JP4214923B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Exhaust gas purification device for internal combustion engine Download PDF

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JP4214923B2
JP4214923B2 JP2004033949A JP2004033949A JP4214923B2 JP 4214923 B2 JP4214923 B2 JP 4214923B2 JP 2004033949 A JP2004033949 A JP 2004033949A JP 2004033949 A JP2004033949 A JP 2004033949A JP 4214923 B2 JP4214923 B2 JP 4214923B2
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裕 澤田
大介 柴田
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Toyota Motor Corp
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Description

本発明は、内燃機関の排気浄化装置に関する。   The present invention relates to an exhaust emission control device for an internal combustion engine.

吸蔵還元型NOx触媒(以下、NOx触媒とする。)を内燃機関の排気通路に配置し、酸化雰囲気のときに排気中の窒素酸化物(NOx)を該NOx触媒に吸蔵し、還元雰囲気となったときは該NOx触媒に吸蔵されていたNOxを還元して排気中のNOxを浄化する技術が知られている。   A NOx storage reduction catalyst (hereinafter referred to as NOx catalyst) is disposed in the exhaust passage of the internal combustion engine, and nitrogen oxide (NOx) in the exhaust gas is stored in the NOx catalyst in an oxidizing atmosphere to form a reducing atmosphere. In this case, a technique is known in which NOx stored in the NOx catalyst is reduced to purify NOx in the exhaust gas.

このNOx触媒は、熱劣化や経年変化による劣化とともにNOxの吸蔵能力が低下することが知られており、この劣化の検出を該NOx触媒前後に取り付けた酸素センサの出力に基づいて行う技術が知られている(例えば、特許文献1参照。)。
特開平11−93742号公報 特許第2692380号公報 特開2002−309928号公報
This NOx catalyst is known to have a NOx occlusion capability that decreases with thermal deterioration and deterioration due to aging, and a technology for detecting this deterioration based on the outputs of oxygen sensors attached before and after the NOx catalyst is known. (For example, refer to Patent Document 1).
JP-A-11-93742 Japanese Patent No. 2692380 JP 2002-309928 A

ところで、NOx触媒へ燃料を供給してNOxの還元を行うときには、排気中の空燃比を空燃比センサにより正確に検出することが求められる。しかし、排気中に含まれる燃料のクラッキングが十分でないと、一部の燃料が空燃比センサの拡散抵抗層を通過できなくなってしまう。そのため、燃料が実際よりも少なく測定され、空燃比センサにより検出される空燃比は、実際よりもリーン側へずれることになる。なお、このような空燃比のずれを以下、「リーンずれ」という。そして、このリーンずれのため、フィードバック制御による燃料添加量の補正が困難となる。   By the way, when the fuel is supplied to the NOx catalyst and NOx is reduced, it is required to accurately detect the air-fuel ratio in the exhaust gas by the air-fuel ratio sensor. However, if the cracking of the fuel contained in the exhaust gas is not sufficient, some fuel cannot pass through the diffusion resistance layer of the air-fuel ratio sensor. Therefore, the fuel is measured less than the actual amount, and the air-fuel ratio detected by the air-fuel ratio sensor is shifted to the lean side from the actual value. Such an air-fuel ratio shift is hereinafter referred to as “lean shift”. This lean shift makes it difficult to correct the fuel addition amount by feedback control.

また、前記従来技術によりNOx触媒の劣化判定を行うときには、排気中の酸素濃度や空燃比を酸素センサや空燃比センサにより正確に検出することが求められる。しかし、上記理由によりNOx触媒の劣化判定を正確に行うことが困難な場合がある。   In addition, when the deterioration determination of the NOx catalyst is performed according to the conventional technique, it is required to accurately detect the oxygen concentration and the air-fuel ratio in the exhaust gas using an oxygen sensor or an air-fuel ratio sensor. However, there are cases where it is difficult to accurately determine the deterioration of the NOx catalyst for the above reasons.

本発明は、上記したような問題点に鑑みてなされたものであり、内燃機関の排気浄化装置において、NOx触媒への燃料供給時に該NOx触媒へ流入する排気の空燃比の正確な値を得ることにより、該NOx触媒の劣化判定の精度を向上させることができる技術を提供することを目的とする。   The present invention has been made in view of the above-described problems, and in an exhaust gas purification apparatus for an internal combustion engine, obtains an accurate value of the air-fuel ratio of exhaust flowing into the NOx catalyst when fuel is supplied to the NOx catalyst. Accordingly, an object of the present invention is to provide a technique capable of improving the accuracy of deterioration determination of the NOx catalyst.

上記課題を達成するために本発明による内燃機関の排気浄化装置は、以下の手段を採用した。すなわち、
内燃機関の排気通路に設けられた第1NOx吸蔵触媒と、
前記第1NOx吸蔵触媒よりも下流の排気の空燃比を検出する第1空燃比検出手段と、
前記第1NOx吸蔵触媒よりも上流から該第1NOx吸蔵触媒へ燃料を添加する燃料添加手段と、
前記燃料添加手段により燃料を添加して前記第1NOx吸蔵触媒に吸蔵されたNOxを還元するNOx還元手段と、
前記燃料添加手段により燃料が添加されているときの前記第1空燃比検出手段の検出値がストイキよりリッチ空燃比であって所定の変化幅内で安定しているか否か判定する出力安定判定手段と、
前記出力安定判定手段により前記第1空燃比検出手段の検出値がストイキよりリッチな空燃比で安定していると判定されたときの前記第1空燃比検出手段により検出される空燃比に基づいて、前記燃料添加手段により燃料が添加されていた期間中の前記第1NOx吸蔵触媒に流入する排気の空燃比を推定する流入排気空燃比推定手段と、
前記出力安定判定手段により前記第1空燃比検出手段の検出値がストイキよりリッチな空燃比で安定していると判定されたときの前記第1空燃比検出手段により得られる排気の空燃比および前記流入排気空燃比推定手段により推定される空燃比に基づいて前記第1NOx吸蔵触媒の劣化を判定する劣化判定手段と、
を具備することを特徴とする。
In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention employs the following means. That is,
A first NOx storage catalyst provided in an exhaust passage of the internal combustion engine;
First air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the first NOx storage catalyst;
Fuel addition means for adding fuel to the first NOx storage catalyst from the upstream side of the first NOx storage catalyst;
NOx reduction means for reducing NOx stored in the first NOx storage catalyst by adding fuel by the fuel addition means;
Output stability determination means for determining whether or not the detected value of the first air-fuel ratio detection means when the fuel is added by the fuel addition means is richer than stoichiometric and stable within a predetermined change range. When,
Based on the air-fuel ratio detected by the first air-fuel ratio detecting means when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric. An inflow exhaust air / fuel ratio estimating means for estimating an air / fuel ratio of exhaust flowing into the first NOx storage catalyst during a period in which fuel is added by the fuel addition means;
When the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detecting means and the Deterioration determining means for determining deterioration of the first NOx storage catalyst based on the air-fuel ratio estimated by the inflowing exhaust air-fuel ratio estimating means;
It is characterized by comprising.

本発明の最大の特徴は、第1NOx吸蔵触媒よりも下流で検出される排気の空燃比がリッチ空燃比であって所定の変化幅内で安定しているときの空燃比から該第1NOx吸蔵触媒に流入する排気の空燃比を推定し、この推定値に基づいて該第1NOx吸蔵触媒の劣化判定を行うことにある。   The greatest feature of the present invention is that the first NOx storage catalyst is determined from the air-fuel ratio when the air-fuel ratio of the exhaust gas detected downstream of the first NOx storage catalyst is a rich air-fuel ratio and is stable within a predetermined variation range. The air-fuel ratio of the exhaust gas flowing into the exhaust gas is estimated, and the deterioration determination of the first NOx storage catalyst is performed based on the estimated value.

ここで、燃料添加手段により燃料の添加が行われているときに、第1空燃比検出手段により検出される空燃比がリッチ側に移行した場合には、第1NOx吸蔵触媒に吸蔵されているNOxおよび酸素の放出が完了している。そして、このときに第1空燃比検出手段により検出される空燃比は、第1NOx吸蔵触媒に吸蔵されていた酸素やNOxの影響を受けない。さらに、第1NOx吸蔵触媒により燃料が十分にクラッキングされるため、第1空燃比検出手段の検出値のリーンずれが生じるのを抑制できる。   Here, when the fuel addition is performed by the fuel addition means, if the air-fuel ratio detected by the first air-fuel ratio detection means shifts to the rich side, the NOx stored in the first NOx storage catalyst. And the release of oxygen is complete. At this time, the air-fuel ratio detected by the first air-fuel ratio detection means is not affected by oxygen or NOx stored in the first NOx storage catalyst. Furthermore, since the fuel is sufficiently cracked by the first NOx storage catalyst, it is possible to suppress the occurrence of lean deviation of the detection value of the first air-fuel ratio detection means.

従って、出力安定判定手段により第1空燃比検出手段の検出値が安定していると判定された場合には、該第1空燃比検出手段により検出される空燃比は実際に第1NOx吸蔵触媒に流入する排気の空燃比とほぼ等しい値を示していることになる。すなわち、出力安定判定手段により第1空燃比検出手段の検出値が安定していると判定された場合には、第1空燃比検出手段から得られる排気の空燃比は、第1NOx吸蔵触媒に流入している排気の空燃比とすることができる。   Therefore, when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable, the air-fuel ratio detected by the first air-fuel ratio detecting means is actually supplied to the first NOx storage catalyst. This indicates a value almost equal to the air-fuel ratio of the inflowing exhaust gas. That is, when the output stability determination means determines that the detection value of the first air-fuel ratio detection means is stable, the exhaust air-fuel ratio obtained from the first air-fuel ratio detection means flows into the first NOx storage catalyst. The air-fuel ratio of the exhaust gas being discharged can be set.

そして、流入排気空燃比推定手段は、出力安定判定手段により第1空燃比検出手段の検出値が安定していると判定されたときの排気の空燃比に基づいて、第1空燃比検出手段の検出値が安定する前であって燃料添加手段により燃料の添加が行われているときに第1NOx吸蔵触媒に流入する排気の空燃比を推定する。この推定は、例えば、燃料添加手段から燃料が添加されている間に第1NOx吸蔵触媒に流入する排気の空燃比の変化を実験的に求めておくことにより可能となる。すなわち、出力安定判定手段により第1空燃比検出手段の検出値が安定していると判定されたときの該第1空燃比検出手段により得られる空燃比と、実験的に求めた排気の空燃比が最終的に到達する空燃比と、が等しくなるように、実験的に求めた排気の空燃比を補正することにより、燃料添加中であって第1空燃比検出手段の検出値が安定する前の期間における第1NOx吸蔵触媒に流入する排気の空燃比を得ることができる。   The inflow exhaust air-fuel ratio estimating means is configured to detect the first air-fuel ratio detecting means based on the air-fuel ratio of the exhaust gas when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable. The air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated before the detected value is stabilized and the fuel addition means is adding fuel. This estimation is possible, for example, by experimentally determining the change in the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst while the fuel is being added from the fuel addition means. That is, the air-fuel ratio obtained by the first air-fuel ratio detection means when the output stability judgment means determines that the detection value of the first air-fuel ratio detection means is stable, and the exhaust air-fuel ratio obtained experimentally By correcting the experimentally obtained air-fuel ratio of the exhaust gas so that the air-fuel ratio that finally reaches is equalized, before the detected value of the first air-fuel ratio detecting means is stabilized during fuel addition In this period, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be obtained.

このようにして得られた第1NOx吸蔵触媒に流入する排気の空燃比を用いて該第1NOx吸蔵触媒の劣化判定を行うことにより、劣化判定の精度を向上させることができる。
なお、「所定の変化幅内で安定している」とは、空燃比の変化幅が所定値よりも小さくなったことを例示することができる。
By determining the deterioration of the first NOx storage catalyst using the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst thus obtained, the accuracy of the deterioration determination can be improved.
“Stable within a predetermined change width” can be exemplified by the fact that the change width of the air-fuel ratio is smaller than a predetermined value.

本発明においては、前記第1空燃比検出手段よりも下流の排気通路に設けられ、酸化能
力を有する第2触媒と、
前記第2触媒よりも下流の排気の空燃比を検出する第2空燃比検出手段と、
をさらに備え、
前記出力安定手段は、前記燃料添加手段により燃料が添加されているときに前記第2空燃比検出手段により検出される空燃比がストイキよりリッチに変化したときに前記第1空燃比検出手段により検出された排気の空燃比がストイキよりリッチな空燃比で安定していると判定することができる。
In the present invention, a second catalyst provided in an exhaust passage downstream of the first air-fuel ratio detection means and having an oxidation ability;
Second air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the second catalyst;
Further comprising
The output stabilizing means is detected by the first air-fuel ratio detecting means when the air-fuel ratio detected by the second air-fuel ratio detecting means changes richer than stoichiometric when fuel is added by the fuel adding means. It can be determined that the air-fuel ratio of the exhaust gas is stable at an air-fuel ratio richer than stoichiometric.

ここで、第2触媒がNOx吸蔵能力を有するようにしてもよい。そして、第2空燃比検出手段により検出された空燃比がリッチ空燃比となった場合には、第2触媒にはリッチ空燃比の排気が流入したことを示している。この場合、第1NOx吸蔵触媒から流出する排気の空燃比はリッチ空燃比となっている。このことは、第1NOx吸蔵触媒に吸蔵されていたNOxの還元が完了していることを示しており、第1空燃比検出手段に検出される排気の空燃比が一定のリッチ空燃比で安定していることをも示している。これにより、出力安定手段は前記第1空燃比検出手段により検出された排気の空燃比が安定していると判定することができる。   Here, the second catalyst may have a NOx storage capacity. When the air-fuel ratio detected by the second air-fuel ratio detection means becomes a rich air-fuel ratio, it indicates that the rich air-fuel ratio exhaust gas has flowed into the second catalyst. In this case, the air-fuel ratio of the exhaust gas flowing out from the first NOx storage catalyst is a rich air-fuel ratio. This indicates that the reduction of NOx stored in the first NOx storage catalyst has been completed, and the air-fuel ratio of the exhaust detected by the first air-fuel ratio detection means is stabilized at a constant rich air-fuel ratio. It also shows that. As a result, the output stabilization means can determine that the air-fuel ratio of the exhaust detected by the first air-fuel ratio detection means is stable.

本発明においては、前記第1NOx吸蔵触媒よりも上流の排気の空燃比を検出する第3空燃比検出手段をさらに備え、前記流入排気空燃比推定手段は前記第3空燃比検出手段により検出された空燃比と前記第1空燃比検出手段により検出された空燃比とから前記第1NOx吸蔵触媒に流入する排気の空燃比を推定することができる。   In the present invention, there is further provided a third air-fuel ratio detecting means for detecting an air-fuel ratio of the exhaust upstream of the first NOx storage catalyst, and the inflowing exhaust air-fuel ratio estimating means is detected by the third air-fuel ratio detecting means. The air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be estimated from the air-fuel ratio and the air-fuel ratio detected by the first air-fuel ratio detection means.

第3空燃比検出手段は、第1NOx吸蔵触媒よりも上流に設けられているため、燃料のクラッキングが十分になされていない状態の排気の空燃比を検出することとなるが、該第1NOx吸蔵触媒に吸蔵されたNOxおよび酸素の放出による影響は受けない。   Since the third air-fuel ratio detection means is provided upstream of the first NOx storage catalyst, the third air-fuel ratio detection means detects the air-fuel ratio of the exhaust when the fuel is not sufficiently cracked. It is not affected by the release of NOx and oxygen stored in the water.

そのため、第3空燃比検出手段により得られた空燃比のみでは、第1吸蔵NOx触媒に流入する排気の空燃比を正確に得ることは困難であるが、空燃比の変化を検出することはできる。また、第1空燃比検出手段により検出される空燃比に基づいて第3空燃比検出手段により得られる排気の空燃比を補正することにより第1NOx吸蔵触媒に流入する排気の空燃比をより正確に得ることができる。そのため、第1NOx吸蔵触媒の劣化判定をより正確に行うことが可能となる。   Therefore, it is difficult to accurately obtain the air-fuel ratio of the exhaust gas flowing into the first storage NOx catalyst only with the air-fuel ratio obtained by the third air-fuel ratio detection means, but it is possible to detect a change in the air-fuel ratio. . Further, by correcting the air-fuel ratio of the exhaust gas obtained by the third air-fuel ratio detection means based on the air-fuel ratio detected by the first air-fuel ratio detection means, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be more accurately determined. Obtainable. Therefore, it is possible to more accurately determine the deterioration of the first NOx storage catalyst.

本発明においては、前記流入排気空燃比推定手段は、前記出力安定判定手段により前記第1空燃比検出手段により検出された空燃比がストイキよりリッチな空燃比で安定していると判定されたときの該第1空燃比検出手段により得られる排気の空燃比と、このときの前記第3空燃比検出手段により検出された空燃比と、が等しくなるように、前記燃料添加手段による燃料添加中の前記第3空燃比検出手段により検出された空燃比を補正することにより、前記第1NOx吸蔵触媒に流入する排気の空燃比を推定することができる。   In the present invention, when the inflow exhaust air-fuel ratio estimating means determines that the air-fuel ratio detected by the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric by the output stability determining means. During the addition of fuel by the fuel addition means, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detection means is equal to the air-fuel ratio detected by the third air-fuel ratio detection means at this time. By correcting the air-fuel ratio detected by the third air-fuel ratio detection means, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be estimated.

前記したように、出力安定判定手段により第1空燃比検出手段の検出値が安定していると判定された場合には、第1NOx吸蔵触媒に吸蔵されていたNOxおよび酸素の放出が完了し、第1空燃比検出手段により検出される空燃比と、第1NOx吸蔵触媒に流入する排気の空燃比と、はほぼ等しい値となる。そこで、第1空燃比検出手段の検出値が安定しているときの第3空燃比検出手段の検出値が第1空燃比検出手段の検出値と等しくなるように、第3空燃比検出手段の検出値を補正する。そして、このときに第3空燃比検出手段の補正を行うための係数を得る。この係数を燃料添加時の第3空燃比検出手段の検出値全てに適用することにより、第1空燃比検出手段の検出値が安定する前の第3空燃比検出手段の検出値を補正することが可能となる。すなわち、第3空燃比検出手段の検出値を記憶させておき、この記憶値を補正することにより、燃料添加時に第1NOx吸蔵触媒に流入し
ていた排気の空燃比を得ることが可能となる。
As described above, when it is determined by the output stability determination means that the detection value of the first air-fuel ratio detection means is stable, the release of NOx and oxygen stored in the first NOx storage catalyst is completed, The air-fuel ratio detected by the first air-fuel ratio detection means and the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst are substantially equal. Therefore, the third air-fuel ratio detection means is set so that the detection value of the third air-fuel ratio detection means when the detection value of the first air-fuel ratio detection means is stable becomes equal to the detection value of the first air-fuel ratio detection means. Correct the detection value. At this time, a coefficient for correcting the third air-fuel ratio detecting means is obtained. By applying this coefficient to all detection values of the third air-fuel ratio detection means at the time of fuel addition, the detection value of the third air-fuel ratio detection means before the detection value of the first air-fuel ratio detection means stabilizes is corrected. Is possible. That is, by storing the detected value of the third air-fuel ratio detecting means and correcting this stored value, it becomes possible to obtain the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst at the time of fuel addition.

本発明においては、前記第2空燃比検出手段により検出される排気の空燃比が所定空燃比よりもリッチとなったときに前記燃料添加手段による燃料の添加を終了させる燃料添加終了手段をさらに備えることができる。   The present invention further includes fuel addition ending means for ending fuel addition by the fuel addition means when the air-fuel ratio of the exhaust detected by the second air-fuel ratio detection means becomes richer than a predetermined air-fuel ratio. be able to.

ここで、第2空燃比検出手段の検出値がリッチ空燃比となった場合には、第1NOx吸蔵触媒に吸蔵されていたNOxの放出が完了している。そして、このときには、流入排気空燃比推定手段による第1NOx吸蔵触媒に流入する排気の空燃比の推定も完了している。そのため、燃料添加を終了させても第1NOx吸蔵触媒の劣化判定を行うことができる。また、このような終了時期とすることにより、過剰な燃料が第1および第2触媒を通過して、下流へと流出することを抑制することができる。   Here, when the detection value of the second air-fuel ratio detection means becomes the rich air-fuel ratio, the release of NOx stored in the first NOx storage catalyst is completed. At this time, the estimation of the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst by the inflowing exhaust air-fuel ratio estimation means is also completed. Therefore, it is possible to determine the deterioration of the first NOx storage catalyst even when the fuel addition is terminated. Moreover, by setting it as such an end time, it can suppress that excess fuel passes a 1st and 2nd catalyst, and flows out downstream.

本発明に係る内燃機関の排気浄化装置では、NOx触媒への燃料供給時に該NOx触媒へ流入する排気の空燃比の正確な値を得ることにより、該NOx触媒の劣化判定の精度を向上させることができる。   In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, by obtaining an accurate value of the air-fuel ratio of the exhaust gas flowing into the NOx catalyst when the fuel is supplied to the NOx catalyst, the accuracy of determining the deterioration of the NOx catalyst is improved. Can do.

以下、本発明に係る内燃機関の排気浄化装置の具体的な実施態様について図面に基づいて説明する。   Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described with reference to the drawings.

図1は、本実施例に係る内燃機関の排気浄化装置を適用する内燃機関1とその排気系の概略構成を示す図である。
図1に示す内燃機関1は、水冷式の4サイクル・ディーゼルエンジンである。
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 to which an exhaust gas purification apparatus for an internal combustion engine according to this embodiment is applied and an exhaust system thereof.
The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

内燃機関1には、燃焼室へ通じる排気通路2が接続されている。この排気通路2は、下流にて大気へと通じている。
前記排気通路2の途中には、内燃機関1側から順に酸化触媒3、第1NOx触媒4及び第2NOx触媒5が備えられている。
An exhaust passage 2 leading to the combustion chamber is connected to the internal combustion engine 1. This exhaust passage 2 communicates with the atmosphere downstream.
In the middle of the exhaust passage 2, an oxidation catalyst 3, a first NOx catalyst 4, and a second NOx catalyst 5 are provided in order from the internal combustion engine 1 side.

第1NOx触媒4及び第2NOx触媒5は、流入する排気の酸素濃度が高いときは排気中のNOxを吸蔵し、流入する排気の酸素濃度が低下し且つ還元剤が存在するときは吸蔵していたNOxを還元する機能を有する吸蔵還元型NOx触媒である。なお、第1NOx触媒4および/または、パティキュレートフィルタに吸蔵還元型NOx触媒を担持させたものであってもよい。また、第1NOx触媒の劣化判定のみを行う場合には、第2NOx触媒5は単なる酸化能力を有する触媒であってもよい。さらに、酸化触媒3は、酸化能力を有する触媒であれば三元触媒、NOx触媒等の他の種類の触媒であってもよい。   The first NOx catalyst 4 and the second NOx catalyst 5 occluded NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and occluded when the oxygen concentration of the inflowing exhaust gas decreased and a reducing agent was present. This is a NOx storage reduction catalyst having a function of reducing NOx. Note that the NOx storage reduction type NOx catalyst may be supported on the first NOx catalyst 4 and / or the particulate filter. When only the deterioration determination of the first NOx catalyst is performed, the second NOx catalyst 5 may be a catalyst having a simple oxidation ability. Furthermore, the oxidation catalyst 3 may be another type of catalyst such as a three-way catalyst or a NOx catalyst as long as it has an oxidation ability.

また、酸化触媒3よりも下流で且つ第1NOx触媒4よりも上流の排気通路2には、該排気通路2を流通する排気の空燃比を検出する第3空燃比センサ6が取り付けられている。また、第1NOx触媒4よりも下流で且つ第2NOx触媒5よりも上流の排気通路2には、該排気通路2を流通する排気の空燃比を検出する第1空燃比センサ7が取り付けられている。さらに、第2NOx触媒5よりも下流の排気通路2には、該排気通路2を流通する排気の空燃比を検出する第2空燃比センサ8が取り付けられている。   A third air / fuel ratio sensor 6 for detecting the air / fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the first NOx catalyst 4. A first air-fuel ratio sensor 7 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the first NOx catalyst 4 and upstream of the second NOx catalyst 5. . Further, a second air-fuel ratio sensor 8 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the second NOx catalyst 5.

ところで、内燃機関1が希薄燃焼運転されている場合は、第1NOx触媒4及び第2NOx触媒5のNOx吸蔵能力が飽和する前に第1NOx触媒4及び第2NOx触媒5に吸蔵されたNOxを還元させる必要がある。   By the way, when the internal combustion engine 1 is operated in lean combustion, the NOx occluded in the first NOx catalyst 4 and the second NOx catalyst 5 is reduced before the NOx occlusion capacity of the first NOx catalyst 4 and the second NOx catalyst 5 is saturated. There is a need.

そこで、本実施例では、酸化触媒3より上流の排気通路2を流通する排気中に還元剤たる燃料(軽油)を添加する燃料添加弁9を備えている。ここで、燃料添加弁9は、後述するECU10からの信号により開弁して燃料を噴射する。燃料添加弁9から排気通路2内へ噴射された燃料は、排気通路2の上流から流れてきた排気の空燃比をリッチにすると共に、第1NOx触媒4及び第2NOx触媒5に吸蔵されていたNOxを還元する。NOx還元時には、第1NOx触媒4及び第2NOx触媒5に流入する排気の空燃比を比較的に短い周期でスパイク的(短時間)にリッチとする、所謂リッチスパイク制御を実行する。   Therefore, in this embodiment, a fuel addition valve 9 for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage 2 upstream from the oxidation catalyst 3 is provided. Here, the fuel addition valve 9 is opened by a signal from the ECU 10 described later to inject fuel. The fuel injected into the exhaust passage 2 from the fuel addition valve 9 makes the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 2 rich, and the NOx stored in the first NOx catalyst 4 and the second NOx catalyst 5. Reduce. During NOx reduction, so-called rich spike control is performed in which the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 and the second NOx catalyst 5 is made rich in a spike (short time) in a relatively short cycle.

以上述べたように構成された内燃機関1には、該内燃機関1を制御するための電子制御ユニットであるECU10が併設されている。このECU10は、内燃機関1の運転条件や運転者の要求に応じて内燃機関1の運転状態を制御するユニットである。   The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.

ECU10には、前記したセンサ等が電気配線を介して接続され、該センサ等の出力信号が入力されるようになっている。
一方、ECU10には、燃料添加弁9が電気配線を介して接続され、ECU10により制御することが可能になっている。
The above-described sensor or the like is connected to the ECU 10 via an electrical wiring, and an output signal from the sensor or the like is input thereto.
On the other hand, the fuel addition valve 9 is connected to the ECU 10 via an electric wiring, and can be controlled by the ECU 10.

ところで、第1NOx触媒4及び第2NOx触媒5は、経年変化や熱により劣化することがある。この劣化は、NOxの吸蔵能力に顕著に現れる。そして、NOxの吸蔵能力が低下し、排気中のNOxの一部が該第1NOx触媒4及び第2NOx触媒5の下流へ流出することがある。これに対し、例えば第1NOx触媒4のNOx吸蔵能力の低下を、第1NOx触媒4前後の空燃比センサ6、7を用いて検出することができる。これにより、劣化の度合いに応じた燃料の添加を行うことが可能となる。また、運転者等に第1NOx触媒4の交換を促すことも可能となる。   By the way, the first NOx catalyst 4 and the second NOx catalyst 5 may be deteriorated by aging or heat. This deterioration appears significantly in the NOx storage capacity. Then, the NOx occlusion capacity decreases, and a part of the NOx in the exhaust gas may flow out downstream of the first NOx catalyst 4 and the second NOx catalyst 5. In contrast, for example, a decrease in the NOx storage capacity of the first NOx catalyst 4 can be detected using the air-fuel ratio sensors 6 and 7 before and after the first NOx catalyst 4. Thereby, it becomes possible to add fuel according to the degree of deterioration. It is also possible to prompt the driver or the like to replace the first NOx catalyst 4.

ここで、第1NOx触媒4にNOxが吸蔵されている場合に、該第1NOx触媒4にリッチ空燃比の排気を供給すると、該第1NOx触媒4に吸蔵されているNOx及び酸素が放出される。リッチスパイクによりリッチ空燃比の排気が第1NOx触媒4に流入し、該第1NOx触媒4からNOx及び酸素が放出されている間は、第1NOx触媒4の下流の空燃比すなわち第1空燃比センサ7により検出される空燃比はストイキとなることが知られている。そして、第1NOx触媒4からのNOx及び酸素の放出が完了した後に、第1空燃比センサ7により検出される空燃比がリッチ空燃比に移行する。このように第1空燃比センサ7によりストイキが検出され、リッチ空燃比に移行するまでの時間は、第1NOx触媒4に吸蔵されているNOx及び酸素の量が多いほど長くなる。   Here, when NOx is occluded in the first NOx catalyst 4, if rich air-fuel ratio exhaust gas is supplied to the first NOx catalyst 4, NOx and oxygen occluded in the first NOx catalyst 4 are released. The rich air-fuel ratio exhaust gas flows into the first NOx catalyst 4 due to the rich spike, and while the NOx and oxygen are released from the first NOx catalyst 4, the air-fuel ratio downstream of the first NOx catalyst 4, that is, the first air-fuel ratio sensor 7. It is known that the air-fuel ratio detected by the above is stoichiometric. Then, after the release of NOx and oxygen from the first NOx catalyst 4 is completed, the air-fuel ratio detected by the first air-fuel ratio sensor 7 shifts to the rich air-fuel ratio. In this way, the time from when the first air-fuel ratio sensor 7 detects the stoichiometric gas and shifts to the rich air-fuel ratio becomes longer as the amount of NOx and oxygen stored in the first NOx catalyst 4 increases.

そして、第1NOx触媒4の劣化の度合いが大きくなるほど、該第1NOx触媒4が吸蔵できるNOx量及び酸素量が減少する。従って、第1NOx触媒4の劣化の度合いが大きくなるほど、第1NOx触媒4に吸蔵されていたNOxを還元するため及び酸素を放出させるために必要となる燃料量が減る。また、リッチスパイク時に第1空燃比センサ7によりストイキが検出された後リッチ空燃比に移行するまでの時間、すなわちストイキの継続時間が短くなる。以上より、第1NOx触媒4に吸蔵されていたNOxおよび酸素を還元もしくは放出させるために要した燃料量、もしくは第1NOx触媒4に流入する排気の空燃比およびストイキの継続時間に基づいて、第1NOx触媒4の劣化の度合いを判定することが可能となる。   As the degree of deterioration of the first NOx catalyst 4 increases, the amount of NOx and oxygen that can be stored by the first NOx catalyst 4 decreases. Therefore, as the degree of deterioration of the first NOx catalyst 4 increases, the amount of fuel necessary for reducing NOx stored in the first NOx catalyst 4 and releasing oxygen decreases. Further, the time until the shift to the rich air-fuel ratio after the stoichiometric detection is detected by the first air-fuel ratio sensor 7 during the rich spike, that is, the stoichiometric duration time is shortened. From the above, based on the amount of fuel required to reduce or release NOx and oxygen stored in the first NOx catalyst 4, or the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 and the duration of the stoichiometry, the first NOx. The degree of deterioration of the catalyst 4 can be determined.

ところで、第1NOx触媒4の劣化の度合いを判定するには、第1NOx触媒4に流入する排気の空燃比を正確に知る必要がある。しかし、リッチスパイク時には、排気中に添加された燃料のクラッキングが十分に行われず第3空燃比センサ6はリーンずれを起こす場合がある。このリーンずれは、酸化触媒3を設けることによりある程度は改善されるが、
第3空燃比センサ6のリーンずれを全くなくすことは困難である。そのため、リッチスパイク制御中に第1NOx触媒4に流入する排気の空燃比を第3空燃比センサ6のみにより検出することは困難となる。
Incidentally, in order to determine the degree of deterioration of the first NOx catalyst 4, it is necessary to accurately know the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4. However, at the time of rich spike, cracking of the fuel added to the exhaust gas is not sufficiently performed, and the third air-fuel ratio sensor 6 may cause a lean shift. This lean shift is improved to some extent by providing the oxidation catalyst 3,
It is difficult to eliminate the lean deviation of the third air-fuel ratio sensor 6 at all. Therefore, it becomes difficult to detect the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 during the rich spike control only by the third air-fuel ratio sensor 6.

その点、本実施例によれば、第2空燃比センサ8の出力信号がリッチ空燃比となった後で所定の変化幅内に安定したときの該第2空燃比センサ8により検出される排気の空燃比に基づいて、第3空燃比センサ8の検出値を補正することができる。   In that respect, according to the present embodiment, the exhaust gas detected by the second air-fuel ratio sensor 8 when the output signal of the second air-fuel ratio sensor 8 becomes stable within a predetermined change width after the rich air-fuel ratio becomes stable. The detected value of the third air-fuel ratio sensor 8 can be corrected based on the air-fuel ratio.

ここで、図2は、本実施例による燃料添加量補正制御を行ったときに第3空燃比センサ6、第1空燃比センサ7、および第2空燃比センサ8から得られる空燃比の時間推移を示したタイムチャート図である。実線が第3空燃比センサ6、破線が第1空燃比センサ7、一点鎖線が第2空燃比センサ8の出力値を夫々示し、点線が目標空燃比を示している。   Here, FIG. 2 shows the time transition of the air-fuel ratio obtained from the third air-fuel ratio sensor 6, the first air-fuel ratio sensor 7, and the second air-fuel ratio sensor 8 when the fuel addition amount correction control according to this embodiment is performed. It is the time chart figure which showed. The solid line indicates the output value of the third air-fuel ratio sensor 6, the broken line indicates the output value of the first air-fuel ratio sensor 7, the alternate long and short dash line indicates the output value of the second air-fuel ratio sensor 8, and the dotted line indicates the target air-fuel ratio.

第1空燃比センサ7により検出される空燃比は、燃料添加と共に低下する。そして、ストイキとなると第1NOx触媒4からNOxおよび酸素が放出される。換言すると、第1NOx触媒4からのNOxおよび酸素の放出が完了するまで間、第1空燃比センサ7の検出値はストイキとなる。その後、第1NOx触媒4からのNOxおよび酸素の放出が完了すると、第1空燃比センサ7により検出される空燃比はリッチ空燃比に低下し、ほぼ一定のリッチ空燃比で推移する。この一定のリッチ空燃比で推移している間の第1空燃比センサ7による検出値は、第1NOx触媒4に流入する実際の排気の空燃比とほぼ等しい値となる。   The air-fuel ratio detected by the first air-fuel ratio sensor 7 decreases with fuel addition. When stoichiometric, NOx and oxygen are released from the first NOx catalyst 4. In other words, the detected value of the first air-fuel ratio sensor 7 becomes stoichiometric until the release of NOx and oxygen from the first NOx catalyst 4 is completed. Thereafter, when the release of NOx and oxygen from the first NOx catalyst 4 is completed, the air-fuel ratio detected by the first air-fuel ratio sensor 7 decreases to a rich air-fuel ratio and changes at a substantially constant rich air-fuel ratio. The detected value by the first air-fuel ratio sensor 7 while changing at this constant rich air-fuel ratio becomes a value substantially equal to the air-fuel ratio of the actual exhaust gas flowing into the first NOx catalyst 4.

次に第2空燃比センサ8により検出される空燃比は、第2NOx触媒5からのNOxおよび酸素の放出が完了するまで間ストイキとなる。そして第2NOx触媒5からのNOxおよび酸素の放出が完了するとリッチ空燃比となるが、その頃には燃料の添加が停止されているので、すぐに空燃比が高まる。   Next, the air-fuel ratio detected by the second air-fuel ratio sensor 8 becomes stoichiometric until the release of NOx and oxygen from the second NOx catalyst 5 is completed. When the release of NOx and oxygen from the second NOx catalyst 5 is completed, the air / fuel ratio becomes rich. At that time, the addition of fuel is stopped, so the air / fuel ratio immediately increases.

また、第3空燃比センサ6により検出される空燃比は、燃料添加によるリーンずれが生じている。
一方、図3は、本実施例による燃料添加量補正制御を行ったときであって、第3空燃比センサの出力を補正したときの空燃比の時間推移を示したタイムチャート図である。
Further, the air-fuel ratio detected by the third air-fuel ratio sensor 6 has a lean shift due to fuel addition.
On the other hand, FIG. 3 is a time chart showing the time transition of the air-fuel ratio when the fuel addition amount correction control according to this embodiment is performed and when the output of the third air-fuel ratio sensor is corrected.

ここで、図3においてAで示した期間(以下、期間Aとする。)では、第1空燃比センサ7からの出力信号は、ほぼ一定の値を示している。この場合、第1空燃比センサ7の検出値はリッチ空燃比であって所定の変化幅内で安定しているといえる。そして、本実施例においては、期間Aにおいて、第1空燃比センサ7の検出値と第3空燃比センサ6の検出値とが等しくなるように、第3空燃比センサ6の検出値を補正する。このときに補正するのは、期間Aに限らずその前のリッチスパイク中に検出された第3空燃比センサ6の検出値の補正をも行う。   Here, in the period indicated by A in FIG. 3 (hereinafter referred to as period A), the output signal from the first air-fuel ratio sensor 7 shows a substantially constant value. In this case, it can be said that the detected value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change width. In the present embodiment, in the period A, the detection value of the third air-fuel ratio sensor 6 is corrected so that the detection value of the first air-fuel ratio sensor 7 and the detection value of the third air-fuel ratio sensor 6 become equal. . The correction at this time is not limited to the period A, and the detection value of the third air-fuel ratio sensor 6 detected during the rich spike before that is also corrected.

すなわち、図3において、「第3空燃比センサ補正前」の空燃比が期間Aにおいて「第1空燃比センサ」の空燃比と等しくなるように、燃料添加開始前のリーン空燃比(以下、ベース空燃比という。)を基準に、よりリッチとなる方向に「第3空燃比センサ補正前」の空燃比を同割合で移動させて「第3空燃比センサ補正後」の空燃比を得る。   That is, in FIG. 3, the lean air-fuel ratio before starting fuel addition (hereinafter referred to as the base air-fuel ratio) so that the air-fuel ratio before “third air-fuel ratio sensor correction” becomes equal to the air-fuel ratio of “first air-fuel ratio sensor” in period A. The air-fuel ratio “before the third air-fuel ratio sensor correction” is moved at the same ratio in the direction of becoming richer on the basis of the air-fuel ratio), and the air-fuel ratio “after the third air-fuel ratio sensor correction” is obtained.

この場合、まずベース空燃比と期間Aにおける第1空燃比センサ7から得られる空燃比との差Xを求める。次に、ベース空燃比と期間Aにおける第3空燃比センサ6から得られる空燃比との差Yを求める。そして、差Xを差Yで除した値を補正係数Zとして求める。この補正係数Zを、期間A以外にも適用する。すなわち、ベース空燃比と燃料添加中の第3空燃比センサ6から得られる空燃比との差Sに補正係数Z乗じて新たな差Tを求め、この差Tをベース空燃比から減じた値を補正後の空燃比として得ることができる。   In this case, first, a difference X between the base air-fuel ratio and the air-fuel ratio obtained from the first air-fuel ratio sensor 7 in the period A is obtained. Next, a difference Y between the base air-fuel ratio and the air-fuel ratio obtained from the third air-fuel ratio sensor 6 in the period A is obtained. Then, a value obtained by dividing the difference X by the difference Y is obtained as a correction coefficient Z. This correction coefficient Z is applied to other than the period A. That is, a new difference T is obtained by multiplying the difference S between the base air-fuel ratio and the air-fuel ratio obtained from the third air-fuel ratio sensor 6 during fuel addition by a correction coefficient Z, and a value obtained by subtracting the difference T from the base air-fuel ratio is obtained. It can be obtained as a corrected air-fuel ratio.

このように、期間Aにおいて算出した補正係数Zを燃料添加時の全ての期間に適用して第3空燃比センサ6の出力値を補正することができる。
この場合、第3空燃比センサ6の検出値をECU10に記憶しておき、期間Aで補正係数Zを算出した後に、記憶しておいた第3空燃比センサ6の検出値の補正を行う。そして、その後に第1NOx触媒4に吸蔵されていたNOxおよび酸素の放出もしくは還元に要した燃料量を算出し、この算出された燃料量を基準に第1NOx触媒4の劣化判定を行う。
As described above, the output value of the third air-fuel ratio sensor 6 can be corrected by applying the correction coefficient Z calculated in the period A to all the periods at the time of fuel addition.
In this case, the detection value of the third air-fuel ratio sensor 6 is stored in the ECU 10, and after the correction coefficient Z is calculated in the period A, the stored detection value of the third air-fuel ratio sensor 6 is corrected. Then, the amount of fuel required for the release or reduction of NOx and oxygen stored in the first NOx catalyst 4 is calculated, and the deterioration of the first NOx catalyst 4 is determined based on the calculated fuel amount.

ここで、図4は、第1NOx触媒4に吸蔵されていたNOxおよび酸素の放出もしくは還元に要した燃料量を示した図である。
「第1NOx触媒分」としてハッチングを施した面積より、第1NOx触媒4に吸蔵されていたNOxおよび酸素の放出に要した燃料添加量が求められ、「第2NOx触媒分」としてハッチングを施した面積より、第2NOx触媒5に吸蔵されていたNOxおよび酸素の放出に要した燃料添加量が求められる。
Here, FIG. 4 is a diagram showing the amount of fuel required for releasing or reducing NOx and oxygen stored in the first NOx catalyst 4.
The amount of fuel added to release NOx and oxygen stored in the first NOx catalyst 4 is obtained from the area hatched as “first NOx catalyst component”, and the hatched area as “second NOx catalyst component”. Thus, the fuel addition amount required for releasing the NOx and oxygen stored in the second NOx catalyst 5 is obtained.

そして、第1NOx触媒4に吸蔵されていたNOxおよび酸素の放出に要した燃料添加量が多いほど、第1NOx触媒4に多くのNOxおよび酸素が吸蔵されていたことが分かり、この燃料添加量に基づいて第1NOx触媒4の劣化の度合いを判定することができる。   It can be seen that the greater the amount of fuel added required to release the NOx and oxygen stored in the first NOx catalyst 4, the more NOx and oxygen were stored in the first NOx catalyst 4. Based on this, the degree of deterioration of the first NOx catalyst 4 can be determined.

なお、本実施例においては、第2空燃比センサ8により得られる空燃比がストイキからリッチとなったときの直前に検出された第1空燃比センサにより得られる空燃比により第3空燃比センサ6の検出値を補正するようにしてもよい。   In this embodiment, the third air-fuel ratio sensor 6 is obtained by the air-fuel ratio obtained by the first air-fuel ratio sensor detected immediately before the air-fuel ratio obtained by the second air-fuel ratio sensor 8 becomes rich from stoichiometric. The detected value may be corrected.

ここで、第2空燃比センサ8の検出値がストイキからリッチに移行するのは、第1NOx触媒4および第2NOx触媒5に吸蔵されていたNOxおよび酸素の放出が完了したことを意味している。すなわち、この直前には、第1NOx触媒4に吸蔵されていたNOxおよび酸素の放出が完了しており、第1空燃比センサ7により検出される排気の空燃比と第1NOx触媒4に流入する排気の空燃比とは、ほぼ等しい値となる。また、第1空燃比センサ7の検出値はリッチ空燃比であって所定の変化幅内で安定しているといえる。さらに、第2空燃比センサ8の検出値は温度変化等による影響を受け難く、第1空燃比センサ7の検出値がリッチ空燃比であって所定の変化幅内で安定していることを精度よく検出することができる。   Here, the detected value of the second air-fuel ratio sensor 8 shifting from stoichiometric to rich means that the release of NOx and oxygen stored in the first NOx catalyst 4 and the second NOx catalyst 5 has been completed. . That is, immediately before this, the release of NOx and oxygen stored in the first NOx catalyst 4 has been completed, and the air-fuel ratio of the exhaust detected by the first air-fuel ratio sensor 7 and the exhaust flowing into the first NOx catalyst 4 The air-fuel ratio is substantially equal. Further, it can be said that the detected value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change width. Further, the detection value of the second air-fuel ratio sensor 8 is not easily affected by temperature changes or the like, and it is accurate that the detection value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change range. Can be detected well.

したがって、このときの第1空燃比センサ7により得られる排気の空燃比に基づいて、第3空燃比センサ6の検出値を補正することが可能となる。この補正は、前記期間Aのときに検出される第1空燃比センサにより得られる排気の空燃比による補正と同様にして行うことができる。   Therefore, the detection value of the third air-fuel ratio sensor 6 can be corrected based on the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio sensor 7 at this time. This correction can be performed in the same manner as the correction by the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio sensor detected during the period A.

また、本実施例においては、第1空燃比センサ7により検出される排気の空燃比がストイキよりリッチへ移行した後所定時間経過後に、第1空燃比センサ7の検出値はリッチ空燃比であって所定の変化幅内で安定しているとして、このときに第1空燃比センサ7により検出される排気の空燃比を基準として、第3空燃比センサ6の検出値を補正してもよい。   Further, in this embodiment, the detected value of the first air-fuel ratio sensor 7 is the rich air-fuel ratio after a predetermined time has elapsed after the air-fuel ratio of the exhaust gas detected by the first air-fuel ratio sensor 7 has shifted from rich to rich. Therefore, the detected value of the third air-fuel ratio sensor 6 may be corrected on the basis of the air-fuel ratio of the exhaust detected by the first air-fuel ratio sensor 7 at this time.

さらに、第3空燃比センサ8の検出値の補正を行うための第1空燃比センサ7の検出値は、期間Aの任意の時における値であってもよく、期間Aの平均値であってもよい。
なお、本実施例においては、第1空燃比センサ7の検出値により第3空燃比センサ6の検出値を補正しているが、これに代えて、燃料添加弁9から燃料が添加されている間に第1NOx触媒4に流入する排気の空燃比を実験的に求めておき、この実験的に求めた値を補正するようにしてもよい。
Further, the detection value of the first air-fuel ratio sensor 7 for correcting the detection value of the third air-fuel ratio sensor 8 may be a value at an arbitrary time in the period A, and is an average value of the period A. Also good.
In this embodiment, the detection value of the third air-fuel ratio sensor 6 is corrected by the detection value of the first air-fuel ratio sensor 7, but instead, fuel is added from the fuel addition valve 9. In the meantime, the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 may be experimentally obtained, and the experimentally obtained value may be corrected.

すなわち、期間Aにおける第1空燃比センサ7により得られる空燃比と、実験的に求めた排気の空燃比が最終的に到達する空燃比と、が等しくなるように、実験的に求めた排気の空燃比を補正することにより、燃料添加中であって期間Aよりも前の期間における第1NOx触媒4に流入する排気の空燃比を得ることができる。   That is, the exhaust gas experimentally determined so that the air-fuel ratio obtained by the first air-fuel ratio sensor 7 in the period A is equal to the air-fuel ratio finally reached by the experimentally determined air-fuel ratio. By correcting the air-fuel ratio, it is possible to obtain the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 during the fuel addition and before the period A.

以上説明したように、本実施例によれば、第1空燃比センサ7からの検出値がリッチ空燃比で且つ安定している期間の検出値により、第3空燃比センサ6の検出値を補正することができる。これにより、第1NOx触媒4のより正確な劣化判定を行うことができる。   As explained above, according to the present embodiment, the detection value of the third air-fuel ratio sensor 6 is corrected by the detection value of the period when the detection value from the first air-fuel ratio sensor 7 is rich and stable. can do. Thereby, a more accurate deterioration determination of the first NOx catalyst 4 can be performed.

本実施例においては、燃料添加の終了時期を第2空燃比センサ8の出力信号により決定する。
ここで、排気系を流通する排気の空燃比を一番正確に検出することができるのは第2空燃比センサ8である。これは、酸化触媒3、第1NOx触媒4および第2NOx触媒5により燃料が十分にクラッキングされるからである。しかし、第2空燃比センサ8は、燃料添加弁9から離れた位置にあり、燃料添加弁9から添加された燃料が第2空燃比センサ8に到達するまでに時間がかかる。そのため、第2空燃比センサ8の出力信号によりリッチスパイク時の空燃比制御を行うと、空燃比の変動が起こることがある。
In this embodiment, the end time of fuel addition is determined by the output signal of the second air-fuel ratio sensor 8.
Here, the second air-fuel ratio sensor 8 can most accurately detect the air-fuel ratio of the exhaust gas flowing through the exhaust system. This is because the fuel is sufficiently cracked by the oxidation catalyst 3, the first NOx catalyst 4, and the second NOx catalyst 5. However, the second air-fuel ratio sensor 8 is located away from the fuel addition valve 9, and it takes time for the fuel added from the fuel addition valve 9 to reach the second air-fuel ratio sensor 8. Therefore, if the air-fuel ratio control during rich spike is performed by the output signal of the second air-fuel ratio sensor 8, the air-fuel ratio may vary.

その点、第2空燃比センサ8の出力信号を燃料添加の終了時期の判定にのみ使用することにより、空燃比の変動を抑制することができる。
ここで、本実施例では、第2空燃比センサ8の出力信号がストイキからリッチに移行したときに、燃料添加弁9からの燃料添加を終了させる。第2空燃比センサ8の出力信号がストイキからリッチに移行したときには、第1NOx触媒4および第2NOx触媒5に吸蔵されていたNOxおよび酸素の放出が完了したことを意味しており、触媒劣化判定に必要な燃料添加量を算出することが可能となっている。したがって、このときに燃料添加を終了させても、実施例1で説明した触媒劣化判定を行うことができ、且つ過剰な燃料の添加を抑制できる。
In that respect, fluctuations in the air-fuel ratio can be suppressed by using the output signal of the second air-fuel ratio sensor 8 only for determining the end timing of fuel addition.
Here, in this embodiment, when the output signal of the second air-fuel ratio sensor 8 shifts from stoichiometric to rich, the fuel addition from the fuel addition valve 9 is terminated. When the output signal of the second air-fuel ratio sensor 8 shifts from stoichiometric to rich, it means that the release of NOx and oxygen stored in the first NOx catalyst 4 and the second NOx catalyst 5 has been completed, and catalyst deterioration determination It is possible to calculate the amount of fuel added necessary for the operation. Therefore, even if fuel addition is terminated at this time, the catalyst deterioration determination described in the first embodiment can be performed, and excessive fuel addition can be suppressed.

実施例に係る内燃機関の排気浄化装置を適用する内燃機関とその排気系の概略構成を示す図である。It is a figure which shows schematic structure of the internal combustion engine which applies the exhaust gas purification apparatus of the internal combustion engine which concerns on an Example, and its exhaust system. 実施例による燃料添加量補正制御を行ったときに第3空燃比センサ、第1空燃比センサ、および第2空燃比センサから得られる空燃比の時間推移を示したタイムチャート図である。FIG. 6 is a time chart showing the time transition of the air-fuel ratio obtained from the third air-fuel ratio sensor, the first air-fuel ratio sensor, and the second air-fuel ratio sensor when the fuel addition amount correction control according to the embodiment is performed. 実施例による燃料添加量補正制御を行ったときであって、第3空燃比センサの出力を補正したときの空燃比の時間推移を示したタイムチャート図である。FIG. 6 is a time chart showing the time transition of the air-fuel ratio when the fuel addition amount correction control according to the embodiment is performed and the output of the third air-fuel ratio sensor is corrected. 第1NOx触媒に吸蔵されていたNOxおよび酸素の放出もしくは還元に要した燃料量を示した図である。It is the figure which showed the amount of fuel required for discharge | release or reduction | restoration of NOx and oxygen which were occluded by the 1st NOx catalyst.

符号の説明Explanation of symbols

1 内燃機関
2 排気通路
3 酸化触媒
4 第1NOx触媒
5 第2NOx触媒
6 第3空燃比センサ
7 第1空燃比センサ
8 第2空燃比センサ
9 燃料添加弁
10 ECU
1 internal combustion engine 2 exhaust passage 3 oxidation catalyst 4 first NOx catalyst 5 second NOx catalyst 6 third air-fuel ratio sensor 7 first air-fuel ratio sensor 8 second air-fuel ratio sensor 9 fuel addition valve 10 ECU

Claims (5)

内燃機関の排気通路に設けられた第1NOx吸蔵触媒と、
前記第1NOx吸蔵触媒よりも下流の排気の空燃比を検出する第1空燃比検出手段と、
前記第1NOx吸蔵触媒よりも上流から該第1NOx吸蔵触媒へ燃料を添加する燃料添加手段と、
前記燃料添加手段により燃料を添加して前記第1NOx吸蔵触媒に吸蔵されたNOxを還元するNOx還元手段と、
前記燃料添加手段により燃料が添加されているときの前記第1空燃比検出手段の検出値がストイキよりリッチ空燃比であって所定の変化幅内で安定しているか否か判定する出力安定判定手段と、
前記出力安定判定手段により前記第1空燃比検出手段の検出値がストイキよりリッチな空燃比で安定していると判定されたときの前記第1空燃比検出手段により検出される空燃比に基づいて、前記燃料添加手段により燃料が添加されていた期間中の前記第1NOx吸蔵触媒に流入する排気の空燃比を推定する流入排気空燃比推定手段と、
前記出力安定判定手段により前記第1空燃比検出手段の検出値がストイキよりリッチな空燃比で安定していると判定されたときの前記第1空燃比検出手段により得られる排気の空燃比および前記流入排気空燃比推定手段により推定される空燃比に基づいて前記第1NOx吸蔵触媒の劣化を判定する劣化判定手段と、
を具備することを特徴とする内燃機関の排気浄化装置。
A first NOx storage catalyst provided in an exhaust passage of the internal combustion engine;
First air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the first NOx storage catalyst;
Fuel addition means for adding fuel to the first NOx storage catalyst from the upstream side of the first NOx storage catalyst;
NOx reduction means for reducing NOx stored in the first NOx storage catalyst by adding fuel by the fuel addition means;
Output stability determination means for determining whether or not the detected value of the first air-fuel ratio detection means when the fuel is added by the fuel addition means is richer than stoichiometric and stable within a predetermined change range. When,
Based on the air-fuel ratio detected by the first air-fuel ratio detecting means when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric. An inflow exhaust air / fuel ratio estimating means for estimating an air / fuel ratio of exhaust flowing into the first NOx storage catalyst during a period in which fuel is added by the fuel addition means;
When the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detecting means and the Deterioration determining means for determining deterioration of the first NOx storage catalyst based on the air-fuel ratio estimated by the inflowing exhaust air-fuel ratio estimating means;
An exhaust emission control device for an internal combustion engine, comprising:
前記第1空燃比検出手段よりも下流の排気通路に設けられ、酸化能力を有する第2触媒と、
前記第2触媒よりも下流の排気の空燃比を検出する第2空燃比検出手段と、
をさらに備え、
前記出力安定手段は、前記燃料添加手段により燃料が添加されているときに前記第2空燃比検出手段により検出される空燃比がストイキよりリッチに変化したときに前記第1空燃比検出手段により検出された排気の空燃比がストイキよりリッチな空燃比で安定していると判定することを特徴とする請求項1に記載の内燃機関の排気浄化装置。
A second catalyst provided in an exhaust passage downstream of the first air-fuel ratio detection means and having an oxidation ability;
Second air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the second catalyst;
Further comprising
The output stabilizing means is detected by the first air-fuel ratio detecting means when the air-fuel ratio detected by the second air-fuel ratio detecting means changes richer than stoichiometric when fuel is added by the fuel adding means. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas is determined to be stable at an air-fuel ratio richer than stoichiometric.
前記第1NOx吸蔵触媒よりも上流の排気の空燃比を検出する第3空燃比検出手段をさらに備え、前記流入排気空燃比推定手段は前記第3空燃比検出手段により検出された空燃比と前記第1空燃比検出手段により検出された空燃比とから前記第1NOx吸蔵触媒に流入する排気の空燃比を推定することを特徴とする請求項1または2に記載の内燃機関の排気浄化装置。   Third air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust upstream of the first NOx storage catalyst is further provided, wherein the inflowing exhaust air-fuel ratio estimating means is configured to detect the air-fuel ratio detected by the third air-fuel ratio detecting means and the first air-fuel ratio. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated from the air-fuel ratio detected by one air-fuel ratio detection means. 前記流入排気空燃比推定手段は、前記出力安定判定手段により前記第1空燃比検出手段により検出された空燃比がストイキよりリッチな空燃比で安定していると判定されたときの該第1空燃比検出手段により得られる排気の空燃比と、このときの前記第3空燃比検出手段により検出された空燃比と、が等しくなるように、前記燃料添加手段による燃料添加中の前記第3空燃比検出手段により検出された空燃比を補正することにより、前記第1NOx吸蔵触媒に流入する排気の空燃比を推定することを特徴とする請求項3に記載の内燃機関の排気浄化装置。   The inflow exhaust air / fuel ratio estimation means is configured to detect the first air / fuel ratio when the output stability determination means determines that the air / fuel ratio detected by the first air / fuel ratio detection means is stable at an air / fuel ratio richer than stoichiometry. The third air-fuel ratio during fuel addition by the fuel addition means so that the air-fuel ratio of the exhaust gas obtained by the fuel-fuel ratio detection means is equal to the air-fuel ratio detected by the third air-fuel ratio detection means at this time. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated by correcting the air-fuel ratio detected by the detection means. 前記第2空燃比検出手段により検出される排気の空燃比が所定空燃比よりもリッチとなったときに前記燃料添加手段による燃料の添加を終了させる燃料添加終了手段をさらに備えることを特徴とする請求項2から4の何れかに記載の内燃機関の排気浄化装置。   The fuel addition means further comprises a fuel addition end means for ending the fuel addition by the fuel addition means when the air-fuel ratio of the exhaust detected by the second air-fuel ratio detection means becomes richer than a predetermined air-fuel ratio. The exhaust emission control device for an internal combustion engine according to any one of claims 2 to 4.
JP2004033949A 2004-02-10 2004-02-10 Exhaust gas purification device for internal combustion engine Expired - Fee Related JP4214923B2 (en)

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JP5039367B2 (en) * 2006-11-24 2012-10-03 本田技研工業株式会社 Exhaust gas purification device for internal combustion engine
GB2478719A (en) * 2010-03-15 2011-09-21 Gm Global Tech Operations Inc Method of operating an internal combustion engine having a lean NOx trap (LNT)
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