An exhaust gas filter attached to an exhaust passage of a diesel engine for trapping particulate matter (to be abbreviated to PM hereafter) contained in exhaust gas to prevent the PM from being discharged into the atmosphere is known. As the exhaust gas filter continues to trap the PM, a blockage eventually occurs. Therefore, when the timing for performing regeneration processing on the exhaust gas filter is reached, for example, a post-injection for injecting additional fuel is performed in the diesel engine during an expansion stroke in addition to a main fuel injection. The fuel that is post-injected during the expansion stroke reaches a catalyst disposed upstream of the exhaust gas filter without being burned in a cylinder, and generates heat through a reaction with the catalyst. The heat burns the PM that has accumulated in the filter, and thus the exhaust gas filter is regenerated.
If the fuel reaches the catalyst during an inactive period when the temperature of the catalyst is low, unburned hydrocarbon contained in the fuel covers the surface of the catalyst, causing hydrocarbon poisoning (to be abbreviated to HC poisoning hereafter) which leads to a deterioration in the performance of the catalyst.
When the vehicle decelerates, the exhaust gas temperature decreases, leading to a reduction in the temperature of the catalyst, and hence HC poisoning is particularly likely to occur at this time.
In JP2002-364436A, published by the Japan Patent Office in 2002, additional fuel is not injected when the vehicle decelerates during filter regeneration to such an extent that the temperature of the catalyst falls below an activation temperature.