Exhaust gas purification system for internal combustion engine

The present invention relates to an exhaust gas purification system for an internal combustion engine that has a supercharger and an EGR apparatus.

In an exhaust gas purification system for an internal combustion engine, there is known one which has an EGR apparatus that introduces at least part of the exhaust gas of the internal combustion engine as EGR gas into an air intake system. It is possible to reduce the amount of NOx emissions by introducing the EGR gas into the air intake system.

Japanese Patent Application Laid-Open No. 5-263716 discloses techniques which, in exhaust gas purification system for an internal combustion engine, limit the amount of change when changing an EGR gas amount to be introduced to the air intake system.

In an exhaust gas purification system for an internal combustion engine that has a supercharger and an EGR apparatus, a charging pressure and an EGR gas amount are changed depending on the operating state of the internal combustion engine. However, the response when changing the charging pressure is low compared to the response when changing the EGR gas amount.

Therefore, when the charging pressure is raised and the EGR gas amount is increased at the time in which the operating state of the internal combustion engine becomes transient, the EGR gas amount is sometimes excessively large with respect to the actual charging pressure. In such cases, the EGR gas amount is excessively large with respect to an amount of intake air flowing into a cylinder. As a consequence, there is a risk of an increase in unburned fuel emissions and accidental fire.

The present invention has been made in view of the above described problem and has as an object to provide art for an exhaust gas purification system for an internal combustion engine that has a supercharger and an EGR apparatus, wherein such art is capable of suppressing NOx emissions while also suppressing an increase in unburned fuel emissions and accidental fire when the internal combustion engine is in a transient operating state.

In the present invention, the following means are employed to solve the above problems.

Namely, in the present invention, a target charging pressure and a target EGR gas amount are calculated based on the operating state of the internal combustion engine. If the actual charging pressure has not reached the target charging pressure when the internal combustion engine is operating in a transient operating state, the EGR gas amount is controlled to an amount smaller than the target EGR gas amount.

More specifically, an exhaust gas purification system for an internal combustion engine according to the present invention is characterized by including the following: an EGR apparatus for introducing at least part of the exhaust gas of the internal combustion engine as EGR gas into an air intake system of the internal combustion engine; a supercharger for supercharging intake air using energy from the exhaust gas of the internal combustion engine; EGR gas amount control means for controlling the EGR gas amount introduced into the air intake system of the internal combustion engine by the EGR apparatus; target EGR gas amount calculation means for calculating a target EGR gas amount that is a target value of the EGR gas amount, based on the operating state of the internal combustion engine; target charging pressure calculation means for calculating a target charging pressure that is a target value of the charging pressure, based on the operating state of the internal combustion engine; and charging pressure detection means for detecting the actual charging pressure, wherein if the actual charging pressure detected by the charging pressure detection means is lower than the target charging pressure when the internal combustion engine is in a transient operating state, the EGR gas amount control means controls the EGR gas amount to an amount smaller than the target EGR gas amount.

According to the present invention, it is possible to suppress the EGR gas amount introduced into the air intake system from becoming an excessively large amount with respect to the actual charging pressure (i.e., with respect to the intake air amount), when the internal combustion engine is in a transient operating state. Therefore, when the internal combustion engine is operating in a transient state, NOx emissions can be suppressed while also suppressing an increase in unburned fuel emissions and accidental fire.

In the present invention, if the actual charging pressure is lower than the target charging pressure when the internal combustion engine is operating in a transient state, the EGR gas amount may be controlled such that the lower the actual charging pressure, the smaller the EGR gas amount. Thus, it is possible to achieve an EGR gas amount more appropriate to the actual charging pressure.

The present invention may further include, in cases where a fuel injection valve in the internal combustion engine directly injects fuel into a cylinder: sub fuel injection execution means for performing a sub fuel injection at a timing ahead of a main fuel injection that is performed by the fuel injection valve at a timing near top dead center in a compression stroke; sub fuel injection timing control means for controlling an execution timing of the sub fuel injection is performed by the sub fuel injection execution means; and target sub fuel injection timing calculation means for calculating a target sub fuel injection timing that is a target value of the execution timing of the sub fuel injection, based on the operating state of the internal combustion engine.

In addition, if the actual charging pressure detected by the charging pressure detection means is lower than the target charging pressure when the internal combustion engine is in a transient operating state, the sub fuel injection timing control means may delay the execution timing of the sub fuel injection more than the target sub fuel injection timing.

Delaying the execution timing of the sub fuel injection shortens an interval between the execution timing of the sub fuel injection and the execution timing of the main fuel injection. Therefore, fuel injected by the sub fuel injection combusts more easily.

Therefore, according to the above, it is possible to suppress an increase in unburned fuel emissions and accidental fire in the case where the actual charging pressure is lower than the target charging pressure while the sub fuel injection is being performed.

Here, the intake air amount becomes smaller as the charging pressure becomes lower. Therefore, fuel injected by the sub fuel injection becomes more difficult to combust as the charging pressure becomes lower.

Hence, when the execution timing of the sub fuel injection is delayed more than the target sub fuel injection timing as in the above description, the execution timing of the sub fuel injection may be controlled such that the lower the actual charging pressure, the more delayed the execution timing of the sub fuel injection. Thus, fuel injected by the sub fuel injection can combust even more easily.

The present invention may further include: sub fuel injection amount control means for controlling a sub fuel injection amount; and target sub fuel injection amount calculation means for calculating a target sub fuel injection amount that is a target value of the sub fuel injection amount, based on the operating state of the internal combustion engine. Moreover, if the execution timing of the sub fuel injection is delayed more than the target sub fuel injection timing, the sub fuel injection amount control means may control the sub fuel injection amount to an amount smaller than the target sub fuel injection amount.

As described above, delaying the execution timing of the sub fuel injection shortens the interval between the execution timing of the sub fuel injection and the execution timing of the main fuel injection. Therefore, oxygen is consumed due to the combustion of fuel injected by the sub fuel injection, and it is highly possible that the main fuel injection is executed in such a state. As a consequence, there is a risk of an increase in particulate matter (hereinafter referred to as PM).

Hence, the oxygen amount consumed for the combustion of fuel injected by the sub fuel injection is decreased by decreasing the sub fuel injection amount. Thus, the generation of PM can be suppressed.