JP4304793B2

JP4304793B2 - Air-fuel ratio control device for internal combustion engine

Info

Publication number: JP4304793B2
Application number: JP32874099A
Authority: JP
Inventors: 司窪島
Original assignee: Denso Corp
Current assignee: Denso Corp
Priority date: 1999-11-18
Filing date: 1999-11-18
Publication date: 2009-07-29
Anticipated expiration: 2019-11-18
Also published as: DE10057013B4; DE10057013A1; JP2001140687A

Description

【０００１】
【発明の属する技術分野】
本発明は、内燃機関の燃料噴射量あるいはＥＧＲ量を調整して、空燃比を所定値に制御する内燃機関の空燃比制御装置に関する。
【０００２】
【従来の技術】
内燃機関の燃料噴射量は、機関出力やエミッションの排出量に、また内燃機関のＥＧＲ量はエミッションの排出量に直接影響を与えるため、燃料噴射量及びＥＧＲ量を精度良く所定値にコントロールすることが非常に重要である。しかし、これらは、燃料噴射弁やＥＧＲ弁等の特性のバラツキや環境条件の変化、あるいは経時劣化等により変化する。
一方、エミッションは、排気中の酸素濃度（空燃比）と強い相関がある。特にディーゼルエンジンでは、排気中のスモーク（黒煙）を低減することが重要課題であるが、スモークは、燃料燃焼時にシリンダ内部が局所的に酸素不足となることにより発生するため、その発生量は排気中の酸素濃度と非常に強い相関を有している。従って、内燃機関におけるエミッション（特にディーゼルエンジンのスモーク）を所定値とするための指標として空燃比を用いることは極めて有用である。
【０００３】
そこで、空燃比に基づいて燃料噴射量及びＥＧＲ量をフィードバック制御する従来技術がある。例えば特開平２−６１３４７号公報に開示されたディーゼルエンジンの燃料噴射制御装置では、空燃比が所定値となるようにＥＧＲ領域ではＥＧＲ量を調整し、非ＥＧＲ領域では全気筒の平均燃料噴射量を調整することでエミッションを所定値に制御している。
しかし、ディーゼルエンジンの非ＥＧＲ領域（高負荷域）あるいはＥＧＲ領域の高負荷運転下においては、センサで検出される空燃比（全気筒の平均空燃比）が目標値となっていても、気筒間で空燃比にばらつきがあると、図６に示すように、そのばらつきが大きくなる程、スモーク量が急激に増加するという問題がある。この問題に対し、前記従来技術では気筒間空燃比のばらつきを修正できないため、スモークを十分に低減することが困難であった。
【０００４】
一方、従来技術として特開平９−２０３３３７号公報に開示された内燃機関の空燃比制御装置がある。この従来技術は、気筒間空燃比のばらつきを解消するもので、空燃比センサの出力と各気筒の排気タイミングとから気筒別に空燃比を算出し、その算出された気筒別の空燃比に基づいて、気筒毎に燃料噴射量をフィードバック制御している。
しかし、この従来技術をディーゼルエンジンに適用した場合、エンジンの低負荷時には排気中の酸素濃度が高いため、その中で気筒間空燃比のずれを検出するには、非常に高い空燃比の検出精度が必要となり、これは実用上困難である。
【０００５】
また、内燃機関の空燃比をセンサで検出する場合、以下の問題が生じる。
▲１▼内燃機関の空燃比は、例えば限界電流式の酸素濃度センサにより検出するのが一般的であり、このセンサの出力は、原理上、雰囲気圧力すなわち排気圧力の大きさにより変化し、排気圧力が高いほど大きくなる。従って、この方式のセンサをディーゼルエンジンに適用した場合、ディーゼルエンジンは圧縮比が高いことから、排気圧力の脈動が大きいという特徴を有するため、たとえ空燃比が一定でも、センサ出力には排気圧脈動に起因する１サイクル／気筒数に対応した周期的な変動（例えば４気筒内燃機関では１８０クランク角度毎の変動）が発生する。この変動の大きさは、エンジンの運転条件によって変化するため、正確な空燃比に関する情報を得ることが困難である。特に、１個のセンサの出力から気筒毎の空燃比を分離して求めようとする場合には、上記問題が顕著となり、気筒毎の空燃比の正確な検出及びその検出した空燃比に基づく制御が非常に困難となる。
【０００６】
▲２▼内燃機関の排気弁から排出された排気が排気管中に設けたセンサに到達するまでには時間がかかるため、時間的な遅れが発生し、この遅れは内燃機関の運転状態によって変化する。従って、この遅れを考慮しないと、１個のセンサの出力から気筒毎の空燃比を分離して求めようとする場合には、気筒毎の空燃比を正確に検出できない。
▲３▼特にディーゼルエンジンにおいては、その燃焼が燃料の自己着火によるため、サイクル毎の時間的あるいは空間的な燃焼の安定性が低く、得られる空燃比の情報がサイクル毎にばらつく。
【０００７】
▲４▼特にディーゼルエンジンでは、例えば高地において大気圧が変化したり、噴射系の仕様や特性により燃料噴射圧力が変化することでシリンダ内の空気が有効に利用される度合い（即ち、空気利用率）が変化すると、図３及び図４に示すように、空燃比とスモーク発生量との関係が変化してしまう。従って、これらの変化によらず目標とする空燃比を一定とすると、スモーク量を所定値に制御できない。
以上の説明のとおり、前記従来技術を用いて、検出した空燃比を基にディーゼルエンジンのエミッションを制御しようとした場合に多くの問題が存在する。
【０００８】
【発明が解決しようとする課題】
本発明は、上記の問題点を解決するためのものであり、内燃機関の各運転状態に応じて空燃比検出手段の出力を適切に処理し、且つ気筒毎の燃料噴射量及びＥＧＲ量の制御により空燃比を所望値に制御することで、内燃機関のエミッション（特にディーゼルエンジンにおけるスモーク）を低減できる内燃機関の空燃比制御装置を提供することにある。
【０００９】
【課題を解決するための手段】
（請求項１の手段）
空燃比制御手段は、ＥＧＲ領域のうち比較的負荷が低い領域では、第２の特性値に基づいてＥＧＲ量を補正し、ＥＧＲ領域のうち比較的負荷が高い領域では、第１の特性値に基づいて気筒毎の燃料噴射量を補正するとともに、第２の特性値に基づいてＥＧＲ量を補正し、非ＥＧＲ領域では、第１の特性値に基づいて気筒毎の燃料噴射量を補正するとともに、第２の特性値に基づいて平均噴射量を補正する。
【００１０】
非ＥＧＲ領域（高負荷領域）及びＥＧＲ領域のうち比較的高負荷領域では、図６に示したように、気筒間空燃比のばらつきが大きくなると、スモークの発生量が急激に増加するため、気筒毎の燃料噴射量を補正して、気筒間空燃比のばらつきを抑える必要がある。ここで、特にディーゼルエンジンでは、高負荷時の方が、低負荷時と比較して排気中の酸素濃度が低いため、各気筒毎の空燃比のずれを検出するために、極めて高い検出精度が要求されることはなく、実用的な空燃比検出手段の出力に基づいて、各気筒毎の空燃比に対応する第１の特性値を算出し、その第１の特性値に基づいて気筒毎の燃料噴射量を補正することができる。
【００１１】
一方、ＥＧＲ領域のうち比較的低負荷領域では、前述したように排気中の酸素濃度が高く、その中で空燃比のずれを検出するためには、非常に高い空燃比の検出精度が必要となるため、気筒毎に空燃比を検出することは実用上困難である。しかし、低負荷領域では、図６に示したように、気筒毎の空燃比にばらつきが生じても、高負荷時と比較してスモークの発生量が低く、且つ気筒間空燃比のばらつきが大きくなってもスモーク発生量は殆ど変化しない。従って、気筒毎の噴射量補正を行わず、ＥＧＲ量を補正して気筒間の平均空燃比を制御する。
これにより、実用可能な精度の空燃比検出手段で、内燃機関の運転状態に応じた適切な気筒毎あるいは平均の空燃比制御が可能となり、エミッション改善が可能となる。
【００１２】
ＥＧＲ領域のうち比較的低負荷領域では、目標空燃比に対応する目標値と、全気筒の平均空燃比に対応する第２の特性値との差が小さくなるようにＥＧＲ量を補正している。これにより、気筒間の平均空燃比のずれ（目標空燃比に対するずれ）に起因するエミッションの悪化を改善できる。
【００１３】
ＥＧＲ領域のうち比較的高負荷領域では、第１の特性値の気筒間のばらつきが所定値以上と判断されていた場合、気筒毎の空燃比に対応する第１の特性値のばらつきが各気筒間で小さくなるように気筒毎の燃料噴射量を補正し、各気筒の第１の特性値を揃えたその後、目標空燃比に対応する目標値と第２の特性値との差が小さくなるようにＥＧＲ量を補正している。これにより、気筒間空燃比のばらつき、及び気筒間の平均空燃比のずれ（目標空燃比に対するずれ）に起因するエミッションの悪化を改善できる。
【００１４】
また、気筒毎の燃料噴射量を補正した後、平均空燃比と目標空燃比とのずれが小さくなるようにＥＧＲ量を補正することにより、空燃比の補正中にスモーク発生量が増加することを防止できる。即ち、内燃機関の負荷が大きいため、スモークが発生しやすい状態で、且つ気筒間空燃比がばらついたままで、全気筒に対するＥＧＲ量を先に補正すると、全気筒の平均空燃比を低く修正する場合には、空燃比のばらつきにより酸素が不足している気筒では、さらに酸素不足状態となり、多量のスモークが発生することとなる。これに対し、先ず気筒毎の燃料噴射量を調整して気筒毎の空燃比を補正した後、全気筒に対するＥＧＲ量を補正すれば、上記の不具合を解消することができる。
【００１５】
非ＥＧＲ領域では、第１の特性値の気筒間のばらつきが所定値以上と判断されていた場合、気筒毎の空燃比に対応する第１の特性値のばらつきが各気筒間で小さくなるように気筒毎の燃料噴射量を補正し、各気筒の第１の特性値を揃えたその後、目標空燃比に対応する目標値と第２の特性値との差が小さくなるように気筒間の平均噴射量を補正している。これにより、気筒間空燃比のばらつき、及び気筒間の平均空燃比のずれ（目標空燃比に対するずれ）に起因するエミッションの悪化を改善できる。
また、気筒毎の燃料噴射量を補正した後、平均空燃比と目標空燃比とのずれが小さくなるように気筒間の平均噴射量を補正することにより、空燃比の補正中にスモーク発生量が増加することを防止できる。
【００１６】
（請求項２の手段）
請求項１に記載した内燃機関の空燃比制御装置において、
気筒毎空燃比算出手段は、内燃機関の１サイクル中に複数回サンプルした空燃比検出手段の出力を内燃機関の１サイクル／気筒数に対応した区間毎に平均化して第１の特性値を算出すると共に、平均化する区間の開始タイミングを内燃機関の運転条件毎に変更する。これにより、一般的な限界電流式の酸素濃度センサを空燃比検出手段として用いた場合に、排気圧の脈動が大きいディーゼルエンジンにおいても排気圧脈動の影響を除去した上で、正確な気筒毎の空燃比を求めることが可能となる。また、排気が空燃比検出手段に到達するまでの遅れが内燃機関の運転状態に応じて変化することを考慮するため、運転状態によらず正確な気筒毎の空燃比を求めることが可能となる。
【００１８】
（請求項３の手段）
請求項２に記載した内燃機関の空燃比制御装置において、
気筒毎空燃比算出手段は、区間毎に得られた第１の特性値を、対応する気筒毎に複数サイクルに渡って平均化して各気筒の特性値とする。これにより、その燃焼が燃料の自己着火によるために燃焼毎の時間的あるいは空間的な安定性が低く、得られる空燃比の情報がサイクル毎にばらつくディーゼルエンジンにおいても、正確な空燃比を検出することが可能となる。
【００１９】
（請求項４の手段）
請求項２または３に記載した内燃機関の空燃比制御装置において、
気筒毎空燃比算出手段は、空燃比検出手段の出力を平均化する区間の開始タイミングを、内燃機関の回転数あるいは負荷が大きいほど早期とする。この場合、内燃機関の回転数および負荷が大きいほど、排気が空燃比検出手段に到達するまでの遅れが小さくなることを考慮するため、内燃機関の運転状態によらず正確な気筒毎の空燃比を求めることが可能となる。
【００２０】
（請求項５の手段）
請求項１〜４に記載した何れかの内燃機関の空燃比制御装置において、
目標空燃比算出手段は、吸気圧検出手段の出力が小さいほど目標空燃比（目標とする排気酸素濃度）を大きくする。
高地などで大気圧が低下し、それに伴って内燃機関、特にディーゼルエンジンの吸気圧が低下すると、空燃比（排気中の酸素濃度）が同一でも、シリンダ内に吸入される酸素の絶対量が低下する。このため、燃料燃焼時のシリンダ内の酸素利用度が低下し、結果的にシリンダ内で局所的に酸素不足となる領域が増加し、スモーク発生量が増加してしまう。これに対し、本発明によれば、吸気圧が小さいほど、目標空燃比を大きくするために、上記の不具合を防止してスモーク発生量を所定値以下とすることが可能となる。
【００２１】
（請求項６の手段）
請求項１〜５に記載した何れかの内燃機関の空燃比制御装置において、
目標空燃比算出手段は、噴射圧検出手段の出力が小さいほど目標空燃比を大きくする。内燃機関の特性変化等の理由により、燃料噴射圧力が低下すると、空燃比（排気中の酸素濃度）が同一でも、燃料噴霧の微粒化および空気との混合状態が悪化する。このため、燃料燃焼時のシリンダ内の酸素利用度が低下し、結果的にシリンダ内で局所的に酸素不足となる領域が増加し、スモーク発生量が増加してしまう。
これに対し、本発明によれば、噴射圧が小さいほど目標空燃比を大きくするために、上記の不具合を防止してスモーク発生量を所定値以下とすることが可能である。
【００２２】
【発明の実施の形態】
次に、本発明を４気筒のディーゼルエンジンに適用した実施形態を図面に基づいて説明する。
図２はディーゼルエンジンの空燃比制御装置のシステム図である。
本発明の内燃機関であるディーゼルエンジン１は、各気筒毎にそれぞれ電磁弁式の燃料噴射弁２が取り付けられている。燃料噴射弁２は、高圧燃料ポンプ３から圧送された燃料を蓄圧するコモンレール４（蓄圧室）を介して高圧燃料が供給され、電磁弁の開閉動作に伴って気筒内への燃料噴射を行う。
コモンレール４には、コモンレール４の内圧（噴射圧力）を検出する噴射圧センサ５が設置されている。
【００２３】
ディーゼルエンジン１には、各気筒内に空気を導入する吸気通路と、各気筒内での燃焼によって生じた排気ガスを排出する排気通路が接続されている。
吸気通路は、図示しないエアクリーナを介して空気を導入する吸気管６と、この吸気管６と各気筒とを分岐接続する吸気マニホールド７を有し、吸気管６には吸気圧センサ８が設置されている。
排気通路は、大気に開口する排気管９と、各気筒より排出される排気ガスを排気管９へ送る排気マニホールド１０を有し、排気管９には排気中の酸素濃度（空燃比）を検出する酸度濃度センサ１１（本発明の空燃比検出手段）が設置されている。
【００２４】
酸素濃度センサ１１は、特にガソリンエンジン用として広く使われている限界電流形の酸素濃度センサ１１である。本センサ１１は、酸素濃度に対応した出力が得られるが、原理上、出力に圧力依存性があるため、たとえ酸素濃度が一定でも、圧力が高いほど出力が大きくなるという特徴を持つ。
排気管９と吸気管６との間には、排気の一部を吸気管６に還流させるためのＥＧＲ配管１２が接続され、このＥＧＲ配管１２の途中にＥＧＲ弁１３が設けられている。
ＥＧＲ弁１３は、ＥＧＲ制御弁１４により弁開度が調整され、その弁開度に応じてＥＧＲ配管１２を流れる排気量（以下ＥＧＲ量と呼ぶ）が制御される。
【００２５】
本システムは、空燃比制御手段としての機能を有するエンジン電子制御回路（以下ＥＣＵ１５と呼ぶ）により制御される。ＥＣＵ１５には、酸素濃度センサ１１、噴射圧センサ５、吸気圧センサ８、エンジン回転数センサ１６、アクセル開度センサ１７、その他、通常の電子制御式ディーゼルエンジン１に用いられる各種センサ類が入力回路に接続され、燃料噴射弁２、ＥＧＲ制御弁１４、高圧燃料ポンプ３等が出力回路に接続されている。
このＥＣＵ１５は、上記の各センサから入力したセンサ信号を基に燃料噴射時期、燃料噴射量、及びＥＧＲ量等を演算し、その演算結果に基づいて高圧燃料ポンプ３、燃料噴射弁２、及びＥＧＲ制御弁１４の作動を電子制御する。
【００２６】
ところが、燃料噴射弁２やＥＧＲ弁１３等の特性のバラツキや環境条件の変化、あるいは経時劣化等により、各気筒の燃料噴射量あるいはＥＧＲ量が予め計画された所定量からずれると、ディーゼルエンジン１にとって重要課題であるスモークを増加させてしまう可能性がある。
そこで、本システムでは、ディーゼルエンジン１のエミッション（特にスモーク）と相関が強い空燃比を酸素濃度センサ１１により検出し、その空燃比に基づいて各気筒の燃料噴射量及びＥＧＲ量をフィードバック制御して、エミッション（特にスモーク）が所定値となるようにするものである。
【００２７】
次に、空燃比に基づいて各気筒の燃料噴射量及びＥＧＲ量を補正する本システムの作動をＥＣＵ１５の処理手順に従って説明する。
図１はＥＣＵ１５の処理手順を示すフローチャートである。
Step100 ：エンジン回転数、アクセル開度、吸気圧、その他各種センサからの情報を基に現在のエンジン１の運転条件を読み込む。
Step101 ：予めＥＣＵ１５内に記憶されているマップ等とStep100 で得られた情報とを合わせて目標空燃比（目標排気酸素濃度）を算出する。
Step102 ：酸素濃度センサ１１の信号から実際の空燃比を読み込む。
Step103 ：後述する方法により全気筒の平均空燃比と各気筒毎の空燃比とをそれぞれ算出する。
【００２８】
Step104 ：エンジン負荷（出力トルク）が所定値より大きいか否かを判定する。具体的には、アクセル開度やＥＣＵ１５内で計算される燃料噴射指令量と所定値との比較により、その大小が判定される。
Step104 でエンジン負荷が低いと判定された場合（判定結果ＮＯ）は、Step105 へ進む。
エンジン負荷が低い場合は、排気中のＮＯ_Xを低減するためにＥＧＲ制御が実行される（図７に示す▲１▼と▲２▼の領域）。また、エンジン負荷が低い場合は、図６に示すように、気筒間空燃比がばらついていても、スモークの発生量にそれほど影響を与えない。これに対し、ＥＧＲ量のずれにより各気筒間の平均空燃比が目標値からずれてしまうと、図５に示すように、エミッション（特にスモーク）が大きく悪化する。
【００２９】
そこで、Step105 では、ＥＧＲ量を補正する必要があるか否かを判断するために、以下の判定を行う。
Step105 ：Step101 で算出した目標空燃比とStep103 で算出した平均空燃比とのずれが所定値より小さいか否かを判定する。
このStep105 で平均空燃比のずれが所定値以上と判定された場合（判定結果ＮＯ）は、Step106 へ進む。
Step106 （本発明の低負荷補正手段）：ここでは、目標空燃比に対する平均空燃比のずれが大きいため、そのずれ量に基づいてＥＧＲ制御弁１４を操作し、ＥＧＲ弁１３の弁開度を変更してＥＧＲ量を補正する。これにより、平均空燃比を所定値に制御できるため、エミッションを改善できる。
【００３０】
一方、前記Step104 において、エンジン負荷が高いと判定された場合（判定結果ＹＥＳ）は、Step107 へ進む。
Step107 ：Step103 で算出した各気筒毎の空燃比のばらつきが予め決められた所定値と比較して大きいか否かを判定する。ここで、各気筒間空燃比のばらつきが所定値以上と判定された場合（判定結果ＮＯ）は、Step108 へ進む。
Step108 （本発明の高負荷補正手段と非ＥＧＲ補正手段）：各気筒毎の噴射量を補正する。即ち、エンジン負荷が大きい場合は、図６に示したように、気筒間空燃比のばらつきが大きいとエミッションが大きく悪化するため、先ず各気筒の空燃比を揃える（ばらつきを無くす）ことで、エミッションを改善する。具体的には、空燃比が低く酸素濃度が低い気筒は噴射量を減らし、逆に空燃比が高く酸素濃度が高い気筒は噴射量を増やす。
【００３１】
前記Step107 において、気筒間の空燃比ばらつきが所定値より小さいと判定された場合（判定結果ＹＥＳ）は、Step109 へ進む。
Step109 ：Step105 と同様に、Step101 で算出した目標空燃比とStep103 で算出した平均空燃比とのずれが所定値より小さいか否かを判定する。
なお、Step107 とStep109 は、この順序、つまりStep107 の後にStep109 を設定した方が良い。その理由は、気筒間空燃比のばらつきが所定値以上ある場合、先に平均空燃比のずれを調整すると、全気筒の平均空燃比を低く補正する場合には、空燃比のばらつきにより酸素が不足している気筒では、さらに酸素不足状態となり、多量のスモークが発生することとなる。これに対し、先ず気筒毎の燃料噴射量を調整して気筒毎の空燃比を補正した後、全気筒の平均空燃比を補正すれば、上記の不具合を解消することができるためである。
【００３２】
Step109 で目標空燃比に対する平均空燃比のずれが所定値以上と判定された場合（判定結果ＮＯ）は、Step110 へ進む。
Step110 ：運転条件がＥＧＲ領域にあるか否かを判定する。ここで運転条件がＥＧＲ領域にあると判定された場合（判定結果ＹＥＳ）は、Step111 へ進む。
Step111 （本発明の高負荷補正手段）：ＥＧＲ量を補正する。
一方、Step110 で運転条件がＥＧＲ領域に無いと判定された場合（判定結果ＮＯ）は、Step112 へ進む。
【００３３】
Step112 （非ＥＧＲ補正手段）：全気筒の平均噴射量を補正する。即ち、全気筒の噴射量を一律に増減補正する。
なお、平均空燃比のずれを補正するために、Step106 及びStep111 において噴射量ではなくＥＧＲ量を補正することには、以下の理由がある。
つまり、ＥＧＲが行われる領域は、図７に示すように、全負荷条件に対して比較的エンジン負荷が低いため、こういった領域で噴射量を増減すると、その増減に対してエンジン出力が敏感に変化し、運転性に悪影響を与える場合がある。これを防ぐために、噴射量ではなくＥＧＲ量を補正している。
【００３４】
次に、上記Step103 における平均空燃比及び気筒毎空燃比の算出方法について図８に示すフローチャートに基づいて詳細に説明する。
Step102 ：例えば１０クランク角（ＣＡ）毎に酸素濃度センサ１１の出力をサンプルする。
Step200 ：Step102 でサンプルした信号（センサ出力）を１サイクル分（４気筒エンジンでは７２０ＣＡ）に渡り平均化して平均空燃比を算出する。
Step201 ：続いて、気筒毎の空燃比を算出する。
【００３５】
各気筒から排出された排気ガスは、各気筒での燃焼順序に従い、例えば図９に示すように＃１気筒→＃３気筒→＃４気筒→＃２気筒の順に酸素濃度センサ１１に到達する。この時、排気ガスが酸素濃度センサに到達するまでは時間遅れ（図９のΔＣＡ）があり、この遅れはエンジン回転数が高い程、エンジン負荷が高い程、小さくなる。従って、前記のStep100 で読み込んだエンジン条件により、ΔＣＡ（＃１ＴＤＣを検出してから＃１気筒の排気ガスが酸素濃度センサ１１に到達するまでにエンジン１が回転するクランク角度）を算出する。
【００３６】
Step202 ：Step201 で算出したΔＣＡを基にサンプル信号を１８０ＣＡ毎に平均化して気筒毎の空燃比に対応する値を算出する。例えば、０〜１８０ＣＡの平均値→＃１気筒、１８０〜３６０ＣＡの平均値→＃３気筒、３６０〜５４０ＣＡの平均値→＃４気筒、５４０〜７２０ＣＡの平均値→＃２気筒とする。
これにより、排気圧脈動の影響を除去することができる。
続いて、特にディーゼルエンジン１の場合、図１０に示すように、サイクル毎の燃焼ばらつきが大きいため、正確な空燃比（特に気筒毎の空燃比情報）を得にくい。そこで、Step203 〜207 では、以下に示すように、Step200 で算出した平均空燃比、及びStep202 で算出した気筒毎空燃比を複数サイクル（例えば５サイクル）に渡って平均化する。
【００３７】
Step203 ：回転数、アクセル開度等の変化により、エンジン１の運転条件が変化したか否かを検出する。ここで、運転条件が大きく変化した場合（判定結果ＹＥＳ）は、Step204 へ進む。
Step204 ：カウンタＮをリセット（Ｎ＝０）して本制御を終了する。これは、例えば車両加速時等で運転条件が大きく変化している間は、空燃比が刻々と変化するため、正確な情報が得られないためである。
一方、Step203 で運転条件が変化していないと判定された場合（判定結果ＮＯ）は、Step205 へ進む。
【００３８】
Step205 ：カウンタＮが所定値（例えばＮ＝５）となったか否かを判定する。ここで、所定値となっていない場合（判定結果ＮＯ）は、今回算出した平均空燃比及び気筒毎空燃比の値を記憶した上で、Step206 へ進む。
Step206 ：カウンタＮをインクリメント（Ｎ＝Ｎ＋１）して、再びStep102 へ戻り、上述のStep102 〜203 の処理を繰り返す。
一方、Step205 でカウンタＮが所定値となった場合（判定結果ＹＥＳ）は、Step207 へ進む。
Step207 ：Step205 で記憶されたＮ回分の平均空燃比及び気筒毎空燃比の値を平均して複数サイクルの平均空燃比及び気筒毎空燃比を算出する。
これにより、図１１に示すように、排気圧脈動の影響とサイクル毎のバラツキを除去した上で、各気筒毎の空燃比に相当する値を安定的に求めることができる。
【００３９】
（本実施形態の効果）
上述したように、空燃比とスモーク発生量との関係では、目標空燃比に対する気筒間の平均空燃比のずれが大きくなると、高負荷時及び低負荷時ともにスモーク発生量が増大する（図５参照）。一方、気筒間空燃比のばらつきが大きくなると、高負荷時ではスモーク発生量が増大するが、低負荷時ではスモーク発生量が殆ど変化しない（図６参照）。この空燃比とスモークとの相関より、エンジン１の低負荷時、つまりＥＧＲ領域のうち低負荷領域では、気筒毎の噴射量を補正して気筒間空燃比のばらつきを小さくしても、スモークを低減できる効果が極めて小さいことが分かる。
【００４０】
そこで、本システムは、ＥＧＲ領域のうち低負荷領域では、気筒毎の噴射量補正を行わず、ＥＧＲ量を補正して気筒間の平均空燃比を制御し、エンジン１の高負荷時（非ＥＧＲ領域及びＥＧＲ領域のうち高負荷領域）では、気筒毎の噴射量補正によって気筒間空燃比のばらつきを小さくし、且つ気筒間の平均空燃比のずれを補正することで、それぞれスモーク発生量を所定値に制御している。これにより、実用可能な精度の酸素濃度センサ１１を用いて、エンジン１の運転状態に応じた適切な気筒毎あるいは平均の空燃比制御が可能となり、エミッションの改善が可能となる。
また、エンジン１の高負荷時（非ＥＧＲ領域及びＥＧＲ領域のうち高負荷領域）では、気筒間空燃比のばらつきを補正してから気筒間の平均空燃比のずれを補正しているので、空燃比補正中にスモーク発生量が増加することを防止できる。
【００４１】
本システムは、排気圧の脈動が大きいディーゼルエンジン１に適用しているが、酸素濃度センサ１１の出力を１サイクル／気筒数＝１８０ＣＡ毎に平均化して気筒毎の空燃比に対応する値を算出しているので、空燃比検出手段として一般的な限界電流式の酸素濃度センサ１１を用いた場合でも、排気圧脈動の影響を除去して正確な空燃比を検出することが可能である。
また、上述のStep200 で算出した平均空燃比、及びStep202 で算出した気筒毎の空燃比を複数サイクル（例えば５サイクル）に渡って平均化することにより、サイクル毎の燃焼ばらつきの影響を小さくできるので、各気筒毎の空燃比を安定的に求めることが可能である。
【００４２】
（変形例）
排気管９の形状の影響や過給機付きのエンジンにおいては排気タービンの影響により、各気筒の排気が気筒毎に明確に分離されず、燃焼順序が前後の気筒（例えば＃１気筒に対し＃３気筒と＃２気筒）の排気が混ざり合って酸素濃度センサ１１に到達する場合がある。また、酸素濃度センサ１１の応答遅れのために、特にエンジン回転数が高い場合には、気筒毎の空燃比変化にセンサ出力変化が追いつかない場合がある。これらいずれの場合でも、図８に示す方法で求めた気筒毎空燃比は、正確な意味での各気筒の空燃比と等しくなく、各気筒の空燃比の相対的な大小に対応する特性値（本発明の第１の特性値）となる。
【００４３】
本システムでは、各気筒の空燃比を直接、目標空燃比へと補正するのではなく、各気筒の空燃比に対応する特性値（センサ出力を１８０ＣＡに渡り平均した値）の偏差が極小となるように補正している。従って、前述したケース等にも対応可能な汎用性が非常に大きな空燃比制御装置とすることができる。
また、図１のStep101 において目標空燃比を算出する場合、エンジン回転数やアクセル開度等のエンジン条件だけから目標空燃比を決めると、スモークを所定値以下に抑制できない場合がある。具体的には、吸気圧あるいは燃料噴射圧が変化した場合である。例えば高地などで大気圧が低下し、それに伴い吸気圧が低下すると、たとえ空燃比が同一でもシリンダ内に吸入される酸素の絶対量が低下することから、燃料燃焼時のシリンダ内の酸素の利用率が低下し、結果的にシリンダ内で局所的に酸素不足となる領域が増加してスモーク発生量が増加してしまう。
【００４４】
一方、燃料噴射弁２の特性変化あるいは噴射圧目標値の変更等の理由により、燃料噴射圧力が低下すると、例えば空燃比が同一でも燃料噴霧の微粒化及び空気との混合状態が悪化することから、燃料燃焼時のシリンダ内の酸素の利用率が低下し、結果的にシリンダ内で局所的に酸素不足となる領域が増加してスモーク発生量が増加してしまう。
そこで、本システムでは、これらに対応するために、図１２に示すように、吸気圧が小さいほど目標空燃比を大きくするように補正変更し、また図１３に示すように、燃料噴射圧力が小さいほど目標空燃比を大きくするように補正変更している。これにより、吸気圧や噴射圧の変化によらず、スモーク量を所定値とすることが可能である。
【００４５】
なお、上述の実施形態では、４気筒のコモンレール式ディーゼルエンジン１を例に説明したが、それ以外の気筒毎に噴射量を調整可能な多気筒内燃機関へも適用可能である。例えば６気筒のエンジンに適用すると、気筒毎空燃比に対応する値を求める際には、排気圧脈動の影響を除去するための平均化区間を７２０ＣＡ／６＝１２０ＣＡとする。
【図面の簡単な説明】
【図１】空燃比制御の処理手順を示すフローチャートである。
【図２】空燃比制御装置のシステム図である。
【図３】吸気圧とスモークとの相関図である。
【図４】燃料噴射圧とスモークとの相関図である。
【図５】平均空燃比のずれとスモークとの相関図である。
【図６】気筒間空燃比のばらつきとスモークとの相関図である。
【図７】ＥＧＲ領域を示すトルク特性図である。
【図８】空燃比を算出するための処理手順を示すフローチャートである。
【図９】酸素濃度センサの時間的な遅れを説明するタイムチャートである。
【図１０】酸素濃度センサの出力を１０ＣＡ毎にサンプルしたものを複数サイクル分、重ね書きした図面である。
【図１１】気筒毎に酸素濃度センサの出力を平均化した図面である。
【図１２】吸気圧と目標空燃比との相関図である。
【図１３】燃料噴射圧と目標空燃比との相関図である。
【符号の説明】
１ディーゼルエンジン（内燃機関）
５噴射圧センサ（噴射圧検出手段）
６吸気管（吸気通路）
７吸気マニホールド（吸気通路）
８吸気圧センサ（吸気圧検出手段）
１１酸素濃度センサ（空燃比検出手段）
１３ＥＧＲ弁（ＥＧＲ手段）
１４ＥＧＲ制御弁（ＥＧＲ手段）
１５ＥＣＵ（空燃比制御手段）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that controls the air-fuel ratio to a predetermined value by adjusting the fuel injection amount or EGR amount of the internal combustion engine.
[0002]
[Prior art]
The fuel injection amount of the internal combustion engine directly affects the engine output and emission emission amount, and the EGR amount of the internal combustion engine directly affects the emission emission amount. Therefore, the fuel injection amount and the EGR amount are accurately controlled to predetermined values. Is very important. However, these change due to variations in characteristics such as fuel injection valves and EGR valves, changes in environmental conditions, deterioration with time, and the like.
On the other hand, the emission has a strong correlation with the oxygen concentration (air-fuel ratio) in the exhaust gas. In diesel engines in particular, reducing smoke (black smoke) in exhaust is an important issue, but smoke is generated when oxygen is locally deficient in the cylinder during fuel combustion. It has a very strong correlation with the oxygen concentration in the exhaust. Therefore, it is very useful to use the air-fuel ratio as an index for setting the emission (especially smoke of the diesel engine) in the internal combustion engine to a predetermined value.
[0003]
Therefore, there is a conventional technique in which the fuel injection amount and the EGR amount are feedback controlled based on the air-fuel ratio. For example, in the fuel injection control device for a diesel engine disclosed in Japanese Patent Laid-Open No. 2-61347, the EGR amount is adjusted in the EGR region so that the air-fuel ratio becomes a predetermined value, and the average fuel injection amount of all cylinders in the non-EGR region. By adjusting the emission, the emission is controlled to a predetermined value.
However, even when the air-fuel ratio (average air-fuel ratio of all cylinders) detected by the sensor is a target value during non-EGR region (high load region) or high-load operation in the EGR region of the diesel engine, If the air-fuel ratio varies, as shown in FIG. 6, there is a problem that the smoke amount increases rapidly as the variation increases. In order to solve this problem, it is difficult to sufficiently reduce the smoke because the conventional technology cannot correct the variation in the air-fuel ratio between the cylinders.
[0004]
On the other hand, there is an air-fuel ratio control device for an internal combustion engine disclosed in Japanese Patent Laid-Open No. 9-203337 as a prior art. This prior art eliminates the variation in the air-fuel ratio between cylinders, calculates the air-fuel ratio for each cylinder from the output of the air-fuel ratio sensor and the exhaust timing of each cylinder, and based on the calculated air-fuel ratio for each cylinder. The fuel injection amount is feedback controlled for each cylinder.
However, when this conventional technology is applied to a diesel engine, the oxygen concentration in the exhaust gas is high when the engine is under a low load. This is difficult in practice.
[0005]
Further, when the air-fuel ratio of the internal combustion engine is detected by a sensor, the following problem occurs.
(1) The air-fuel ratio of an internal combustion engine is generally detected by, for example, a limit current type oxygen concentration sensor, and the output of this sensor changes in principle according to the atmospheric pressure, that is, the exhaust pressure. The higher the pressure, the greater. Therefore, when this type of sensor is applied to a diesel engine, since the diesel engine has a high compression ratio, the exhaust pressure pulsation is large. Therefore, even if the air-fuel ratio is constant, the sensor output has an exhaust pressure pulsation. Periodic fluctuations corresponding to one cycle / the number of cylinders due to (for example, fluctuations every 180 crank angles in a four-cylinder internal combustion engine) occur. Since the magnitude of this variation varies depending on the engine operating conditions, it is difficult to obtain accurate information on the air-fuel ratio. In particular, when the air-fuel ratio for each cylinder is to be obtained separately from the output of one sensor, the above problem becomes significant, and accurate detection of the air-fuel ratio for each cylinder and control based on the detected air-fuel ratio. Becomes very difficult.
[0006]
(2) Since it takes time for the exhaust discharged from the exhaust valve of the internal combustion engine to reach the sensor provided in the exhaust pipe, a time delay occurs, and this delay varies depending on the operating state of the internal combustion engine. To do. Therefore, if this delay is not taken into account, the air-fuel ratio for each cylinder cannot be detected accurately when the air-fuel ratio for each cylinder is obtained separately from the output of one sensor.
{Circle around (3)} Particularly in a diesel engine, since the combustion is due to self-ignition of fuel, the temporal or spatial combustion stability of each cycle is low, and the obtained air-fuel ratio information varies from cycle to cycle.
[0007]
(4) Particularly in a diesel engine, for example, the degree to which the air in the cylinder is effectively used by changing the atmospheric pressure at high altitudes or by changing the fuel injection pressure depending on the specifications and characteristics of the injection system (that is, the air utilization rate). ) Changes, the relationship between the air-fuel ratio and the amount of smoke generated changes as shown in FIGS. Therefore, if the target air-fuel ratio is constant regardless of these changes, the smoke amount cannot be controlled to a predetermined value.
As described above, there are many problems when trying to control the emission of a diesel engine based on the detected air-fuel ratio using the conventional technology.
[0008]
[Problems to be solved by the invention]
The present invention is for solving the above-described problems, appropriately processing the output of the air-fuel ratio detection means in accordance with each operating state of the internal combustion engine, and controlling the fuel injection amount and the EGR amount for each cylinder. An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine that can reduce the emission of the internal combustion engine (especially smoke in a diesel engine) by controlling the air-fuel ratio to a desired value.
[0009]
[Means for Solving the Problems]
(Means of Claim 1)
The air-fuel ratio control unit corrects the EGR amount based on the second characteristic value in a region where the load is relatively low in the EGR region, and changes to the first characteristic value in a region where the load is relatively high in the EGR region. Based on this, the fuel injection amount for each cylinder is corrected, the EGR amount is corrected based on the second characteristic value, and the fuel injection amount for each cylinder is corrected based on the first characteristic value in the non-EGR region. The average injection amount is corrected based on the second characteristic value.
[0010]
In the relatively high load region of the non-EGR region (high load region) and the EGR region, as shown in FIG. 6, when the variation in the air-fuel ratio between the cylinders increases, the amount of smoke generated increases rapidly. It is necessary to correct the fuel injection amount for each to suppress the variation in the air-fuel ratio between the cylinders. Here, particularly in a diesel engine, the oxygen concentration in the exhaust gas is lower when the load is high than when the load is low. Therefore, extremely high detection accuracy is required to detect the deviation of the air-fuel ratio for each cylinder. The first characteristic value corresponding to the air-fuel ratio for each cylinder is calculated based on the output of the practical air-fuel ratio detection means without being required, and for each cylinder based on the first characteristic value. The fuel injection amount can be corrected.
[0011]
On the other hand, in the relatively low load region of the EGR region, the oxygen concentration in the exhaust gas is high as described above, and in order to detect the deviation of the air-fuel ratio in that region, a very high air-fuel ratio detection accuracy is required. Therefore, it is practically difficult to detect the air-fuel ratio for each cylinder. However, in the low load region, as shown in FIG. 6, even if the air-fuel ratio varies from cylinder to cylinder, the amount of smoke generated is low and the variation in the cylinder-to-cylinder air-fuel ratio is large compared to when the load is high. Even so, the amount of smoke generated hardly changes. Accordingly, the injection amount correction for each cylinder is not performed, but the EGR amount is corrected to control the average air-fuel ratio between the cylinders.
As a result, the air-fuel ratio detection means with practical accuracy can perform appropriate cylinder-by-cylinder or average air-fuel ratio control in accordance with the operating state of the internal combustion engine, and emission can be improved.
[0012]
  EIn the relatively low load region of the GR region, the EGR amount is corrected so that the difference between the target value corresponding to the target air-fuel ratio and the second characteristic value corresponding to the average air-fuel ratio of all the cylinders becomes small. . Thereby, it is possible to improve the deterioration of the emission due to the deviation of the average air-fuel ratio between the cylinders (deviation from the target air-fuel ratio).
[0013]
  In a relatively high load area in the EGR areaWhen the variation between the cylinders of the first characteristic value is determined to be a predetermined value or more,Correcting the fuel injection amount for each cylinder so that the variation in the first characteristic value corresponding to the air-fuel ratio for each cylinder is reduced between the cylinders;Align the first characteristic value of each cylinderThereafter, the EGR amount is corrected so that the difference between the target value corresponding to the target air-fuel ratio and the second characteristic value becomes small. As a result, it is possible to improve the emission deterioration caused by the variation in the air-fuel ratio between the cylinders and the deviation in the average air-fuel ratio between the cylinders (deviation from the target air-fuel ratio).
[0014]
In addition, after correcting the fuel injection amount for each cylinder, correcting the EGR amount so as to reduce the difference between the average air-fuel ratio and the target air-fuel ratio increases the amount of smoke generated during the correction of the air-fuel ratio. Can be prevented. That is, when the load on the internal combustion engine is large, smoke is likely to occur and the air-fuel ratio between the cylinders remains varied, and if the EGR amount for all the cylinders is corrected first, the average air-fuel ratio of all the cylinders is corrected to be low. In a cylinder where oxygen is deficient due to variations in the air-fuel ratio, the oxygen further becomes deficient and a large amount of smoke is generated. On the other hand, if the fuel injection amount for each cylinder is first adjusted to correct the air-fuel ratio for each cylinder and then the EGR amount for all the cylinders is corrected, the above problem can be solved.
[0015]
  In non-EGR areasWhen the variation between the cylinders of the first characteristic value is determined to be a predetermined value or more,Correcting the fuel injection amount for each cylinder so that the variation in the first characteristic value corresponding to the air-fuel ratio for each cylinder is reduced between the cylinders;Align the first characteristic value of each cylinderThereafter, the average injection amount between the cylinders is corrected so that the difference between the target value corresponding to the target air-fuel ratio and the second characteristic value becomes small. As a result, it is possible to improve the emission deterioration caused by the variation in the air-fuel ratio between the cylinders and the deviation in the average air-fuel ratio between the cylinders (deviation from the target air-fuel ratio).
  In addition, after correcting the fuel injection amount for each cylinder, by correcting the average injection amount between the cylinders so that the difference between the average air-fuel ratio and the target air-fuel ratio becomes small, the amount of smoke generated during the correction of the air-fuel ratio can be reduced. It can be prevented from increasing.
[0016]
  (Claims2Means)
  The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
  The cylinder air-fuel ratio calculating means averages the output of the air-fuel ratio detecting means sampled a plurality of times during one cycle of the internal combustion engine for each section corresponding to one cycle / number of cylinders of the internal combustion engine.FirstWhile calculating a characteristic value, the start timing of the area to average is changed for every operating condition of an internal combustion engine. As a result, when a general limit current type oxygen concentration sensor is used as the air-fuel ratio detection means, even in a diesel engine having a large exhaust pressure pulsation, the influence of the exhaust pressure pulsation is removed, It becomes possible to obtain the air-fuel ratio. In addition, since it takes into account that the delay until the exhaust reaches the air-fuel ratio detection means changes according to the operating state of the internal combustion engine, it is possible to obtain an accurate air-fuel ratio for each cylinder regardless of the operating state. .
[0018]
  (Claims3Means)
  Claim2In the air-fuel ratio control device for an internal combustion engine described in 1.
  The air-fuel ratio calculation means for each cylinder was obtained for each section.FirstThe characteristic value is averaged over a plurality of cycles for each corresponding cylinder to obtain the characteristic value of each cylinder. As a result, since the combustion is based on the self-ignition of the fuel, the temporal or spatial stability of each combustion is low, and the accurate air-fuel ratio is detected even in a diesel engine in which the obtained air-fuel ratio information varies from cycle to cycle. It becomes possible.
[0019]
  (Claims4Means)
  Claim2 or 3Described inTauchiIn an air-fuel ratio control device for a fuel engine,
  The cylinder air-fuel ratio calculating means sets the start timing of the section in which the output of the air-fuel ratio detecting means is averaged earlier as the engine speed or load of the internal combustion engine increases. In this case, since the delay until the exhaust reaches the air-fuel ratio detecting means becomes smaller as the rotational speed and load of the internal combustion engine are larger, an accurate air-fuel ratio for each cylinder is obtained regardless of the operating state of the internal combustion engine. Can be obtained.
[0020]
  (Claims5Means)
  Claims 1 to4In the air-fuel ratio control device for any of the internal combustion engines described in 1.
  The target air-fuel ratio calculating means increases the target air-fuel ratio (target exhaust oxygen concentration) as the output of the intake pressure detecting means decreases.
  When the atmospheric pressure drops at high altitudes and the intake pressure of an internal combustion engine, especially a diesel engine, decreases, the absolute amount of oxygen sucked into the cylinder decreases even if the air-fuel ratio (oxygen concentration in the exhaust) is the same. To do. For this reason, the oxygen utilization in the cylinder at the time of fuel combustion falls, As a result, the area | region where oxygen runs short locally in a cylinder will increase, and the amount of smoke generation will increase. On the other hand, according to the present invention, as the intake pressure is smaller, the target air-fuel ratio is increased, so that the above-described problem can be prevented and the amount of smoke generated can be reduced to a predetermined value or less.
[0021]
  (Claims6Means)
  Claims 1 to5In the air-fuel ratio control device for any of the internal combustion engines described in 1.
  The target air-fuel ratio calculation means increases the target air-fuel ratio as the output of the injection pressure detection means decreases. If the fuel injection pressure decreases due to a change in the characteristics of the internal combustion engine, etc., the atomization of the fuel spray and the mixed state with air will deteriorate even if the air-fuel ratio (oxygen concentration in the exhaust gas) is the same. For this reason, the oxygen utilization in the cylinder at the time of fuel combustion falls, As a result, the area | region where oxygen runs short locally in a cylinder will increase, and the amount of smoke generation will increase.
  On the other hand, according to the present invention, since the target air-fuel ratio is increased as the injection pressure is decreased, the above-described problem can be prevented and the smoke generation amount can be set to a predetermined value or less.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Next, an embodiment in which the present invention is applied to a four-cylinder diesel engine will be described with reference to the drawings.
FIG. 2 is a system diagram of an air-fuel ratio control device for a diesel engine.
A diesel engine 1 which is an internal combustion engine of the present invention is provided with a solenoid valve type fuel injection valve 2 for each cylinder. The fuel injection valve 2 is supplied with high-pressure fuel via a common rail 4 (accumulation chamber) for accumulating fuel pumped from the high-pressure fuel pump 3, and injects fuel into the cylinder in accordance with the opening / closing operation of the electromagnetic valve.
The common rail 4 is provided with an injection pressure sensor 5 that detects an internal pressure (injection pressure) of the common rail 4.
[0023]
The diesel engine 1 is connected to an intake passage for introducing air into each cylinder and an exhaust passage for discharging exhaust gas generated by combustion in each cylinder.
The intake passage has an intake pipe 6 that introduces air through an air cleaner (not shown), and an intake manifold 7 that branches and connects the intake pipe 6 and each cylinder. An intake pressure sensor 8 is installed in the intake pipe 6. ing.
The exhaust passage has an exhaust pipe 9 that opens to the atmosphere, and an exhaust manifold 10 that sends exhaust gas discharged from each cylinder to the exhaust pipe 9. The exhaust pipe 9 detects the oxygen concentration (air-fuel ratio) in the exhaust. An acidity concentration sensor 11 (air-fuel ratio detection means of the present invention) is installed.
[0024]
The oxygen concentration sensor 11 is a limiting current type oxygen concentration sensor 11 that is widely used especially for gasoline engines. Although this sensor 11 can provide an output corresponding to the oxygen concentration, in principle, since the output is pressure-dependent, the output increases as the pressure increases even if the oxygen concentration is constant.
An EGR pipe 12 for returning a part of the exhaust gas to the intake pipe 6 is connected between the exhaust pipe 9 and the intake pipe 6, and an EGR valve 13 is provided in the middle of the EGR pipe 12.
The EGR valve 13 has its valve opening adjusted by the EGR control valve 14, and the exhaust amount (hereinafter referred to as EGR amount) flowing through the EGR pipe 12 is controlled according to the valve opening.
[0025]
This system is controlled by an engine electronic control circuit (hereinafter referred to as ECU 15) having a function as air-fuel ratio control means. The ECU 15 includes an oxygen concentration sensor 11, an injection pressure sensor 5, an intake pressure sensor 8, an engine speed sensor 16, an accelerator opening sensor 17, and other various sensors used for a normal electronically controlled diesel engine 1. The fuel injection valve 2, the EGR control valve 14, the high-pressure fuel pump 3 and the like are connected to the output circuit.
The ECU 15 calculates the fuel injection timing, the fuel injection amount, the EGR amount, and the like based on the sensor signals input from the respective sensors, and based on the calculation results, the high pressure fuel pump 3, the fuel injection valve 2, and the EGR The operation of the control valve 14 is electronically controlled.
[0026]
However, if the fuel injection amount or EGR amount of each cylinder deviates from a predetermined amount due to variations in characteristics of the fuel injection valve 2, the EGR valve 13, etc., changes in environmental conditions, deterioration with time, etc., the diesel engine 1 May increase smoke, which is an important issue.
Therefore, in this system, the air-fuel ratio that has a strong correlation with the emission (especially smoke) of the diesel engine 1 is detected by the oxygen concentration sensor 11, and the fuel injection amount and the EGR amount of each cylinder are feedback-controlled based on the air-fuel ratio. The emission (especially smoke) is set to a predetermined value.
[0027]
Next, the operation of this system for correcting the fuel injection amount and the EGR amount of each cylinder based on the air-fuel ratio will be described according to the processing procedure of the ECU 15.
FIG. 1 is a flowchart showing a processing procedure of the ECU 15.
Step 100: The current operating conditions of the engine 1 are read based on information from the engine speed, accelerator opening, intake pressure, and other various sensors.
Step 101: A target air-fuel ratio (target exhaust oxygen concentration) is calculated by combining a map stored in advance in the ECU 15 and the information obtained in Step 100.
Step 102: The actual air-fuel ratio is read from the signal of the oxygen concentration sensor 11.
Step 103: The average air-fuel ratio of all cylinders and the air-fuel ratio for each cylinder are calculated by the method described later.
[0028]
Step 104: It is determined whether or not the engine load (output torque) is larger than a predetermined value. Specifically, the magnitude is determined by comparing the accelerator opening and the fuel injection command amount calculated in the ECU 15 with a predetermined value.
If it is determined in step 104 that the engine load is low (determination result NO), the process proceeds to step 105.
When engine load is low, NO in exhaust_XEGR control is executed to reduce the above (regions (1) and (2) shown in FIG. 7). Further, when the engine load is low, as shown in FIG. 6, even if the inter-cylinder air-fuel ratio varies, the amount of smoke generated is not significantly affected. On the other hand, if the average air-fuel ratio between the cylinders deviates from the target value due to the deviation of the EGR amount, the emission (especially smoke) is greatly deteriorated as shown in FIG.
[0029]
Therefore, in Step 105, the following determination is performed to determine whether or not the EGR amount needs to be corrected.
Step 105: It is determined whether or not the difference between the target air-fuel ratio calculated in Step 101 and the average air-fuel ratio calculated in Step 103 is smaller than a predetermined value.
If it is determined in step 105 that the average air-fuel ratio shift is greater than or equal to a predetermined value (determination result NO), the process proceeds to step 106.
Step 106 (low load correction means of the present invention): Here, since the deviation of the average air-fuel ratio with respect to the target air-fuel ratio is large, the EGR control valve 14 is operated based on the deviation amount, and the valve opening degree of the EGR valve 13 is changed. Then, the EGR amount is corrected. Thereby, since the average air-fuel ratio can be controlled to a predetermined value, emission can be improved.
[0030]
On the other hand, if it is determined in Step 104 that the engine load is high (determination result YES), the process proceeds to Step 107.
Step 107: It is determined whether or not the variation in air-fuel ratio for each cylinder calculated in Step 103 is larger than a predetermined value determined in advance. Here, when it is determined that the variation in the air-fuel ratio between the cylinders is equal to or greater than the predetermined value (determination result NO), the process proceeds to Step 108.
Step 108 (High load correction means and non-EGR correction means of the present invention): The injection amount for each cylinder is corrected. That is, when the engine load is large, as shown in FIG. 6, if the variation in the air-fuel ratio between the cylinders is large, the emission is greatly deteriorated. Therefore, by first adjusting the air-fuel ratio of each cylinder (eliminating the variation), To improve. Specifically, a cylinder with a low air-fuel ratio and a low oxygen concentration reduces the injection amount, and conversely, a cylinder with a high air-fuel ratio and a high oxygen concentration increases the injection amount.
[0031]
If it is determined in Step 107 that the air-fuel ratio variation among the cylinders is smaller than the predetermined value (determination result YES), the process proceeds to Step 109.
Step 109: Similar to Step 105, it is determined whether or not the difference between the target air-fuel ratio calculated in Step 101 and the average air-fuel ratio calculated in Step 103 is smaller than a predetermined value.
Step 107 and Step 109 should be set in this order, that is, Step 109 after Step 107. The reason for this is that if the variation in the air-fuel ratio between cylinders is greater than or equal to a predetermined value, and if the deviation in the average air-fuel ratio is adjusted first, then if the average air-fuel ratio of all cylinders is corrected to a low level, the lack of oxygen due to the variation in air-fuel ratio In the cylinder that is running, the oxygen further becomes deficient and a large amount of smoke is generated. On the other hand, if the fuel injection amount for each cylinder is first adjusted to correct the air-fuel ratio for each cylinder and then the average air-fuel ratio for all cylinders is corrected, the above problem can be solved.
[0032]
If it is determined in Step 109 that the deviation of the average air-fuel ratio from the target air-fuel ratio is greater than or equal to a predetermined value (determination result NO), the process proceeds to Step 110.
Step 110: It is determined whether or not the operating condition is in the EGR region. When it is determined that the operating condition is in the EGR region (determination result YES), the process proceeds to Step 111.
Step 111 (High load correcting means of the present invention): EGR amount is corrected.
On the other hand, when it is determined in Step 110 that the operating condition is not in the EGR region (determination result NO), the process proceeds to Step 112.
[0033]
Step 112 (Non-EGR correcting means): The average injection amount of all cylinders is corrected. That is, the injection amount of all cylinders is uniformly increased or decreased.
In order to correct the deviation of the average air-fuel ratio, the EGR amount, not the injection amount, is corrected in Step 106 and Step 111 for the following reason.
That is, in the region where EGR is performed, as shown in FIG. 7, the engine load is relatively low with respect to the full load condition. Therefore, if the injection amount is increased or decreased in such a region, the engine output is sensitive to the increase or decrease. May adversely affect driving performance. In order to prevent this, not the injection amount but the EGR amount is corrected.
[0034]
Next, the calculation method of the average air-fuel ratio and the cylinder-by-cylinder air-fuel ratio in Step 103 will be described in detail based on the flowchart shown in FIG.
Step 102: For example, the output of the oxygen concentration sensor 11 is sampled every 10 crank angles (CA).
Step 200: The signal (sensor output) sampled in Step 102 is averaged over one cycle (720 CA for a four-cylinder engine) to calculate an average air-fuel ratio.
Step 201: Subsequently, the air-fuel ratio for each cylinder is calculated.
[0035]
Exhaust gas discharged from each cylinder reaches the oxygen concentration sensor 11 in the order of # 1 cylinder → # 3 cylinder → # 4 cylinder → # 2 cylinder, for example, as shown in FIG. 9 according to the combustion order in each cylinder. At this time, there is a time delay (ΔCA in FIG. 9) until the exhaust gas reaches the oxygen concentration sensor, and this delay decreases as the engine speed increases and the engine load increases. Therefore, ΔCA (a crank angle at which the engine 1 rotates from when # 1 TDC is detected until the exhaust gas of the # 1 cylinder reaches the oxygen concentration sensor 11) is calculated according to the engine conditions read at Step 100 described above.
[0036]
Step 202: The sample signal is averaged every 180 CA based on ΔCA calculated in Step 201, and a value corresponding to the air-fuel ratio for each cylinder is calculated. For example, the average value of 0 to 180 CA, the average value of # 1 cylinder, 180 to 360 CA, the average value of # 3 cylinder, the average value of 360 to 540 CA, the average value of # 4 cylinder, the average of 540 to 720 CA, and the # 2 cylinder.
Thereby, the influence of exhaust pressure pulsation can be eliminated.
Subsequently, particularly in the case of the diesel engine 1, as shown in FIG. 10, since the combustion variation for each cycle is large, it is difficult to obtain an accurate air-fuel ratio (particularly, air-fuel ratio information for each cylinder). Therefore, in Steps 203 to 207, as shown below, the average air-fuel ratio calculated in Step 200 and the cylinder air-fuel ratio calculated in Step 202 are averaged over a plurality of cycles (for example, five cycles).
[0037]
Step 203: It is detected whether or not the operating conditions of the engine 1 have changed due to changes in the rotational speed, accelerator opening, and the like. Here, if the operating condition has changed significantly (determination result YES), the routine proceeds to Step 204.
Step 204: The counter N is reset (N = 0) and this control is finished. This is because, for example, the air-fuel ratio changes every moment while the driving conditions change greatly, for example, when the vehicle is accelerated, so accurate information cannot be obtained.
On the other hand, when it is determined in Step 203 that the operating condition has not changed (determination result NO), the process proceeds to Step 205.
[0038]
Step 205: It is determined whether or not the counter N has reached a predetermined value (for example, N = 5). Here, when it is not the predetermined value (determination result NO), the process proceeds to Step 206 after storing the calculated average air-fuel ratio and cylinder-by-cylinder air-fuel ratio.
Step 206: Increment the counter N (N = N + 1), return to Step 102 again, and repeat the processing of Steps 102 to 203 described above.
On the other hand, when the counter N reaches a predetermined value in Step 205 (determination result YES), the process proceeds to Step 207.
Step 207: The average air-fuel ratio and cylinder air-fuel ratio for N times stored in Step 205 are averaged to calculate the average air-fuel ratio and cylinder air-fuel ratio for a plurality of cycles.
As a result, as shown in FIG. 11, the value corresponding to the air-fuel ratio for each cylinder can be stably obtained after removing the influence of the exhaust pressure pulsation and the variation for each cycle.
[0039]
(Effect of this embodiment)
As described above, in the relationship between the air-fuel ratio and the amount of smoke generated, if the deviation of the average air-fuel ratio between the cylinders relative to the target air-fuel ratio increases, the amount of smoke generated increases at both high and low loads (see FIG. 5). ). On the other hand, when the variation in the air-fuel ratio between cylinders increases, the amount of smoke generated increases at high loads, but the amount of smoke generated hardly changes at low loads (see FIG. 6). From the correlation between the air-fuel ratio and the smoke, when the engine 1 is under a low load, that is, in the low-load region of the EGR region, the smoke can be reduced even if the injection amount for each cylinder is corrected to reduce the variation in the air-fuel ratio between the cylinders. It turns out that the effect which can be reduced is very small.
[0040]
Therefore, in the low load region of the EGR region, the present system does not perform the injection amount correction for each cylinder, but corrects the EGR amount to control the average air-fuel ratio between the cylinders, so that the engine 1 is under high load (non-EGR). In the high load region of the region and the EGR region), the variation in the air-fuel ratio between the cylinders is reduced by correcting the injection amount for each cylinder, and the deviation in the average air-fuel ratio between the cylinders is corrected to thereby determine the amount of smoke generated in advance. The value is controlled. As a result, it is possible to perform appropriate cylinder-by-cylinder or average air-fuel ratio control according to the operating state of the engine 1 using the oxygen concentration sensor 11 with practical accuracy, and to improve emissions.
Further, when the engine 1 is at a high load (a high load region of the non-EGR region and the EGR region), the variation in the air-fuel ratio between the cylinders is corrected and the deviation in the average air-fuel ratio between the cylinders is corrected. It is possible to prevent the smoke generation amount from increasing during the fuel ratio correction.
[0041]
This system is applied to a diesel engine 1 having a large exhaust pressure pulsation. The output of the oxygen concentration sensor 11 is averaged every cycle / number of cylinders = 180 CA to calculate a value corresponding to the air-fuel ratio of each cylinder. Therefore, even when a general limit current type oxygen concentration sensor 11 is used as the air-fuel ratio detection means, it is possible to remove the influence of the exhaust pressure pulsation and detect the accurate air-fuel ratio.
In addition, by averaging the average air-fuel ratio calculated in Step 200 described above and the air-fuel ratio for each cylinder calculated in Step 202 over a plurality of cycles (for example, five cycles), it is possible to reduce the influence of combustion variation for each cycle. It is possible to stably obtain the air-fuel ratio for each cylinder.
[0042]
(Modification)
Due to the influence of the shape of the exhaust pipe 9 and the influence of the exhaust turbine in an engine with a supercharger, the exhaust of each cylinder is not clearly separated for each cylinder, and the combustion order of the cylinders before and after (for example, # 1 cylinder ## 3 cylinders and # 2 cylinders) may be mixed and reach the oxygen concentration sensor 11 in some cases. In addition, due to the response delay of the oxygen concentration sensor 11, the sensor output change may not catch up with the air-fuel ratio change for each cylinder, particularly when the engine speed is high. In any of these cases, the air-fuel ratio per cylinder obtained by the method shown in FIG. 8 is not equal to the air-fuel ratio of each cylinder in the accurate sense, and the characteristic value corresponding to the relative magnitude of the air-fuel ratio of each cylinder ( (First characteristic value of the present invention).
[0043]
In this system, the deviation of the characteristic value corresponding to the air-fuel ratio of each cylinder (the value obtained by averaging the sensor output over 180 CA) is minimized instead of directly correcting the air-fuel ratio of each cylinder to the target air-fuel ratio. It is corrected as follows. Therefore, the air-fuel ratio control apparatus can be made to be very versatile and can be applied to the above-described cases.
When calculating the target air-fuel ratio in Step 101 of FIG. 1, if the target air-fuel ratio is determined only from engine conditions such as the engine speed and the accelerator opening, smoke may not be suppressed below a predetermined value. Specifically, this is a case where the intake pressure or the fuel injection pressure changes. For example, if the atmospheric pressure decreases at high altitudes and the intake pressure decreases accordingly, even if the air-fuel ratio is the same, the absolute amount of oxygen sucked into the cylinder decreases, so the use of oxygen in the cylinder during fuel combustion The rate decreases, and as a result, the region where oxygen is locally deficient in the cylinder increases and the amount of smoke generated increases.
[0044]
On the other hand, if the fuel injection pressure decreases due to a change in the characteristics of the fuel injection valve 2 or a change in the injection pressure target value, for example, even if the air-fuel ratio is the same, the atomization of the fuel spray and the mixed state with air deteriorate. Then, the utilization rate of oxygen in the cylinder at the time of fuel combustion decreases, and as a result, a region where oxygen is locally deficient in the cylinder increases and the amount of smoke generated increases.
Therefore, in this system, in order to cope with these, as shown in FIG. 12, a correction change is made so that the target air-fuel ratio increases as the intake pressure decreases, and the fuel injection pressure decreases as shown in FIG. The correction is changed so as to increase the target air-fuel ratio. As a result, the smoke amount can be set to a predetermined value regardless of changes in the intake pressure or the injection pressure.
[0045]
In the above-described embodiment, the 4-cylinder common rail diesel engine 1 has been described as an example. However, the present invention can be applied to a multi-cylinder internal combustion engine in which the injection amount can be adjusted for each of the other cylinders. For example, when applied to a 6-cylinder engine, when obtaining a value corresponding to the air-fuel ratio per cylinder, an averaging interval for removing the influence of exhaust pressure pulsation is set to 720 CA / 6 = 120 CA.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a processing procedure of air-fuel ratio control.
FIG. 2 is a system diagram of an air-fuel ratio control apparatus.
FIG. 3 is a correlation diagram between intake pressure and smoke.
FIG. 4 is a correlation diagram between fuel injection pressure and smoke.
FIG. 5 is a correlation diagram between a deviation in average air-fuel ratio and smoke.
FIG. 6 is a correlation diagram between variations in air-fuel ratio between cylinders and smoke.
FIG. 7 is a torque characteristic diagram showing an EGR region.
FIG. 8 is a flowchart showing a processing procedure for calculating an air-fuel ratio.
FIG. 9 is a time chart for explaining a time delay of the oxygen concentration sensor.
FIG. 10 is a drawing in which the output of an oxygen concentration sensor is sampled every 10 CA and overwritten for a plurality of cycles.
FIG. 11 is an average of the output of the oxygen concentration sensor for each cylinder.
FIG. 12 is a correlation diagram between intake pressure and target air-fuel ratio.
FIG. 13 is a correlation diagram between a fuel injection pressure and a target air-fuel ratio.
[Explanation of symbols]
1 Diesel engine (internal combustion engine)
5 Injection pressure sensor (Injection pressure detection means)
6 Intake pipe (intake passage)
7 Intake manifold (intake passage)
8 Intake pressure sensor (Intake pressure detection means)
11 Oxygen concentration sensor (air-fuel ratio detection means)
13 EGR valve (EGR means)
14 EGR control valve (EGR means)
15 ECU (air-fuel ratio control means)

Claims

Air-fuel ratio detection means for detecting the air-fuel ratio in the exhaust gas discharged from each cylinder of the internal combustion engine;
EGR means for recirculating a part of the exhaust gas to the intake passage of the internal combustion engine;
Target air-fuel ratio calculating means for calculating a target air-fuel ratio based on the operating state of the internal combustion engine;
An air-fuel ratio calculating unit for each cylinder for calculating a first characteristic value corresponding to the air-fuel ratio for each cylinder based on the output of the air-fuel ratio detecting unit;
Average air-fuel ratio calculating means for calculating a second characteristic value corresponding to the average air-fuel ratio of all cylinders based on the output of the air-fuel ratio detecting means;
By correcting the fuel injection amount to each cylinder or the EGR amount to be recirculated to the intake passage by the EGR means, the air-fuel ratio detected by the air-fuel ratio detecting means is set to the target air-fuel ratio for each cylinder. Air-fuel ratio control means for controlling the air-fuel ratio of the cylinder and the average air-fuel ratio of all cylinders,
This air-fuel ratio control means
A low load correction means for correcting an EGR amount so that a difference between a target value corresponding to the target air-fuel ratio and the second characteristic value is small in a region where the load is relatively low in the EGR region;
In a region where the load is relatively high in the EGR region , based on the first characteristic value calculated by the cylinder air-fuel ratio calculation means, it is determined that the variation of the first characteristic value between the cylinders is greater than or equal to a predetermined value. If not, a fuel injection amount of each of the cylinders is corrected as the variation of the first characteristic value among the cylinders is small, after its having uniform first characteristic value of each cylinder, the target air High load correction means for correcting the EGR amount so that the difference between the target value corresponding to the fuel ratio and the second characteristic value is small;
In the non-EGR region, when the variation between the cylinders of the first characteristic value is determined to be greater than or equal to a predetermined value based on the first characteristic value calculated by the air-fuel ratio calculating unit for each cylinder, in after its said first variations of characteristic values of the fuel injection amount of each of the cylinders is corrected to be smaller, stocked first characteristic value of each cylinder, and a target value corresponding to the target air-fuel ratio An air-fuel ratio control apparatus for an internal combustion engine, comprising non-EGR correction means for correcting an average injection amount between the cylinders so that a difference from the second characteristic value is small.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The cylinder air-fuel ratio calculating means averages the output of the air-fuel ratio detecting means sampled a plurality of times during one cycle of the internal combustion engine for each section corresponding to one cycle / number of cylinders of the internal combustion engine. The air-fuel ratio control apparatus for an internal combustion engine is characterized in that the start timing of the averaging section is changed for each operating condition of the internal combustion engine.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 2,
The cylinder air-fuel ratio calculating means averages the first characteristic value obtained for each section over a plurality of cycles for each corresponding cylinder to obtain a characteristic value of each cylinder. Air-fuel ratio control device.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 2 or 3,
The air-fuel ratio calculation means for each cylinder makes the start timing of the section for averaging the output of the air-fuel ratio detection means earlier as the rotational speed or load of the internal combustion engine is larger. Control device.

In the air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 4,
Intake pressure detection means for detecting the intake pressure of the internal combustion engine,
The air-fuel ratio control apparatus for an internal combustion engine, wherein the target air-fuel ratio calculating means increases the target air-fuel ratio as the output of the intake pressure detecting means decreases.

The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 5,
An injection pressure detecting means for detecting a fuel injection pressure of the internal combustion engine;
The air-fuel ratio control apparatus for an internal combustion engine, wherein the target air-fuel ratio calculating means increases the target air-fuel ratio as the output of the injection pressure detecting means decreases.