JP3621864B2

JP3621864B2 - Exhaust gas purification method for diesel engine

Info

Publication number: JP3621864B2
Application number: JP2000135763A
Authority: JP
Inventors: 満細谷; 浩平林
Original assignee: Hino Motors Ltd
Current assignee: Hino Motors Ltd
Priority date: 2000-05-09
Filing date: 2000-05-09
Publication date: 2005-02-16
Anticipated expiration: 2020-05-09
Also published as: JP2001317396A

Description

【０００１】
【発明の属する技術分野】
本発明は、ディーゼルエンジンの排気浄化方法に関するものである。
【０００２】
【従来の技術】
従来より、ディーゼルエンジンにおいては、排気ガスが流通する排気管の途中にＮＯｘ還元触媒を装備し、該ＮＯｘ還元触媒の上流側に必要量の還元剤を添加して該還元剤をＮＯｘ還元触媒上で排気ガス中のＮＯｘ（窒素酸化物）と還元反応させることによりＮＯｘの排出濃度を低減し得るようにしたものがある。
【０００３】
他方、理論空燃比より薄い空燃比で希薄燃焼（リーンバーン）を行わせることにより大幅な燃費の向上を図り得ることが広く知られているが、このような希薄燃焼運転を行うことを想定したディーゼルエンジンに関してもＮＯｘの排出濃度を低減することは重要な課題となっている。
【０００４】
しかしながら、一般的に、希薄燃焼運転時における酸素過剰存在下では、ＮＯｘ還元触媒上において、還元剤がＮＯｘと反応するよりも先に酸素と反応してしまうので、希薄燃焼運転時における実用化レベルの高いＮＯｘ低減効果を得ることが難しかった。
【０００５】
そこで、希薄燃焼運転時においてもＮＯｘを低減し得る触媒として、排気ガス中の酸素過剰率が高い時にＮＯｘを酸化して硝酸塩の状態で一時的に吸蔵し且つ排気ガス中の酸素過剰率が低い時に還元剤の介在によりＮＯｘを分解放出して還元浄化する性質を備えたＮＯｘ吸蔵還元触媒の実用化が現在検討されている。
【０００６】
尚、この種のＮＯｘ吸蔵還元触媒としては、白金・バリウム・アルミナ触媒や、イリジウム・白金・バリウム・アルミナ触媒などが前述した如き性質を有するものとして既に知られている。
【０００７】
【発明が解決しようとする課題】
しかしながら、ＮＯｘ吸蔵還元触媒に吸蔵されたＮＯｘを放出させて該ＮＯｘ吸蔵還元触媒の再生を図るのに際し、従来においては、排気ガス再循環装置（ＥＧＲ装置）により排気ガスを排気側から吸気側へ多量に再循環させることで酸素過剰率を下げてＮＯｘ吸蔵還元触媒を再生することが検討されているが、このように多量の排気ガスを再循環させて酸素過剰率を下げるだけでは、気筒内での燃焼状態の大幅な悪化を招いて多量の黒煙が発生してしまうことが避けられないため、未だＮＯｘ吸蔵還元触媒のディーゼルエンジンでの実用化の目処が立っていないのが実情である。
【０００８】
本発明は、上述の実情に鑑みてなされたものであり、黒煙を発生させることなくＮＯｘ吸蔵還元触媒を良好に再生し得るようにした実用性の高いディーゼルエンジンの排気浄化方法を提供することを目的としている。
【０００９】
【課題を解決するための手段】
本発明は、圧縮上死点より若干早いタイミングで主噴射を行う通常モードにより希薄燃焼で通常運転を行いつつＮＯx吸蔵還元触媒により排気ガス中のＮＯxを酸化して硝酸塩の状態で一時的に吸蔵し、ＮＯx吸蔵還元触媒の吸蔵量が限界に近づいた時に運転状態を通常モードから触媒再生モードに切り換え、排気ガスの排気側から吸気側への再循環量を増加し且つ吸入空気量を減らして酸素過剰率を下げ、前記通常モードの主噴射より更に早いタイミングで主噴射を行うと共に、該主噴射に続いて圧縮上死点より若干遅いタイミングでポスト噴射を行い、排気ガス中に生成された未燃燃料の熱分解成分を還元剤としてＮＯx吸蔵還元触媒からＮＯxを分解放出させて該ＮＯx吸蔵還元触媒の再生を図り、その放出したＮＯxをＮＯx吸蔵還元触媒上で前記未燃燃料の熱分解成分と反応させて還元浄化することを特徴とするディーゼルエンジンの排気浄化方法、に係るものである。
【００１０】
而して、このようにすれば、希薄燃焼による通常運転時に、排気ガス中の酸素過剰率が高く維持されることにより、排気ガス中のＮＯｘがＮＯｘ吸蔵還元触媒により良好に酸化されて硝酸塩の状態で一時的に吸蔵され、通常運転時における燃費向上が図られつつ排気ガス中のＮＯｘが大幅に低減されることになる。
【００１１】
又、十分な量のＮＯxを硝酸塩の状態で吸蔵してＮＯx吸蔵還元触媒の吸蔵能力が低下してきた時には、運転状態を通常モードから触媒再生モードに切り換えて、排気ガスの排気側から吸気側への再循環量を増加し且つ吸入空気量を減らし、これにより気筒内の酸素過剰率を下げた状態として、通常モードの主噴射より更に早いタイミングで主噴射を行い、次いで、圧縮上死点より若干遅いタイミングでポスト噴射を行うようにすると、主噴射の燃料が早めに気筒内に投入されることにより燃料の予混合化が促進されて主噴射による黒煙の発生が著しく抑制され、しかも、主としてポスト噴射の燃料が完全燃焼せずに熱分解されてＨ₂，ＣＯ，ＨＣなど未燃燃料の熱分解成分が多く残留した排気ガスが生成されることになる。
【００１２】
そして、これらのＨ_２，ＣＯ，ＨＣを多く含む排気ガスが排気側から吸気側へ向け多量に再循環されると、気筒内がＨ_２，ＣＯ，ＨＣなどの未燃燃料の熱分解成分が均等に分散した酸素過剰率の低い状態となり、斯かる状態で気筒内に次の主噴射が行われて燃料が燃焼すると、酸素過剰率が低いために完全燃焼とはならないものの、その燃焼状態自体は良好なものとなって黒煙などを発生せずに燃焼が完了することになり、しかも、その燃焼後に排気ガス中に残存したままＮＯｘ吸蔵還元触媒へと導かれた未燃燃料の熱分解成分が還元剤として機能し、ＮＯｘ吸蔵還元触媒からＮＯｘが分解放出されて該ＮＯｘ吸蔵還元触媒の再生が図られ、その放出されたＮＯｘがＮＯｘ吸蔵還元触媒上で前記未燃燃料の熱分解成分と反応して還元浄化されることになる。
【００１３】
更に、本発明においては、ディーゼルエンジンの回転数と負荷とを検出して運転状態を監視し、その運転状態に応じたＮＯｘ吸蔵還元触媒の吸蔵量を推定して積算し、その積算された吸蔵量に基づきＮＯｘ吸蔵還元触媒の吸蔵量が限界に近づいたと判断することが好ましく、このようにすれば、ＮＯｘ吸蔵還元触媒を再生すべき時期を明確に判定することが可能となる。
【００１４】
又、ＮＯｘ吸蔵還元触媒に流通する排気ガスの温度を検出し、ＮＯｘ吸蔵還元触媒がＮＯｘの分解放出及び還元浄化に関し活性を有する温度域であることを確認した上で運転状態を通常モードから触媒再生モードに切り換えることが好ましく、このようにすれば、ＮＯｘ吸蔵還元触媒の活性が低下した状態を避けて効率の良い再生を行うことが可能となる。
【００１５】
【発明の実施の形態】
以下本発明の実施の形態を図面を参照しつつ説明する。
【００１６】
図１は本発明を実施する形態の一例を示すもので、本形態例の排気浄化装置においては、ディーゼルエンジン１からの排気ガス２が流通する排気管３の途中に、ＮＯｘ吸蔵還元触媒４がケーシング５に抱持されて装備されている。
【００１７】
又、ここに図示しているディーゼルエンジン１では、排気ガス再循環装置（ＥＧＲ装置）を成すＥＧＲパイプ６により排気管３と吸気管７との間が接続されており、このＥＧＲパイプ６の途中に備えた常時閉のＥＧＲバルブ８を吸気行程中に開け、前記ＥＧＲパイプ６を通して排気ガス２の一部を吸入空気９と一緒に再循環してディーゼルエンジン１の気筒１０内に送り込み、該気筒１０内での燃焼温度を下げてＮＯｘの低減化を図るようにしてある。
【００１８】
ここで、吸気管７におけるＥＧＲパイプ６の接続位置より上流側には、吸気量を調節する吸気バルブ１１が装備されており、又、排気管３におけるＥＧＲパイプ６の分岐位置より下流側には、排気量を調節する排気バルブ１２が装備されている。
【００１９】
更に、ディーゼルエンジン１の各気筒１０の頂部には、図示しない燃料タンクの燃料（軽油）をサプライポンプ１３によりコモンレール式の蓄圧タンク１４（実質的な燃料噴射装置）を介し導いて気筒１０内に噴射するインジェクタ１５（燃焼噴射ノズル）が装備されている。
【００２０】
そして、運転席のアクセルには、アクセル開度をディーゼルエンジン１の負荷として検出するアクセルセンサ１６（負荷センサ）が備えられていると共に、ディーゼルエンジン１の適宜位置には、その回転数を検出する回転センサ１７が装備されており、これらアクセルセンサ１６からのアクセル開度信号１６ａと回転センサ１７からの回転数信号１７ａとが制御装置１８に入力されるようになっている。
【００２１】
又、特に本形態例においては、前記ケーシング５のＮＯｘ吸蔵還元触媒４を挟んだ前後位置に、排気ガス２の温度を検出する温度センサ１９が装備されており、この温度センサ１９からの温度信号１９ａが前記制御装置１８に入力されるようになっている。
【００２２】
一方、前記制御装置１８においては、各気筒１０のインジェクタ１５に向け燃料のタイミング及び噴射量を指令する燃料噴射信号１５ａが出力されるようになっていると共に、ＥＧＲバルブ８と吸気バルブ１１と排気バルブ１２とに対し、夫々の開度を指令する開度指令信号８ａ，１１ａ，１２ａが出力されるようになっている。
【００２３】
即ち、この制御装置１８では、アクセル開度信号１６ａ及び回転数信号１７ａから判断されるディーゼルエンジン１の運転状態を監視して、その運転状態に応じたＮＯｘ吸蔵還元触媒４の吸蔵量を推定して積算し、その積算された吸蔵量に基づきＮＯｘ吸蔵還元触媒４の吸蔵量が限界に近づいたと判断された時に、温度信号１９ａにより排気ガス２の温度を更に確認し、ＮＯｘ吸蔵還元触媒４がＮＯｘの分解放出及び還元浄化に関し活性を有する温度域にあることを確認した上で、各気筒１０のインジェクタ１５に向け新たなタイミング及び噴射量を指令する燃料噴射信号１５ａが出力されると共に、ＥＧＲバルブ８と吸気バルブ１１と排気バルブ１２とに向け開度指令信号８ａ，１１ａ，１２ａが出力され、ディーゼルエンジン１の運転状態が通常モードから触媒再生モードに切り換えられるようにしてある。
【００２４】
ここで、ディーゼルエンジン１の運転状態が通常モードの場合においては、ＥＧＲバルブ８を０％開度の全閉とし且つ吸気バルブ１１及び排気バルブ１２を１００％の開度として、図２に破線で示す如き圧縮上死点より若干早いタイミングで主噴射を行う運転状態に制御されるようになっているが、運転状態を触媒再生モードに切り換えた場合においては、ＥＧＲバルブ８を５０％開度に開け且つ吸気バルブ１１及び排気バルブ１２を５０％以下の開度に絞り、図２に実線で示す如き通常モードの主噴射より更に早いタイミングで且つ噴射量を減らして主噴射を行い、該主噴射に続いて圧縮上死点より若干遅いタイミングでポスト噴射を行う運転状態に制御されるようになっている。
【００２５】
尚、図２においては、その縦軸をインジェクタの針弁のリフト（揚程）とし且つ横軸をクランクシャフトの回転角度としており、圧縮上死点の左側（タイミングの早い側）が圧縮工程を示し、圧縮上死点の右側（タイミングの遅い側）が爆発工程を示している。
【００２６】
而して、このようにすれば、通常運転時において、ＥＧＲバルブ８が全閉にされてＥＧＲパイプ６による排気ガス２の再循環が止められ、吸気バルブ１１及び排気バルブ１２が１００％の開度に開けられて、圧縮上死点より若干早いタイミングで主噴射を行う通常モードに制御されるので、このような希薄燃焼の通常モードで排気ガス２中の酸素過剰率が高く維持されることにより、排気ガス２中のＮＯｘがＮＯｘ吸蔵還元触媒４により良好に酸化されて硝酸塩の状態で一時的に吸蔵され、通常運転時における燃費向上が図られつつ排気ガス２中のＮＯｘが大幅に低減されることになる。
【００２７】
そして、アクセル開度信号１６ａ及び回転数信号１７ａに基づきディーゼルエンジン１の運転状態が制御装置１８により監視され、その運転状態に応じたＮＯｘ吸蔵還元触媒４の吸蔵量が推定されて積算されているので、その積算された吸蔵量に基づきＮＯｘ吸蔵還元触媒４の吸蔵量が限界に近づいたと判断された時に、温度信号１９ａに基づきＮＯｘ吸蔵還元触媒４が所定の活性温度域にあることが更に確認されて、ディーゼルエンジン１の運転状態が通常モードから触媒再生モードに切り換えられる。
【００２８】
即ち、制御装置１８により、ＥＧＲバルブ８が５０％開度に開けられてＥＧＲパイプ６を通し多量の排気ガス２が排気側から吸気側へ再循環量され、且つ吸気バルブ１１及び排気バルブ１２が５０％以下の開度に絞られて吸入空気量が減らされ、これにより気筒１０内の酸素過剰率がλ＝１以下で下げられた状態となり、しかも、新たなタイミング及び噴射量を指令する燃料噴射信号１５ａを受けたインジェクタ１５により、通常モードの主噴射より更に早いタイミングで且つ噴射量を減らして主噴射が行われ、該主噴射に続いて圧縮上死点より若干遅いタイミングでポスト噴射が行われることになる。
【００２９】
このようにディーゼルエンジン１の運転状態が通常モードから触媒再生モードに切り換えられると、主噴射の燃料が早めに気筒１０内に投入されることにより燃料の予混合化が促進されて主噴射による黒煙の発生が著しく抑制され、しかも、主としてポスト噴射の燃料が完全燃焼せずに熱分解されてＨ_２，ＣＯ，ＨＣなど未燃燃料の熱分解成分が多く残留した排気ガス２が生成されることになる。
【００３０】
そして、これらのＨ_２，ＣＯ，ＨＣを多く含む排気ガス２がＥＧＲパイプ６を通し排気側から吸気側へ向け多量に再循環されると、気筒１０内がＨ_２，ＣＯ，ＨＣなどの未燃燃料の熱分解成分が均等に分散した酸素過剰率の低い状態となり、斯かる状態で気筒１０内に次の主噴射が行われて燃料が燃焼すると、酸素過剰率が低いために完全燃焼とはならないものの、その燃焼状態自体は良好なものとなって黒煙などを発生せずに燃焼が完了することになり、しかも、その燃焼後に排気ガス２中に残存したままＮＯｘ吸蔵還元触媒４へと導かれた未燃燃料の熱分解成分が還元剤として機能し、ＮＯｘ吸蔵還元触媒４からＮＯｘが分解放出されて該ＮＯｘ吸蔵還元触媒４の再生が図られ、その放出されたＮＯｘがＮＯｘ吸蔵還元触媒４上で前記未燃燃料の熱分解成分と反応して還元浄化されることになる。
【００３１】
事実、本発明者らが行った実験結果によれば、図３に示す如く、通常モードによる主噴射のみで酸素過剰率を下げていった場合に、既にλ＝１．４付近から急激に黒煙の発生率が上昇し、λ＝１以下の領域では、実用性のある排気浄化装置として到底供し得ないほどの激しい黒煙の発生が確認されたが、触媒再生モードに切り換えて主噴射とポスト噴射とを行いつつ酸素過剰率を下げていった場合には、λ＝０．８付近まで下げても黒煙の発生率が極めて低く抑えられることが確認されている。
【００３２】
従って、上記形態例によれば、気筒１０内での燃焼状態の大幅な悪化を招くことなく排気ガス２を排気側から吸気側へ多量に再循環させて酸素過剰率を下げることができるので、黒煙を発生させることなくＮＯｘ吸蔵還元触媒４を良好に再生することができ、これによりＮＯｘ吸蔵還元触媒４のディーゼルエンジン１への適用を実用化することができる。
【００３３】
又、特に本形態例においては、ディーゼルエンジン１の回転数と負荷とを検出して運転状態を監視し、その運転状態に応じたＮＯｘ吸蔵還元触媒４の吸蔵量を推定して積算し、その積算された吸蔵量に基づきＮＯｘ吸蔵還元触媒４の吸蔵量が限界に近づいたと判断するようにしているので、ＮＯｘ吸蔵還元触媒４を再生すべき時期を明確に判定することができる。
【００３４】
更には、ＮＯｘ吸蔵還元触媒４に流通する排気ガス２の温度についても検出し、ＮＯｘ吸蔵還元触媒４がＮＯｘの分解放出及び還元浄化に関し活性を有する温度域であることを確認した上で運転状態を通常モードから触媒再生モードに切り換えるようにしているので、ＮＯｘ吸蔵還元触媒４の活性が低下した状態を避けて効率の良い再生を行うことができる。
【００３５】
尚、本発明のディーゼルエンジンの排気浄化方法は、上述の形態例にのみ限定されるものではなく、本発明の要旨を逸脱しない範囲内において種々変更を加え得ることは勿論である。
【００３６】
【発明の効果】
上記した本発明のディーゼルエンジンの排気浄化方法によれば、下記の如き種々の優れた効果を奏し得る。
【００３７】
（Ｉ）気筒内での燃焼状態の大幅な悪化を招くことなく排気ガスを排気側から吸気側へ多量に再循環させて酸素過剰率を下げることができるので、黒煙を発生させることなくＮＯｘ吸蔵還元触媒を良好に再生することができ、これによりＮＯｘ吸蔵還元触媒のディーゼルエンジンへの適用を実用化することができる。
【００３８】
（ＩＩ）ディーゼルエンジンの回転数と負荷とを検出して運転状態を監視し、その運転状態に応じたＮＯｘ吸蔵還元触媒の吸蔵量を推定して積算し、その積算された吸蔵量に基づきＮＯｘ吸蔵還元触媒の吸蔵量が限界に近づいたと判断するようにすれば、ＮＯｘ吸蔵還元触媒を再生すべき時期を明確に判定することができる。
【００３９】
（ＩＩＩ）ＮＯｘ吸蔵還元触媒に流通する排気ガスの温度を検出し、ＮＯｘ吸蔵還元触媒がＮＯｘの分解放出及び還元浄化に関し活性を有する温度域であることを確認した上で運転状態を通常モードから触媒再生モードに切り換えるようにすれば、ＮＯｘ吸蔵還元触媒の活性が低下した状態を避けて効率の良い再生を行うことができる。
【図面の簡単な説明】
【図１】本発明を実施する形態の一例を示す概略図である。
【図２】触媒再生モードの有効性を通常モードと比較して示したグラフである。
【図３】通常モードと触媒再生モードとによる燃焼の噴射タイミングを説明するグラフである。
【符号の説明】
１ディーゼルエンジン
２排気ガス
３排気管
４ＮＯｘ吸蔵還元触媒
６ＥＧＲパイプ
７吸気管
９吸入空気
１２排気バルブ
１５インジェクタ
１６アクセルセンサ（負荷センサ）
１７回転センサ
１９温度センサ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust purification method for a diesel engine.
[0002]
[Prior art]
Conventionally, in a diesel engine, a NOx reduction catalyst is provided in the middle of an exhaust pipe through which exhaust gas flows, and a necessary amount of a reducing agent is added upstream of the NOx reduction catalyst so that the reducing agent is placed on the NOx reduction catalyst. And NOx (nitrogen oxide) in the exhaust gas can be reduced to reduce the NOx emission concentration.
[0003]
On the other hand, it is widely known that lean combustion can be performed at an air / fuel ratio thinner than the stoichiometric air / fuel ratio, so that significant improvement in fuel efficiency can be achieved. However, it is assumed that such lean combustion operation is performed. Reducing the NOx emission concentration is also an important issue for diesel engines.
[0004]
However, in general, in the presence of excess oxygen during lean combustion operation, the reducing agent reacts with oxygen before reacting with NOx on the NOx reduction catalyst. It was difficult to obtain a high NOx reduction effect.
[0005]
Therefore, as a catalyst that can reduce NOx even during lean-burn operation, when the oxygen excess rate in the exhaust gas is high, NOx is oxidized and temporarily stored in the form of nitrate, and the oxygen excess rate in the exhaust gas is low At present, practical application of NOx occlusion reduction catalysts having the property of decomposing and releasing NOx and reducing and purifying it through the intervention of a reducing agent is currently under investigation.
[0006]
As this type of NOx occlusion reduction catalyst, platinum / barium / alumina catalyst, iridium / platinum / barium / alumina catalyst, etc. are already known as having the above-mentioned properties.
[0007]
[Problems to be solved by the invention]
However, when NOx occluded in the NOx occlusion reduction catalyst is released to regenerate the NOx occlusion reduction catalyst, conventionally, the exhaust gas is recirculated from the exhaust side to the intake side by an exhaust gas recirculation device (EGR device). It has been studied to regenerate the NOx occlusion reduction catalyst by reducing the oxygen excess rate by recirculating a large amount. However, by simply recirculating a large amount of exhaust gas and reducing the oxygen excess rate, It is inevitable that the NOx storage reduction catalyst will be put to practical use in diesel engines because it is unavoidable that a large amount of black smoke is generated due to a significant deterioration in the combustion state of the engine. .
[0008]
The present invention has been made in view of the above-described circumstances, and provides a highly practical diesel engine exhaust gas purification method capable of satisfactorily regenerating the NOx storage reduction catalyst without generating black smoke. It is an object.
[0009]
[Means for Solving the Problems]
In the present invention, NOx in the exhaust gas is oxidized by the NOx occlusion reduction catalyst and temporarily occluded in the form of nitrate while performing normal operation with lean combustion in the normal mode in which the main injection is performed slightly earlier than the compression top dead center. When the storage amount of the NOx storage reduction catalyst approaches the limit, the operating state is switched from the normal mode to the catalyst regeneration mode, the amount of exhaust gas recirculated from the exhaust side to the intake side is increased, and the amount of intake air is decreased. The excess oxygen ratio is lowered, and main injection is performed at a timing earlier than the main injection in the normal mode, and post injection is performed slightly later than the compression top dead center following the main injection, and is generated in the exhaust gas. NOx is decomposed and released from the NOx occlusion reduction catalyst using the pyrolysis component of unburned fuel as a reducing agent to regenerate the NOx occlusion reduction catalyst, and the released NOx is recovered on the NOx occlusion reduction catalyst. Serial exhaust gas purifying method for a diesel engine which comprises reducing purified is reacted with thermal decomposition component of unburned fuel which relate to.
[0010]
Thus, in this way, during normal operation by lean combustion, the oxygen excess rate in the exhaust gas is maintained high, so that NOx in the exhaust gas is oxidized well by the NOx storage reduction catalyst and the nitrates are reduced. Therefore, NOx in the exhaust gas is greatly reduced while improving fuel efficiency during normal operation.
[0011]
Also, when a sufficient amount of NOx is occluded in the state of nitrate and the storage capacity of the NOx occlusion reduction catalyst has declined, the operating state is switched from the normal mode to the catalyst regeneration mode, and the exhaust gas is changed from the exhaust side to the intake side. The amount of intake air is decreased and the oxygen excess rate in the cylinder is reduced, so that the main injection is performed at an earlier timing than the main injection in the normal mode , and then from the compression top dead center. If post-injection is performed at a slightly later timing, the fuel of the main injection is introduced into the cylinder early to promote the premixing of the fuel, and the generation of black smoke due to the main injection is remarkably suppressed, Mainly, post-injected fuel is pyrolyzed without being completely burned, and exhaust gas in which many pyrolytic components of unburned fuel such as H ₂ , CO, and HC remain is generated.
[0012]
And these H _2, CO, the exhaust gas containing a large amount of HC is large amount of recirculated toward the intake side from the exhaust side, the cylinder is H _2, CO, thermal decomposition component of unburned fuel such as HC Even if the oxygen excess rate is evenly dispersed and the fuel is burned by the next main injection in the cylinder in this state, the oxygen excess rate is low, so complete combustion is not possible, but the combustion state itself The combustion is completed without generating black smoke and the like, and the pyrolysis of unburned fuel led to the NOx occlusion reduction catalyst remains in the exhaust gas after the combustion. The component functions as a reducing agent, NOx is decomposed and released from the NOx storage reduction catalyst, the NOx storage reduction catalyst is regenerated, and the released NOx is a thermal decomposition component of the unburned fuel on the NOx storage reduction catalyst Reduced and purified by reacting with Will be.
[0013]
Furthermore, in the present invention, the rotational speed and load of the diesel engine are detected to monitor the operation state, and the storage amount of the NOx storage reduction catalyst corresponding to the operation state is estimated and integrated, and the integrated storage is performed. It is preferable to determine that the storage amount of the NOx storage reduction catalyst has approached the limit based on the amount, and in this way, it becomes possible to clearly determine when the NOx storage reduction catalyst should be regenerated.
[0014]
In addition, the temperature of the exhaust gas flowing through the NOx storage reduction catalyst is detected, and it is confirmed that the NOx storage reduction catalyst is in a temperature range that is active in the decomposition and release of NOx and the reduction purification. It is preferable to switch to the regeneration mode. In this way, efficient regeneration can be performed while avoiding a state in which the activity of the NOx storage reduction catalyst is lowered.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0016]
FIG. 1 shows an example of an embodiment for carrying out the present invention. In the exhaust purification apparatus of this embodiment, a NOx occlusion reduction catalyst 4 is placed in the middle of an exhaust pipe 3 through which exhaust gas 2 from a diesel engine 1 flows. The casing 5 is held and equipped.
[0017]
Further, in the diesel engine 1 shown here, the exhaust pipe 3 and the intake pipe 7 are connected by an EGR pipe 6 constituting an exhaust gas recirculation device (EGR device). The normally closed EGR valve 8 provided for the engine is opened during the intake stroke, and a part of the exhaust gas 2 is recirculated together with the intake air 9 through the EGR pipe 6 and fed into the cylinder 10 of the diesel engine 1. The combustion temperature in 10 is lowered to reduce NOx.
[0018]
Here, an intake valve 11 for adjusting the intake air amount is provided on the upstream side of the connection position of the EGR pipe 6 in the intake pipe 7, and further on the downstream side of the branch position of the EGR pipe 6 in the exhaust pipe 3. An exhaust valve 12 for adjusting the exhaust amount is provided.
[0019]
Furthermore, fuel (light oil) in a fuel tank (not shown) is guided to the top of each cylinder 10 of the diesel engine 1 by a supply pump 13 via a common rail pressure accumulation tank 14 (substantial fuel injection device). An injector 15 (combustion injection nozzle) for injection is provided.
[0020]
The accelerator in the driver's seat is provided with an accelerator sensor 16 (load sensor) that detects the accelerator opening as a load of the diesel engine 1 and detects the rotational speed at an appropriate position of the diesel engine 1. A rotation sensor 17 is provided, and an accelerator opening signal 16 a from the accelerator sensor 16 and a rotation speed signal 17 a from the rotation sensor 17 are input to the control device 18.
[0021]
In particular, in the present embodiment, a temperature sensor 19 for detecting the temperature of the exhaust gas 2 is provided at the front and rear positions of the casing 5 with the NOx occlusion reduction catalyst 4 interposed therebetween, and a temperature signal from the temperature sensor 19 is provided. 19 a is input to the control device 18.
[0022]
On the other hand, in the control device 18, a fuel injection signal 15 a for instructing the fuel timing and the injection amount is output to the injector 15 of each cylinder 10, and the EGR valve 8, the intake valve 11, and the exhaust gas are output. Opening command signals 8a, 11a, and 12a for commanding the respective opening are output to the valve 12.
[0023]
That is, the control device 18 monitors the operating state of the diesel engine 1 determined from the accelerator opening signal 16a and the rotational speed signal 17a, and estimates the storage amount of the NOx storage reduction catalyst 4 according to the operating state. When it is determined that the storage amount of the NOx storage reduction catalyst 4 is approaching the limit based on the stored storage amount, the temperature of the exhaust gas 2 is further confirmed by the temperature signal 19a, and the NOx storage reduction catalyst 4 After confirming that the temperature is in an active temperature range for NOx decomposition and reduction and reduction and purification, a fuel injection signal 15a for commanding a new timing and injection amount is output to the injector 15 of each cylinder 10, and EGR The opening command signals 8a, 11a, 12a are output to the valve 8, the intake valve 11, and the exhaust valve 12, and the operation state of the diesel engine 1 is output. There are to be switched from the normal mode to the catalyst regeneration mode.
[0024]
Here, when the operating state of the diesel engine 1 is in the normal mode, the EGR valve 8 is fully closed at 0% opening, and the intake valve 11 and the exhaust valve 12 are set to 100% opening, with a broken line in FIG. As shown in the figure, the operation state is controlled so that the main injection is performed slightly earlier than the compression top dead center. However, when the operation state is switched to the catalyst regeneration mode, the EGR valve 8 is set to 50% opening. Open and throttle the intake valve 11 and the exhaust valve 12 to 50% or less, perform main injection at a timing earlier than the main injection in the normal mode as shown by the solid line in FIG. Subsequently, the operation state is controlled such that the post injection is performed at a timing slightly later than the compression top dead center.
[0025]
In FIG. 2, the vertical axis represents the lift (lift) of the needle valve of the injector and the horizontal axis represents the rotation angle of the crankshaft, and the left side of compression top dead center (early timing side) indicates the compression process. The right side of compression top dead center (late timing side) shows the explosion process.
[0026]
Thus, in this way, during normal operation, the EGR valve 8 is fully closed, the recirculation of the exhaust gas 2 by the EGR pipe 6 is stopped, and the intake valve 11 and the exhaust valve 12 are opened 100%. Since the engine is controlled to the normal mode in which the main injection is performed at a timing slightly earlier than the compression top dead center, the oxygen excess rate in the exhaust gas 2 is kept high in such a normal mode of lean combustion. As a result, NOx in the exhaust gas 2 is satisfactorily oxidized by the NOx occlusion reduction catalyst 4 and temporarily stored in the form of nitrate, and NOx in the exhaust gas 2 is greatly reduced while improving fuel efficiency during normal operation. Will be.
[0027]
The operation state of the diesel engine 1 is monitored by the control device 18 based on the accelerator opening signal 16a and the rotation speed signal 17a, and the storage amount of the NOx storage reduction catalyst 4 corresponding to the operation state is estimated and integrated. Therefore, when it is determined that the storage amount of the NOx storage reduction catalyst 4 is approaching the limit based on the accumulated storage amount, it is further confirmed that the NOx storage reduction catalyst 4 is in the predetermined active temperature range based on the temperature signal 19a. Thus, the operation state of the diesel engine 1 is switched from the normal mode to the catalyst regeneration mode.
[0028]
That is, the EGR valve 8 is opened to 50% by the control device 18, a large amount of exhaust gas 2 is recirculated from the exhaust side to the intake side through the EGR pipe 6, and the intake valve 11 and the exhaust valve 12 are The amount of intake air is reduced by reducing the opening to 50% or less, so that the oxygen excess rate in the cylinder 10 is reduced to λ = 1 or less, and a fuel that commands a new timing and injection amount The injector 15 that has received the injection signal 15a performs main injection at a timing earlier than the main injection in the normal mode and by reducing the injection amount, and post injection at a timing slightly later than the compression top dead center following the main injection. Will be done.
[0029]
As described above, when the operation state of the diesel engine 1 is switched from the normal mode to the catalyst regeneration mode, the fuel of the main injection is introduced into the cylinder 10 earlier, so that the premixing of the fuel is promoted and the black by the main injection is promoted. The generation of smoke is remarkably suppressed, and the exhaust gas 2 is generated in which mainly the post-injected fuel is thermally decomposed without being completely burned and a large amount of unburned fuel such as H ₂ , CO, HC remains. It will be.
[0030]
And these _H 2, CO, when the exhaust gas 2 containing a large amount of HC is large amount of recirculated toward the intake side from the exhaust side through the EGR pipe 6, the cylinder 10 is such as _H 2, CO, HC Not When the thermal decomposition component of the fuel is evenly dispersed and the oxygen excess rate is low, in this state, when the next main injection is performed in the cylinder 10 and the fuel burns, the oxygen excess rate is low, and thus complete combustion However, the combustion state itself is good and combustion is completed without generating black smoke or the like, and the NOx occlusion reduction catalyst 4 remains in the exhaust gas 2 after the combustion. The thermal decomposition component of the unburned fuel thus led functions as a reducing agent, NOx is decomposed and released from the NOx storage reduction catalyst 4 to regenerate the NOx storage reduction catalyst 4, and the released NOx is stored in the NOx storage. On the reduction catalyst 4 It reacts with the thermal decomposition component of unburned fuel and is reduced and purified.
[0031]
In fact, according to the results of experiments conducted by the present inventors, as shown in FIG. 3, when the oxygen excess rate is reduced only by the main injection in the normal mode, the black has already been rapidly increased from around λ = 1.4. In the area where the smoke generation rate increased and λ = 1 or less, intense black smoke generation that could not be provided as a practical exhaust gas purification device was confirmed. When the oxygen excess rate is lowered while performing post injection, it has been confirmed that the generation rate of black smoke can be suppressed to a very low level even if the oxygen excess rate is lowered to around λ = 0.8.
[0032]
Therefore, according to the above embodiment, the excess oxygen ratio can be reduced by recirculating the exhaust gas 2 in a large amount from the exhaust side to the intake side without causing significant deterioration of the combustion state in the cylinder 10. The NOx occlusion reduction catalyst 4 can be regenerated well without generating black smoke, whereby the application of the NOx occlusion reduction catalyst 4 to the diesel engine 1 can be put into practical use.
[0033]
Further, particularly in the present embodiment, the rotational speed and load of the diesel engine 1 are detected to monitor the operation state, and the storage amount of the NOx storage reduction catalyst 4 corresponding to the operation state is estimated and integrated. Since it is determined that the storage amount of the NOx storage reduction catalyst 4 is approaching the limit based on the integrated storage amount, it is possible to clearly determine when the NOx storage reduction catalyst 4 should be regenerated.
[0034]
Further, the temperature of the exhaust gas 2 flowing through the NOx occlusion reduction catalyst 4 is also detected, and it is confirmed that the NOx occlusion reduction catalyst 4 is in a temperature range that is active in the decomposition and release of NOx and reduction purification. Is switched from the normal mode to the catalyst regeneration mode, it is possible to avoid the state in which the activity of the NOx occlusion reduction catalyst 4 is reduced and perform efficient regeneration.
[0035]
It should be noted that the exhaust purification method for a diesel engine of the present invention is not limited to the above-described embodiment, and it is needless to say that various changes can be made without departing from the gist of the present invention.
[0036]
【The invention's effect】
According to the exhaust purification method for a diesel engine of the present invention described above, various excellent effects as described below can be obtained.
[0037]
(I) Exhaust gas can be recirculated in a large amount from the exhaust side to the intake side without causing a significant deterioration in the combustion state in the cylinder, so that the oxygen excess rate can be reduced, so that NOx is not generated without generating black smoke. The storage reduction catalyst can be regenerated satisfactorily, whereby the application of the NOx storage reduction catalyst to a diesel engine can be put into practical use.
[0038]
(II) The rotational speed and load of the diesel engine are detected to monitor the operating state, the NOx storage reduction catalyst corresponding to the operating state is estimated and accumulated, and NOx is based on the accumulated stored amount. If it is determined that the storage amount of the storage reduction catalyst has approached the limit, it is possible to clearly determine when the NOx storage reduction catalyst should be regenerated.
[0039]
(III) The temperature of the exhaust gas flowing through the NOx occlusion reduction catalyst is detected, and after confirming that the NOx occlusion reduction catalyst is in a temperature range that is active for NOx decomposition and release and reduction purification, the operation state is changed from the normal mode. By switching to the catalyst regeneration mode, efficient regeneration can be performed while avoiding a state in which the activity of the NOx storage reduction catalyst is reduced.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an example of an embodiment for carrying out the present invention.
FIG. 2 is a graph showing the effectiveness of a catalyst regeneration mode in comparison with a normal mode.
FIG. 3 is a graph for explaining combustion injection timings in a normal mode and a catalyst regeneration mode.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Diesel engine 2 Exhaust gas 3 Exhaust pipe 4 NOx storage reduction catalyst 6 EGR pipe 7 Intake pipe 9 Intake air 12 Exhaust valve 15 Injector 16 Accelerator sensor (load sensor)
17 Rotation sensor 19 Temperature sensor

Claims

NOx in the exhaust gas is oxidized by the NOx occlusion reduction catalyst and temporarily occluded in the form of nitrate while performing normal operation with lean combustion in the normal mode in which the main injection is performed slightly earlier than the compression top dead center , and NOx occlusion When the storage amount of the reduction catalyst approaches the limit, the operating state is switched from the normal mode to the catalyst regeneration mode, the recirculation amount of exhaust gas from the exhaust side to the intake side is increased, and the intake air amount is decreased to increase the oxygen excess rate. The main injection is performed at a timing earlier than that of the main injection in the normal mode, and the post injection is performed at a timing slightly later than the compression top dead center following the main injection, so that the unburned fuel generated in the exhaust gas NOx is decomposed and released from the NOx occlusion reduction catalyst using the pyrolysis component as a reducing agent to regenerate the NOx occlusion reduction catalyst, and the released NOx is unburned on the NOx occlusion reduction catalyst. Exhaust gas purifying method of diesel engine, characterized in that is reacted with the pyrolysis component reduces and purifies it.

The rotational speed and load of the diesel engine are detected to monitor the operation state, and the NOx storage reduction catalyst according to the operation state is estimated and integrated, and the NOx storage reduction catalyst is based on the integrated storage amount. The exhaust purification method for a diesel engine according to claim 1, wherein the storage amount of the diesel engine is determined to approach a limit.

The temperature of the exhaust gas flowing through the NOx storage reduction catalyst is detected, and after confirming that the NOx storage reduction catalyst is in a temperature range that is active in the decomposition and release of NOx and reduction purification, the operation state is changed from the normal mode to the catalyst regeneration mode. 3. The exhaust gas purification method for a diesel engine according to claim 1 or 2, wherein