A 3-D CFD model of spark-ignition hydraulic free piston engine(HFPE) was built by the simulation software of Converge. The impact of different piston motion laws on HFPE combustion process

knock tendency and thermal power conversion efficiency was investigated to control knock. The simulation results show that the knocking process of spark-ignition HFPE begins at the end of mixture region near piston edge. It's developed by the swift expansion of several knocking cores

and has the same location and mechanism as a crank-connecting-rod type spark-ignition engine does. By reducing piston mass

which can speed up the piston movement in compression stoke and reduce the duration near top dead center

the effect of decreasing knock tendency and intensity is obvious. Taking the advantage of mass-reduced piston movement together with high compression ratio (14)

the engine thermal power conversion efficiency can be promoted to some extent.