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JP2005002968A - Exhaust emission control device of internal combustion engine - Google Patents

Exhaust emission control device of internal combustion engine Download PDF

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Publication number
JP2005002968A
JP2005002968A JP2003170244A JP2003170244A JP2005002968A JP 2005002968 A JP2005002968 A JP 2005002968A JP 2003170244 A JP2003170244 A JP 2003170244A JP 2003170244 A JP2003170244 A JP 2003170244A JP 2005002968 A JP2005002968 A JP 2005002968A
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Japan
Prior art keywords
exhaust gas
oxidation catalyst
internal combustion
combustion engine
temperature
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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JP2003170244A
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Japanese (ja)
Inventor
Ritsuko Shinozaki
律子 篠▼崎▲
Shinichi Saito
真一 斎藤
Yoshihisa Takeda
好央 武田
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Mitsubishi Fuso Truck and Bus Corp
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Mitsubishi Fuso Truck and Bus Corp
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Priority to JP2003170244A priority Critical patent/JP2005002968A/en
Publication of JP2005002968A publication Critical patent/JP2005002968A/en
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
    • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion
    • F01N3/2053—By-passing catalytic reactors, e.g. to prevent overheating
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features having two or more separate purifying devices arranged in series
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/105—General auxiliary catalysts, e.g. upstream or downstream of the main catalyst
    • F01N3/106—Auxiliary oxidation catalysts
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
    • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion
    • F01N3/206—Adding periodically or continuously substances to exhaust gases for promoting purification, e.g. catalytic material in liquid form, NOx reducing agents
    • F01N3/2066—Selective catalytic reduction [SCR]
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N9/00—Electrical control of exhaust gas treating apparatus
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N2410/00—By-passing, at least partially, exhaust from inlet to outlet of apparatus, to atmosphere or to other device
    • F01N2410/02—By-passing, at least partially, exhaust from inlet to outlet of apparatus, to atmosphere or to other device in case of high temperature, e.g. overheating of catalytic reactor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N2900/00—Details of electrical control or of the monitoring of the exhaust gas treating apparatus
    • F01N2900/06—Parameters used for exhaust control or diagnosing
    • F01N2900/14—Parameters used for exhaust control or diagnosing said parameters being related to the exhaust gas
    • F01N2900/1402—Exhaust gas composition
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
    • F01N2900/00—Details of electrical control or of the monitoring of the exhaust gas treating apparatus
    • F01N2900/06—Parameters used for exhaust control or diagnosing
    • F01N2900/14—Parameters used for exhaust control or diagnosing said parameters being related to the exhaust gas
    • F01N2900/1404—Exhaust gas temperature
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/40—Engine management systems

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Health & Medical Sciences (AREA)
  • Toxicology (AREA)
  • Materials Engineering (AREA)
  • Exhaust Gas Treatment By Means Of Catalyst (AREA)
  • Exhaust Gas After Treatment (AREA)

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine which can clean up NOx with high efficiency. <P>SOLUTION: The exhaust emission control device 1 has an SCR catalyst 7 and a powerful oxidation catalyst 8 on the upstream side of the SCR catalyst 7 in an exhaust system of the engine 2. Further, the device is provided with an oxidation catalyst bypass 15 which bypasses the powerful oxidation catalyst 8, and a changeover valve 16 which changes an exhaust gas passage. When the exhaust gas is at such a temperature that an NO<SB>2</SB>invert ratio of the powerful oxidation catalyst 8 is 50% or more, the exhaust gas is let to flow to the oxidation catalyst bypass 15 by the changeover valve 16, so that it is possible to prevent excessive generation of NO<SB>2</SB>which results in reduction of the NOx cleaning efficiency in SCR catalyst 7. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、内燃機関の排気浄化装置に関する。
【０００２】
【従来の技術】
ディーゼルエンジン等の内燃機関から排出される排気ガスには、ＨＣ（炭化水素）、ＣＯ（一酸化炭素）、ＮＯｘ（窒素酸化物）及びＰＭ（Ｐａｒｔｉｃｕｌａｔｅ Ｍａｔｔｅｒ：パティキュレート）等の汚染物質が含まれる。これらの汚染物質の中でもＮＯｘは、酸化触媒やガソリン自動車で実用化されている三元触媒では浄化が難しく、ＮＯｘを浄化することができる有望な触媒として選択還元型ＮＯｘ触媒（ＳＣＲ触媒）の開発が行われている。
【０００３】
ＳＣＲ触媒はアンモニアなどの還元剤の存在下でＮＯｘを浄化する触媒である。尿素水タンクからＳＣＲ触媒の上流側の排気系に添加された尿素水は、排気ガスの熱により加水分解されアンモニアを生成する。このアンモニアが還元剤として働き、排気ガス中のＮＯｘと反応することにより排気ガス中のＮＯｘが浄化される。
【０００４】
このＮＯｘ浄化については、下記（１）〜（４）式に示す反応により浄化反応が進行することが知られている。
４ＮＯ＋４ＮＨ３＋Ｏ２ → ４Ｎ２＋６Ｈ２Ｏ （１）
ＮＯ＋ＮＯ２＋２ＮＨ３ → ２Ｎ２＋３Ｈ２Ｏ （２）
２ＮＯ２＋４ＮＨ３＋Ｏ２ → ３Ｎ２＋６Ｈ２Ｏ （３）
６ＮＯ２＋８ＮＨ３ → ７Ｎ２＋１２Ｈ２Ｏ （４）
【０００５】
また、上記（１）〜（４）式の浄化反応は、その反応速度に差があることが知られており、（２）式による浄化反応が最も反応速度が速く、次に（１）式による浄化反応が速く、（３）又は（４）式の浄化反応は他の反応と比較して遅いことが分かっている。最も反応速度が速い（２）式は、ＮＯ（一酸化窒素）とＮＯ２（二酸化窒素）とが１対１で共存しこれらと同量のアンモニアとが反応する浄化反応であり、したがって、排気ガス中のＮＯｘにおけるＮＯとＮＯ２との比が１：１のときに、最もＮＯｘ浄化反応が良くなる（下記、特許文献１参照。）。
【０００６】
一方、内燃機関から排出されるＮＯｘはほとんどがＮＯである。このため、ＳＣＲ触媒の上流側に酸化触媒を設けてＮＯの一部をＮＯ２に酸化することにより、排気ガス中のＮＯとＮＯ２との比を１：１に近づけるようにしている。しかしながら、酸化触媒を併用したこの浄化システムでは、排気ガスの温度が高温のときに酸化触媒による酸化能力が強くなりすぎて、ＮＯが過剰に酸化され多量のＮＯ２が生成してしまう。この結果、反応速度の遅い上記（３）又は（４）式による浄化反応が起こるようになり、ＳＣＲ触媒によるＮＯｘ浄化性能が減少してしまう。
【０００７】
また、上記（１）、（２）式の浄化反応では、ＮＨ３と浄化されるＮＯｘ（ＮＯとＮＯ２の総量）との比が１対１の関係（ＮＨ３／ＮＯｘ＝１）にあるが、上記（３）、（４）式の浄化反応では浄化されるＮＯｘの総量に対してより多くのＮＨ３が必要（ＮＨ３／ＮＯｘ＞１）となる。このため、当初、（１）、（２）式の浄化反応を想定して添加する尿素水を制御していても、（３）、（４）式の浄化反応が起こることでアンモニア量が不足してしまい、ＳＣＲ触媒によるＮＯｘ浄化性能が減少してしまう。
【０００８】
【特許文献１】
特表２００１−５２５９０２号公報
【０００９】
【発明が解決しようとする課題】
本発明は、上記状況に鑑みてなされたものであり、過剰のＮＯ酸化（ＮＯ２生成）を行わず、なおかつＮＯとＮＯ２とをＮＯｘ浄化に理想的な１対１となるように共存させることができる内燃機関の排気浄化装置を提供することを目的とする。
【００１０】
【課題解決するための手段】
上記課題を解決する第１の発明は、内燃機関の排気系に設けられアンモニアを還元剤として排気ガス中のＮＯｘを選択還元するＮＯｘ触媒と、前記排気系のうち当該ＮＯｘ触媒よりも上流側に設けられる第１酸化触媒とを有する内燃機関の排気浄化装置において、
前記第１酸化触媒をバイパスする酸化触媒バイパスと、
前記内燃機関からの排気ガスの流路を前記第１酸化触媒または前記酸化触媒バイパスのいずれかに切り替える切替手段と、
前記内燃機関からの排気ガスの温度を検出又は推測する排気ガス温度検出手段と、
前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記内燃機関からの排気ガスが前記酸化触媒バイパスを通過するように前記切替手段を制御する切替制御手段とを備えたことを特徴とする内燃機関の排気浄化装置である。
【００１１】
ＳＣＲ触媒におけるＮＯｘ浄化率を向上させるため、ＮＯｘ触媒の上流側に酸化触媒（第１酸化触媒）を設置してＮＯｘの一部をＮＯ２に酸化しているが、排気ガスの温度が所定温度以上の場合には、このＮＯ２の生成が過剰となり、ＮＯｘ浄化性能が低下する。そこで、第１の発明では、排気ガスの温度が所定温度以上の場合には、排気ガスの流路を第１酸化触媒側から酸化触媒バイパス側へ切り替えて、ＮＯｘの酸化（ＮＯ２の生成）を抑制するようにする。
【００１２】
酸化触媒バイパスを設置した第１の発明では、排気ガス温度が所定温度以上の場合に排気ガス中のＮＯを酸化することなくＳＣＲ触媒へ送ることとなるが、排気ガスが既に高温状態となっておりＳＣＲ触媒の活性も高いため、酸化触媒（第１酸化触媒）を介さずとも十分にＮＯｘ浄化を行うことができる。
【００１３】
排気ガス温度検出手段は、内燃機関からの排気ガスの温度を検出又は推測する機能を有し、排気系であればいずれの場所に設けても本発明の効果を得ることができる。好ましくは、内燃機関から排出された直後の排気ガス温度を検出等することができる位置、第１酸化触媒の直上流の位置又は切替手段の直上流の位置がよい。
【００１４】
切替手段は、内燃機関からの排気ガスの流路を第１酸化触媒または酸化触媒バイパスのいずれかに切り替える機能を有するが、当該機能としては、流路を完全に切り替える機能のほかに、第１酸化触媒と酸化触媒バイパスとを流れる排気ガスの流量比を制御する機能であっても良い。
【００１５】
上記課題を解決する第２の発明は、内燃機関の排気系に設けられアンモニアを還元剤として排気ガス中のＮＯｘを選択還元するＮＯｘ触媒と、前記排気系のうち当該ＮＯｘ触媒よりも上流側に設けられる第１酸化触媒とを有する内燃機関の排気浄化装置において、
前記排気系のうち前記第１酸化触媒と並列に設けられ、前記第１酸化触媒よりも比較的酸化力が弱い第２酸化触媒と、
前記内燃機関からの排気ガスの流路を前記第１酸化触媒または前記第２酸化触媒のいずれかに切り替える切替手段と、
前記内燃機関からの排気ガスの温度を検出又は推測する排気ガス温度検出手段と、
前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記内燃機関からの排気ガスが前記第２酸化触媒を通過するように前記切替手段を制御する切替制御手段とを備えたことを特徴とする内燃機関の排気浄化装置である。
【００１６】
排気ガスの温度が所定温度以上の場合には、酸化触媒（第１酸化触媒）によるＮＯ２の生成が過剰となり、ＮＯｘ浄化性能が低下する。そこで、第２の発明では、排気ガスの温度が所定温度以上の場合には、排気ガスの流路を第１酸化触媒側から酸化力が弱い第２酸化触媒側へ切り替えて、ＮＯｘの酸化（ＮＯ２の生成）効率を下げるようにする。
【００１７】
第１酸化触媒、第２酸化触媒の酸化力の強弱は、例えば、担持させるＰｔ等の触媒量で調整する。例えば、第１酸化触媒には低温から高い活性を与えるため触媒量を比較的多くして強力酸化触媒とする一方、第２酸化触媒は、第１酸化触媒において過剰な酸化反応が始まる温度あたりから活性化されるように触媒量を少なく調整して弱力酸化触媒とする。
【００１８】
上記課題を解決する第３の発明は、第２の発明に係る内燃機関の排気浄化装置において、
前記第２酸化触媒は、前記排気ガス中のＮＯを酸化してＮＯ２に転化する転化率が最大でも５０％以下であることを特徴とする内燃機関の排気浄化装置である。
【００１９】
酸化触媒の性能を示すＮＯ→ＮＯ２転化率は排気ガスの温度により変化する。第２酸化触媒として、ＮＯ→ＮＯ２転化率の最大値が５０％以下の酸化触媒を採用することにより、第１酸化触媒におけるＮＯ過剰酸化（ＮＯの５０％より多くが酸化されてしまう状態）を回避する役割を有する第２酸化触媒においてＮＯの過剰酸化を起こさないようにする。
【００２０】
上記課題を解決する第４の発明は、第１ないし第３のいずれかの発明に係る内燃機関の排気浄化装置において、
前記所定温度は、前記第１酸化触媒が前記排気ガス中のＮＯを酸化してＮＯ２に転化する転化率が５０％となる温度であることを特徴とする内燃機関の排気浄化装置である。
【００２１】
所定温度を第１酸化触媒のＮＯ→ＮＯ２転化率が５０％となる温度に設定し、排気ガスの温度が当該所定温度以上となったときに、排気ガスの流路を切替手段により酸化触媒バイパス側又は第２酸化触媒側に切替えて、ＮＯ２の過剰生成を抑制してＳＣＲ触媒におけるＮＯｘ浄化性能の低下を防止する。なお、酸化触媒のＮＯ→ＮＯ２転化率は排気ガスの温度により変化するが、転化率と排気ガス温度との関係がやまなりのグラフとなり、転化率が５０％となる排気ガス温度が２つ以上存在する場合には、それらのうち最も低い温度を所定温度とする。
【００２２】
上記課題を解決する第５の発明は、第１ないし第４のいずれかの発明に係る内燃機関の排気浄化装置において、
前記ＮＯｘ触媒は、主として下記（１）式又は（２）式の少なくとも一方により排気ガス中のＮＯｘを浄化することを特徴とする内燃機関の排気浄化装置である。
４ＮＯ＋４ＮＨ３＋Ｏ２ → ４Ｎ２＋６Ｈ２Ｏ （１）
ＮＯ＋ＮＯ２＋２ＮＨ３ → ２Ｎ２＋３Ｈ２Ｏ （２）
【００２３】
ＳＣＲ触媒においてＮＯｘ浄化速度が比較的速い反応を主として用いるように、内燃機関の排気浄化装置を制御することにより、下記（３）、（４）式に示す反応速度が比較的遅く、ＮＯｘ浄化性能の低下の原因となる反応が起こることを防止する。
２ＮＯ２＋４ＮＨ３＋Ｏ２ → ３Ｎ２＋６Ｈ２Ｏ （３）
６ＮＯ２＋８ＮＨ３ → ７Ｎ２＋１２Ｈ２Ｏ （４）
【００２４】
上記課題を解決する第６の発明は、第１ないし第４のいずれかの発明に係る内燃機関の排気浄化装置において、
前記切替制御手段は、前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記切替手段により、前記第１酸化触媒を流れる排気ガスと前記第２酸化触媒又は前記酸化触媒バイパスを流れる排気ガスとの流量比を調整して、前記ＮＯｘ触媒に流入する排気ガス中のＮＯとＮＯ２との比が略１対１となるように制御することを特徴とする内燃機関の排気浄化装置である。
【００２５】
排気ガスの温度が所定温度以上であるときに、ＮＯｘ触媒に流入する排気ガス中のＮＯとＮＯ２との比が略１対１となるように排気系の流路制御をすることにより、最も効率よくＮＯｘを浄化することができる排気浄化装置とする。排気ガス中のＮＯとＮＯ２との比率の検出手段としては、直接検出することができるＮＯｘセンサによる手段や、排気ガスの状態（排気ガス温度、酸化触媒におけるＮＯ→ＮＯ２転化率及び各流路を通過する排気ガス流量等）から推測する手段等が挙げられる。
【００２６】
【発明の実施の形態】
以下、図面に基づき本発明の好適な実施例を例示的に詳しく説明する。
【００２７】
＜第１の実施形態＞
図１は、本発明の第１の実施形態に係る内燃機関の排気浄化装置を示した構成図である。同図に示すように、排気浄化装置１は、内燃機関としてのエンジン２の排気系に設けられたＮＯｘ触媒としてのＳＣＲ触媒７と、排気系のうちＳＣＲ触媒７よりも上流側に設けられた第１酸化触媒としての強力酸化触媒８と、排気系のうちＳＣＲ触媒７よりも上流側であり強力酸化触媒８よりも下流側に還元剤としてのアンモニアを供給する尿素水添加装置９等と、強力酸化触媒８をバイパスする酸化触媒バイパス１５と、強力酸化触媒８と酸化触媒バイパス１５との分岐部分に設けられエンジン２からの排気ガスの流路として強力酸化触媒８又は酸化触媒バイパス１５のいずれかを選択する切替手段としての切替弁１６と、切替弁１６の上流側の排気ガスの温度を検出する排気ガス温度検出手段としての排気ガス温度センサ１７と、尿素水添加装置９に内蔵され所定の条件によって切替弁１６を制御する切替制御手段とから構成される。なお、切替弁１６の切替制御は、図示していないエンジンＥＣＵにて行ってもよい。
【００２８】
エアクリーナー３から吸引された空気は、ターボチャージャー４によって過給され、インタークーラー５を介してエンジン２に供給された後、エンジン２内で燃料と共に燃焼し、排気ガスとして排気系へ排出される。
【００２９】
還元剤を供給するユニットとしては、還元剤としてのアンモニアの素となる尿素水を貯蔵する尿素水タンク１０と、ＳＣＲ触媒７の温度を検出又は推測する触媒温度センサ１３と、尿素水を排気系におけるＳＣＲ触媒７の上流側に供給する尿素水噴射ノズル１２と、触媒温度センサ１３により推測される温度により添加する尿素水量を制御する尿素水添加装置９とから構成される。
【００３０】
触媒温度センサ１３は、ＳＣＲ触媒７の上流側と下流側とに設けられ、当該２つのポイントにおける排気ガスの温度の平均値をＳＣＲ触媒７の温度として推測している。なお、触媒温度センサとしては、ＳＣＲ触媒７の温度を直接検出できるセンサであってもよい。
【００３１】
また、ＳＣＲ触媒７の下流側には、酸化触媒１４が設けられ、過剰な尿素水の添加による余剰のアンモニア（アンモニアスリップの発生）を分解するようになっている。なお、アンモニアスリップを発生させない尿素水添加制御を行うことができる還元剤を供給するユニットであれば、酸化触媒１４を省略することができる。
【００３２】
尿素水添加装置９は、切替制御手段としての機能も有し、排気ガス温度センサ１７により検出される排気ガスの温度が所定温度以上の場合には、切替弁１６を制御し、エンジン２からの排気ガスを酸化触媒バイパス１５へ流入させると共に、排気ガスの温度が所定温度より低い場合には、排気ガスが強力酸化触媒８を通過するように切替弁１６を制御する。
【００３３】
図２は、強力酸化触媒の一例に係るＮＯ→ＮＯ２転化率と排気ガス温度との関係図である。なお、同図には、下記第２の実施形態で説明する弱力酸化触媒のＮＯ２転化率と排気ガス温度との関係も示してある。
【００３４】
同図に示すように、強力酸化触媒の一例に係るＮＯ２転化率は、排気ガス温度が約３００℃までは温度の上昇に従い高くなり、排気ガス温度が３００℃付近で約７０％以上の転化率となり、さらに排気ガス温度が高くなると転化率が低下していくという、排気ガス温度に対して転化率がやまなりに変化する関係を有する。また、排気ガス温度が約２３０℃〜約３９０℃の範囲では転化率が５０％以上となり、過剰のＮＯ２を生成してＮＯｘ浄化効率を低下させる温度範囲となっている。なお、同図に示す転化率特性は、強力酸化触媒の一例を示すものであり、酸化触媒に担持させるＰｔ等の触媒量で調整することができる。
【００３５】
切替制御手段としての尿素水添加装置９は、排気ガス温度センサ１７により検出される強力酸化触媒８の直上流を流れる排気ガスの温度が所定温度以上か否かにより切替弁１６の制御を行うが、この所定温度としては、強力酸化触媒８のＮＯ２転化率が５０％となる温度を設定してある。例えば、図２に示す転化率特性を有する強力酸化触媒の場合には、転化率が５０％となる排気ガス温度としては約２３０℃と約３９０℃の２つの温度が存在するが、最も低い温度である約２３０℃を所定温度とする。
【００３６】
すなわち、切替制御手段は、強力酸化触媒８のＮＯ２転化率が５０％以上となるような排気ガス温度（図２に示す強力酸化触媒では約２３０℃以上）である場合には、切替弁１６によりエンジン２からの排気ガスを酸化触媒バイパス１５へ流入させると共に、強力酸化触媒８のＮＯ２転化率が５０％より低くなるような排気ガス温度（図２に示す強力酸化触媒では約２３０℃より低温）である場合には、切替弁１６により排気ガスが強力酸化触媒８を通過するように制御する。
【００３７】
この結果、強力酸化触媒８のＮＯ２転化率が５０％より低い排気ガス温度である場合には、排気ガスを強力酸化触媒８を流通させることにより、より多くのＮＯをＮＯ２へ酸化してＳＣＲ触媒７におけるＮＯｘ浄化効率を向上させることができる。この際には、過剰のＮＯ２が生成していないため、ＳＣＲ触媒７上では比較的反応速度の速い上記（１）、（２）式による浄化反応が行われる。
【００３８】
一方、強力酸化触媒８のＮＯ２転化率が５０％以上となるような排気ガス温度である場合には、排気ガスを酸化触媒バイパス１５を流通させてＮＯ２の生成を抑制することにより、ＳＣＲ触媒７におけるＮＯｘ浄化率の低下を防止することができる。この際には、排気ガス中にＮＯ２はほとんど含まれていないため、ＳＣＲ触媒７上では主として上記（１）式による浄化反応が行われるが、排気ガスが高温状態となっているためＳＣＲ触媒７は十分に活性化されており、強力酸化触媒を介さずとも高いＮＯｘ浄化効率を得ることができる。
【００３９】
＜第２の実施形態＞
次に、第２の実施形態について説明する。本実施形態では、図１に示した第１の実施形態に係る排気浄化装置における酸化触媒バイパス１５の代わりに第２酸化触媒としての「弱力酸化触媒」を設けて（すなわち、排気系において強力酸化触媒８と並列に弱力酸化触媒を設置する）、強力酸化触媒８のＮＯ２転化率が５０％以上となるような排気ガス温度である場合には、切替弁１６によりエンジン２からの排気ガスを「弱力酸化触媒」へ流入させると共に、強力酸化触媒８のＮＯ２転化率が５０％より低くなるような排気ガス温度である場合には、切替弁１６により排気ガスが強力酸化触媒８を通過するように、切替制御手段により制御する。
【００４０】
ここで、弱力酸化触媒とは担持させる触媒量を減少させるなどして、強力酸化触媒８よりも酸化力を弱めた触媒であり、例えば、図２に示す転化率特性を有する弱力酸化触媒を用いる。
【００４１】
同図に示すように、弱力酸化触媒の一例に係るＮＯ２転化率は、排気ガス温度が約３００℃までは温度の上昇に従い高くなり、排気ガス温度が３００℃付近で約５０％の転化率となり、さらに排気ガス温度が高くなると転化率が低下していくという、排気ガス温度に対して転化率がやまなりに変化する関係を有する。なお、同図に示す転化率特性は、弱力酸化触媒の一例を示すものであり、酸化触媒に担持させるＰｔ等の触媒量で調整することができる。
【００４２】
この結果、強力酸化触媒８のＮＯ２転化率が５０％以上となるような排気ガス温度である場合には、排気ガスを弱力酸化触媒を流通させてＮＯ２の生成を抑制することにより、ＳＣＲ触媒７におけるＮＯｘ浄化率の低下を防止することができる。
【００４３】
この際には、排気ガス中のＮＯは弱力酸化触媒により酸化され、上記（２）式による反応速度の最も速い浄化反応を利用することができるため、単に酸化触媒バイパスを流通させるよりも更に高いＮＯｘ浄化効率を得ることができる。また、弱力酸化触媒は、転化率が５０％以上とならないように酸化力を調整した酸化触媒であるため、過剰のＮＯ２を生成することがなく、ＮＯｘ浄化効率が低下するおそれもない。
【００４４】
第１、第２の実施形態では、切替制御手段により、所定温度を境にして排気ガス流路を強力酸化触媒８側または酸化触媒バイパス１５（又は弱力酸化触媒）側へ変更する構成としたが、切替弁１６の開度を調整することにより、強力酸化触媒８側と酸化触媒バイパス１５（又は弱力酸化触媒）側とを流れる排気ガスの流量比を制御するようにしてもよい。
【００４５】
流量比を制御する際には、排気ガスの温度が所定温度以上であるときに、例えば、ＳＣＲ触媒に流入する排気ガス中のＮＯとＮＯ２との比が略１対１となるように制御することにより、最も効率よくＮＯｘを浄化することができる排気浄化装置とすることができる。排気ガス中のＮＯとＮＯ２との比率は、例えば、排気ガスの状態（排気ガス温度、酸化触媒におけるＮＯ→ＮＯ２転化率及び各流路を通過する排気ガス流量等）から推測することができる。
【００４６】
【発明の効果】
第１の発明では、排気ガスの温度が所定温度以上の場合には、排気ガスの流路を第１酸化触媒側から酸化触媒バイパス側へ切り替えて、排気ガス中のＮＯを酸化することなくＳＣＲ触媒へ送ることとしたので、ＳＣＲ触媒においてＮＯｘ浄化効率を低下させる原因である過剰なＮＯ２の生成を防止して、ＮＯｘ浄化効率を低下させることを防止することができる。
【００４７】
第２の発明では、排気ガスの温度が所定温度以上の場合には、排気ガスの流路を第１酸化触媒側から酸化力が弱い第２酸化触媒側へ切り替えて、ＮＯｘの酸化（ＮＯ２の生成）効率を下げることにより、ＳＣＲ触媒においてＮＯｘ浄化効率を低下させる原因である過剰なＮＯ２の生成を抑制して、ＮＯｘ浄化効率を高い状態で維持することができる。
【００４８】
第３の発明では、第２の発明に係る内燃機関の排気浄化装置において、第２酸化触媒として、ＮＯ→ＮＯ２転化率の最大値が５０％以下の酸化触媒を採用することにより、第１酸化触媒におけるＮＯ過剰酸化（ＮＯの５０％より多くが酸化されてしまう状態）を回避する役割を有する第２酸化触媒においてＮＯの過剰酸化を起こさないようにして、ＳＣＲ触媒におけるＮＯｘ浄化効率の低下を防止することができる。
【００４９】
第４の発明では、第１ないし第３のいずれかの発明に係る内燃機関の排気浄化装置において、所定温度を第１酸化触媒のＮＯ→ＮＯ２転化率が５０％となる温度に設定することにより、排気ガスの温度が当該所定温度以上となったときに、排気ガスの流路を第２酸化触媒側又は酸化触媒バイパス側に切替えて、効果的にＮＯ２の過剰生成を抑制して、ＳＣＲ触媒におけるＮＯｘ浄化性能の低下を防止することができる。
【００５０】
第５の発明では、第１ないし第４のいずれかの発明に係る内燃機関の排気浄化装置において、種々のＮＯｘ浄化反応のうち反応速度の速い浄化反応である下記（１）式又は（２）式の少なくとも一方により排気ガス中のＮＯｘを浄化するように制御することとしたので、ＳＣＲ触媒におけるＮＯｘ浄化性能を効果的に向上させることができる。
４ＮＯ＋４ＮＨ３＋Ｏ２ → ４Ｎ２＋６Ｈ２Ｏ （１）
ＮＯ＋ＮＯ２＋２ＮＨ３ → ２Ｎ２＋３Ｈ２Ｏ （２）
【００５１】
第６の発明では、第１ないし第４のいずれかの発明に係る内燃機関の排気浄化装置において、排気ガスの温度が所定温度以上であるときに、ＮＯｘ触媒に流入する排気ガス中のＮＯとＮＯ２との比が略１対１となるように排気系の流路を制御することにより、最も効率よくＮＯｘを浄化することができる排気浄化装置とすることができる。
【図面の簡単な説明】
【図１】本発明の実施形態に係る内燃機関の排気浄化装置の概略構成図である。
【図２】酸化触媒のＮＯ→ＮＯ２転化率と排気ガス温度との関係図である。
【符号の説明】
１ 排気浄化装置
２ エンジン
３ エアクリーナー
４ ターボチャージャー
５ インタークーラー
７ ＳＣＲ触媒
８ 強力酸化触媒（前段）
９ 尿素水添加装置
１０ 尿素水タンク
１２ 尿素水噴射ノズル
１３ 触媒温度センサ
１４ 酸化触媒（後段）
１５ 酸化触媒バイパス
１６ 切替弁
１７ 排気ガス温度センサ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
Exhaust gas discharged from an internal combustion engine such as a diesel engine contains pollutants such as HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxide), and PM (Particulate Matter). . Among these pollutants, NOx is difficult to purify with oxidation catalysts and three-way catalysts put to practical use in gasoline automobiles, and a selective reduction type NOx catalyst (SCR catalyst) is developed as a promising catalyst that can purify NOx. Has been done.
[0003]
The SCR catalyst is a catalyst that purifies NOx in the presence of a reducing agent such as ammonia. The urea water added from the urea water tank to the exhaust system upstream of the SCR catalyst is hydrolyzed by the heat of the exhaust gas to generate ammonia. This ammonia acts as a reducing agent and reacts with NOx in the exhaust gas to purify NOx in the exhaust gas.
[0004]
Regarding this NOx purification, it is known that the purification reaction proceeds by the reactions shown in the following formulas (1) to (4).
4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (1)
NO + NO 2 + 2NH 3 → 2N 2 + 3H 2 O (2)
2NO 2 + 4NH 3 + O 2 → 3N 2 + 6H 2 O (3)
6NO 2 + 8NH 3 → 7N 2 + 12H 2 O (4)
[0005]
Further, it is known that the purification reactions of the above formulas (1) to (4) have different reaction rates. The purification reaction according to the formula (2) has the fastest reaction rate, and then the formula (1) It has been found that the purification reaction according to (3) is fast, and the purification reaction of formula (3) or (4) is slower than other reactions. Equation (2), which has the fastest reaction rate, is a purification reaction in which NO (nitrogen monoxide) and NO 2 (nitrogen dioxide) coexist on a one-to-one basis and react with the same amount of ammonia. the ratio of NO to NO 2 in the NOx in the gas is 1: when 1, most NOx purification reaction is improved (below, see Patent Document 1.).
[0006]
On the other hand, most of NOx discharged from the internal combustion engine is NO. Thus, by oxidizing a portion of NO is provided an oxidation catalyst on the upstream side of the SCR catalyst NO 2, the ratio of NO to NO 2 in the exhaust gas 1: as close to 1. However, in this purification system using an oxidation catalyst in combination, the oxidation capability of the oxidation catalyst becomes too strong when the temperature of the exhaust gas is high, and NO is excessively oxidized and a large amount of NO 2 is generated. As a result, the purification reaction according to the above formula (3) or (4) with a slow reaction rate occurs, and the NOx purification performance by the SCR catalyst is reduced.
[0007]
In the purification reactions of the above formulas (1) and (2), the ratio between NH 3 and NOx to be purified (total amount of NO and NO 2 ) is in a one-to-one relationship (NH 3 / NOx = 1). However, in the purification reactions of the above formulas (3) and (4), more NH 3 is required (NH 3 / NOx> 1) with respect to the total amount of NOx to be purified. For this reason, even if the urea water to be added is controlled initially assuming the purification reaction of formulas (1) and (2), the amount of ammonia is insufficient due to the purification reaction of formulas (3) and (4). As a result, the NOx purification performance of the SCR catalyst is reduced.
[0008]
[Patent Document 1]
JP 2001-525902 A [0009]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, and does not perform excessive NO oxidation (NO 2 generation), and coexists with NO and NO 2 so as to be in an ideal 1: 1 relationship for NOx purification. An object of the present invention is to provide an exhaust emission control device for an internal combustion engine.
[0010]
[Means for solving the problems]
A first invention that solves the above-described problems includes a NOx catalyst that is provided in an exhaust system of an internal combustion engine and selectively reduces NOx in exhaust gas using ammonia as a reducing agent, and is located upstream of the NOx catalyst in the exhaust system. In an exhaust gas purification apparatus for an internal combustion engine having a first oxidation catalyst provided,
An oxidation catalyst bypass for bypassing the first oxidation catalyst;
Switching means for switching the flow path of the exhaust gas from the internal combustion engine to either the first oxidation catalyst or the oxidation catalyst bypass;
Exhaust gas temperature detection means for detecting or estimating the temperature of the exhaust gas from the internal combustion engine;
Switching control for controlling the switching means so that the exhaust gas from the internal combustion engine passes through the oxidation catalyst bypass when the temperature of the exhaust gas detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature. And an exhaust gas purifying device for an internal combustion engine.
[0011]
To improve the NOx purification rate of the SCR catalyst, but by installing the catalytic oxidation upstream of the NOx catalyst (first oxidation catalyst) which oxidizes part of NOx in NO 2, the temperature of the exhaust gas is a predetermined temperature In the above case, the production of NO 2 becomes excessive, and the NOx purification performance is lowered. Therefore, in the first invention, when the temperature of the exhaust gas is equal to or higher than the predetermined temperature, the exhaust gas flow path is switched from the first oxidation catalyst side to the oxidation catalyst bypass side to oxidize NOx (generate NO 2 ). To suppress.
[0012]
In the first invention in which the oxidation catalyst bypass is installed, when the exhaust gas temperature is equal to or higher than the predetermined temperature, NO in the exhaust gas is sent to the SCR catalyst without being oxidized, but the exhaust gas is already in a high temperature state. Since the cage SCR catalyst has high activity, NOx purification can be sufficiently performed without using an oxidation catalyst (first oxidation catalyst).
[0013]
The exhaust gas temperature detecting means has a function of detecting or estimating the temperature of the exhaust gas from the internal combustion engine, and the effect of the present invention can be obtained regardless of the location of the exhaust system. Preferably, a position where the exhaust gas temperature immediately after being discharged from the internal combustion engine can be detected, a position immediately upstream of the first oxidation catalyst, or a position immediately upstream of the switching means is preferable.
[0014]
The switching means has a function of switching the flow path of the exhaust gas from the internal combustion engine to either the first oxidation catalyst or the oxidation catalyst bypass. The function includes the first function in addition to the function of completely switching the flow path. The function of controlling the flow rate ratio of the exhaust gas flowing through the oxidation catalyst and the oxidation catalyst bypass may be used.
[0015]
A second invention that solves the above-described problem is a NOx catalyst that is provided in an exhaust system of an internal combustion engine and selectively reduces NOx in exhaust gas using ammonia as a reducing agent, and is located upstream of the NOx catalyst in the exhaust system. In an exhaust gas purification apparatus for an internal combustion engine having a first oxidation catalyst provided,
A second oxidation catalyst provided in parallel with the first oxidation catalyst in the exhaust system and having a relatively weak oxidizing power than the first oxidation catalyst;
Switching means for switching the flow path of the exhaust gas from the internal combustion engine to either the first oxidation catalyst or the second oxidation catalyst;
Exhaust gas temperature detection means for detecting or estimating the temperature of the exhaust gas from the internal combustion engine;
Switching for controlling the switching means so that the exhaust gas from the internal combustion engine passes through the second oxidation catalyst when the temperature of the exhaust gas detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature. An exhaust emission control device for an internal combustion engine, comprising: a control means.
[0016]
When the temperature of the exhaust gas is equal to or higher than the predetermined temperature, NO 2 is excessively generated by the oxidation catalyst (first oxidation catalyst), and the NOx purification performance is deteriorated. Therefore, in the second invention, when the temperature of the exhaust gas is equal to or higher than the predetermined temperature, the exhaust gas flow path is switched from the first oxidation catalyst side to the second oxidation catalyst side having a weak oxidizing power to oxidize NOx ( NO 2 production) Reduce efficiency.
[0017]
The strength of the oxidizing power of the first oxidation catalyst and the second oxidation catalyst is adjusted by the amount of catalyst such as Pt to be supported, for example. For example, since the first oxidation catalyst is given a high activity from a low temperature, the amount of the catalyst is made relatively strong to make it a strong oxidation catalyst, while the second oxidation catalyst starts from around the temperature at which an excessive oxidation reaction starts in the first oxidation catalyst. A weak oxidation catalyst is prepared by adjusting the amount of the catalyst so as to be activated.
[0018]
A third invention for solving the above-described problems is an internal combustion engine exhaust gas purification apparatus according to the second invention.
The second oxidation catalyst is an exhaust gas purification apparatus for an internal combustion engine, wherein a conversion rate for oxidizing NO in the exhaust gas and converting it into NO 2 is 50% or less at the maximum.
[0019]
The NO → NO 2 conversion rate indicating the performance of the oxidation catalyst varies depending on the temperature of the exhaust gas. By employing an oxidation catalyst having a maximum NO → NO 2 conversion rate of 50% or less as the second oxidation catalyst, NO excessive oxidation in the first oxidation catalyst (a state where more than 50% of NO is oxidized) In the second oxidation catalyst having a role of avoiding NO, excessive oxidation of NO is prevented.
[0020]
According to a fourth aspect of the present invention for solving the above problems, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to third aspects of the invention,
The predetermined temperature is an exhaust gas purification apparatus for an internal combustion engine, wherein a conversion rate at which the first oxidation catalyst oxidizes NO in the exhaust gas and converts it into NO 2 is 50%.
[0021]
The predetermined temperature is set to a temperature at which the NO → NO 2 conversion rate of the first oxidation catalyst becomes 50%. By switching to the bypass side or the second oxidation catalyst side, excessive generation of NO 2 is suppressed to prevent the NOx purification performance of the SCR catalyst from being lowered. The NO → NO 2 conversion rate of the oxidation catalyst varies depending on the temperature of the exhaust gas, but the relationship between the conversion rate and the exhaust gas temperature becomes a graph with a lot of difference, and there are two exhaust gas temperatures at which the conversion rate becomes 50%. If there is more than one, the lowest temperature is set as the predetermined temperature.
[0022]
A fifth invention for solving the above-described problems is an internal combustion engine exhaust gas purification apparatus according to any one of the first to fourth inventions,
The NOx catalyst is an exhaust gas purification apparatus for an internal combustion engine that purifies NOx in exhaust gas mainly by at least one of the following formulas (1) and (2).
4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (1)
NO + NO 2 + 2NH 3 → 2N 2 + 3H 2 O (2)
[0023]
By controlling the exhaust purification device of the internal combustion engine so as to mainly use a reaction with a relatively high NOx purification rate in the SCR catalyst, the reaction rate shown in the following formulas (3) and (4) is relatively slow, and the NOx purification performance. To prevent reactions that cause a decrease in
2NO 2 + 4NH 3 + O 2 → 3N 2 + 6H 2 O (3)
6NO 2 + 8NH 3 → 7N 2 + 12H 2 O (4)
[0024]
According to a sixth aspect of the present invention for solving the above problems, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to fourth aspects of the invention,
When the exhaust gas temperature detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature, the switching control means is configured to switch the exhaust gas flowing through the first oxidation catalyst and the second oxidation by the switching means. The flow rate ratio with the exhaust gas flowing through the catalyst or the oxidation catalyst bypass is adjusted to control the ratio of NO and NO 2 in the exhaust gas flowing into the NOx catalyst to be approximately 1: 1. An exhaust purification device for an internal combustion engine.
[0025]
By controlling the flow path of the exhaust system so that the ratio of NO to NO 2 in the exhaust gas flowing into the NOx catalyst is approximately 1: 1 when the temperature of the exhaust gas is equal to or higher than a predetermined temperature, An exhaust purification device capable of efficiently purifying NOx is provided. As a means for detecting the ratio of NO to NO 2 in the exhaust gas, a means using a NOx sensor that can be directly detected, a state of the exhaust gas (exhaust gas temperature, NO → NO 2 conversion rate in the oxidation catalyst, and each flow) And a means for inferring from the exhaust gas flow rate passing through the road.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
[0027]
<First Embodiment>
FIG. 1 is a configuration diagram showing an exhaust emission control device for an internal combustion engine according to a first embodiment of the present invention. As shown in the figure, the exhaust purification device 1 is provided on the upstream side of the SCR catalyst 7 in the exhaust system, and the SCR catalyst 7 as the NOx catalyst provided in the exhaust system of the engine 2 as the internal combustion engine. A strong oxidation catalyst 8 as a first oxidation catalyst, a urea water addition device 9 for supplying ammonia as a reducing agent upstream of the SCR catalyst 7 and downstream of the strong oxidation catalyst 8 in the exhaust system, etc. An oxidation catalyst bypass 15 that bypasses the strong oxidation catalyst 8, and a strong oxidation catalyst 8 or an oxidation catalyst bypass 15 that is provided at a branch portion between the strong oxidation catalyst 8 and the oxidation catalyst bypass 15 as an exhaust gas flow path from the engine 2. A switching valve 16 as a switching means for selecting the above, an exhaust gas temperature sensor 17 as an exhaust gas temperature detection means for detecting the temperature of the exhaust gas upstream of the switching valve 16, and urea water Composed of the switching control means for controlling the switching valve 16 by a predetermined condition is incorporated in the pressure device 9. The switching control of the switching valve 16 may be performed by an engine ECU (not shown).
[0028]
The air sucked from the air cleaner 3 is supercharged by the turbocharger 4, supplied to the engine 2 through the intercooler 5, burns with fuel in the engine 2, and is discharged to the exhaust system as exhaust gas.
[0029]
The unit for supplying the reducing agent includes a urea water tank 10 for storing urea water as a raw material of ammonia as a reducing agent, a catalyst temperature sensor 13 for detecting or estimating the temperature of the SCR catalyst 7, and an urea water exhaust system. The urea water injection nozzle 12 supplied to the upstream side of the SCR catalyst 7 and the urea water addition device 9 that controls the amount of urea water to be added according to the temperature estimated by the catalyst temperature sensor 13.
[0030]
The catalyst temperature sensor 13 is provided on the upstream side and the downstream side of the SCR catalyst 7, and estimates the average value of the exhaust gas temperature at the two points as the temperature of the SCR catalyst 7. The catalyst temperature sensor may be a sensor that can directly detect the temperature of the SCR catalyst 7.
[0031]
Further, an oxidation catalyst 14 is provided on the downstream side of the SCR catalyst 7 to decompose excess ammonia (generation of ammonia slip) due to addition of excess urea water. Note that the oxidation catalyst 14 can be omitted if it is a unit that supplies a reducing agent capable of performing urea water addition control that does not generate ammonia slip.
[0032]
The urea water addition device 9 also has a function as a switching control means. When the temperature of the exhaust gas detected by the exhaust gas temperature sensor 17 is equal to or higher than a predetermined temperature, the urea water addition device 9 controls the switching valve 16 to The exhaust gas is caused to flow into the oxidation catalyst bypass 15 and the switching valve 16 is controlled so that the exhaust gas passes through the strong oxidation catalyst 8 when the temperature of the exhaust gas is lower than a predetermined temperature.
[0033]
FIG. 2 is a graph showing the relationship between the NO → NO 2 conversion rate and the exhaust gas temperature according to an example of the strong oxidation catalyst. In addition, the same figure also shows the relationship between the NO 2 conversion rate of the weak oxidation catalyst described in the second embodiment below and the exhaust gas temperature.
[0034]
As shown in the figure, the NO 2 conversion rate according to an example of the strong oxidation catalyst increases as the exhaust gas temperature rises to about 300 ° C., and the conversion is about 70% or more when the exhaust gas temperature is around 300 ° C. The conversion rate decreases with increasing exhaust gas temperature, and the conversion rate decreases with increasing exhaust gas temperature. Further, when the exhaust gas temperature is in the range of about 230 ° C. to about 390 ° C., the conversion rate is 50% or more, which is a temperature range in which excessive NO 2 is generated and NOx purification efficiency is lowered. The conversion characteristics shown in the figure show an example of a strong oxidation catalyst, and can be adjusted by the amount of catalyst such as Pt supported on the oxidation catalyst.
[0035]
The urea water addition device 9 as the switching control means controls the switching valve 16 depending on whether or not the temperature of the exhaust gas flowing immediately upstream of the strong oxidation catalyst 8 detected by the exhaust gas temperature sensor 17 is equal to or higher than a predetermined temperature. As the predetermined temperature, a temperature at which the NO 2 conversion rate of the strong oxidation catalyst 8 becomes 50% is set. For example, in the case of the strong oxidation catalyst having the conversion rate characteristic shown in FIG. 2, there are two exhaust gas temperatures of about 230 ° C. and about 390 ° C. at which the conversion rate is 50%, but the lowest temperature The predetermined temperature is about 230 ° C.
[0036]
In other words, the switching control means switches the switching valve 16 when the exhaust gas temperature is such that the NO 2 conversion rate of the strong oxidation catalyst 8 is 50% or more (about 230 ° C. or more for the strong oxidation catalyst shown in FIG. 2). As a result, exhaust gas from the engine 2 flows into the oxidation catalyst bypass 15 and the exhaust gas temperature at which the NO 2 conversion rate of the strong oxidation catalyst 8 becomes lower than 50% (from about 230 ° C. in the strong oxidation catalyst shown in FIG. 2). If the temperature is low), the switching valve 16 controls the exhaust gas to pass through the strong oxidation catalyst 8.
[0037]
As a result, when the NO 2 conversion rate of the strong oxidation catalyst 8 is lower than 50%, the exhaust gas is passed through the strong oxidation catalyst 8 to oxidize more NO to NO 2 . The NOx purification efficiency in the SCR catalyst 7 can be improved. At this time, since excessive NO2 is not generated, the purification reaction according to the above equations (1) and (2) is performed on the SCR catalyst 7 at a relatively high reaction rate.
[0038]
On the other hand, when the NO2 conversion of strong oxidation catalyst 8 is an exhaust gas temperature such that 50% or more, by the exhaust gas is passed through an oxidation catalyst bypass 15 to suppress the formation of NO 2, SCR catalyst 7 can be prevented from lowering the NOx purification rate. At this time, since NO 2 is hardly contained in the exhaust gas, the purification reaction according to the above equation (1) is mainly performed on the SCR catalyst 7, but the SCR catalyst is in a high temperature state because the exhaust gas is in a high temperature state. 7 is sufficiently activated, and high NOx purification efficiency can be obtained without using a strong oxidation catalyst.
[0039]
<Second Embodiment>
Next, a second embodiment will be described. In the present embodiment, a “weak oxidation catalyst” as a second oxidation catalyst is provided instead of the oxidation catalyst bypass 15 in the exhaust gas purification apparatus according to the first embodiment shown in FIG. If the exhaust gas temperature is such that the NO 2 conversion rate of the strong oxidation catalyst 8 is 50% or more, an exhaust gas from the engine 2 is exhausted by the switching valve 16. When the gas flows into the “weak oxidation catalyst” and the exhaust gas temperature is such that the NO 2 conversion rate of the strong oxidation catalyst 8 is lower than 50%, the switching valve 16 causes the exhaust gas to flow into the strong oxidation catalyst 8. Is controlled by the switching control means.
[0040]
Here, the weak oxidation catalyst is a catalyst whose oxidizing power is weaker than that of the strong oxidation catalyst 8 by reducing the amount of the catalyst to be supported. For example, the weak oxidation catalyst having the conversion rate characteristic shown in FIG. Is used.
[0041]
As shown in the figure, the NO 2 conversion rate according to an example of the weak oxidation catalyst increases as the temperature rises until the exhaust gas temperature reaches about 300 ° C, and the conversion is about 50% when the exhaust gas temperature is around 300 ° C. The conversion rate decreases with increasing exhaust gas temperature, and the conversion rate decreases with increasing exhaust gas temperature. The conversion characteristics shown in the figure show an example of a weak oxidation catalyst, and can be adjusted by the amount of catalyst such as Pt supported on the oxidation catalyst.
[0042]
As a result, when the exhaust gas temperature is such that the NO 2 conversion rate of the strong oxidation catalyst 8 is 50% or more, the exhaust gas is allowed to flow through the weak oxidation catalyst to suppress the generation of NO 2. A reduction in the NOx purification rate in the catalyst 7 can be prevented.
[0043]
In this case, NO in the exhaust gas is oxidized by the weak oxidation catalyst, and the purification reaction having the fastest reaction rate according to the above equation (2) can be used, so that it is more than simply circulating the oxidation catalyst bypass. High NOx purification efficiency can be obtained. Moreover, since the weak oxidation catalyst is an oxidation catalyst whose oxidizing power is adjusted so that the conversion rate does not become 50% or more, it does not generate excessive NO 2 and the NOx purification efficiency does not decrease.
[0044]
In the first and second embodiments, the switching control means changes the exhaust gas flow path to the strong oxidation catalyst 8 side or the oxidation catalyst bypass 15 (or weak oxidation catalyst) side at a predetermined temperature. However, the flow rate ratio of the exhaust gas flowing through the strong oxidation catalyst 8 side and the oxidation catalyst bypass 15 (or weak oxidation catalyst) side may be controlled by adjusting the opening degree of the switching valve 16.
[0045]
When controlling the flow ratio, when the temperature of the exhaust gas is equal to or higher than a predetermined temperature, for example, the control is performed so that the ratio of NO to NO 2 in the exhaust gas flowing into the SCR catalyst is approximately 1: 1. By doing so, it is possible to provide an exhaust purification device capable of purifying NOx most efficiently. The ratio of NO to NO 2 in the exhaust gas can be estimated from, for example, the state of the exhaust gas (exhaust gas temperature, NO → NO 2 conversion rate in the oxidation catalyst, exhaust gas flow rate passing through each flow path, etc.). it can.
[0046]
【The invention's effect】
In the first invention, when the temperature of the exhaust gas is equal to or higher than the predetermined temperature, the flow path of the exhaust gas is switched from the first oxidation catalyst side to the oxidation catalyst bypass side, and the SCR is oxidized without oxidizing NO in the exhaust gas. Since it is sent to the catalyst, it is possible to prevent the NOx purification efficiency from being lowered by preventing the generation of excessive NO 2 that is the cause of the NOx purification efficiency being lowered in the SCR catalyst.
[0047]
In the second invention, when the temperature of the exhaust gas is equal to or higher than the predetermined temperature, the flow path of the exhaust gas is switched from the first oxidation catalyst side to the second oxidation catalyst side having a weak oxidizing power to oxidize NOx (NO 2 By reducing the production efficiency, it is possible to suppress the production of excess NO 2 that is a cause of lowering the NOx purification efficiency in the SCR catalyst and maintain the NOx purification efficiency in a high state.
[0048]
In the third invention, in the exhaust gas purification apparatus for an internal combustion engine according to the second invention, as the second oxidation catalyst, an oxidation catalyst having a maximum value of NO → NO 2 conversion of 50% or less is adopted. Reduction of NOx purification efficiency in the SCR catalyst by preventing excessive oxidation of NO in the second oxidation catalyst having a role of avoiding NO excessive oxidation (a state in which more than 50% of NO is oxidized) in the oxidation catalyst Can be prevented.
[0049]
In a fourth invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to third inventions, the predetermined temperature is set to a temperature at which the NO → NO 2 conversion rate of the first oxidation catalyst becomes 50%. Thus, when the temperature of the exhaust gas becomes equal to or higher than the predetermined temperature, the exhaust gas flow path is switched to the second oxidation catalyst side or the oxidation catalyst bypass side, effectively suppressing excessive generation of NO 2 , A decrease in the NOx purification performance of the SCR catalyst can be prevented.
[0050]
In a fifth aspect of the invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to fourth aspects of the present invention, the following (1) or (2) Since control is performed so as to purify NOx in the exhaust gas by at least one of the equations, the NOx purification performance of the SCR catalyst can be effectively improved.
4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (1)
NO + NO 2 + 2NH 3 → 2N 2 + 3H 2 O (2)
[0051]
According to a sixth invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to fourth inventions, when the temperature of the exhaust gas is equal to or higher than a predetermined temperature, NO in the exhaust gas flowing into the NOx catalyst By controlling the flow path of the exhaust system so that the ratio with NO 2 is approximately 1: 1, it is possible to provide an exhaust purification device that can purify NOx most efficiently.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an exhaust emission control device for an internal combustion engine according to an embodiment of the present invention.
FIG. 2 is a graph showing the relationship between the NO → NO 2 conversion rate of the oxidation catalyst and the exhaust gas temperature.
[Explanation of symbols]
1 Exhaust Purification Device 2 Engine 3 Air Cleaner 4 Turbocharger 5 Intercooler 7 SCR Catalyst 8 Powerful Oxidation Catalyst (front)
9 Urea water addition device 10 Urea water tank 12 Urea water injection nozzle 13 Catalyst temperature sensor 14 Oxidation catalyst (rear stage)
15 Oxidation catalyst bypass 16 Switching valve 17 Exhaust gas temperature sensor

Claims (6)

内燃機関の排気系に設けられアンモニアを還元剤として排気ガス中のＮＯｘを選択還元するＮＯｘ触媒と、前記排気系のうち当該ＮＯｘ触媒よりも上流側に設けられる第１酸化触媒とを有する内燃機関の排気浄化装置において、
前記第１酸化触媒をバイパスする酸化触媒バイパスと、
前記内燃機関からの排気ガスの流路を前記第１酸化触媒または前記酸化触媒バイパスのいずれかに切り替える切替手段と、
前記内燃機関からの排気ガスの温度を検出又は推測する排気ガス温度検出手段と、
前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記内燃機関からの排気ガスが前記酸化触媒バイパスを通過するように前記切替手段を制御する切替制御手段とを備えたことを特徴とする内燃機関の排気浄化装置。An internal combustion engine having an NOx catalyst provided in an exhaust system of the internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, and a first oxidation catalyst provided upstream of the NOx catalyst in the exhaust system. In the exhaust purification device of
An oxidation catalyst bypass for bypassing the first oxidation catalyst;
Switching means for switching the flow path of the exhaust gas from the internal combustion engine to either the first oxidation catalyst or the oxidation catalyst bypass;
Exhaust gas temperature detection means for detecting or estimating the temperature of the exhaust gas from the internal combustion engine;
Switching control for controlling the switching means so that the exhaust gas from the internal combustion engine passes through the oxidation catalyst bypass when the temperature of the exhaust gas detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature. And an exhaust emission control device for an internal combustion engine. 内燃機関の排気系に設けられアンモニアを還元剤として排気ガス中のＮＯｘを選択還元するＮＯｘ触媒と、前記排気系のうち当該ＮＯｘ触媒よりも上流側に設けられる第１酸化触媒とを有する内燃機関の排気浄化装置において、
前記排気系のうち前記第１酸化触媒と並列に設けられ、前記第１酸化触媒よりも比較的酸化力が弱い第２酸化触媒と、
前記内燃機関からの排気ガスの流路を前記第１酸化触媒または前記第２酸化触媒のいずれかに切り替える切替手段と、
前記内燃機関からの排気ガスの温度を検出又は推測する排気ガス温度検出手段と、
前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記内燃機関からの排気ガスが前記第２酸化触媒を通過するように前記切替手段を制御する切替制御手段とを備えたことを特徴とする内燃機関の排気浄化装置。An internal combustion engine having an NOx catalyst provided in an exhaust system of the internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, and a first oxidation catalyst provided upstream of the NOx catalyst in the exhaust system. In the exhaust purification device of
A second oxidation catalyst provided in parallel with the first oxidation catalyst in the exhaust system and having a relatively weak oxidizing power than the first oxidation catalyst;
Switching means for switching the flow path of the exhaust gas from the internal combustion engine to either the first oxidation catalyst or the second oxidation catalyst;
Exhaust gas temperature detection means for detecting or estimating the temperature of the exhaust gas from the internal combustion engine;
Switching for controlling the switching means so that the exhaust gas from the internal combustion engine passes through the second oxidation catalyst when the temperature of the exhaust gas detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature. An exhaust emission control device for an internal combustion engine, comprising: a control means. 請求項２に記載する内燃機関の排気浄化装置において、
前記第２酸化触媒は、前記排気ガス中のＮＯを酸化してＮＯ２に転化する転化率が最大でも５０％以下であることを特徴とする内燃機関の排気浄化装置。The exhaust gas purification apparatus for an internal combustion engine according to claim 2,
The second oxidation catalyst, the exhaust gas purifying apparatus for an internal combustion engine conversion to convert by oxidizing NO in the exhaust gas to NO 2 is equal to or less than 50% at maximum. 請求項１ないし３のいずれかに係る内燃機関の排気浄化装置において、
前記所定温度は、前記第１酸化触媒が前記排気ガス中のＮＯを酸化してＮＯ２に転化する転化率が５０％となる温度であることを特徴とする内燃機関の排気浄化装置。The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3,
The exhaust gas purification apparatus for an internal combustion engine, wherein the predetermined temperature is a temperature at which a conversion rate at which the first oxidation catalyst oxidizes NO in the exhaust gas and converts it into NO 2 is 50%. 請求項１ないし４のいずれかに係る内燃機関の排気浄化装置において、
前記ＮＯｘ触媒は、主として下記（１）式又は（２）式の少なくとも一方により排気ガス中のＮＯｘを浄化することを特徴とする内燃機関の排気浄化装置。
４ＮＯ＋４ＮＨ３＋Ｏ２ → ４Ｎ２＋６Ｈ２Ｏ （１）
ＮＯ＋ＮＯ２＋２ＮＨ３ → ２Ｎ２＋３Ｈ２Ｏ （２）The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4,
The exhaust gas purification apparatus for an internal combustion engine, wherein the NOx catalyst purifies NOx in exhaust gas mainly by at least one of the following formulas (1) and (2).
4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O (1)
NO + NO 2 + 2NH 3 → 2N 2 + 3H 2 O (2) 請求項１ないし４のいずれかに係る内燃機関の排気浄化装置において、
前記切替制御手段は、前記排気ガス温度検出手段により検出又は推測される排気ガスの温度が所定温度以上であるときに、前記切替手段により、前記第１酸化触媒を流れる排気ガスと前記第２酸化触媒又は前記酸化触媒バイパスを流れる排気ガスとの流量比を調整して、前記ＮＯｘ触媒に流入する排気ガス中のＮＯとＮＯ２との比が略１対１となるように制御することを特徴とする内燃機関の排気浄化装置。The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4,
When the exhaust gas temperature detected or estimated by the exhaust gas temperature detection means is equal to or higher than a predetermined temperature, the switching control means is configured to switch the exhaust gas flowing through the first oxidation catalyst and the second oxidation by the switching means. The flow rate ratio with the exhaust gas flowing through the catalyst or the oxidation catalyst bypass is adjusted to control the ratio of NO and NO 2 in the exhaust gas flowing into the NOx catalyst to be approximately 1: 1. An exhaust purification device for an internal combustion engine.
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