1. Field of the Invention
The present invention relates to a system and a method for purifying exhaust gas from an internal combustion engine using a catalyst. More particularly, the invention relates to an exhaust gas purification system for an internal combustion engine that includes a supplemental fuel valve for supplying supplemental fuel to an exhaust passage, as well as relates to an exhaust gas purification method.
2. Description of the Related Art
The exhaust gas purification system for purifying exhaust gas from the internal combustion engine, such as a diesel engine, (hereinafter sometimes called engine) has, as one example, an NOx storage reduction catalyst and a particulate filter for trapping particulate matter (hereinafter referred to as PM) contained in exhaust gas.
The NOx storage reduction catalyst is designed to purify exhaust gas by means of storing NOx in the presence of when the oxygen concentration in the exhaust gas is high and reducing NOx to N2 when the concentration of oxygen of the exhaust gas is low and a large amount of reductants (e.g., unburned component of fuel, such as HC) are available in the exhaust gas. For example, a diesel particulate filter (DPF) or a diesel particulate-NOx reduction system (DPNR) catalyst is used as the particulate filter (hereinafter referred to as filter) for trapping PM.
The aforementioned exhaust gas purification system, which has an NOx storage reduction catalyst and a filter in the exhaust passage, performs a PM catalyst regeneration control, a sulfur poisoning recovery control and an NOx reduction control (hereinafter collectively referred to as catalyst control). The PM catalyst regeneration control regenerates the filter by oxidizing and removing PM deposits in the filter. The sulfur poisoning recovery control recovers the NOx storage reduction catalyst from sulfur poisoning by eliminating SOx stored in the NOx storage reduction catalyst. The NOx reduction control chemically reduces NOx stored in the NOx storage reduction catalyst.
One of these catalyst controls, the PM catalyst regeneration control, involves estimating the amount of PM deposits in the filter based on the operating conditions of the engine. The PM catalyst regeneration control further involves supplying supplemental fuel from a supplemental fuel valve to the exhaust passage (upstream of the filter) when the estimated amount of PM deposits is equal to or greater than a specified value (maximum allowable deposit amount). The supply of supplemental fuel increases bed temperature, thereby promoting oxidization (combustion) of the PM deposits in the filter.
The sulfur poisoning recovery control recovers the NOx storage reduction catalyst from sulfur poisoning. The sulfur poisoning recovery control involves supplying supplemental fuel from the supplemental fuel valve to the exhaust passage. This increases the catalyst bed temperature and simultaneously adjusts the air-fuel ratio of exhaust gas to a stoichiometric or richer ratio. The sulfur poisoning recovery control further involves eliminating SOx stored in the NOx storage reduction catalyst. In turn, the NOx reduction control chemically reduces NOx. In the NOx reduction control, supplemental fuel is supplied from the supplemental fuel valve to the exhaust passage and fed to the NOx storage reduction catalyst where the NOx stored in the catalyst reacts with a fuel component (HC).
The exhaust gas purification system also has a disadvantage. When the amount of PM deposits is greater than a specified value (e.g. immediately before or in the course of the PM catalyst regeneration control), and the volume of intake air decreases due to deceleration of the engine, the reduced volume of intake air may prevent the quantity of heat generated by burning the PM deposits from being transferred. This may cause an excessive increase in catalyst bed temperature. To overcome this problem, a related art is proposed in JP-A-2005-155500, paragraph [0003] as follows. During the PM catalyst regeneration control, if the engine is decelerating, the throttle valve in the intake passage is opened to increase the amount of air in exhaust gas, thereby increasing the amount of heat that is transferred by air passing through the catalyst. This prevents an excessive increase in catalyst bed temperature.
In addition, JP-A-2005-155500 further describes the situation where the volume of intake air is increased during the PM catalyst regeneration control when the engine is decelerating, the catalyst bed temperature is high, and the amount of PM deposits is large. Under these conditions, the quantity of heat generated by burning the PM deposits is greater than the quantity of heat that can be transferred by the air contained in exhaust gas. This can cause an abrupt increase in the catalyst bed temperature. Under such circumstances, it is necessary to inhibit the increase in volume of intake air.
Before the sulfur poisoning recovery control, the engine exhaust gas purification system performs the PM catalyst regeneration control to burn the PM deposits in the filter. This is called a “soot removal” process. Thereby, the amount of PM deposits is already reduced when the sulfur poisoning recovery control performed after the PM catalyst regeneration control. There is thus no excessive increase in catalyst bed temperature that occurs as a result of burning the PM deposits. In such situation, however, if the volume of intake air increases during deceleration in the same manner as under the PM catalyst regeneration control, an excess of air would pass through the filter. This prevents the catalyst bed temperature from being kept high, so that the temperature decreases. As a result of such reduced catalyst bed temperatures, the recovery of the catalyst from sulfur poisoning is more difficult. Accordingly, it takes longer for the NOx storage reduction catalyst to recover from sulfur poisoning, which increases the supplemental fuel supply quantity. Because additional supplemental fuel is required for raising the catalyst bed temperature, fuel economy is reduced.