To promptly change engine speed of an internal combustion engine to the engine speed at which the system efficiency expressed by multiplication value of thermal efficiency of the internal combustion engine and transmission efficiency of a power transmission mechanism becomes maximum, in a vehicle driving system which can change the engine speed, while fixing output of the internal combustion engine.
The allowable maximum variation of the engine speed is associated with output and is stored in a storage device. The allowable maximum variation according to the requested output is read from the storage device, and the engine speed is changed by the allowance maximum variation by making the engine speed on line of operation set beforehand into an initial value. Then whenever the engine speed is changed, the system efficiency expressed by a multiplication value of the thermal efficiency of the internal combustion engine and the transmission efficiency of the power transmission mechanism is computed, and the engine speed at which the system efficiency becomes the maximum is specified, and the engine speed at which the system efficiency becomes the maximum is decided as the optimal engine speed.
Mamoru Takada
Hidekazu Onishi