JP2004245133A - Diesel engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for accelerating warming-up of a catalyst and maintaining its activity. <P>SOLUTION: In an early warming-up period after the start of a diesel engine, a first warming-up operation mode for accelerating the warming-up of the catalyst is carried out, and a second warming-up operation mode for maintaining the activity of the catalyst is carried out in a latter half warming-up period. The first warming-up operation mode for accelerating the warming-up of the catalyst is a mode higher in the energy of exhaust gas flowing in the catalyst than a normal operation mode. The second warming-up operation mode for maintaining the activity of the catalyst is a mode higher in the temperature of exhaust gas flowing in the catalyst than the normal operation mode. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

ここで、Ｃ１，Ｃ２は定数、Ｒｅはレイノズル数、Ｐｒはプラントル数、Ｔｇは排気ガスの温度、Ｔｃは触媒の温度、Ｖは排気ガスの流量である。なお、プラントル数は流体の物性値だけで決まるので、定数Ｃ２で代表されている。また、レイノルズ数Ｒｅは、排気ガスの流量Ｖに比例する。
【００６２】
伝熱量Ｑが多いほど、触媒４０の暖機が促進される。従って、触媒４０の暖機を促進するためには、次の２つの手段を採用することが可能である。
（ａ）排気ガスの温度Ｔｇを上昇させる。
（ｂ）排気ガスの流量Ｖを増大させる。
【００６３】
以下に説明する各種の実施例は、これらの２つの手段の少なくとも一方を利用して触媒４０の暖機を促進する。また、触媒４０が活性温度まで達した後も、しばらくの間は活性を維持するための運転を実行する。この触媒活性維持のための運転では、排気ガスの温度Ｔｇを上昇させる手段が利用される。
【００６４】
図５は、本発明の実施例における触媒暖機の概要を示している。横軸は、始動時（コールドスタート）からの経過時間であり、縦軸は車速と触媒床温である。グラフの上部に示されているように、暖機期間は、早期暖機期間（早期暖機フェーズ）と後期暖機期間（後期暖機フェーズ）とに分かれている。この例における車速の変化は、所定の車両性能テストパターンに適合するものである。
【００６５】
早期暖機期間ｔ０〜ｔ１は、触媒床温を常温から活性温度（例えば約２５０℃）まで上昇させるための期間である。この早期暖機期間では、触媒暖機促進運転モード（後述する）が実行され、これによって、早期暖機期間の終了時には触媒床温が活性温度に達する。後期暖機期間ｔ１〜ｔ２は、触媒床温を活性温度以上に維持するための期間である。この後期暖機期間では、エンジンへの要求負荷が所定値よりも低いことを含む所定の運転条件を満たしたときに、触媒活性維持運転モード（後述する）が実行される。図５の例では、図の上部に示すように、触媒活性維持運転モードは、エンジンの要求負荷が低く回転数がほぼ一定である運転条件（具体的には、定速走行時とアイドリング時）に触媒活性維持運転モードが実行されている。
【００６６】
後期暖機期間の後（時刻ｔ２以降）は、通常の運転が行われる。後期暖機期間の終了時には、触媒床温が十分に上昇しており、また、エンジンの冷却水や排気経路なども十分に昇温している。従って、触媒暖機促進運転モードや触媒活性時運転モードを行わずに、通常の運転を行っても触媒床温を活性温度以上に維持することが可能である。なお、本明細書において、後期暖機期間後において通常行われる運転モードを「通常運転モード」と呼ぶ。また、早期暖機期間と後期暖機期間とをまとめて単に「暖機期間」と呼ぶことがある。
【００６７】
早期暖機期間中における触媒暖機促進運転モードとしては、暖機期間後の通常運転モードに比べて触媒を流れる排気のエネルギがより高い運転モードを採用することができる。ここで、「通常運転モードに比べて」という文言は、「通常運転モードとほぼ同じエンジン負荷および回転数の場合と比べて」、という意味である。触媒暖機促進運転モードの具体例については後に詳述する。後期暖機期間中における触媒活性維持運転モードとしては、暖機期間後の通常運転モードに比べて触媒を流れる排気の温度がより高い運転モードを採用することができる。例えば、触媒活性維持運転モードとして、図２、図３で説明した低温燃焼を採用することができる。
【００６８】
触媒活性維持運転モードにおいて排気ガス温度を高めるためには、一般に、通常運転モードよりも空気過剰率を低下（すなわち空燃比を低下）させれば良い。特に、触媒活性維持運転モードとして、通常運転モードと触媒暖機促進運転モードのいずれよりも空気過剰率を低下させることが好ましい。この理由は、空気過剰率が高いと多量の空気によって排気温度が低下してしまい、逆に、空気過剰率を低くすれば排気温度を高められるからである。図３に示したように、低温燃焼は空燃比がかなり低く、排気温度が通常運転モードよりも高くなるので、触媒活性維持運転モードとして採用することができる。また、触媒活性維持運転モードとして低温燃焼以外の燃焼モードを採用し、その際、空気過剰率を暖機期間後の通常運転モードよりも低下させることによって、排気温度を高めることが可能である。空気過剰率を低下させる手段としては、スロットル弁２８（図１）の開度の低下や、ＥＧＲ率の上昇があり、これらの２つを同時に採用することも可能である。
【００６９】
また、触媒活性維持運転モードとしては、通常運転モードよりも触媒を流れる排気ガス量を低下させるモードを採用することも可能である。特に、触媒活性維持運転モードとして、通常運転モードと触媒暖機促進運転モードのいずれよりも触媒を流れる排気ガス量を低下させることが好ましい。こうすれば、多量の排気によって触媒を冷却してしまうことを防止することができるので、触媒の活性を維持することが可能である。排気ガス量を低下させる手段としては、スロットル弁２８の開度の低下や、ＥＧＲ率の増大を利用することができる。
【００７０】
なお、触媒活性維持運転モードは、触媒４０の活性温度を維持することを意図しているので、触媒暖機促進運転モードよりも触媒４０に与えるエネルギは少なくて良い。従って、触媒活性維持運転モードは、通常運転モードよりも燃費は多少悪化するが、触媒暖機促進運転モードよりも燃費が良好な（燃料効率の良い）運転モードとすることが好ましい。
【００７１】
Ｃ．暖機制御の第１実施例：
図６は、暖機制御の第１実施例におけるディーゼルエンジン１０の運転状態を示す説明図である。図５の横軸は始動（コールドスタート）からの時間であり、縦軸は、上から順に、エンジン冷却水温、燃料噴射時期、および、ＥＧＲ率である。時刻ｔ０でエンジンが始動すると、エンジン冷却水温が所定の温度ＣＡＴｏｎに到達する時刻ｔ１までの早期暖機期間では、触媒４０を暖機するための触媒暖機促進運転（触媒暖機用燃焼）が行われる。なお、早期暖機期間は、エンジン冷却水温が所定の温度ＣＡＴｏｎに達するまでの期間として設定される。あるいは、触媒床温が所定の温度に達するまでの期間してもよい。時刻ｔ１から時刻ｔ２までの後期暖機期間では、触媒活性維持運転モードと通常運転モードとが行われる。本実施例では、触媒活性維持運転モードでは、図２、図３で説明した低温燃焼が行われ、通常運転モードでは通常燃焼が行われる。時刻ｔ２以降は、暖機後の通常の運転モード（通常燃焼）が行われる。なお、図６では、後期暖機期間ｔ１〜ｔ２における低温燃焼と通常燃焼の期間は、図示の便宜上、簡略化されて描かれている。
【００７２】
早期暖機期間ｔ０〜ｔ１中においては、燃料噴射は、パイロット噴射とメイン噴射とに分割して行なわれる。パイロット噴射の噴射量は、１燃焼サイクルの全噴射量の約３０％に設定されており、メイン噴射の噴射量は約７０％に設定されている。また、パイロット噴射の噴射時期は、圧縮上死点ＴＤＣの約１０°手前（すなわちＴＤＣ−約１０°）であり、メイン噴射の噴射時期は圧縮上死点ＴＤＣの約１０°後ろ（すなわちＴＤＣ＋約１０°）に設定されている。なお、本明細書において、「噴射時期」とは噴射開始時期を意味する。暖機期間中における噴射量や噴射時期の設定値の意味については後述する。
【００７３】
早期暖機期間中は、ＥＧＲ率が０に設定されている。すなわち、ＥＧＲ弁６２が閉鎖されており、燃焼排ガスの還流がカットされている。この理由は、早期暖機期間中にＥＧＲを行うと、ＥＧＲクーラ６４（図１）によって排気温度が低下してしまうので、却って暖機が抑制される可能性があるからである。これから理解できるように、暖機期間中のＥＧＲカット運転は、上述した（１）式における排気ガスの温度Ｔｇを上昇させる手段として使用することができる。但し、暖機期間中に、ある程度のＥＧＲを行うようにしても良い。例えば、加速中はＥＧＲ率を１０〜２０％に設定しても良い。ＥＧＲ率をこのような低い値に設定することによって、触媒４０の暖機を促進するとともに、ＥＧＲカットによるＮＯｘの過度の増加を防止することができる。
【００７４】
後期暖機期間ｔ１〜ｔ２では、比較的低負荷の運転条件において低温燃焼が行われ、比較的高負荷の運転条件では通常燃焼が行われる。図３で説明したように、低温燃焼は、通常燃焼に比べて触媒入ガス温度が高いので、触媒４０の活性を維持することができる。また、触媒４０は早期暖機期間中に活性温度に達しているので、後期暖機期間において低温燃焼（触媒活性維持運転）を継続的に行う必要はなく、間欠的に低温燃焼を行えば十分である。これに対して、早期暖機期間ｔ０〜ｔ１では、エンジンの負荷に拘わらず、触媒暖機促進運転を継続的に行うことが好ましい。なお、後期暖機期間の終了時期は、単に後期暖機期間の時間を計測して決定することができる。あるいは、触媒床温やエンジン冷却水温が所定の値に達したときに後期暖機期間を終了させるようにしてもよい。
【００７５】
後期暖機期間中の通常燃焼の際にも、早期暖機期間中の触媒暖機用燃焼と同様に、パイロット噴射とメイン噴射とが行われる。但し、通常燃焼時には、パイロット噴射の噴射量は、１燃焼サイクルの全噴射量の約１０％に設定され、メイン噴射の噴射量は約９０％に設定される。また、パイロット噴射の噴射時期は、（ＴＤＣ−約１５°）であり、メイン噴射の噴射時期は（ＴＤＣ＋約５°）である。時刻ｔ２以降の通常運転モードでは、後期暖機期間中と同じ通常燃焼が行われる。
【００７６】
図７（Ａ）は、早期暖機期間中の燃焼室内の圧力変化を示しており、図７（Ｂ）は熱発生量の変化を示している。図７（Ａ），（Ｂ）の横軸はクランク角度θである。早期暖機期間中においては、パイロット噴射もメイン噴射も圧縮上死点ＴＤＣから離れた時点で行われるので、圧縮上死点ＴＤＣ付近における圧力上昇率は低い。このため、いわゆる等容度が低下する。ディーゼルエンジンでは、一般に、等容度が低下すると熱効率が低下し、排気エネルギが増加することが知られている。具体的には、パイロット噴射された燃料は、圧縮上死点ＴＤＣ前に燃焼して熱を発生するが、この熱はほとんど外部に対して仕事をせず、燃焼室内の温度・圧力の上昇に利用される。メイン噴射も、圧縮上死点ＴＤＣからかなり離れた時点で行われるので、その熱発生量のかなりの部分が排気の温度上昇に利用される。
【００７７】
早期暖機期間中のパイロット噴射およびメイン噴射の噴射時期と噴射量とは、以下のような事項を考慮して決定されている。排気エネルギを増加させるためには、メイン噴射を圧縮上死点ＴＤＣ以降のなるべく遅い時点で（すなわち、なるべく大幅に遅角させて）行うことが望ましい。但し、メイン噴射をあまり遅角させると失火する可能性がある。そこで、パイロット噴射の噴射量を増加させると燃焼室の温度が上昇するので、メイン噴射を遅角させたときに失火する可能性を低下させることができる。また、パイロット噴射とメイン噴射の噴射時期の間隔には、ある程度の好ましい値（例えばクランク角度で２０°）が存在する。従って、パイロット噴射とメイン噴射の噴射時期は、失火する可能性が低く、かつ、メイン噴射の噴射時期がなるべく遅くなるように決定される。また、パイロット噴射とメイン噴射の噴射時期と噴射量は、排気エネルギの増大分を含み、かつ、要求負荷を満足するように決定される。この結果、早期暖機期間中では、通常燃焼に比べて、パイロット噴射の噴射量が多く、かつ、メイン噴射の噴射時期がより遅い時期に設定される。
【００７８】
なお、等負荷の運転条件では、早期暖機期間中の触媒暖機用燃焼の方が、通常燃焼よりも全噴射量は多くなる。早期暖機期間中のパイロット噴射の噴射割合（パイロット噴射量／全噴射量）は、通常燃焼におけるパイロット噴射の噴射割合の約２倍〜約５倍であることが好ましい。
【００７９】
以上の説明から理解できるように、第１実施例の早期暖機期間中の燃料噴射には、以下のような特徴がある。
（１）早期暖機期間中におけるパイロット噴射の噴射量割合は、早期暖機期間後における通常燃焼において実行されるパイロット噴射の噴射量割合よりも高い。（２）早期暖機期間中におけるメイン噴射の噴射時期は、暖機期間後における通常燃焼において実行されるメイン噴射の噴射量時期よりも遅い。
【００８０】
このような制御を行うことによって、メイン噴射をかなりの遅角させて、排気エネルギを増大させることが可能であり、触媒４０をより効率的に暖機することが可能である。また、パイロット噴射の噴射量が多いので、メイン噴射の際には、燃焼室内の温度と圧力がかなり上昇している。このため、メイン噴射で噴射された燃料が十分に燃焼するので、燃焼排ガス中のＨＣ濃度を低減することができる。すなわち、第１実施例では、ＨＣ濃度を抑制しつつ、触媒の暖機を効率良く行うことが可能である。
【００８１】
なお、パイロット噴射とメイン噴射の時期にはある程度の融通性があるが、触媒の暖機のためには、パイロット噴射を圧縮上死点の手前で行い、メイン噴射を圧縮上死点以降で行うことが好ましい。こうすることによって、ＨＣ濃度を抑制しつつ、触媒の暖機をより効率良く行うことが可能である。
【００８２】
図８は、第１実施例に従って暖機運転（触媒暖機促進運転および触媒活性維持運転）の実行例を示している。車速の変化は図５に示したものと同じである。暖機運転を行った場合には、始動後約７０秒経過した早期暖機期間の終了時ｔ１に、触媒床温が活性温度に達しており、その後も活性温度以上に維持されている。一方、暖機運転を行わない場合には、始動後約１０００秒経過した後に活性温度に到達している。これから理解できるように、第１実施例の触媒暖機運転を行うことによって、短時間で触媒４０を活性化させることが可能であり、また、その後も活性を維持することができる。
【００８３】
なお、後期暖機期間を行わずに、触媒が活性温度に達した後は通常燃焼（通常運転モード）を行うことも可能である。但し、図３で説明したように、通常燃焼では、触媒入りガス温度が低温燃焼よりも低く、また、ＥＧＲ率が低温燃焼よりも低いので触媒４０を通過する排気ガス量が多い。従って、早期暖機期間の後にすぐに通常燃焼のみで運転を行うと、図８に一点鎖線で示すように、触媒４０が多量の排気ガスで冷却されて活性温度以下になる可能性がある。これに対して、本実施例では、後期暖機期間において、比較的低負荷の運転条件において低温燃焼を行うので、触媒４０の活性を維持することが可能である。
【００８４】
なお、図６の例では、早期暖機期間中は、パイロット噴射とメイン噴射の噴射割合および噴射時期が一定に保たれるものとしたが、暖機の程度に応じてこれらを変更するようにしてもよい。図９は、エンジン冷却水温に応じて噴射割合と噴射時期とを変更する場合の例を示すグラフである。この例では、エンジン冷却水温が上昇するにつれて、パイロット噴射の噴射割合が低下し、一方、メイン噴射の噴射時期が進角する。換言すれば、エンジン冷却水温が上昇するにつれて、通常燃焼の噴射状態に近づくように調整が行われる。こうすることによって、暖機の程度に応じた適切な噴射噴射で触媒を暖機することが可能である。なお、エンジン冷却水温の代わりに、触媒床温に応じてパイロット噴射とメイン噴射の噴射割合および噴射時期を変更するようにしてもよい。
【００８５】
Ｄ．暖機制御の第２実施例：
図１０は、暖機制御の第２実施例におけるディーゼルエンジン１０の運転状態を示す説明図である。第２実施例では、以下に説明するように、早期暖機期間におけるパイロット噴射とメイン噴射の噴射時期および噴射量が、第１実施例よりも最適化されている。
【００８６】
図１１は、メイン噴射時期およびメイン噴射量と失火領域との関係を示す説明図である。この図から解るように、あるメイン噴射量のときにメイン噴射時期を遅角させると、失火限界を超えて失火領域に入ってしまう。但し、メイン噴射量が少ないほど、失火限界のクランク角度は遅角側にずれる。図１１の実線は、パイロット噴射量を一定としたときに、同一のトルクを発生させるためのメイン噴射の時期と噴射量との関係を示している。一般に、メイン噴射の噴射時期を遅角させると熱効率が低下する。従って、パイロット噴射量が一定値に保たれる条件下では、メイン噴射の噴射時期を遅角させるほど、同一のトルクを発生するために必要なメイン噴射量が増大する。また、パイロット噴射量を増加させた場合には、同一のメイン噴射量でメイン噴射時期を遅角させても同一のトルクを発生することができる。
【００８７】
ところで、失火限界の近傍の臨界領域では、メイン噴射が十分に遅角しているので、排気ガス温度が高められる。臨界領域における排気ガス温度は、各種の燃料噴射条件に依存している。図１２は、燃料噴射条件による排気ガス温度Ｔｇの変化を示す説明図である。図１２の横軸は、メイン噴射時期であり、縦軸は燃焼排ガスの温度である。また、実線は、パイロット噴射割合Ｑｐ（全噴射量に対するパイロット噴射量の割合）が０％のときのグラフであり、一点鎖線はこれが２０％、二点鎖線は４０％のときのグラフである。この図から理解できるように、パイロット噴射量割合Ｑｐを０％から２０％に増加させると、失火限界となるメイン噴射時期が遅角するので、失火限界の臨界領域における排気ガス温度Ｔｇも上昇する。ところが、パイロット噴射量割合Ｑｐを過度に（例えば４０％に）増大させると、メイン噴射量の割合が低下してしまうので、逆に排気ガス温度Ｔｇが低下することになる。図１３は、図１２の関係を、パイロット噴射量割合と排気ガス温度Ｔｇとの関係に書き直したものである。この図からも理解できるように、メイン噴射の条件を図１１の臨界領域内に設定したときに、排気ガス温度Ｔｇが最も高くなるようなパイロット噴射量割合の最適な条件が存在する。このような最適な噴射条件は、各エンジン毎に実験的に決定される。また、この最適な噴射条件はエンジンの温度にも依存するので、例えばエンジン冷却水温度に応じて変更することが好ましい。
【００８８】
なお、メイン噴射量とパイロット噴射量との合計が一定であるときには、以下の２つの場合を考慮して、噴射条件を最適化することが可能である。すなわち、パイロット噴射量が多い場合には、メイン噴射量は少なくなるがメイン噴射の時期は遅らせることが可能である。一方、パイロット噴射量が少ない場合には、メイン噴射量が多くなるが、メイン噴射の時期は進める必要が生じる。排気ガス温度Ｔｇが最も高くなるようなパイロット噴射とメイン噴射の最適な条件は、これらの２つの場合のバランスを考慮して決定される。すなわち、この最適な条件は、失火が生じない範囲においてパイロット噴射量が最大可能量となる条件と、最小可能量となる条件と、の中間に存在する。従って、これらの２つの極端な条件の中間を探索することによって、最適な条件を見いだすことが可能である。
【００８９】
以上のように、第２実施例では、早期暖機期間中において、排気ガス温度Ｔｇが最も高くなるように、パイロット噴射の噴射量割合とメイン噴射の噴射時期とが、失火限界直前の臨界領域の時期に相当するように設定されている。これによって、排気エネルギが増加し、触媒の暖機を効率的に促進することが可能である。なお、この臨界領域の範囲としては、クランク角度で失火限界から失火限界の手前約２°までの範囲とすることが好ましく、失火限界からその手前の約１°までの範囲とすることがさらに好ましい。
【００９０】
第２実施例では、さらに、排気ガスの温度上昇を促進する手段として、噴射条件の調整の他に、ＥＧＲカット運転と、アイドル回転数の上昇と、グロープラグの継続使用と、の３つの手段が利用されている。
【００９１】
図１０に示したように、第２実施例では、第１実施例と同様に、早期暖機期間中においてＥＧＲがカットされている。この結果、ＥＧＲクーラ６４（図１）における熱損失によって排気温度が低下してしまうことを防止し、触媒の暖機を促進することが可能である。また、図１０の例では、アイドル回転数は、早期暖機期間中は１２００ｒｐｍに設定されており、早期暖機期間後は９５０ｒｐｍに設定されている。このように、早期暖機期間中にアイドル回転数を増加させると、上述した（１）式における排気ガス流量Ｖを増大させることができ、これによって触媒４０の暖機を促進することが可能である。
【００９２】
図１０の最下部に示すように、早期暖機期間中は、さらに、グロープラグが継続的に利用されている。図１４は、燃焼室の縦断面を示している。燃焼室１１０は、シリンダ壁１１２と、シリンダヘッド１１４と、ピストン１１６とで構成されている。シリンダヘッド１１４には、吸気ポート１２２と排気ポート１２４とが設けられている。吸気ポート１２２には吸気弁１３２とが設けられており、排気ポート１２４には排気弁１３４が設けられている。また、シリンダヘッド１１４の燃焼室のほぼ中心位置には、燃料噴射弁１４が設けられており、燃料噴射弁１４のノズル１５に隣接した位置にはグロープラグ１２６が設けられている。なお、グロープラグ１２６は、吸気を加熱できる位置に配置されていればよく、例えば、吸気ポート１２２に設けられていてもよい。
【００９３】
グロープラグ１２６は、通常は、エンジン１０の始動性の向上を図るために、始動時の極く短い時間のみに利用される。これに対して、第２実施例では、早期暖機期間中において、グロープラグ１２６がかなり長時間の間、継続的にオン状態に保たれている。こうすることによって、排気ガス温度をさらに高めて、触媒４０の暖機を促進することが可能である。
【００９４】
なお、ＥＧＲカット運転と、グロープラグによる継続的な吸気加熱運転と、アイドル回転数上昇運転と、の３種類の運転は、同時に行う必要はないが、これらのうちの少なくとも１つを早期暖機期間中の少なくとも一部の期間で実行することが好ましい。また、これらの運転の程度は、エンジン冷却水温度や触媒昇温に依存して変化させることが好ましく、早期暖機期間中に必要がなくなったときにはこれらの運転を停止してもよい。
【００９５】
Ｅ．暖機制御の第３実施例：
図１５は、暖機制御の第３実施例におけるディーゼルエンジン１０の運転状態を示す説明図である。第３実施例では、燃料噴射の時期や噴射量は図６に示した第１実施例と同じであるが、可変ノズルターボ（ＶＮＴ）２０の入口ノズル２５（図１）と吸気絞り（スロットル弁２８）の開度を早期暖機期間中に絞っている点に特徴がある。なお、早期暖機期間後は、負荷要求に応じてＶＮＴや吸気絞りの開度が調整されるが、図１５の例では簡略化して描かれている。
【００９６】
図１６は、ＶＮＴと吸気絞りの調整による触媒の暖機効果を説明している。ＶＮＴのノズル開度を低下させると、エンジンの背圧（排気管１６の圧力）が上昇する（ブロックＢ２）。また、吸気絞りの開度が低下すると、吸気圧が低下する。エンジン背圧の上昇と吸気圧の低下は、エンジンのポンプ損失の増加を引き起こし（ブロックＢ３）、必要な動力を発生するために要する燃料を増加させる（ブロックＢ４）。この結果、排気エネルギが増加し（ブロックＢ５）、触媒の暖機を促進させることができる（ブロックＢ６）。なお、吸気絞りの開度低下は、吸気量を減らして吸気温度を上昇させる効果も有している。
【００９７】
一方、エンジン背圧の上昇は、タービン回転数を増加させるという効果もある（ブロックＢ７）。タービン回転数が増加すると、これに伴って吸気圧が上昇する（ブロックＢ８）。また、吸気絞り弁が絞られているので、吸気管内の流速は増加する（ブロックＢ９）。これは吸気のエントロピが増加することを意味するので、吸気温度が上昇する（ブロックＢ１０，Ｂ１１）。この結果、排気エネルギが増加し（ブロックＢ５）、触媒の暖機を促進させることができる（ブロックＢ６）。
【００９８】
このように、早期暖機期間中にＶＮＴと吸気絞りを暖機後よりも絞ることによって、触媒の暖機を促進させることが可能である。また、これらの制御は、ＨＣ濃度を増加させる可能性が低い。従って、ＨＣ濃度を抑制しつつ触媒の暖機を促進することができる。
【００９９】
Ｆ．暖機制御の第４実施例：
図１７は、暖機制御の第４実施例におけるディーゼルエンジン１０の運転状態を示す説明図である。第４実施例は、燃料噴射の時期や噴射量は図６に示した第１実施例と同じであるが、早期暖機期間中に吸気絞り（スロットル弁２８）を高速で開閉している点に特徴がある。吸気絞りを高速で開閉すると、そのエネルギが吸気に与えられて吸気のエントロピが増加し、吸気温度が上昇する。従って、排気温度も上昇し、触媒の暖機を促進させることができる。また、この制御はＨＣ濃度を増加させる可能性が低いので、ＨＣ濃度を抑制しつつ触媒の暖機を促進することができる。
【０１００】
第３実施例や第４実施例の説明から理解できるように、早期暖機期間中に、吸気絞りやＶＮＴの開度を調整することによって、排気温度を上昇させ、ＨＣ濃度を増加させること無く触媒の暖機を促進することが可能である。なお、第３実施例や第４実施例で採用した制御は、燃料噴射の時期や噴射量とは直接的な関係が無いので、第１および第２実施例における制御と任意に組み合わせて利用することが可能である。
【０１０１】
Ｇ．変形例：
なお、この発明は上記の実施例や実施形態に限られるものではなく、その要旨を逸脱しない範囲において種々の態様において実施することが可能であり、例えば次のような変形も可能である。
【０１０２】
Ｇ１．変形例１：
また、本発明は、自動車に限らず、自動車以外の車両や、飛行機、船舶などの種々の移動体のエンジンにも適用可能である。
【０１０３】
Ｇ２．変形例２：
上記実施例では、エンジン冷却水温や触媒床温を用いて運転期間の切換を行っていたが、他の測定値を用いて運転期間の切換を行うようにしてもよい。また、ある運転期間が所定の長さに達したときに他の運転期間に切換えるようにしてもよい。
【０１０４】
Ｇ３．変形例３：
上記実施例では、温度センサを用いて触媒床温やエンジン冷却水温を直接測定していたが、これらの温度は間接的に測定してもよい。すなわち、触媒やエンジン冷却水以外の温度を測定して、これから触媒床温やエンジン冷却水温を推定してもよい。換言すれば、触媒の床温やエンジン冷却水温を実質的に示す温度を検出するための温度検出部を用いるようにしてもよい。
【０１０５】
Ｇ４．変形例４：
上記実施例で、採用した各種の設定値（例えば燃料噴射量や噴射時期）は、単なる例示であり、これ以外の種々の値を採用することが可能である。また、上述した各種の実施例に採用されている各種の運転を、様々に組み合わせて触媒の暖機を行うことが可能である。
【０１０６】
Ｇ５．変形例５：
上述した触媒暖機用の各種の運転（運転モード）は、暖機期間中にわたって継続して行われる必要はなく、少なくともその一部の期間において実行されていればよい。すなわち、本明細書において、「暖機期間において」あるいは「暖機期間中に」という文言は、特に断らない限り「暖機期間の少なくとも一部において」という意味を有している。
【図面の簡単な説明】
【図１】実施例のディーゼルエンジン１０の概略構成を表わす説明図。
【図２】ＥＧＲ率を次第に増加させていったときに、燃焼排ガス中の種々の物質の濃度が変化する様子を概念的に示した説明図。
【図３】空燃比（Ａ／Ｆ）を変化させたときに、燃焼排ガス中のスモークと燃費が変化する様子を概念的に示した説明図。
【図４】低温燃焼運転領域と通常燃焼運転領域とを示すグラフ。
【図５】触媒暖機運転の概要を示す説明図。
【図６】暖機制御の第１実施例におけるディーゼルエンジン１０の運転状態を示す説明図。
【図７】第１実施例における暖機期間中の燃焼室内の圧力変化と、熱発生量との関係を示す説明図。
【図８】触媒暖機運転の実行結果の一例を示すグラフ。
【図９】エンジン冷却水温に応じて噴射割合と噴射時期とを変更する場合の例を示すグラフ。
【図１０】暖機制御の第２実施例におけるディーゼルエンジン１０の運転状態を示す説明図。
【図１１】メイン噴射時期およびメイン噴射量と失火領域との関係を示す説明図。
【図１２】燃料噴射条件による排気ガス温度Ｔｇの変化を示す説明図。
【図１３】パイロット噴射量割合と排気ガス温度Ｔｇとの関係を示す説明図。
【図１４】燃焼室の縦断面を示す説明図。
【図１５】暖機制御の第３実施例におけるディーゼルエンジン１０の運転状態を示す説明図。
【図１６】ＶＮＴと吸気絞りの調整による触媒の暖機効果の説明図。
【図１７】暖機制御の第４実施例におけるディーゼルエンジン１０の運転状態を示す説明図。
【符号の説明】
１０…ディーゼルエンジン
１２…吸気管
１４…燃料噴射弁
１５…ノズル
１６…排気管
１８…燃料ポンプ
１９…コモンレール
２０…過給機
２１…タービン
２２…コンプレッサ
２３…シャフト
２４…インタークーラ
２５…入口ノズル
２６…アクチュエータ
２８…スロットル弁
３０…制御ユニット
４０…メイン触媒
４２…温度センサ
５０…圧力センサ
５２…エアフロメータ
６０…ＥＧＲ流路
６２…ＥＧＲ弁
６４…ＥＧＲクーラ
６６…ＥＧＲ触媒
７２…空燃比センサ
１１０…燃焼室
１１２…シリンダ壁
１１４…シリンダヘッド
１１６…ピストン
１２２…吸気ポート
１２４…排気ポート
１２６…グロープラグ
１３２…吸気弁
１３４…排気弁[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a technology for warming up a catalyst in a diesel engine.
[0002]
[Prior art]
In diesel engines, a catalyst is usually used to purify exhaust gas. Such a catalyst cannot exhibit a purifying function unless it reaches a certain temperature or higher. Therefore, usually, at the time of starting the diesel engine, a warm-up operation for increasing the bed temperature of the catalyst is performed.
[0003]
As a technique for warming up a catalyst in a diesel engine, there is a technique described in Patent Document 1, for example. In this technique, a second EGR passage for raising the temperature of the catalyst is provided separately from the normal first EGR passage. The first EGR passage recirculates the exhaust gas from the upstream side of the catalyst to the intake side, while the second EGR passage recirculates the exhaust gas from the downstream side of the catalyst to the intake side. . When the temperature of the catalyst is low, the exhaust gas is recirculated from the downstream side of the catalyst using the second EGR passage so that the entire amount of the exhaust gas discharged from the engine flows through the catalyst. The rise is hastening.
[0004]
[Patent Document 1] JP-A-11-229973
[Patent Document 2] JP-A-5-4448
[Patent Document 3] Japanese Patent Application Laid-Open No. 10-274086
[Patent Document 4] JP-A-10-274087
[Patent Document 5] Japanese Patent No. 3092569
[Patent Document 6] Japanese Patent No. 3331935
[0005]
[Problems to be solved by the invention]
However, if the normal operation is performed immediately after warming up the catalyst, the catalyst may be cooled by a large amount of exhaust gas, and the activity of the catalyst may be reduced. Therefore, there has been a demand for a technique capable of promoting warm-up of a catalyst and maintaining its activity.
[0006]
The present invention has been made to solve the above-described conventional problems, and has as its object to provide a technique capable of promoting warm-up of a catalyst and maintaining its activity.
[0007]
[Means for Solving the Problems and Their Functions and Effects]
In order to achieve the above object, the diesel engine of the present invention
A combustion chamber,
A fuel injection device for injecting fuel into the combustion chamber,
A catalyst for purifying exhaust gas from the combustion chamber,
A control unit that controls a plurality of devices of the diesel engine including the fuel injection device;
With
The control unit includes:
In order to warm up the catalyst and maintain the activity, the first warm-up operation mode is executed during the early warm-up period after the start of the diesel engine, and the second warm-up operation mode is performed during the late warm-up period after the early warm-up period. The machine operation mode,
The first warm-up operation mode is a mode in which the energy of exhaust gas flowing through the catalyst is higher than the energy of exhaust gas flowing through the catalyst in a normal operation mode of the diesel engine after the late warm-up period. ,
The second warm-up operation mode is an operation mode in which the temperature of the exhaust gas flowing through the catalyst is higher than the temperature of the exhaust gas flowing through the catalyst in the normal operation mode.
[0008]
During the early warm-up period, the energy of the exhaust gas flowing through the catalyst is higher than in the normal operation mode, so that the catalyst can be quickly activated. Further, during the late warm-up period, the temperature of the exhaust gas flowing through the catalyst is higher than in the normal operation mode, so that the activity of the catalyst can be maintained without excessive cooling.
[0009]
The second warm-up operation mode may be a mode in which the flow rate of exhaust gas flowing through the catalyst is smaller than any of the first warm-up operation mode and the normal operation mode.
[0010]
In the mode in which the flow rate of the exhaust gas flowing through the catalyst is small, the effect of cooling the catalyst is small even when the exhaust gas temperature is somewhat low, so that the activity of the catalyst can be maintained.
[0011]
In addition, the second warm-up operation mode may be an operation mode in which the excess air ratio in the combustion chamber is smaller than any of the first warm-up operation mode and the normal operation mode.
[0012]
In the operation mode in which the excess air ratio is small, the exhaust gas temperature is high, so that the activity of the catalyst can be efficiently maintained.
[0013]
The second warm-up operation mode may have a higher fuel efficiency than the first warm-up operation mode and a lower fuel efficiency than the normal operation mode.
[0014]
According to this configuration, the activity of the catalyst can be maintained without significantly deteriorating the fuel efficiency.
[0015]
The early warm-up period may be a period until the coolant temperature of the diesel engine reaches a predetermined temperature.
[0016]
Alternatively, the early warm-up period may be a period until the temperature of the catalyst reaches a predetermined temperature.
[0017]
According to these configurations, the catalyst can be sufficiently activated during the early warm-up period.
[0018]
The second warm-up operation mode may be executed when a predetermined operation condition including that a required load on the diesel engine is lower than a predetermined value is satisfied.
[0019]
In this configuration, in the second warm-up period, the second warm-up operation mode is executed only when the load is relatively low, so that the activity of the catalyst can be reduced without excessively sacrificing the engine operation performance and fuel efficiency. Can be maintained.
[0020]
Note that the second warm-up operation mode may be a low-temperature combustion mode.
[0021]
In the low-temperature combustion mode, the amount of flue gas tends to be smaller and the temperature of the flue gas tends to be higher than in the normal operation mode. Therefore, if the low-temperature combustion mode is adopted as the second warm-up operation mode, the activity of the catalyst can be maintained.
[0022]
In the first warm-up operation mode, the control unit controls the fuel injection device so as to perform (i) fuel injection in a divided manner into pilot injection and main injection, and (ii) injection of the pilot injection. The amount ratio is set to a value higher than the injection amount ratio of the pilot injection executed in the normal operation mode, and (iii) the injection timing of the main injection is set to the main injection executed in the normal operation mode. It is preferable to set a time point later than the injection amount timing.
[0023]
In this configuration, since the injection amount ratio of the pilot injection is set to a high value in the first warm-up operation mode in the early warm-up period, the temperature and pressure of the combustion chamber can be sufficiently increased before the main injection. . As a result, the fuel injected by the main injection can be sufficiently burned to suppress the HC concentration, and even if the injection timing of the main injection is retarded in order to increase the exhaust energy, no misfire occurs. The engine can be driven. As a result, it is possible to promote the warm-up of the catalyst while suppressing the HC concentration in the exhaust gas.
[0024]
In addition, the injection amount ratio of the pilot injection in the first warm-up operation mode is a value in a range of about 2 to about 5 times the injection amount ratio of the pilot injection executed in the normal operation mode. Is preferred.
[0025]
If the injection ratio of the pilot injection is set to a value within this range, it is possible to set the injection timing of the main injection sufficiently late so that the exhaust energy becomes sufficiently high.
[0026]
Preferably, the injection timing of the pilot injection in the first warm-up operation mode is set before compression top dead center, and the injection timing of the main injection is set after compression top dead center.
[0027]
According to this configuration, by setting the pilot injection before the compression top dead center, the temperature and pressure in the combustion chamber can be sufficiently increased before the main injection, and the main injection is set after the compression top dead center. By doing so, the exhaust energy can be increased.
[0028]
The diesel engine further includes a temperature detection unit for detecting a temperature substantially indicating a bed temperature of the catalyst, and the control unit controls the first warm-up in accordance with the catalyst bed temperature. The injection ratio of the pilot injection and the timing of the main injection in the machine operation mode may be adjusted.
[0029]
Alternatively, the diesel engine further includes a temperature detection unit for detecting a temperature substantially indicating a cooling water temperature of the combustion chamber, and the control unit determines the first temperature in accordance with the cooling water temperature. The injection ratio of the pilot injection and the timing of the main injection in the warm-up operation mode may be adjusted.
[0030]
According to these configurations, it is possible to set an appropriate fuel injection state according to the warm-up state, and thereby it is possible to more efficiently promote the warm-up of the catalyst.
[0031]
In addition, the injection amount ratio of the pilot injection and the injection timing of the main injection in the first warm-up operation mode may be set such that the main injection corresponds to a time immediately before a misfire limit.
[0032]
If the main injection timing is set immediately before the misfire limit, exhaust energy can be increased as much as possible without misfire. Therefore, warming up of the catalyst can be further promoted.
[0033]
The timing immediately before the misfire limit is preferably in the range of the crank angle from the misfire limit to about 2 ° before the misfire limit.
[0034]
In this range, the exhaust energy can be sufficiently increased.
[0035]
The diesel engine further includes an EGR device for recirculating exhaust gas from the combustion chamber to an intake passage to the combustion chamber, and a glow plug provided at an intake port of the combustion chamber. In the first warm-up operation mode, the control unit includes: (a) an EGR cut operation for stopping the recirculation of exhaust gas by the EGR device; and (b) a continuous intake air heating operation by the glow plug. (C) at least one of an idling speed increasing operation for increasing the idling speed of the diesel engine to be higher than the idling speed in the normal operation mode may be executed.
[0036]
Since these three types of operations all contribute to an increase in exhaust energy, it is possible to further promote warm-up of the catalyst.
[0037]
The diesel engine further includes a supercharger that compresses intake air supplied to the combustion chamber using energy of exhaust gas from the combustion chamber, and the supercharger is supplied to the supercharger. A variable supercharger that can adjust the supercharging pressure of the intake air by adjusting the operating pressure of the exhaust gas may be used. At this time, in the first warm-up operation mode, the control unit may make the operating pressure of the exhaust of the variable supercharger higher than the operating pressure in the normal operation mode.
[0038]
When the operating pressure of the exhaust of the variable supercharger is increased, the pump loss increases and the exhaust energy increases. As a result, it is possible to promote warm-up of the catalyst without increasing the HC concentration.
[0039]
The diesel engine further includes an intake throttle valve for narrowing an intake flow path to the combustion chamber, and the control unit further controls a throttle amount of the intake throttle valve in the first warm-up operation mode. The throttle amount in the normal operation mode may be increased.
[0040]
Increasing the intake throttle amount increases the pump loss, thereby increasing the exhaust energy. In addition, when the intake air is throttled, the intake air amount decreases, so that the exhaust gas temperature increases. As a result, it is possible to further promote warm-up of the catalyst without increasing the HC concentration.
[0041]
The diesel engine further includes an intake throttle valve for narrowing an intake passage to the combustion chamber, and the control unit continuously opens and closes the intake throttle valve in the first warm-up operation mode. You may do so.
[0042]
In this diesel engine, since the intake throttle valve is continuously opened and closed, the exhaust energy can be increased by the mechanical energy. As a result, it is possible to promote warm-up of the catalyst without increasing the HC concentration.
[0043]
The present invention can be realized in various modes, for example, a diesel engine (compression ignition type internal combustion engine), a catalyst warm-up control device or method for a diesel engine, a vehicle using a diesel engine or It can be realized in a mode such as a moving body.
[0044]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described in the following order based on examples.
A. Overview of device configuration and operation:
B. Concept of promoting catalyst warm-up and maintaining activity when starting the engine:
C. First embodiment of warm-up control:
D. Second embodiment of warm-up control:
E. FIG. Third embodiment of warm-up control:
F. Fourth embodiment of warm-up control:
G. FIG. Modified example
[0045]
A. Overview of device configuration and operation:
FIG. 1 is an explanatory diagram illustrating a schematic configuration of a diesel engine 10 according to an embodiment of the present invention. The diesel engine 10 is a so-called four-cylinder engine and has four combustion chambers # 1 to # 4. Air is supplied to each combustion chamber via an intake pipe 12. The fuel injection device includes a fuel pump 18, a common rail 19, and a fuel injection valve 14. The fuel is pressurized by the fuel pump 18 and distributed to the fuel injection valves 14 of each combustion chamber via the common rail 19. The fuel injection valve 14 is controlled by the control unit 30 to inject fuel into each combustion chamber at an appropriate injection timing and injection amount. The exhaust gas generated by the combustion is discharged to the exhaust pipe 16. Note that the diesel engine 10 of the present embodiment is used as a prime mover for generating a driving force for a vehicle.
[0046]
The engine 10 is provided with a supercharger 20. The supercharger 20 adjusts the opening area of the turbine 21 provided in the exhaust pipe 16, the compressor 22 provided in the intake pipe 12, the shaft 23 connecting the two, and the inlet nozzle 25 of the turbine 21. Actuator 26. When the combustion exhaust gas discharged from the combustion chamber turns the turbine 21 of the supercharger 20, the compressor 22 rotates via the shaft 23, compresses the air, and supplies the air to each combustion chamber. The supercharger 20 is a variable supercharger in which the driving pressure of exhaust gas can be changed by an actuator 26, and is generally called a variable nozzle turbo (VNT) or a variable capacity turbo.
[0047]
An air cleaner (not shown) is provided upstream of the compressor 22. The compressor 22 compresses the air taken in through the air cleaner and supplies the compressed air to each combustion chamber. An intercooler 24 is provided downstream of the compressor 22. The air compressed by the compressor 22 is supplied to each combustion chamber after being cooled by the intercooler 24. Downstream of the intercooler 24, a throttle valve 28 (an intake throttle valve) is provided.
[0048]
A main catalyst 40 is provided in the exhaust pipe 16 on the downstream side of the turbine 21. As the main catalyst 40, for example, a NOx occlusion reduction type catalyst that can remove both NOx and particulate matter such as soot and SOF (Soluble Organic Fraction) contained in the combustion exhaust gas is used.
[0049]
The NOx storage reduction catalyst accumulates NOx when the air-fuel ratio when the combustion reaction proceeds in the combustion chamber is smaller than the stoichiometric air-fuel ratio (so-called lean case). When the air-fuel ratio is higher than the stoichiometric air-fuel ratio (so-called rich case) or at the stoichiometric air-fuel ratio (so-called stoichiometry), the accumulated NOx is released. By such an operation, NOx in the combustion exhaust gas is purified. Further, the NOx storage reduction type catalyst also has an activity as an oxidation catalyst. By oxidizing soot and SOF in the flue gas on the catalyst, the flue gas is purified.
[0050]
The exhaust pipe 16 and the intake pipe 12 are connected by an EGR flow path 60, and a part of the combustion exhaust gas can be recirculated to the intake pipe 12 via the EGR flow path 60. The amount of exhaust gas returning to the intake pipe 12 is controlled by adjusting the opening of the EGR valve 62 and the throttle valve 28. An EGR cooler 64 is further provided in the EGR passage 60, and the EGR cooler 64 cools the combustion exhaust gas before being introduced into the intake pipe 12. An EGR catalyst 66 is provided upstream of the EGR cooler 64. The EGR catalyst 66 includes an oxidation catalyst, and oxidizes and removes particulate matter such as SOF in the combustion exhaust gas introduced into the intake pipe 12. Note that the EGR catalyst 66 may be omitted.
[0051]
The control unit 30 receives measured values from sensors for detecting the engine speed Ne, the accelerator depression amount L, and the engine cooling water temperature Ten, respectively, and according to these measured values, the fuel pump 18 and the fuel It controls the injection valve 14, the EGR valve 62, the throttle valve 28, and the like. Various other sensors are provided as sensors. For example, the common rail 19 is provided with a pressure sensor 50 for detecting the pressure of the fuel. An air flow meter 52 for detecting the amount of intake air is provided upstream of the compressor 22 in the intake pipe 12. The exhaust pipe 16 is provided with an air-fuel ratio sensor 72 for detecting an air-fuel ratio. Further, the main catalyst 40 is provided with a temperature sensor 42 for detecting a catalyst bed temperature.
[0052]
FIG. 2 shows that when the EGR rate ([EGR gas amount] / [EGR gas amount + intake air amount]) is gradually increased, the NOx concentration, smoke, CO (carbon monoxide) concentration, FIG. 4 is an explanatory diagram conceptually showing a state in which the concentration of HC (unburned hydrocarbon-based compound) changes. In the example of FIG. 2, the combustion injection timing is fixed. Here, smoke is an index indicating the concentration of suspended carbon-containing particles such as soot in exhaust gas, and is measured by a dedicated measuring device called a smoke meter. When the carbon dioxide-containing suspended particulates such as soot are not contained in the exhaust gas at all, the smoke value is 0, and the smoke value increases as the concentration of the particulates increases. In the example of FIG. 2, the smoke value starts to increase around the time when the EGR rate exceeds 40%, but when the EGR rate is further increased and exceeds about 60%, almost no smoke is generated. The NOx concentration decreases as the EGR rate increases. That is, when the EGR rate is sufficiently increased, the NOx emission amount can be reduced to almost 0 (at most about 10 ppm) and the smoke can be reduced to almost 0. In this specification, a combustion mode in which the EGR rate is set to a value of 60% or more to reduce smoke and NOx concentration is referred to as “low temperature combustion”. A normal combustion mode performed at a lower EGR rate than low-temperature combustion is referred to as “normal combustion”. In normal combustion, the EGR rate is suppressed to 50% or less. The name "low temperature combustion" is derived from the fact that in this combustion mode, the local combustion temperature in the combustion chamber is suppressed to a considerably lower temperature than usual by the action of EGR gas. It is known that when the local combustion temperature is low, the amounts of smoke and NOx generated tend to be reduced. The low-temperature combustion is described in detail, for example, in Japanese Patent No. 3092569 disclosed by the present applicant.
[0053]
As shown at the top of FIG. 2, the air-fuel ratio decreases as the EGR rate increases. Assuming that the amount of intake air (the sum of the amount of intake air and the amount of EGR gas) supplied to the combustion chamber in one intake process is constant, the amount of intake air decreases as the amount of EGR gas increases. Since the oxygen concentration in the EGR gas is lower than the oxygen concentration in normal air, the air-fuel ratio approaches the rich side as the EGR gas amount increases, that is, as the EGR rate increases. As shown in FIG. 2, since the EGR rate is higher in the low-temperature combustion than in the normal combustion, the air-fuel ratio is shifted to a rich side as compared with the normal combustion.
[0054]
Diesel engine combustion usually occurs under lean air-fuel ratios. Also when performing EGR, the air-fuel ratio is kept lean during normal combustion. When the EGR gas amount is further increased, combustion can be performed under the condition that the air-fuel ratio becomes the stoichiometric air-fuel ratio (stoichiometry). Strictly, the air-fuel ratio also depends on the composition of the fuel, but the stoichiometric air-fuel ratio takes a value in the vicinity of 14.7 to 14.8 for most fuels. When the EGR gas amount is further increased from the state of the stoichiometric air-fuel ratio, the air-fuel ratio becomes rich.
[0055]
FIG. 3 shows various combustion-related characteristics when the air-fuel ratio (A / F) is changed by changing the EGR rate (amount of smoke in combustion exhaust gas, fuel efficiency of a diesel engine, a catalyst bed temperature, and a temperature of a gas containing a catalyst). FIG. 4 is an explanatory diagram conceptually showing a state of changing. Here, the catalyst bed temperature and the catalyst-containing gas temperature represent the catalyst bed temperature of the main catalyst 40 in FIG. 1 and the temperature of the combustion exhaust gas flowing into the catalyst, respectively. As shown in FIG. 3, low-temperature combustion is classified into lean low-temperature combustion performed leaner than the stoichiometric air-fuel ratio and rich low-temperature combustion performed richer than the stoichiometric air-fuel ratio.
[0056]
Lean low-temperature combustion can sufficiently suppress smoke, and also exhibits sufficiently excellent fuel economy as compared with normal combustion. Note that, when performing lean low-temperature combustion, as shown in FIG. 2, the HC concentration and the CO concentration in the combustion exhaust gas are higher than in the normal combustion. However, in low-temperature combustion, there is a tendency that the amount of flue gas is smaller and the temperature of flue gas is higher than in normal combustion. Therefore, when performing low-temperature combustion, the catalyst bed temperature can be maintained higher (FIG. 3). By keeping the catalyst bed temperature high, it is possible to easily reduce the HC concentration and the CO concentration in the exhaust gas discharged to the outside. As described above, lean low-temperature combustion is an excellent combustion mode in which extremely clean exhaust gas can be discharged when combined with the main catalyst 40.
[0057]
As described above, the low temperature combustion is a combustion mode in which a large amount of EGR gas is circulated to lower the combustion temperature. Therefore, low-temperature combustion can be adopted only when the load is low. That is, in order to operate the engine with a higher load, it is necessary to increase the fuel injection amount and the intake air amount. Here, since the total value of the amount of air sucked into the combustion chamber in one intake and the amount of exhaust gas recirculated to the intake side does not change in principle, if the amount of air increases, the amount of exhaust gas recirculated will increase. It decreases by that much. Therefore, when the load is high, the EGR rate cannot be maintained at a value high enough to establish low-temperature combustion. Therefore, at the time of medium to high load, normal combustion is performed. NOx generated by performing normal combustion at medium to high loads is stored in the main catalyst, which is a NOx storage reduction catalyst. The NOx thus occluded is reduced by HC and CO discharged when low-temperature combustion is performed under a low load.
[0058]
FIG. 4 is a graph showing a low-temperature combustion operation region and a normal combustion operation region. As shown in FIG. 4, low-temperature combustion can be performed when the engine operating condition is in a relatively low output region R1, and normal combustion is performed when the engine operation condition is in a relatively high output region R2. However, it is also possible to execute normal combustion in the low-temperature combustion operation region. Therefore, in the low temperature combustion operation region, either low temperature combustion or normal combustion is selected as necessary. More specifically, a map in which the exhaust gas air-fuel ratio to be controlled is set using the engine speed and the required torque of the engine as parameters is stored as a low-temperature combustion map and a normal combustion map, respectively. In addition, in the selected combustion mode, control is performed so that the air-fuel ratio detected by the air-fuel ratio sensor 72 attached to the exhaust pipe 16 becomes the target air-fuel ratio. Such a map is stored in the ROM (not shown) of the control unit 30.
[0059]
Note that rich low-temperature combustion is not normally used because it is a combustion mode in which fuel efficiency deteriorates as shown in FIG. 3, but since the catalyst bed temperature can be raised as shown in FIG. This is usually done for a short time. That is, when the normal combustion is continued, the catalyst exhaust gas temperature is low, the catalyst bed temperature gradually decreases, and there is a possibility that the catalyst activity decreases. The catalyst activity can be restored by raising the catalyst bed temperature.
[0060]
B. Concept of promoting catalyst warm-up and maintaining activity when starting the engine:
The warm-up of the catalyst 40 is performed by the heat of the exhaust gas passing through the catalyst 40. Here, if a turbulent flow model in the pipe is adopted as the flow of the exhaust gas in the catalyst 40, the heat transfer amount Q from the exhaust gas to the catalyst 40 is given by the following equation (1).
[0061]

Here, C1 and C2 are constants, Re is the Reynolds number, Pr is the Prandtl number, Tg is the exhaust gas temperature, Tc is the catalyst temperature, and V is the exhaust gas flow rate. Since the Prandtl number is determined only by the physical properties of the fluid, it is represented by a constant C2. The Reynolds number Re is proportional to the flow rate V of the exhaust gas.
[0062]
As the heat transfer amount Q increases, the warm-up of the catalyst 40 is promoted. Therefore, the following two means can be adopted to promote the warm-up of the catalyst 40.
(A) The temperature Tg of the exhaust gas is increased.
(B) Increase the flow rate V of the exhaust gas.
[0063]
Various embodiments described below utilize at least one of these two means to promote warm-up of the catalyst 40. Further, even after the catalyst 40 has reached the activation temperature, an operation for maintaining the activity is executed for a while. In the operation for maintaining the catalyst activity, means for increasing the temperature Tg of the exhaust gas is used.
[0064]
FIG. 5 shows an outline of catalyst warm-up in the embodiment of the present invention. The horizontal axis represents the elapsed time from the start (cold start), and the vertical axis represents the vehicle speed and the catalyst bed temperature. As shown at the top of the graph, the warm-up period is divided into an early warm-up period (early warm-up phase) and a late warm-up period (late warm-up phase). The change in vehicle speed in this example conforms to a predetermined vehicle performance test pattern.
[0065]
The early warm-up period t0 to t1 is a period for raising the catalyst bed temperature from room temperature to an activation temperature (for example, about 250 ° C.). During the early warm-up period, a catalyst warm-up promoting operation mode (described later) is executed, whereby the catalyst bed temperature reaches the activation temperature at the end of the early warm-up period. The late warm-up period t1 to t2 is a period for maintaining the catalyst bed temperature at or above the activation temperature. In the latter warm-up period, when a predetermined operating condition including that a required load on the engine is lower than a predetermined value is satisfied, a catalyst activation maintaining operation mode (described later) is executed. In the example of FIG. 5, as shown in the upper part of the figure, in the catalyst activity maintaining operation mode, the operating conditions in which the required load of the engine is low and the rotation speed is almost constant (specifically, at the time of constant speed driving and idling) The catalyst activity maintaining operation mode is executed.
[0066]
After the late warm-up period (after time t2), normal operation is performed. At the end of the late warm-up period, the catalyst bed temperature has risen sufficiently, and the temperature of the engine coolant and the exhaust passage have also risen sufficiently. Therefore, the catalyst bed temperature can be maintained at the activation temperature or higher even when the normal operation is performed without performing the catalyst warm-up promotion operation mode or the catalyst activation operation mode. In this specification, an operation mode normally performed after the late warm-up period is referred to as a “normal operation mode”. In addition, the early warm-up period and the late warm-up period may be simply referred to as a “warm-up period”.
[0067]
As the catalyst warm-up promotion operation mode during the early warm-up period, an operation mode in which the energy of exhaust gas flowing through the catalyst is higher than in the normal operation mode after the warm-up period can be adopted. Here, the phrase "compared to the normal operation mode" means "compared to the case of the engine load and the rotation speed substantially equal to those in the normal operation mode". A specific example of the catalyst warm-up promotion operation mode will be described later in detail. As the catalyst activity maintaining operation mode during the late warm-up period, an operation mode in which the temperature of exhaust flowing through the catalyst is higher than in the normal operation mode after the warm-up period can be adopted. For example, the low-temperature combustion described in FIGS. 2 and 3 can be adopted as the catalyst activity maintaining operation mode.
[0068]
In order to raise the exhaust gas temperature in the catalyst activity maintaining operation mode, it is generally sufficient to lower the excess air ratio (that is, lower the air-fuel ratio) as compared with the normal operation mode. In particular, as the catalyst activity maintaining operation mode, it is preferable to reduce the excess air ratio more than any of the normal operation mode and the catalyst warm-up promotion operation mode. The reason is that if the excess air ratio is high, the exhaust temperature is reduced by a large amount of air, and conversely, if the excess air ratio is reduced, the exhaust temperature can be increased. As shown in FIG. 3, the low-temperature combustion has a considerably low air-fuel ratio, and the exhaust gas temperature becomes higher than in the normal operation mode. Further, a combustion mode other than the low-temperature combustion is adopted as the catalyst activity maintaining operation mode, and in this case, the exhaust gas temperature can be increased by lowering the excess air ratio from that in the normal operation mode after the warm-up period. Means for reducing the excess air ratio include a decrease in the opening of the throttle valve 28 (FIG. 1) and an increase in the EGR ratio, and these two can be employed simultaneously.
[0069]
Further, as the catalyst activity maintaining operation mode, a mode in which the amount of exhaust gas flowing through the catalyst is reduced as compared with the normal operation mode can be adopted. In particular, as the catalyst activity maintaining operation mode, it is preferable to reduce the amount of exhaust gas flowing through the catalyst more than in either the normal operation mode or the catalyst warm-up promotion operation mode. This can prevent the catalyst from being cooled by a large amount of exhaust gas, so that the activity of the catalyst can be maintained. As a means for reducing the amount of exhaust gas, a reduction in the opening degree of the throttle valve 28 or an increase in the EGR rate can be used.
[0070]
Since the catalyst activity maintaining operation mode is intended to maintain the activation temperature of the catalyst 40, the energy given to the catalyst 40 may be smaller than in the catalyst warm-up promotion operation mode. Therefore, in the catalyst activity maintaining operation mode, although the fuel efficiency is slightly worse than the normal operation mode, it is preferable to set the operation mode in which the fuel efficiency is better (the fuel efficiency is higher) than the catalyst warm-up promotion operation mode.
[0071]
C. First embodiment of warm-up control:
FIG. 6 is an explanatory diagram illustrating an operation state of the diesel engine 10 in the first embodiment of the warm-up control. The horizontal axis in FIG. 5 is the time from the start (cold start), and the vertical axis is the engine coolant temperature, the fuel injection timing, and the EGR rate in order from the top. When the engine is started at time t0, a catalyst warm-up promotion operation (catalyst warm-up combustion) for warming up the catalyst 40 is performed in an early warm-up period until time t1 when the engine coolant temperature reaches the predetermined temperature CAton. Done. The early warm-up period is set as a period until the engine coolant temperature reaches a predetermined temperature CAton. Alternatively, it may be a period until the catalyst bed temperature reaches a predetermined temperature. During the late warm-up period from time t1 to time t2, the catalyst activity maintaining operation mode and the normal operation mode are performed. In the present embodiment, the low-temperature combustion described in FIGS. 2 and 3 is performed in the catalyst activity maintaining operation mode, and the normal combustion is performed in the normal operation mode. After time t2, the normal operation mode (normal combustion) after warm-up is performed. In FIG. 6, the period of the low-temperature combustion and the period of the normal combustion in the late warm-up period t1 to t2 are illustrated in a simplified manner for convenience of illustration.
[0072]
During the early warm-up period t0 to t1, the fuel injection is divided into pilot injection and main injection. The injection amount of the pilot injection is set to about 30% of the total injection amount in one combustion cycle, and the injection amount of the main injection is set to about 70%. The injection timing of the pilot injection is about 10 ° before the compression top dead center TDC (that is, TDC minus about 10 °), and the injection timing of the main injection is about 10 ° behind the compression top dead center TDC (that is, TDC + about 10 °). In addition, in this specification, "injection timing" means injection start timing. The meaning of the set values of the injection amount and the injection timing during the warm-up period will be described later.
[0073]
During the early warm-up period, the EGR rate is set to zero. That is, the EGR valve 62 is closed, and the recirculation of the combustion exhaust gas is cut. The reason for this is that if EGR is performed during the early warm-up period, the exhaust gas temperature is reduced by the EGR cooler 64 (FIG. 1), so that the warm-up may be suppressed. As can be understood, the EGR cut operation during the warm-up period can be used as a means for increasing the exhaust gas temperature Tg in the above-described equation (1). However, a certain amount of EGR may be performed during the warm-up period. For example, the EGR rate may be set to 10 to 20% during acceleration. By setting the EGR rate to such a low value, warming up of the catalyst 40 can be promoted, and an excessive increase in NOx due to the EGR cut can be prevented.
[0074]
During the late warm-up period t1 to t2, low-temperature combustion is performed under relatively low-load operation conditions, and normal combustion is performed under relatively high-load operation conditions. As described with reference to FIG. 3, the low-temperature combustion has a higher catalyst-input gas temperature than the normal combustion, so that the activity of the catalyst 40 can be maintained. In addition, since the catalyst 40 has reached the activation temperature during the early warm-up period, it is not necessary to continuously perform low-temperature combustion (catalyst activation maintaining operation) during the late warm-up period. It is. On the other hand, in the early warm-up period t0 to t1, it is preferable to continuously perform the catalyst warm-up promotion operation regardless of the engine load. The end time of the late warm-up period can be determined simply by measuring the time of the late warm-up period. Alternatively, the late warm-up period may be ended when the catalyst bed temperature or the engine cooling water temperature reaches a predetermined value.
[0075]
The pilot injection and the main injection are also performed during the normal combustion during the late warm-up period, similarly to the catalyst warm-up combustion during the early warm-up period. However, during normal combustion, the injection amount of the pilot injection is set to about 10% of the total injection amount in one combustion cycle, and the injection amount of the main injection is set to about 90%. Further, the injection timing of the pilot injection is (TDC−about 15 °), and the injection timing of the main injection is (TDC + about 5 °). In the normal operation mode after time t2, the same normal combustion as during the latter warm-up period is performed.
[0076]
FIG. 7A shows a pressure change in the combustion chamber during the early warm-up period, and FIG. 7B shows a change in the amount of heat generated. The horizontal axis in FIGS. 7A and 7B is the crank angle θ. During the early warm-up period, since both the pilot injection and the main injection are performed at a point away from the compression top dead center TDC, the pressure rise rate near the compression top dead center TDC is low. Therefore, the so-called isocapacity is reduced. In a diesel engine, it is generally known that when the isocapacity decreases, the thermal efficiency decreases and the exhaust energy increases. Specifically, the fuel injected by the pilot burns before compression top dead center TDC to generate heat, but this heat does little work to the outside and increases the temperature and pressure in the combustion chamber. Used. Since the main injection is also performed at a point far away from the compression top dead center TDC, a significant part of the heat generation is used for increasing the temperature of the exhaust gas.
[0077]
The injection timing and the injection amount of the pilot injection and the main injection during the early warm-up period are determined in consideration of the following matters. In order to increase the exhaust energy, it is desirable that the main injection be performed at a time as late as possible after the compression top dead center TDC (that is, as greatly retarded as possible). However, if the main injection is retarded too much, there is a possibility of misfire. Therefore, when the injection amount of the pilot injection is increased, the temperature of the combustion chamber increases, so that the possibility of misfiring when the main injection is retarded can be reduced. In addition, the interval between the injection timings of the pilot injection and the main injection has a certain desirable value (for example, 20 ° in crank angle). Therefore, the injection timings of the pilot injection and the main injection are determined so that the possibility of misfiring is low and the injection timing of the main injection is delayed as much as possible. Further, the injection timing and the injection amount of the pilot injection and the main injection are determined so as to include the increase in the exhaust energy and to satisfy the required load. As a result, during the early warm-up period, the injection amount of the pilot injection is set to be larger and the injection timing of the main injection is set later than in the normal combustion.
[0078]
Note that, under the operating condition of equal load, the total injection amount is larger in the catalyst warm-up combustion during the early warm-up period than in the normal combustion. The injection ratio of the pilot injection during the early warm-up period (pilot injection amount / total injection amount) is preferably about 2 to about 5 times the injection ratio of the pilot injection in the normal combustion.
[0079]
As can be understood from the above description, the fuel injection during the early warm-up period of the first embodiment has the following features.
(1) The injection amount ratio of the pilot injection during the early warm-up period is higher than the injection amount ratio of the pilot injection executed in the normal combustion after the early warm-up period. (2) The injection timing of the main injection during the early warm-up period is later than the injection amount timing of the main injection executed in the normal combustion after the warm-up period.
[0080]
By performing such control, the main injection can be considerably retarded, the exhaust energy can be increased, and the catalyst 40 can be warmed up more efficiently. Further, since the injection amount of the pilot injection is large, the temperature and the pressure in the combustion chamber are considerably increased during the main injection. For this reason, the fuel injected by the main injection sufficiently burns, so that the HC concentration in the combustion exhaust gas can be reduced. That is, in the first embodiment, it is possible to efficiently warm up the catalyst while suppressing the HC concentration.
[0081]
Although there is some flexibility in the timing of the pilot injection and the main injection, in order to warm up the catalyst, the pilot injection is performed before the compression top dead center, and the main injection is performed after the compression top dead center. Is preferred. By doing so, it is possible to more efficiently warm up the catalyst while suppressing the HC concentration.
[0082]
FIG. 8 shows an execution example of the warm-up operation (catalyst warm-up promotion operation and catalyst activity maintaining operation) according to the first embodiment. The change in vehicle speed is the same as that shown in FIG. When the warm-up operation is performed, the catalyst bed temperature has reached the activation temperature at the end time t1 of the early warm-up period after the elapse of about 70 seconds from the start, and is maintained at or above the activation temperature thereafter. On the other hand, when the warm-up operation is not performed, the activation temperature has been reached about 1000 seconds after the start. As can be understood, by performing the catalyst warm-up operation of the first embodiment, the catalyst 40 can be activated in a short time, and the activity can be maintained thereafter.
[0083]
Note that it is also possible to perform normal combustion (normal operation mode) after the catalyst reaches the activation temperature without performing the late warm-up period. However, as described with reference to FIG. 3, in normal combustion, the temperature of the gas containing the catalyst is lower than in low-temperature combustion, and the EGR rate is lower than in low-temperature combustion, so that the amount of exhaust gas passing through the catalyst 40 is large. Therefore, if the operation is performed only by the normal combustion immediately after the early warm-up period, there is a possibility that the catalyst 40 is cooled by a large amount of exhaust gas and becomes lower than the activation temperature, as shown by a dashed line in FIG. On the other hand, in the present embodiment, during the latter warm-up period, low-temperature combustion is performed under relatively low-load operating conditions, so that the activity of the catalyst 40 can be maintained.
[0084]
In the example of FIG. 6, during the early warm-up period, the injection ratio between the pilot injection and the main injection and the injection timing are kept constant. However, these are changed according to the degree of warm-up. You may. FIG. 9 is a graph illustrating an example in which the injection ratio and the injection timing are changed according to the engine coolant temperature. In this example, as the engine coolant temperature rises, the injection ratio of the pilot injection decreases, while the injection timing of the main injection advances. In other words, the adjustment is performed so as to approach the normal combustion injection state as the engine coolant temperature rises. By doing so, it is possible to warm up the catalyst with an appropriate injection injection according to the degree of warm-up. Note that, instead of the engine coolant temperature, the injection ratio between the pilot injection and the main injection and the injection timing may be changed according to the catalyst bed temperature.
[0085]
D. Second embodiment of warm-up control:
FIG. 10 is an explanatory diagram illustrating an operation state of the diesel engine 10 in the second embodiment of the warm-up control. In the second embodiment, as described below, the injection timing and the injection amount of the pilot injection and the main injection during the early warm-up period are more optimized than in the first embodiment.
[0086]
FIG. 11 is an explanatory diagram showing the relationship between the main injection timing, the main injection amount, and the misfire area. As can be seen from this figure, if the main injection timing is retarded at a certain main injection amount, the main injection amount exceeds the misfire limit and enters the misfire region. However, the smaller the main injection amount, the more the misfire limit crank angle shifts to the retard side. The solid line in FIG. 11 shows the relationship between the main injection timing and the injection amount for generating the same torque when the pilot injection amount is constant. Generally, when the injection timing of the main injection is retarded, the thermal efficiency decreases. Therefore, under the condition that the pilot injection amount is maintained at a constant value, the more the injection timing of the main injection is retarded, the more the main injection amount necessary to generate the same torque increases. When the pilot injection amount is increased, the same torque can be generated even if the main injection timing is retarded with the same main injection amount.
[0087]
By the way, in the critical region near the misfire limit, the main injection is sufficiently retarded, so that the exhaust gas temperature is increased. Exhaust gas temperature in the critical region depends on various fuel injection conditions. FIG. 12 is an explanatory diagram showing a change in the exhaust gas temperature Tg depending on the fuel injection conditions. The horizontal axis in FIG. 12 is the main injection timing, and the vertical axis is the temperature of the combustion exhaust gas. The solid line is a graph when the pilot injection ratio Qp (the ratio of the pilot injection amount to the total injection amount) is 0%, the one-dot chain line is a graph when this is 20%, and the two-dot chain line is a graph when it is 40%. As can be understood from this figure, when the pilot injection amount ratio Qp is increased from 0% to 20%, the main injection timing at the misfire limit is retarded, so that the exhaust gas temperature Tg in the critical region of the misfire limit also increases. . However, if the pilot injection amount ratio Qp is excessively increased (for example, to 40%), the ratio of the main injection amount decreases, and conversely, the exhaust gas temperature Tg decreases. FIG. 13 rewrites the relationship of FIG. 12 into the relationship between the pilot injection amount ratio and the exhaust gas temperature Tg. As can be understood from this figure, when the main injection condition is set in the critical region of FIG. 11, there is an optimum condition of the pilot injection amount ratio at which the exhaust gas temperature Tg becomes highest. Such optimum injection conditions are experimentally determined for each engine. Since the optimum injection condition also depends on the temperature of the engine, it is preferable to change the optimum injection condition according to, for example, the temperature of the engine cooling water.
[0088]
When the sum of the main injection amount and the pilot injection amount is constant, it is possible to optimize the injection conditions in consideration of the following two cases. That is, when the pilot injection amount is large, the main injection amount is small but the timing of the main injection can be delayed. On the other hand, when the pilot injection amount is small, the main injection amount increases, but the timing of the main injection needs to be advanced. The optimum conditions for the pilot injection and the main injection at which the exhaust gas temperature Tg becomes highest are determined in consideration of the balance between these two cases. That is, the optimum condition is intermediate between the condition where the pilot injection amount is the maximum possible amount and the condition where the pilot injection amount is the minimum possible amount within a range where misfire does not occur. Therefore, by searching for an intermediate point between these two extreme conditions, it is possible to find the optimal condition.
[0089]
As described above, in the second embodiment, during the early warm-up period, the injection amount ratio of the pilot injection and the injection timing of the main injection are set in the critical region immediately before the misfire limit so that the exhaust gas temperature Tg becomes the highest. It is set to correspond to the period of. As a result, the exhaust energy is increased, and the warm-up of the catalyst can be efficiently promoted. The range of the critical region is preferably a range from the misfire limit to approximately 2 ° before the misfire limit at the crank angle, and more preferably a range from the misfire limit to approximately 1 ° before the misfire limit. .
[0090]
In the second embodiment, as means for promoting the temperature rise of the exhaust gas, in addition to the adjustment of the injection condition, three means of EGR cut operation, increase in idle speed, and continuous use of the glow plug are provided. Is used.
[0091]
As shown in FIG. 10, in the second embodiment, similarly to the first embodiment, the EGR is cut during the early warm-up period. As a result, it is possible to prevent the exhaust gas temperature from being lowered due to the heat loss in the EGR cooler 64 (FIG. 1), and to promote the warm-up of the catalyst. In the example of FIG. 10, the idle speed is set to 1200 rpm during the early warm-up period, and is set to 950 rpm after the early warm-up period. As described above, when the idling speed is increased during the early warm-up period, the exhaust gas flow rate V in the above-described equation (1) can be increased, and thereby the warm-up of the catalyst 40 can be promoted. is there.
[0092]
As shown in the lowermost part of FIG. 10, during the early warm-up period, the glow plug is further continuously used. FIG. 14 shows a longitudinal section of the combustion chamber. The combustion chamber 110 includes a cylinder wall 112, a cylinder head 114, and a piston 116. The cylinder head 114 has an intake port 122 and an exhaust port 124. The intake port 122 is provided with an intake valve 132, and the exhaust port 124 is provided with an exhaust valve 134. Further, a fuel injection valve 14 is provided substantially at the center of the combustion chamber of the cylinder head 114, and a glow plug 126 is provided at a position adjacent to the nozzle 15 of the fuel injection valve 14. The glow plug 126 only needs to be arranged at a position where the intake air can be heated. For example, the glow plug 126 may be provided at the intake port 122.
[0093]
The glow plug 126 is normally used only for a very short time at the time of starting in order to improve the startability of the engine 10. On the other hand, in the second embodiment, during the early warm-up period, the glow plug 126 is kept on for a long time. By doing so, it is possible to further increase the temperature of the exhaust gas and promote the warm-up of the catalyst 40.
[0094]
The three types of operations of the EGR cut operation, the continuous intake heating operation using the glow plug, and the idling speed increasing operation do not need to be performed at the same time. It is preferable that the process is executed at least for a part of the period. The degree of these operations is preferably changed depending on the temperature of the engine cooling water or the temperature rise of the catalyst, and these operations may be stopped when it becomes unnecessary during the early warm-up period.
[0095]
E. FIG. Third embodiment of warm-up control:
FIG. 15 is an explanatory diagram illustrating an operation state of the diesel engine 10 in the third embodiment of the warm-up control. In the third embodiment, the timing and amount of fuel injection are the same as those in the first embodiment shown in FIG. 6, but the inlet nozzle 25 (FIG. 1) of the variable nozzle turbo (VNT) 20 and the intake throttle (throttle valve) 28) is characterized in that the opening is narrowed during the early warm-up period. After the early warm-up period, the VNT and the opening degree of the intake throttle are adjusted in accordance with the load request, but are simplified in the example of FIG.
[0096]
FIG. 16 illustrates the catalyst warm-up effect by adjusting the VNT and the intake throttle. When the nozzle opening of the VNT is reduced, the back pressure of the engine (the pressure of the exhaust pipe 16) increases (block B2). Further, when the opening degree of the intake throttle is reduced, the intake pressure is reduced. The increase in engine back pressure and the decrease in intake pressure cause an increase in engine pump loss (block B3), increasing the fuel required to generate the required power (block B4). As a result, the exhaust energy increases (block B5), and the warm-up of the catalyst can be promoted (block B6). The reduction in the opening degree of the intake throttle also has the effect of reducing the intake air amount and increasing the intake air temperature.
[0097]
On the other hand, an increase in the engine back pressure also has the effect of increasing the turbine speed (block B7). When the turbine speed increases, the intake pressure increases accordingly (block B8). Further, since the intake throttle valve is throttled, the flow velocity in the intake pipe increases (block B9). This means that the entropy of the intake air increases, so that the intake air temperature increases (blocks B10 and B11). As a result, the exhaust energy increases (block B5), and the warm-up of the catalyst can be promoted (block B6).
[0098]
In this way, by narrowing the VNT and the intake throttle during the early warm-up period from after the warm-up, it is possible to promote the warm-up of the catalyst. Also, these controls are less likely to increase the HC concentration. Therefore, warming up of the catalyst can be promoted while suppressing the HC concentration.
[0099]
F. Fourth embodiment of warm-up control:
FIG. 17 is an explanatory diagram illustrating an operation state of the diesel engine 10 in the fourth embodiment of the warm-up control. The fourth embodiment has the same fuel injection timing and injection amount as the first embodiment shown in FIG. 6, except that the intake throttle (throttle valve 28) is opened and closed at a high speed during the early warm-up period. There is a feature. When the intake throttle is opened and closed at a high speed, the energy is given to the intake air, the entropy of the intake air increases, and the intake air temperature rises. Therefore, the exhaust gas temperature also increases, and the warm-up of the catalyst can be promoted. Further, since this control is unlikely to increase the HC concentration, it is possible to promote the warm-up of the catalyst while suppressing the HC concentration.
[0100]
As can be understood from the description of the third embodiment and the fourth embodiment, during the early warm-up period, by adjusting the opening degree of the intake throttle and the VNT, the exhaust gas temperature is raised, and the HC concentration is not increased. It is possible to promote warm-up of the catalyst. Note that the control adopted in the third and fourth embodiments has no direct relation to the fuel injection timing and the injection amount, and is used in any combination with the control in the first and second embodiments. It is possible.
[0101]
G. FIG. Modification:
The present invention is not limited to the above-described examples and embodiments, but can be implemented in various modes without departing from the gist of the invention, and for example, the following modifications are possible.
[0102]
G1. Modification 1
In addition, the present invention is not limited to automobiles, but is also applicable to engines of various vehicles such as vehicles other than automobiles, airplanes, and ships.
[0103]
G2. Modified example 2:
In the above embodiment, the operation period is switched using the engine cooling water temperature or the catalyst bed temperature. However, the operation period may be switched using another measured value. Further, when a certain operation period reaches a predetermined length, the operation period may be switched to another operation period.
[0104]
G3. Modification 3:
In the above embodiment, the catalyst bed temperature and the engine cooling water temperature are directly measured using the temperature sensor, but these temperatures may be measured indirectly. That is, the temperature other than the catalyst and the engine cooling water may be measured, and the catalyst bed temperature and the engine cooling water temperature may be estimated from this. In other words, a temperature detector for detecting a temperature substantially indicating the catalyst bed temperature or the engine cooling water temperature may be used.
[0105]
G4. Modification 4:
The various set values (for example, the fuel injection amount and the injection timing) employed in the above-described embodiment are merely examples, and various other values may be employed. Further, it is possible to warm up the catalyst by variously combining the various operations employed in the various embodiments described above.
[0106]
G5. Modification 5:
The various operations (operation modes) for catalyst warm-up described above need not be continuously performed during the warm-up period, but may be performed at least during a part of the warm-up period. That is, in this specification, the phrase "during the warm-up period" or "during the warm-up period" means "at least a part of the warm-up period" unless otherwise specified.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a schematic configuration of a diesel engine 10 according to an embodiment.
FIG. 2 is an explanatory diagram conceptually showing how the concentrations of various substances in the combustion exhaust gas change when the EGR rate is gradually increased.
FIG. 3 is an explanatory diagram conceptually showing a state in which smoke and fuel efficiency in combustion exhaust gas change when an air-fuel ratio (A / F) is changed.
FIG. 4 is a graph showing a low-temperature combustion operation region and a normal combustion operation region.
FIG. 5 is an explanatory diagram showing an outline of a catalyst warm-up operation.
FIG. 6 is an explanatory diagram showing an operation state of the diesel engine 10 in the first embodiment of the warm-up control.
FIG. 7 is an explanatory diagram showing a relationship between a pressure change in a combustion chamber and a heat generation amount during a warm-up period in the first embodiment.
FIG. 8 is a graph showing an example of an execution result of a catalyst warm-up operation.
FIG. 9 is a graph showing an example in which the injection ratio and the injection timing are changed according to the engine coolant temperature.
FIG. 10 is an explanatory diagram showing an operation state of a diesel engine in a second embodiment of the warm-up control.
FIG. 11 is an explanatory diagram showing a relationship between a main injection timing, a main injection amount, and a misfire region.
FIG. 12 is an explanatory diagram showing a change in exhaust gas temperature Tg depending on fuel injection conditions.
FIG. 13 is an explanatory diagram showing a relationship between a pilot injection amount ratio and an exhaust gas temperature Tg.
FIG. 14 is an explanatory view showing a vertical section of a combustion chamber.
FIG. 15 is an explanatory diagram showing an operation state of the diesel engine in a third embodiment of the warm-up control.
FIG. 16 is an explanatory diagram of a catalyst warm-up effect by adjusting VNT and an intake throttle.
FIG. 17 is an explanatory diagram showing an operation state of the diesel engine in a fourth embodiment of the warm-up control.
[Explanation of symbols]
10. Diesel engine
12 ... intake pipe
14 ... Fuel injection valve
15 ... Nozzle
16 ... exhaust pipe
18 ... Fuel pump
19 ... Common rail
20 ... Supercharger
21 ... Turbine
22 ... Compressor
23 ... Shaft
24 ... Intercooler
25 ... Inlet nozzle
26 ... Actuator
28 ... Throttle valve
30 ... Control unit
40 ... Main catalyst
42 ... Temperature sensor
50 ... Pressure sensor
52 ... Air flow meter
60: EGR flow path
62 ... EGR valve
64: EGR cooler
66 ... EGR catalyst
72 ... Air-fuel ratio sensor
110 ... combustion chamber
112 ... Cylinder wall
114 ... Cylinder head
116 ... Piston
122 ... intake port
124 ... exhaust port
126 ... Glow plug
132 ... intake valve
134 ... exhaust valve

Claims (19)

A diesel engine,
A combustion chamber,
A fuel injection device for injecting fuel into the combustion chamber,
A catalyst for purifying exhaust gas from the combustion chamber,
A control unit that controls a plurality of devices of the diesel engine including the fuel injection device;
With
The control unit includes:
In order to warm up the catalyst and maintain the activity, the first warm-up operation mode is executed during the early warm-up period after the start of the diesel engine, and the second warm-up operation mode is performed during the late warm-up period after the early warm-up period. The machine operation mode,
The first warm-up operation mode is a mode in which the energy of exhaust gas flowing through the catalyst is higher than the energy of exhaust gas flowing through the catalyst in a normal operation mode of the diesel engine after the late warm-up period. ,
The diesel engine, wherein the second warm-up operation mode is an operation mode in which the temperature of exhaust gas flowing through the catalyst is higher than the temperature of exhaust gas flowing through the catalyst in the normal operation mode. The diesel engine according to claim 1,
The diesel engine, wherein the second warm-up operation mode is a mode in which the flow rate of exhaust gas flowing through the catalyst is smaller than any of the first warm-up operation mode and the normal operation mode. The diesel engine according to claim 1 or 2,
The diesel engine, wherein the second warm-up operation mode is a mode in which the excess air ratio in the combustion chamber is smaller than any of the first warm-up operation mode and the normal operation mode. The diesel engine according to any one of claims 1 to 3, wherein
The diesel engine, wherein the second warm-up operation mode has a higher fuel efficiency than the first warm-up operation mode and a lower fuel efficiency than the normal operation mode. The diesel engine according to any one of claims 1 to 4, wherein
The diesel engine, wherein the early warm-up period is a period until the cooling water temperature of the diesel engine reaches a predetermined temperature. The diesel engine according to any one of claims 1 to 4, wherein
The diesel engine, wherein the early warm-up period is a period until the temperature of the catalyst reaches a predetermined temperature. The diesel engine according to any one of claims 1 to 6, wherein
The diesel engine, wherein the second warm-up operation mode is executed when a predetermined operation condition including that a required load on the diesel engine is lower than a predetermined value is satisfied. The diesel engine according to claim 7, wherein
The diesel engine, wherein the second warm-up operation mode is a low-temperature combustion mode. The diesel engine according to claim 1, wherein:
In the first warm-up operation mode, the control unit includes:
(I) controlling the fuel injection device so as to divide fuel injection into pilot injection and main injection, and
(Ii) setting the injection amount ratio of the pilot injection to a value higher than the injection amount ratio of the pilot injection executed in the normal operation mode;
(Iii) setting the injection timing of the main injection to a time later than the injection amount timing of the main injection executed in the normal operation state;
diesel engine. The diesel engine according to claim 9, wherein
The diesel engine, wherein the injection amount ratio of the pilot injection in the first warm-up operation mode is a value in a range of about 2 to about 5 times the injection amount ratio of the pilot injection executed in the normal operation mode. . The diesel engine according to claim 9 or 10, wherein
A diesel engine, wherein the injection timing of the pilot injection in the first warm-up operation mode is set before compression top dead center, and the injection timing of the main injection is set after compression top dead center. The diesel engine according to any one of claims 9 to 11, further comprising:
A temperature detecting unit for detecting a temperature substantially indicating the bed temperature of the catalyst,
The diesel engine, wherein the control unit adjusts an injection ratio of the pilot injection and a timing of the main injection in the first warm-up operation mode according to a bed temperature of the catalyst. The diesel engine according to any one of claims 9 to 11, further comprising:
A temperature detection unit for detecting a temperature substantially indicating a cooling water temperature of the combustion chamber,
The diesel engine, wherein the control unit adjusts an injection ratio of the pilot injection and a timing of the main injection in the first warm-up operation mode according to the cooling water temperature. The diesel engine according to claim 9, wherein
The diesel engine, wherein an injection amount ratio of the pilot injection and an injection timing of the main injection in the first warm-up operation mode are set such that the main injection corresponds to a timing immediately before a misfire limit. The diesel engine according to claim 14, wherein
The diesel engine, wherein the time immediately before the misfire limit is a range of the crank angle from the misfire limit to about 2 ° before the misfire limit. The diesel engine according to claim 14 or 15, further comprising:
An EGR device for recirculating exhaust gas from the combustion chamber to an intake passage to the combustion chamber;
A glow plug provided at an intake port of the combustion chamber;
With
In the first warm-up operation mode, the control unit includes:
(A) an EGR cut operation for stopping the recirculation of exhaust gas by the EGR device;
(B) continuous intake air heating operation by the glow plug;
(C) an idling speed increasing operation for increasing the idling speed of the diesel engine higher than the idling speed in the normal operation mode;
A diesel engine that performs at least one of the following. The diesel engine according to any one of claims 9 to 16, further comprising:
A supercharger that compresses intake air supplied to the combustion chamber using energy of exhaust gas from the combustion chamber,
The supercharger is a variable supercharger capable of adjusting a supercharging pressure of the intake air by adjusting an operating pressure of exhaust gas supplied to the supercharger,
The diesel engine according to claim 1, wherein the control unit increases the operating pressure of the exhaust of the variable supercharger in the first warm-up operation mode to be higher than the operation pressure in the normal operation mode. The diesel engine according to claim 17, further comprising:
An intake throttle valve for restricting an intake passage to the combustion chamber,
The diesel engine, wherein the control unit further causes the throttle amount of the intake throttle valve to be larger than the throttle amount in the normal operation mode in the first warm-up operation mode. The diesel engine according to claim 17, further comprising:
An intake throttle valve for restricting an intake passage to the combustion chamber,
The diesel engine according to claim 1, wherein the control unit continuously opens and closes the intake throttle valve in the first warm-up operation mode.

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