JP2004011435A - Air-fuel ratio control device for multi-cylinder internal combustion engine - Google Patents

Air-fuel ratio control device for multi-cylinder internal combustion engine Download PDF

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JP2004011435A
JP2004011435A JP2002162037A JP2002162037A JP2004011435A JP 2004011435 A JP2004011435 A JP 2004011435A JP 2002162037 A JP2002162037 A JP 2002162037A JP 2002162037 A JP2002162037 A JP 2002162037A JP 2004011435 A JP2004011435 A JP 2004011435A
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air
fuel ratio
cylinder
engine
valve
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JP4126963B2 (en
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Shinichi Mitani
三谷　信一
Toshiaki Asada
浅田　俊昭
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Toyota Motor Corp
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Toyota Motor Corp
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  • Valve Device For Special Equipments (AREA)
  • Output Control And Ontrol Of Special Type Engine (AREA)
  • Electrical Control Of Air Or Fuel Supplied To Internal-Combustion Engine (AREA)
  • Combined Controls Of Internal Combustion Engines (AREA)

Abstract

<P>PROBLEM TO BE SOLVED: To restrain variation of an air-fuel ratio of each of cylinders, in an engine wherein a suction air amount is controlled by changing an open valve characteristic value. <P>SOLUTION: A valve lift amount and a working angle (vale opening period) of an intake valve 2 of the engine 1 is changed during operation using a variable valve mechanism 9, and an amount of intake air filling each of the cylinder is controlled. The valve lift amount and the working angle are changed by delaying an ignition timing of each of the cylinders at idle operating of the engine, to measure the air-fuel ratio of each of the cylinders by a single air-fuel ratio sensor 57 arranged to an exhaust passage, and air-fuel ratio correction coefficient for correcting the variation of the air-fuel ratio is calculated. At an operation other than the idle operation, the air-fuel ratio correction coefficient found at the idle operation is corrected in accordance with an engine operation condition(engine speed, load, and temperature), whereby a fuel injection amount of each of the cylinders is corrected without actually measuring the variation of the air-fuel ratio of each of the cylinders by the air-fuel ratio sensor. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、吸気弁開弁特性を変化させる可変バルブ機構を有する多気筒内燃機関の空燃比制御装置に関し、詳細には運転状態の変化にかかわらず各気筒の運転空燃比のばらつきを排除することが可能な多気筒内燃機関の空燃比制御装置に関する。
【０００２】
【従来の技術】
内燃機関の各気筒の吸入空気量に影響を与える吸気弁の開弁特性を変化させることにより、吸気通路の絞り弁（スロットル弁）による絞り損失を生じることなく各気筒の吸入空気量を調節するようにした内燃機関の制御装置が知られている。例えば、吸気弁のバルブリフト量、開弁期間（吸気弁カムの作用角）、バルブオーバラップ量等の開弁特性値を変化させると、他の条件が同一であっても気筒内に吸入される空気量は変化する。このため、これらの吸気弁開弁特性値のうち一つまたはそれ以上を運転中に変化させることにより、スロットル弁を用いることなく機関吸入空気量を制御する、いわゆる機関のノンスロットル運転が可能となる。このように、スロットル弁を用いないノンスロットル運転を行うことにより、スロットル弁による吸気絞り損失を低減して機関の熱効率を向上させることが可能となる。
【０００３】
機関のノンスロットル運転を行う場合には、各気筒に吸入される空気量はそれぞれの気筒毎の吸気弁の開弁特性値により決定されることとなる。ところが、各気筒の吸気弁、あるいは吸気弁の開弁特性値を変化させる可変バルブ機構には製造上及び制御上の誤差が生じるため各気筒の開弁特性値を同一に制御した場合でも実際には各気筒の開弁特性値にはばらつきを生じる。このため、各気筒毎の吸入空気量もそれぞれの吸気弁の開弁特性値のばらつきに応じてばらつくようになる。このため、仮に各気筒への燃料供給量が等しい場合であっても各気筒では気筒毎に運転空燃比がばらつくようになり、各気筒の発生トルクが変動する問題が生じる。
【０００４】
この種の空燃比制御装置の例としては、例えば特開平６−２１３０４４号公報に記載されたものがある。同公報の空燃比制御装置は、各気筒の吸気弁バルブリフトを変化させてノンスロットル運転を行う多気筒内燃機関の排気通路に酸素濃度センサを配置し、単一の酸素濃度センサを用いて各気筒の排気空燃比を測定するとともに、この排気空燃比のばらつきに応じて各気筒の吸気弁のバルブリフトを調節することにより、各気筒の吸入空気量を同一の値に調節している。これにより、各気筒の吸入空気量と空燃比とのばらつきが防止されるようになる。
【０００５】
【発明が解決しようとする課題】
ところが、上記特開平６−２１３０４４号公報の装置のように、機関のそれぞれの運転状態で実際に測定した各気筒の排気空燃比に基づいて各気筒のバルブリフトを調整することにより各気筒間の空燃比のばらつきを無くすようにしていると問題が生じる。
【０００６】
例えば、各運転状態における吸入空気量のばらつきは酸素濃度センサ等で測定した各気筒の排気空燃比のばらつきに基づいて算出されるが、実際には機関の運転状態が異なると排気ガスの流速が変化するため、酸素濃度センサの検出遅れ時間が変化する。また、特に特開平６−２１３０４号公報の装置のように複数の気筒の排気ガスを単一のセンサを用いて測定していると、上記検出遅れにも気筒毎のばらつきが生じ、しかもこのばらつきが機関回転数、負荷などによっても変化してしまう。このため、各運転状態においてセンサで検出した排気空燃比に基づいて吸入空気量を補正していると、例えば高回転、高負荷運転時等には補正誤差が大きくなり、極端な場合には制御にハンチングを生じるような場合もある。
【０００７】
また、機関が過渡状態にある場合には運転状態の変化に応じてセンサの検出遅れも変化の過渡状態にあるため、測定値の信頼性が低下し吸入空気量の補正精度が悪化するようになり、同様に誤補正や制御のハンチングが生じる場合がある。
【０００８】
本発明は、上記従来技術の問題に鑑み、吸気弁の開弁特性を変化させノンスロットル運転を行う多気筒内燃機関において、機関運転状態の変化にかかわらず正確に各気筒の空燃比のばらつきを無くすことが可能な多気筒内燃機関の空燃比制御装置を提供することを目的としている。
【０００９】
【課題を解決するための手段】
請求項１に記載の発明によれば、気筒内吸入空気量に影響を与える吸気弁の開弁特性値を変化させる可変バルブ機構を備えた多気筒機関の空燃比制御装置であって、機関の予め定めた基準運転状態において各気筒毎に測定した排気空燃比を用いて基準運転状態における各開弁特性値毎の各気筒の排気空燃比のばらつきを算出するとともに、算出したばらつきに基づいて、前記基準運転状態において各気筒の運転空燃比のばらつきを低減するための燃料噴射量の空燃比補正係数を各開弁特性値毎に算出する補正係数算出手段と、機関が前記基準運転状態以外の運転状態にあるときに、各開弁特性値における前記空燃比補正係数を機関運転状態を表す所定のパラメータの値に基づいて修正することにより、基準運転状態以外の運転状態における各気筒の運転空燃比のばらつきを低減するように各気筒の燃料噴射量を制御する、多気筒内燃機関の空燃比制御装置が提供される。
【００１０】
すなわち、請求項１の発明では基準運転状態において各気筒毎に測定した排気空燃比にもとづいて各開弁特性値毎の各気筒の空燃比のばらつきが算出され、このばらつきに基づいて各気筒の空燃比ばらつきを無くすための燃料噴射量の補正係数である空燃比補正係数が算出される。しかし、本発明では実際に測定した排気空燃比に基づいて各気筒の空燃比補正係数を算出するのは基準運転状態においてのみであり、他の運転状態における空燃比補正係数は、基準運転状態で求めた各開弁特性値毎の空燃比補正係数を機関の運転状態に応じて補正することにより求めるようにしている。
【００１１】
このため、基準運転状態として、例えば定常運転状態で、しかも排気ガスのセンサへの到達遅れ（輸送遅れ）が既知の、正確に各気筒毎の空燃比のばらつきの検出が可能な運転状態をとることにより、正確に基準運転状態における空燃比補正係数を算出することが可能となる。
なお、各気筒間の開弁特性値のばらつきの大きさは、開弁特性値に応じて変るため、基準運転状態における各気筒の空燃比補正係数も各開弁特性値毎に求められる。
【００１２】
本発明では更に、基準運転状態からの運転状態の変化と、その運転状態変化があった場合にも各気筒の空燃比のばらつきを低減するために必要な空燃比補正係数の修正を予め実験等により求めてある。そして、基準運転状態と異なる運転状態における空燃比補正係数の値は、基準運転状態における同一の開弁特性値における空燃比補正係数を運転状態に応じて修正することにより求められる。このため、基準運転状態における各開弁特性値の空燃比補正係数に基づいて、定常、過渡を問わず各運転状態における空燃比補正係数を正確に求めることができ、運転状態にかかわらず正確に各気筒の空燃比のばらつきをなくすことが可能となる。
【００１３】
請求項２に記載の発明によれば、前記補正係数算出手段は各気筒の空燃比を実質的に同一にするように前記燃料噴射量の空燃比補正係数を各開弁特性値毎に算出する、請求項１に記載の多気筒内燃機関の空燃比制御装置が提供される。
すなわち請求項２の発明では各気筒の空燃比を実質的に同一にするように空燃比補正係数が算出される。これにより、各気筒間の空燃比のばらつきが完全に防止されるようになる。
請求項３に記載の発明によれば、前記補正係数算出手段は、機関が前記予め定めた基準運転状態において機関が気筒内吸入空気量が最大になる基準開弁特性値で運転されているときの各気筒毎の空燃比ばらつきに基づいて、各気筒の空燃比のばらつきを低減するための燃料噴射量の基準補正係数を算出し、機関が前記基準開弁特性値以外で運転されている場合には、前記基準補正係数を用いて補正した後の燃料噴射量に対する前記空燃比補正係数を算出する、請求項１または請求項２に記載の多気筒内燃機関の空燃比制御装置が提供される。
【００１４】
すなわち、請求項３の発明では基準運転状態において機関が、筒内吸入空気量が最大になる開弁特性値である基準開弁特性値で運転されているときの各気筒毎の空燃比ばらつきに基づいて基準補正係数が算出される。機関が基準開弁特性値で運転されている状態とは、例えば開弁特性値としてバルブリフトを制御する場合には、バルブリフトが最大になるように制御された状態、開弁期間（作用角）を制御する場合には、作用角が最大になるように制御された状態である。
【００１５】
このように吸入空気量が最大になるように開弁特性値が制御された状態では、開弁特性値による吸入空気量のばらつきはほとんど無視できるほど小さくなり、各気筒における吸入空気量のばらつきは各気筒に至る吸気通路の長さや形状のわずかな差異によるもののみとなる。また、この状態では各気筒の吸入空気量のばらつきは小さいため、各気筒の燃料噴射弁などの特性ばらつきが空燃比のばらつきに大きく現れるようになる。
【００１６】
従って、基準開弁特性値での運転中に測定した各気筒毎の空燃比に基づいて算出した基準補正係数を用いて燃料噴射量を補正することにより、各気筒の開弁特性値のばらつき以外の原因による固有の空燃比ばらつきを補正することが可能となる。これにより、基準開弁特性値での運転以外の運転時に、基準開弁特性値での運転で求めた基準補正係数を用いてまず燃料噴射量を補正し、この補正後の燃料噴射量に対する空燃比補正係数を算出するようにすることにより、開弁特性値の変化による空燃比のばらつきを正確に補正することが可能となる。
【００１７】
請求項４に記載の発明によれば、前記機関運転状態を表すパラメータは、機関回転数、機関負荷又はアクセル開度のうち少なくとも１つを含む請求項１から請求項３に記載の多気筒内燃機関の空燃比制御装置が提供される。
【００１８】
すなわち、請求項４の発明では基準運転状態において算出した各気筒の各開弁特性値毎の空燃比補正係数は、機関回転数、機関負荷またはアクセル開度（アクセルペダルの踏込み量）のいずれか一つ以上のパラメータに応じて修正される。例えば、機関負荷またはアクセル開度が増大すると各気筒の吸入空気量はそれに応じて増大する。このため、機関負荷またはアクセル開度が増大すると開弁特性値が同一であっても各気筒の吸入空気量のばらつきは大きくなる。このため、空燃比補正係数の修正量は機関負荷またはアクセル開度が増大するとそれにつれて大きくなる。
【００１９】
一方、機関回転数が上昇すると、それに応じて吸入空気量が増大するため気筒間の吸入空気量のばらつきも大きくなるが、吸気ポートへの筒内既燃ガスの吹返しの量も回転数により変化するため、実際には回転数が一様に増加しても吸入空気量のばらつきは一様には増大せず、ばらつきが最大になる回転数が存在する。このため、開弁特性値が同一であった場合、各気筒の空燃比補正係数はある回転数までは回転数の上昇に応じて増大するが、この回転数を越えると回転数の上昇とともに空燃比補正係数が減少する。すなわち、空燃比補正係数が最大となるピーク回転数が存在する。
【００２０】
このように、基準運転状態における各気筒の空燃比補正係数を、機関回転数、機関負荷またはアクセル開度に応じて予め求めておいた関係に基づいて運転状態に応じて修正するようにしたことにより、運転状態にかかわらず正確に各気筒の空燃比のばらつきを低減することが可能となる。
【００２１】
請求項５に記載の発明によれば、前記機関運転状態を表すパラメータは、更に機関温度を含む、請求項４に記載の多気筒内燃機関の空燃比制御装置が提供される。
【００２２】
すなわち、請求項５の発明では請求項４の発明において更に、機関温度に応じて空燃比補正係数が修正される。可変バルブ機構を備えた機関では、機構の構成部品の熱膨張量が各気筒で必ずしも一様ではなく、気筒毎にばらつきが生じる。このため、機関温度上昇による各気筒の吸気弁の開弁特性値の変化は同一にはならず開弁特性値のばらつきが生じる。このばらつきは熱膨張量が大きいほど、すなわち温度が高いほど大きくなるため、空燃比補正係数の修正量も機関温度が高いほど大きくなる。
【００２３】
本発明では、機関温度に応じて空燃比補正係数を修正するようにしたことにより、各運転状態において更に正確に各気筒の空燃比のばらつきを低減することが可能となる。
【００２４】
請求項６に記載の発明によれば、前記補正係数算出手段は、排気通路に配置された単一の空燃比センサを用いて複数気筒の排気空燃比を測定する、請求項１または請求項２に記載の多気筒内燃機関の空燃比制御装置が提供される。
【００２５】
すなわち、請求項６の発明では排気通路に配置された単一の空燃比センサを用いて複数の気筒の排気空燃比を測定する。
本来、各気筒毎の排気空燃比を全ての運転状態において正確に測定できれば、その測定結果を用いて正確な空燃比のばらつき補正を行うことができる。
各気筒の排気行程の位相が異なっている場合、各気筒からの排気は時間差を持って排気通路の空燃比センサ設置位置に到達するため、機関回転速度に同期して空燃比センサの出力をサンプリングすることにより、単一の空燃比センサを用いて複数気筒の排気空燃比を個別に測定することができる。しかし、単一の空燃比センサを用いた場合には、運転状態によりセンサでの空燃比センサ検出遅れや、各気筒の排気のセパレーションが大きく変化するため、特定の測定条件が成立する場合以外の運転条件では各気筒毎の排気空燃比を正確に測定することはできない。
【００２６】
本発明では、実際に各気筒毎の空燃比を測定するのは基準運転状態においてのみであるため、上記の各気筒毎の排気空燃比を正確に測定できる特定の条件が成立する運転状態を基準運転状態として設定するようにすることにより、単一のセンサを用いた場合でも正確に各気筒毎の排気空燃比を正確に測定することが可能となる。
【００２７】
請求項７に記載の発明によれば、前記基準運転状態は、機関のアイドル運転状態である請求項６に記載の多気筒内燃機関の空燃比制御装置が提供される。
【００２８】
すなわち、本発明では基準運転状態として機関のアイドル運転状態が採用される。アイドル運転状態では、通常運転状態には変化が少なく、定常運転が行われる。また、回転数が低く、各気筒からの排気ガスが単一のセンサに到達する際の時間差が大きく、各気筒からの排気ガスのセパレーションが良好になるため気筒毎の排気ガス空燃比を正確に測定することができる。このため、アイドル運転状態を基準運転状態に設定して各気筒の排気空燃比を測定することにより、正確な空燃比制御が可能となる。
【００２９】
なお、例えば機関の点火時期を遅角させることにより、機関出力の増大を抑制しながら各気筒の吸入空気量を増大したアイドル運転を行うようにすれば、各気筒の排気ガス量が増大するため各気筒毎の空燃比の測定精度が更に向上する。
【００３０】
【発明の実施の形態】
以下、添付図面を用いて本発明の実施形態について説明する。
図１は本発明の空燃比制御装置を自動車用４気筒ガソリン機関に適用した場合の概略構成図、図２は図１の機関の吸気系統の概略構成を示す模式図、図３は図２の吸気系における空燃比センサ５７の配置を示す平面図である。
【００３１】
図１〜図３において、１は内燃機関、８は機関１の気筒内に形成された燃焼室、２は吸気弁、３は排気弁をそれぞれ示している。本実施形態では、吸気弁２の駆動用カムシャフト６と排気弁駆動用カムシャフト７とが独立して設けられている。図１〜図３において、４はカムシャフト６に設けられた吸気弁駆動用カム、５はカムシャフト７に設けられた排気弁駆動用カムを示している。
【００３２】
また、１３はクランクシャフト、１５は燃料噴射弁、１７は機関回転数を検出する回転数センサである。１９は機関全体の吸入空気量を検出するエアフローメータ、２０は内燃機関冷却水の温度を検出するための冷却水温センサ、２２はＥＣＵ（電子制御装置）である。５０はシリンダ、５２は吸気管、５３はサージタンク、５１はサージタンクと各気筒の吸気ポートとを接続する吸気マニホルドである。また、５４は排気管、５５は点火栓、５６は独立したアクチュエータ（図示せず）を備え、後述するＥＣＵ２２からの制御信号に応じてアクセル開度（アクセルペダル踏込み量）とは無関係に開度を変更可能なスロットル弁、５７は排気ガス空燃比を検出するための空燃比センサである。
【００３３】
本実施形態においては、排気弁駆動用カム５はカムシャフト軸線方向に一様なカムプロファイルを有する通常のカムが用いられているのに対して、吸気弁駆動用カム４は、カムシャフト６の軸線方向に沿ってカムプロファイルが変化する形状とされている。
図４は、吸気弁駆動用カム４の詳細形状を示す図である。図４に示すように、本実施形態の吸気弁カム４のカムプロファイルは、カムシャフト中心軸線方向に沿って変化しており、カムプロファイルのノーズ高さと作用角とは図４の右端から左端に向けて連続的に増大するようにカムプロファイルが設定されている。このため、吸気弁２のバルブリフト量と開弁期間とは、吸気弁２のバルブリフタのカム４との接触位置に応じて変化し、バルブリフタの接触位置がカムの右端から左端に移動するに従ってバルブリフト量は大きく、かつ吸気弁の開弁期間は長くなる。
【００３４】
本実施形態では、可変バルブ機構９を用いて機関運転中にカムシャフトを軸線方向に移動させることにより、吸気弁２のバルブリフト量と開弁期間等の開弁特性値を変化させることが可能となっている。すなわち、可変バルブ機構９を用いて、カムシャフト６を機関運転中に軸線方向にスライドさせることにより吸気弁カム４とバルブリフタとの接触位置を変化させ、吸気弁２の駆動に使用するカムプロファイルを変化させることが可能となっている。
【００３５】
吸気弁２のバルブリフト量が増大すると、吸気弁の開弁期間が同一であっても気筒内に吸入される空気量が増大する。また、カムの作用角（吸気弁の開弁期間）が大きく（長く）なると、バルブリフト量が同一であっても気筒内に吸入される空気量は増大する。本明細書では、上記の吸気弁バルブリフト量、作用角（開弁期間）等のように気筒内吸入空気量に影響を与える吸気弁動作パラメータを開弁特性値と称している。
【００３６】
図５は可変バルブ機構９の動作原理を示す断面図である。図５において、３０は吸気弁用カムシャフト６に連結された磁性体、３１は磁性体３０を駆動するためのソレノイド、３２は磁性体３０を図５右側方向に向けて付勢するための圧縮ばねである。本実施形態の可変バルブ機構では、コイル３１に通電が行われると磁性体３０は、ばね３２の付勢力に抗して図５左方向に移動し、吸気弁２のバルブリフタとカム４との接触位置はカムシャフト軸線方向に変位する。磁性体３０の移動量はソレノイド３１への通電電流に応じて変化するため、本実施形態では、ソレノイド３１への通電電流を制御することにより吸気弁２のバルブリフタとカム４との接触位置、すなわち吸気弁２の開弁特性値を制御することが可能となっている。本実施形態では、ソレノイド３１への通電電流が増大するに従って、カムシャフト６は図４、図５において左側方向に移動し、吸気弁２のバルブリフト量と開弁期間とが減少する。このため、本実施形態では、ソレノイド３１に通電していない状態で機関１の各気筒の吸入空気量は最大になり、通電電流が増大するにつれて各気筒の吸入空気量は減少する。
【００３７】
図１に１６で示すのは、吸気弁２の開弁特性値（バルブリフト量、開弁期間）を検出する開弁特性値センサである。前述したように、本実施形態では吸気弁２の開弁特性値はカムシャフト６の軸線方向移動量に応じて変化するため、カムシャフト６の軸線方向位置が定れば吸気弁２の開弁特性値も決定される。このため、本実施形態では開弁特性値センサ１６としては、吸気弁カムシャフト６の軸線方向位置（移動量）を検出する軸位置センサが用いられ、ＥＣＵ２２は開弁特性値センサ１６で検出したカムシャフト位置を用いて、予め記憶した関係に基づいて吸気弁２のバルブリフト量、開弁期間などの開弁特性値を算出している。
【００３８】
なお、本実施形態では、吸気弁２のカム４のプロファイルは軸線方向に沿ってノーズ高さと作用角との両方が同時に変化するように設定されているが、ノーズ高さ（バルブリフト量）のみ、あるいは作用角（開弁期間）のみが変化するようにしても、可変バルブ機構９を用いて各気筒の吸入空気量を制御することが可能である。
また、本実施形態では吸気弁の開弁特性値のみを変更しているが、排気弁にも９と同様な可変バルブ機構を設け、排気弁の開弁特性値をも変更するようにすることも可能である。
更に、それぞれの弁毎に例えば電磁アクチュエータ等の駆動装置を有する独立駆動式の吸気弁または／及び排気弁を有する機関では、それぞれの駆動装置を制御することにより個々の弁の開弁特性値を変更することが可能である。
【００３９】
前述したように、本実施形態では、スロットル弁５６は独立したアクチュエータを備えており、運転者のアクセルペダル踏込み量（アクセル開度）と機関運転状態とに基づいてＥＣＵ２２により制御される。
本実施形態では気筒吸気弁のバルブリフト量、開弁期間などの開弁特性値を変更することにより各気筒の吸入空気量を制御することができる。このため、本実施形態では吸入空気量が比較的少ない領域ではスロットル弁５６は全開状態に保持したまま吸気弁の開弁特性値を変更することにより吸入空気量を制御するようにして、吸気管絞りによるポンピング損失を低減した、いわゆるノンスロットル運転を行うことが可能となる。
【００４０】
ところが、実際の機関では各気筒の吸気弁２やカム４等を含む動弁系には、製作誤差、運転上の熱変形等によるばらつきが生じるため各気筒の吸気弁開弁特性値を同一に制御しても気筒内に充填される空気量にはばらつきが生じ、同一にはならない。
更に、動弁系のばらつき以外にも例えば、各気筒に至る吸気通路の長さの差等により、各気筒の吸入空気量には差が生じている。また、吸入空気量のばらつきに加えて各気筒に噴射される燃料量にも各気筒の燃料噴射弁の製作公差内でばらつきが生じている。
【００４１】
このため、実際の機関では各気筒の吸気弁開弁特性と燃料噴射量との設定値を同一に制御していても、気筒毎の吸入空気量と燃料噴射量とにはばらつきを生じるため、各気筒の燃焼空燃比にもばらつきを生じるようになる。このため、気筒間の空燃比ずれによる排気性状の悪化や発生トルクの不均一が生じる問題がある。
特に、本実施形態のように吸気絞りを排除したノンスロットル運転を行う場合には、各気筒の吸入空気量は吸気弁の開弁特性値により決定されるため、開弁特性値の各気筒でのばらつきが吸入空気量のばらつきに直接影響するようになる。
このような吸入空気量等のばらつきによる各気筒毎の空燃比のばらつきは、各気筒からの排気空燃比を計測して空燃比のばらつきを低減するように燃料噴射量を各気筒毎に補正することにより解消することができる。
【００４２】
本実施形態では、機関１の排気マニホルドには排気空燃比を検出する空燃比センサ５７が配置されている。本来、各気筒の排気空燃比を正確に検出するためには、各気筒の排気系毎に空燃比センサを配置することが好ましい。しかし、各気筒では気筒の工程サイクルは所定のクランク角ずつ異なっており、例えば本実施形態のような４サイクル４気筒機関では各気筒の排気行程はクランク角１８０度（１８０ＣＡ）ずつずれている。
従って、単一の空燃比センサ５７を用いた場合でも、測定条件が整っている場合には正確に各気筒の燃焼空燃比を計測することが可能となる。
【００４３】
上記の、単一の空燃比センサ５７を用いて各気筒の燃焼空燃比を正確に計測することが可能な条件とは、例えば
（Ａ）機関が定常運転されていること。
（Ｂ）機関回転数が低速であること。
（Ｃ）各気筒からの排気流量が多いこと。
等である。
すなわち、単一の空燃比センサ５７を用いて各気筒の空燃比を正確に検出するためには、空燃比センサ５７への各気筒からの排気到達時期（輸送遅れなど）が一定である必要があるため、機関が定常運転されていること（上記（Ａ））が必要となる。
【００４４】
また、機関回転数が高くなると各気筒から排出された排気が空燃比センサ５７の位置に到達する前に混合してしまい、それぞれの気筒からの排気のセパレーションが悪化するため気筒毎の空燃比を正確に計測できなくなる。従って、単一の空燃比センサ５７を用いて各気筒の排気空燃比を正確に検出するためには機関が低速で運転されている状態であること（上記（Ｂ））が好ましい。
【００４５】
更に、空燃比センサ５７で各気筒からの排気空燃比を検出する際には、各気筒からの排気流量が多ければ多いほど検出精度が向上する。このため、単一の空燃比センサ５７を用いて各気筒の排気空燃比を検出する際にも出来るだけ排気流量が多い状態で（上記条件（Ｃ））計測を行うことが好ましい。
【００４６】
なお、仮に各気筒の排気系毎に空燃比センサを設けた場合にも正確な計測を行うためには上記（Ａ）から（Ｃ）の条件が成立した状態で計測を行うことが好ましいが、本実施形態のように単一の空燃比センサ５７を用いて各気筒の排気空燃比を計測する際には、上記（Ａ）から（Ｃ）の条件が特に重要となる。
【００４７】
ところが、実際の運転では上記の（Ａ）から（Ｃ）の条件が常に成立するわけではなく、機関の高速運転、過渡運転なども頻繁に行われる。
一方、吸気弁の動弁系のばらつきに起因する各気筒毎の空燃比のばらつきは、吸気弁のバルブリフト量や開弁期間（作用角）等の開弁特性値だけでなく、機関回転数、負荷などの機関運転状態に応じて変化する。また、これらの空燃比のばらつきは機関の稼働時間によっても変化する。従って、気筒毎の空燃比のばらつきを補正するためには本来、全ての運転状態でその都度各気筒の排気空燃比を計測することが必要となるが、上述のように単一の空燃比センサを用いて各気筒の排気空燃比を計測する場合、高速運転時、過渡運転時などでは正確に各気筒の排気空燃比を検出することができない。このため、単一の空燃比センサ出力に基づいて気筒毎の空燃比のばらつきを補正していると、機関の運転状態によっては誤補正や制御のハンチングを生じる場合がある。
【００４８】
本実施形態では、各気筒毎の空燃比のばらつきを補正する際に、以下の方法で単一の空燃比センサを用いながら正確に各運転状態における空燃比のばらつきを補正する。
以下、本実施形態における気筒毎の空燃比ばらつき補正操作について説明する。
本実施形態における空燃比ばらつき補正は、１．基準運転状態ににおける各気筒毎の空燃比計測と空燃比補正係数の算出と、２．機関運転状態に応じた空燃比補正係数の修正、の２段階で行われる。
【００４９】
以下、それぞれについて説明する。
１．基準運転状態における気筒毎空燃比計測と空燃比補正係数の算出。
本実施形態では、まず、上記の（Ａ）から（Ｃ）の条件を満足する機関運転状態（基準運転状態）で機関を運転し、この運転状態における空燃比センサ５７の出力から各気筒の排気空燃比を計測する。
【００５０】
すなわち、本実施形態では基準運転状態として、機関が必ず定常（条件（Ａ））かつ低速（条件（Ｂ））で運転されるアイドル運転時に排気流量を増大（条件（Ｃ））させるために点火時期を所定量遅角した状態をとり、この基準運転状態で吸気弁のバルブリフト、作用角等の開弁特性値を変化させて各気筒の空燃比を計測する。
この状態では、各気筒の排気空燃比を最も正確に計測することができるため、空燃比ばらつきをなくすための各気筒の燃料噴射量の補正量（補正係数）を正確に算出することができる。
【００５１】
なお、本実施形態ではＥＣＵ２２は別途実行される図示しない燃料噴射量演算操作により、エアフローメータ１９で検出した吸入空気量Ｑと機関回転数Ｎとに基づいて燃料噴射量設定値Ｆを、Ｆ＝（Ｑ／Ｎ）×Ｋ×ＦＡＦの形で算出する。ここで係数Ｋは、１回転当りの吸入空気量（Ｑ／Ｎ）に対して各気筒での燃焼空燃比を目標空燃比（例えば理論空燃比）にするために必要とされる１回当りの燃料噴射量、ＦＡＦは空燃比センサ５７出力に基づいて検出した機関全体としての平均運転空燃比を目標空燃比にするためのフィードバック補正係数である。本実施形態ではフィードバック補正係数ＦＡＦについては、公知の適宜な方法で設定されるが、本発明の技術的特徴とは直接関係しないため詳細な説明は省略する。
【００５２】
燃料噴射演算操作により算出される燃料噴射量設定値Ｆは各気筒に共通の値となる。各気筒の実際の吸入空気量と実際の燃料噴射量が同一であれば、各気筒の燃料噴射弁に燃料噴射量設定値Ｆに対応する燃料噴射信号を入力することにより、各気筒の空燃比は同一になる。しかし、実際には各気筒の吸入空気量にはばらつきがあり、更に、同一の燃料噴射信号Ｆを入力した場合でも各燃料噴射弁の燃料噴射量にはばらつきが生じるため、各気筒の空燃比は同一にはならない。
【００５３】
本実施形態では、基準運転状態で計測した各気筒の排気空燃比から、各気筒の空燃比のばらつきを算出し、空燃比のばらつきを低減するために必要とされる燃料噴射量の補正係数（空燃比補正係数）Ａｉ（ｉは気筒番号）を気筒毎に算出する。本発明では空燃比補正係数の算出方法として、公知の任意の方法を採用することができるが、本実施形態では、各気筒の空燃比のばらつきを完全になくして各気筒の空燃比を一致させるようにしており、例えば各気筒の排気空燃比をＡＦｉとしたときの各気筒の空燃比補正係数Ａｉを、
【００５４】
Ａｉ＝ＡＦｉ／（（１／ｎ）Σ（１〜ｎ）ＡＦｉ）として算出する。
ここで、（１／ｎ）Σ（１〜ｎ）ＡＦｉは全気筒の空燃比の算術平均値である。上記により算出した空燃比補正係数Ａｉを用いて上記の各気筒共通の燃料噴射量設定値Ｆを補正し、各気筒の燃料噴射弁にＡｉ×Ｆの大きさの燃料噴射信号を供給することにより、各気筒の空燃比は上記平均空燃比（１／ｎ）Σ（１〜ｎ）ＡＦｉに一致するようになる。
【００５５】
上述の空燃比補正係数Ａｉは、基準運転状態において吸気弁の開弁特性値を変えて、各開弁特性値毎に作成する。
ところで、前述したように、機関の基準運転状態においては各気筒の空燃比のばらつきを正確に測定することができる。しかし、空燃比のばらつきには（１）作用角等の開弁特性値のばらつきに起因するものと、（２）各気筒の吸気通路の長さの差による空気量のばらつきや燃料噴射弁の特性のばらつきに起因するものとの２種類がある。
【００５６】
しかも、これらのばらつきのうち、各気筒毎の作用角などの開弁特性値に起因する空燃比のばらつきは開弁特性値が変化するとばらつきの大きさも変化するのに対して、吸気通路長さや燃料噴射弁特性によるばらつきは作用角が変化してもほとんど変化しない。
このため、正確に空燃比のばらつきを補正するためにはこれらの２種類のばらつきを区別して取扱う必要がある。そこで、本実施形態ではまず、上記（２）の、開弁特性値とは無関係なものに起因する空燃比のばらつきを求め、このばらつきを補正した状態で（１）の開弁特性値毎のばらつきに対する補正係数を求めることとしている。
【００５７】
作用角、バルブリフトなどの開弁特性値のばらつきによる空燃比（吸入空気量）のばらつきは、吸入空気量が少ない状態ほど、言換えれば作用角及びバルブリフト量が小さいほど大きくなり、逆に吸入空気量が最大となった状態、すなわち作用角とバルブリフトと量とが最大になった状態では、開弁特性値に起因する各気筒の吸入空気量のばらつきはほとんど無視できる程度になる。すなわち、作用角とバルブリフト量とが最大の状態では、各気筒における空燃比のばらつきは、吸気通路長さや燃料噴射弁特性のばらつきに起因するもの（上記（２））のみになる。
【００５８】
そこで、本実施形態ではまず、基準運転状態において吸入空気量が最大になる開弁特性値（基準開弁特性値）で機関を運転し、このときの空燃比補正係数を各気筒の基準補正係数Ｘｉとして算出する。
【００５９】
すなわち、Ｘｉ＝ＡＦｉ／（（１／ｎ）Σ（１〜ｎ）ＡＦｉ）
そして、上記吸入空気量が最大になる開弁特性値以外の開弁特性値では、予め基準補正係数Ｘｉを用いて燃料噴射設定値Ｆを補正した量の燃料Ｘｉ×Ｆをそれぞれの気筒に噴射した状態での空燃比のばらつきをもとめ、空燃比補正係数Ａｉを算出する。これにより、空燃比補正係数Ａｉは各気筒の開弁特性値のばらつきのみに起因する空燃比のばらつきに対応した値となる。なお、基準開弁特性値における各気筒の空燃比補正係数Ａｉは１．０とする。
これにより、基準運転状態における各開弁特性値毎の各気筒の空燃比補正係数Ａｉが求められる。
【００６０】
２．機関運転状態に応じた空燃比補正係数Ａｉの修正。
次に機関運転状態に応じた空燃比補正係数Ａｉの修正について説明する。上述したように、基準運転状態で求めた空燃比補正係数Ａｉは、開弁特性値のばらつきのみに起因する各気筒の空燃比のばらつきに対応したものとなっている。
ところが、各気筒の開弁特性値のばらつきは、開弁特性値そのものの値により変化するだけでなく、機関の運転状態によっても変化する。
【００６１】
例えば、機関負荷が増大するとも開弁特性値が同一であっても吸入空気量は増大する。このため、開弁特性値が同一であっても各気筒の空燃比のばらつきは大きくなるため、空燃比補正係数による燃料噴射量の補正量も開弁特性値の値が同一であっても機関負荷とともに大きくする必要がある。
【００６２】
また、機関回転数が増大すると負荷が増大したと場合と同様に吸入空気量は増大する。しかし、この場合には回転数により吸気ポートへの既燃ガスの吹返し量が変化するため、回転数が増大した場合気筒内吸入空気量はある回転数までは一様に増大するが、それ以上では回転数とともに減少するようになる。従って、開弁特性値が同一であっても各気筒の空燃比のばらつきは、回転数と共に変化し、ばらつきが最大になるピーク回転数が存在する。
【００６３】
更に、各気筒の空燃比のばらつきは機関温度によっても変化する。例えば、機関温度が上昇するとカムシャフトも熱膨張する。本実施形態では、図５に示した可変バルブ機構９を用いて開弁特性値を変化させているため、カムシャフト６は磁性体３０を固定点として図５の左側に熱膨張することになる。このため、各気筒におけるカム４の熱膨張による移動量（すなわち熱膨張による各気筒の開弁特性値変化量）は、磁性体３０から離れた側の気筒ほど大きくなる。
従って、機関温度に応じて各気筒での熱膨張による開弁特性値の変化を気筒毎に補正する必要が生じる。
【００６４】
本実施形態では、予め機関負荷、回転数、機関温度による空燃比補正係数の変化を実験または計算により求めてあり、基準運転状態における空燃比補正係数に対する修正係数の形でＥＣＵ２２のＲＯＭに格納してある。そして、基準運転状態と異なる負荷、回転数、機関温度の運転では、基準運転状態において算出した空燃比補正係数をこれらの修正係数に基づいて補正するようにしている。
これにより、本実施形態では正確に各気筒の排気空燃比を計測することが困難な運転状態においても正確に各気筒の空燃比を一致させるように各気筒の燃料噴射量を補正することが可能となっている。
【００６５】
図６、図７は、上記に説明した空燃比制御操作を説明するフローチャートであり、図６は空燃比補正係数Ａｉ及び基準補正係数Ｘｉの学習操作を、図７は図６の操作で学習したＡｉ及びＸｉを用いた燃料噴射量補正操作を、それぞれ示している。図６、図７の操作は、ＥＣＵ２２により一定時間毎または一定クランク回転角毎に実行されるルーチンにより行われる。
【００６６】
まず、図６の学習操作について説明する。
図６において操作がスタートすると、ステップ６０１では、まず今回機関を始動してから各気筒の補正係数Ａｉ、Ｘｉの学習が完了しているか否かが判定され、すでに完了している場合はそのまま本操作を終了する。
【００６７】
ステップ６０１で、まだ補正係数の学習が完了していない場合には、次にステップ６０３に進み、現在機関が基準運転状態で運転されているか否かが判定される。前述したように空燃比補正係数Ａｉ及び基準補正係数Ｘｉは各気筒毎の空燃比を空燃比センサ５７で正確に検出できる状態で学習する必要がある。本実施形態では、車両が停止中であり（例えば車速が３ｋｍ／ｈ以下）、かつ機関がアイドル運転中（スロットル弁開度がゼロ）の状態のときを基準運転状態と判断し、学習を行う。機関が基準運転状態にないときには、本操作はそのまま終了する。
次に、ステップ６０５では学習を行うべき値に各気筒の吸気弁の開弁特性値（バルブリフト、開弁期間）を設定する。
【００６８】
前述したように、空燃比センサ５７で正確に各気筒の空燃比を検出するためにはできるだけ各気筒の排気流量が大きいことが好ましい。そこで、本実施形態では機関回転数をアイドル回転数に維持しながら各気筒の点火時期を遅延させた状態で学習を行う。点火時期を遅延させると各気筒の出力トルクが低下するため、トルクの低下を補いつつアイドル回転数を一定に維持するために各気筒の開弁特性値は大流量側にシフトされ、排気流量が増大する。ステップ６０５では、吸気弁の開弁特性値を大流量側にシフトさせ、各気筒の吸入空気量を増大させると同時に、各気筒の点火時期を調節（遅角）することにより機関のアイドル回転数を一定に維持する。
【００６９】
ステップ６０７では、上記の開弁特性値設定により機関のアイドル回転数を維持したまま気筒の吸入空気量が充分に増大したか否か、すなわち学習条件が成立したか否かを判定し、成立していない場合には成立するまで待つこととし、今回の操作は終了する。これにより、学習条件が成立するまで図６の操作が繰返される。
【００７０】
なお、本実施形態ではステップ６０７で学習条件が成立した場合には、ステップ６１９から６２５により、そのときの開弁特性値θにおける空燃比補正係数Ａｉ（θ）が算出され、その後ステップ６０５での開弁特性値の設定を所定量ずつ変更して、各気筒毎に各開弁特性値θにおける空燃比補正係数Ａｉが全部算出されるまで図６の操作を繰返す。
【００７１】
また、ステップ６０５では、まず現在の運転状態でアイドル回転数を維持しながら点火時期遅角により吸入空気量が最大になる開弁特性値（基準開弁特性値）の空燃比補正係数を算出し、この基準開弁特性値における各気筒の空燃比補正係数を基準補正係数Ｘｉとして記憶し、次回の図６の操作実行時からは、ステップ６０５では開弁特性値を所定量ずつ、吸入空気量が低下する側に変更するとともに、機関回転数が変化しないように点火時期を調整する操作を行う。
【００７２】
すなわち、本実施形態では補正係数の学習時にはまず各気筒の吸入空気量が現在の回転数で最大になるように開弁特性値を調整して各基準補正係数を算出し、その後吸入空気量を減少させながら他の開弁特性値における空燃比補正係数を算出するようにしている。
前述したように、まず学習時に基準補正係数を算出するのは気筒の吸入空気量がある程度大きくなる開弁特性値での運転では、各気筒間の空燃比のばらつきは、吸気通路長さや燃料噴射弁の特性ばらつきによるもののみとなるためである。これらのばらつきは開弁特性値を変化させても変化しない。このため、本実施形態ではまず基準補正係数をＸｉを求め、このＸｉで各気筒の空燃比が等しくなるように燃料噴射量を補正した状態で他の開弁特性値における空燃比補正係数Ａｉを算出するようにしている。
すなわち、本実施形態における空燃比補正係数Ａｉは、各開弁特性値における各気筒の、開弁特性値のみに起因する空燃比のばらつきに対応した値となっている。
【００７３】
上記の操作を具体的に説明すると、図６ステップ６０７で学習条件が成立した場合には、ステップ６０９に進み、現在の開弁特性値が各気筒の最大吸入空気量に対応する、予め定めた値（基準開弁特性値）になっているか否かを判定し、基準開弁特性値になっている場合には、ステップ６１３で空燃比センサ５７出力に基づいて各気筒の排気空燃比を計測する。
【００７４】
すなわち、ステップ６１３では、各気筒の排気行程に排出された排気が、空燃比センサ５７に到達するタイミング毎にクランク回転角に同期して空燃比センサ５７出力をサンプリングし、各気筒の排気空燃比ＡＦｉを求め、ステップ６１５では各気筒の排気空燃比の平均値（（１／ｎ）Σ（１〜ｎ）ＡＦｉ）と、それぞれの気筒の排気空燃比ＡＦｉとから各気筒の空燃比が同一なるように燃料噴射量を修正するための基準補正係数Ｘｉを、Ｘｉ＝ＡＦｉ／（（１／ｎ）Σ（１〜ｎ）ＡＦｉ）として算出する。前述したように、基準補正係数Ｘｉは、各気筒の開弁特性値以外の要素（吸気通路長さや燃料噴射弁特性のばらつき）に起因する各空燃比のばらつきに対応する値となる。
そして、ステップ６１５で各気筒の基準補正係数Ｘｉを算出後、ステップ６１７では算出したＸｉを記憶（学習）して、今回の操作は終了する。
【００７５】
次回に図６の操作が開始されると、ステップ６０５では開弁特性値θが、吸入空気量が低下する側（例えばバルブリフト量が減少、及びカム作用角が減少する側）に所定量シフトされる。これにより、ステップ６０９の次にはステップ６１９から６２５が実行され、各気筒の各開弁特性値における空燃比補正係数Ａｉ（θ）が求められる。
すなわち、ステップ６１９では各気筒の排気空燃比が計測され、ステップ６２１で、現在の開弁特性値θにおける仮の空燃比補正係数Ａｉ（θ）′が、
Ａｉ（θ）′＝ＡＦｉ／（（１／ｎ）Σ（１〜ｎ）ＡＦｉ）
として算出される。
【００７６】
ステップ６１９で算出された仮の空燃比補正係数Ａｉ（θ）′は、機関の熱膨張による各気筒により生じる各気筒の空燃比ばらつきと、各気筒の開弁特性値以外の要素に起因する各気筒の空燃比のばらつきとを含んだ値となっている。そこで、ステップ６２３では、各気筒の、後述する熱変形に対する補正係数Ｒｉ（ＴＬ）とステップ６１７で記憶した基準補正係数ＸｉＪとを用いて、仮の空燃比補正係数Ａｉ（θ）′を補正し、各気筒の真の空燃比補正係数Ａｉ（θ）を算出する。
すなわち、Ａｉ（θ）＝Ａｉ（θ）′／（Ｘｉ×Ｒｉ（ＴＬ））
【００７７】
なお、上記の各式において、添字ｉは気筒番号を、θは開弁特性値を表し、ＴＬは機関温度（潤滑油温度）を表している。上記により、ステップ６２３で算出された空燃比補正係数Ａｉ（θ）は、機関の熱膨張によるばらつきや、吸気通路長さや燃料噴射弁特性のばらつきによる各気筒の空燃比のばらつきへの影響を排除した、純粋に開弁特性値のみに起因する各気筒の空燃比のばらつきに対応したものとなる。
【００７８】
ステップ６２５では、ステップ６２３で算出した空燃比補正係数を記憶した後、本操作の今回の実行は終了する。なお、本操作は予め定めた開弁特性値の値全てについて各気筒の空燃比補正係数Ａｉ（θ）を算出するまで繰返された後終了する。
【００７９】
図７は、図６の学習操作により記憶した補正係数ＸｉとＡｉ（θ）とを用いた燃料噴射量補正操作を示すフローチャートである。
図７の操作では、基準運転状態で求めた各気筒の空燃比補正係数Ａｉ（θ）を機関運転状態を表すパラメータの値に基づいて補正することにより、基準運転状態以外の運転状態においても、正確に各気筒の空燃比が一致するように燃料噴射量を気筒毎に補正する。
【００８０】
図７において、ステップ７０１では機関回転数Ｎ、吸入空気量Ｑ、機関潤滑油温度ＴＬがそれぞれのセンサにより検出され、ステップ７０３では現在の吸気弁の開弁特性値θが、開弁特性値センサ１６（図１）により検出される。
そして、ステップ７０５では、図６のステップ６１７で記憶した各気筒の基準補正係数Ｘｉが読出される。
また、ステップ７０７では、ステップ７０３で検出した現在の開弁特性値θに基づいて、ステップ６２５で記憶した各気筒の空燃比補正係数Ａｉ（θ）が読出される。
【００８１】
更に、ステップ７０９では、機関回転数Ｎと機関吸入空気量Ｑとに基づいて空燃比補正係数の回転数Ｎと負荷（Ｑ／Ｎ）における各気筒の修正係数Ｌｉが、決定される。本実施形態では、前述のように機関回転数Ｎと吸入空気量Ｑとを変えて機関を運転し、基準運転状態で求めた空燃比補正係数Ａｉ（θ）がどのように変化するかを予め求めてあり、ＮとＱとをパラメータとする２次元数値マップの形で回転数負荷修正係数Ｌｉとして予めＥＣＵ２２のＲＯＭに格納してある。ステップ７１３では、機関回転数Ｎと機関吸入空気量Ｑとに基づいて上記マップから現在の回転数と負荷とにおける回転数負荷修正係数Ｌｉが算出される。
なお、ステップ７０９では機関負荷を表すパラメータとして機関１回転当りの吸入空気量（Ｑ／Ｎ）をとっているが、機関負荷を表すパラメータとしてＱ／Ｆの代りにアクセル開度（アクセルペダルの踏込み量）を用いても良い。
また、ステップ７０９では、機関回転数と負荷とに対する修正を１つの修正係数Ｌｉで行っているが、機関回転数に対する修正係数と機関負荷に対する修正係数とを個別に設けるようにすることも可能である。
【００８２】
更に、ステップ７１１では、機関の熱膨張を表す各気筒の機関温度修正係数Ｒｉが、ステップ７０１で検出した潤滑油温度（機関温度）ＴＬに基づいて算出される。前述したように、機関の熱膨張による空燃比のばらつきは各気筒の位置関係により異なってくる。本実施形態では、予め空燃比補正係数に対する機関の熱膨張の影響を各機関温度（潤滑油温度）毎に実験により求め、潤滑油温度ＴＬをパラメータとした１次元マップとしてＥＣＵ２２のＲＯＭに格納してあり、ステップ７１１ではこのマップに基づいて現在の潤滑油温度ＴＬにおける各気筒毎の機関温度修正係数Ｒｉ（ＴＬ）が算出される。
【００８３】
次いで、ステップ７１３では、ステップ７０７で読出した現在の開弁特性値θにおける基準運転状態の各気筒の空燃比補正係数Ａｉ（θ）が、ステップ７０９で算出した回転数負荷修正係数Ｌｉ、ステップ７１１で算出した機関温度修正係数Ｒｉ（ＴＬ）とを用いて、Ａｉ（θ）×Ｌｉ×Ｒｉ（ＴＬ）に修正され、修正後の空燃比補正係数を用いて、別途図示しない操作により機関回転数と負荷（Ｑ／Ｎ）とに基づいて算出された燃料噴射量設定値Ｆが修正される。これにより、現在の運転状態で各気筒の空燃比を一致させるために必要とされる各気筒の燃料噴射量Ｆｉ（θ）が、
【００８４】
Ｆｉ（θ）＝Ｆ×Ｘｉ×Ａｉ（θ）×Ｌｉ×Ｒｉ（ＴＬ）
として算出される。
上述のように、本実施形態によれば、単一の空燃比センサ５７を用いた場合でも正確に各気筒毎の空燃比を検出可能な基準運転状態で各気筒の空燃比補正係数を求め、この空燃比補正係数を機関運転状態を表す所定のパラメータを用いて修正することにより、単一の空燃比センサ５７では正確に各気筒の空燃比を検出することのできない運転状態においても、正確に各気筒の空燃比を一致させることが可能となっている。
【００８５】
ところで、前述のように、本実施形態では吸気弁の開弁特性値を変化させることにより、スロットル弁を使用せずに、可変バルブ機構９を使用して機関の吸入空気量を制御することが可能となっており、スロットル弁による絞り損失をなくした熱効率の高いノンスロットル運転が可能となっている。
しかし、可変バルブ機構９を用いて吸入空気量を制御する場合には、スロットル弁を用いて吸入空気量を制御する場合に較べて上記ノンスロットル運転が可能であることの他にも有利な点がある。
【００８６】
可変バルブ機構９を用いた場合にはスロットル弁による吸気絞りに較べて、極めて短時間で気筒の吸入空気量を変化させることが可能となる。
通常、スロットル弁を用いて気筒の吸入空気量を変化させる場合には、吸気弁下流側のサージタンクや吸気マニホルドなどの容積がデッドボリュームとなるため、スロットル弁の開度を変えてから実際に気筒内吸入空気量が変化するまでに時間を要し、吸入空気量は比較的緩やかに変化するようになる。
【００８７】
これに対して、吸気弁の開弁特性値を変化させると気筒内吸入空気量は極めて短時間にステップ状変化に近い変化をするようになる。
このため、例えばＮＯＸ吸蔵触媒から吸蔵したＮＯＸを放出させ、還元浄化する場合には吸気弁の開弁特性値を変化させることにより気筒内吸入空気量を変化させるほうが良好な結果を得ることができる。
【００８８】
気筒内で理論空燃比より希薄（リーン）な燃焼を行う内燃機関の排気系に、流入する排気の空燃比がリーンのときに排気中のＮＯＸを吸着、吸収またはその両方にて選択的に保持し、流入する排気の空燃比が理論空燃比またはリッチ空燃比となったときに、吸蔵したＮＯＸを排気中の還元成分を用いて還元浄化するＮＯＸ吸蔵触媒を設け、機関のリーン空燃比運転中にＮＯＸ吸蔵触媒に排気中のＮＯＸを吸蔵除去するリーンバーン機関の排気浄化装置が知られている。
【００８９】
このようなＮＯＸ吸蔵触媒を使用した排気浄化装置では、ＮＯＸ吸蔵触媒が吸蔵したＮＯＸで飽和してしまうともはや排気中のＮＯＸを除去できなくなるため、ＮＯＸ吸蔵触媒に吸蔵したＮＯＸの量が所定量に到達する毎に機関をリーン空燃比運転から理論空燃比またはリッチ空燃比に切換えて、ＮＯＸ吸蔵触媒に理論空燃比またはリッチ空燃比の排気を供給して歯意中の還元成分によりＮＯＸ吸蔵触媒に吸蔵されたＮＯＸを還元浄化する必要がある。
【００９０】
通常、機関をリーン空燃比運転から理論またはリッチ空燃比運転に切換えるためには、各気筒に供給する燃料を増大すると同時に、気筒内に吸入される空気量を低減する必要がある。
この場合、例えば通常の機関では、スロットル弁を絞り、吸入空気量を低減するとともに、ＥＧＲ（排気ガス再循環）装置により吸気系に排気ガスの一部を循環させて気筒に吸入される新気の量を低減することにより、気筒内に吸入される空気（新気）量を低減する。しかし、スロットル弁下流側にはサージタンク、吸気マニホルドなどの比較的大きな容積が存在する。このため、スロットル弁開度を急激に変えた場合でも実際に気筒内に吸入される空気量は比較的緩やかにしか低下せず、機関の空燃比を急激にリーンから理論空燃比またはリッチ空燃比にすることはできない。
【００９１】
このため、ＮＯＸ吸蔵触媒が吸収したＮＯＸを還元浄化する際に、スロットル弁を用いて吸入空気量を制御していると、排気空燃比をリーンからリッチに急激に切換えることが出来ず、切換の際にＮＯＸ吸蔵触媒のＮＯＸの還元浄化に寄与しないリーンとリッチとの間の中間空燃比での運転を経てから切換が行われることとなり、無駄な燃料消費の増大を招くとともに、ＮＯＸ吸蔵触媒の吸蔵したＮＯＸの還元浄化に要する時間が増大する問題が生じる。
これに対して、例えば図５の可変バルブ機構９を用いた場合には、カムの短い距離の移動で気筒の吸入空気量を急激に減少することができるため、リーンから理論空燃比またはリッチ空燃比への切換も極めて短時間で行うことができる。
このため、バルブの開弁特性値を変更することにより気筒内吸入空気量を制御する機関では、ＮＯＸ吸蔵触媒に吸蔵したＮＯＸを短時間で効率的に還元浄化することが可能となる。
【００９２】
ところで、上述のように開弁特性値毎に空燃比補正係数を求める場合には、開弁特性値センサ１６の検出精度が問題となる。例えば、開弁特性値センサ１６の出力特性が変化してしまったような場合には、気筒の開弁特性値を正確にフィードバック制御することができないため、各気筒の空燃比補正係数を正確に求めることができなくなるのみならず、機関の吸入空気量を正確に制御することができなくなり、機関性能や排気性状が悪化する問題がある。
【００９３】
図１から図５に示した実施形態では、機関運転中に開弁特性値センサ１６の実際の出力特性を検出し、検出した出力特性に基づいてセンサ１６出力値を補正するようにしている。これにより、開弁特性値センサ１６の出力特性が変化したような場合にも、正確に開弁特性値を検出することが可能となる。
【００９４】
以下、本実施形態における開弁特性値センサ１６の実際の出力特性の検出方法について説明する。
開弁特性値センサ１６の実際の出力特性は、例えば機関の開弁特性値（本実施形態では、バルブリフト量と作用角）が最大になったときのセンサ１６出力と最小になったときの出力とにより規定される。このため、機関運転中に開弁特性値を最大値と最小値に設定することが可能であれば、開弁特性値センサ１６の実際の出力特性を機関運転中に検出することが可能となる。
【００９５】
ところが、各気筒の開弁特性値を変化させると気筒内に吸入される空気量が変化するため、機関出力トルクや回転数が大幅に変化してしまい、通常は運転中に開弁特性値を最大値と最小値との間で変化させることは困難な問題がある。
本実施形態では、この問題を、例えば機関のフュエルカット運転中に行うことにより解決している。
【００９６】
以下、図８を用いて本実施形態の開弁特性値センサ１６の出力特性検出操作について説明する。図８の操作はＥＣＵ２２により一定時間毎または一定クランク回転角毎に実行されるルーチンとして行われる。
【００９７】
図８の操作では、
イ）機関が減速中などのフュエルカット運転を実施していること、または
ロ）フュエルカット以外の運転が実施されており、前回の出力特性計測から所定時間以上経過していて、しかもスロットル弁開度操作等により機関の出力トルクを一定に維持しながら開弁特性値を最小値及び最大値に設定することが可能であること、
のいずれか一方の条件が成立したときに、機関の開弁特性を実際に最小値から最大値まで変化させて、最小値と最大値とにおける開弁特性値センサ１６出力値を求めることによりセンサ１６の出力特性の計測を行う。
前述したように、本実施形態ではセンサ出力特性を検出する際に実際に機関の開弁特性値を最小値と最大値にセットする必要があり、機関出力に影響が出る可能性がある。
【００９８】
そこで、本実施形態では機関で燃焼が行われておらず開弁特性値を最小または最大にセットしても機関出力に影響が生じないフュエルカット運転中、若しくは開弁特性値を最大値及び最小値にセットした場合でもスロットル弁開度を調整することにより機関吸入空気量（すなわち機関出力）を一定に維持できる運転状態で機関が運転されている場合のみセンサ出力特性の検出操作を行うようにしている。
これにより、機関の運転に影響を生じることなくセンサ出力特性を検出可能となるため、出力特性が変化した場合でも特性変化に応じた出力補正を行い、正確に機関の開弁特性値を検出することが可能となる。
【００９９】
以下、図８の操作を具体的に説明する。
操作がスタートすると、まずステップ８０１では現在機関のフュエルカット運転（Ｆ／Ｃ運転）が行われているか否かが判定される。現在Ｆ／Ｃ運転が実行されている場合には、開弁特性値を変化させても機関の運転に影響がでることがないため、ステップ８０３から８０５でセンサ１６の出力特性の計測を行う。
【０１００】
すなわち、ステップ８０３では、可変バルブ機構９を駆動して開弁特性値を最小値（吸入空気量が最小になる開弁特性値、すなわち本実施形態ではバルブリフト量と作用角との両方が最小になる位置に相当し、図５ではカムシャフト６が最も図５の左側に移動したときの開弁特性値）に制御する。そして、ステップ８０５では可変バルブ機構９により開弁特性値が最小値に到達するのに充分な時間が経過後開弁特性値センサ１６の出力θを、最小開弁特性値出力θｍｉｎとして記憶する。
【０１０１】
そして、最小開弁特性値出力を記憶した後、ステップ８０７では開弁特性値を最大値（吸入空気量が最大になる開弁特性値、すわなち本実施形態ではバルブリフト量と作用角との両方が最大になる位置であり、図５でカムシャフト６が最も右側に移動したときの開弁特性値）に制御し、充分な時間が経過した後に開弁特性値センサ１６の出力θを、最大開弁特性値出力値θｍａｘとして記憶する。
【０１０２】
また、ステップ８０１で現在フュエルカット運転中でない場合には、次にステップ８１１に進み、フラグＸの値が１にセットされているか否かを判定する。Ｘはステップ８１３の計測実行条件が成立した場合にはステップ８１７で１に、計測実行条件が成立しない場合にはステップ８１５で、出力特性の検出が終了した場合にはステップ８２９で、それぞれゼロにセットされるフラグである。フラグＸは、センサ出力特性を検出中であるか否かを示し、一旦ステップ８１３の計測実行条件が成立した場合には、その後ステップ８１３の条件が成立しなくなっても、ステップ８１９から８２７のセンサ出力特性検出操作を完了するようにする機能を有している。
【０１０３】
ステップ８１１でＸ≠１であった場合には、センサ出力特性を検出中でないため、ステップ８１３に進み、現在センサ出力特性検出を実行する条件が成立しているか否かを判定する。前述したように、本実施形態ではセンサ出力特性を検出するためには、吸気弁の開弁特性値を変更する必要があるため、フュエルカット運転時以外はあまり頻繁に実施することは好ましくない。そこで、ステップ８１３で判定する検出実行条件は、機関始動時からの運転時間が所定値（例えば１０分）の整数倍であること（この場合には、機関運転中１０分毎に計測実行条件が成立する）、あるいは、機関冷却水温度または潤滑油温度が１０°Ｋの整数倍であること（この場合には、機関運転中冷却水温度または潤滑油温度が１０°Ｋ上昇する毎に計測実行条件が成立する）とされている。なお、ステップ８１３の計測実行条件としては、機関運転中に適宜な時間間隔で計測実行条件が成立するような条件であれば他の条件を設定することも可能である。
【０１０４】
ステップ８１３で計測実行条件が成立していない場合には、ステップ８１５でフラグＸの値を０にセットして、本操作はステップ８２１から８２９のセンサ出力特性検出を行うことなく終了する。
【０１０５】
また、ステップ８１３で計測実行条件が成立していた場合には、ステップ８１７に進み、フラグＸを１にセットした後ステップ８１９に進む。これにより、一旦ステップ８１３の計測実行条件が成立すると、次回からは図８の操作を実行する毎に、ステップ８１１の次に直接ステップ８１９が実行されるようになり、出力特性の検出が完了してステップ８２９でＸの値が１にセットされるまで、ステップ８１３の判定はバイパスされるようになる。
【０１０６】
ステップ８１９では、スロットル弁の開度を制御することにより現在の機関出力トルクを変化させずに開弁特性値を最小から最大まで変化させることが可能か否か、すなわち等出力制御条件が成立しているか否かが判定される。例えば、現在の機関運転状態（回転数、負荷）では開弁特性値を最小にセットするとスロットル弁を全開にしても現在の吸入空気量を維持できずに吸入空気量が減少してしまうような場合、あるいはスロットル弁を全閉にしても開弁特性値を最大にすると吸入空気量が現在の吸入空気量より増大してしまうような場合は、現在の機関出力トルクを変化させずに開弁特性値を最小から最大まで変化させる制御（等出力制御）を行うことはできない。
本実施形態では、予め等出力制御が可能な回転数Ｎと負荷（Ｑ／Ｎ）との範囲を予め実験等により求め、等出力制御条件としてＥＣＵ２２のＲＯＭに格納してある。ステップ８１９では、現在の回転数、負荷がこの等出力制御条件に合致しているか否かが判定される。
【０１０７】
ステップ８１９で現在等出力制御条件が成立していない場合には、本操作の今回の実行は終了する。この場合、次回以降の運転でステップ８１９の等出力制御条件が成立すればステップ８２１以下のセンサ出力特性検出操作が実行される。
ステップ８１０で等出力制御条件が成立している場合には、ステップ８２１で開弁特性値が最小になるように可変バルブ機構９が制御されるとともに機関の出力トルクが一定に維持されるようにスロットル弁５６開度が調整される。そして、ステップ８２３では開弁特性値が最小値に到達するのに充分な時間が経過した後に現在のセンサ１６出力をθｍｉｎとして記憶（学習）する。
【０１０８】
また、ステップ８２５と８２７では、ステップ８２１，８２３と同様に機関の出力を一定に維持しながら開弁特性値を最大になるようにして、この状態でのセンサ１６出力をθｍａｘとして記憶（学習）する。そして、θｍｉｎとθｍａｘとの値の学習が完了するとステップ８２９ではフラグＸの値が０にセットされる。
【０１０９】
上述のように、図８の操作により、機関がフュエルカット運転する毎に、及びそれ以外の運転状態では適宜な間隔でセンサ出力特性の検出が行われ、開弁特性の最小値と最大値とにおけるセンサ出力θｍａｘおよびθｍｉｎの値が更新される。これにより、最大値と最小値との間の任意の開弁特性値に対するセンサ出力値を公知の適宜な方法で求めることができるようになり、機関運転中にセンサ出力特性が変化した場合にもセンサ出力を補正して正確な開弁特性値を検出することが可能となる。
【０１１０】
【発明の効果】
各請求項に記載の発明によれば、機関の運転状態にかかわらず各気筒間の空燃比のばらつきを低減することが可能となる共通の効果を奏する。
【図面の簡単な説明】
【図１】本発明の空燃比制御装置を自動車用４気筒ガソリン機関に適用した実施形態の概略構成図である。
【図２】図１の機関の吸気系統の概略構成を説明する模式図である。
【図３】図２の吸気系における空燃比センサの配置を示す平面図である。
【図４】図１の機関の吸気弁駆動用カムの詳細形状を示す図である。
【図５】可変バルブ機構９の動作原理を示す断面図である。
【図６】図１の実施形態の空燃比制御操作を説明するフローチャートである。
【図７】図１の実施形態の空燃比制御操作を説明するフローチャートである。
【図８】図１の実施形態の開弁特性値センサの出力特性検出操作を説明するフローチャートである。
【符号の説明】
１…内燃機関
２…吸気弁
３…排気弁
４…吸気弁駆動カム
６…吸気カム
９…可変バルブ機構
１６…開弁特性値センサ
２２…電子制御ユニット（ＥＣＵ）
５６…スロットル弁[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an air-fuel ratio control device for a multi-cylinder internal combustion engine having a variable valve mechanism that changes intake valve opening characteristics, and more specifically, to eliminate variations in operating air-fuel ratio of each cylinder regardless of changes in operating conditions. The present invention relates to an air-fuel ratio control device for a multi-cylinder internal combustion engine that is capable of performing the following.
[0002]
[Prior art]
By changing the opening characteristics of the intake valves that affect the intake air amount of each cylinder of the internal combustion engine, the intake air amount of each cylinder is adjusted without causing a throttle loss due to the throttle valve (throttle valve) in the intake passage. A control device for an internal combustion engine as described above is known. For example, when the valve opening characteristic values such as the valve lift amount of the intake valve, the valve opening period (operating angle of the intake valve cam), and the valve overlap amount are changed, the intake valve is sucked into the cylinder even if other conditions are the same. The amount of air varies. Therefore, by changing one or more of these intake valve opening characteristic values during operation, the so-called non-throttle operation of the engine, which controls the engine intake air amount without using a throttle valve, becomes possible. Become. As described above, by performing the non-throttle operation without using the throttle valve, it is possible to reduce the intake throttle loss caused by the throttle valve and improve the thermal efficiency of the engine.
[0003]
When performing non-throttle operation of the engine, the amount of air taken into each cylinder is determined by the valve opening characteristic value of the intake valve for each cylinder. However, the intake valve of each cylinder or the variable valve mechanism that changes the valve opening characteristic value of the intake valve causes errors in manufacturing and control, so that even if the valve opening characteristic value of each cylinder is controlled to be the same, Causes variation in the valve opening characteristic value of each cylinder. For this reason, the intake air amount for each cylinder also varies according to the variation in the valve opening characteristic value of each intake valve. Therefore, even if the amount of fuel supplied to each cylinder is equal, the operating air-fuel ratio of each cylinder varies from cylinder to cylinder, causing a problem that the generated torque of each cylinder varies.
[0004]
An example of this type of air-fuel ratio control device is disclosed in, for example, Japanese Patent Application Laid-Open No. 6-213044. The air-fuel ratio control device of the publication discloses an oxygen concentration sensor disposed in an exhaust passage of a multi-cylinder internal combustion engine that performs non-throttle operation by changing an intake valve valve lift of each cylinder. The intake air amount of each cylinder is adjusted to the same value by measuring the exhaust air-fuel ratio of the cylinder and adjusting the valve lift of the intake valve of each cylinder according to the variation of the exhaust air-fuel ratio. As a result, variation between the intake air amount and the air-fuel ratio of each cylinder is prevented.
[0005]
[Problems to be solved by the invention]
However, by adjusting the valve lift of each cylinder based on the exhaust air-fuel ratio of each cylinder actually measured in each operating state of the engine, as in the device disclosed in Japanese Patent Application Laid-Open No. 6-213044, the A problem arises if variations in the air-fuel ratio are eliminated.
[0006]
For example, the variation in the amount of intake air in each operating state is calculated based on the variation in the exhaust air-fuel ratio of each cylinder measured by an oxygen concentration sensor or the like. Therefore, the detection delay time of the oxygen concentration sensor changes. In particular, when the exhaust gas of a plurality of cylinders is measured using a single sensor as in the apparatus disclosed in Japanese Patent Application Laid-Open No. Hei 6-21304, the above-described detection delay varies from cylinder to cylinder. Changes depending on the engine speed, load, and the like. For this reason, if the intake air amount is corrected based on the exhaust air-fuel ratio detected by the sensor in each operation state, for example, a correction error becomes large at the time of high rotation, high load operation, and the like. In some cases, hunting may occur.
[0007]
Also, when the engine is in a transient state, the detection delay of the sensor is also in a transitional state in accordance with the change in the operating state, so that the reliability of the measured value is reduced and the correction accuracy of the intake air amount is deteriorated. Similarly, erroneous correction and control hunting may occur.
[0008]
The present invention has been made in view of the above-mentioned problems of the prior art, and in a multi-cylinder internal combustion engine that performs non-throttle operation by changing the valve opening characteristics of an intake valve, the variation in air-fuel ratio of each cylinder is accurately determined regardless of a change in engine operation state. It is an object of the present invention to provide an air-fuel ratio control device for a multi-cylinder internal combustion engine that can be eliminated.
[0009]
[Means for Solving the Problems]
According to the first aspect of the present invention, there is provided an air-fuel ratio control device for a multi-cylinder engine including a variable valve mechanism for changing a valve opening characteristic value of an intake valve that affects an intake air amount in a cylinder. Using the exhaust air-fuel ratio measured for each cylinder in the predetermined reference operation state to calculate the variation in the exhaust air-fuel ratio of each cylinder for each valve-opening characteristic value in the reference operation state, based on the calculated variation, Correction coefficient calculating means for calculating an air-fuel ratio correction coefficient of a fuel injection amount for each valve-opening characteristic value for reducing the variation of the operating air-fuel ratio of each cylinder in the reference operation state; When in the operating state, by correcting the air-fuel ratio correction coefficient at each valve opening characteristic value based on the value of a predetermined parameter representing the engine operating state, each of the values in the operating state other than the reference operating state is corrected. Controlling the fuel injection quantity of each cylinder so as to reduce variations in the operating air-fuel ratio of the cylinder, the air-fuel ratio control apparatus for a multi-cylinder internal combustion engine is provided.
[0010]
That is, according to the first aspect of the invention, the variation of the air-fuel ratio of each cylinder for each valve-opening characteristic value is calculated based on the exhaust air-fuel ratio measured for each cylinder in the reference operation state, and the variation of each cylinder is calculated based on this variation. An air-fuel ratio correction coefficient, which is a correction coefficient of the fuel injection amount for eliminating the air-fuel ratio variation, is calculated. However, in the present invention, the air-fuel ratio correction coefficient of each cylinder is calculated based on the actually measured exhaust air-fuel ratio only in the reference operation state, and the air-fuel ratio correction coefficient in other operation states is different in the reference operation state. The obtained air-fuel ratio correction coefficient for each valve opening characteristic value is corrected by correcting it in accordance with the operating state of the engine.
[0011]
For this reason, as the reference operation state, for example, an operation state in which the exhaust gas reaches the sensor in a steady operation state (transfer delay) is known and the variation in the air-fuel ratio of each cylinder can be accurately detected is taken. This makes it possible to accurately calculate the air-fuel ratio correction coefficient in the reference operation state.
Since the magnitude of the variation of the valve opening characteristic value among the cylinders changes according to the valve opening characteristic value, the air-fuel ratio correction coefficient of each cylinder in the reference operating state is also obtained for each valve opening characteristic value.
[0012]
According to the present invention, furthermore, a change in the operating state from the reference operating state and a correction of the air-fuel ratio correction coefficient necessary for reducing the variation in the air-fuel ratio of each cylinder even when the operating state changes are performed in advance by experiments or the like. Is required by Then, the value of the air-fuel ratio correction coefficient in an operation state different from the reference operation state is obtained by correcting the air-fuel ratio correction coefficient at the same valve opening characteristic value in the reference operation state according to the operation state. For this reason, based on the air-fuel ratio correction coefficient of each valve opening characteristic value in the reference operation state, the air-fuel ratio correction coefficient in each operation state can be accurately obtained regardless of whether the operation is steady or transient, and the air-fuel ratio correction coefficient can be accurately obtained regardless of the operation state. It is possible to eliminate variations in the air-fuel ratio of each cylinder.
[0013]
According to the second aspect of the present invention, the correction coefficient calculating means calculates the air-fuel ratio correction coefficient of the fuel injection amount for each valve opening characteristic value so that the air-fuel ratio of each cylinder becomes substantially the same. An air-fuel ratio control device for a multi-cylinder internal combustion engine according to claim 1 is provided.
That is, in the second aspect of the present invention, the air-fuel ratio correction coefficient is calculated so that the air-fuel ratio of each cylinder becomes substantially the same. As a result, variation in the air-fuel ratio between the cylinders is completely prevented.
According to the third aspect of the present invention, the correction coefficient calculating means determines that the engine is operating with the reference valve opening characteristic value at which the in-cylinder intake air amount becomes maximum in the predetermined reference operating state. Calculating a reference correction coefficient of the fuel injection amount for reducing the variation of the air-fuel ratio of each cylinder based on the variation of the air-fuel ratio of each cylinder, and when the engine is operated at a value other than the reference valve opening characteristic value. The air-fuel ratio control apparatus for a multi-cylinder internal combustion engine according to claim 1 or 2, wherein the air-fuel ratio correction coefficient for the fuel injection amount after being corrected using the reference correction coefficient is calculated. .
[0014]
That is, in the invention of claim 3, the air-fuel ratio variation of each cylinder when the engine is operated with the reference valve opening characteristic value that is the valve opening characteristic value that maximizes the in-cylinder intake air amount in the reference operation state. A reference correction coefficient is calculated based on the reference correction coefficient. For example, when the engine is operated at the reference valve opening characteristic value, when the valve lift is controlled as the valve opening characteristic value, the state in which the valve lift is controlled to be maximum, the valve opening period (operating angle) Is controlled so that the operating angle is maximized.
[0015]
In the state where the valve opening characteristic value is controlled so that the intake air amount is maximized, the variation in the intake air amount due to the valve opening characteristic value becomes almost negligible, and the variation in the intake air amount in each cylinder becomes small. This is only due to slight differences in the length and shape of the intake passage leading to each cylinder. Also, in this state, since the variation in the intake air amount of each cylinder is small, the variation in the characteristics of the fuel injection valves and the like in each cylinder appears largely in the variation in the air-fuel ratio.
[0016]
Therefore, by correcting the fuel injection amount using the reference correction coefficient calculated based on the air-fuel ratio of each cylinder measured during operation at the reference valve opening characteristic value, the variation other than the variation of the valve opening characteristic value of each cylinder is obtained. It is possible to correct the inherent air-fuel ratio variation due to the cause. Thus, during an operation other than the operation with the reference valve opening characteristic value, the fuel injection amount is first corrected by using the reference correction coefficient obtained in the operation with the reference valve opening characteristic value, and the air amount corresponding to the corrected fuel injection amount is By calculating the fuel ratio correction coefficient, it is possible to accurately correct variations in the air-fuel ratio due to changes in the valve opening characteristic value.
[0017]
According to the fourth aspect of the invention, the parameter representing the engine operating state includes at least one of an engine speed, an engine load, and an accelerator opening degree. An air-fuel ratio control device for an engine is provided.
[0018]
That is, in the invention of claim 4, the air-fuel ratio correction coefficient for each valve opening characteristic value of each cylinder calculated in the reference operation state is one of the engine speed, the engine load, and the accelerator opening (accelerator pedal depression amount). Modified according to one or more parameters. For example, when the engine load or the accelerator opening increases, the intake air amount of each cylinder increases accordingly. For this reason, when the engine load or the accelerator opening increases, the variation in the intake air amount of each cylinder increases even if the valve opening characteristic values are the same. For this reason, the correction amount of the air-fuel ratio correction coefficient increases as the engine load or the accelerator opening increases.
[0019]
On the other hand, when the engine speed increases, the intake air amount increases accordingly, and the variation in the intake air amount among the cylinders also increases.However, the amount of in-cylinder burned gas returning to the intake port also depends on the engine speed. Therefore, even if the rotational speed actually increases uniformly, the variation in the intake air amount does not increase uniformly, and there is a rotational speed at which the variation becomes maximum. For this reason, when the valve opening characteristic values are the same, the air-fuel ratio correction coefficient of each cylinder increases as the rotational speed increases up to a certain rotational speed. The fuel ratio correction coefficient decreases. That is, there is a peak rotation speed at which the air-fuel ratio correction coefficient is maximized.
[0020]
As described above, the air-fuel ratio correction coefficient of each cylinder in the reference operating state is corrected according to the operating state based on a relationship previously obtained according to the engine speed, the engine load or the accelerator opening. Accordingly, it is possible to accurately reduce the variation in the air-fuel ratio of each cylinder regardless of the operation state.
[0021]
According to the fifth aspect of the present invention, there is provided the air-fuel ratio control apparatus for a multi-cylinder internal combustion engine according to the fourth aspect, wherein the parameter representing the engine operating state further includes an engine temperature.
[0022]
That is, in the fifth aspect of the invention, the air-fuel ratio correction coefficient is further corrected according to the engine temperature in the fourth aspect of the invention. In an engine equipped with a variable valve mechanism, the amount of thermal expansion of the components of the mechanism is not always uniform in each cylinder, and varies from cylinder to cylinder. For this reason, the change in the valve opening characteristic value of the intake valve of each cylinder due to the rise in the engine temperature is not the same, and the valve opening characteristic value varies. This variation increases as the amount of thermal expansion increases, that is, as the temperature increases. Therefore, the correction amount of the air-fuel ratio correction coefficient also increases as the engine temperature increases.
[0023]
In the present invention, the air-fuel ratio correction coefficient is corrected according to the engine temperature, so that it is possible to more accurately reduce the variation in the air-fuel ratio of each cylinder in each operating state.
[0024]
According to the invention described in claim 6, the correction coefficient calculating means measures the exhaust air-fuel ratio of a plurality of cylinders using a single air-fuel ratio sensor disposed in an exhaust passage. The present invention provides an air-fuel ratio control device for a multi-cylinder internal combustion engine.
[0025]
That is, in the invention of claim 6, the exhaust air-fuel ratio of a plurality of cylinders is measured using a single air-fuel ratio sensor disposed in the exhaust passage.
Essentially, if the exhaust air-fuel ratio of each cylinder can be accurately measured in all operating states, accurate measurement of the air-fuel ratio variation can be performed using the measurement results.
If the exhaust stroke of each cylinder has a different phase, the exhaust from each cylinder reaches the air-fuel ratio sensor installation position in the exhaust passage with a time lag, so the output of the air-fuel ratio sensor is sampled in synchronization with the engine speed. By doing so, the exhaust air-fuel ratio of a plurality of cylinders can be individually measured using a single air-fuel ratio sensor. However, when a single air-fuel ratio sensor is used, the detection delay of the air-fuel ratio sensor by the sensor and the separation of the exhaust gas of each cylinder greatly change depending on the operating state, so that a specific measurement condition is not satisfied. Under operating conditions, the exhaust air-fuel ratio for each cylinder cannot be measured accurately.
[0026]
In the present invention, the actual measurement of the air-fuel ratio of each cylinder is performed only in the reference operation state. Therefore, the operation state in which the specific conditions for accurately measuring the exhaust air-fuel ratio of each cylinder are satisfied is referred to as the reference operation state. By setting the operating state, even when a single sensor is used, the exhaust air-fuel ratio of each cylinder can be accurately measured.
[0027]
According to a seventh aspect of the present invention, there is provided the air-fuel ratio control device for a multi-cylinder internal combustion engine according to the sixth aspect, wherein the reference operation state is an idle operation state of the engine.
[0028]
That is, in the present invention, the idle operation state of the engine is adopted as the reference operation state. In the idling operation state, there is little change in the normal operation state, and the steady operation is performed. In addition, the rotation speed is low, the time difference between the exhaust gas from each cylinder reaching a single sensor is large, and the separation of the exhaust gas from each cylinder is improved. Can be measured. For this reason, by setting the idle operation state to the reference operation state and measuring the exhaust air-fuel ratio of each cylinder, accurate air-fuel ratio control becomes possible.
[0029]
Note that, for example, by performing the idle operation in which the intake air amount of each cylinder is increased while suppressing an increase in the engine output by retarding the ignition timing of the engine, the exhaust gas amount of each cylinder increases. The measurement accuracy of the air-fuel ratio for each cylinder is further improved.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram in which the air-fuel ratio control device of the present invention is applied to a four-cylinder gasoline engine for an automobile, FIG. 2 is a schematic diagram showing a schematic configuration of an intake system of the engine in FIG. 1, and FIG. FIG. 3 is a plan view showing an arrangement of an air-fuel ratio sensor 57 in an intake system.
[0031]
1 to 3, reference numeral 1 denotes an internal combustion engine, 8 denotes a combustion chamber formed in a cylinder of the engine 1, 2 denotes an intake valve, and 3 denotes an exhaust valve. In this embodiment, the driving camshaft 6 for the intake valve 2 and the camshaft 7 for driving the exhaust valve are provided independently. 1 to 3, reference numeral 4 denotes an intake valve driving cam provided on the camshaft 6, and reference numeral 5 denotes an exhaust valve driving cam provided on the camshaft 7.
[0032]
Reference numeral 13 denotes a crankshaft, 15 denotes a fuel injection valve, and 17 denotes a rotation speed sensor for detecting an engine rotation speed. 19 is an air flow meter for detecting the intake air amount of the entire engine, 20 is a cooling water temperature sensor for detecting the temperature of cooling water of the internal combustion engine, and 22 is an ECU (electronic control unit). 50 is a cylinder, 52 is an intake pipe, 53 is a surge tank, and 51 is an intake manifold connecting the surge tank to the intake port of each cylinder. Reference numeral 54 denotes an exhaust pipe, 55 denotes an ignition plug, and 56 denotes an independent actuator (not shown). The opening is independent of an accelerator opening (accelerator pedal depression amount) according to a control signal from the ECU 22 described later. Is a throttle valve, and 57 is an air-fuel ratio sensor for detecting the exhaust gas air-fuel ratio.
[0033]
In the present embodiment, the exhaust valve driving cam 5 is a normal cam having a uniform cam profile in the camshaft axial direction, whereas the intake valve driving cam 4 is a camshaft 6. The cam profile changes along the axial direction.
FIG. 4 is a diagram showing a detailed shape of the intake valve driving cam 4. As shown in FIG. 4, the cam profile of the intake valve cam 4 of this embodiment changes along the center axis direction of the camshaft, and the nose height and the working angle of the cam profile change from the right end to the left end in FIG. The cam profile is set so as to continuously increase toward the center. For this reason, the valve lift amount and the valve opening period of the intake valve 2 change according to the contact position of the valve lifter of the intake valve 2 with the cam 4, and the valve position increases as the contact position of the valve lifter moves from the right end to the left end of the cam. The lift amount is large, and the opening period of the intake valve is long.
[0034]
In the present embodiment, the valve opening characteristic values such as the valve lift amount and the valve opening period of the intake valve 2 can be changed by moving the camshaft in the axial direction during the operation of the engine using the variable valve mechanism 9. It has become. That is, by using the variable valve mechanism 9 to slide the camshaft 6 in the axial direction during engine operation, the contact position between the intake valve cam 4 and the valve lifter is changed, and the cam profile used for driving the intake valve 2 is changed. It is possible to change.
[0035]
When the valve lift of the intake valve 2 increases, the amount of air drawn into the cylinder increases even if the opening period of the intake valve is the same. Further, when the working angle of the cam (opening period of the intake valve) becomes large (long), the amount of air drawn into the cylinder increases even if the valve lift is the same. In this specification, an intake valve operation parameter that affects the intake air amount in the cylinder, such as the above-described intake valve valve lift amount, operating angle (valve opening period), etc., is referred to as a valve opening characteristic value.
[0036]
FIG. 5 is a sectional view showing the operation principle of the variable valve mechanism 9. 5, reference numeral 30 denotes a magnetic material connected to the intake valve camshaft 6, reference numeral 31 denotes a solenoid for driving the magnetic material 30, and reference numeral 32 denotes a compression for urging the magnetic material 30 rightward in FIG. It is a spring. In the variable valve mechanism of the present embodiment, when the coil 31 is energized, the magnetic body 30 moves to the left in FIG. 5 against the urging force of the spring 32, and the contact between the valve lifter of the intake valve 2 and the cam 4. The position is displaced in the camshaft axial direction. Since the amount of movement of the magnetic body 30 changes according to the current supplied to the solenoid 31, in the present embodiment, the contact position between the valve lifter of the intake valve 2 and the cam 4 by controlling the current supplied to the solenoid 31, that is, The valve opening characteristic value of the intake valve 2 can be controlled. In this embodiment, as the energizing current to the solenoid 31 increases, the camshaft 6 moves to the left in FIGS. 4 and 5, and the valve lift of the intake valve 2 and the valve opening period decrease. For this reason, in the present embodiment, the intake air amount of each cylinder of the engine 1 becomes maximum when the solenoid 31 is not energized, and the intake air amount of each cylinder decreases as the energizing current increases.
[0037]
Reference numeral 16 in FIG. 1 denotes a valve opening characteristic value sensor that detects a valve opening characteristic value (valve lift amount, valve opening period) of the intake valve 2. As described above, in the present embodiment, the valve opening characteristic value of the intake valve 2 changes according to the amount of movement of the camshaft 6 in the axial direction. Characteristic values are also determined. For this reason, in this embodiment, as the valve opening characteristic value sensor 16, an axial position sensor that detects the axial position (movement amount) of the intake valve camshaft 6 is used, and the ECU 22 detects the valve opening characteristic value sensor 16. Using the camshaft position, a valve opening characteristic value such as a valve lift amount and a valve opening period of the intake valve 2 is calculated based on a relationship stored in advance.
[0038]
In the present embodiment, the profile of the cam 4 of the intake valve 2 is set so that both the nose height and the working angle change at the same time along the axial direction, but only the nose height (valve lift amount) is set. Alternatively, even if only the operating angle (valve opening period) changes, the intake air amount of each cylinder can be controlled using the variable valve mechanism 9.
In this embodiment, only the opening characteristic value of the intake valve is changed. However, a variable valve mechanism similar to 9 is provided for the exhaust valve, and the opening characteristic value of the exhaust valve is also changed. Is also possible.
Further, in an engine having an independently driven intake valve and / or exhaust valve having a driving device such as an electromagnetic actuator for each valve, the valve opening characteristic value of each valve is controlled by controlling each driving device. It is possible to change.
[0039]
As described above, in the present embodiment, the throttle valve 56 includes an independent actuator, and is controlled by the ECU 22 based on the accelerator pedal depression amount (accelerator opening) of the driver and the engine operating state.
In this embodiment, the intake air amount of each cylinder can be controlled by changing the valve opening characteristic value such as the valve lift amount and the valve opening period of the cylinder intake valve. For this reason, in the present embodiment, in a region where the intake air amount is relatively small, the intake air amount is controlled by changing the valve opening characteristic value of the intake valve while maintaining the throttle valve 56 in the fully open state. It is possible to perform a so-called non-throttle operation in which the pumping loss due to the throttle is reduced.
[0040]
However, in an actual engine, a valve train including the intake valve 2 and the cam 4 of each cylinder has a variation due to a manufacturing error, thermal deformation during operation, and the like. Even if the control is performed, the amount of air charged into the cylinder varies, and does not become the same.
Further, in addition to the variation in the valve operating system, for example, a difference occurs in the intake air amount of each cylinder due to a difference in the length of the intake passage leading to each cylinder. Further, in addition to the variation in the amount of intake air, the amount of fuel injected into each cylinder also varies within the manufacturing tolerance of the fuel injection valve of each cylinder.
[0041]
For this reason, in an actual engine, even if the set values of the intake valve opening characteristic and the fuel injection amount of each cylinder are controlled to be the same, the intake air amount and the fuel injection amount for each cylinder vary, so that The combustion air-fuel ratio of each cylinder also varies. For this reason, there is a problem that the exhaust characteristics deteriorate and the generated torque becomes uneven due to the air-fuel ratio deviation between the cylinders.
In particular, when performing a non-throttle operation without the intake throttle as in the present embodiment, the amount of intake air of each cylinder is determined by the valve opening characteristic value of the intake valve. Will directly affect the variation in the amount of intake air.
The variation in the air-fuel ratio for each cylinder due to the variation in the intake air amount and the like is corrected by measuring the exhaust air-fuel ratio of each cylinder and reducing the fuel injection amount for each cylinder so as to reduce the variation in the air-fuel ratio. This can be solved by doing so.
[0042]
In the present embodiment, an air-fuel ratio sensor 57 that detects an exhaust air-fuel ratio is disposed in the exhaust manifold of the engine 1. Originally, in order to accurately detect the exhaust air-fuel ratio of each cylinder, it is preferable to dispose an air-fuel ratio sensor for each exhaust system of each cylinder. However, in each cylinder, the process cycle of the cylinder differs by a predetermined crank angle. For example, in a four-cycle four-cylinder engine as in this embodiment, the exhaust stroke of each cylinder is shifted by a crank angle of 180 degrees (180 CA).
Therefore, even when a single air-fuel ratio sensor 57 is used, it is possible to accurately measure the combustion air-fuel ratio of each cylinder when the measurement conditions are satisfied.
[0043]
The conditions under which the combustion air-fuel ratio of each cylinder can be accurately measured using the single air-fuel ratio sensor 57 include, for example,
(A) The engine is operating normally.
(B) The engine speed is low.
(C) The exhaust flow rate from each cylinder is large.
And so on.
That is, in order to accurately detect the air-fuel ratio of each cylinder using the single air-fuel ratio sensor 57, it is necessary that the exhaust arrival timing from each cylinder to the air-fuel ratio sensor 57 (transport delay, etc.) be constant. Therefore, it is necessary that the engine be operated in a steady state ((A) above).
[0044]
Also, when the engine speed increases, the exhaust gas discharged from each cylinder mixes before reaching the position of the air-fuel ratio sensor 57, and the separation of the exhaust gas from each cylinder deteriorates. Measurement cannot be performed accurately. Therefore, in order to accurately detect the exhaust air-fuel ratio of each cylinder using the single air-fuel ratio sensor 57, it is preferable that the engine be operated at a low speed ((B) above).
[0045]
Further, when the air-fuel ratio sensor 57 detects the exhaust air-fuel ratio from each cylinder, the detection accuracy improves as the exhaust gas flow from each cylinder increases. For this reason, when detecting the exhaust air-fuel ratio of each cylinder using the single air-fuel ratio sensor 57, it is preferable to perform the measurement with the exhaust gas flow rate as large as possible (condition (C)).
[0046]
In addition, in order to perform accurate measurement even if an air-fuel ratio sensor is provided for each exhaust system of each cylinder, it is preferable to perform measurement in a state where the above conditions (A) to (C) are satisfied. When measuring the exhaust air-fuel ratio of each cylinder using a single air-fuel ratio sensor 57 as in this embodiment, the conditions (A) to (C) are particularly important.
[0047]
However, the conditions (A) to (C) are not always satisfied in actual operation, and high-speed operation and transient operation of the engine are frequently performed.
On the other hand, the variation in the air-fuel ratio for each cylinder due to the variation in the valve system of the intake valve is caused not only by the valve opening characteristic values such as the valve lift amount and the valve opening period (operating angle) of the intake valve, but also by the , Depending on the engine operating state such as load. These variations in the air-fuel ratio also change depending on the operating time of the engine. Therefore, in order to correct the variation of the air-fuel ratio for each cylinder, it is originally necessary to measure the exhaust air-fuel ratio of each cylinder in every operating state. When the exhaust air-fuel ratio of each cylinder is measured using, the exhaust air-fuel ratio of each cylinder cannot be accurately detected during high-speed operation, transient operation, and the like. For this reason, if the variation in the air-fuel ratio for each cylinder is corrected based on the output of a single air-fuel ratio sensor, erroneous correction or control hunting may occur depending on the operating state of the engine.
[0048]
In the present embodiment, when correcting the variation in the air-fuel ratio for each cylinder, the variation in the air-fuel ratio in each operating state is accurately corrected using a single air-fuel ratio sensor by the following method.
Hereinafter, the air-fuel ratio variation correction operation for each cylinder in the present embodiment will be described.
The air-fuel ratio variation correction according to the present embodiment is performed as follows: 1. Air-fuel ratio measurement and calculation of air-fuel ratio correction coefficient for each cylinder in the reference operation state; Correction of the air-fuel ratio correction coefficient according to the engine operating state.
[0049]
Hereinafter, each will be described.
1. Measurement of the air-fuel ratio for each cylinder and calculation of the air-fuel ratio correction coefficient in the reference operation state.
In the present embodiment, first, the engine is operated in an engine operating state (reference operating state) that satisfies the above conditions (A) to (C), and the exhaust of each cylinder is obtained from the output of the air-fuel ratio sensor 57 in this operating state. Measure the air-fuel ratio.
[0050]
That is, in the present embodiment, the ignition is performed as the reference operating state in order to increase the exhaust flow rate (condition (C)) during the idle operation in which the engine is always operated at a steady state (condition (A)) and at a low speed (condition (B)). The air-fuel ratio of each cylinder is measured by changing the valve opening characteristic values such as the valve lift and the operating angle of the intake valve in this reference operation state in a state where the timing is delayed by a predetermined amount.
In this state, since the exhaust air-fuel ratio of each cylinder can be measured most accurately, the correction amount (correction coefficient) of the fuel injection amount of each cylinder for eliminating the air-fuel ratio variation can be accurately calculated.
[0051]
In this embodiment, the ECU 22 performs a fuel injection amount calculation operation (not shown), which is separately executed, to set a fuel injection amount set value F based on the intake air amount Q detected by the air flow meter 19 and the engine speed N, and F = It is calculated in the form of (Q / N) × K × FAF. Here, the coefficient K is a value per one time required for setting the combustion air-fuel ratio in each cylinder to a target air-fuel ratio (for example, a stoichiometric air-fuel ratio) with respect to the intake air amount per rotation (Q / N). The fuel injection amount and FAF are feedback correction coefficients for setting the average operating air-fuel ratio of the entire engine detected based on the output of the air-fuel ratio sensor 57 to the target air-fuel ratio. In the present embodiment, the feedback correction coefficient FAF is set by a known appropriate method, but a detailed description is omitted because it is not directly related to the technical features of the present invention.
[0052]
The fuel injection amount set value F calculated by the fuel injection calculation operation is a value common to each cylinder. If the actual intake air amount and the actual fuel injection amount of each cylinder are the same, a fuel injection signal corresponding to the fuel injection amount set value F is input to the fuel injection valve of each cylinder, thereby obtaining the air-fuel ratio of each cylinder. Are the same. However, in practice, the intake air amount of each cylinder varies, and furthermore, even when the same fuel injection signal F is input, the fuel injection amount of each fuel injection valve varies. Are not the same.
[0053]
In the present embodiment, the variation of the air-fuel ratio of each cylinder is calculated from the exhaust air-fuel ratio of each cylinder measured in the reference operation state, and the correction coefficient (for the fuel injection amount required to reduce the variation of the air-fuel ratio) An air-fuel ratio correction coefficient) Ai (i is a cylinder number) is calculated for each cylinder. In the present invention, any known method can be adopted as a method of calculating the air-fuel ratio correction coefficient. In the present embodiment, the air-fuel ratio of each cylinder is made equal by completely eliminating the variation of the air-fuel ratio of each cylinder. For example, when the exhaust air-fuel ratio of each cylinder is AFi, the air-fuel ratio correction coefficient Ai of each cylinder is
[0054]
Ai = AFi / ((1 / n) Σ (1 to n) AFi).
Here, (1 / n) Σ (1 to n) AFi is an arithmetic average value of the air-fuel ratios of all the cylinders. The fuel injection amount setting value F common to each cylinder is corrected using the air-fuel ratio correction coefficient Ai calculated as described above, and a fuel injection signal having a size of Ai × F is supplied to the fuel injection valve of each cylinder. The air-fuel ratio of each cylinder becomes equal to the average air-fuel ratio (1 / n) Σ (1-n) AFi.
[0055]
The above-described air-fuel ratio correction coefficient Ai is created for each valve opening characteristic value by changing the valve opening characteristic value of the intake valve in the reference operation state.
By the way, as described above, in the reference operation state of the engine, it is possible to accurately measure the variation in the air-fuel ratio of each cylinder. However, the variation in air-fuel ratio is caused by (1) variation in valve opening characteristic value such as operating angle, and (2) variation in air amount due to difference in length of intake passage of each cylinder, and variation in fuel injection valve. There are two types, one caused by variation in characteristics.
[0056]
In addition, among these variations, the variation in the air-fuel ratio caused by the valve opening characteristic value such as the operating angle of each cylinder changes as the valve opening characteristic value changes. The variation due to the fuel injector characteristics hardly changes even when the operating angle changes.
For this reason, in order to accurately correct the variation in the air-fuel ratio, it is necessary to treat these two types of variations separately. Therefore, in the present embodiment, first, the variation of the air-fuel ratio due to the above-mentioned (2) that is unrelated to the valve opening characteristic value is obtained, and the variation is corrected to obtain the air-fuel ratio for each valve opening characteristic value of (1). A correction coefficient for the variation is determined.
[0057]
The variation of the air-fuel ratio (intake air amount) due to the variation of the valve opening characteristic values such as the operating angle and the valve lift increases as the intake air amount decreases, in other words, the operating angle and the valve lift amount decrease. In the state where the intake air amount is maximized, that is, the operating angle, the valve lift and the amount are maximized, the variation in the intake air amount of each cylinder due to the valve opening characteristic value is almost negligible. That is, when the operating angle and the valve lift amount are maximum, the variation in the air-fuel ratio in each cylinder is only caused by the variation in the intake passage length and the characteristics of the fuel injection valve ((2) above).
[0058]
Therefore, in the present embodiment, first, the engine is operated with the valve opening characteristic value (reference valve opening characteristic value) that maximizes the intake air amount in the reference operation state, and the air-fuel ratio correction coefficient at this time is set to the reference correction coefficient of each cylinder. Xi is calculated.
[0059]
That is, Xi = AFi / ((1 / n) Σ (1−n) AFi)
For the valve opening characteristic values other than the valve opening characteristic value at which the intake air amount becomes the maximum, fuel Xi × F is injected into each cylinder in an amount obtained by correcting the fuel injection set value F using the reference correction coefficient Xi in advance. The air-fuel ratio correction coefficient Ai is calculated based on the variation in the air-fuel ratio in the state. Thus, the air-fuel ratio correction coefficient Ai becomes a value corresponding to the variation in the air-fuel ratio caused only by the variation in the valve opening characteristic value of each cylinder. The air-fuel ratio correction coefficient Ai of each cylinder at the reference valve opening characteristic value is set to 1.0.
Thus, the air-fuel ratio correction coefficient Ai of each cylinder for each valve opening characteristic value in the reference operation state is obtained.
[0060]
2. Correction of the air-fuel ratio correction coefficient Ai according to the engine operating state.
Next, the correction of the air-fuel ratio correction coefficient Ai according to the engine operating state will be described. As described above, the air-fuel ratio correction coefficient Ai obtained in the reference operation state corresponds to the variation in the air-fuel ratio of each cylinder caused only by the variation in the valve opening characteristic value.
However, the variation in the valve opening characteristic value of each cylinder varies not only with the value of the valve opening characteristic value itself, but also with the operating state of the engine.
[0061]
For example, even when the engine load increases and the valve opening characteristic value remains the same, the intake air amount increases. For this reason, even if the valve opening characteristic value is the same, the variation in the air-fuel ratio of each cylinder becomes large, so that even when the correction amount of the fuel injection amount by the air-fuel ratio correction coefficient is the same, the engine Must increase with load.
[0062]
Further, when the engine speed increases, the intake air amount increases as in the case where the load increases. However, in this case, the amount of burned gas blown back to the intake port changes depending on the number of revolutions. Therefore, when the number of revolutions increases, the amount of intake air in the cylinder increases uniformly up to a certain number of revolutions. Above, it decreases with the rotation speed. Therefore, even if the valve opening characteristic value is the same, the variation in the air-fuel ratio of each cylinder changes with the rotational speed, and there is a peak rotational speed at which the variation becomes maximum.
[0063]
Further, the variation in the air-fuel ratio of each cylinder also changes depending on the engine temperature. For example, when the engine temperature rises, the camshaft also thermally expands. In this embodiment, since the valve opening characteristic value is changed using the variable valve mechanism 9 shown in FIG. 5, the camshaft 6 thermally expands to the left in FIG. . Therefore, the amount of movement of the cam 4 in each cylinder due to thermal expansion (that is, the amount of change in the valve opening characteristic value of each cylinder due to thermal expansion) increases as the cylinder moves away from the magnetic body 30.
Therefore, it is necessary to correct a change in the valve opening characteristic value due to thermal expansion in each cylinder according to the engine temperature for each cylinder.
[0064]
In the present embodiment, a change in the air-fuel ratio correction coefficient depending on the engine load, the number of revolutions, and the engine temperature is obtained in advance by experiment or calculation, and stored in the ROM of the ECU 22 in the form of a correction coefficient for the air-fuel ratio correction coefficient in the reference operation state. It is. When the load, rotation speed, and engine temperature are different from those in the reference operation state, the air-fuel ratio correction coefficient calculated in the reference operation state is corrected based on these correction coefficients.
Thus, in the present embodiment, even in an operating state where it is difficult to accurately measure the exhaust air-fuel ratio of each cylinder, it is possible to correct the fuel injection amount of each cylinder so that the air-fuel ratio of each cylinder is accurately matched. It has become.
[0065]
6 and 7 are flowcharts for explaining the air-fuel ratio control operation described above. FIG. 6 shows the learning operation of the air-fuel ratio correction coefficient Ai and the reference correction coefficient Xi, and FIG. A fuel injection amount correction operation using Ai and Xi is shown, respectively. 6 and 7 are performed by a routine that is executed by the ECU 22 at regular intervals or at constant crank rotation angles.
[0066]
First, the learning operation of FIG. 6 will be described.
When the operation is started in FIG. 6, in step 601, it is first determined whether or not learning of the correction coefficients Ai and Xi of each cylinder has been completed after starting the engine this time. End the operation.
[0067]
If it is determined in step 601 that the learning of the correction coefficient has not been completed, the process proceeds to step 603, where it is determined whether the engine is currently operating in the reference operating state. As described above, the air-fuel ratio correction coefficient Ai and the reference correction coefficient Xi need to be learned in a state where the air-fuel ratio of each cylinder can be accurately detected by the air-fuel ratio sensor 57. In this embodiment, learning is performed by determining that the vehicle is stopped (for example, the vehicle speed is 3 km / h or less) and the engine is idling (the throttle valve opening is zero) as the reference operation state. . When the engine is not in the reference operation state, this operation ends as it is.
Next, in step 605, the valve opening characteristic value (valve lift, valve opening period) of the intake valve of each cylinder is set to a value to be learned.
[0068]
As described above, in order for the air-fuel ratio sensor 57 to accurately detect the air-fuel ratio of each cylinder, it is preferable that the exhaust flow rate of each cylinder be as large as possible. Therefore, in the present embodiment, learning is performed in a state where the ignition timing of each cylinder is delayed while maintaining the engine speed at the idle speed. If the ignition timing is delayed, the output torque of each cylinder decreases, so the valve opening characteristic value of each cylinder is shifted to the large flow rate side in order to maintain the idle speed constant while compensating for the decrease in torque, and the exhaust flow rate is reduced. Increase. In step 605, the idle opening speed of the engine is adjusted by shifting the valve opening characteristic value of the intake valve to the large flow rate side to increase the intake air amount of each cylinder and adjusting (retarding) the ignition timing of each cylinder. Is kept constant.
[0069]
In step 607, it is determined whether or not the intake air amount of the cylinder has been sufficiently increased while maintaining the idle speed of the engine by the above-described setting of the valve opening characteristic value, that is, whether or not the learning condition has been satisfied. If not, the process waits until the condition is satisfied, and the current operation ends. Thus, the operation of FIG. 6 is repeated until the learning condition is satisfied.
[0070]
In the present embodiment, when the learning condition is satisfied in step 607, the air-fuel ratio correction coefficient Ai (θ) at the valve opening characteristic value θ at that time is calculated in steps 619 to 625, and then in step 605 The setting of the valve opening characteristic value is changed by a predetermined amount, and the operation of FIG. 6 is repeated until the air-fuel ratio correction coefficient Ai at each valve opening characteristic value θ is completely calculated for each cylinder.
[0071]
In step 605, first, the air-fuel ratio correction coefficient of the valve opening characteristic value (reference valve opening characteristic value) at which the intake air amount becomes maximum due to the ignition timing retard while maintaining the idling speed in the current operation state is calculated. The air-fuel ratio correction coefficient of each cylinder at this reference valve opening characteristic value is stored as a reference correction coefficient Xi, and from the next execution of the operation of FIG. And the operation of adjusting the ignition timing so that the engine speed does not change.
[0072]
That is, in the present embodiment, at the time of learning the correction coefficient, first, the valve opening characteristic value is adjusted so that the intake air amount of each cylinder becomes the maximum at the current rotational speed, and each reference correction coefficient is calculated. While decreasing, the air-fuel ratio correction coefficient at another valve opening characteristic value is calculated.
As described above, first, the reference correction coefficient is calculated at the time of learning in the operation with the valve opening characteristic value in which the intake air amount of the cylinder is increased to some extent, because the variation in the air-fuel ratio between the cylinders depends on the length of the intake passage and the fuel injection. This is because it is only due to the characteristic variation of the valve. These variations do not change even when the valve opening characteristic value is changed. For this reason, in the present embodiment, first, the reference correction coefficient Xi is obtained, and the air-fuel ratio correction coefficient Ai at the other valve opening characteristic values is corrected while the fuel injection amount is corrected so that the air-fuel ratio of each cylinder becomes equal with the Xi. It is calculated.
That is, the air-fuel ratio correction coefficient Ai in the present embodiment is a value corresponding to the variation of the air-fuel ratio of each cylinder at each valve opening characteristic value, which is caused only by the valve opening characteristic value.
[0073]
More specifically, when the learning condition is satisfied in step 607 in FIG. 6, the process proceeds to step 609, in which the current valve opening characteristic value corresponds to the maximum intake air amount of each cylinder. It is determined whether the value is equal to the reference value (reference valve opening characteristic value). If the reference value is equal to the reference valve opening characteristic value, the exhaust air-fuel ratio of each cylinder is measured in step 613 based on the output of the air-fuel ratio sensor 57. I do.
[0074]
That is, in step 613, the output of the air-fuel ratio sensor 57 is sampled in synchronization with the crank rotation angle at each timing when the exhaust gas discharged in the exhaust stroke of each cylinder reaches the air-fuel ratio sensor 57, and the exhaust air-fuel ratio of each cylinder is sampled. AFi is obtained, and in step 615, the air-fuel ratio of each cylinder becomes the same from the average value of the exhaust air-fuel ratio of each cylinder ((1 / n) Σ (1 to n) AFi) and the exhaust air-fuel ratio AFi of each cylinder. Thus, the reference correction coefficient Xi for correcting the fuel injection amount is calculated as Xi = AFi / ((1 / n) Σ (1-n) AFi). As described above, the reference correction coefficient Xi is a value corresponding to a variation in each air-fuel ratio caused by an element (variation in intake passage length or fuel injection valve characteristic) other than the valve opening characteristic value of each cylinder.
Then, after calculating the reference correction coefficient Xi of each cylinder in step 615, the calculated Xi is stored (learned) in step 617, and the current operation ends.
[0075]
When the operation in FIG. 6 is started next time, in step 605, the valve opening characteristic value θ is shifted by a predetermined amount to a side where the intake air amount decreases (for example, a side where the valve lift amount decreases and the cam working angle decreases). Is done. Thus, after step 609, steps 619 to 625 are executed, and the air-fuel ratio correction coefficient Ai (θ) at each valve opening characteristic value of each cylinder is obtained.
That is, in step 619, the exhaust air-fuel ratio of each cylinder is measured, and in step 621, the provisional air-fuel ratio correction coefficient Ai (θ) ′ at the current valve opening characteristic value θ is calculated as
Ai (θ) ′ = AFi / ((1 / n) Σ (1-n) AFi)
Is calculated as
[0076]
The provisional air-fuel ratio correction coefficient Ai (θ) ′ calculated in step 619 is obtained by calculating the air-fuel ratio variation of each cylinder caused by the thermal expansion of the engine and the factors other than the valve opening characteristic value of each cylinder. The value includes the variation in the air-fuel ratio of the cylinder. Therefore, in step 623, the provisional air-fuel ratio correction coefficient Ai (θ) 'is corrected using the correction coefficient Ri (TL) for thermal deformation of each cylinder described later and the reference correction coefficient XiJ stored in step 617. , The true air-fuel ratio correction coefficient Ai (θ) of each cylinder is calculated.
That is, Ai (θ) = Ai (θ) ′ / (Xi × Ri (TL))
[0077]
In each of the above equations, the subscript i represents the cylinder number, θ represents the valve opening characteristic value, and TL represents the engine temperature (lubricating oil temperature). As described above, the air-fuel ratio correction coefficient Ai (θ) calculated in step 623 excludes the influence of the variation due to the thermal expansion of the engine and the variation in the air-fuel ratio of each cylinder due to the variation of the intake passage length and the characteristics of the fuel injection valve. This corresponds to the variation in the air-fuel ratio of each cylinder caused purely by only the valve opening characteristic value.
[0078]
In step 625, after the air-fuel ratio correction coefficient calculated in step 623 is stored, the current execution of this operation ends. Note that this operation is repeated until the air-fuel ratio correction coefficient Ai (θ) of each cylinder is calculated for all of the predetermined valve opening characteristic values, and ends.
[0079]
FIG. 7 is a flowchart showing a fuel injection amount correction operation using the correction coefficients Xi and Ai (θ) stored by the learning operation of FIG.
In the operation of FIG. 7, by correcting the air-fuel ratio correction coefficient Ai (θ) of each cylinder obtained in the reference operation state based on the value of the parameter representing the engine operation state, even in operation states other than the reference operation state, The fuel injection amount is corrected for each cylinder so that the air-fuel ratio of each cylinder exactly matches.
[0080]
In FIG. 7, at step 701, the engine speed N, the intake air amount Q, and the engine lubricating oil temperature TL are detected by the respective sensors. At step 703, the current opening characteristic value θ of the intake valve is determined by the valve opening characteristic value sensor. 16 (FIG. 1).
Then, in step 705, the reference correction coefficient Xi of each cylinder stored in step 617 of FIG. 6 is read.
In step 707, the air-fuel ratio correction coefficient Ai (θ) of each cylinder stored in step 625 is read based on the current valve opening characteristic value θ detected in step 703.
[0081]
Further, in step 709, the correction coefficient Li of each cylinder at the rotation speed N and the load (Q / N) of the air-fuel ratio correction coefficient is determined based on the engine speed N and the engine intake air amount Q. In the present embodiment, the engine is operated while changing the engine speed N and the intake air amount Q as described above, and it is determined in advance how the air-fuel ratio correction coefficient Ai (θ) obtained in the reference operating state changes. It has been obtained and stored in advance in the ROM of the ECU 22 as a rotational speed load correction coefficient Li in the form of a two-dimensional numerical map using N and Q as parameters. In step 713, a rotational speed load correction coefficient Li at the current rotational speed and load is calculated from the map based on the engine rotational speed N and the engine intake air amount Q.
In step 709, the intake air amount per engine revolution (Q / N) is used as a parameter representing the engine load. However, instead of Q / F, the accelerator opening (depression of the accelerator pedal) is used as the parameter representing the engine load. Amount) may be used.
Further, in step 709, the correction for the engine speed and the load is performed using one correction coefficient Li. However, the correction coefficient for the engine speed and the correction coefficient for the engine load may be separately provided. is there.
[0082]
Further, at step 711, an engine temperature correction coefficient Ri of each cylinder representing the thermal expansion of the engine is calculated based on the lubricating oil temperature (engine temperature) TL detected at step 701. As described above, the variation in the air-fuel ratio due to the thermal expansion of the engine differs depending on the positional relationship between the cylinders. In the present embodiment, the effect of the thermal expansion of the engine on the air-fuel ratio correction coefficient is determined in advance by experiment for each engine temperature (lubricating oil temperature), and stored in the ROM of the ECU 22 as a one-dimensional map using the lubricating oil temperature TL as a parameter. In step 711, an engine temperature correction coefficient Ri (TL) for each cylinder at the current lubricating oil temperature TL is calculated based on this map.
[0083]
Next, at step 713, the air-fuel ratio correction coefficient Ai (θ) of each cylinder in the reference operating state at the current valve opening characteristic value θ read at step 707 is calculated by the rotational speed load correction coefficient Li calculated at step 709, and step 711. The engine speed is corrected to Ai (θ) × Li × Ri (TL) using the engine temperature correction coefficient Ri (TL) calculated in (1), and the engine speed is separately calculated by an operation not shown separately using the corrected air-fuel ratio correction coefficient. The fuel injection amount set value F calculated based on the load and the load (Q / N) is corrected. As a result, the fuel injection amount Fi (θ) of each cylinder required to match the air-fuel ratio of each cylinder in the current operation state becomes:
[0084]
Fi (θ) = F × Xi × Ai (θ) × Li × Ri (TL)
Is calculated as
As described above, according to the present embodiment, even when the single air-fuel ratio sensor 57 is used, the air-fuel ratio correction coefficient of each cylinder is obtained in the reference operation state in which the air-fuel ratio of each cylinder can be accurately detected. By correcting the air-fuel ratio correction coefficient using a predetermined parameter representing the engine operating state, even in an operating state in which the single air-fuel ratio sensor 57 cannot accurately detect the air-fuel ratio of each cylinder, It is possible to make the air-fuel ratio of each cylinder equal.
[0085]
By the way, as described above, in this embodiment, the intake air amount of the engine can be controlled by using the variable valve mechanism 9 without using the throttle valve by changing the valve opening characteristic value of the intake valve. This makes possible non-throttle operation with high thermal efficiency that eliminates throttle loss due to the throttle valve.
However, when the intake air amount is controlled using the variable valve mechanism 9, the non-throttle operation is possible as compared with the case where the intake air amount is controlled using the throttle valve. There is.
[0086]
When the variable valve mechanism 9 is used, it is possible to change the intake air amount of the cylinder in a very short time as compared with the intake throttle using the throttle valve.
Normally, when changing the intake air amount of a cylinder using a throttle valve, the volume of the surge tank or intake manifold downstream of the intake valve becomes a dead volume. It takes time for the amount of intake air in the cylinder to change, and the amount of intake air changes relatively slowly.
[0087]
On the other hand, when the valve opening characteristic value of the intake valve is changed, the in-cylinder intake air amount changes in a very short time close to a step change.
For this reason, for example, NO X NO stored from the storage catalyst X In the case of discharging and reducing and purifying, the better result can be obtained by changing the intake air amount in the cylinder by changing the valve opening characteristic value of the intake valve.
[0088]
When the air-fuel ratio of exhaust gas flowing into the exhaust system of an internal combustion engine that performs leaner combustion than the stoichiometric air-fuel ratio in the cylinder is lean, the NO X Is selectively retained by adsorption, absorption or both, and when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the stored NO X Of reducing and purifying methane by using the reducing component in the exhaust gas X A storage catalyst is provided, and NO during the lean air-fuel ratio operation of the engine. X NO in exhaust gas to storage catalyst X There is known an exhaust gas purification device for a lean burn engine that stores and removes air.
[0089]
Such NO X In an exhaust gas purification device using an occlusion catalyst, NO X NO stored by the storage catalyst X NO in exhaust when saturated with X Can not be removed, so NO X NO stored in the storage catalyst X The engine is switched from the lean air-fuel ratio operation to the stoichiometric air-fuel ratio or the rich air-fuel ratio every time the amount of X By supplying exhaust gas with a stoichiometric air-fuel ratio or rich air-fuel ratio to the storage catalyst, NO X NO stored in the storage catalyst X Needs to be reduced and purified.
[0090]
Normally, in order to switch the engine from the lean air-fuel ratio operation to the theoretical or rich air-fuel ratio operation, it is necessary to increase the amount of fuel supplied to each cylinder and at the same time reduce the amount of air taken into the cylinder.
In this case, for example, in a normal engine, the throttle valve is throttled to reduce the amount of intake air, and a portion of the exhaust gas is circulated to the intake system by an EGR (exhaust gas recirculation) device, and the fresh air sucked into the cylinder. , The amount of air (fresh air) drawn into the cylinder is reduced. However, a relatively large volume such as a surge tank and an intake manifold exists downstream of the throttle valve. Therefore, even when the throttle valve opening is suddenly changed, the amount of air actually sucked into the cylinder decreases only relatively slowly, and the air-fuel ratio of the engine rapidly changes from lean to stoichiometric or rich air-fuel ratio. Can not be.
[0091]
Therefore, NO X NO absorbed by the storage catalyst X If the intake air amount is controlled by using the throttle valve when reducing and purifying the exhaust gas, the exhaust air-fuel ratio cannot be rapidly switched from lean to rich, and NO X NO of the storage catalyst X The switching is performed after operation at an intermediate air-fuel ratio between lean and rich, which does not contribute to the reduction and purification of NO, resulting in wasteful increase in fuel consumption and NO. X NO stored by the storage catalyst X There is a problem that the time required for reduction purification of the catalyst increases.
On the other hand, for example, when the variable valve mechanism 9 shown in FIG. 5 is used, the intake air amount of the cylinder can be sharply reduced by moving the cam for a short distance. Switching to the fuel ratio can be performed in a very short time.
For this reason, in an engine that controls the in-cylinder intake air amount by changing the valve opening characteristic value of the valve, NO X NO stored in the storage catalyst X Can be efficiently reduced and purified in a short time.
[0092]
When the air-fuel ratio correction coefficient is determined for each valve opening characteristic value as described above, the detection accuracy of the valve opening characteristic value sensor 16 becomes a problem. For example, when the output characteristic of the valve opening characteristic value sensor 16 has changed, the valve opening characteristic value of the cylinder cannot be accurately feedback-controlled, so that the air-fuel ratio correction coefficient of each cylinder can be accurately adjusted. In addition to this, there is a problem that it becomes impossible to accurately control the intake air amount of the engine, and the engine performance and the exhaust characteristics are deteriorated.
[0093]
In the embodiment shown in FIGS. 1 to 5, the actual output characteristic of the valve opening characteristic value sensor 16 is detected during the operation of the engine, and the output value of the sensor 16 is corrected based on the detected output characteristic. Thus, even when the output characteristic of the valve opening characteristic value sensor 16 changes, the valve opening characteristic value can be accurately detected.
[0094]
Hereinafter, a method of detecting the actual output characteristics of the valve opening characteristic value sensor 16 in the present embodiment will be described.
The actual output characteristics of the valve opening characteristic value sensor 16 include, for example, the output of the sensor 16 when the valve opening characteristic value of the engine (in the present embodiment, the valve lift and the operating angle) are maximum, and the output when the sensor 16 is minimum Output. For this reason, if it is possible to set the valve opening characteristic value to the maximum value and the minimum value during engine operation, it becomes possible to detect the actual output characteristic of the valve opening characteristic value sensor 16 during engine operation. .
[0095]
However, when the valve opening characteristic value of each cylinder is changed, the amount of air taken into the cylinder changes, so that the engine output torque and the number of revolutions change significantly. It is difficult to change between the maximum value and the minimum value.
In the present embodiment, this problem is solved by, for example, performing the fuel cut operation of the engine.
[0096]
Hereinafter, the output characteristic detection operation of the valve opening characteristic value sensor 16 of the present embodiment will be described with reference to FIG. The operation in FIG. 8 is performed as a routine executed by the ECU 22 at regular intervals or at constant crank rotation angles.
[0097]
In the operation of FIG.
B) The engine is performing fuel cut operation such as deceleration, or
B) An operation other than fuel cut is being performed, and a predetermined time or more has elapsed since the previous output characteristic measurement, and the valve opening characteristic value is maintained while maintaining the engine output torque constant by operating the throttle valve opening. Can be set to minimum and maximum values,
When either of the conditions is satisfied, the valve opening characteristic of the engine is actually changed from the minimum value to the maximum value, and the output value of the valve opening characteristic value sensor 16 at the minimum value and the maximum value is obtained. 16 output characteristics are measured.
As described above, in the present embodiment, it is necessary to actually set the valve opening characteristic value of the engine to the minimum value and the maximum value when detecting the sensor output characteristic, which may affect the engine output.
[0098]
Therefore, in this embodiment, during the fuel cut operation in which combustion is not performed in the engine and the engine output is not affected even if the valve opening characteristic value is set to the minimum or the maximum, or the valve opening characteristic value is set to the maximum value and the minimum value. Even when set to a value, the sensor output characteristic detection operation is performed only when the engine is operating in an operating state in which the engine intake air amount (ie, engine output) can be maintained constant by adjusting the throttle valve opening. ing.
As a result, the sensor output characteristics can be detected without affecting the operation of the engine, so that even when the output characteristics change, the output is corrected according to the characteristic change, and the valve opening characteristic value of the engine is accurately detected. It becomes possible.
[0099]
Hereinafter, the operation of FIG. 8 will be specifically described.
When the operation is started, first, in step 801, it is determined whether or not the fuel cut operation (F / C operation) of the engine is currently being performed. When the F / C operation is currently being performed, the output characteristics of the sensor 16 are measured in steps 803 to 805 because changing the valve opening characteristic value does not affect the operation of the engine.
[0100]
That is, in step 803, the variable valve mechanism 9 is driven to reduce the valve opening characteristic value to the minimum value (the valve opening characteristic value that minimizes the intake air amount, ie, in this embodiment, both the valve lift amount and the operating angle are minimized). In FIG. 5, the valve opening characteristic value is controlled when the camshaft 6 moves to the leftmost position in FIG. In step 805, the output θ of the valve opening characteristic value sensor 16 is stored as the minimum valve opening characteristic value output θmin after a lapse of time sufficient for the variable valve mechanism 9 to reach the minimum value.
[0101]
Then, after storing the minimum valve opening characteristic value output, in step 807, the valve opening characteristic value is set to the maximum value (the valve opening characteristic value at which the intake air amount is maximized, that is, the valve lift amount, the operating angle, and Are the maximum positions, and the valve opening characteristic value when the camshaft 6 is moved to the rightmost position in FIG. 5) is controlled. After a sufficient time has passed, the output θ of the valve opening characteristic value sensor 16 is controlled. , The maximum valve opening characteristic value output value θmax.
[0102]
If it is determined in step 801 that the fuel cut operation is not currently being performed, the process proceeds to step 811 to determine whether the value of the flag X is set to 1. X is set to 1 in step 817 when the measurement execution condition is satisfied in step 813, is set to step 815 when the measurement execution condition is not satisfied, and is set to zero in step 829 when the output characteristic detection is completed. This flag is set. The flag X indicates whether or not the sensor output characteristic is being detected. If the measurement execution condition of step 813 is once satisfied, the sensor X of steps 819 to 827 will not be satisfied even if the condition of step 813 is no longer satisfied. It has a function to complete the output characteristic detection operation.
[0103]
If X ≠ 1 in step 811, since the sensor output characteristics are not being detected, the process proceeds to step 813, where it is determined whether the conditions for executing the sensor output characteristics detection are satisfied. As described above, in this embodiment, in order to detect the sensor output characteristic, it is necessary to change the valve opening characteristic value of the intake valve. Therefore, it is not preferable to perform the operation very frequently except during the fuel cut operation. Therefore, the detection execution condition determined in step 813 is that the operation time from the start of the engine is an integral multiple of a predetermined value (for example, 10 minutes) (in this case, the measurement execution condition is set every 10 minutes during the engine operation). Holds, or the engine cooling water temperature or lubricating oil temperature is an integral multiple of 10 ° K (in this case, measurement is performed every time the cooling water temperature or lubricating oil temperature increases by 10 ° K during engine operation) Condition is satisfied). As the measurement execution condition in step 813, another condition can be set as long as the measurement execution condition is satisfied at appropriate time intervals during engine operation.
[0104]
If the measurement execution condition is not satisfied in step 813, the value of the flag X is set to 0 in step 815, and this operation ends without detecting the sensor output characteristics in steps 821 to 829.
[0105]
If the measurement execution condition is satisfied in step 813, the process proceeds to step 817, where the flag X is set to 1, and then the process proceeds to step 819. Thus, once the measurement execution condition of step 813 is satisfied, the next time the operation of FIG. 8 is executed, step 819 is directly executed after step 811 and the detection of the output characteristic is completed. Until the value of X is set to 1 in step 829, the determination in step 813 is bypassed.
[0106]
In step 819, it is determined whether or not it is possible to change the valve opening characteristic value from the minimum to the maximum without changing the current engine output torque by controlling the opening degree of the throttle valve, that is, the equal output control condition is satisfied. Is determined. For example, in the current engine operating state (rotational speed, load), if the valve opening characteristic value is set to the minimum, the current intake air amount cannot be maintained even if the throttle valve is fully opened, and the intake air amount decreases. If the intake air amount increases beyond the current intake air amount when the valve opening characteristic value is maximized even when the throttle valve is fully closed, the valve is opened without changing the current engine output torque. Control (equal output control) for changing the characteristic value from the minimum to the maximum cannot be performed.
In the present embodiment, the range between the rotational speed N and the load (Q / N) at which equal output control is possible is obtained in advance by an experiment or the like, and stored in the ROM of the ECU 22 as equal output control conditions. In step 819, it is determined whether or not the current rotational speed and load match these equal output control conditions.
[0107]
If the current equal output control condition is not satisfied at step 819, the current execution of this operation ends. In this case, if the equal output control condition in step 819 is satisfied in the next and subsequent operations, the sensor output characteristic detection operation in step 821 and subsequent steps is executed.
If the equal output control condition is satisfied in step 810, the variable valve mechanism 9 is controlled so that the valve opening characteristic value is minimized in step 821, and the output torque of the engine is maintained constant. The opening of the throttle valve 56 is adjusted. In step 823, the current output of the sensor 16 is stored (learned) as θmin after a sufficient time has elapsed for the valve opening characteristic value to reach the minimum value.
[0108]
In steps 825 and 827, the valve opening characteristic value is maximized while maintaining the output of the engine constant, similarly to steps 821 and 823, and the sensor 16 output in this state is stored as θmax (learning). I do. When the learning of the values of θmin and θmax is completed, the value of the flag X is set to 0 in step 829.
[0109]
As described above, the sensor output characteristic is detected by the operation of FIG. 8 every time the engine performs the fuel cut operation and at appropriate intervals in other operation states, and the minimum value and the maximum value of the valve opening characteristic are determined. The values of the sensor outputs θmax and θmin at are updated. As a result, the sensor output value for an arbitrary valve opening characteristic value between the maximum value and the minimum value can be obtained by a known appropriate method, and even when the sensor output characteristic changes during engine operation, By correcting the sensor output, an accurate valve opening characteristic value can be detected.
[0110]
【The invention's effect】
According to the invention described in each claim, there is a common effect that it is possible to reduce the variation in the air-fuel ratio between the cylinders regardless of the operating state of the engine.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an embodiment in which an air-fuel ratio control device of the present invention is applied to a four-cylinder gasoline engine for an automobile.
FIG. 2 is a schematic diagram illustrating a schematic configuration of an intake system of the engine of FIG. 1;
FIG. 3 is a plan view showing an arrangement of an air-fuel ratio sensor in the intake system of FIG. 2;
FIG. 4 is a view showing a detailed shape of an intake valve driving cam of the engine of FIG. 1;
FIG. 5 is a sectional view showing the operation principle of the variable valve mechanism 9;
FIG. 6 is a flowchart illustrating an air-fuel ratio control operation of the embodiment of FIG. 1;
FIG. 7 is a flowchart illustrating an air-fuel ratio control operation of the embodiment of FIG. 1;
8 is a flowchart illustrating an output characteristic detection operation of the valve opening characteristic value sensor according to the embodiment of FIG. 1;
[Explanation of symbols]
1. Internal combustion engine
2. Intake valve
3. Exhaust valve
4: Intake valve drive cam
6 ... intake cam
9… Variable valve mechanism
16 ... Valve opening characteristic value sensor
22 Electronic control unit (ECU)
56 ... Throttle valve

Claims (7)

気筒内吸入空気量に影響を与える吸気弁の開弁特性値を変化させる可変バルブ機構を備えた多気筒機関の空燃比制御装置であって、
機関の予め定めた基準運転状態において各気筒毎に測定した排気空燃比を用いて基準運転状態における各開弁特性値毎の各気筒の排気空燃比のばらつきを算出するとともに、算出したばらつきに基づいて、前記基準運転状態において各気筒の運転空燃比のばらつきを低減するための燃料噴射量の空燃比補正係数を各開弁特性値毎に算出する補正係数算出手段と、
機関が前記基準運転状態以外の運転状態にあるときに、各開弁特性値における前記空燃比補正係数を機関運転状態を表す所定のパラメータの値に基づいて修正することにより、基準運転状態以外の運転状態における各気筒の運転空燃比のばらつきを低減するように各気筒の燃料噴射量を制御する、多気筒内燃機関の空燃比制御装置。An air-fuel ratio control device for a multi-cylinder engine including a variable valve mechanism that changes a valve opening characteristic value of an intake valve that affects an intake air amount in a cylinder,
Using the exhaust air-fuel ratio measured for each cylinder in the predetermined reference operating state of the engine, calculate the variation in the exhaust air-fuel ratio of each cylinder for each valve-opening characteristic value in the reference operating state, and based on the calculated variation. Correction coefficient calculation means for calculating an air-fuel ratio correction coefficient of the fuel injection amount for each valve-opening characteristic value for reducing the variation of the operating air-fuel ratio of each cylinder in the reference operating state;
When the engine is in an operation state other than the reference operation state, by correcting the air-fuel ratio correction coefficient at each valve opening characteristic value based on a value of a predetermined parameter representing the engine operation state, An air-fuel ratio control device for a multi-cylinder internal combustion engine that controls a fuel injection amount of each cylinder so as to reduce a variation in an operating air-fuel ratio of each cylinder in an operating state. 前記補正係数算出手段は各気筒の空燃比を実質的に同一にするように前記燃料噴射量の空燃比補正係数を各開弁特性値毎に算出する、請求項１に記載の多気筒内燃機関の空燃比制御装置。2. The multi-cylinder internal combustion engine according to claim 1, wherein the correction coefficient calculation unit calculates an air-fuel ratio correction coefficient of the fuel injection amount for each valve opening characteristic value so that an air-fuel ratio of each cylinder becomes substantially the same. Air-fuel ratio control device. 前記補正係数算出手段は、機関が前記予め定めた基準運転状態において機関が気筒内吸入空気量が最大になる基準開弁特性値で運転されているときの各気筒毎の空燃比ばらつきに基づいて、各気筒の空燃比のばらつきを低減するための燃料噴射量の基準補正係数を算出し、機関が前記基準開弁特性値以外で運転されている場合には、前記基準補正係数を用いて補正した後の燃料噴射量に対する前記空燃比補正係数を算出する、請求項１または請求項２に記載の多気筒内燃機関の空燃比制御装置。The correction coefficient calculating means is configured to calculate the correction coefficient on the basis of the air-fuel ratio variation of each cylinder when the engine is operated at the reference valve opening characteristic value at which the in-cylinder intake air amount is maximized in the predetermined reference operation state. Calculating a reference correction coefficient of the fuel injection amount for reducing the variation in the air-fuel ratio of each cylinder, and correcting the correction using the reference correction coefficient when the engine is operated at a value other than the reference valve opening characteristic value. The air-fuel ratio control device for a multi-cylinder internal combustion engine according to claim 1 or 2, wherein the air-fuel ratio correction coefficient for the fuel injection amount after the calculation is calculated. 前記機関運転状態を表すパラメータは、機関回転数、機関負荷又はアクセル開度のうち少なくとも１つを含む請求項１から請求項３のいずれか１項に記載の多気筒内燃機関の空燃比制御装置。The air-fuel ratio control device for a multi-cylinder internal combustion engine according to any one of claims 1 to 3, wherein the parameter representing the engine operation state includes at least one of an engine speed, an engine load, and an accelerator opening. . 前記機関運転状態を表すパラメータは、更に機関温度を含む、請求項４に記載の多気筒内燃機関の空燃比制御装置。The air-fuel ratio control device for a multi-cylinder internal combustion engine according to claim 4, wherein the parameter indicating the engine operation state further includes an engine temperature. 前記補正係数算出手段は、排気通路に配置された単一の空燃比センサを用いて複数気筒の排気空燃比を測定する、請求項１または請求項２に記載の多気筒内燃機関の空燃比制御装置。3. The air-fuel ratio control of a multi-cylinder internal combustion engine according to claim 1, wherein the correction coefficient calculation unit measures exhaust air-fuel ratios of a plurality of cylinders using a single air-fuel ratio sensor disposed in an exhaust passage. 4. apparatus. 前記基準運転状態は、機関のアイドル運転状態である請求項６に記載の多気筒内燃機関の空燃比制御装置。The air-fuel ratio control apparatus for a multi-cylinder internal combustion engine according to claim 6, wherein the reference operation state is an idle operation state of the engine.
JP2002162037A 2002-06-03 2002-06-03 Air-fuel ratio control device for multi-cylinder internal combustion engine Expired - Fee Related JP4126963B2 (en)

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JP2007154779A (en) * 2005-12-06 2007-06-21 Toyota Motor Corp Control device for internal combustion engine
JP2007247480A (en) * 2006-03-15 2007-09-27 Hitachi Ltd Control device and control method of internal combustion engine
JP4663557B2 (en) * 2006-03-15 2011-04-06 日立オートモティブシステムズ株式会社 Control device and control method for internal combustion engine
JP2008144639A (en) * 2006-12-08 2008-06-26 Denso Corp Control device for internal combustion engine
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JP2010001861A (en) * 2008-06-23 2010-01-07 Mazda Motor Corp Method for controlling air fuel ratio of multi-cylinder internal combustion engine
JP2010174743A (en) * 2009-01-29 2010-08-12 Mitsubishi Motors Corp Air fuel ratio control device for multi-cylinder engine
JP2011089443A (en) * 2009-10-21 2011-05-06 Mitsubishi Motors Corp Detector for detecting variations in air-fuel ratio between cylinders
JP2011179357A (en) * 2010-02-26 2011-09-15 Honda Motor Co Ltd Control device for internal combustion engine
US20130197783A1 (en) * 2012-02-01 2013-08-01 Toshikazu Kato Air-fuel ratio imbalance detecting device and air-fuel ratio imbalance detecting method for internal combustion engine
US9074545B2 (en) * 2012-02-01 2015-07-07 Toyota Jidosha Kabushiki Kaisha Air-fuel ratio imbalance detecting device and air-fuel ratio imbalance detecting method for internal combustion engine
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