Compression Ratio Control of Free Piston Linear Generator with In-Cylinder Pressure Feedforward

The free piston linear generator (FPLG) is a novel machine that functions as an Auxiliary Power Unit (APU) for hybrid electric vehicles, which contains two opposed free piston engines and one linear generator between them. FPLG has attracted extensive interest for its potential advantages in terms of high power density and multi-fuel flexibility. The guarantee of FPLG generating electricity steadily and efficiently is the high controllability of compression ratio. In this article, a control-oriented discrete-time model was established based on Otto cycle. Since the fluctuation of in-cylinder pressure caused by instable fuel injection mass and combustion process is the main disturbance, a composite controller is designed to precisely control the compression ratio of FPLG. The composite controller is made up of a feedforward controller and a feedback tracking controller. Finally, the control performance of the composite controller and a general proportional-integral (PI) controller was compared by simulation. The simulation results indicate that the composite controller shows better performance in transient process when fuel injection mass or reference compression ratio changes suddenly. Furthermore, the standard deviation of the compression ratio controlled by the composite controller is smaller in the case of the fuel injection mass or the efficiency parameter of Wiebe combustion model obeying normal distribution.