Abstract: In order to analyze the combustion performance of a spark ignited free-piston gasoline engine for linear alternator, according to the piston motion characteristics, a coupled multidimensional computational fluid dynamics model is used to simulate the combustion process. The effect of free-piston motion on the combustion process is analyzed in comparison with a conventional crankshaft-driven gasoline engine. The results show that, compared with the conventional engine, the peak in-cylinder pressure and the mean temperature of the free-piston linear alternator are lower, and the period of the peak in-cylinder pressure is shorter, which benefits the reduction of emission of NO exhaust. The heat release rate of the free-piston linear alternator has the characteristics of slow heat release at prophase, gentle heat release at metaphase and long combustion at anaphase. Less gas mixture burns before the piston reaches the top dead centre. The loss work in compression stroke is also less than that of the conventional engine. However, the indicated efficiency of the free-piston linear alternator is lower because of its serious after-combustion period.

Key words: engineering of power and machine, free-piston gasoline engine, combustion characteristics, dynamics feature of piston, ignition schedule