Exhaust gas purification system for an internal combustion engine

The present invention relates to an exhaust gas purification system for an internal combustion engine which includes an NOx storage-reduction catalyst arranged on an exhaust passage of the internal combustion engine, and a precatalyst that is arranged on an exhaust passage at a location upstream of the NOx storage-reduction catalyst and has an oxidation function.

In Japanese patent application laid-open No. 2000-310113, there is disclosed an exhaust gas purification system for an internal combustion engine which includes an exhaust gas purification catalyst that is arranged on an exhaust passage of the internal combustion engine, and an adsorption means that is arranged on the exhaust passage at a location downstream of the exhaust gas purification catalyst and adsorbs prescribed components of an exhaust gas, wherein the degradation of the exhaust gas purification catalyst is determined based on an amount of the prescribed components adsorbed by the adsorption means.

Also, in Japanese patent application laid-open No. 2001-342879, there is disclosed an exhaust gas purification system for an internal combustion engine which includes an NOx storage-reduction catalyst (hereinafter simply referred to as a NOx catalyst) that is arranged on an exhaust passage of the internal combustion engine, and a precatalyst that is arranged on the exhaust passage at a location upstream of the NOx catalyst and has an oxidation function, wherein when the NOx occluded in the NOx catalyst is reduced, the air fuel ratio of an exhaust gas is controlled in accordance with the level of degradation of the precatalyst.

The present invention is intended to provide a technique which can detect the level of degradation of a precatalyst in an exhaust gas purification system for an internal combustion engine that includes a NOx catalyst arranged on an exhaust passage of the internal combustion engine, and the precatalyst arranged on the exhaust passage at a location upstream of the NOx catalyst and having an oxidation function.

The present invention detects the level of degradation of the precatalyst based on the width of change in temperature of the NOx catalyst at the time when a reducing agent is intermittently supplied from an upstream side of the precatalyst to the precatalyst and the NOx catalyst.

More specifically, an exhaust gas purification system for an internal combustion engine according to the present invention is characterized by comprising:

an NOx storage-reduction catalyst that is arranged on an exhaust passage of the internal combustion engine,

a precatalyst that is arranged on an exhaust passage at a location upstream of said NOx storage-reduction catalyst and has an oxidation function,

a temperature detecting means that detects the temperature of said NOx storage-reduction catalyst,

a reducing agent supplying means that intermittently supplies a reducing agent to said precatalyst and said NOx storage-reduction catalyst from an upstream side of said precatalyst, and

a degradation level detection means that detects the level of degradation of said precatalyst based on the width of change in temperature of said NOx storage-reduction catalyst at the time when said reducing agent is intermittently supplied by said reducing agent supplying means.

When the level of degradation of the precatalyst is relatively low, the oxidation of the reducing agent supplied from the reducing agent supplying means is liable to be facilitated in the precatalyst. When the reducing agent is oxidized in said precatalyst, the temperature of the precatalyst rises due to the oxidation heat, and at the same time the temperature of the NOx catalyst rises, too. In case where the oxidation of the reducing agent in the precatalyst is liable to be facilitated, the amount of the reducing agent and the amount of oxygen in the exhaust gas which reach up to the NOx catalyst become relatively small. Therefore, the amount of the reducing agent oxidized in the NOx catalyst necessarily decreases. As a result, the amount of oxidation heat generated due to the oxidation of the reducing agent in the NOx catalyst decreases. Accordingly, in case where the level of degradation of the precatalyst is relatively low, the temperature itself of the NOx catalyst rises higher when the intermittent supply of the reducing agent is executed by the reducing agent supplying means as compared with the time when the supply of said reducing agent is not executed, but the width of change in temperature of the NOx catalyst during the time when the reducing agent is intermittently supplied becomes small.

On the other hand, when the level of degradation of the precatalyst is relatively high, the reducing agent supplied from the reducing agent supplying means is difficult to be oxidized in the precatalyst. Thus, the temperature of the precatalyst is also difficult to rise. In such a case, the amount of the reducing agent and the amount of oxygen in the exhaust gas which reach up to the NOx catalyst become relatively large. As a result, the amount of oxidation heat generated due to the oxidation of the reducing agent in the NOx catalyst increases. Accordingly, a difference in temperature of the NOx catalyst between the times when the reducing agent is supplied to the NOx catalyst and when it is not supplied becomes large. In other words, when the level of degradation of the precatalyst is relatively high, the width of change in temperature of the NOx catalyst during the time when the reducing agent is intermittently supplied from the reducing agent supplying means becomes large.

In this manner, the width of change in temperature of the NOx catalyst at the time when the reducing agent is intermittently supplied by the reducing agent supplying means varies according to the level of degradation of the precatalyst. Accordingly, the level of degradation of the precatalyst can be detected based on the width of change in temperature of the NOx catalyst at this time.

The reducing agent becomes less liable to be oxidized in the precatalyst in accordance with the increasing level of degradation of the precatalyst. Thus, the temperature of the precatalyst becomes difficult to rise, and the amount of the reducing agent and the amount of oxygen in the exhaust gas, which reach the NOx catalyst when the reducing agent is supplied by the reducing agent supplying means, increase. Accordingly, when the reducing agent is intermittently supplied by the reducing agent supplying means, the temperature of the NOx catalyst at the time when the reducing agent is not supplied becomes lower and the temperature of the NOx catalyst when the reducing agent is supplied becomes higher, in accordance with the increasing level of degradation of the precatalyst.

Accordingly, in the present invention, it can be determined that the larger the width of change in temperature of the NOx catalyst at the time when the reducing agent is intermittently supplied by the reducing agent supplying means, the higher the level of degradation of the precatalyst is.

In the present invention, a NOx reduction control execution means that executes NOx reduction control for reducing the NOx occluded in the NOx catalyst by intermittently supplying the reducing agent by means of the reducing agent supplying means may further be provided.

As stated above, the higher the level of degradation of the precatalyst, the more the amount of the reducing agent and the amount of oxygen in the exhaust gas become, which can reach the NOx catalyst when the reducing agent is supplied by the reducing agent supplying means. In this case, there is fear that even if the reducing agent is supplied so as to reduce the NOx occluded in the NOx catalyst, the efficiency of NOx reduction control might decrease because of the oxidation reaction of the reducing agent also occurring with the reduction reaction of NOx in the NOx catalyst.

Accordingly, in the above case, a precatalyst temperature raising means that serves to make the temperature of the precatalyst higher, in case where the width of change in temperature of the NOx catalyst at the time when the reducing agent is intermittently supplied by the reducing agent supplying means (hereinafter simply referred to as the temperature change width of the NOx catalyst) is equal to or larger than a predetermined change width when the NOx reduction control is executed by the NOx reduction control execution means, than in case where the temperature change width of the NOx catalyst is less than the predetermined change width may further be provided.

Here, the predetermined change width is a threshold value with which it can be determined that the degradation of the precatalyst has progressed to such an extent that the efficiency of NOx reduction control is lowered excessively.