Among hybrid vehicles running on both an engine (internal combustion engine) and an electrical motor, a series hybrid electrical vehicle is a vehicle that has its electrical motor working as a power source for traveling and that is supplied with electrical power from a generator connected mechanically with its engine. This type of series hybrid electrical vehicle allows the electrical motor to generate braking power through regenerative braking, which can reduce load distribution on friction brakes (i.e., braking device) and thereby lower their attrition. Also, the regenerative electrical power produced by regenerative braking is charged into an electrical storage device for effective use in powering the electrical motor so that fuel consumption may be reduced.
However, when regenerative electrical power turns out to be greater than charging electrical power acceptable to the electrical storage device (i.e., acceptable charging electrical power), regenerative braking power may be limited to protect the electrical storage device. Then the load distribution on the friction brakes is increased to make up for the amount by which the regenerative braking power is limited, which can promote the attrition of the friction brakes.
In view of the problem above, it has been proposed that the above-mentioned technical structure should have the generator replaced with a motor generator capable of rotating the engine so as to reduce the load distribution on the friction brakes. When excess regenerative electrical power that cannot be charged into the electrical storage device is generated, the excess regenerative electrical power is used to let the motor generator rotate the engine so that excess electrical power may be consumed by engine braking (see JP-2000-291451-A). This literature also describes a technique by which, if regenerative electrical power is more than what can be consumed by engine braking, an exhaust valve is closed to increase the load torque of the engine so that greater regenerative electrical power is consumed than when the exhaust value is left open, which prevents over speed of the engine.