JP2009250135A - Exhaust emission control device - Google Patents

Exhaust emission control device Download PDF

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JP2009250135A
JP2009250135A JP2008099878A JP2008099878A JP2009250135A JP 2009250135 A JP2009250135 A JP 2009250135A JP 2008099878 A JP2008099878 A JP 2008099878A JP 2008099878 A JP2008099878 A JP 2008099878A JP 2009250135 A JP2009250135 A JP 2009250135A
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exhaust
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JP5155718B2 (en
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Takatoshi Furukawa
卓俊 古川
Nobuhiro Funayama
悦弘 舩山
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Hino Motors Ltd
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    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02ATECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02TCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00Road transport of goods or passengers
    • Y02T10/10Internal combustion engine [ICE] based vehicles
    • Y02T10/12Improving ICE efficiencies

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  • Exhaust Gas After Treatment (AREA)
  • Processes For Solid Components From Exhaust (AREA)
  • Electrical Control Of Air Or Fuel Supplied To Internal-Combustion Engine (AREA)
  • Exhaust Gas Treatment By Means Of Catalyst (AREA)

Abstract

<P>PROBLEM TO BE SOLVED: To prevent discharge of HC to the outside of a vehicle by premixed compression ignition, the deterioration in fuel economy and an increase in an HC discharge quantity by useless fuel addition in a temperature area of the low NOx reduction ratio. <P>SOLUTION: This exhaust emission control device has temperature sensors 23 and 24 for detecting the exhaust temperature in a proper position of an exhaust pipe 11 as a substitute value for estimating the catalyst bed temperature of a selective reduction type catalyst 13, and has a control device 22 for permitting premixed compression ignition control only when estimating that the catalyst bed temperature of the selective reduction type catalyst 13 is the active temperature or more based on a detection value of the temperature sensors 23 and 24, permitting fuel addition only when its estimated catalyst bed temperature of the selective reduction type catalyst 13 falls within an effective temperature range sandwiched between a high temperature area unable to provide the required NOx reduction ratio and a low temperature area of not expecting sure combustion of fuel and controlling a fuel adding quantity so that the adding fuel/NOx ratio becomes a right optimal value in response to the NOx reduction ratio in its estimated catalyst bed temperature of the selective reduction type catalyst 13. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

本発明は、排気浄化装置に関するものである。   The present invention relates to an exhaust emission control device.

従来より、自動車のディーゼルエンジン等では、排気側から排気ガスの一部を抜き出して吸気側へと戻し、その吸気側に戻された排気ガスでエンジン内での燃料の燃焼を抑制させて燃焼温度を下げることによりNOxの発生を低減するようにした、いわゆる排気ガス再循環(EGR:Exhaust Gas Recirculation)を採用したものがある。   Conventionally, in an automobile diesel engine or the like, a part of the exhaust gas is extracted from the exhaust side and returned to the intake side, and the exhaust gas returned to the intake side suppresses the combustion of fuel in the engine to reduce the combustion temperature. Some of them adopt so-called exhaust gas recirculation (EGR), which reduces the generation of NOx by lowering.

ただし、排気ガスの再循環によりNOxの低減化を図ることは、気筒内での燃焼不良により黒煙を発生してしまうこととトレードオフの関係にあるので、黒煙の発生を抑制する観点から排気ガスの再循環量に制限がかかるという不具合があり、単純に排気ガスの再循環を行うだけで大幅なNOxの低減化を図ることは困難である。   However, reducing NOx by exhaust gas recirculation is in a trade-off relationship with the generation of black smoke due to poor combustion in the cylinder, so from the viewpoint of suppressing the generation of black smoke. There is a problem in that the amount of exhaust gas recirculation is limited, and it is difficult to significantly reduce NOx simply by recirculating exhaust gas.

このため、近年においては、通常であれば圧縮上死点近辺で行われるべき燃料噴射を圧縮上死点より早いタイミングで行い、気筒内への燃料の先行投入により燃料の予混合化を促進してから着火燃焼させて黒煙の発生を抑制するようにした予混合圧縮着火の採用が検討されている(例えば、特許文献1参照)。   For this reason, in recent years, fuel injection that should normally be performed near the compression top dead center is performed at a timing earlier than the compression top dead center, and fuel premixing is promoted by prior injection of fuel into the cylinder. Adoption of premixed compression ignition that suppresses the generation of black smoke by igniting and burning after that has been studied (for example, see Patent Document 1).

即ち、このような予混合圧縮着火を採用して燃焼を行うと、燃料が良好に分散混合して均等に薄まった状態で燃焼が行われることになるので、燃焼温度が比較的低く抑制されてNOxの発生が少なくなり、しかも、局所的に燃料の濃い部分が生じ難くなって黒煙の発生を抑制する上でも有効となる。   That is, if combustion is performed using such premixed compression ignition, combustion is performed in a state where the fuel is well dispersed and mixed and evenly diluted, so the combustion temperature is suppressed to a relatively low level. The generation of NOx is reduced, and moreover, it is difficult to produce a fuel-rich portion locally, which is effective in suppressing the generation of black smoke.

尚、予混合圧縮着火を採用するに際しては、燃料の投入タイミングを圧縮上死点より早めても、その着火については圧縮上死点付近で行われることが望ましいが、通常のエンジンの圧縮比では、圧縮上死点より早いタイミングで着火する過早着火が起こって燃焼騒音の悪化や熱効率的な損失が生じるという問題があり、特にエンジン温度が高くなる高負荷の運転条件では、圧縮に伴い混合気の温度が早期に着火温度に達してしまうので、圧縮上死点よりもかなり早いタイミングで過早着火が起こって大幅な燃焼騒音の悪化を招いてしまう結果となり、現状では軽負荷の運転条件でしか予混合圧縮着火を採用する目処が立っていない。   When adopting premixed compression ignition, it is desirable that the ignition be performed near the compression top dead center even if the fuel injection timing is advanced from the compression top dead center. There is a problem that pre-ignition occurs at a timing earlier than the compression top dead center, resulting in deterioration of combustion noise and loss of heat efficiency, especially in high-load operating conditions where the engine temperature is high, mixing with compression As the temperature of the air reaches the ignition temperature early, pre-ignition occurs at a timing much earlier than the compression top dead center, resulting in a significant deterioration in combustion noise. However, there is no prospect of adopting premixed compression ignition.

ただし、前述した如き予混合圧縮着火の運転においては、NOxを低減できる背反として大量のHCとCOが発生してしまうという不具合があり、これらのHC及びCOが車外へ排出されてしまわないよう排気管途中に酸化触媒を設置する必要があるが、一般的に予混合圧縮着火が適用される軽負荷の運転領域では排気温度が低く、酸化触媒に良好な触媒活性を発揮させることができないことから十分な排気浄化性能が得られないという問題があった。   However, in the operation of the premixed compression ignition as described above, there is a problem that a large amount of HC and CO is generated as a contradiction that can reduce NOx, and the exhaust is performed so that these HC and CO are not discharged outside the vehicle. Although it is necessary to install an oxidation catalyst in the middle of the pipe, the exhaust temperature is generally low in the light load operation region where premixed compression ignition is applied, and the oxidation catalyst cannot exhibit good catalytic activity. There was a problem that sufficient exhaust purification performance could not be obtained.

そこで、本発明者らは、ディーゼルエンジンの排気管上流部にHCよりもCOを優先して酸化処理するプレ酸化触媒を設けると共に、該プレ酸化触媒の下流側に酸素共存下でも選択的にNOxをHCと反応させる触媒機能とNOx吸着能とを備えた選択還元型触媒を設け、更には、選択還元型触媒と前記プレ酸化触媒との間に排気ガス中に燃料を添加する燃料添加手段を設けてなる排気浄化装置を創案するに到った。   Accordingly, the present inventors have provided a pre-oxidation catalyst that preferentially oxidizes CO over HC at the upstream portion of the exhaust pipe of a diesel engine, and selectively NOx downstream of the pre-oxidation catalyst even in the presence of oxygen. A selective reduction catalyst having a catalytic function for reacting HC with HC and NOx adsorption ability is provided, and further a fuel addition means for adding fuel to the exhaust gas between the selective reduction catalyst and the pre-oxidation catalyst. The inventor has come up with an exhaust purification device.

このようにすれば、排気温度の低い軽負荷時等に予混合圧縮着火が適用されてHCとCOが大量に発生したとしても、排気管上流部に設けられているプレ酸化触媒は、ディーゼルエンジンから排出された直後の高温の排気ガスに晒されてCO,HCを酸化処理するに十分な触媒活性を既に得ているので、このプレ酸化触媒にてCOとHCが酸化処理され、その酸化反応熱により排気ガスの昇温化が図られることになる。   In this way, even if premixed compression ignition is applied at a light load with a low exhaust temperature and a large amount of HC and CO is generated, the pre-oxidation catalyst provided in the upstream portion of the exhaust pipe is a diesel engine. Since it has been exposed to high-temperature exhaust gas immediately after being discharged from the catalyst and has already obtained sufficient catalytic activity to oxidize CO and HC, CO and HC are oxidized by this pre-oxidation catalyst, and the oxidation reaction The temperature of the exhaust gas is increased by heat.

この際、COはHCよりも反応性が高く、COからCO2への酸化処理の反応速度も速いため、プレ酸化触媒を排気管上流部に設けておくだけで得られる触媒活性だけでもCOの大半を酸化処理することが可能であるが、反応速度の遅いHCの多くをプレ酸化触媒で酸化処理することは困難であり、HCの多くはプレ酸化触媒を通り抜けて下流側へ向かうことになる。 At this time, CO is more reactive than HC, and the reaction rate of the oxidation treatment from CO to CO 2 is fast. Therefore, the catalytic activity obtained only by providing a pre-oxidation catalyst upstream of the exhaust pipe is sufficient for CO. Although most of the HC can be oxidized, it is difficult to oxidize most of the slow HC with the pre-oxidation catalyst, and most of the HC passes through the pre-oxidation catalyst and goes downstream. .

そして、このプレ酸化触媒を通り抜けたHCは、該プレ酸化触媒で昇温された排気ガスと共に下流側の選択還元型触媒に到り、これまでに選択還元型触媒に吸着されたNOxを還元浄化するための還元剤として利用される。   The HC passing through the pre-oxidation catalyst reaches the downstream selective reduction catalyst together with the exhaust gas heated by the pre-oxidation catalyst, and reduces and purifies NOx adsorbed on the selective reduction catalyst so far. It is used as a reducing agent.

即ち、下流側の選択還元型触媒では、予混合圧縮着火によりNOxの低減化が図られる前からのNOxの吸着が進んでいるので、プレ酸化触媒で昇温された排気ガスに晒されて触媒床温度が上げられた選択還元型触媒にHCが導かれてくると、このHCを還元剤として吸着NOxの還元浄化が図られる。   That is, in the selective catalytic reduction catalyst on the downstream side, NOx adsorption has progressed before the reduction of NOx by premixed compression ignition, so the catalyst is exposed to the exhaust gas heated by the pre-oxidation catalyst. When HC is led to the selective catalytic reduction catalyst whose bed temperature has been raised, the reduced purification of adsorbed NOx is achieved using this HC as a reducing agent.

尚、選択還元型触媒へのNOxの吸着は、雰囲気中のCOの存在により阻害される(吸着NOxの放出が促される)ことが本発明者らの知見として得られているが、プレ酸化触媒により先行してCOを除去しておけば、このような懸念を払拭して選択還元型触媒のNOx吸着能を最大限に利用することが可能となる。   Although the present inventors have obtained that NOx adsorption on the selective catalytic reduction catalyst is inhibited by the presence of CO in the atmosphere (the release of adsorbed NOx is promoted), the pre-oxidation catalyst If CO is removed in advance, it is possible to eliminate such a concern and make maximum use of the NOx adsorption ability of the selective catalytic reduction catalyst.

また、予混合圧縮着火が適用されない中高負荷時においては、燃料添加手段により燃料を添加し、この添加燃料から生じたHCを還元剤として選択還元型触媒でNOxの還元浄化を行わせるようにすれば良い。
特開平11−107792号公報
Further, at the time of medium and high loads where premixed compression ignition is not applied, fuel is added by the fuel addition means, and the selective reduction catalyst is used to reduce and purify NOx using HC generated from the added fuel as a reducing agent. It ’s fine.
Japanese Patent Application Laid-Open No. 11-107772

しかしながら、前述の如き新たに創案された排気浄化装置においても、長時間のエンジン停止後の始動時(コールドスタート時)等では、排気系の暖機が十分に進むまでの間に選択還元型触媒の触媒床温度が活性温度域まで上昇しきらない時間帯ができてしまい、この間に実施された予混合圧縮着火により発生したHCの車外への排出を防止できないという問題がある。   However, even in the newly-developed exhaust gas purification apparatus as described above, the selective catalytic reduction catalyst is required until the exhaust system warms up sufficiently at the time of start after a long engine stop (during cold start) or the like. Thus, there is a problem that the catalyst bed temperature does not fully rise to the activation temperature range, and it is not possible to prevent the discharge of HC generated by the premixed compression ignition performed during this time.

また、従来の選択還元型触媒を備えた排気浄化装置にあっては、NOx流量に対し、ある一定の割合で燃料添加を実施しているのが通常であるが、本発明者らの知見によれば、選択還元型触媒のNOx低減率が、ある特定の触媒床温度の近辺で高くなり、この近辺から離れるに従い著しく低下してしまうことが確認されているため、従前通りに単純に一定の割合で燃料を添加し続けてしまうと、NOx低減率の低い温度域での添加燃料の多くが無駄になり、燃費の悪化やHC排出量の増加といった不具合を招く虞れがある。   In addition, in an exhaust emission control device equipped with a conventional selective reduction catalyst, it is usual to add fuel at a certain ratio with respect to the NOx flow rate. According to this, it has been confirmed that the NOx reduction rate of the selective catalytic reduction catalyst becomes high near a specific catalyst bed temperature and decreases remarkably as it goes away from this vicinity. If fuel is continuously added at a rate, much of the added fuel in the temperature range where the NOx reduction rate is low is wasted, and there is a risk of inconveniences such as deterioration in fuel consumption and increase in HC emissions.

本発明は上述の実情に鑑みてなしたもので、長時間のエンジン停止後の始動時等で予混合圧縮着火により発生したHCが車外へ多く排出されてしまったり、NOx低減率の低い温度域での無駄な燃料添加により燃費の悪化やHC排出量の増加を招いてしまったりする問題を解決し得る排気浄化装置を提供することを目的としている。   The present invention has been made in view of the above circumstances, and a large amount of HC generated by premixed compression ignition at the time of starting after a long engine stop or the like is discharged to the outside of the vehicle, or a temperature range where the NOx reduction rate is low. It is an object of the present invention to provide an exhaust emission control device that can solve the problem of wasteful fuel addition in which the deterioration of fuel consumption and the increase of HC emissions are caused.

本発明は、少なくとも軽負荷時に予混合圧縮着火制御が適用されるディーゼルエンジンの排気管上流部にHCよりもCOを優先して酸化処理するプレ酸化触媒を設け、該プレ酸化触媒の下流側に酸素共存下でも選択的にNOxをHCと反応させる触媒機能とNOx吸着能とを備えた選択還元型触媒を設け、該選択還元型触媒と前記プレ酸化触媒との間に排気ガス中に燃料を添加する燃料添加手段を設けた排気浄化装置であって、選択還元型触媒の触媒床温度を推定するための代用値として排気管の適宜位置で排気温度を検出する温度センサを備え、該温度センサの検出値に基づき選択還元型触媒の触媒床温度が活性温度以上であると推定された場合に限り予混合圧縮着火制御を許可し、その推定された選択還元型触媒の触媒床温度が必要なNOx低減率を得られない高温域と燃料の確実な燃焼が見込めない低温域とに挟まれた有効温度範囲にある場合に限り燃料添加を許可すると共に、その推定された選択還元型触媒の触媒床温度でのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加量を制御する制御装置を備えたことを特徴とするものである。   The present invention provides a pre-oxidation catalyst that preferentially oxidizes CO over HC over an exhaust pipe upstream portion of a diesel engine to which premixed compression ignition control is applied at least at a light load, and downstream of the pre-oxidation catalyst. A selective reduction type catalyst having a catalytic function for selectively reacting NOx with HC even in the presence of oxygen and an NOx adsorption capability is provided, and fuel is introduced into the exhaust gas between the selective reduction type catalyst and the pre-oxidation catalyst. An exhaust purification device provided with a fuel addition means for adding, comprising a temperature sensor for detecting an exhaust temperature at an appropriate position of an exhaust pipe as a substitute value for estimating a catalyst bed temperature of a selective catalytic reduction catalyst, the temperature sensor The premixed compression ignition control is permitted only when the catalyst bed temperature of the selective catalytic reduction catalyst is estimated to be equal to or higher than the activation temperature based on the detected value, and the estimated catalytic bed temperature of the selective catalytic reduction catalyst is required. NOx Fuel addition is permitted only when the temperature is between the high temperature range where the reduction rate cannot be obtained and the low temperature range where reliable combustion of the fuel cannot be expected, and the catalyst bed of the estimated selective catalytic reduction catalyst is used. A control device is provided that controls the amount of fuel added so that the added fuel / NOx ratio becomes an optimum value without excess or deficiency corresponding to the NOx reduction rate at temperature.

このようにすれば、排気温度の低い軽負荷時等で予混合圧縮着火を適用するに際し、温度センサの検出値に基づき選択還元型触媒の触媒床温度が活性温度以上であるかどうかが制御装置で確認され、この活性温度を下まわっている場合には予混合圧縮着火制御が許可されないため、長時間のエンジン停止後の始動時(コールドスタート時)等に、選択還元型触媒の触媒床温度が活性温度域まで上昇していない状況下で予混合圧縮着火が実施されることがなくなり、予混合圧縮着火を要因とするHCの車外への排出が防止される。   In this way, when premixed compression ignition is applied at a light load with a low exhaust temperature or the like, it is determined whether or not the catalyst bed temperature of the selective catalytic reduction catalyst is equal to or higher than the activation temperature based on the detection value of the temperature sensor. When the temperature is below this activation temperature, premixed compression ignition control is not permitted. Therefore, the catalyst bed temperature of the selective catalytic reduction catalyst during a start after a long engine stop (cold start), etc. Premixed compression ignition is no longer performed in a situation where the temperature does not rise to the active temperature range, and HC is prevented from being discharged outside the vehicle due to premixed compression ignition.

また、一般的に予混合圧縮着火が適用されない中高負荷時等において、燃料添加手段により燃料を添加し、この添加燃料から生じたHCを還元剤として選択還元型触媒でNOxの還元浄化を行わせるに際し、温度センサの検出値に基づき推定された選択還元型触媒の触媒床温度が必要なNOx低減率を得られない高温域と燃料の確実な燃焼が見込めない低温域とに挟まれた有効温度範囲にあるかどうかが制御装置で確認され、この有効温度範囲を外れている場合には燃料添加が許可されず、しかも、有効温度範囲に入っていても、その推定された選択還元型触媒の触媒床温度でのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加量が制御されるため、無駄な燃料添加が極力回避されて燃費の悪化やHC排出量の増加が防止される。   Also, in the case of medium to high loads where premixed compression ignition is not generally applied, fuel is added by the fuel addition means, and NOx is reduced and purified by the selective reduction catalyst using HC generated from the added fuel as a reducing agent. In this case, the effective temperature sandwiched between the high temperature range where the required NOx reduction rate is not obtained and the low temperature range where reliable combustion of the fuel cannot be expected. The controller checks whether the temperature is within the effective temperature range, and if it is outside this effective temperature range, fuel addition is not permitted, and even if it is within the effective temperature range, the estimated selective catalytic reduction catalyst The amount of fuel added is controlled so that the added fuel / NOx ratio becomes an optimum value with no excess or deficiency corresponding to the NOx reduction rate at the catalyst bed temperature, so that unnecessary fuel addition is avoided as much as possible and fuel consumption deteriorates. HC emissions Increase in volume is prevented.

更に、本発明においては、選択還元型触媒の前段にパティキュレートフィルタを設けると共に、該パティキュレートフィルタの更に前段にメイン酸化触媒を設け、該メイン酸化触媒とプレ酸化触媒との間に燃料添加手段が配置されるように構成することが好ましい。   Furthermore, in the present invention, a particulate filter is provided in the preceding stage of the selective catalytic reduction catalyst, and a main oxidation catalyst is provided in the further preceding stage of the particulate filter, and fuel adding means is provided between the main oxidation catalyst and the pre-oxidation catalyst. Are preferably arranged.

このようにすれば、NOxとパティキュレートの同時低減を図ることが可能となり、しかも、パティキュレートフィルタの強制再生を行う必要が生じた際に、燃料添加手段により燃料を添加すると、この添加燃料から生じた高濃度のHCがメイン酸化触媒を通過する間に酸化反応し、その反応熱で昇温した排気ガスの流入により直後のパティキュレートフィルタの触媒床温度が上げられてパティキュレートが燃やし尽くされ、パティキュレートフィルタの再生化が図られることになる。   In this way, it is possible to simultaneously reduce NOx and particulates. Moreover, when it is necessary to perform forced regeneration of the particulate filter, if fuel is added by the fuel addition means, the added fuel is The generated high-concentration HC undergoes an oxidation reaction while passing through the main oxidation catalyst, and the inflow of exhaust gas heated by the reaction heat raises the catalyst bed temperature of the particulate filter immediately after that to burn out the particulates. Thus, regeneration of the particulate filter is achieved.

上記した本発明の排気浄化装置によれば、下記の如き種々の優れた効果を奏し得る。   According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

(I)本発明の請求項1に記載の発明によれば、温度センサにより排気管の適宜位置で選択還元型触媒の触媒床温度の代用値として排気温度を検出し、その検出値に基づき予混合圧縮着火制御及び燃料添加の許可判定と燃料添加量制御を適切に行うようにしているので、長時間のエンジン停止後の始動時等で予混合圧縮着火により発生したHCが車外へ多く排出されてしまう問題や、NOx低減率の低い温度域での無駄な燃料添加により燃費の悪化やHC排出量の増加を招いてしまう問題を確実に解決することができる。   (I) According to the first aspect of the present invention, the exhaust gas temperature is detected as a substitute value for the catalyst bed temperature of the selective catalytic reduction catalyst at an appropriate position of the exhaust pipe by the temperature sensor, and based on the detected value, Since mixed compression ignition control and fuel addition permission determination and fuel addition amount control are appropriately performed, a large amount of HC generated by premixed compression ignition at the start after a long engine stop is discharged to the outside of the vehicle. And the problem of causing deterioration of fuel consumption and increase in HC emissions due to wasteful fuel addition in a temperature range where the NOx reduction rate is low can be reliably solved.

(II)本発明の請求項2に記載の発明によれば、NOxとパティキュレートの同時低減を図ることができ、しかも、予混合圧縮着火を適用しない場合に選択還元型触媒への還元剤として燃料を添加するための燃料添加手段を流用してパティキュレートフィルタの強制再生を実施することができる。   (II) According to the invention described in claim 2 of the present invention, NOx and particulates can be simultaneously reduced, and as a reducing agent for the selective catalytic reduction catalyst when premixed compression ignition is not applied. The forced regeneration of the particulate filter can be performed by diverting the fuel addition means for adding the fuel.

以下本発明の実施の形態を図面を参照しつつ説明する。   Embodiments of the present invention will be described below with reference to the drawings.

図1は本発明を実施する形態の一例を示すもので、図中1はターボチャージャ2を装備したディーゼルエンジンを示しており、エアクリーナ3から導かれた吸気4が吸気管5を通し前記ターボチャージャ2のコンプレッサ2aへと送られ、該コンプレッサ2aで加圧された吸気4がインタークーラ6へと送られて冷却され、該インタークーラ6から更に吸気マニホールド7へと吸気4が導かれてディーゼルエンジン1の各気筒8(図1では直列6気筒の場合を例示している)に分配されるようになっている。   FIG. 1 shows an example of an embodiment of the present invention. In FIG. 1, reference numeral 1 shows a diesel engine equipped with a turbocharger 2, and intake air 4 guided from an air cleaner 3 passes through an intake pipe 5 and the turbocharger. 2, the intake air 4 pressurized by the compressor 2 a is sent to the intercooler 6 to be cooled, and the intake air 4 is further guided from the intercooler 6 to the intake manifold 7. 1 is distributed to each cylinder 8 (FIG. 1 illustrates the case of inline 6 cylinders).

また、前記ディーゼルエンジン1の各気筒8から排出された排気ガス9は、排気マニホールド10を介しターボチャージャ2のタービン2bへと送られ、該タービン2bを駆動した排気ガス9が排気管11を介し車外へ排出されるようにしてある。   Further, the exhaust gas 9 discharged from each cylinder 8 of the diesel engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 10, and the exhaust gas 9 driving the turbine 2b passes through the exhaust pipe 11. It is designed to be discharged outside the vehicle.

そして、前記排気管11におけるタービン2bの出口(排気管11の上流部)には、HCよりもCOを優先して酸化処理するプレ酸化触媒12が装備されており、このプレ酸化触媒12は、例えば、Pt/Pd/アルミナを原料として担持させた酸化触媒とすれば良い。   A pre-oxidation catalyst 12 that preferentially oxidizes CO over HC is provided at the outlet of the turbine 2b in the exhaust pipe 11 (upstream portion of the exhaust pipe 11). For example, an oxidation catalyst in which Pt / Pd / alumina is supported as a raw material may be used.

更に、このプレ酸化触媒12より下流側の排気管11には、酸素共存下でも選択的にNOxをHCと反応させる触媒機能とNOx吸着能とを備えた選択還元型触媒13が介装されており、この選択還元型触媒13は、例えば、Pt/ゼオライト触媒等により構成されている。   Further, the exhaust pipe 11 downstream of the pre-oxidation catalyst 12 is provided with a selective reduction catalyst 13 having a catalytic function for selectively reacting NOx with HC and coexisting with NOx even in the presence of oxygen. The selective reduction catalyst 13 is composed of, for example, a Pt / zeolite catalyst.

また、特に本形態例においては、選択還元型触媒13の前段に、酸化触媒を一体的に担持して成る触媒再生型のパティキュレートフィルタ14が収容されており、このパティキュレートフィルタ14は、セラミックから成る多孔質のハニカム構造を有し、格子状に区画された各流路の入口が交互に目封じされ、入口が目封じされていない流路については、その出口が目封じされるようになっており、各流路を区画する多孔質薄壁を透過した排気ガス9のみが下流側へ排出されるようにしてある。尚、このパティキュレートフィルタ14には、例えば、Pt/Pd/アルミナにセリア(CeO2)を加えた酸化触媒等を一体的に担持させておけば良い。 In particular, in the present embodiment, a catalyst regeneration type particulate filter 14 that integrally supports an oxidation catalyst is accommodated in the preceding stage of the selective reduction catalyst 13, and the particulate filter 14 is made of ceramic. So that the inlets of the respective flow paths partitioned in a lattice pattern are alternately sealed, and the outlets of the flow paths that are not sealed are sealed. Thus, only the exhaust gas 9 that has permeated through the porous thin wall partitioning each flow path is discharged to the downstream side. The particulate filter 14 may be integrally supported with, for example, an oxidation catalyst obtained by adding ceria (CeO 2 ) to Pt / Pd / alumina.

更に、このパティキュレートフィルタ14の前段には、フロースルー型のメイン酸化触媒15が装備されており、このメイン酸化触媒15には、例えば、Pt/アルミナにセリア(CeO2)を加えた原料を担持させておけば良い。 Further, a flow-through type main oxidation catalyst 15 is provided in the preceding stage of the particulate filter 14, and the main oxidation catalyst 15 is made of, for example, a raw material obtained by adding ceria (CeO 2 ) to Pt / alumina. It only has to be supported.

また、メイン酸化触媒15とプレ酸化触媒12との間には、排気ガス9中に燃料を添加する燃料添加手段として燃料添加弁16が装備されており、この燃料添加弁16には、所要場所に配置された燃料タンク17から導いた燃料添加ライン18が接続されており、該燃料添加ライン18の途中に装備したポンプ19の駆動により燃料タンク17内の燃料が抜き出されて前記燃料添加弁16に向けて供給され、該燃料添加弁16のノズル先端から排気管11内に噴射されるようになっている。   Further, a fuel addition valve 16 is provided between the main oxidation catalyst 15 and the pre-oxidation catalyst 12 as a fuel addition means for adding fuel to the exhaust gas 9. A fuel addition line 18 led from a fuel tank 17 disposed in the fuel addition line 18 is connected, and fuel in the fuel tank 17 is extracted by driving a pump 19 provided in the middle of the fuel addition line 18 so that the fuel addition valve is connected. 16 and is injected into the exhaust pipe 11 from the nozzle tip of the fuel addition valve 16.

そして、前記ディーゼルエンジン1には、各気筒8毎に装備されたインジェクタ20から成る燃料噴射装置21が搭載されており、該燃料噴射装置21における各インジェクタ20の電磁弁が、エンジン制御コンピュータ(ECU:Electronic Control Unit)を成す制御装置22により負荷や回転数に基づいて噴射タイミングや噴射量(開弁時間)が制御信号21aを介して適切に制御されるようにしてあるが、特に軽負荷時においては、圧縮上死点近辺で行われるべきメイン噴射を圧縮上死点より早いタイミングで行う予混合圧縮着火が適用されるようになっている。   The diesel engine 1 is equipped with a fuel injection device 21 composed of an injector 20 provided for each cylinder 8, and an electromagnetic valve of each injector 20 in the fuel injection device 21 is connected to an engine control computer (ECU). : The control device 22 forming an electronic control unit) appropriately controls the injection timing and the injection amount (valve opening time) based on the load and the rotational speed via the control signal 21a. , Premixed compression ignition in which main injection to be performed in the vicinity of the compression top dead center is performed at a timing earlier than the compression top dead center is applied.

ただし、この制御装置22には、メイン酸化触媒15の入口と出口に夫々配置した温度センサ23,24からの検出信号23a,24aが入力されるようになっており、これら各温度センサ23,24で検出される平均ガス温度を、選択還元型触媒13の触媒床温度を推定するための代用値とし、この平均ガス温度が所定値以上となった時に選択還元型触媒13の触媒床温度が活性温度以上であると推定し、このような活性温度以上であるとの推定が成された場合に限り予混合圧縮着火制御を燃料噴射装置21に対し許可するようにしてある。   However, the control device 22 is supplied with detection signals 23a and 24a from temperature sensors 23 and 24 arranged at the inlet and the outlet of the main oxidation catalyst 15, respectively. Is used as a substitute value for estimating the catalyst bed temperature of the selective catalytic reduction catalyst 13, and the catalytic bed temperature of the selective catalytic reduction catalyst 13 is activated when the average gas temperature exceeds a predetermined value. The premixed compression ignition control is permitted to the fuel injection device 21 only when it is estimated that the temperature is higher than the temperature and that the temperature is higher than the activation temperature.

また、その推定された選択還元型触媒13の触媒床温度が必要なNOx低減率を得られない高温域と燃料の確実な燃焼が見込めない低温域とに挟まれた有効温度範囲にある場合に限り燃料添加を許可すると共に、その推定された選択還元型触媒13の触媒床温度でのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加弁16及びポンプ19を制御信号16a,19aを介して制御するようになっている。   Further, when the estimated catalyst bed temperature of the selective catalytic reduction catalyst 13 is in an effective temperature range sandwiched between a high temperature range where the required NOx reduction rate cannot be obtained and a low temperature range where reliable combustion of the fuel cannot be expected. As long as fuel addition is permitted, the fuel addition valve 16 is adjusted so that the added fuel / NOx ratio becomes an optimum value with no excess or deficiency corresponding to the estimated NOx reduction rate at the catalyst bed temperature of the selective catalytic reduction catalyst 13. The pump 19 is controlled through control signals 16a and 19a.

即ち、図2のグラフに、縦軸にNOx低減率をとり、横軸に選択還元型触媒13の触媒床温度をとって、添加燃料/NOx比が「2」の場合Aと、添加燃料/NOx比が「4」の場合Bと、添加燃料/NOx比が「6」の場合Cとを夫々示している通り、選択還元型触媒13の触媒床温度のT1℃近辺でA,B,Cの何れの添加燃料/NOx比の場合もNOx低減率がピークとなる。 That is, in the graph of FIG. 2, the vertical axis represents the NOx reduction rate, the horizontal axis represents the catalyst bed temperature of the selective catalytic reduction catalyst 13, and the additive fuel / NOx ratio is “2”. and when NOx ratio is "4" B, added fuel / as NOx ratio indicates people each case and C "6", a at around T 1 ° C. of the catalyst bed temperature of the selective reduction catalyst 13, B, The NOx reduction rate reaches a peak at any added fuel / NOx ratio of C.

そして、ここから触媒床温度が上昇するのに従いNOx低減率は低下していくことになるが、NOx低減率がピークとなるT1℃近辺では、添加燃料/NOx比が高いものほど極めて高いNOx低減率が得られるが、T2℃まで温度上昇してしまうと、A,B,Cの何れの添加燃料/NOx比もNOx低減率が略同程度まで下がり切ってしまい、添加燃料/NOx比を減らしてもNOx低減率にそれほどの差が生じないことが判る。 The NOx reduction rate decreases as the catalyst bed temperature rises from here, but the higher the added fuel / NOx ratio is, the higher the NOx reduction rate is in the vicinity of T 1 ° C where the NOx reduction rate peaks. Although a reduction rate can be obtained, if the temperature rises to T 2 ° C, the added fuel / NOx ratio of any of A, B, and C decreases to almost the same level as the added fuel / NOx ratio, and the added fuel / NOx ratio It can be seen that there is no significant difference in the NOx reduction rate even if the amount is reduced.

また、T1℃近辺から触媒床温度が低下した場合もNOx低減率が急激に低下していくことになるが、T1℃近辺より温度が低い領域では、添加燃料の確実な燃焼が見込めなくなってしまうという別の側面もあり、選択還元型触媒13に添加燃料が残留してしまう懸念がある。 Also, when the catalyst bed temperature decreases from around T 1 ° C, the NOx reduction rate will drop sharply, but in the region where the temperature is lower than around T 1 ° C, reliable combustion of the added fuel cannot be expected. There is another aspect that the added fuel remains in the selective catalytic reduction catalyst 13.

そこで、必要なNOx低減率を得られないT2℃より高い高温域と、燃料の確実な燃焼が見込めないT1℃より低い低温域とに挟まれた有効温度範囲T1℃〜T2℃に触媒床温度がある場合に限り燃料添加を許可し、しかも、そのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加量を制御している。 Therefore, an effective temperature range T 1 ° C. to T 2 ° C. sandwiched between a high temperature range higher than T 2 ° C where the required NOx reduction rate cannot be obtained and a low temperature range lower than T 1 ° C where reliable combustion of the fuel cannot be expected. Fuel addition is permitted only when the catalyst bed temperature is present, and the amount of fuel added is controlled so that the added fuel / NOx ratio becomes an optimum value with no excess or deficiency corresponding to the NOx reduction rate.

ここで、制御装置22で添加燃料/NOx比を決定するにあたり、現在のNOx流量を算出する必要があるが、負荷や回転数等の入力信号に基づき現在の運転状態が制御装置22により把握されているので、例えば、現在の運転状態に照らしてNOxの発生マップ等から現在のNOx流量を読み出すようにすれば良い。尚、NOxセンサと吸気流量センサとを備えて、両者の実測値からNOx流量を算出することも可能である。   Here, in determining the added fuel / NOx ratio by the control device 22, it is necessary to calculate the current NOx flow rate, but the current operation state is grasped by the control device 22 based on input signals such as the load and the rotational speed. Therefore, for example, the current NOx flow rate may be read from a NOx generation map or the like in light of the current operation state. It is also possible to provide a NOx sensor and an intake flow rate sensor, and calculate the NOx flow rate from the measured values of both.

尚、燃料添加を許可するか否かに関する判定は、制御装置22において、例えば、図3に示す如く、メイン酸化触媒15の入口ガス温度がT1℃以上であるか、入口と出口のガス平均温度がT1℃〜T2℃の範囲にあるかの何れかが確認された時に、最終的に出口ガス温度がT2℃以下であることを確かめてから燃料添加を許可するようにしている。 Whether or not fuel addition is permitted is determined by the control device 22, for example, as shown in FIG. 3, whether the inlet gas temperature of the main oxidation catalyst 15 is T 1 ° C or higher, or the average gas at the inlet and outlet. When it is confirmed that the temperature is in the range of T 1 ° C to T 2 ° C, the fuel addition is permitted after confirming that the outlet gas temperature is finally T 2 ° C or lower. .

また、ここに図示している例では、排気マニホールド10における各気筒8の並び方向の一端部と、吸気マニホールド7に接続されている吸気管5の一端部との間がEGRライン25で接続されており、排気マニホールド10から抜き出した排気ガス9の一部をEGR用酸化触媒28に通して排気ガス9のHCとSOF分(Soluble Organic Fraction:可溶性有機成分)を酸化浄化し、次いで、水冷式のEGRクーラ26で冷却してEGRバルブ27を介し吸気管5に再循環するようにしてある。   Further, in the example shown here, an EGR line 25 connects between one end of the exhaust manifold 10 in the direction in which the cylinders 8 are arranged and one end of the intake pipe 5 connected to the intake manifold 7. A part of the exhaust gas 9 extracted from the exhaust manifold 10 is passed through an EGR oxidation catalyst 28 to oxidize and purify HC and SOF (Soluble Organic Fraction) of the exhaust gas 9, and then water-cooled This is cooled by the EGR cooler 26 and recirculated to the intake pipe 5 via the EGR valve 27.

而して、このように排気浄化装置を構成すれば、排気温度の低い軽負荷時に予混合圧縮着火を適用するに際し、温度センサ23,24の検出値に基づき選択還元型触媒13の触媒床温度が活性温度以上であるかどうかが制御装置22で確認され、この活性温度を下まわっている場合には予混合圧縮着火制御が許可されないため、長時間のエンジン停止後の始動時等に、選択還元型触媒13の触媒床温度が活性温度域まで上昇していない状況下で予混合圧縮着火が実施されることがなくなり、予混合圧縮着火を要因とするHC及びCOの車外への排出が防止される。   Thus, if the exhaust gas purification apparatus is configured in this way, the catalyst bed temperature of the selective catalytic reduction catalyst 13 based on the detection values of the temperature sensors 23 and 24 when applying premixed compression ignition at a light load with a low exhaust gas temperature. It is confirmed by the control device 22 whether or not the temperature is higher than the activation temperature. When the temperature is lower than the activation temperature, the premix compression ignition control is not permitted. Premixed compression ignition is no longer performed in a situation where the catalyst bed temperature of the reduction catalyst 13 has not risen to the active temperature range, and the discharge of HC and CO outside the vehicle due to premixed compression ignition is prevented. Is done.

また、一般的に予混合圧縮着火が適用されない中高負荷時等において、燃料添加弁16により燃料を添加し、この添加燃料から生じたHCを還元剤として選択還元型触媒13でNOxの還元浄化を行わせるに際し、温度センサ23,24の検出値に基づき推定された選択還元型触媒13の触媒床温度が必要なNOx低減率を得られない高温域と燃料の確実な燃焼が見込めない低温域とに挟まれた有効温度範囲にあるかどうかが制御装置22で確認され、この有効温度範囲を外れている場合には燃料添加が許可されず、しかも、有効温度範囲に入っていても、その推定された選択還元型触媒13の触媒床温度でのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加量が制御されるため、無駄な燃料添加が極力回避されて燃費の悪化やHC排出量の増加が防止される。   Further, in the case of medium and high loads where premixed compression ignition is not generally applied, fuel is added by the fuel addition valve 16, and NOx is reduced and purified by the selective reduction catalyst 13 using HC generated from the added fuel as a reducing agent. When performing, the catalyst bed temperature of the selective catalytic reduction catalyst 13 estimated based on the detection values of the temperature sensors 23 and 24 is a high temperature region where a required NOx reduction rate cannot be obtained, and a low temperature region where reliable combustion of fuel is not expected. Whether or not the temperature is within the effective temperature range is confirmed by the control device 22, and if it is outside this effective temperature range, fuel addition is not permitted, and even if it is within the effective temperature range, its estimation The amount of added fuel is controlled so that the added fuel / NOx ratio becomes an optimum value without excess or deficiency in accordance with the NOx reduction rate at the catalyst bed temperature of the selective catalytic reduction catalyst 13, so that wasteful fuel addition is prevented. As much as possible This avoids deterioration in fuel consumption and increase in HC emissions.

更に、本形態例においては、選択還元型触媒13の前段にパティキュレートフィルタ14を設けると共に、該パティキュレートフィルタ14の更に前段にメイン酸化触媒15を設け、該メイン酸化触媒15とプレ酸化触媒12との間に燃料添加弁16が配置されるように構成しているので、NOxとパティキュレートの同時低減を図ることが可能となり、しかも、パティキュレートフィルタ14の強制再生を行う必要が生じた際に、燃料添加弁16により燃料を添加すると、この添加燃料から生じた高濃度のHCがメイン酸化触媒15を通過する間に酸化反応し、その反応熱で昇温した排気ガス9の流入により直後のパティキュレートフィルタ14の触媒床温度が上げられてパティキュレートが燃やし尽くされ、パティキュレートフィルタ14の再生化が図られることになる。   Further, in the present embodiment, a particulate filter 14 is provided in front of the selective catalytic reduction catalyst 13, and a main oxidation catalyst 15 is provided further in front of the particulate filter 14, and the main oxidation catalyst 15 and the pre-oxidation catalyst 12 are provided. Since the fuel addition valve 16 is arranged between the NOx and the particulates, it is possible to simultaneously reduce NOx and the particulates, and when the particulate filter 14 needs to be forcibly regenerated. In addition, when fuel is added by the fuel addition valve 16, high-concentration HC generated from the added fuel undergoes an oxidation reaction while passing through the main oxidation catalyst 15, and immediately after the exhaust gas 9 heated by the reaction heat flows in. The catalyst bed temperature of the particulate filter 14 is raised, the particulates are burned out, and the particulate filter 14 Will be regenerated.

従って、上記形態例によれば、温度センサ23,24により排気管11の適宜位置で選択還元型触媒13の触媒床温度の代用値として排気温度を検出し、その検出値に基づき予混合圧縮着火制御及び燃料添加の許可判定と燃料添加量制御を適切に行うようにしているので、長時間のエンジン停止後の始動時等で予混合圧縮着火により発生したHCが車外へ多く排出されてしまう問題や、NOx低減率の低い温度域での無駄な燃料添加により燃費の悪化やHC排出量の増加を招いてしまう問題を確実に解決することができる。   Therefore, according to the above embodiment, the exhaust temperature is detected as a substitute value for the catalyst bed temperature of the selective catalytic reduction catalyst 13 at an appropriate position of the exhaust pipe 11 by the temperature sensors 23 and 24, and premixed compression ignition is performed based on the detected value. Control and fuel addition permission judgment and fuel addition amount control are performed appropriately, so that a lot of HC generated by premixed compression ignition at the start after a long engine stop etc. is discharged outside the vehicle In addition, it is possible to reliably solve the problem that fuel consumption is deteriorated and HC emission amount is increased due to wasteful fuel addition in a temperature range where the NOx reduction rate is low.

更に、本形態例では、NOxとパティキュレートの同時低減を図ることができ、しかも、予混合圧縮着火を適用しない場合に選択還元型触媒13への還元剤として燃料を添加するための燃料添加弁16を流用してパティキュレートフィルタ14の強制再生を実施することができる。   Furthermore, in this embodiment, NOx and particulates can be reduced simultaneously, and a fuel addition valve for adding fuel as a reducing agent to the selective catalytic reduction catalyst 13 when premixed compression ignition is not applied. The forced regeneration of the particulate filter 14 can be performed by diverting 16.

尚、本発明の排気浄化装置は、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。   Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

本発明を実施する形態の一例を示す概略図である。It is the schematic which shows an example of the form which implements this invention. NOx低減率と選択還元型触媒の触媒床温度との関係を示すグラフである。It is a graph which shows the relationship between NOx reduction rate and the catalyst bed temperature of a selective reduction catalyst. 燃料添加の許可判定の具体的な手順を示すフローチャートである。It is a flowchart which shows the specific procedure of permission determination of fuel addition.

符号の説明Explanation of symbols

1 ディーゼルエンジン
9 排気ガス
11 排気管
12 プレ酸化触媒
13 選択還元型触媒
14 パティキュレートフィルタ
15 メイン酸化触媒
16 燃料添加弁(燃料添加手段)
22 制御装置
23 温度センサ
23a 検出信号
24 温度センサ
24a 検出信号
DESCRIPTION OF SYMBOLS 1 Diesel engine 9 Exhaust gas 11 Exhaust pipe 12 Pre oxidation catalyst 13 Selective reduction type catalyst 14 Particulate filter 15 Main oxidation catalyst 16 Fuel addition valve (fuel addition means)
22 control device 23 temperature sensor 23a detection signal 24 temperature sensor 24a detection signal

Claims (2)

少なくとも軽負荷時に予混合圧縮着火制御が適用されるディーゼルエンジンの排気管上流部にHCよりもCOを優先して酸化処理するプレ酸化触媒を設け、該プレ酸化触媒の下流側に酸素共存下でも選択的にNOxをHCと反応させる触媒機能とNOx吸着能とを備えた選択還元型触媒を設け、該選択還元型触媒と前記プレ酸化触媒との間に排気ガス中に燃料を添加する燃料添加手段を設けた排気浄化装置であって、選択還元型触媒の触媒床温度を推定するための代用値として排気管の適宜位置で排気温度を検出する温度センサを備え、該温度センサの検出値に基づき選択還元型触媒の触媒床温度が活性温度以上であると推定された場合に限り予混合圧縮着火制御を許可し、その推定された選択還元型触媒の触媒床温度が必要なNOx低減率を得られない高温域と燃料の確実な燃焼が見込めない低温域とに挟まれた有効温度範囲にある場合に限り燃料添加を許可すると共に、その推定された選択還元型触媒の触媒床温度でのNOx低減率に対応させて添加燃料/NOx比が過不足のない最適値となるように燃料添加量を制御する制御装置を備えたことを特徴とする排気浄化装置。   A pre-oxidation catalyst that preferentially oxidizes CO over HC is provided at the upstream side of the exhaust pipe of a diesel engine to which premixed compression ignition control is applied at least at light load, and even in the presence of oxygen on the downstream side of the pre-oxidation catalyst A fuel addition for providing a selective reduction type catalyst having a catalytic function for selectively reacting NOx with HC and an NOx adsorption capability, and adding fuel into the exhaust gas between the selective reduction type catalyst and the pre-oxidation catalyst An exhaust purification device provided with a means, comprising a temperature sensor for detecting the exhaust temperature at an appropriate position of the exhaust pipe as a substitute value for estimating the catalyst bed temperature of the selective catalytic reduction catalyst, and the detected value of the temperature sensor Based on this, only when it is estimated that the catalyst bed temperature of the selective catalytic reduction catalyst is higher than the activation temperature, the premixed compression ignition control is permitted. Gain The addition of fuel is permitted only when the temperature is between the high temperature range and the low temperature range where reliable combustion of the fuel cannot be expected, and NOx at the estimated catalyst bed temperature of the selective catalytic reduction catalyst is allowed. An exhaust emission control device comprising a control device for controlling the amount of fuel added so that the added fuel / NOx ratio becomes an optimum value without excess or deficiency corresponding to the reduction rate. 選択還元型触媒の前段にパティキュレートフィルタを設けると共に、該パティキュレートフィルタの更に前段にメイン酸化触媒を設け、該メイン酸化触媒とプレ酸化触媒との間に燃料添加手段が配置されるように構成したことを特徴とする請求項1に記載の排気浄化装置。   A particulate filter is provided in front of the selective catalytic reduction catalyst, a main oxidation catalyst is provided in front of the particulate filter, and fuel adding means is arranged between the main oxidation catalyst and the pre-oxidation catalyst. The exhaust emission control device according to claim 1, wherein
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