In recent years, a request for improving the fuel economy of vehicles and a demand for environmental-friendly vehicles according to the strengthening of regulations on toxic exhaust gases become increased. Therefore, hybrid electric vehicles have attracted considerable attentions as specific alternative plans.
Hybrid electric vehicles are driven by an engine and an electrical motor, with the driving state is maintained by appropriately changing a driving mode such as an engine mode, a motor mode, or a hybrid mode corresponding to the driver's need or the driving condition.
The driving mode of the hybrid electric vehicles is classified into a HEV (hybrid emission vehicle) mode and a ZEV (zero emission vehicle). In the HEV mode, the vehicle is driven by an engine, and the exhaust gas is inevitably emitted. In the ZEV mode, the vehicle is driven by a power of the electric motor supplied with an electric power of a battery without operating the engine. Therefore, in the ZEV mode, the exhaust gas is not emitted. As a result, when the hybrid electric vehicle is driven in ZEV mode, it is possible to reduce air pollution due to the exhaust gas as compared with the conventional vehicles.
Since a generator is provided in the hybrid electric vehicle, the battery is charged by electricity generated from the generator when being driven in an engine drive mode or in an inertial drive mode.
The power transmission device of the hybrid electric vehicle is largely classified into a serial type, a parallel type, and a hybrid type. Among these, the hybrid type adopts both an advantage of the serial type that has high output power generation efficiency and an advantage of the parallel type that can simultaneously use an engine and a motor. And thus, the excellence in maximization of the hybrid effect is widely accepted.
In the hybrid power transmission device of the conventional hybrid electric vehicle, two motor generators serve as motors and generators to generate a driving force and serve as transmissions. When the motor generators serve as transmissions, the energy flow to the electrical path increases. Therefore, the conversion efficiency that should be necessarily considered in the conversion process of the kinetic energy into the electrical energy and the inverse conversion process thereof is applied to decrease the efficiency of total power transmission system.
The decrease in the efficiency of the power transmission system is remarkable in the low load and high speed driving area and the high load and low speed driving area. The increase in the electrical transmission path requires the increase in the capacity of the motor generator, which causes a bad fuel economy.