Waste Energy Recovery Through Turbo Generation: “Unexpected Fuel Efficiency Sweet Spot for Transient Control.”

Abstract

A non-intuitive aspect of the transient management of a waste energy recovery (WER) system on a small turbocharged gasoline engine is presented in this work. The WER system is realized with a positive displacement expander and an electric motor, named exhaust turbo generating (ETG) system. The steady state simulations indicate that for a range of medium loads it can provide up to 5.5% reduction in the fuel consumption. For this, the ETG system set point and the engine spark timing, which is affected by the extra imposed backpressure, are optimized offline. The decentralized control design presented in this work warrants effective actuator coordination including five controlling actuators on the engine level and two for the ETG system. A sliding mode controller is designed for controlling the ETG speed with different closed loop bandwidths. The results indicate that the optimum control strategy is not the one with the fastest closed loop ETG response. Specifically, one would expect that the fastest transients would be better in maintaining the benefits that were projected in steady state GT-Power simulations. With this work, it is clarified for the first time that the ETG control design tuned to optimize the transitions to optimal rotational speed is as important as the actual selection of these steady state optimal set points, which has been studied extensively in the past.