1. Field of the Invention
The present invention relates to a device for controlling an internal combustion engine and, more specifically, to a device for controlling an internal combustion engine provided with a drive assist unit for assisting the rotation of the engine.
2. Description of the Related Art
In internal combustion engines, it is often required to raise the temperature of the exhaust gas. In the engine equipped with an exhaust gas purifying catalyst in the exhaust gas passage, for example, it is required, at the start of the engine, to raise the temperature of the exhaust gas to activate the catalyst by elevating the catalyst temperature. Further, a NOx occluding and reducing catalyst is known which occludes NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and purifies the occluded NOx by reduction with reducing components in the exhaust gas when the air-fuel ratio of the exhaust gas that is flowing in becomes rich. When the NOx occluding and reducing catalyst is used for a exhaust gas purifying catalyst, the sulfur oxide in the exhaust gas is occluded by the catalyst together with the NOx and builds up in the catalyst. In order to conduct a sulfur contamination-removing operation for releasing the occluded sulfur-oxide from the catalyst, it is necessary to maintain the catalyst at a high temperature by elevating the temperature of the exhaust gas.
When the engine is operated at a high load, the temperature of the exhaust gas rises without conducting any particular operation. When the engine is operated at a low load, however, the temperature of the exhaust gas is usually low and must be forcibly raised.
In order to raise the temperature of the engine exhaust gas, for example, usually the ignition timing of the engine is retarded (i.e., delayed)), as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-221062.
When the ignition timing of the engine is delayed, the air-fuel mixture in each cylinder starts burning in a delayed timing. When the exhaust valve in each cylinder is opened, therefore, the combustion gas that has not been completely burned yet is discharged into the exhaust gas passage from the cylinder. Therefore, an increased proportion of the combustion energy of the mixture is discharged into the exhaust gas passage together with the exhaust gas without being converted into the mechanical energy (i.e., an engine output).
Thus, the catalyst temperature can be raised by delaying the engine ignition timing when it becomes necessary to conduct the sulfur contamination-removing operation for the NOx occluding and reducing catalyst during the warming up period after the cold start of the engine or when the engine is operated at conditions where the exhaust gas temperature is low.
Japanese Unexamined Patent Publication (Kokai) No. 2002-285883 teaches a control device for elevating the temperature of the exhaust gas at the cold start of the internal combustion engine mounted on a so-called hybrid vehicle equipped with an auxiliary drive unit such as an electric motor.
Namely, in the engine of the '883 publication, the fuel necessary for obtaining a target air-fuel ratio of the engine is partly injected in the expansion stroke of the engine during the warming up after the start of the engine, in order to raise the temperature of the exhaust gas discharged from the cylinder when the exhaust valve is opened, so that the exhaust gas of a high temperature arrives at the catalyst.
When the ignition timing of the engine is delayed as described in the '062 publication, however, the combustion usually loses stability. Therefore, if the temperature of the exhaust gas is raised by delaying the ignition timing during the warming-up period after the cold start of the engine, the revolution of the engine fluctuates and misfiring occurs in an extreme case. In the engine mounted on the vehicle, further, if the temperature of the exhaust gas is raised by delaying the ignition timing in order to remove the sulfur contamination from the NOx occluding and reducing catalyst when the vehicle is running, the output torque fluctuates due to unstable combustion, and the operability of the vehicle is deteriorated.
According to '883 publication, further, the temperature of the exhaust gas is raised by injecting a part of the fuel during the cylinder expansion stroke of the internal combustion engine for a hybrid vehicle. In order to raise the temperature of the exhaust gas by injecting the fuel during the cylinder expansion stroke, however, direct cylinder fuel injection valves are necessary for injecting the fuel directly into the cylinders.
Further, operating modes of a hybrid vehicle usually include a charge traveling mode operation in which the vehicle is driven by the output of the internal combustion engine and, at the same time, the generator is driven by the engine and the generated electric power is stored in a rechargeable battery, as well as an EV (electric vehicle) traveling mode operation in which the vehicle is driven by an electric motor using the electric power stored in the rechargeable battery while halting the engine when the rechargeable battery is in a fully charged state (when the state of charge (hereinafter referred to as “SOC”) is full).
If it becomes necessary to raise the temperature of the exhaust gas to remove the sulfur contamination from the NOx occluding and reducing catalyst, the temperature of the exhaust gas can be raised by injecting the fuel in the expansion stroke, as taught in '883 publication, or by delaying the ignition timing, as taught in '062 publication, when the vehicle is running in the charge traveling mode. However, if the SOC is full and the vehicle is running in the EV traveling mode, the engine must be halted or must be operated at a low load.
When the engine is halted, the exhaust gas of a high temperature cannot be supplied to the catalyst, as a matter of course. When the operation of the engine is continued at a low load, the temperature of the exhaust gas cannot be raised to a sufficient degree even if the ignition timing is delayed as taught in '062 publication or the fuel is injected during the expansion stroke as taught in '883 publication.
In the engine of the hybrid vehicle, therefore, when the EV traveling mode operation takes place due to a full SOC, the temperature of the exhaust gas drops and the sulfur contamination-removing operation for the NOx occluding and reducing catalyst is interrupted if it is being executed.
The sulfur contamination-removing operation in this case is resumed when the charge traveling mode operation starts. However, the temperature of the catalyst that was high when the operation was interrupted, has now been lowered after the EV traveling mode operation. To resume the sulfur contamination-removing operation, therefore, the temperature of the catalyst must be raised again with extra energy and time and, thereby, the sulfur contamination-removing operation cannot be conducted efficiently.