＳパージモードがＯＮ（ステップＳ４でＹｅｓと）なる。すると、点火時期をリタードして吸蔵型ＮＯｘ触媒２５を昇温すると共に空燃比をリッチ化するため、吸蔵型ＮＯｘ触媒２５に吸蔵されたＳＯｘが放出される。そして、Ｓパージモードが継続するとＳ再生が進み、７００℃換算Ｓ再生時間が増加してＳ再生頻度が大きくなり、このＳ再生頻度が大きくなると、図４のマップに基づいてＺＳＴＥＭＰが上昇し、触媒温度Ｔcat を越える。
【００４９】
すると、ＳパージモードがＯＦＦ（ステップＳ４でＮｏ）となり、吸蔵型ＮＯｘ触媒２５の昇温をやめるため、触媒温度Ｔcat が低下する。その後、触媒温度Ｔcat が低い状態で走行距離が延びると、Ｓ再生頻度が小さくなると共に、ＺＳＴＥＭＰが低下して６００℃に戻る。
【００５０】
また、ドライバの運転状態により触媒温度Ｔcat が再び上昇し、触媒温度Ｔcat がＺＳＴＥＭＰを越えるとＳパージモードがＯＮ（ステップＳ４でＹｅｓ）される。このとき、例えば、ドライバが市街地のような場所で加減速を繰り返す運転状態を行っていると、吸蔵型ＮＯｘ触媒２５はＳＯｘを放出するのに必要な高温状態になりにくい。即ち、この場合、触媒温度Ｔcat は最初ＺＳＴＥＭＰ（６００℃）を越えたものの６５０℃よりも低い状態が継続することとなり、この温度域ではＳ再生はされるもののＳ再生効率が低く、燃費悪化度合が多いわりにあまりＳ再生されない。そのため、この状態の継続時間が設定時間Ｃを経過したら、ＳパージモードをＯＦＦ（ステップＳ４でＮｏ）とすることで、点火時期リタード及びリッチ化を中止しＳ再生効率が低い温度域では燃費の悪化を抑制できる。
【００５１】
なお、Ｓパージモードにて、空燃比をリッチ空燃比モードとしてリーン空燃比モードへの移行を禁止しているが、吸蔵型ＮＯｘ触媒２５が高温であるときには、吸蔵型ＮＯｘ触媒２５の熱劣化対策のために既にリーン空燃比モードは禁止となっている場合もある。
【００５２】
このように本実施形態では、吸蔵型ＮＯｘ触媒２５からＳＯｘを放出する再生度合が所定範囲に保たれるように制御、つまり、吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat が活性温度（例えば、２５０〜３５０℃）より高く、且つ、吸蔵されたＳＯｘを脱離するのに適した温度（例えば、６５０℃〜８００℃）あるいはそれより低く設定された昇温設定温度ＺＳＴＥＭＰ以上となった場合に、点火時期をリタードして吸蔵型ＮＯｘ触媒２５を昇温させると共に空燃比をリッチ化してＳＯｘを放出する。
【００５３】
そして、吸蔵型ＮＯｘ触媒２５が強制Ｓパージ制御にてイオウ成分を放出する状態になった頻度は、この吸蔵型ＮＯｘ触媒２５からＨ₂Ｓが生成される特性と相関があることを発見し、このイオウ成分を放出する状態になった頻度（Ｓ再生頻度）に応じて点火時期を徐々に所定の小さなリタード量に向けて移行させ、吸蔵型ＮＯｘ触媒２５を徐々に昇温して触媒温度を従来より低い温度まで上昇させるように、触媒温度及びまたは昇温速度を制御している。
【００５４】
上述したように本実施形態では、吸蔵型ＮＯｘ触媒２５がある程度高温状態にあるときに、アシスト的に吸蔵型ＮＯｘ触媒２５を昇温してＳＯｘ放出に適した触媒温度域においてＳＯｘを放出して再生している。そのため、吸蔵型ＮＯｘ触媒２５が低温状態でＳＯｘの放出に最適な温度まで昇温するのに長時間を要するような場合に無理にＳ再生を実施するものではない。これによって長時間のリッチ化及び点火時期リタード、及び低温から大きく昇温しなければならないための大幅なリタードを行う必要がなく、燃費の悪化を抑制できる。
【００５５】
そして、このＳ再生時に、点火時期リタード直後に大量のＨ₂Ｓの発生現象はなく、このときのＨ₂Ｓの発生濃度を低くすると共にＨ₂Ｓを低い濃度のまま保持することができ、Ｈ₂Ｓ濃度を低く抑えて排気ガスが異臭を放つことを確実に防止する一方で、吸蔵型ＮＯｘ触媒２５の再生時間の長時間化を抑制して安定した触媒の再生が可能となる。
【００５６】
また、Ｓパージモードで触媒温度Ｔcat がイオウ成分の脱離に最適な温度（例えば、６５０℃以上）へ近づかない場合、設定時間Ｃを経過した後にＳパージモードを停止させるようにしている。従って、例えば、車両が市街地のような場所で加減速を繰り返す運転状態のときは、吸蔵型ＮＯｘ触媒２５がＳＯｘを放出するのに最適な高温状態になりにくいため、設定時間Ｃの経過を待ってＳパージモードを中止させており、これによってＳ再生効率が良くない温度帯ではリッチ化及び昇温の継続に制限を設けることにより燃費の悪化を抑制できる。
【００５７】
なお、本実施形態では、吸蔵型ＮＯｘ触媒２５の温度が活性温度を２５０℃〜３５０℃とし、吸蔵されたＳＯｘを脱離するのに適した温度を６５０℃から８００℃（より好ましくは７００℃以上）、昇温設定温度ＺＳＴＥＭＰを６００℃から開始してＳ再生頻度により変化させるようにしたが、各温度は吸蔵型ＮＯｘ触媒２５の特性、あるいはエンジン形態や排気温度などにより適宜設定すればよいものである。
【００５８】
また、上述の実施形態では、強制Ｓパージ制御時に、排気空燃比がリッチまたはストイキのときに吸蔵型ＮＯｘ触媒２５が所定の温度領域になるように昇温する特定運転状態にて、Ｓ再生頻度に応じて硫化水素の生成を抑制するように触媒の昇温最高温度あるいはその昇温速度を制御したが、Ｓ再生頻度に代えて吸蔵型ＮＯｘ触媒２５に吸蔵されたＳ吸蔵量を推定し、このＳ吸蔵量に応じて硫化水素の生成を抑制するように触媒の昇温最高温度あるいはその昇温速度を制御するようにしてもよいので、簡単に説明しておく。
【００５９】
ここで、吸蔵型ＮＯx触媒２５に吸蔵されたＳＯｘの吸蔵量（被毒Ｓ量Ｑｓ）の推定方法について説明する。この被毒Ｓ量Ｑｓは、基本的には燃料噴射積算量Ｑｆに基づき設定されるものであり、燃料噴射制御ルーチン（図示せず）の実行周期毎に次式により演算される。
Ｑｓ＝Ｑｓ（ｎ−１）＋ΔＱｆ・Ｋ−Ｒｓ ・・・（６）
ここに、Ｑｓ（ｎ−１）は被毒Ｓ量の前回値であり、ΔＱｆは実行周期当たりの燃料噴射積算量、Ｋは補正係数、Ｒｓは実行周期当たりの放出Ｓ量を示している。つまり、現在の被毒Ｓ量Ｑｓは、実行周期当たりの燃料噴射積算量ΔＱｆを補正係数Ｋで補正して積算するとともに、該積算値から実行周期当たりの放出Ｓ量Ｒｓを減算することで求められる。
【００６０】
この補正係数Ｋは、例えば、次式に示すように、空燃比Ａ／Ｆに応じたＳ被毒係数Ｋ１、燃料中のＳ含有量に応じたＳ被毒係数Ｋ２及び触媒温度Ｔcatに応じたＳ被毒係数Ｋ３の３つの補正係数の積からなっている。
Ｋ＝Ｋ１・Ｋ２・Ｋ３ ・・・（７）
また、実行周期当たりの放出Ｓ量Ｒｓは次式から演算される。
Ｒｓ＝α・Ｒ１・Ｒ２・ｄＴ ・・・（８）
ここに、αは単位時間当たりの放出率（設定値）であり、ｄＴは燃料噴射制御ルーチンの実行周期を示しており、Ｒ１及びＲ２はそれぞれ触媒温度Ｔcatに応じた放出能力係数及び空燃比Ａ／Ｆに応じた放出能力係数を示している。
【００６１】
従って、このＳ吸蔵量（被毒Ｓ量Ｑｓ）に応じて点火時期リタードの変化速度を遅くして吸蔵型ＮＯｘ触媒２５の昇温最高温度及び昇温速度を遅くすると、ＳＯ₂ の放出量が一気に増加してＨ₂Ｓが多量に発生し易いＳパージ制御の開始時において、ＳＯ₂ の放出量を抑制してＨ₂Ｓの発生濃度を低く抑えることができる。
【００６２】
また、空燃比をリッチ化させる手法はオープンループで目標Ａ／Ｆを変更する方式に限られるものではなく、例えば、Ｏ₂ センサを利用したストイキフィードバック制御において、フィードバック制御用の積分ゲイン（係数）をリッチ側に大きくリーン側に小さく変更する方法を用いてもよい。そして、この場合は積分ゲインや比例ゲインの速度をＳ再生頻度に応じて設定することで平均空燃比をＳ再生頻度に応じて徐々にリッチ化させたり、ゲイン値そのものをＳ再生頻度に応じて設定することで平均空燃比自体がＳ再生頻度に応じてリッチ空燃比になるように制御すればよい。また、リッチ化手段として前述の２段噴射あるいは排気管２１内への直接燃料噴射を用いてもよい。
【００６３】
また、本発明は、Ｓ再生時に、イオウ放出頻度またはイオウ吸蔵量に関連する指標に応じて硫化水素の生成を抑制するように触媒の温度あるいは昇温速度を制御することが特徴であるため、上述した実施形態のように、Ｓパージ制御を開始する条件としてＳ再生頻度を用いなくてもよい。例えば、ＮＯｘ吸蔵能力を再生するために吸蔵型ＮＯｘ触媒２５に吸蔵されたＳ吸蔵量（被毒Ｓ量Ｑｓ）が所定量以上になったことを条件としてＳパージ制御を開始するようにしてもよい。
【００６４】
ところで、吸蔵型ＮＯｘ触媒２５における車両の加速時などに伴うリッチ化及び触媒温度による自然Ｓパージ時にも、ＳＯｘはＳＯ₂ として放出されることから排気ガスが高温で且つ水素や一酸化炭素や炭化水素等が多く存在する還元雰囲気のもとでは、硫化水素（Ｈ₂Ｓ）が発生して臭気を放ってしまう。そして、触媒が直ちに昇温され（触媒昇温温度が早い）、触媒温度が高温であるほど水素等の還元剤も多くＳＯ₂ からＨ₂Ｓが急激に多量に生成される。図８及び図９は、本発明の他の実施形態を示すものであり、自然Ｓパージ時、つまり、触媒温度が高い状態でリッチ化する場合は、そのリッチ度合を更に大きくして排気空燃比のリッチ化による燃料冷却効果により触媒の温度あるいは昇温速度を抑制するようにしたものである。
【００６５】
即ち、この自然Ｓパージ制御について、図８のフローチャートに示すように、ステップＳ１１では、高温センサ２４により検出された排気温度情報から吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat を推定する。ステップＳ１２では、目標平均有効圧Ｐｅとエンジン回転速度Ｎｅとに応じて決定される最終目標Ａ／Ｆがリーン空燃比からリッチ空燃比（ストイキ空燃比を含んでもよい）に切り換わった否か、即ち、運転者の加速操作に伴って機関運転状態に対応した最終目標Ａ／Ｆがリッチ空燃比に切り換わった否かが判定され、そうでない場合はそのままリターンし、リッチ空燃比に切り換わった場合は、ステップＳ１３に進む。
【００６６】
このステップＳ１３では、推定された触媒温度Ｔcatが所定の高温度ＴcatH（例えば、６５０℃）以上であるか否かを判定し、触媒温度Ｔcatが高温度ＴcatHより低ければステップＳ１４に進んで通常パターンでリッチ化して最終目標Ａ／Ｆとする一方、触媒温度Ｔcatが高温度ＴcatH以上であればステップＳ１５に進み、最終目標Ａ／Ｆより大きな目標Ａ／Ｆに向けてリッチ化する。このときの大きな目標Ａ／Ｆは、現時点での触媒温度Ｔcat、エンジン負荷、エンジン回転数等の運転条件に基づいて設定すればよい。そして、ステップＳ１４、Ｓ１５を経過した後はリターンする。ここで、前述のステップＳ１２、Ｓ１３では、エンジン１１の運転状態がエンリッチ運転とされ、吸蔵型ＮＯｘ触媒２５が吸蔵したイオウ成分を放出可能な高温状態に達しているか否かを判別している。
【００６７】
触媒温度Ｔcatが所定の高温度ＴcatHを超えているときにリッチ空燃比化されると、吸蔵型ＮＯｘ触媒２５に吸蔵されたＳＯｘはＳＯ₂として放出されると同時にＨ₂Ｓが大量発生してしまう。そのため、車両の加速時などのエンリッチ運転のもとでは、前述のステップＳ１５のように、最終目標Ａ／Ｆより大きな目標のリッチ空燃比に向けてリッチ化するようにし、この燃料冷却効果により吸蔵型ＮＯｘ触媒２５の温度あるいは昇温速度を低く抑えることで、Ｈ₂Ｓが多量に発生し易い自然Ｓパージの開始時において、Ｈ₂Ｓの発生量を少なく抑えるようにする。これによってＨ₂Ｓが多量に発生しやすい自然Ｓパージ開始時のＨ₂Ｓ濃度を低く抑えることができる。
【００６８】
つまり、図９に、加速運転時における目標Ａ／Ｆの時間変化（ａ）に基づいた触媒温度の時間変化（ｂ）とＨ₂Ｓ発生量の時間変化（ｃ）とがタイムチャートとして示してある。この場合、従来（通常のリッチ化）のように最終目標Ａ／Ｆの切換時から所定のリッチ空燃比ＡＦ₁（例えば、値１４）とした場合を破線で示し、本実施形態のように最終目標Ａ／Ｆより大きな目標のリッチ空燃比に向けてリッチ化させた場合を実線で示してある。このタイムチャートからわかるように、従来の制御（破線）では、最終目標Ａ／Ｆの切換直後に一時的に大量にＨ₂Ｓが発生していたが、本実施形態の制御（実線）では、切換直後のＨ₂Ｓの発生濃度を低くすると共にこのまま保持するようにでき、エンリッチ運転の場合においてもＨ₂Ｓ濃度を低く抑えて排ガスが異臭を放つことを確実に防止することができる。
【００６９】
なお、上述実施形態においては、ステップＳ１１にて、現時点の吸蔵型ＮＯｘ触媒２５の触媒温度Ｔcat を推定し、ステップＳ１３にて、触媒温度Ｔcatと高温度ＴcatHとを比較して目標Ａ／Ｆ以上にリッチするかどうかを判定するようにしたが、制御の遅れ、排気系の遅れ、触媒の熱容量が大きいことによる昇温の遅れ等があるため、現時点での触媒温度Ｔcatに基づいた制御では触媒昇温に対して目標Ａ／Ｆ増大による冷却が間に合わない場合がある。そこで、ステップＳ１３での判定条件をステップＳ１１で算出した現在の触媒温度Ｔcat、目標Ａ／Ｆ、運転条件（負荷・回転数、あるいは車速、水温等）に基づいて、吸蔵型ＮＯｘ触媒２５の触媒温度最高温度予測値を算出し、その触媒最高温度予測値が所定値以上のとき、ステップＳ１５で目標Ａ／Ｆ以上にリッチ化するように変更してもよい。また、ステップＳ１３にて、吸蔵型ＮＯｘ触媒２５の触媒最高温度予測値に代えて、ステップＳ１１で算出した現在の触媒温度Ｔcat、目標Ａ／Ｆ、運転条件（負荷・回転数、あるいは車速、水温等）に基づいて、吸蔵型ＮＯｘ触媒２５の触媒温度上昇率予測値を算出し、その触媒温度上昇率予測値が所定値以上のとき、ステップＳ１５で目標Ａ／Ｆ以上にリッチ化するようにしてもよい。
【００７０】
更に、ステップＳ１３にて、触媒最高温度予測値が所定値以上で且つ触媒温度上昇率予測値が所定値以上のとき、ステップＳ１５で目標Ａ／Ｆ以上にリッチ化するようにしてもよいし、ステップＳ１３にて、触媒最高温度予測値が所定値以上または触媒温度上昇率予測値が所定値以上のとき、ステップＳ１５で目標Ａ／Ｆ以上にリッチ化するようにしてもよい。また、ステップＳ１３の判定条件として、「Ｓ再生頻度が所定値以下のとき」を追加してもよい。このときは、ステップＳ１５での「目標Ａ／Ｆ以上にリッチ化」の終了条件として、「Ｓ再生頻度が所定値以上になったとき」としてもよい。更に、ステップＳ１３での判定条件として、「目標Ａ／Ｆが所定Ａ／Ｆ（燃料冷却効果の大きなＡ／Ｆ、例えば、Ａ／Ｆ＝１３）を追加してもよい。
【００７１】
なお、リッチ化による燃料冷却効果で吸蔵型ＮＯｘ触媒２５の温度あるいは昇温速度を低く抑えるようにしたが、この手法以外に、エンジン冷却水を触媒の周辺に流動させたり、別途触媒のための冷却手段を設けてもよい。
【００７２】
このように本実施形態では、吸蔵型ＮＯｘ触媒２５が高温状態でリッチ化されて自然に触媒から吸蔵したイオウ成分が放出される場合には、空燃比を目標Ａ／Ｆよりも大きな値でリッチ化させるようにする。従って、自然Ｓパージに伴って大量のＨ₂Ｓが急激に発生する現象はなく、このＨ₂Ｓの発生濃度を小さくすると共にＨ₂Ｓ濃度を小さい値のまま保持することができ、Ｈ₂Ｓ濃度を低く抑えて排気ガスが異臭を放つことを確実に防止することができ、このＨ₂Ｓの大量発生の防止制御は、車両の加速時などに伴うリッチ化で自然にＳパージする場合に適用すると有効的である。
【００７３】
なお、上述の実施形態においては、Ｓ再生頻度またはＳ吸蔵量に基づいて触媒温度あるいは昇温速度を制御することでＨ₂Ｓの放出を制御するようにしたが、これらの方法とＳ再生頻度またはＳ吸蔵量に基づいてＳパージ時のＡ／Ｆを徐々に変更させる方法、あるいはＳ再生頻度またはＳ吸蔵量に基づいてＳパージ時のＡ／Ｆのリッチ度合を変化させる方法を組み合わせてもよい。
【００７４】
また、上述の各実施形態では、エンジン１１を筒内噴射型ガソリンエンジンとしたが、これに限られず、吸気管噴射型ガソリンエンジンであってもよく、また、上記実施形態では、三元触媒２２を設けて排気ガスの浄化効率を向上させているが、必ずしも三元触媒２２を設けなくてもよく、三元触媒２２がなくても本発明を好適に実現可能である。また、イオウ成分を吸蔵して放出する触媒としては吸蔵型ＮＯｘ触媒２５を上述の実施形態において取り上げたが、触媒としてはその他のものであってもよく、吸着したＮＯｘを直接接触還元する吸着型ＮＯｘ触媒やイオウ成分を吸蔵しやすいタイプの三元触媒であってもよい。更には、エンジンとしては、ディーゼルエンジンに適用することも可能である。
【００７５】
【発明の効果】
以上、実施形態において詳細に説明したように本発明の内燃機関の排気浄化装置によれば、触媒の温度が所定の触媒昇温設定温度以上となる特定運転状態において、触媒におけるイオウ放出頻度またはイオウ吸蔵量に関連する指標に応じて硫化水素の生成を抑制するように触媒の温度あるいは昇温速度を制御するようにしたので、触媒からＨ₂Ｓが一度に大量に発生するのを防止して放出濃度を低く抑え、排気ガスの異臭を防止する一方で、触媒の再生時間の長時間化を抑制して安定した触媒装置の再生を可能とすることができる。
【図面の簡単な説明】
【図１】本発明の一実施形態に係る内燃機関の排気浄化装置の概略構成図である。
【図２】本実施形態の排気浄化装置による強制Ｓパージ制御のフローチャートである。
【図３】Ｓパージ制御のタイムチャートである。
【図４】Ｓ再生頻度とＮＯｘ排出量との関係を表すグラフである。
【図５】Ｓ再生頻度に対する昇温設定温度を表すグラフである。
【図６】触媒温度推定値に対する反映係数を表すグラフである。
【図７】強制Ｓパージ制御における点火時期リタード量及び触媒温度の時間変化に対するＨ₂Ｓ発生濃度の時間変化を表すタイムチャートである。
【図８】本発明の他の実施形態による自然Ｓパージ制御のフローチャートである。
【図９】自然Ｓパージ制御における目標Ａ／Ｆ及び触媒温度の時間変化に対するＨ₂Ｓ発生濃度の時間変化を表すタイムチャートである。
【符号の説明】
１１ エンジン（内燃機関）
２１ 排気管（排気通路）
２２ 三元触媒
２３ 排気浄化触媒装置
２４ 高温センサ
２５ 吸蔵型ＮＯｘ触媒（触媒）
２６ 三元触媒
２７ ＮＯｘセンサ
２８ 電子コントロールユニット，ＥＣＵ（触媒状態検知手段、特定運転状態検出手段、温度制御手段）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine having a catalyst that has a characteristic of storing sulfur components in exhaust gas in an exhaust passage and releasing the stored sulfur components when the exhaust air-fuel ratio is rich or stoichiometric at high temperatures. .
[0002]
[Prior art]
In recent years, lean combustion internal combustion engines in which an internal combustion engine is operated at a lean air-fuel ratio to improve fuel efficiency have been put into practical use. In this lean combustion internal combustion engine, when operating at a lean air-fuel ratio, there is a problem that the three-way catalyst cannot sufficiently purify NOx (nitrogen oxide) in the exhaust gas due to its purification characteristics. Recently, it has been operated at a lean air-fuel ratio. Storage type NOx catalysts that store NOx in exhaust gas and release and reduce NOx stored during operation at a stoichiometric or rich air-fuel ratio have been adopted.
[0003]
This storage-type NOx catalyst converts NOx in exhaust gas to nitrate (X-NO) in an excess state of oxygen in the internal combustion engine._Three), And the stored NOx is released in an excess of carbon monoxide (CO) to form nitrogen (N₂) To reduce (at the same time carbonate X-CO_ThreeIs produced). However, sulfur (S) component is contained in the fuel, and this S component reacts with oxygen to become sulfur oxide (SOx), and this SOx is stored as a sulfate instead of nitrate as a sulfate instead of NOx. There is a problem that the NOx catalyst is occluded and the purification efficiency of the catalyst is lowered. However, the SOx occluded in the catalyst can be obtained by making the air-fuel ratio rich or stoichiometric and bringing the catalyst to a high temperature state.₂Is released (S purge). For example, it is disclosed by Unexamined-Japanese-Patent No. 7-217474.
[0004]
[Problems to be solved by the invention]
By the way, SO₂Is released under a reducing atmosphere in which the exhaust gas is at a high temperature and a large amount of carbon monoxide, hydrocarbons, etc. are present, for example, hydrogen sulfide (H₂S) occurs.
SO₂+ 3H₂→ H₂S + 2H₂O
SO₂+ 2CO + H₂→ H₂S + CO₂
3SO₂+ C_ThreeH₆→ 3H₂S + 3CO₂
This H₂It is desirable that S is not generated as much as possible in order to emit a strong odor as is generally known.
[0005]
However, in the technique disclosed in the above-mentioned publication, the stored SOx is SO4.₂As a result, the released SO₂And H around the catalyst₂Chemical reaction with CO, HC, etc. will proceed rapidly, and H₂S will be generated in large quantities abruptly. H like this₂When S is rapidly generated in large quantities,₂The S concentration is locally increased, and the exhaust gas discharged into the atmosphere emits a very strong odor, which is not preferable.
[0006]
In addition, the “engine control device” disclosed in Japanese Patent Application Laid-Open No. 11-107809 relates to NOx catalyst regeneration control.₂Since S occurs when the exhaust gas temperature is high, when the temperature is high, the degree of enrichment of the exhaust air-fuel ratio is reduced to about the stoichiometric level.₂A technique for suppressing the refinement of S is disclosed. However, in our study, H₂It was found that the generation characteristic of S shows a characteristic that increases exponentially with temperature. Therefore, as in the technique described in the above-mentioned publication, simply reducing the degree of enrichment based on temperature,₂The generation of S cannot be suppressed efficiently. That is, in a region where the catalyst temperature is particularly high, while releasing S, H₂It is difficult to suppress the formation of S, and H₂If the air-fuel ratio can suppress the generation of S, the S release time becomes extremely long. On the other hand, if the air-fuel ratio is set such that the S release time does not become long, H₂There is a problem that the generation of S cannot be sufficiently suppressed. For this reason, only the control as described above can be performed with H.₂There is a limit to the method for suppressing the release of S into the atmosphere, and there is a problem that a sufficient effect cannot be obtained. In addition, H at the time of S release of such a catalyst₂Although the S generation characteristics vary depending on the catalyst state other than the temperature, the above-mentioned gazette does not consider such a point, and there is sufficient room for improvement.
[0007]
As a result of studying the above point, the present inventors have found that H at the time of release of the sulfur component from the catalyst.₂It was found that the generation characteristics of S have a correlation with the frequency at which the catalyst is released from the sulfur component and the sulfur storage amount of the catalyst. Therefore, in order to solve the above-mentioned problems, the present invention controls the catalyst temperature or the heating rate so as to suppress the generation of hydrogen sulfide according to the frequency at which the sulfur component can be released and the amount of stored sulfur. Thus, the H produced with the release of SOx from the catalyst₂To provide an exhaust gas purification apparatus for an internal combustion engine that can suppress regeneration of the exhaust gas by suppressing the length of the regeneration time of the catalyst while suppressing the odor of exhaust gas by suppressing the concentration of S to be low, and enabling stable regeneration of the catalyst device. Objective.
[0008]
[Means for Solving the Problems]
  In the exhaust gas purification apparatus for an internal combustion engine according to the present invention for achieving the above object, the sulfur component in the exhaust gas is occluded in the exhaust passage of the internal combustion engine.If the temperature is equal to or higher than the preset catalyst temperature riseIn addition, a catalyst with the characteristic of releasing the sulfur component stored when the exhaust air-fuel ratio is rich or stoichiometric is provided, and the catalyst state detection means detects or estimates an indicator related to the sulfur release frequency or sulfur storage amount in the catalyst, and identifies it Operating state detection meansThe catalyst temperature is equal to or higher than the preset catalyst temperature rise temperature.Detecting specific operating conditions, temperature control meansWhen the specific operating state detection means detects the specific operating stateThe temperature of the catalyst or the rate of temperature rise is controlled so as to suppress the production of hydrogen sulfide according to the above index.
[0009]
The sulfur release frequency and sulfur storage amount in the catalyst have a correlation with the characteristics of hydrogen sulfide generation when sulfur is released from the catalyst, and the specific sulfur release is determined according to the index related to the sulfur release frequency or sulfur storage amount. Since the temperature or rate of temperature increase of the catalyst in the operating state is controlled so as to suppress the generation of hydrogen sulfide, regeneration of the catalyst can be performed at a temperature or rate of temperature increase that can achieve both catalyst regeneration by releasing sulfur components and suppression of hydrogen sulfide. This makes it possible to suppress the emission concentration of hydrogen sulfide to prevent a bad odor of the exhaust gas, while suppressing an increase in the regeneration time of the catalyst and to stably regenerate the catalyst device.
[0010]
In this case, the catalyst state detection means detects or estimates the frequency with which the sulfur component is released from the catalyst, and the temperature control means controls so that the catalyst temperature or the temperature increase rate decreases as the frequency decreases. It is desirable to work. Further, the catalyst state detection means detects or estimates the amount of sulfur component stored in the catalyst, and the temperature control means controls so that the temperature of the catalyst or the rate of temperature increase decreases as the sulfur storage amount increases. It is desirable to operate on. Further, the specific operation state is an operation state in which the temperature of the catalyst is forcibly increased so as to release the sulfur component from the catalyst, and the temperature control means may control the maximum temperature or the temperature increase rate of the catalyst. . The specific operating state detection means detects a shift to a specific operating state accompanying acceleration of the internal combustion engine, etc., and the temperature control means controls the catalyst temperature or the temperature increase rate by the fuel cooling effect due to the richness of the exhaust air / fuel ratio. You may do. Further, the temperature control means may control either one or both of the temperature of the catalyst and the rate of temperature increase so as to suppress the generation of hydrogen sulfide according to the index in the specific operation state.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0012]
FIG. 1 is a schematic configuration of an exhaust purification device for an internal combustion engine according to an embodiment of the present invention, FIG. 2 is a flowchart of forced S purge control by the exhaust purification device of the present embodiment, FIG. 3 is a time chart of S purge control, and FIG. 4 is a graph showing the relationship between the S regeneration frequency and the NOx emission amount, FIG. 5 is a graph showing the temperature rise set temperature with respect to the S regeneration frequency, FIG. 6 is a graph showing the reflection coefficient for the estimated catalyst temperature value, and FIG. H with respect to time change of ignition timing retard amount and catalyst temperature in purge control₂FIG. 8 is a flow chart of the natural S purge control according to another embodiment of the present invention, and FIG. 9 is a graph showing the H with respect to the time change of the target A / F and the catalyst temperature in the natural S purge control.₂The time chart showing the time change of S generation density is shown.
[0013]
The internal combustion engine (hereinafter referred to as an engine) according to the present embodiment switches, for example, a fuel injection mode (operation mode), thereby fuel injection in an intake stroke (intake stroke injection mode) or fuel injection in a compression stroke ( This is an in-cylinder injection type spark ignition type in-line four-cylinder gasoline engine capable of performing a compression stroke injection mode). The in-cylinder injection engine 11 is operated not only at a stoichiometric air fuel ratio (stoichiometric) or at a rich air fuel ratio (rich air fuel ratio operation) but also at a lean air fuel ratio (lean air fuel ratio operation). In particular, in the compression stroke injection mode, operation at an ultra-lean air-fuel ratio is possible.
[0014]
In the present embodiment, as shown in FIG. 1, an electromagnetic fuel injection valve 14 is attached to the cylinder head 12 of the engine 11 together with a spark plug 13 for each cylinder. The fuel can be directly injected into 15. A fuel supply device (fuel pump) is connected to the fuel injection valve 14 via a fuel pipe (not shown), and the fuel in the fuel tank is supplied at a high fuel pressure. The fuel is supplied from the fuel injection valve 14 to the combustion chamber. 15 is injected at a desired fuel pressure. At this time, the fuel injection amount is determined from the fuel discharge pressure of the fuel pump and the valve opening time (fuel injection time) of the fuel injection valve 14.
[0015]
An intake port is formed in the cylinder head 12 in a substantially upright direction for each cylinder, and one end of an intake manifold 16 is connected so as to communicate with each intake port. A drive-by-wire (DBW) type electric throttle valve 17 is connected to the other end of the intake manifold 16, and the throttle valve 17 is provided with a throttle sensor 18 for detecting a throttle opening θth. Further, an exhaust port is formed in the cylinder head 12 in a substantially horizontal direction for each cylinder, and one end of an exhaust manifold 19 is connected to communicate with each exhaust port.
[0016]
The engine 11 is provided with a crank angle sensor 20 that detects a crank angle. The crank angle sensor 20 can detect the engine rotational speed Ne. Note that the above-described in-cylinder injection engine 11 is already known, and a detailed description thereof will be omitted here.
[0017]
An exhaust pipe (exhaust passage) 21 is connected to the exhaust manifold 19 of the engine 11, and the exhaust pipe 21 is illustrated via a small three-way catalyst 22 and an exhaust purification catalyst device 23 close to the engine 11. No muffler is connected. A portion of the exhaust pipe 21 between the three-way catalyst 22 and the exhaust purification catalyst device 23 is located immediately upstream of the exhaust purification catalyst device 23, that is, immediately upstream of the storage type NOx catalyst 25 described later. A high temperature sensor 24 for detecting the exhaust temperature is provided.
[0018]
The exhaust purification catalyst device 23 includes two catalysts, a storage type NOx catalyst 25 and a three-way catalyst 26, and the three-way catalyst 26 is arranged downstream of the storage type NOx catalyst 25. It is installed. If the storage-type NOx catalyst 25 has a sufficient function as a three-way catalyst, a three-way catalyst function-integrated storage-type NOx catalyst including only this storage-type NOx catalyst 25 may be used. This NOx storage catalyst 25 temporarily stores NOx in an oxidizing atmosphere, releases NOx in a reducing atmosphere mainly containing CO, and N₂It has a function of reducing to (nitrogen) or the like. Specifically, the storage-type NOx catalyst 25 is configured as a catalyst having platinum (Pt), rhodium (Rh), palladium (Pd), etc. as noble metals, and barium (Ba), potassium (K) as storage agents. Alkali metals such as alkaline earth metals are employed. A NOx sensor 27 that detects the NOx concentration is provided downstream of the exhaust purification catalyst device 23.
[0019]
Further, an ECU (electronic control unit) 28 having an input / output device, a storage device (ROM, RAM, nonvolatile RAM, etc.), a central processing unit (CPU), a timer counter, etc. is provided. Inclusive control of the exhaust emission control device of the present embodiment is included. That is, various sensors such as the high temperature sensor 24 and the NOx sensor 27 described above are connected to the input side of the ECU 28, and detection information from these sensors is input. On the other hand, the ignition plug 13 and the fuel injection valve 14 described above are connected to the output side of the ECU 28 via an ignition coil. The ignition coil, the fuel injection valve 14 and the like are detected information from various sensors. The optimum values such as the fuel injection amount and ignition timing calculated based on the above are output. Accordingly, an appropriate amount of fuel is injected from the fuel injection valve 14 at an appropriate timing, and ignition is performed at an appropriate timing by the spark plug 13.
[0020]
Actually, the ECU 28 determines the target in-cylinder pressure corresponding to the engine load, that is, the target average effective pressure Pe, based on accelerator opening information from an accelerator opening sensor (not shown) and engine rotational speed information Ne from the crank angle sensor 20. Further, the fuel injection mode is set from a map (not shown) according to the target average effective pressure Pe and the engine rotational speed information Ne. For example, when both the target average effective pressure Pe and the engine rotational speed Ne are small, the fuel injection mode is set to the compression stroke injection mode, and fuel is injected in the compression stroke, while the target average effective pressure Pe increases, or the engine When the rotational speed Ne increases, the fuel injection mode is changed to the intake stroke injection mode, and fuel is injected in the intake stroke.
[0021]
Then, a target air-fuel ratio (target A / F) as a control target is set from the target average effective pressure Pe and the engine speed Ne, and an appropriate amount of fuel injection is determined based on the target A / F. Further, the catalyst temperature Tcat is estimated from the exhaust gas temperature information detected by the high temperature sensor 24. Specifically, in order to correct an error caused by the high temperature sensor 24 and the storage-type NOx catalyst 25 being somewhat apart from each other, the temperature is determined according to the target average effective pressure Pe and the engine rotational speed information Ne. The difference map is set in advance by experiments or the like, and the catalyst temperature Tcat is uniquely estimated when the target average effective pressure Pe and the engine rotational speed information Ne are determined.
[0022]
Hereinafter, the operation of the exhaust gas purification apparatus for an internal combustion engine of the present embodiment configured as described above will be described.
[0023]
In the storage type NOx catalyst 25 of the exhaust purification catalyst device 23, nitrate is generated from NOx in the exhaust gas in an oxygen concentration excess atmosphere as in the super lean combustion operation in the lean mode, whereby NOx is stored and the exhaust gas is purified. Is done. On the other hand, in an atmosphere where the oxygen concentration is reduced, nitrate stored in the storage-type NOx catalyst 25 reacts with CO in the exhaust to produce carbonate, and NOx is released, and the released NOx is reduced in the exhaust. It is reduced by the agent and the three-way catalyst 26. Accordingly, when NOx occlusion progresses in the occlusion-type NOx catalyst 25, the oxygen concentration is lowered by enriching the air-fuel ratio or performing additional fuel injection, etc., and CO is supplied into the exhaust gas. NOx is released from the gas to maintain the function.
[0024]
By the way, the sulfur component (SOx) contained in the fuel and the lubricating oil is also present in the exhaust gas, and the occlusion-type NOx catalyst 25 occludes SOx together with NOx occlusion in an oxygen concentration excess atmosphere. In other words, the sulfur component is oxidized to SOx, and a part of this SOx reacts with the storage agent that is originally used for NOx storage on the storage type NOx catalyst 25 to become a sulfate, and is stored in place of NOx. The NOx catalyst 25 is occluded.
[0025]
Further, the storage type NOx catalyst 25 tends to release the stored SOx when the oxygen concentration decreases. In other words, in an atmosphere where the oxygen concentration is reduced and CO is excessive, a portion of the sulfate stored in the storage-type NOx catalyst 25 reacts with CO in the exhaust, so that carbonate is easily generated and SOx is generated. Easily detached. However, sulfate has higher stability as a salt than nitrate, and only a part of it is decomposed only in an atmosphere in which the oxygen concentration is lowered. Therefore, it is necessary to simultaneously set the storage-type NOx catalyst 25 to a predetermined high temperature state. is there. Therefore, when the operation in which the catalyst temperature does not increase continues, the amount of sulfate remaining in the storage NOx catalyst 25 increases with time. Thereby, the storage capability of the storage-type NOx catalyst 25 decreases with time, and the performance as the storage-type NOx catalyst 25 deteriorates (S poisoning).
[0026]
In the present embodiment, in order to regenerate the NOx storage capacity, the catalyst state detection means estimates the catalyst temperature increase set temperature as an index that correlates with the S regeneration frequency, and the catalyst temperature Tcat becomes equal to or higher than the catalyst temperature increase set temperature. In this case (specific operation state), the temperature of the catalyst is raised and the air-fuel ratio is enriched to release the stored SOx to recover the NOx storage capacity. In this embodiment, the catalyst state detection means detects or estimates an index related to the sulfur release frequency in the catalyst, but the SOx stored in the storage-type NOx catalyst 25 instead of the S regeneration frequency. In this case, when the stored SOx amount exceeds a predetermined value (S poisoning progresses to some extent), the catalyst is heated and the air-fuel ratio is enriched to release SOx. do it.
[0027]
The catalyst regeneration control is executed so that the regeneration degree for releasing SOx from the storage-type NOx catalyst 25 is maintained within a predetermined range. That is, the temperature of the storage-type NOx catalyst 25 is higher than the activation temperature (for example, 250 to 350 ° C.) and is suitable for desorbing the stored SOx (for example, 650 to 800 ° C.) or lower. When the temperature reaches a set temperature (for example, 600 ° C.) or higher, the storage-type NOx catalyst 25 is heated and the air-fuel ratio is enriched to release SOx. In other words, when the original catalyst temperature is high to some extent, the catalyst is slightly raised in an assist manner to reach the catalyst temperature range where the S regeneration speed is fast, and SOx can be efficiently performed with a small temperature increase (a small degree of fuel consumption deterioration). It is what is released.
[0028]
Further, when SOx stored in the NOx storage catalyst 25 is raised in temperature and the air-fuel ratio is made rich to release SOx, SOx is released as SOx.₂In a reducing atmosphere where the exhaust gas is high in temperature and contains a large amount of hydrogen, carbon monoxide, hydrocarbons, etc., particularly in an atmosphere rich in hydrogen (hydrogen increases proportionally as the air-fuel ratio is rich) And hydrogen sulfide (H₂S) occurs and gives off odor. The higher the temperature rise maximum temperature of the catalyst, or the higher the temperature rise rate, the higher the SO₂Release amount of SO₂To H₂S is rapidly generated in large quantities. For this reason, in the present embodiment, the temperature control means performs the S regeneration frequency in the specific operation state in which the temperature of the storage NOx catalyst 25 is increased to the above-described predetermined temperature range when the exhaust air-fuel ratio is rich or stoichiometric. Accordingly, the maximum temperature rise of the catalyst or the rate of temperature rise is controlled so as to suppress the production of hydrogen sulfide.
[0029]
Here, the S purge control will be described in detail based on the flowchart shown in FIG. 2 and the time chart shown in FIG. As shown in FIG. 2, first, in step S1, the catalyst temperature Tcat of the storage NOx catalyst 25 is estimated from the exhaust gas temperature information detected by the high temperature sensor 24. In this case, as described above, the error between the high temperature sensor 24 and the actual catalyst temperature is corrected based on the temperature difference map set according to the target average effective pressure Pe and the engine rotational speed information Ne.
[0030]
Next, in step S2, the S regeneration frequency indicating the degree of regeneration (S regeneration) from the S poison of the storage NOx catalyst 25 is calculated (frequency calculation means). The calculation method will be described below. This S regeneration frequency is calculated based on the following equation (1).
S regeneration frequency (s / km) = 700 ° C conversion S regeneration time / travel distance (1)
Here, the 700 ° C. converted S regeneration time is the sum of the times obtained by converting the S regeneration time at the estimated catalyst temperature Tcat into the S regeneration time when the catalyst temperature Tcat of the storage NOx catalyst 25 is 700 ° C. By dividing the 700 ° C. converted S regeneration time by the travel distance, the S regeneration time per 1 km of travel distance, that is, the S regeneration frequency can be obtained. When this S regeneration frequency increases, SOx is well released (purged). It has been regenerated well from S poisoning.
[0031]
As a specific method of calculating the 700 ° C. converted S regeneration time, the S regeneration time is calculated based on the following formula (2).

Here, the S regeneration rate coefficient is for correcting that the S regeneration rate varies depending on the temperature of the occlusion-type NOx catalyst 25, and the S regeneration time at each catalyst temperature is converted into an S regeneration time equivalent to 700 ° C. Is for. Since the S regeneration rate increases exponentially as the temperature of the storage NOx catalyst 25 increases, it is set to 0 when the catalyst temperature Tcat of the storage NOx catalyst 25 is 580 ° C. or less, and at a temperature higher than that, It calculates with the following Formula (3) approximated by the exponential function.
S reproduction speed coefficient = exp {−kk × (1 / T₁)-(1 / T₀)} (3)
Here, kk is a predetermined coefficient set according to the S regeneration reaction of the storage-type NOx catalyst 25, T₁Is the catalyst temperature Tcat (K), T of the NOx storage catalyst 25₀Is 973 (700 + 273) (K).
In the present embodiment, calculation by an exponential function is not performed in the ECU 28, and a value calculated in advance is obtained from an S regeneration rate coefficient map for the stored catalyst temperature. The A / F coefficient described above indicates the degree of S regeneration by A / F, and is 0 when the air-fuel ratio is in the lean mode or when the fuel is cut, and 1 in other modes. The A / F coefficient may be classified more finely, and may be set to 0 when the air-fuel ratio is in the lean mode or when the fuel is cut, 2/3 in the stoichiometric F / B mode, and 1 in the rich (O / L) mode.
[0032]
The S regeneration frequency is reset at a predetermined value D (for example, 1.5 s / km) or more, and the 700 ° C. converted S regeneration time is 0 s and the travel distance is 0 km. That is, if the S regeneration frequency is equal to or higher than the predetermined value D, the storage type NOx catalyst 25 is sufficiently purged and the regeneration is stopped. A method for setting the threshold value D when resetting the S reproduction frequency may be as follows. That is, as shown in FIG. 4, it is known that the NOx emission value decreases as the S regeneration frequency increases, and the S regeneration frequency may be set to be equal to or less than the NOx emission regulation value. Although this threshold value D is influenced by the catalyst characteristics and the target value of the NOx amount, it is generally known that the threshold value D should be set to approximately 1.5 s / km or more.
[0033]
When the S regeneration frequency is obtained in this way, the temperature rise set temperature ZSTTEMP (° C.) is calculated in step S3. In this case, as shown in FIG. 5, a map of ZSTEMP with respect to the S reproduction frequency is set in advance. According to this map, when the S regeneration frequency is low, ZSTEMP is set to 600 ° C., and ZSTEMP increases as the S regeneration frequency increases, and the predetermined value D is set to 800 ° C. In other words, the temperature rise setting temperature ZSTTEMP is changed based on the S regeneration frequency.
[0034]
In step S4, it is determined whether or not the catalyst temperature Tcat of the storage type NOx catalyst 25 is equal to or higher than ZSTTEMP. If the catalyst temperature Tcat is equal to or higher than ZSTTEMP, the S regeneration frequency is somewhat low and the storage type NOx catalyst 25 is Since it is estimated that the temperature has risen to some extent and is easily purged with S, the process proceeds to step S5, and the control mode is switched to the S purge mode. As a result, removal of SOx stored in the storage-type NOx catalyst 25 (S purge control) is started.
[0035]
That is, in step S6, the storage timing NOx catalyst 25 is heated by controlling the ignition timing, that is, retarding. That is, by sufficiently raising the temperature of the exhaust gas flowing into the storage NOx catalyst 25 by the ignition timing retard, the storage NOx catalyst 25 is heated to a temperature suitable for S purge control (for example, 650 ° C. to 800 ° C.). Will be. In this case, the SO released by the S purge control₂Reacts in a reducing atmosphere as described above.₂S is generated, and at this time, the retard amount of the ignition timing retard is reduced according to the S regeneration degree (S regeneration frequency), or the change rate is slowed to increase the maximum temperature rise of the storage-type NOx catalyst 25. If the temperature is lowered or the heating rate is slowed, this SO₂Release amount increases at a stretch and H₂At the start of S purge control, where a large amount of S is likely to occur, SO₂Suppressing the release amount of H₂The generation concentration of S can be kept low. In general, H₂The higher the peak concentration of S, the more H₂The odor of S is likely to be a problem, but this control is particularly H₂This is effective for keeping the peak concentration of S low.
[0036]
The ignition timing is set so as to retard according to the following equation (4).

Here, ZSSA is set based on the retard map set in accordance with the target average effective pressure Pe and the engine rotational speed information Ne, and is an amount that takes into account the allowance amount to the retard amount that becomes the combustion limit. Is set. Further, as shown in FIG. 6, the reflection coefficient is set based on the estimated catalyst temperature, that is, a map corresponding to the catalyst temperature Tcat, and is set to 0 when the fuel is cut or when the throttle opening is equal to or larger than a predetermined value. Yes. Further, the reflection coefficient A is changed from 0 to 1 over a predetermined time (predetermined number of strokes). The smaller the S regeneration frequency at the start of the S purge control, the longer the predetermined time and the catalyst temperature increase rate. The temperature of the storage-type NOx catalyst 25 is slowly increased by lowering the temperature. The reflection coefficient B is obtained from a map of the S regeneration frequency at the start of the S purge control. The smaller the S regeneration frequency at the start of the S purge control, the smaller the reflection coefficient B and the final ignition timing. Reduce the retard amount and lower the maximum catalyst temperature.
[0037]
That is, in FIG. 7, the change in catalyst temperature with time (b) based on the result of execution of steps S1 to S4, the change in ignition timing (a) at the time of execution of S purge control after step S5, and H₂The time variation (c) of the generated concentration of S is shown as a time chart. In this case, the case where the ignition timing is retarded by a large amount immediately after the start of the S purge control (or even when it is gradually changed, as fast as several cycles) is indicated by a broken line, as in the prior art. A solid line indicates a case where the ignition timing is gradually shifted toward a predetermined small retard amount according to the S regeneration frequency as in the embodiment. As can be seen from this time chart, in the conventional control (broken line), the temperature of the storage NOx catalyst 25 is raised immediately after the start of the S purge control, and the catalyst temperature immediately rises to a predetermined high temperature. Large amount of H₂S was generated, but in the control (solid line) of the present embodiment, immediately after the start of the S purge control, the storage-type NOx catalyst 25 is gradually heated, and the catalyst temperature slowly rises to a predetermined temperature lower than the conventional temperature. A lot of H₂There is no occurrence of S, H immediately after the start of S purge₂The generation concentration of S can be lowered and maintained as it is.₂It is possible to reliably prevent the exhaust gas from giving off an odor by keeping the S concentration low.
[0038]
In particular, the ignition timing is gradually retarded to gradually increase the temperature of the storage NOx catalyst 25 and suppress the catalyst temperature.₂Keeping the generated concentration of S at a low value, SO₂Can be efficiently promoted. As a result, the S purge control is prevented from being prolonged as much as possible and the fuel consumption is preferably prevented from deteriorating.₂The S concentration can be kept low.
[0039]
In the above-described embodiment, the reflection coefficient A and the reflection coefficient B are set to open loop control using values previously determined by a map or the like, but may be feedback control. That is, the target catalyst temperature increase rate and the maximum catalyst temperature are first set according to the S regeneration frequency, and the set target temperature obtained from them and the current catalyst bottom temperature (estimated based on the output of the high temperature sensor 24). From the deviation, the ignition timing retard amount is changed by feedback control so that the estimated catalyst temperature becomes the set target temperature. As this feedback control method, general integral control, proportional control, differential control, etc. may be used in combination.
[0040]
When performing the S purge, the throttle valve 17 is operated in accordance with the ignition timing retard to adjust the throttle opening θth with respect to the base throttle opening, and the intake air amount is manipulated. The torque is reduced so that the torque is compensated for and the torque is controlled to be substantially constant. In this case, the throttle opening is set by the following equation (5).
Throttle opening = base throttle opening-ZSETV x reflection coefficient (5)
Here, ZSETV is set based on the throttle opening correction map set according to the target average effective pressure Pe and the engine speed information Ne, and the reflection coefficient is as described above. It is set based on the map shown in FIG. In this case, the throttle opening θth may be set with respect to the corrected ignition timing (ignition timing after retarding).
[0041]
Subsequently, in step S7, control is performed so that the air-fuel ratio becomes a predetermined rich A / F. That is, lean operation is prohibited and only stoichiometric F / B (including rich shift) or open loop rich operation is performed so that the air-fuel ratio becomes stoichiometric or rich.
[0042]
Returning to FIG. 2, when the catalyst temperature Tcat is lower than the predetermined value K (for example, 650 ° C.) after the predetermined set time C (for example, 45 seconds) has elapsed in Step S8, the storage type NOx is determined in Step S9. The temperature of the catalyst 25 is stopped and the air-fuel ratio is made lean to exit this routine. That is, when the catalyst temperature Tcat is not close to the optimum temperature for desorption of sulfur components (for example, 650 ° C. or higher) even in the S purge mode, the S purge mode is stopped in order to suppress fuel consumption deterioration. ing. On the other hand, if the catalyst temperature Tcat becomes equal to or higher than the predetermined value K before the predetermined time C elapses, the temperature rise of the storage NOx catalyst 25 and the enrichment of the air-fuel ratio are continued.
[0043]
In this way, in the S purge mode, the routine of the S purge control is repeated. However, when the SOx stored in the storage NOx catalyst 25 is released, the S regeneration frequency of the storage NOx catalyst 25 increases, and FIG. Based on the map, ZSTEMP also increases. When ZSTEMP exceeds the catalyst temperature Tcat, the catalyst temperature Tcat of the storage type NOx catalyst 25 becomes lower than ZSTTEMP in step S4, the control routine is exited, and the process proceeds to step S9 to stop the S purge control.
[0044]
The three-way catalyst 26 is an O 2 such as ceria or zirconia.₂Since it has a function of storing oxygen inside when it contains a storage agent, H generated in the storage-type NOx catalyst 25₂S is satisfactorily oxidized by oxygen inside the three-way catalyst 26. This also reliably prevents the exhaust gas from giving off an odor.
[0045]
By the way, in the above-described embodiment, during the S purge control, the ignition timing is gradually retarded to gradually raise the storage NOx catalyst 25 and the retard amount is limited to suppress the maximum catalyst temperature. For example, when the S purge control is started in step S6, the ignition timing of the S purge control is gradually retarded to raise the maximum catalyst temperature to the same catalyst temperature as before, or The retard amount for the purge control may be obtained as a fixed value lower than the conventional one, and the retard amount changing speed may be uniquely set according to the target retard amount. Even in this case, SO₂Conventionally, the discharge time of H is increased immediately after the start of S purge control.₂S to suppress the temporarily large amount of S, H₂The generation concentration of S can be made sufficiently low, and H₂It is possible to reliably prevent the exhaust gas from giving off an odor by keeping the S concentration low.
[0046]
In the above-described embodiment, the retard amount is continuously and smoothly shifted toward a predetermined value in accordance with the S regeneration frequency during the S purge control. However, the retard amount is divided into a plurality of steps (stepped). Also good. Further, the method for raising the temperature of the occlusion-type NOx catalyst 25 is not limited to the ignition timing retard, and may be, for example, two-stage injection in which additional fuel injection is performed in the expansion stroke after main fuel injection. The catalyst temperature increase temperature may be controlled by gradually increasing the additional fuel injection amount as the value of is smaller. Further, the lower the S regeneration frequency, the lower the maximum catalyst temperature if the final additional fuel injection amount is reduced. In addition, the fuel may be directly injected into the exhaust pipe 21 upstream of the occlusion-type NOx catalyst 25, or a heating means such as an electric heating catalyst or a burner may be used.
[0047]
Further, as a method of obtaining the catalyst temperature, in addition to the method of obtaining from the high temperature sensor 24 as in the above-described embodiment, the catalyst temperature is experimentally determined beforehand as a map of the target average effective pressure Pe (engine load) and the engine rotational speed Ne. The temperature may be obtained from a value stored in the ECU 28. Alternatively, the catalyst temperature may be obtained from a value obtained by experimentally storing the catalyst temperature in advance in the ECU 28 as a map for the vehicle speed. In this case, although the accuracy is somewhat lowered, there is an advantage that the cost is reduced because the high temperature sensor 24 is not necessary.
[0048]
Here, the switching control to the above-described S purge mode will be specifically described based on the time chart of FIG. As shown in FIG. 3, when the S purge mode is OFF and the S regeneration frequency is low, the temperature rise set temperature ZSTEMP is set to 600 ° C. Then, when the driving temperature of the driver, for example, during acceleration or traveling on a suburban road, highway or mountain road, the catalyst temperature Tcat rises and this catalyst temperature Tcat exceeds ZSTEMP, the S purge mode is turned on (step S4). Yes). Then, in order to retard the ignition timing to raise the temperature of the storage type NOx catalyst 25 and to enrich the air-fuel ratio, SOx stored in the storage type NOx catalyst 25 is released. When the S purge mode continues, S regeneration proceeds, the 700 ° C. converted S regeneration time increases, and the S regeneration frequency increases. When this S regeneration frequency increases, ZSTEMP increases based on the map of FIG. Exceeds catalyst temperature Tcat.
[0049]
Then, the S purge mode is turned off (No in step S4), and the temperature of the storage NOx catalyst 25 is stopped, so that the catalyst temperature Tcat is lowered. Thereafter, when the travel distance is extended while the catalyst temperature Tcat is low, the S regeneration frequency is reduced, and ZSTEMP is lowered to 600 ° C.
[0050]
Further, when the catalyst temperature Tcat rises again due to the operating state of the driver and the catalyst temperature Tcat exceeds ZSTTEMP, the S purge mode is turned ON (Yes in step S4). At this time, for example, if the driver is in an operation state in which acceleration / deceleration is repeated in a place such as an urban area, the storage-type NOx catalyst 25 is unlikely to reach a high temperature state necessary for releasing SOx. That is, in this case, although the catalyst temperature Tcat initially exceeds ZSTTEMP (600 ° C.), it continues to be lower than 650 ° C. In this temperature range, although S regeneration is performed, the S regeneration efficiency is low, and the degree of deterioration of fuel consumption Although there are many, S reproduction is not so much. For this reason, when the set time C has elapsed in this state, the S purge mode is turned OFF (No in step S4), so that the ignition timing retard and enrichment are stopped, and the fuel efficiency is reduced in the temperature range where the S regeneration efficiency is low. Deterioration can be suppressed.
[0051]
In the S purge mode, the air-fuel ratio is set to the rich air-fuel ratio mode and the transition to the lean air-fuel ratio mode is prohibited. However, when the storage NOx catalyst 25 is at a high temperature, measures against thermal deterioration of the storage NOx catalyst 25 are performed. Therefore, the lean air-fuel ratio mode may already be prohibited.
[0052]
Thus, in the present embodiment, control is performed so that the regeneration degree for releasing SOx from the storage-type NOx catalyst 25 is maintained within a predetermined range, that is, the catalyst temperature Tcat of the storage-type NOx catalyst 25 is set to the activation temperature (for example, 250 to When the temperature is higher than 350 ° C.) and suitable for desorbing the stored SOx (for example, 650 ° C. to 800 ° C.) or lower than the temperature setting temperature ZSTEP set lower than that, ignition is performed The timing is retarded to raise the temperature of the storage NOx catalyst 25, and the air-fuel ratio is enriched to release SOx.
[0053]
The frequency at which the storage-type NOx catalyst 25 is brought into a state in which the sulfur component is released by the forced S purge control is determined from the storage-type NOx catalyst 25 by H₂Discovering that there is a correlation with the characteristics of S being generated, and gradually shifting the ignition timing toward a predetermined small retard amount according to the frequency at which this sulfur component is released (S regeneration frequency), The catalyst temperature and / or the rate of temperature increase are controlled so that the temperature of the storage NOx catalyst 25 is gradually raised to raise the catalyst temperature to a temperature lower than the conventional temperature.
[0054]
As described above, in this embodiment, when the storage NOx catalyst 25 is in a high temperature state to some extent, the storage NOx catalyst 25 is assisted to raise the temperature and release SOx in a catalyst temperature range suitable for SOx release. Playing. For this reason, the S regeneration is not forcedly performed when it takes a long time for the storage-type NOx catalyst 25 to rise to a temperature optimum for SOx release in a low temperature state. As a result, it is not necessary to perform rich enrichment and ignition timing retard for a long period of time and a large retard for having to be greatly raised from a low temperature, thereby suppressing deterioration in fuel consumption.
[0055]
During this S regeneration, a large amount of H immediately after the ignition timing retard₂There is no occurrence of S, H at this time₂Lower the generation concentration of S and H₂S can be kept at a low concentration, and H₂While the S concentration is kept low to reliably prevent the exhaust gas from giving off a strange odor, it is possible to stably regenerate the catalyst by suppressing the regeneration time of the storage NOx catalyst 25 from being prolonged.
[0056]
Further, when the catalyst temperature Tcat does not approach the optimum temperature for desorption of sulfur components (for example, 650 ° C. or more) in the S purge mode, the S purge mode is stopped after the set time C has elapsed. Therefore, for example, when the vehicle is in an operation state where acceleration / deceleration is repeated in a place such as an urban area, the storage-type NOx catalyst 25 is unlikely to reach a high temperature state that is optimal for releasing SOx, so that the set time C has elapsed. Thus, the S purge mode is stopped, and in this manner, in a temperature range where the S regeneration efficiency is not good, it is possible to suppress deterioration of fuel consumption by limiting the enrichment and continuation of the temperature rise.
[0057]
In the present embodiment, the temperature of the storage NOx catalyst 25 is set to an activation temperature of 250 ° C. to 350 ° C., and a temperature suitable for desorbing the stored SOx is 650 ° C. to 800 ° C. (more preferably 700 ° C. As described above, the temperature rise set temperature ZSTTEMP is changed from the S regeneration frequency starting from 600 ° C., but each temperature may be appropriately set depending on the characteristics of the storage NOx catalyst 25, the engine configuration, the exhaust temperature, or the like. Is.
[0058]
Further, in the above-described embodiment, during the forced S purge control, when the exhaust air-fuel ratio is rich or stoichiometric, the S regeneration frequency is increased in a specific operation state in which the temperature is increased so that the storage NOx catalyst 25 is in a predetermined temperature range. The maximum temperature of the catalyst or the rate of temperature increase was controlled so as to suppress the production of hydrogen sulfide according to the above, but the S occlusion amount occluded in the occlusion-type NOx catalyst 25 was estimated instead of the S regeneration frequency, The maximum temperature rise or the temperature rise rate of the catalyst may be controlled so as to suppress the production of hydrogen sulfide in accordance with the S occlusion amount, and will be described briefly.
[0059]
Here, a method of estimating the storage amount of SOx (poisoned S amount Qs) stored in the storage-type NOx catalyst 25 will be described. The poisoning S amount Qs is basically set based on the fuel injection integrated amount Qf, and is calculated by the following equation for each execution cycle of a fuel injection control routine (not shown).
Qs = Qs (n−1) + ΔQf · K−Rs (6)
Here, Qs (n−1) is the previous value of the poisoning S amount, ΔQf is the fuel injection integrated amount per execution cycle, K is the correction coefficient, and Rs is the released S amount per execution cycle. That is, the current poisoning S amount Qs is obtained by correcting and integrating the fuel injection integrated amount ΔQf per execution cycle with the correction coefficient K, and subtracting the released S amount Rs per execution cycle from the integrated value. It is done.
[0060]
The correction coefficient K corresponds to, for example, the S poisoning coefficient K1 corresponding to the air-fuel ratio A / F, the S poisoning coefficient K2 corresponding to the S content in the fuel, and the catalyst temperature Tcat as shown in the following equation. It consists of the product of three correction coefficients of S poison coefficient K3.
K = K1, K2, K3 (7)
Further, the released S amount Rs per execution cycle is calculated from the following equation.
Rs = α · R1 · R2 · dT (8)
Here, α is the release rate (set value) per unit time, dT indicates the execution cycle of the fuel injection control routine, and R1 and R2 are the discharge capacity coefficient and air-fuel ratio A corresponding to the catalyst temperature Tcat, respectively. The release capacity coefficient according to / F is shown.
[0061]
Therefore, if the change rate of the ignition timing retard is slowed according to this S occlusion amount (poisoned S amount Qs) to slow down the maximum temperature rise and temperature rise rate of the storage NOx catalyst 25, SO₂Release amount increases at a stretch and H₂At the start of S purge control, where a large amount of S is likely to occur, SO₂Suppressing the release amount of H₂The generation concentration of S can be kept low.
[0062]
Further, the method of enriching the air-fuel ratio is not limited to the method of changing the target A / F in an open loop.₂In stoichiometric feedback control using a sensor, a method may be used in which an integral gain (coefficient) for feedback control is changed to a large value on the rich side and a small value on the lean side. In this case, the average air-fuel ratio is gradually enriched according to the S regeneration frequency by setting the speed of the integral gain or proportional gain according to the S regeneration frequency, or the gain value itself is varied according to the S regeneration frequency. By setting, the average air-fuel ratio itself may be controlled to become a rich air-fuel ratio according to the S regeneration frequency. Further, the aforementioned two-stage injection or direct fuel injection into the exhaust pipe 21 may be used as the enrichment means.
[0063]
Further, the present invention is characterized by controlling the temperature of the catalyst or the heating rate so as to suppress the generation of hydrogen sulfide according to the index related to the sulfur release frequency or the amount of stored sulfur during the S regeneration. As in the above-described embodiment, the S regeneration frequency may not be used as a condition for starting the S purge control. For example, the S purge control may be started on the condition that the S occlusion amount (poisoned S amount Qs) occluded in the occlusion-type NOx catalyst 25 in order to regenerate the NOx occlusion capacity becomes a predetermined amount or more. Good.
[0064]
By the way, even when the storage type NOx catalyst 25 is enriched when the vehicle is accelerated and when the natural S purge is caused by the catalyst temperature, SOx is SO.₂In a reducing atmosphere where the exhaust gas is at a high temperature and a large amount of hydrogen, carbon monoxide, hydrocarbons, etc. are present, hydrogen sulfide (H₂S) occurs and gives off odor. The catalyst is immediately heated (the catalyst heating temperature is faster), and the higher the catalyst temperature, the more reducing agent such as hydrogen and SO.₂To H₂S is rapidly generated in large quantities. FIG. 8 and FIG. 9 show another embodiment of the present invention. When the natural S purge is performed, that is, when the catalyst is rich at a high temperature, the richness is further increased and the exhaust air-fuel ratio is increased. The temperature of the catalyst or the heating rate is suppressed by the fuel cooling effect due to the enrichment of the catalyst.
[0065]
That is, regarding the natural S purge control, as shown in the flowchart of FIG. 8, in step S11, the catalyst temperature Tcat of the storage NOx catalyst 25 is estimated from the exhaust gas temperature information detected by the high temperature sensor 24. In step S12, whether or not the final target A / F determined according to the target average effective pressure Pe and the engine speed Ne is switched from a lean air-fuel ratio to a rich air-fuel ratio (which may include a stoichiometric air-fuel ratio); That is, it is determined whether or not the final target A / F corresponding to the engine operating state is switched to the rich air-fuel ratio in accordance with the acceleration operation of the driver. Otherwise, the process returns as it is and the rich air-fuel ratio is switched. If yes, go to Step S13.
[0066]
In step S13, it is determined whether or not the estimated catalyst temperature Tcat is equal to or higher than a predetermined high temperature TcatH (for example, 650 ° C.). If the catalyst temperature Tcat is lower than the high temperature TcatH, the process proceeds to step S14 and the normal pattern is reached. On the other hand, if the catalyst temperature Tcat is equal to or higher than the high temperature TcatH, the process proceeds to step S15 to enrich toward the target A / F larger than the final target A / F. The large target A / F at this time may be set based on operating conditions such as the current catalyst temperature Tcat, engine load, and engine speed. And after step S14, S15, it returns. Here, in the above-described steps S12 and S13, it is determined whether or not the operating state of the engine 11 is an enriched operation and the storage NOx catalyst 25 has reached a high temperature state in which the stored sulfur component can be released.
[0067]
When the rich air-fuel ratio is achieved when the catalyst temperature Tcat exceeds the predetermined high temperature TcatH, the SOx stored in the storage-type NOx catalyst 25 becomes SO.₂At the same time released as H₂A large amount of S is generated. Therefore, under the enrichment operation such as when the vehicle is accelerated, the enrichment is performed toward the target rich air-fuel ratio larger than the final target A / F as in step S15 described above. By suppressing the temperature of the NOx catalyst 25 or the temperature rising rate to a low level,₂At the start of a natural S purge where a large amount of S is likely to occur,₂Reduce the amount of S generated. H₂H at the start of natural S purge, where a large amount of S is likely to occur₂The S concentration can be kept low.
[0068]
That is, FIG. 9 shows the time variation (b) of the catalyst temperature based on the time variation (a) of the target A / F during acceleration operation and H₂The time variation (c) of the amount of S generated is shown as a time chart. In this case, a predetermined rich air-fuel ratio AF from when the final target A / F is switched as in the conventional case (normal enrichment).₁The case of (for example, value 14) is indicated by a broken line, and the case of enrichment toward a target rich air-fuel ratio larger than the final target A / F as in the present embodiment is indicated by a solid line. As can be seen from this time chart, in the conventional control (broken line), a large amount of H is temporarily stored immediately after the final target A / F is switched.₂S was generated, but in the control (solid line) of this embodiment, H immediately after switching₂The generation concentration of S can be lowered and maintained as it is, and even in the case of enrichment operation, H₂It is possible to reliably prevent the exhaust gas from giving off an odor by keeping the S concentration low.
[0069]
In the above-described embodiment, the current catalyst temperature Tcat of the NOx storage catalyst 25 is estimated in step S11, and the catalyst temperature Tcat and the high temperature TcatH are compared in step S13 to achieve a target A / F or higher. However, there is a control delay, an exhaust system delay, a temperature rise delay due to a large heat capacity of the catalyst, etc., so in the control based on the current catalyst temperature Tcat, the catalyst There are cases where the cooling due to the increase in the target A / F is not in time for the temperature rise. Therefore, the catalyst of the NOx storage catalyst 25 is determined based on the current catalyst temperature Tcat, the target A / F calculated in step S11, and the operating conditions (load / rotation speed, vehicle speed, water temperature, etc.). A predicted temperature maximum value may be calculated, and when the predicted maximum catalyst temperature is equal to or greater than a predetermined value, the temperature may be changed to be richer than the target A / F in step S15. In step S13, the current catalyst temperature Tcat, target A / F calculated in step S11, operation conditions (load / rotation speed, vehicle speed, water temperature) are substituted for the predicted maximum catalyst temperature of the storage NOx catalyst 25. Etc.), the predicted catalyst temperature increase rate of the storage-type NOx catalyst 25 is calculated, and when the estimated catalyst temperature increase rate is equal to or greater than a predetermined value, the target A / F is enriched in step S15. May be.
[0070]
Furthermore, in step S13, when the catalyst maximum temperature predicted value is equal to or greater than a predetermined value and the catalyst temperature increase rate predicted value is equal to or greater than a predetermined value, the target A / F may be enriched in step S15. In step S13, when the catalyst maximum temperature predicted value is equal to or greater than a predetermined value or the catalyst temperature increase rate predicted value is equal to or greater than a predetermined value, the target A / F may be enriched in step S15. Further, “when the S reproduction frequency is equal to or lower than a predetermined value” may be added as the determination condition in step S13. At this time, as a condition for ending “enrichment beyond the target A / F” in step S15, “when the S reproduction frequency exceeds a predetermined value” may be set. Furthermore, “target A / F may be a predetermined A / F (A / F having a large fuel cooling effect, for example, A / F = 13) may be added as a determination condition in step S13.
[0071]
Although the temperature or temperature increase rate of the storage-type NOx catalyst 25 is kept low due to the fuel cooling effect due to enrichment, in addition to this method, engine cooling water can be flown around the catalyst, or separately for the catalyst. A cooling means may be provided.
[0072]
Thus, in the present embodiment, when the storage NOx catalyst 25 is enriched at a high temperature and the sulfur component stored naturally is released from the catalyst, the air-fuel ratio is rich at a value larger than the target A / F. To make it. Therefore, a large amount of H with natural S purge₂There is no phenomenon in which S suddenly occurs.₂Reduce the generation concentration of S and H₂S concentration can be kept small, H₂It is possible to reliably prevent the exhaust gas from giving off an odor by keeping the S concentration low.₂The control for preventing the occurrence of a large amount of S is effective when applied to a case where the S purge is naturally performed due to enrichment associated with acceleration of the vehicle.
[0073]
In the above-described embodiment, H is controlled by controlling the catalyst temperature or the heating rate based on the S regeneration frequency or the S occlusion amount.₂Although the release of S is controlled, based on these methods and the method of gradually changing the A / F during the S purge based on the S regeneration frequency or the S occlusion amount, or on the basis of the S regeneration frequency or the S occlusion amount You may combine the method of changing the richness of A / F at the time of S purge.
[0074]
Further, in each of the above-described embodiments, the engine 11 is a cylinder injection gasoline engine. However, the present invention is not limited to this, and an intake pipe injection gasoline engine may be used. In the above embodiment, the three-way catalyst 22 is used. However, the three-way catalyst 22 is not necessarily provided, and the present invention can be suitably realized without the three-way catalyst 22. Further, although the storage-type NOx catalyst 25 is taken up as the catalyst for storing and releasing the sulfur component in the above-described embodiment, other catalysts may be used, and an adsorption-type that directly reduces the adsorbed NOx by contact. It may be a NOx catalyst or a three-way catalyst that easily stores sulfur components. Furthermore, as an engine, it is also possible to apply to a diesel engine.
[0075]
【The invention's effect】
  As described above in detail in the embodiment, according to the exhaust gas purification apparatus for an internal combustion engine of the present invention,The catalyst temperature is equal to or higher than the preset catalyst temperature rise temperature.In a specific operating state, the catalyst temperature or the heating rate is controlled so as to suppress the production of hydrogen sulfide in accordance with an index related to the sulfur release frequency or sulfur storage amount in the catalyst.₂Prevents the generation of a large amount of S at a time to keep the emission concentration low and prevents the exhaust gas odor, while reducing the catalyst regeneration time and enabling stable regeneration of the catalyst device. can do.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention.
FIG. 2 is a flowchart of forced S purge control by the exhaust gas purification apparatus of the present embodiment.
FIG. 3 is a time chart of S purge control.
FIG. 4 is a graph showing a relationship between S regeneration frequency and NOx emission amount.
FIG. 5 is a graph showing a temperature rise setting temperature with respect to S regeneration frequency.
FIG. 6 is a graph showing a reflection coefficient with respect to an estimated catalyst temperature value.
FIG. 7 shows the H with respect to the time variation of the ignition timing retard amount and the catalyst temperature in the forced S purge control.₂It is a time chart showing the time change of S generation density.
FIG. 8 is a flowchart of natural S purge control according to another embodiment of the present invention.
FIG. 9 shows the target A / F and H with respect to the time change of the catalyst temperature in the natural S purge control.₂It is a time chart showing the time change of S generation density.
[Explanation of symbols]
11 Engine (Internal combustion engine)
21 Exhaust pipe (exhaust passage)
22 Three-way catalyst
23 Exhaust purification catalytic equipment
24 High temperature sensor
25 NOx storage catalyst (catalyst)
26 Three-way catalyst
27 NOx sensor
28 Electronic control unit, ECU (catalyst state detection means, specific operation state detection means, temperature control means)

Claims (1)

A characteristic that is provided in the exhaust passage of the internal combustion engine and stores the sulfur component in the exhaust gas , and also releases the stored sulfur component when the temperature is equal to or higher than a predetermined catalyst temperature rise temperature and the exhaust air-fuel ratio is rich or stoichiometric. A catalyst having
Catalyst state detection means for detecting or estimating an index related to sulfur release frequency or sulfur storage amount in the catalyst;
A specific operation state detecting means for detecting a specific operation state in which the temperature of the catalyst is equal to or higher than the predetermined catalyst temperature increase set temperature ;
Temperature control means for controlling the temperature of the catalyst or the rate of temperature rise so as to suppress the production of hydrogen sulfide according to the index when the specific operating state detecting means detects the specific operating state. An exhaust gas purification apparatus for an internal combustion engine characterized by the above.

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