An optimization of free-piston engine generator combustion using variable piston motion

The free-piston engine generator has been a research focus lately due to its crankless structure and unconstrained piston motion, which brings about some weaknesses for the combustion process. This study aims to explore the optimization possibility of combustion using its free-piston motion. Three potential piston motion trajectories were obtained by the free-piston engine generator dynamic model and then served as the boundary condition to establish the combustion model. Through the simulations and comparisons of combustion processes in different piston motions, a manner to optimize the free-piston engine generator combustion performance, that is, by adjusting the piston motion trajectory, was proposed and verified effectively. The results show that under the situation of the slowest velocity and smallest acceleration around the top dead center, the piston moves with a smallest change in in-cylinder gas volume but greatest heat release around the top dead center, during which the free-piston engine generator gains the greatest peak values of in-cylinder pressure, temperature, and the rate of heat release and pressure rise in the three, naturally leading to the highest NO emission and soot emission; the late combustion duration and heat released in the late combustion are smaller than the other two cases. But the fastest piston motion around the top dead center shortens the duration of combustion and heat release and brings about greater soot emission than the case of conventional sinusoidal trajectory.