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A hybrid system that has been put on the market by Nissan Motor Company was configured by removing the torque convertor with a lockup clutch from a conventional 7-speed automatic transmission and installing a clutch and a motor in its place. This hybrid electric vehicle (HEV) has a simple structure and is expected to improve fuel economy and responsiveness because it eliminates the torque convertor. One issue for this system is that an abrupt change in the input torque could cause torsional vibration of the drive shaft, resulting in a severe degradation of ride comfort. To solve this problem, an original vibration control system that was adapted for the mass-produced LEAF electric vehicle was also adapted for use on this HEV fitted with an engine and a 7-speed automatic transmission. This control system enables the hybrid vehicle to generate maximum motor torque at launch and also provides significant advantages for vehicle design.

The one-motor, two-clutch hybrid system adopted by Nissan requires high control performance for the entire system, because the single motor provides the various functions of a drive motor, generator and engine starter depending on the driving situation. An especially challenging problem is to prevent the motor torque variation caused by engine start while transmitting driving torque to the drive wheels in a situation where the engine is started in response to the driver's demand for acceleration during motor-only operation. In this case, it is effective to let the clutch on the driving side slip until engine start is completed. For that purpose, we designed the slip control system described here. This system controls driving torque by the clutch while controlling the slip speed by means of the motor speed control. The slip speed can be controlled with high accuracy because the motor speed is controlled by the motor torque which is easily controllable.