1. Field of the Invention
The invention relates to a hybrid vehicle (HV) on which an internal combustion engine and a motor are mounted as drive sources.
2. Description of Related Art
A HV is capable of electric running and hybrid running. The electric running is a running in which the operation of an internal combustion engine is stopped and only the output of a motor is used. The hybrid running is a running in which the outputs of both of the internal combustion engine and the motor are used.
More specifically, the HV starts the engine to thereby start the hybrid running when “vehicle request power that changes according to an accelerator operation amount and a vehicle speed” becomes equal to or more than “an engine start power threshold value”. In addition, the HV stops the operation of the engine and performs the electric running when the vehicle request power becomes equal to or less than an engine stop power threshold value during the hybrid running. That is, the HV performs the operation of the engine when the engine can be efficiently operated and/or when a sufficient vehicle driving force cannot be obtained only with the motor. On the other hand, the HV stops the operation of the engine when the engine cannot be efficiently operated and/or when the sufficient vehicle driving force can be obtained only with the motor (see, e.g., Japanese Patent Application Publication No. 2005-42561 (JP-2005-42561 A)). Thus, the HV intermittently operates the engine. That is, the HV executes “an intermittent operation of the engine”.
The HV has an electric power storage device (e.g., a battery) that can supply electric power to the motor and can be charged. Further, in recent years, a plug-in HV has been developed. In the plug-in HV, the electric power storage device can be charged with electric power supplied from the outside of the vehicle. Hereinbelow, charging of the electric power storage device with the electric power supplied from the outside of the vehicle is referred to as “external charging”.
When the external charging is performed, the electric power storage device is almost fully charged in many cases so that the remaining capacity of the electric power storage device is large. Consequently, the HV runs in an EV (Electric Vehicle) mode (CD mode: Charge Depleting mode) until the remaining capacity of the electric power storage device is reduced to a mode switching threshold value after the external charging. The EV mode is a mode in which the electric running has higher priority than the hybrid running. Thereafter, when the remaining capacity of the electric power storage device becomes equal to or smaller than the mode switching threshold value, the HV runs in an HV mode (CS mode: charge sustaining mode).
For example, in the EV mode, the engine start power threshold value and the engine stop power threshold value are set to be higher than those in the HV mode. Further, for example, an intermittent operation permission vehicle speed in the EV mode is set to be higher than that in the HV mode. The intermittent operation permission vehicle speed is a vehicle speed that permits the intermittent operation. Consequently, in the EV mode, there are cases where the operation of the engine is stopped even when the vehicle speed is considerably high.
As can be seen from the foregoing, in the HV, the frequency of non-operation of the engine is high. In addition, for example, in a case where, even when the operation of the engine is started especially in the EV mode, the engine is operated in an idle state (idle operation) because of the small vehicle request power, the operation of the engine is stopped at the time point at which the vehicle speed becomes equal to or lower than the intermittent operation permission vehicle speed. In the HV, there are cases where the idle operation of the engine is performed when a condition for a specific idle operation such as the idle operation for accelerating the warm-up of a catalyst or performing air conditioning of a vehicle compartment is satisfied.
There is available an ISC (idle speed control) system in which, when the engine is operated in the idle state, “a control parameter that adjusts the rotation speed of the engine (e.g., a throttle valve opening)” is feedback-controlled such that the rotation speed of the engine is maintained at a target idle rotation speed. In the ISC system, when the engine is operated in the idle state, a value corresponding to the control parameter is learned as an idle operation control learned value.
In a case where the engine provided with the ISC system is mounted on the HV, when the vehicle speed is higher than a predetermined learning permission vehicle speed, the learning of the idle operation control learned value is prohibited even when the engine is operated in the idle state. The learning of the idle operation control learned value is permitted only when the vehicle speed is not more than the predetermined learning permission vehicle speed. This is for executing the learning of the idle operation control learned value in the more stable idle operation state. The learning permission vehicle speed is lower than the above-described intermittent operation permission vehicle speed.
However, as described above, the frequency of start of the engine is low in the HV. In addition, even if the engine is started, it is rarely the case that the vehicle speed is reduced to the learning permission vehicle speed or lower while the engine maintains the idle operation state. This is because the vehicle request power in a case where the engine is operated in the idle state is less than the engine stop power threshold value, and hence the operation of the engine is stopped at the time point at which the vehicle speed becomes equal to or lower than the intermittent operation permission vehicle speed. As a result, in the HV, opportunity for learning of the idle operation control learned value is limited.