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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.

A new control system for the coasting range was designed with the μ-synthesis technique to achieve robust stability, based on the slip speed control system that was reported in our previous paper.(1) The results of driving tests conducted with the fuel supply cut off while coasting confirm that the new control system is able to avoid engine stall even under sudden hard braking on a low friction road (μ<0.1) at a vehicle speed of 20 km/h and a turbine speed of 1000 rpm. The system also allows the lock-up clutch to slip stably at a certain target slip speed at anytime while coasting and achieves robust performance against characteristic variations of the lock-up mechanism. This slip speed control system thus makes it possible to extend the fuel cut-off range to a lower engine speed of 800 rpm, down from 950 rpm, thereby improving fuel economy by about 1%.

This paper proposes an innovative and accurate method of estimating the internal conditions of rechargeable batteries for vehicles powered by electric motors, such as electric vehicles (EVs) and hybrid electric vehicles (HEVs). The proposed method is necessary to utilize battery power fully on vehicles powered by electric motors (especially HEVs) and thereby improve fuel economy or reduce the battery size. As the first step in this study, the relationship between the current and terminal voltage of a rechargeable lithium-ion battery was described using a linear parameter varying (LPV) model. That made it possible to reduce the problem of estimating the internal battery conditions (internal resistance, time constant, and so on) to a problem of recursively estimating the model parameters with an adaptive digital filter.