JP4155662B2 - Three-way catalyst oxygen storage control device - Google Patents

Three-way catalyst oxygen storage control device Download PDF

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JP4155662B2
JP4155662B2 JP09087999A JP9087999A JP4155662B2 JP 4155662 B2 JP4155662 B2 JP 4155662B2 JP 09087999 A JP09087999 A JP 09087999A JP 9087999 A JP9087999 A JP 9087999A JP 4155662 B2 JP4155662 B2 JP 4155662B2
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fuel ratio
storage amount
exhaust air
way catalyst
oxygen storage
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JP2000282925A (en
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浩二 高橋
重男 大隈
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Hitachi Ltd
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Description

【０００１】
【発明の属する技術分野】
本発明は三元触媒の酸素ストレージ量制御装置に関し、詳しくは、エンジンの排気通路に介装されて排気中のＣＯ，ＨＣの酸化及びＮＯｘの還元を行う三元触媒において、酸素ストレージ量を最適に制御する技術に関する。
【０００２】
【従来の技術】
従来から、エンジンの排気通路に三元触媒を介装する一方、該三元触媒における酸化と還元とのバランスをとって高い転化率を実現するために、燃焼混合気の空燃比を理論空燃比に保つ空燃比フィードバック制御を行うエンジンの排気浄化システムが知られている。
【０００３】
【発明が解決しようとする課題】
ところで、三元触媒は、酸素を貯蔵する酸素ストレージ能力を有するため、触媒中の酸素ストレージ量が転化率に大きく影響することになり、例えば酸素ストレージ量が最適量よりも多いと、ＣＯ，ＨＣの酸化反応が促進される一方、ＮＯｘの還元反応が鈍り、逆に酸素ストレージ量が最適値よりも少ないと、ＮＯｘの還元反応が促進される一方、ＣＯ，ＨＣの酸化反応が鈍ることになる。
【０００４】
ところが、従来のように、触媒入口での排気空燃比を理論空燃比にフィードバック制御する場合には、そのときの酸素ストレージ量とは無関係に、触媒に導入される排気中の酸素濃度が一定になるように制御するから、酸素ストレージ量を最適値に制御することにはならず、結果的に、三元触媒における転化率を安定的に高いレベルに維持させることができないという問題があった。
【０００５】
この点に鑑み、三元触媒の上流側に排気空燃比をリニアに検出する広域型の酸素濃度センサ、三元触媒の下流側に排気空燃比の理論空燃比に対するリッチ、リーンを検出するストイキ型の酸素濃度センサを設け、上流側の広域型酸素濃度センサの出力に基づいて三元触媒の酸素ストレージ量を推定しつつ下流側のストイキ型酸素濃度センサの出力がリッチを示した場合は酸素ストレージ量を０とし、リーンを示した場合は酸素ストレージ量を最大とする補正を行い、該酸素ストレージ量の推定値に基づいて、酸素ストレージ量を最適値にするように空燃比を制御するようにしたものがある（特開平６−２４９０３２号公報参照）。
【０００６】
しかしながら、上記のものでは、下流側のストイキ型酸素濃度センサがストイキ近傍の出力を出しているときには、補正が行われないため触媒に吸着している酸素ストレージ量の推定値と実際の値とに差を生じてしまい、良好な制御を行えなかった。
【０００７】
触媒の下流側にも広域型の酸素濃度センサを使用すれば、酸素ストレージ量を高精度に推定できるが、コストが高くついてしまう。
本発明は上記問題点に鑑みなされたものであり、触媒上流側に広域型酸素濃度センサ、下流側にストイキ型酸素濃度センサを使用しつつ、酸素ストレージ量を、高精度に推定して、酸化反応と還元反応とをバランスさせる最適量に制御できるようにし、以ってＣＯ，ＨＣ，ＮＯｘを安定的に高い転化率で浄化できるようにすることを目的とする。
【０００８】
【課題を解決するための手段】
そのため請求項１記載の発明は、図１に示すように、
エンジンの排気通路に介装される三元触媒の上流側に設置され、排気空燃比の変化に対して出力値が連続的に変化する特性を有した広域型酸素濃度センサと、
三元触媒の下流側に設置され、排気空燃比の理論空燃比近傍で出力値が急変する特性を有したストイキ型酸素濃度センサと、
前記広域型酸素濃度センサの出力値に対して排気空燃比検出値を割付した第１変換テーブルと、前記ストイキ型酸素濃度センサの出力値に対して、排気空燃比検出値を、理論空燃比近傍では他の空燃比領域より高密度に割付した第２変換テーブルと、を有する変換手段と、
前記広域型酸素濃度センサ及びストイキ型酸素濃度センサの出力値を、それぞれ前記変換手段で変換して求められた三元触媒上流側の排気空燃比と、三元触媒下流側の排気空燃比と、に基づいて前記三元触媒における酸素ストレージ量を推定する酸素ストレージ量推定手段と、
エンジンの負荷及び回転速度に基づいて、ＣＯ，ＨＣの排出量が多くなる運転領域では酸素ストレージ量の目標値を大きい値に設定し、ＮＯｘの排出量が多くなる運転領域では酸素ストレージ量の目標値を小さい値に設定する目標ストレージ量設定手段と、
該目標ストレージ量設定手段で設定された目標値と前記酸素ストレージ量推定手段で推定された酸素ストレージ量との差が少なくなるように、前記三元触媒入口の排気空燃比をフィードバック制御する排気空燃比フィードバック手段と、
を含んで構成した。
【０００９】
かかる構成によると、酸素ストレージ量推定手段が、広域型酸素濃度センサ及びストイキ型酸素濃度センサの出力値を、前記変換手段の第１変換テーブル及び第２変換テーブルよりそれぞれ変換して求めた三元触媒上流側及び下流側の排気空燃比（酸素濃度）に基づいて、前記三元触媒における酸素ストレージ量を推定する。ここで、ストイキ型酸素濃度センサの出力値は、排気空燃比の理論空燃比近傍で急変する特性を有するが、前記第２変換テーブルには、該出力値に対して理論空燃比近傍の排気空燃比検出値を高密度に割付してあるので、排気空燃比を高精度に検出でき、したがって、酸素ストレージ量を高精度に推定できる。
【００１０】
一方、排気空燃比をリーン化して供給酸素量を増大させれば、三元触媒内で酸化反応に必要な酸素量よりも供給される酸素量が多くなって、酸素ストレージ量が増大し、また、排気空燃比をリッチ化すれば供給酸素量が減ることで三元触媒内に貯蔵されていた酸素が酸化反応に用いられて酸素ストレージ量は減少することになる。
【００１１】
そこで、推定された酸素ストレージ量に基づいて入口空燃比制御手段が、触媒入口の排気空燃比、即ち、燃焼混合気の空燃比（より具体的には、エンジンへの燃料噴射量）を制御し、実際の酸素ストレージ量が最適値になるように調整する。
【００１３】
また、三元触媒における酸素ストレージ量が、エンジンの負荷及び回転速度に基づいて、運転状態毎の目標値になるように、触媒入口の排気空燃比が制御される。具体的には、ＮＯｘの排出量が多くなる運転状態では、酸素ストレージ量の目標値を比較的小さくすることで、ＮＯｘを効果的に浄化でき、また、ＣＯ，ＨＣの排出量が多くなる運転状態では、酸素ストレージ量の目標値を比較的大きくすることで、ＣＯ，ＨＣを効果的に浄化できる。
【００１４】
請求項２記載の発明では、前記排気空燃比フィードバック手段が、前記目標ストレージ量設定手段で設定された目標値と前記酸素ストレージ量推定手段で推定された酸素ストレージ量との差を前記三元触媒入口の目標排気空燃比に変換し、該目標排気空燃比と前記三元触媒の上流側の酸素濃度センサで検出される実際の排気空燃比との差に基づいて、前記三元触媒入口の排気空燃比をフィードバック制御する構成とした。
【００１５】
かかる構成によると、酸素ストレージ量の目標値と推定値との差から、目標の酸素ストレージ量を得るための目標排気空燃比を設定し、実際の排気空燃比が目標排気空燃比になるように制御することで、酸素ストレージ量が目標値に一致するように制御される。
【００１６】
【発明の効果】
請求項１記載の発明によると、三元触媒下流側の酸素濃度センサは比較的安価なストイキ型酸素濃度センサを使用して、コストの増加を抑制しつつ、三元触媒における酸素ストレージ量を高精度に推定しながら最適な値に制御でき、以て、ＣＯ，ＨＣ，ＮＯｘを安定的に高い転化率で浄化できるという効果がある。
【００１７】
また、特に、運転状態によるＣＯ，ＨＣ，ＮＯｘ排出量の違いに対応して、三元触媒における酸素ストレージ量を変化させることができ、以て、ＣＯ，ＨＣ，ＮＯｘを効果的に浄化することが可能になるという効果がある。
【００１８】
請求項２記載の発明によると、酸素ストレージ量を目標に一致させるための目標排気空燃比を設定し、実際の排気空燃比を前記目標排気空燃比に一致させるようにフィードバック制御を行うことで、酸素ストレージ量を目標に精度に良く制御できるという効果がある。
【００１９】
【発明の実施の形態】
以下に本発明の実施の形態を説明する。
図２は、実施形態におけるエンジンのシステム構成を示す図である。
【００２０】
この図２において、エンジン１の吸気通路には燃料噴射弁２が設けられており、該燃料噴射弁２から噴射される燃料と空気とが予混合し、シリンダ内に吸気弁３を介して吸引される。シリンダ内の燃焼混合気は、点火栓４による火花点火によって着火燃焼し、燃焼排気は、排気弁５を介して排気通路６に排出される。
【００２１】
排気通路６には、三元触媒７が介装されており、該三元触媒７で排気中のＣＯ，ＨＣ，ＮＯｘが浄化される。
前記三元触媒７の上流側には、排気空燃比の変化に対して出力値が連続的に変化する特性を有した広域型酸素濃度センサ８が介装され、三元触媒７の下流側には、排気空燃比の理論空燃比近傍で出力値が急変する特性を有したストイキ型酸素濃度センサ９が介装されている。
【００２２】
前記広域型酸素濃度センサ８及びストイキ型酸素濃度センサ９の検出信号はコントロールユニット10に入力され、図中に制御ブロック図として示した流れに沿って処理されて、最終的には、燃料噴射弁２による燃料噴射量が制御されるようになっている。
【００２３】
以下、図２に示す制御ブロック図に沿って、処理内容を概説する。
センサ出力変換処理部（変換手段）100では、三元触媒７上流側の広域型酸素濃度センサ８からのＡ／Ｄ変換後の出力を、第１変換テーブルによって排気空燃比の検出値に変換すると共に、三元触媒７下流側のストイキ型酸素濃度センサ９からのＡ／Ｄ変換後の出力を、第２変換テーブルによって排気空燃比の検出値に変換する。ここで、前記第１変換テーブル及び第２変換テーブルは、図３に示すような変換特性を有する。そして、第１変換テーブルは、Ａ／Ｄ変換出力値の変化に対して排気空燃比の検出値が略均等に割付してあるのに対し、第２変換テーブルは、理論空燃比近傍のＡ／Ｄ変換出力値に対して、排気空燃比の検出値が細かく高密度に割付してあり、これにより、理論空燃比近傍の排気空燃比が高分解能で演算される。
【００２４】
酸素ストレージ量推定部101 （酸素ストレージ量推定手段）では、前記センサ出力変換処理部100により変換処理された三元触媒７上流側の及び下流側の排気空燃比（酸素濃度）に基づいて三元触媒７における酸素ストレージ量を推定する。
【００２５】
一方、目標酸素ストレージ量設定部102 （目標ストレージ量設定手段）では、エンジン負荷Ｔｐとエンジン回転速度Ｎｅとに基づいて目標酸素ストレージ量を設定する。尚、ＣＯ，ＨＣが出やすい運転領域では前記目標酸素ストレージ量を比較的大きくして、ＣＯ，ＨＣの酸化反応が促進されるようにし、逆に、ＮＯｘが出やすい領域では前記目標酸素ストレージ量を比較的小さくして、ＮＯｘの還元反応が促進されるようにすると良い。
【００２６】
酸素ストレージ量比較部103 では、前記酸素ストレージ濃度推定部101 で推定された実際の酸素ストレージ量と、前記目標酸素ストレージ量設定部102 で設定された目標酸素ストレージ量との差を演算する。
【００２７】
そして、目標空燃比設定部104 では、前記酸素ストレージ量比較部103 で算出された酸素ストレージ量の実際値と目標値との差を、目標空燃比（目標Ａ／Ｆ）に変換する。尚、前記目標空燃比は、三元触媒７の上流側の酸素濃度センサ８で検出される触媒入口の排気空燃比の目標値である。
【００２８】
空燃比比較部105 では、前記目標空燃比設定部104 で設定された目標空燃比と、前記酸素濃度センサ８からの検出信号に基づいてセンサ出力変換処理部100で検出された三元触媒７入口での実際の空燃比とを比較し、空燃比偏差算出部106 では、前記目標空燃比と実際の空燃比との差を算出する。
【００２９】
そして、噴射量補正部107 （排気空燃比フィードバック手段）では、前記空燃比の差が少なくするように、換言すれば、実際の空燃比を前記目標空燃比に近づけるように、燃料噴射弁２による燃料噴射量を補正するための補正値を決定する。
【００３０】
尚、上記目標酸素ストレージ量設定部102 ，酸素ストレージ量比較部103 ，目標空燃比設定部104 ，空燃比比較部105 ，空燃比偏差算出部106 及び噴射量補正部107 によって入口空燃比制御手段が構成される。
【００３１】
噴射量演算部108 では、エンジン１の吸入空気量Ｑ，回転速度Ｎｅに基づいて基本燃料噴射量を演算すると共に、エンジン１の冷却水温度Ｔｗ等に応じた補正値、及び、前記噴射量補正部107 で設定された補正値によって前記基本燃料噴射量を補正して、最終的な燃料噴射量を算出する。そして、前記燃料噴射量に相当するパルス幅の噴射パルス信号を、所定の噴射タイミングにおいて燃料噴射弁２に出力する。
【００３２】
ここで、図２の制御ブロック図に示した制御内容を、図３のフローチャートに従って詳述する。
図３のフローチャートにおいて、まず、Ｓ１では、三元触媒７上流側の酸素濃度センサ８からの検出信号（Ａ／Ｄ変換値）を、前記第１変換テーブルにより変換して三元触媒７入口での排気空燃比（酸素濃度）を検出する。
【００３３】
Ｓ２では、前記Ｓ１で検出した排気空燃比を平均化処理し、Ｓ３では、前記平均化処理された排気空燃比を、入口空燃比ＡＦＩＮとする。
Ｓ４では、三元触媒７下流側の酸素濃度センサ９からの検出信号（Ａ／Ｄ変換値）を、前記第１変換テーブルにより変換して三元触媒７出口での排気空燃比（酸素濃度）を検出する。
【００３４】
Ｓ５では、前記Ｓ４で検出した排気空燃比を平均化処理し、Ｓ６では、前記平均化処理された排気空燃比を、出口空燃比ＡＦＯＵＴとする。
Ｓ７では、前記入口空燃比ＡＦＩＮと出口空燃比ＡＦＯＵＴとの差を変数とする関数ｆ1 （ＡＦＩＮ−ＡＦＯＵＴ）に基づいて三元触媒７における酸素ストレージ量の推定値ＳＯ2 を算出する。前記関数ｆ1 （ＡＦＩＮ−ＡＦＯＵＴ）は、入口空燃比ＡＦＩＮと出口空燃比ＡＦＯＵＴとの差、即ち、供給酸素量と反応後の残存酸素量との差から、三元触媒における酸素ストレージ量を推定するモデル式である。
【００３５】
Ｓ８では、エンジン負荷Ｔｐとエンジン回転速度Ｎｅとに応じて予め目標酸素ストレージ量ＴＳＯ2 を記憶したマップを参照し、そのときのエンジン負荷Ｔｐとエンジン回転速度Ｎｅに対応する目標酸素ストレージ量ＴＳＯ2 を検索する。
【００３６】
Ｓ９では、前記目標酸素ストレージ量ＴＳＯ2 と推定値ＳＯ2 との差ΔＳＯ2 を算出する。
Ｓ10では、前記酸素ストレージ量の差ΔＳＯ2 を変数とする関数ｆ2 （ΔＳＯ2 ）に基づいて三元触媒７の入口での目標排気空燃比ＴＡＦを算出する。前記関数ｆ2 （ΔＳＯ2 ）は、前記酸素ストレージ量の差ΔＳＯ2 を０ならしめるための、差ΔＳＯ2 から目標排気空燃比ＴＡＦへの変換モデル式である。
【００３７】
Ｓ11では、前記目標排気空燃比ＴＡＦと、実際の触媒入口空燃比である前記入口空燃比ＡＦＩＮとの差ΔＡＦを演算する。
Ｓ12では、前記ΔＡＦに基づいて実際の入口空燃比ＡＦＩＮが、目標排気空燃比ＴＡＦよりもリーンであるかリッチであるかを判別する。
【００３８】
そして、実際の入口空燃比ＡＦＩＮが目標排気空燃比ＴＡＦよりもリーンであるときには、Ｓ13へ進んで燃料噴射量をより増量して入口空燃比をよりリッチに修正すべく補正値をリッチ方向へ積分制御する。
【００３９】
一方、実際の入口空燃比ＡＦＩＮが目標排気空燃比ＴＡＦよりもリッチであるときには、Ｓ14へ進んで燃料噴射量をより減量して入口空燃比をよりリーンに修正すべく補正値をリーン方向へ積分制御する。
【００４０】
また、実際の入口空燃比ＡＦＩＮが略目標排気空燃比ＴＡＦに一致しているときには、Ｓ15へ進んで、それまでの補正値を保持させるようにする。
実際の入口空燃比ＡＦＩＮをリッチ化すれば、三元触媒７に対する供給酸素量が減少して、酸素ストレージ量を減少変化させることができ、また、実際の入口空燃比ＡＦＩＮをリーン化すれば、三元触媒７に対する供給酸素量が増大して、酸素ストレージ量を増大変化させることができる。
【００４１】
従って、前記目標排気空燃比ＴＡＦの設定においては、推定値ＳＯ2 が目標酸素ストレージ量ＴＳＯ2 よりも少ないときには、その差が大きいときほど目標排気空燃比ＴＡＦとしてリーンな空燃比を設定すれば、実際の酸素ストレージ量を増大変化させて目標に近づけることができ、また、推定値ＳＯ2 が目標酸素ストレージ量ＴＳＯ2 よりも多いときには、その差が大きいときほど目標排気空燃比ＴＡＦとしてリッチな空燃比を設定すれば、実際の酸素ストレージ量を減少変化させて目標に近づけることができる。
【００４２】
そして、三元触媒７における酸素ストレージ量を目標値（最適値）に維持することができれば、三元触媒７におけるＣＯ，ＨＣの酸化反応及びＮＯｘの還元反応をバランス良く実行させて、最大限の浄化性能を得ることができる。
【図面の簡単な説明】
【図１】請求項１，２記載の発明に係る装置の基本構成を示すブロック図。
【図２】実施の形態におけるエンジンのシスムテ構成及び制御内容を示すブロック図。
【図３】実施の形態に使用する第１変換テーブル及び第２変換テ−ブルを示す図。
【図４】実施の形態における酸素ストレージ量制御の様子を示すフローチャート。
【符号の説明】
１ エンジン
２ 燃料噴射弁
３ 吸気弁
４ 点火栓
５ 排気弁
６ 排気通路
７ 三元触媒
８ 広域型酸素濃度センサ
９ ストイキ型酸素濃度センサ
10 コントロールユニット
101 酸素ストレージ量推定部
102 目標酸素ストレージ量設定部
103 酸素ストレージ量比較部
104 目標空燃比設定部
105 空燃比比較部
106 空燃比偏差算出部
107 噴射量補正部
108 噴射量演算部
109入口空燃比検出部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oxygen storage amount control device for a three-way catalyst, and more particularly, to optimize the oxygen storage amount in a three-way catalyst that is interposed in an engine exhaust passage and oxidizes CO, HC and NOx in exhaust gas. It relates to the technology to control.
[0002]
[Prior art]
Conventionally, a three-way catalyst is interposed in the exhaust passage of the engine, and in order to achieve a high conversion by balancing oxidation and reduction in the three-way catalyst, the air-fuel ratio of the combustion mixture is set to the stoichiometric air-fuel ratio. 2. Description of the Related Art An engine exhaust purification system that performs air-fuel ratio feedback control to maintain a constant is known.
[0003]
[Problems to be solved by the invention]
By the way, since the three-way catalyst has an oxygen storage capacity for storing oxygen, the oxygen storage amount in the catalyst greatly affects the conversion rate. For example, if the oxygen storage amount is larger than the optimum amount, CO, HC While the oxidation reaction of NOx is promoted, the reduction reaction of NOx becomes dull. Conversely, if the amount of oxygen storage is less than the optimum value, the reduction reaction of NOx is promoted, while the oxidation reaction of CO and HC becomes dull. .
[0004]
However, when the exhaust air-fuel ratio at the catalyst inlet is feedback-controlled to the stoichiometric air-fuel ratio as in the past, the oxygen concentration in the exhaust gas introduced into the catalyst is kept constant regardless of the oxygen storage amount at that time. Therefore, there is a problem that the oxygen storage amount cannot be controlled to an optimum value, and as a result, the conversion rate in the three-way catalyst cannot be stably maintained at a high level.
[0005]
In view of this point, a wide range type oxygen concentration sensor that linearly detects the exhaust air-fuel ratio upstream of the three-way catalyst, and a stoichiometric type that detects rich and lean of the exhaust air-fuel ratio to the stoichiometric air-fuel ratio downstream of the three-way catalyst If the output of the stoichiometric oxygen concentration sensor on the downstream side is rich while estimating the oxygen storage amount of the three-way catalyst based on the output of the wide area oxygen concentration sensor on the upstream side, oxygen storage When the amount is 0 and lean is indicated, correction is performed to maximize the oxygen storage amount, and the air-fuel ratio is controlled based on the estimated value of the oxygen storage amount so as to optimize the oxygen storage amount. (See Japanese Patent Laid-Open No. 6-249032 ).
[0006]
However, in the above, when the stoichiometric oxygen concentration sensor on the downstream side outputs an output in the vicinity of the stoichiometry, no correction is performed, so the estimated value of the oxygen storage amount adsorbed on the catalyst and the actual value are not corrected. A difference was produced, and good control could not be performed.
[0007]
If a wide-area oxygen concentration sensor is used also on the downstream side of the catalyst, the amount of oxygen storage can be estimated with high accuracy, but the cost is high.
The present invention has been made in view of the above-described problems. While using a wide-area oxygen concentration sensor on the upstream side of the catalyst and a stoichiometric oxygen concentration sensor on the downstream side, the oxygen storage amount is estimated with high accuracy, and oxidation is performed. The object is to be able to control to an optimum amount that balances the reaction and the reduction reaction, and thereby to purify CO, HC, NOx stably at a high conversion rate.
[0008]
[Means for Solving the Problems]
Therefore, as shown in FIG.
A wide-area oxygen concentration sensor installed on the upstream side of the three-way catalyst interposed in the exhaust passage of the engine and having a characteristic that the output value continuously changes with respect to the change of the exhaust air-fuel ratio;
A stoichiometric oxygen concentration sensor installed on the downstream side of the three-way catalyst and having a characteristic that the output value changes suddenly in the vicinity of the theoretical air-fuel ratio of the exhaust air-fuel ratio;
The first conversion table in which the exhaust air / fuel ratio detection value is assigned to the output value of the wide-area oxygen concentration sensor, and the exhaust air / fuel ratio detection value is set to the vicinity of the theoretical air / fuel ratio with respect to the output value of the stoichiometric oxygen concentration sensor. Then, the conversion means having a second conversion table assigned at a higher density than the other air-fuel ratio region ,
The exhaust air / fuel ratio on the upstream side of the three-way catalyst and the exhaust air / fuel ratio on the downstream side of the three-way catalyst obtained by converting the output values of the wide-area oxygen concentration sensor and the stoichiometric oxygen concentration sensor, respectively, by the conversion means, Oxygen storage amount estimation means for estimating the oxygen storage amount in the three-way catalyst based on
Based on the engine load and rotation speed, the target value of the oxygen storage amount is set to a large value in the operation region where the CO and HC emission amounts increase, and the oxygen storage amount target is set in the operation region where the NOx emission amount increases. A target storage amount setting means for setting the value to a small value ;
Exhaust air for feedback control of the exhaust air-fuel ratio at the three-way catalyst inlet so that the difference between the target value set by the target storage amount setting means and the oxygen storage amount estimated by the oxygen storage amount estimation means is reduced. Fuel ratio feedback means;
Constructed including.
[0009]
According to this configuration, the oxygen storage amount estimation means converts the output values of the wide area type oxygen concentration sensor and the stoichiometric type oxygen concentration sensor from the first conversion table and the second conversion table of the conversion means, respectively, and obtained the three-way data. Based on the exhaust air-fuel ratio (oxygen concentration) on the upstream side and downstream side of the catalyst, the oxygen storage amount in the three-way catalyst is estimated. Here, the output value of the stoichiometric oxygen concentration sensor has a characteristic that changes suddenly in the vicinity of the stoichiometric air-fuel ratio of the exhaust air-fuel ratio, but in the second conversion table, the exhaust air near the stoichiometric air-fuel ratio is stored in the second conversion table. Since the detected fuel ratio is assigned with high density, the exhaust air-fuel ratio can be detected with high accuracy, and therefore the oxygen storage amount can be estimated with high accuracy.
[0010]
On the other hand, if the exhaust air-fuel ratio is made lean to increase the amount of oxygen supplied, the amount of oxygen supplied in the three-way catalyst will be larger than the amount of oxygen required for the oxidation reaction, increasing the amount of oxygen storage, If the exhaust air-fuel ratio is enriched, the amount of oxygen supplied is reduced, so that the oxygen stored in the three-way catalyst is used for the oxidation reaction and the oxygen storage amount is reduced.
[0011]
Therefore, the inlet air-fuel ratio control means controls the exhaust air-fuel ratio at the catalyst inlet, that is, the air-fuel ratio of the combustion mixture (more specifically, the fuel injection amount to the engine) based on the estimated oxygen storage amount. Adjust the actual oxygen storage amount to the optimum value.
[0013]
Further, the exhaust air / fuel ratio at the catalyst inlet is controlled so that the oxygen storage amount in the three-way catalyst becomes the target value for each operating state based on the load and rotation speed of the engine . Specifically, in an operation state in which the amount of NOx emission increases, the target value of the oxygen storage amount can be made relatively small to effectively purify NOx, and the operation in which the amount of CO and HC emissions increases. In the state, CO and HC can be effectively purified by relatively increasing the target value of the oxygen storage amount .
[0014]
According to a second aspect of the present invention, the exhaust air / fuel ratio feedback means calculates the difference between the target value set by the target storage amount setting means and the oxygen storage amount estimated by the oxygen storage amount estimation means. Based on the difference between the target exhaust air-fuel ratio at the inlet and the actual exhaust air-fuel ratio detected by the oxygen concentration sensor upstream of the three-way catalyst, the exhaust at the three-way catalyst inlet The air-fuel ratio is feedback controlled.
[0015]
According to such a configuration, the target exhaust air-fuel ratio for obtaining the target oxygen storage amount is set from the difference between the target value and the estimated value of the oxygen storage amount so that the actual exhaust air-fuel ratio becomes the target exhaust air-fuel ratio. By controlling, the oxygen storage amount is controlled to coincide with the target value.
[0016]
【The invention's effect】
According to the first aspect of the present invention, the oxygen concentration sensor downstream of the three-way catalyst uses a relatively inexpensive stoichiometric oxygen concentration sensor to suppress an increase in cost and increase the oxygen storage amount in the three-way catalyst. It is possible to control to an optimum value while estimating the accuracy, so that CO, HC and NOx can be stably purified at a high conversion rate.
[0017]
In particular, the amount of oxygen storage in the three-way catalyst can be changed in response to the difference in CO, HC, NOx emissions depending on the operating conditions , thereby effectively purifying CO, HC, NOx. Has the effect of becoming possible.
[0018]
According to the invention of claim 2, by setting a target exhaust air-fuel ratio for making the oxygen storage amount coincide with the target, and performing feedback control so that the actual exhaust air-fuel ratio coincides with the target exhaust air-fuel ratio, There is an effect that the oxygen storage amount can be controlled accurately with a target.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
FIG. 2 is a diagram illustrating a system configuration of the engine in the embodiment.
[0020]
In FIG. 2, a fuel injection valve 2 is provided in the intake passage of the engine 1, and fuel and air injected from the fuel injection valve 2 are premixed and sucked into the cylinder via the intake valve 3. Is done. The combustion mixture in the cylinder is ignited and burned by spark ignition by the spark plug 4, and the combustion exhaust is discharged to the exhaust passage 6 via the exhaust valve 5.
[0021]
A three-way catalyst 7 is interposed in the exhaust passage 6, and the three-way catalyst 7 purifies CO, HC, and NOx in the exhaust gas.
A wide area type oxygen concentration sensor 8 having a characteristic that an output value continuously changes in response to a change in the exhaust air-fuel ratio is interposed on the upstream side of the three-way catalyst 7, and on the downstream side of the three-way catalyst 7. Is equipped with a stoichiometric oxygen concentration sensor 9 having a characteristic that the output value changes suddenly near the stoichiometric air-fuel ratio of the exhaust air-fuel ratio.
[0022]
Detection signals from the wide-area oxygen concentration sensor 8 and the stoichiometric oxygen concentration sensor 9 are input to the control unit 10 and processed along the flow shown as a control block diagram in the figure, and finally the fuel injection valve. The fuel injection amount by 2 is controlled.
[0023]
The processing contents will be outlined below along the control block diagram shown in FIG.
In the sensor output conversion processing unit (conversion means) 100, the output after the A / D conversion from the wide area type oxygen concentration sensor 8 upstream of the three-way catalyst 7 is converted into the detected value of the exhaust air / fuel ratio by the first conversion table. At the same time, the output after the A / D conversion from the stoichiometric oxygen concentration sensor 9 on the downstream side of the three-way catalyst 7 is converted into the detected value of the exhaust air / fuel ratio by the second conversion table. Here, the first conversion table and the second conversion table have conversion characteristics as shown in FIG. In the first conversion table, the detected value of the exhaust air-fuel ratio is assigned substantially evenly with respect to the change in the A / D conversion output value, whereas in the second conversion table, the A / D near the theoretical air-fuel ratio is assigned. The detected value of the exhaust air-fuel ratio is finely and densely assigned to the D-converted output value, whereby the exhaust air-fuel ratio in the vicinity of the theoretical air-fuel ratio is calculated with high resolution.
[0024]
In the oxygen storage amount estimation unit 101 (oxygen storage amount estimation means), three-way based on the upstream and downstream exhaust air-fuel ratios (oxygen concentration) converted by the sensor output conversion processing unit 100. The amount of oxygen storage in the catalyst 7 is estimated.
[0025]
On the other hand, the target oxygen storage amount setting unit 102 (target storage amount setting means) sets the target oxygen storage amount based on the engine load Tp and the engine speed Ne. In the operation region where CO and HC are likely to be emitted, the target oxygen storage amount is made relatively large so that the oxidation reaction of CO and HC is promoted. Is relatively small so that the reduction reaction of NOx is promoted.
[0026]
The oxygen storage amount comparison unit 103 calculates a difference between the actual oxygen storage amount estimated by the oxygen storage concentration estimation unit 101 and the target oxygen storage amount set by the target oxygen storage amount setting unit 102.
[0027]
The target air-fuel ratio setting unit 104 converts the difference between the actual value of the oxygen storage amount calculated by the oxygen storage amount comparison unit 103 and the target value into a target air-fuel ratio (target A / F). The target air-fuel ratio is a target value of the exhaust air-fuel ratio at the catalyst inlet detected by the oxygen concentration sensor 8 on the upstream side of the three-way catalyst 7.
[0028]
In the air-fuel ratio comparison unit 105, the three-way catalyst 7 inlet detected by the sensor output conversion processing unit 100 based on the target air-fuel ratio set by the target air-fuel ratio setting unit 104 and the detection signal from the oxygen concentration sensor 8. The air / fuel ratio deviation calculating unit 106 calculates a difference between the target air / fuel ratio and the actual air / fuel ratio.
[0029]
The injection amount correction unit 107 (exhaust air / fuel ratio feedback means) uses the fuel injection valve 2 to reduce the difference in the air / fuel ratio, in other words, to bring the actual air / fuel ratio closer to the target air / fuel ratio. A correction value for correcting the fuel injection amount is determined.
[0030]
The target oxygen storage amount setting unit 102, the oxygen storage amount comparison unit 103, the target air / fuel ratio setting unit 104, the air / fuel ratio comparison unit 105, the air / fuel ratio deviation calculation unit 106, and the injection amount correction unit 107 constitute an inlet air / fuel ratio control means. Composed.
[0031]
The injection amount calculation unit 108 calculates a basic fuel injection amount based on the intake air amount Q and the rotational speed Ne of the engine 1, a correction value according to the coolant temperature Tw of the engine 1, and the injection amount correction The basic fuel injection amount is corrected by the correction value set by the unit 107 to calculate the final fuel injection amount. Then, an injection pulse signal having a pulse width corresponding to the fuel injection amount is output to the fuel injection valve 2 at a predetermined injection timing.
[0032]
Here, the control contents shown in the control block diagram of FIG. 2 will be described in detail according to the flowchart of FIG.
In the flowchart of FIG. 3, first, in S1, the detection signal (A / D conversion value) from the oxygen concentration sensor 8 upstream of the three-way catalyst 7 is converted by the first conversion table, and then the three-way catalyst 7 inlet. The exhaust air-fuel ratio (oxygen concentration) is detected.
[0033]
In S2, the exhaust air-fuel ratio detected in S1 is averaged, and in S3, the averaged exhaust air-fuel ratio is set as the inlet air-fuel ratio AFIN.
In S4, the detection signal (A / D conversion value) from the oxygen concentration sensor 9 downstream of the three-way catalyst 7 is converted by the first conversion table, and the exhaust air-fuel ratio (oxygen concentration) at the three-way catalyst 7 outlet is converted. Is detected.
[0034]
In S5, the exhaust air-fuel ratio detected in S4 is averaged, and in S6, the averaged exhaust air-fuel ratio is set as the outlet air-fuel ratio AFOUT.
In S7, an estimated value SO2 of the oxygen storage amount in the three-way catalyst 7 is calculated based on a function f1 (AFIN-AFOUT) having the difference between the inlet air-fuel ratio AFIN and the outlet air-fuel ratio AFOUT as a variable. The function f1 (AFIN-AFOUT) estimates the oxygen storage amount in the three-way catalyst from the difference between the inlet air-fuel ratio AFIN and the outlet air-fuel ratio AFOUT, that is, the difference between the supplied oxygen amount and the residual oxygen amount after reaction. It is a model formula.
[0035]
In S8, a map in which the target oxygen storage amount TSO2 is stored in advance according to the engine load Tp and the engine rotational speed Ne is referenced, and the target oxygen storage amount TSO2 corresponding to the engine load Tp and the engine rotational speed Ne at that time is retrieved. To do.
[0036]
In S9, a difference ΔSO2 between the target oxygen storage amount TSO2 and the estimated value SO2 is calculated.
In S10, the target exhaust air-fuel ratio TAF at the inlet of the three-way catalyst 7 is calculated based on the function f2 (ΔSO2) having the oxygen storage amount difference ΔSO2 as a variable. The function f2 (.DELTA.SO2) is a conversion model expression from the difference .DELTA.SO2 to the target exhaust air-fuel ratio TAF in order to make the oxygen storage amount difference .DELTA.SO2 zero.
[0037]
In S11, a difference ΔAF between the target exhaust air-fuel ratio TAF and the inlet air-fuel ratio AFIN that is the actual catalyst inlet air-fuel ratio is calculated.
In S12, it is determined based on the ΔAF whether the actual inlet air-fuel ratio AFIN is leaner or richer than the target exhaust air-fuel ratio TAF.
[0038]
When the actual inlet air-fuel ratio AFIN is leaner than the target exhaust air-fuel ratio TAF, the process proceeds to S13 and the correction value is integrated in the rich direction so as to increase the fuel injection amount to correct the inlet air-fuel ratio to be richer. Control.
[0039]
On the other hand, when the actual inlet air-fuel ratio AFIN is richer than the target exhaust air-fuel ratio TAF, the routine proceeds to S14 where the correction value is integrated in the lean direction so as to reduce the fuel injection amount and correct the inlet air-fuel ratio to be leaner. Control.
[0040]
Further, when the actual inlet air-fuel ratio AFIN substantially matches the target exhaust air-fuel ratio TAF, the routine proceeds to S15, where the correction value so far is held.
If the actual inlet air-fuel ratio AFIN is enriched, the amount of oxygen supplied to the three-way catalyst 7 can be reduced, and the oxygen storage amount can be decreased and changed, and if the actual inlet air-fuel ratio AFIN is made lean, The amount of oxygen supplied to the three-way catalyst 7 is increased, and the amount of oxygen storage can be increased.
[0041]
Accordingly, in setting the target exhaust air / fuel ratio TAF, when the estimated value SO2 is smaller than the target oxygen storage amount TSO2, the leaner the air / fuel ratio is set as the target exhaust air / fuel ratio TAF, the larger the difference is. The oxygen storage amount can be increased and brought closer to the target. When the estimated value SO2 is larger than the target oxygen storage amount TSO2, the richer the difference, the more the rich air-fuel ratio is set as the target exhaust air-fuel ratio TAF. For example, the actual oxygen storage amount can be reduced and brought closer to the target.
[0042]
If the oxygen storage amount in the three-way catalyst 7 can be maintained at the target value (optimum value), the CO, HC oxidation reaction and the NOx reduction reaction in the three-way catalyst 7 are executed in a balanced manner, and the maximum amount is achieved. Purification performance can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a basic configuration of an apparatus according to claims 1 and 2;
FIG. 2 is a block diagram showing a system configuration and control contents of the engine in the embodiment.
FIG. 3 is a diagram showing a first conversion table and a second conversion table used in the embodiment.
FIG. 4 is a flowchart showing a state of oxygen storage amount control in the embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Engine 2 Fuel injection valve 3 Intake valve 4 Spark plug 5 Exhaust valve 6 Exhaust passage 7 Three-way catalyst 8 Wide area type oxygen concentration sensor 9 Stoichi type oxygen concentration sensor
10 Control unit
101 Oxygen storage amount estimation unit
102 Target oxygen storage amount setting section
103 Oxygen storage amount comparison part
104 Target air-fuel ratio setting section
105 Air-fuel ratio comparison unit
106 Air-fuel ratio deviation calculator
107 Injection amount correction unit
108 Injection amount calculator
109 Inlet air-fuel ratio detector

Claims (2)

エンジンの排気通路に介装される三元触媒の上流側に設置され、排気空燃比の変化に対して出力値が連続的に変化する特性を有した広域型酸素濃度センサと、
三元触媒の下流側に設置され、排気空燃比の理論空燃比近傍で出力値が急変する特性を有したストイキ型酸素濃度センサと、
前記広域型酸素濃度センサの出力値に対して排気空燃比検出値を割付した第１変換テーブルと、前記ストイキ型酸素濃度センサの出力値に対して、排気空燃比検出値を、理論空燃比近傍では他の空燃比領域より高密度に割付した第２変換テーブルと、を有する変換手段と、
前記広域型酸素濃度センサ及びストイキ型酸素濃度センサの出力値を、それぞれ前記変換手段で変換して求められた三元触媒上流側の排気空燃比と、三元触媒下流側の排気空燃比と、に基づいて前記三元触媒における酸素ストレージ量を推定する酸素ストレージ量推定手段と、
エンジンの負荷及び回転速度に基づいて、ＣＯ，ＨＣの排出量が多くなる運転領域では酸素ストレージ量の目標値を大きい値に設定し、ＮＯｘの排出量が多くなる運転領域では酸素ストレージ量の目標値を小さい値に設定する目標ストレージ量設定手段と、
該目標ストレージ量設定手段で設定された目標値と前記酸素ストレージ量推定手段で推定された酸素ストレージ量との差が少なくなるように、前記三元触媒入口の排気空燃比をフィードバック制御する排気空燃比フィードバック手段と、
を含んで構成されたことを特徴する三元触媒の酸素ストレージ量制御装置。A wide-area oxygen concentration sensor installed on the upstream side of the three-way catalyst interposed in the exhaust passage of the engine and having a characteristic that the output value continuously changes with respect to the change of the exhaust air-fuel ratio;
A stoichiometric oxygen concentration sensor installed on the downstream side of the three-way catalyst and having a characteristic that the output value changes suddenly in the vicinity of the theoretical air-fuel ratio of the exhaust air-fuel ratio;
The first conversion table in which the exhaust air / fuel ratio detection value is assigned to the output value of the wide-area oxygen concentration sensor, and the exhaust air / fuel ratio detection value is set to the vicinity of the theoretical air / fuel ratio with respect to the output value of the stoichiometric oxygen concentration sensor. Then, the conversion means having a second conversion table assigned at a higher density than the other air-fuel ratio region ,
The exhaust air / fuel ratio on the upstream side of the three-way catalyst and the exhaust air / fuel ratio on the downstream side of the three-way catalyst obtained by converting the output values of the wide-area oxygen concentration sensor and the stoichiometric oxygen concentration sensor, respectively, by the conversion means, Oxygen storage amount estimation means for estimating the oxygen storage amount in the three-way catalyst based on
Based on the engine load and rotation speed, the target value of the oxygen storage amount is set to a large value in the operation region where the CO and HC emission amounts increase, and the oxygen storage amount target is set in the operation region where the NOx emission amount increases. A target storage amount setting means for setting the value to a small value ;
Exhaust air for feedback control of the exhaust air-fuel ratio at the three-way catalyst inlet so that the difference between the target value set by the target storage amount setting means and the oxygen storage amount estimated by the oxygen storage amount estimation means is reduced. Fuel ratio feedback means;
An oxygen storage amount control device for a three-way catalyst, comprising: 前記排気空燃比フィードバック手段が、前記目標ストレージ量設定手段で設定された目標値と前記酸素ストレージ量推定手段で推定された酸素ストレージ量との差を前記三元触媒入口の目標排気空燃比に変換し、該目標排気空燃比と前記三元触媒の上流側の酸素濃度センサで検出される実際の排気空燃比との差に基づいて、前記三元触媒入口の排気空燃比をフィードバック制御することを特徴とする請求項１記載の三元触媒の酸素ストレージ量制御装置。The exhaust air / fuel ratio feedback means converts the difference between the target value set by the target storage amount setting means and the oxygen storage amount estimated by the oxygen storage amount estimation means into a target exhaust air / fuel ratio at the three-way catalyst inlet. Feedback control of the exhaust air / fuel ratio at the inlet of the three-way catalyst based on the difference between the target exhaust air / fuel ratio and the actual exhaust air / fuel ratio detected by the oxygen concentration sensor upstream of the three-way catalyst. The oxygen storage amount control device for a three-way catalyst according to claim 1, characterized in that:
JP09087999A 1999-03-31 1999-03-31 Three-way catalyst oxygen storage control device Expired - Fee Related JP4155662B2 (en)

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