JP2005023819A

JP2005023819A - Air-fuel ratio control device for internal combustion engine

Info

Publication number: JP2005023819A
Application number: JP2003189445A
Authority: JP
Inventors: Yuji Yanagawa; 祐治柳川; Michihiro Hatake; 道博畠
Original assignee: Mitsubishi Motors Corp
Current assignee: Mitsubishi Motors Corp
Priority date: 2003-07-01
Filing date: 2003-07-01
Publication date: 2005-01-27

Abstract

【課題】目標空燃比が変更されたとき、空燃比制御を適切に行い、燃費の悪化を防止する内燃機関の空燃比制御装置を提供する。
【解決手段】エンジン１の運転状態に応じた目標空燃比が変更されたとき、排気通路１３に設けられたＬＡＦＳ１４により検出された排気ガス中のＯ_２濃度から求めた検出空燃比と、気筒２の排気ポート６の出口からＬＡＦＳ１４までの排気ガスの輸送、拡散遅れ１９を、一次遅れとしてモデル化して求めた演算空燃比との偏差が所定値以上である場合、空燃比が目標空燃比になるように、目標空燃比と演算空燃比の偏差に基づいて、インジェクタ３から気筒２に供給する燃料供給量を制御する内燃機関の空燃比制御装置。
【選択図】図１An air-fuel ratio control apparatus for an internal combustion engine that appropriately performs air-fuel ratio control and prevents deterioration of fuel consumption when a target air-fuel ratio is changed is provided.
When a target air-fuel ratio is changed according to an operating state of an engine, a detected air-fuel ratio obtained from an O ₂ concentration in exhaust gas detected by a LAFS provided in an exhaust passage and a cylinder When the deviation from the calculated air-fuel ratio obtained by modeling the transport delay and diffusion delay 19 from the outlet of the exhaust port 6 to the LAFS 14 as a first-order lag is a predetermined value or more, the air-fuel ratio becomes the target air-fuel ratio. As described above, the air-fuel ratio control apparatus for an internal combustion engine that controls the amount of fuel supplied from the injector 3 to the cylinder 2 based on the deviation between the target air-fuel ratio and the calculated air-fuel ratio.
[Selection] Figure 1

Description

【０００１】
【発明の属する技術分野】
本発明は、主噴射後に、触媒のための追加の噴射を行う内燃機関の空燃比制御装置に関する。
【０００２】
【従来の技術】
一般的に、車両の内燃機関においては、所定の目標空燃比になるように、吸入空気量と内燃機関の回転数に応じた燃料量を設定して供給しており、更に、排気通路に設けられたリニアＡ／Ｆセンサにより排気ガス中の空燃比を検出し（以下、検出空燃比と呼ぶ。）、目標空燃比に対する検出空燃比の偏差をなくすようにフィードバック制御を行っている。
【０００３】
又、近年は、排気ガス中のＮＯｘ、ＳＯｘの排出量の低減のため、リーンＮＯｘ触媒等において、空燃比をリッチ側に制御し、排気ガスを還元雰囲気とすることで、ＮＯｘ放出を行い（所謂、ＮＯｘパージ）、又、燃料との反応により触媒の温度を昇温し、且つ、同様に排気ガスを還元雰囲気とすることにより、ＳＯｘの放出（所謂、硫黄被毒防止のためのＳパージ）を行う。又、ＤＰＦ（ディーゼル・パーティキュレート・フィルタ）でも、同様の空燃比制御により、ＤＰＦの温度を昇温して、排気ガス中の有害物質の清浄化を行っている。
【０００４】
上記空燃比の制御では、吸入空気量を絞ると共に、主噴射後に追加の副噴射を行って燃料を供給することで、空燃比をリッチ方向へ制御している。ところが、検出空燃比を求めるために用いるリニアＡ／Ｆセンサが、燃焼室より下流の排気通路に設けられているので、燃焼室での空燃比に対して、排気ガスの輸送、拡散遅れによる応答遅れが生じ、又、リニアＡ／Ｆセンサ単体の電気的な検出遅れによる応答遅れも生じるため、検出空燃比にも遅れを生じて、適切な空燃比制御が行えないおそれがあった。そのため、これらの応答遅れを補正するため、従来から様々な手段が用いられてきた。
【０００５】
例えば、燃料噴射弁からリニアＡ／Ｆセンサまでの排気ガスの輸送、拡散遅れを無駄時間要素とし、リニアＡ／Ｆセンサ単体の電気的な検出遅れを１次遅れ要素とし、これらを併せたモデルを応答遅れモデルとして、応答遅れを補正する技術が開示されている（特許文献１参照）。上記従来技術では、定常運転時には、上記応答遅れモデルを用いて目標空燃比を修正しており、又、目標空燃比を変更する時等の過渡時には、応答遅れモデルの精度が悪くなり、失火判定の精度が低下するため、失火判定を禁止するようにしている。
【０００６】
【特許文献１】
特開平７−２６９４０７号公報（第４−８頁、第１−４図）
【０００７】
【発明が解決しようとする課題】
内燃機関の定常運転時には、リニアＡ／Ｆセンサ等のセンサの検出値に基づく検出空燃比を用いて、空燃比のフィードバック制御を行っても、排気ガスの輸送、拡散遅れやセンサ単体の電気的な検出遅れによる影響は大きくない。
【０００８】
ところが、目標空燃比を変更する等の過渡運転時には、目標空燃比が変化した後の排気ガスが、その排気ガスの輸送、拡散遅れ時間経過後に、リニアＡ／Ｆセンサまで到達しているにもかかわらず、リニアＡ／Ｆセンサ単体の電気的遅れのため、検出空燃比を精度よく求めることが難しい。そこで、上記従来技術の応答遅れモデルでは、リニアＡ／Ｆセンサ単体の電気的な遅れをモデル化して検出空燃比を算出するようにしているが、この応答遅れモデルでは、リニアＡ／Ｆセンサの個体差、経時変化の影響を必然的に受け易い。
【０００９】
又、上記従来技術では、特に、目標空燃比をリーンからリッチ方向に変化させるときには、リニアＡ／Ｆセンサ単体の電気的な遅れを含む検出空燃比を用いて、供給燃料量等が求められて、空燃比が制御されており、実際の空燃比が目標空燃比に到達しているにもかかわらず、この遅れを含む検出空燃比が目標空燃比になっていないと判断され、更に、リッチになるように燃料が供給されて、燃費が悪化していた。
【００１０】
本発明は上記課題に鑑みなされたもので、目標空燃比が変更されたとき、空燃比の制御を適切に行い、燃費の悪化を防止する内燃機関の空燃比制御装置を提供することを目的とする。
【００１１】
【課題を解決するための手段】
上記課題を解決する本発明に係る内燃機関の空燃比制御装置は、
内燃機関の運転状態に応じて、目標空燃比を設定する目標空燃比設定手段と、内燃機関の排気通路に設けられ、排気ガス中の特定成分濃度を検出する特定成分濃度検出手段と、
特定成分濃度検出手段により検出された特定成分濃度から検出空燃比を求める検出空燃比算出手段と、
目標空燃比と検出空燃比の偏差に基づき、空燃比が目標空燃比になるように、内燃機関の燃焼室に供給する燃料供給量を制御する燃料供給量制御手段と、
燃焼室の出口から特定成分濃度検出手段までの排気ガスの輸送、拡散遅れを、一次遅れとしてモデル化して演算空燃比を求める演算空燃比算出手段とを有する。
目標空燃比が変更されたとき、検出空燃比と演算空燃比との偏差が所定値以上である場合、目標空燃比と演算空燃比の偏差に基づき、空燃比が目標空燃比になるように、燃焼室に供給する燃料供給量を制御する。
なお、目標空燃比が変更される場合としては、例えば、内燃機関の運転状態が、加速又は減速状態である場合や、触媒におけるＮＯｘ放出、ＳＯｘ放出を行う場合などがある。
又、特定成分濃度検出手段が検出する排気ガス中の特定成分としては、Ｏ_２等が対象となる。
又、空燃比は、燃料供給量によってのみ制御されるものではなく、吸入される空気量を変更することでも制御する。
【００１２】
上記課題を解決する本発明に係る内燃機関の空燃比制御装置は、
上記内燃機関の空燃比制御装置において、
燃料供給量制御手段が、目標空燃比と演算空燃比の偏差に基づいた燃料供給量の制御を、燃焼のために行う主噴射の後、つまり、追加の噴射となる副噴射時に行う。
【００１３】
【発明の実施の形態】
前述の従来技術において、検出空燃比の応答遅れとしては、排気ガスがリニアＡ／Ｆセンサ等のセンサに到達するまでの輸送、拡散遅れと、センサ単体の電気的な応答遅れの２つが考慮されていた。しかし、従来技術では、検出空燃比がリニアＡ／Ｆセンサ単体の遅れを含んだ値となっており、このようなセンサの検出値に基づいて検出空燃比を求めているため、実際の空燃比がリーンになる傾向があった。又、センサ単体の電気的な検出遅れを考慮してモデル化した場合でも、センサの個体差の影響を排除することができず、補正するのに限界があった。
【００１４】
一方、本発明に係る内燃機関の空燃比制御装置では、従来技術において無駄時間としてモデル化されている排気ガスの輸送、拡散遅れを、排気ガスの濃度の拡散遅れを考慮した輸送、拡散の１次遅れとしてモデル化することで、より正確な輸送、拡散遅れを推測して、精度のよい空燃比（以下、演算空燃比と呼ぶ。）を求めるようにした。又、この演算空燃比は、センサの検出値を直接用いていないため、センサの電気的な検出遅れを考慮する必要がなく、センサの電気的な遅れのモデルを用いる必要もなく、センサの個体差の影響を最小限にすることができた。そこで、上記特徴を有する本発明に係る内燃機関の空燃比制御装置の実施形態例を、以下に示す図を用いて説明する。
【００１５】
図１は、本発明に係る内燃機関の空燃比制御装置の実施形態の一例を示す構成図である。
図１では、内燃機関の一例として、ディーゼルエンジンの概略図を示し、わかりやすくするため、１つの気筒分の構成を図示した。
【００１６】
図１に示すように、本発明に係る内燃機関の空燃比制御装置において、内燃機関であるエンジン１は、燃料の燃焼室となる複数の気筒２と、各々の気筒２に設けられ、気筒２内に直接燃料を供給するインジェクタ３と、気筒２内に供給された燃料を燃焼させることで往復運動されるピストン４と、ピストン４に連結され、往復運動を回転運動に変換するコンロッド及び回転運動をするクランクシャフト（図示せず）と、気筒２の吸気ポート５に設けられ、吸入された空気（以下、吸気と略す。）を導入する吸気弁５ａと、気筒２の排気ポート６に設けられ、排気ガスを排出する排気弁６ａとを有している。なお、インジェクタ３は、燃料供給量制御手段の一部を構成する噴射式の燃料供給装置であり、燃料を加圧する蓄圧装置が接続されているものである。
【００１７】
気筒２の吸気側の吸気通路７には、上流側から、吸気を清浄にするエアクリーナ８と、吸気の過給を行うターボチャージャ９の吸気側タービン室と、過給された吸気を冷却するインタークーラ１０と、アクセル開度と連動せず、運転状態に応じて吸気通路７の開度を変更する吸気絞り弁１１と、後述するＥＧＲ（ＥｘｈａｕｓｔＧａｓＲｅｃｉｒｃｕｌａｔｉｏｎＳｙｓｔｅｍ、排出ガス再循環システム）通路１６の開度を変更するＥＧＲバルブ１２等が設けられている。なお、吸気通路７には、空燃比を算出するため、吸気量を計測する吸気量計測手段を有しているが、図１では省略している。
【００１８】
気筒２の排気側の排気通路１３には、上流側から、排気ガスの排気エネルギーを利用するターボチャージャ９の排気側タービン室と、排気ガス中のＯ_２濃度（特定成分濃度）を検出するリニアＡ／Ｆセンサ１４（特定成分濃度検出手段。以下、ＬＡＦＳと略す。）と、排気ガスを清浄にして排出する触媒１５が設けられている。つまり、ＬＡＦＳ１４は、気筒２の排気ポート６の出口から距離Ｌ離れ、触媒１５の上流側に位置している。なお、ターボチャージャ９及びインタークーラ１０は、必ずしも必要なものではない。
【００１９】
加えて、排気通路１３には、排気ポート６直後の下流側にて、吸気通路７へ分岐され、排気ガスを吸気通路７の吸気絞り弁１１の下流側へ循環させるＥＧＲ通路１６が設けられ、ＥＧＲ通路１４上には、吸気通路７側へ循環する排気ガスを冷却させるＥＧＲクーラ１７が設けられている。
【００２０】
触媒１５は、リーンＮＯｘ触媒と呼ばれるものを用いており、これは、空燃比リーン時（酸化過剰雰囲気下）にＮＯｘを吸蔵し、リッチ時に吸蔵されたＮＯｘを還元、浄化するものである。そのため、ＮＯｘ浄化時には、リッチにして還元雰囲気にしている。
【００２１】
ＥＣＵ（ＥｌｅｃｔｒｏｎｉｃＣｏｎｔｒｏｌＵｎｉｔ、電子制御装置）１８は、プログラムに従って演算を行い、各機器の制御を行うＣＰＵと、プログラムやデータ、演算結果を記録するＲＡＭ、ＲＯＭ等の記憶領域と、信号の入出力を行うインターフェース、タイマカウンタ等を有するものであり、ＥＣＵ１８が上記各機器を制御している。
【００２２】
具体的には、ＥＣＵ１８は、エンジン１の運転状態、例えば、加減速や触媒の状態に応じて、目標空燃比λｔを設定する目標空燃比設定手段と、ＬＡＦＳ１４により検出されたＯ_２濃度から検出空燃比λｓを求める検出空燃比算出手段とを有し、定常運転時には、目標空燃比λｔと検出空燃比λｓの偏差に基づき、空燃比が目標空燃比λｔになるように、気筒２に供給する燃料供給量を、インジェクタ３を用いて制御して、空燃比を制御している（燃料供給量制御手段）。
【００２３】
更に、本発明に係る空燃比制御装置では、ＥＣＵ１８が、気筒２の排気ポート６の出口からＬＡＦＳ１４までの排気ガスの輸送、拡散遅れ１９を、一次遅れとしてモデル化して演算空燃比λｍを求める演算空燃比算出手段を備え、運転状態の過渡時、例えば、目標空燃比λｔが変更されたとき、検出空燃比λｓと演算空燃比λｍとの偏差Δλが、所定値以上である場合、目標空燃比λｔと演算空燃比λｓの偏差に基づき、空燃比が目標空燃比λｔになるように、気筒２に供給する燃料供給量を、インジェクタ３を用いて制御している。
【００２４】
排気ガスの輸送、拡散の遅れ１９を、一次遅れとしてモデル化して演算空燃比λｍを求める上記演算空燃比算出手段では、従来のようなＬＡＦＳ１４単体の電気的な遅れのモデルを用いていないため、ＬＡＦＳ１４の個体差や経時変化の影響を最小限に抑えられる。更に、エンジン１では、排気ガスの流速を測定でき、ＬＡＦＳ１４の取付位置も、図面にて正確に管理できるため、より精度の高い演算空燃比λｍを求めることが可能となる。
【００２５】
上記構成のエンジン１において、インジェクタ３は、吸気、圧縮、排気、膨張の各行程が終了する間に、少なくとも２回の燃料噴射を行うことができるものである。又、空燃比をリッチ方向に制御する時には、吸気絞り１１やＥＧＲバルブ１２を制御して、吸気新気量を絞り、更に、インジェクタ３による追加の噴射（副噴射）を行うことにより燃料量を増やして、所定の空燃比を得るようにしている。特に、本発明の場合、上記空燃比制御のための燃料供給量の制御は、気筒２内にて燃焼させる燃料（主噴射燃料）の供給時に行うものではなく、その後の副噴射時に行い、触媒１５を適切にリッチ状態にするものである。
【００２６】
図２は、本発明に係る内燃機関の空燃比制御装置での空燃比制御の一例を示すフローチャートである。
【００２７】
（ステップＳ１）
本発明に係る空燃比制御において、目標空燃比λｔを変更させる場合、ＬＡＦＳ出力による検出空燃比λｓと、１次遅れモデルによる演算空燃比λｍを求める。
【００２８】
目標空燃比λｔが変化する場合としては、例えば、触媒に吸蔵されたＮＯｘ、ＳＯｘを放出するために、リーンからストイキオ（又はストイキオより若干リッチ）にステップ的に変化するように、ＥＣＵが目標空燃比λｔを設定する場合が考えられる。
【００２９】
実際には、まず、吸気絞り弁、ＥＧＲバルブを併用して吸気量を減少させることで、空燃比をリッチ側へ制御しているが、ここでは、それでも目標空燃比λｔに到達できない場合を想定して、副燃料噴射を行うことで、空燃比をリッチ側へ制御している。このとき、空燃比のフィードバック制御のために、検出空燃比λｓを用いる場合と、演算空燃比λｍを用いる場合がある。
【００３０】
ここで、排気ガスの輸送、拡散遅れが考慮された演算空燃比λｍを求める１次遅れモデルとして、拡散方程式を近似したものを用いる。その近似計算を以下に示す。
【００３１】
まず、一般的な２次元の拡散方程式を以下に示す。
【数１】

ここで、Ｒｅはレイノルズ数、Ｓｃはシュミット数である。
【００３２】
排気通路においては、その輸送、拡散の遅れは、その長さ方向が支配的になるため、１次元で考えても十分であり、上記式は以下のように簡略化できる。
【数２】

【００３３】
又、排気ポート出口からＬＡＦＳ取付位置までの距離をＬとすると（図１参照）、排気ガスの流速、密度、粘性係数より、レイノルズ数Ｒｅが大きく、１／Ｒｅ≒０と近似できるため、上記式は更に以下のように簡略化できる。
【数３】

【００３４】
ＣをＬＡＦＳの取付位置での所定成分（例えば、Ｏ_２、Ｎ_２、ＣＯ_２、Ｈ_２Ｏ等）の濃度、Ｃ_ｉｎを排気ポートでの所定成分の濃度として、上記式を変形すると、以下の式となる。
【数４】

これは、時定数Ｔ＝Ｌ／ｕの１次遅れ系を示す式となる。
【００３５】
更に、この式を差分化すると、以下の式となる。
【数５】

ここで、ｋ＝１／｛１＋（Ｔ／Δｔ）｝とする。
つまり、ＬＡＦＳの取付位置における所定時間前の所定成分濃度と、この位置から所定距離Ｌ上流の排気ポート出口における所定成分濃度から、輸送、拡散の遅れを考慮した現時点での所定成分濃度を導けることとなる。
【００３６】
リーンの場合、所定成分濃度としては、ＣＯ_２の濃度に注目すればよいので、例えば、軽油の完全燃焼を前提にした場合、演算空燃比λｍは以下の式から求められる。
【数６】

上記式（１）において、αはストイキオ燃焼時の燃料（軽油）１ｋｍｏｌに対する空気のモル数、Ｖ_０は標準状態（０℃、１ａｔｍ）での気体の占める体積を表し、「１６」は軽油の分子式をＣ_１６Ｈ_３０で近似した場合の炭素数、「４４」はＣＯ_２の分子量、「７．５」は燃焼方程式から上記式（１）を導く過程で得られる数を表す。
【００３７】
（ステップＳ２）
ステップ１で求められた検出空燃比λｓと、上記式（１）により求められた演算空燃比λｍから、偏差Δλ＝｜λｍ−λｓ｜を計算し、偏差Δλが所定値以上であれば、ステップＳ３へ進み、偏差Δλが所定値より小さければ、ステップＳ４へ進む。ここで、所定値としては、例えば、０．５位に設定する。
【００３８】
（ステップＳ３）
偏差Δλが所定値以上の場合、各物理量の演算、特に、目標空燃比λｔへのフィードバック制御へは、演算空燃比λｍを用いて行う。
【００３９】
（ステップＳ４）
偏差Δλが所定値より小さい場合、各物理量の演算、特に、目標空燃比λｔへのフィードバック制御へは、検出空燃比λｓを用いて行う。
【００４０】
上記空燃比制御は、リーンからリッチ方向へ目標空燃比λｔを変更する場合のみならず、例えば、ＮＯｘ又はＳＯｘの放出を中止するため、目標空燃比λｔをストイキオ（又はリッチ）からリーン方向へ変更する場合にも、繰り返し用いて行う。
【００４１】
図３は、本発明に係る空燃比制御の一例を示すタイムチャートである。
【００４２】
本発明に係る空燃比制御では、例えば、目標空燃比λｔがリーンからリッチ方向に変化した場合、輸送、拡散遅れ時間Ｔ_ｄ１の経過後、ＬＡＦＳ近傍の実際の空燃比は目標空燃比λｔになっていると判断でき、この早いタイミングの時点（図３中Ａ点）で、リッチにするための副燃料噴射量を減少させて、目標空燃比λｔを維持するための最低限の副燃料噴射量を供給するだけでよく、略ＬＡＦＳ単体の遅れ時間Ｔ_ｓ１分、余分な噴射量を追加する必要がなくなり、燃費に有利となる。なお、場合によっては、副燃料噴射自体を停止するようにしてもよい。
【００４３】
又、演算空燃比λｍと検出空燃比λｓとの偏差Δλが所定値以上の間は、演算空燃比λｍを用いて副燃料噴射量を求めるので（図３中のλｍフィードバック参照）、ＬＡＦＳ単体の応答遅れに影響を受けることがなく、単位時間当りの副燃料噴射量を適切に求めることができ、検出空燃比λｓを用いて副燃料噴射量を求めた場合（図３中のλｓフィードバック参照）に比較して、図３中のＢ領域部分に該当する無駄な噴射量を節約できることができ、燃費に有利となる。
【００４４】
ＬＡＦＳの応答遅れ［輸送、拡散遅れ（Ｔ_ｄ１）＋センサ単体遅れ（Ｔ_ｓ１）］後のＣ点においては、検出空燃比λｓがＬＡＦＳの応答遅れの影響を受けにくい状態であるため、このＣ点以降の目標空燃比λｔへの制御は、検出空燃比λｓを用いて目標空燃比λｔへの制御を行うようにしている。具体的には、演算空燃比λｍと検出空燃比λｓとの偏差Δλが所定値より小さい場合、検出空燃比λｓを用いて目標空燃比λｔへの制御を行う。
【００４５】
なお、本発明に係る空燃比制御では、ＮＯｘ、ＳＯｘ放出終了後に、リッチからリーンへ目標空燃比λｔを変更し、副燃料噴射を停止した際にも、演算空燃比λｍを用いてフィードバック制御を行うことで、ＬＡＦＳの応答遅れの影響を受けずに制御することができるため、適切な空燃比の制御を行うことができる。したがって、リッチからリーンへの空燃比切換時に、過剰なリーン制御に基づく失火を未然に回避して、トルクショックの発生を防止することが可能となる。
【００４６】
【発明の効果】
請求項１に係る発明によれば、目標空燃比の変更時に、センサ出力に基づいて求めた空燃比ではなく、排気ガスの輸送、拡散遅れを１次遅れとしてモデル化して求めた空燃比を用いて、目標空燃比への制御を行うので、空燃比を求めるセンサの出力遅れに左右されることがなく、空燃比、つまり噴射燃料量を適切に制御することができ、燃費の悪化を防止できる。
【００４７】
請求項２に係る発明によれば、副噴射時に適切な燃料量を供給することで、無駄な燃料量を供給することなく、触媒に適量の還元剤（燃料）を供給することができ、不必要にリッチにすることなく、ＮＯｘ、ＳＯｘの放出（浄化）を行うことができる。
【図面の簡単な説明】
【図１】本発明に係る内燃機関の空燃比制御装置の実施形態の一例を示す構成図である。
【図２】本発明に係る内燃機関の空燃比制御装置での空燃比制御の一例を示すフローチャートである。
【図３】本発明に係る内燃機関の空燃比制御装置での空燃比制御の一例を示すタイムチャートである。
【符号の説明】
１エンジン
２気筒
３インジェクタ
４ピストン
５吸気ポート
５ａ吸気弁
６排気ポート
６ａ排気弁
７吸気通路
８エアクリーナ
９ターボチャージャ
１０インタークーラ
１１吸気絞り弁
１２ＥＧＲバルブ
１３排気通路
１４ＬＡＦＳ
１５触媒
１６ＥＧＲ通路
１７ＥＧＲクーラ
１８ＥＣＵ
１９排気ガスの輸送、拡散遅れ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that performs additional injection for a catalyst after main injection.
[0002]
[Prior art]
In general, in an internal combustion engine of a vehicle, a fuel amount corresponding to the intake air amount and the rotational speed of the internal combustion engine is set and supplied so as to have a predetermined target air-fuel ratio, and further provided in an exhaust passage. The air-fuel ratio in the exhaust gas is detected by the linear A / F sensor (hereinafter referred to as the detected air-fuel ratio), and feedback control is performed so as to eliminate the deviation of the detected air-fuel ratio from the target air-fuel ratio.
[0003]
In recent years, in order to reduce the amount of NOx and SOx emissions in exhaust gas, NOx is released by controlling the air-fuel ratio to a rich side and making the exhaust gas a reducing atmosphere in a lean NOx catalyst or the like ( SOx release (so-called S purge for prevention of sulfur poisoning) by raising the temperature of the catalyst by reaction with fuel and similarly setting the exhaust gas to a reducing atmosphere )I do. Also in DPF (diesel particulate filter), the temperature of the DPF is raised by the same air-fuel ratio control to clean harmful substances in the exhaust gas.
[0004]
In the air-fuel ratio control, the air-fuel ratio is controlled in the rich direction by reducing the intake air amount and supplying fuel by performing additional sub-injection after main injection. However, since the linear A / F sensor used for obtaining the detected air-fuel ratio is provided in the exhaust passage downstream of the combustion chamber, the response due to the exhaust gas transport and diffusion delay with respect to the air-fuel ratio in the combustion chamber. Since a delay occurs and a response delay due to an electrical detection delay of the linear A / F sensor alone occurs, the detected air-fuel ratio is also delayed, and there is a possibility that appropriate air-fuel ratio control cannot be performed. For this reason, various means have been conventionally used to correct these response delays.
[0005]
For example, the exhaust gas transport from the fuel injection valve to the linear A / F sensor, the diffusion delay is a dead time element, and the electrical detection delay of the linear A / F sensor alone is a primary delay element. A technique for correcting the response delay using the response delay model is disclosed (see Patent Document 1). In the above prior art, the target air-fuel ratio is corrected using the response delay model during steady operation, and the accuracy of the response delay model deteriorates during transients such as when changing the target air-fuel ratio, and misfire detection is performed. Since the accuracy of the system is reduced, misfire determination is prohibited.
[0006]
[Patent Document 1]
JP-A-7-269407 (page 4-8, Fig. 1-4)
[0007]
[Problems to be solved by the invention]
During steady operation of an internal combustion engine, even if air-fuel ratio feedback control is performed using a detected air-fuel ratio based on a detected value of a sensor such as a linear A / F sensor, the exhaust gas transport, diffusion delay, and electrical The influence of the detection delay is not great.
[0008]
However, during transient operation such as changing the target air-fuel ratio, the exhaust gas after the target air-fuel ratio has changed has reached the linear A / F sensor after the exhaust gas transport and diffusion delay time has elapsed. Regardless, because of the electrical delay of the linear A / F sensor alone, it is difficult to accurately determine the detected air-fuel ratio. Therefore, in the response delay model of the above prior art, the detected air-fuel ratio is calculated by modeling the electrical delay of the linear A / F sensor alone, but in this response delay model, the linear A / F sensor Naturally affected by individual differences and changes over time.
[0009]
In the above-described prior art, particularly when the target air-fuel ratio is changed from lean to rich, the amount of fuel to be supplied is determined using the detected air-fuel ratio including the electrical delay of the linear A / F sensor alone. Even if the air-fuel ratio is controlled and the actual air-fuel ratio has reached the target air-fuel ratio, it is determined that the detected air-fuel ratio including this delay has not reached the target air-fuel ratio. The fuel was supplied so that the fuel consumption was getting worse.
[0010]
The present invention has been made in view of the above problems, and an object of the present invention is to provide an air-fuel ratio control apparatus for an internal combustion engine that appropriately controls the air-fuel ratio when the target air-fuel ratio is changed and prevents deterioration of fuel consumption. To do.
[0011]
[Means for Solving the Problems]
An air-fuel ratio control apparatus for an internal combustion engine according to the present invention that solves the above problems is as follows.
A target air-fuel ratio setting means for setting a target air-fuel ratio according to the operating state of the internal combustion engine, a specific component concentration detection means for detecting a specific component concentration in the exhaust gas provided in the exhaust passage of the internal combustion engine,
A detected air-fuel ratio calculating means for obtaining a detected air-fuel ratio from the specific component concentration detected by the specific component concentration detecting means;
Fuel supply amount control means for controlling the fuel supply amount supplied to the combustion chamber of the internal combustion engine so that the air-fuel ratio becomes the target air-fuel ratio based on the deviation between the target air-fuel ratio and the detected air-fuel ratio;
A calculation air / fuel ratio calculating means for modeling the transport / diffusion delay of the exhaust gas from the outlet of the combustion chamber to the specific component concentration detecting means as a first order delay to obtain the calculated air / fuel ratio;
When the target air-fuel ratio is changed, if the deviation between the detected air-fuel ratio and the calculated air-fuel ratio is a predetermined value or more, based on the deviation between the target air-fuel ratio and the calculated air-fuel ratio, the air-fuel ratio becomes the target air-fuel ratio. The amount of fuel supplied to the combustion chamber is controlled.
Note that the target air-fuel ratio may be changed, for example, when the operating state of the internal combustion engine is an acceleration or deceleration state, or when NOx release or SOx release in the catalyst is performed.
Further, the specific component in the exhaust gas detected by the specific component concentration detection means is O ₂ or the like.
Further, the air-fuel ratio is not controlled only by the fuel supply amount, but is also controlled by changing the amount of intake air.
[0012]
An air-fuel ratio control apparatus for an internal combustion engine according to the present invention that solves the above problems is as follows.
In the internal combustion engine air-fuel ratio control apparatus,
The fuel supply amount control means controls the fuel supply amount based on the deviation between the target air-fuel ratio and the calculated air-fuel ratio after the main injection that is performed for combustion, that is, at the time of sub-injection that is an additional injection.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the above-described prior art, the response delay of the detected air-fuel ratio takes into consideration two factors: the transport until the exhaust gas reaches the sensor such as the linear A / F sensor, the diffusion delay, and the electrical response delay of the sensor alone. It was. However, in the prior art, the detected air-fuel ratio is a value including a delay of the linear A / F sensor alone, and the detected air-fuel ratio is obtained based on the detected value of such a sensor. Tended to be lean. Even when modeling is performed in consideration of the electrical detection delay of a single sensor, the influence of individual differences among sensors cannot be eliminated, and there is a limit to correction.
[0014]
On the other hand, in the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the transport and diffusion delay of exhaust gas, which is modeled as dead time in the prior art, is one of transport and diffusion considering the diffusion delay of the exhaust gas concentration. By modeling as the next delay, more accurate transport and diffusion delays were estimated, and an accurate air-fuel ratio (hereinafter referred to as a calculated air-fuel ratio) was obtained. In addition, since the calculated air-fuel ratio does not directly use the detection value of the sensor, it is not necessary to consider the electrical detection delay of the sensor, it is not necessary to use a model of the electrical delay of the sensor, and the individual sensor The effect of the difference could be minimized. An embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention having the above characteristics will be described with reference to the drawings shown below.
[0015]
FIG. 1 is a configuration diagram showing an example of an embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
FIG. 1 shows a schematic diagram of a diesel engine as an example of an internal combustion engine, and a configuration for one cylinder is shown for easy understanding.
[0016]
As shown in FIG. 1, in the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, an engine 1 that is an internal combustion engine is provided in a plurality of cylinders 2 serving as combustion chambers for fuel and in each cylinder 2. An injector 3 that directly supplies fuel into the inside, a piston 4 that is reciprocated by burning the fuel supplied into the cylinder 2, a connecting rod that is connected to the piston 4 and converts the reciprocating motion into rotational motion, and rotational motion A crankshaft (not shown) that performs the above operation, an intake port 5 of the cylinder 2, an intake valve 5 a that introduces intake air (hereinafter abbreviated as intake), and an exhaust port 6 of the cylinder 2. And an exhaust valve 6a for exhausting exhaust gas. The injector 3 is an injection-type fuel supply device that forms part of the fuel supply amount control means, and is connected to a pressure accumulator that pressurizes the fuel.
[0017]
In the intake passage 7 on the intake side of the cylinder 2, an air cleaner 8 that cleans the intake air, an intake side turbine chamber of the turbocharger 9 that performs supercharging of the intake air, and an interface that cools the supercharged intake air. A cooler 10, an intake throttle valve 11 that changes the opening of the intake passage 7 according to the operating state without being linked to the accelerator opening, and an EGR (Exhaust Gas Recirculation System) passage 16 to be described later. An EGR valve 12 or the like for changing the opening is provided. The intake passage 7 has an intake air amount measuring means for measuring the intake air amount in order to calculate the air-fuel ratio, but is omitted in FIG.
[0018]
In the exhaust passage 13 on the exhaust side of the cylinder 2, from the upstream side, the exhaust side turbine chamber of the turbocharger 9 that uses the exhaust energy of the exhaust gas, and the linear that detects the O ₂ concentration (specific component concentration) in the exhaust gas. An A / F sensor 14 (specific component concentration detection means; hereinafter abbreviated as LAFS) and a catalyst 15 that cleans and exhausts exhaust gas are provided. That is, the LAFS 14 is located at a distance L from the outlet of the exhaust port 6 of the cylinder 2 and on the upstream side of the catalyst 15. The turbocharger 9 and the intercooler 10 are not necessarily required.
[0019]
In addition, the exhaust passage 13 is provided with an EGR passage 16 that branches to the intake passage 7 immediately downstream of the exhaust port 6 and circulates the exhaust gas downstream of the intake throttle valve 11 in the intake passage 7. An EGR cooler 17 that cools the exhaust gas that circulates toward the intake passage 7 is provided on the EGR passage 14.
[0020]
The catalyst 15 uses what is called a lean NOx catalyst, which stores NOx when the air-fuel ratio is lean (under an excessive oxidation atmosphere), and reduces and purifies the NOx stored when rich. Therefore, when NOx purification is performed, a reducing atmosphere is made rich.
[0021]
An ECU (Electronic Control Unit) 18 performs an operation according to a program and controls each device, a storage area such as a RAM and a ROM for storing programs, data, and operation results, and signal input / output The ECU 18 controls the above devices.
[0022]
Specifically, the ECU 18 detects from the O ₂ concentration detected by the LAFS 14 and the target air-fuel ratio setting means for setting the target air-fuel ratio λt according to the operating state of the engine 1, for example, the acceleration / deceleration and the catalyst state. A detecting air-fuel ratio calculating means for obtaining the air-fuel ratio λs, and supplying the cylinder 2 with the air-fuel ratio so that the air-fuel ratio becomes the target air-fuel ratio λt based on the difference between the target air-fuel ratio λt and the detected air-fuel ratio λs during steady operation. The fuel supply amount is controlled using the injector 3 to control the air-fuel ratio (fuel supply amount control means).
[0023]
Further, in the air-fuel ratio control device according to the present invention, the ECU 18 calculates the calculated air-fuel ratio λm by modeling the transport of the exhaust gas from the outlet of the exhaust port 6 of the cylinder 2 to the LAFS 14 and the diffusion delay 19 as a first order delay. Air-fuel ratio calculating means, and when the target air-fuel ratio λt is changed, for example, when the target air-fuel ratio λt is changed, if the deviation Δλ between the detected air-fuel ratio λs and the calculated air-fuel ratio λm is greater than or equal to a predetermined value, Based on the deviation between λt and the calculated air-fuel ratio λs, the fuel supply amount supplied to the cylinder 2 is controlled using the injector 3 so that the air-fuel ratio becomes the target air-fuel ratio λt.
[0024]
The calculated air-fuel ratio calculating means for modeling the exhaust gas transport / diffusion delay 19 as a first-order lag to obtain the calculated air-fuel ratio λm does not use the conventional LAFS14 single-phase electrical delay model. The influence of LAFS14 individual differences and changes over time can be minimized. Furthermore, in the engine 1, the exhaust gas flow rate can be measured, and the LAFS 14 mounting position can also be accurately managed in the drawing, so that it is possible to obtain the calculated air-fuel ratio λm with higher accuracy.
[0025]
In the engine 1 having the above-described configuration, the injector 3 can perform fuel injection at least twice during the intake, compression, exhaust, and expansion strokes. Further, when the air-fuel ratio is controlled in the rich direction, the intake throttle 11 and the EGR valve 12 are controlled to reduce the amount of fresh intake air, and further, the fuel amount is reduced by performing additional injection (sub-injection) by the injector 3. It is increased so as to obtain a predetermined air-fuel ratio. In particular, in the case of the present invention, the control of the fuel supply amount for the air-fuel ratio control is not performed at the time of supplying fuel to be combusted in the cylinder 2 (main injection fuel), but at the subsequent sub-injection, 15 is appropriately made rich.
[0026]
FIG. 2 is a flowchart showing an example of air-fuel ratio control in the air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
[0027]
(Step S1)
In the air-fuel ratio control according to the present invention, when the target air-fuel ratio λt is changed, the detected air-fuel ratio λs based on the LAFS output and the calculated air-fuel ratio λm based on the first-order lag model are obtained.
[0028]
When the target air-fuel ratio λt changes, for example, in order to release NOx and SOx occluded in the catalyst, the ECU may cause the target air-fuel ratio to change stepwise from lean to stoichiometric (or slightly richer than stoichiometric). The case where the fuel ratio λt is set can be considered.
[0029]
Actually, the air-fuel ratio is first controlled to the rich side by reducing the intake amount by using both the intake throttle valve and the EGR valve. Here, however, it is assumed that the target air-fuel ratio λt still cannot be reached. Then, by performing the auxiliary fuel injection, the air-fuel ratio is controlled to the rich side. At this time, the detected air-fuel ratio λs or the calculated air-fuel ratio λm may be used for air-fuel ratio feedback control.
[0030]
Here, an approximation of the diffusion equation is used as a first-order lag model for obtaining the calculated air-fuel ratio λm in consideration of the transport and diffusion delay of exhaust gas. The approximate calculation is shown below.
[0031]
First, a general two-dimensional diffusion equation is shown below.
[Expression 1]

Here, Re is the Reynolds number and Sc is the Schmitt number.
[0032]
In the exhaust passage, the transport and diffusion delays are dominant in the length direction, so it is sufficient to consider them in one dimension, and the above equation can be simplified as follows.
[Expression 2]

[0033]
If the distance from the exhaust port outlet to the LAFS mounting position is L (see FIG. 1), the Reynolds number Re is larger than the flow velocity, density, and viscosity coefficient of the exhaust gas, and can be approximated as 1 / Re≈0. The equation can be further simplified as follows:
[Equation 3]

[0034]
When the above equation is modified with C being the concentration of a predetermined component (for example, O ₂ , N ₂ , CO ₂ , H ₂ O, etc.) at the LAFS mounting position and C _in being the concentration of the predetermined component at the exhaust port, It becomes the following formula.
[Expression 4]

This is an expression showing a first-order lag system with a time constant T = L / u.
[0035]
Furthermore, when this equation is differentiated, the following equation is obtained.
[Equation 5]

Here, k = 1 / {1+ (T / Δt)}.
That is, the current predetermined component concentration in consideration of the delay in transportation and diffusion can be derived from the predetermined component concentration before the predetermined time at the LAFS mounting position and the predetermined component concentration at the exhaust port outlet at a predetermined distance L upstream from this position. It becomes.
[0036]
In the case of lean, attention should be paid to the concentration of CO ₂ as the predetermined component concentration. For example, when assuming complete combustion of light oil, the calculated air-fuel ratio λm can be obtained from the following equation.
[Formula 6]

In the above formula (1), α represents the number of moles of air with respect to 1 kmol of fuel (light oil) during stoichiometric combustion, V ₀ represents the volume of gas in the standard state (0 ° C., 1 atm), and “16” represents the light oil The number of carbons when the molecular formula is approximated by C ₁₆ H ₃₀ , “44” is the molecular weight of CO ₂ , and “7.5” is the number obtained in the process of deriving the above formula (1) from the combustion equation.
[0037]
(Step S2)
The deviation Δλ = | λm−λs | is calculated from the detected air-fuel ratio λs obtained in step 1 and the calculated air-fuel ratio λm obtained by the above equation (1). The process proceeds to S3, and if the deviation Δλ is smaller than the predetermined value, the process proceeds to Step S4. Here, the predetermined value is set, for example, to the 0.5th place.
[0038]
(Step S3)
When the deviation Δλ is equal to or greater than a predetermined value, the calculation of each physical quantity, particularly the feedback control to the target air-fuel ratio λt, is performed using the calculated air-fuel ratio λm.
[0039]
(Step S4)
When the deviation Δλ is smaller than a predetermined value, the calculation of each physical quantity, particularly the feedback control to the target air-fuel ratio λt, is performed using the detected air-fuel ratio λs.
[0040]
The air-fuel ratio control is not only performed when the target air-fuel ratio λt is changed from lean to rich, but for example, the target air-fuel ratio λt is changed from stoichiometric (or rich) to lean in order to stop releasing NOx or SOx. Also when used, it is repeated.
[0041]
FIG. 3 is a time chart showing an example of the air-fuel ratio control according to the present invention.
[0042]
In the air-fuel ratio control according to the present invention, for example, when the target air-fuel ratio λt changes from lean to rich, the actual air-fuel ratio in the vicinity of LAFS becomes the target air-fuel ratio λt after the transport and diffusion delay time T _d1 has elapsed. At the time of this early timing (point A in FIG. 3), the sub fuel injection amount for reducing the sub fuel injection amount for enrichment and maintaining the target air-fuel ratio λt is reduced. It is not necessary to add an extra injection amount for the delay time T _s1 of approximately LAFS alone, which is advantageous for fuel consumption. In some cases, the auxiliary fuel injection itself may be stopped.
[0043]
Further, while the difference Δλ between the calculated air-fuel ratio λm and the detected air-fuel ratio λs is equal to or greater than a predetermined value, the sub-fuel injection amount is obtained using the calculated air-fuel ratio λm (see λm feedback in FIG. 3). When the sub fuel injection amount per unit time can be obtained appropriately without being affected by the response delay, and the sub fuel injection amount is obtained using the detected air-fuel ratio λs (see λs feedback in FIG. 3) Compared to the above, it is possible to save a useless injection amount corresponding to the region B in FIG. 3, which is advantageous for fuel consumption.
[0044]
At the point C after LAFS response delay [transportation, diffusion delay (T _d1 ) + sensor unit delay (T _s1 )], the detected air-fuel ratio λs is not easily affected by the LAFS response delay. Control to the target air-fuel ratio λt after the point is performed using the detected air-fuel ratio λs. Specifically, when the deviation Δλ between the calculated air-fuel ratio λm and the detected air-fuel ratio λs is smaller than a predetermined value, control to the target air-fuel ratio λt is performed using the detected air-fuel ratio λs.
[0045]
In the air-fuel ratio control according to the present invention, the feedback control is performed using the calculated air-fuel ratio λm even when the target air-fuel ratio λt is changed from rich to lean after the end of NOx and SOx release and the sub fuel injection is stopped. By performing the control, the control can be performed without being influenced by the response delay of the LAFS, so that the appropriate air-fuel ratio can be controlled. Therefore, when the air-fuel ratio is switched from rich to lean, misfire based on excessive lean control can be avoided and torque shock can be prevented from occurring.
[0046]
【The invention's effect】
According to the first aspect of the present invention, when the target air-fuel ratio is changed, the air-fuel ratio obtained by modeling the exhaust gas transport and diffusion delay as a primary delay is used instead of the air-fuel ratio obtained based on the sensor output. Thus, the control to the target air-fuel ratio is performed, so that the air-fuel ratio, that is, the amount of injected fuel can be appropriately controlled without being influenced by the output delay of the sensor for obtaining the air-fuel ratio, and the deterioration of fuel consumption can be prevented. .
[0047]
According to the invention of claim 2, by supplying an appropriate amount of fuel at the time of sub-injection, an appropriate amount of reducing agent (fuel) can be supplied to the catalyst without supplying an unnecessary amount of fuel. NOx and SOx can be released (purified) without being made rich as necessary.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an example of an embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
FIG. 2 is a flowchart showing an example of air-fuel ratio control in the air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
FIG. 3 is a time chart showing an example of air-fuel ratio control in the air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Engine 2 Cylinder 3 Injector 4 Piston 5 Intake port 5a Intake valve 6 Exhaust port 6a Exhaust valve 7 Intake passage 8 Air cleaner 9 Turbocharger 10 Intercooler 11 Intake throttle valve 12 EGR valve 13 Exhaust passage 14 LAFS
15 Catalyst 16 EGR passage 17 EGR cooler 18 ECU
19 Exhaust gas transport, diffusion delay

Claims

Target air-fuel ratio setting means for setting the target air-fuel ratio according to the operating state of the internal combustion engine;
A specific component concentration detecting means provided in an exhaust passage of the internal combustion engine for detecting a specific component concentration in the exhaust gas;
A detected air-fuel ratio calculating means for obtaining a detected air-fuel ratio from the specific component concentration detected by the specific component concentration detecting means;
Fuel supply amount control means for controlling a fuel supply amount supplied to the combustion chamber of the internal combustion engine based on a deviation between the target air fuel ratio and the detected air fuel ratio so that the air fuel ratio becomes the target air fuel ratio. In an air-fuel ratio control apparatus for an internal combustion engine,
A calculation air / fuel ratio calculating means for modeling a transport delay of the exhaust gas from the outlet of the combustion chamber to the specific component concentration detection means and a diffusion delay as a first order delay to obtain a calculated air / fuel ratio;
When the target air-fuel ratio is changed, if the deviation between the detected air-fuel ratio and the calculated air-fuel ratio is a predetermined value or more, the fuel supply amount control means is configured so that the air-fuel ratio becomes the target air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine, wherein the fuel supply amount supplied to the combustion chamber is controlled based on a deviation between the target air-fuel ratio and the calculated air-fuel ratio.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The fuel supply amount control means controls the fuel supply amount based on a deviation between the target air-fuel ratio and the calculated air-fuel ratio after the main injection performed for combustion. apparatus.