The coupling of an internal combustion engine with a starter/alternator is one of the most easily realizable hybrid electric vehicle configurations to achieve significant fuel economy savings in urban driving. A successful implementation of the starter alternator technology includes controlling the electric motor to start and stop the engine quickly and smoothly, without compromising the vehicle noise, vibration and harshness (NVH) signature. The issue becomes more critical in the case of Diesel hybrids, as the peak compression torque is much larger than in automotive spark ignition engines. This paper presents a model-based approach in control design for engine start/stop operations with a belted starter/alternator. Starting from previous modeling and experimental results, a nonlinear model of a belted starter/alternator coupled with a Diesel engine is developed for control algorithm development. With the introduction of a feed-forward control action proportional to the instantaneous engine torque, the starter/alternator controller is capable of consistently reducing the large torque fluctuations during the engine start. With this feedforward control action, the engine start control problem can be translated into a simpler disturbance rejection problem, given a prescribed speed trajectory. This facilitates a linearization of the complex nonlinear model to produce a control-oriented model on which feedback control can be designed. Using the control-oriented model thus developed, different linear control designs have been developed and compared. Further, a robustness study is conducted to evaluate the effect of noise and uncertainties common to such systems. The final results are tested on the original nonlinear truth model, demonstrating the capability of starting and stopping the engine with very limited torque and speed fluctuations.

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