The test on the piston motion of a hydraulic free piston engine prototype was carried out. The result shows that the piston motion and compression ratio are self-adaptive to the variation of the combustion phase and aggregated heat release. The piston reversion is advanced and the maximum movement of the piston and the compression ratio are reduced with the advance of the combustion phase or the increase of aggregated heat release. This self-adaptive characteristic can effectively avoid the post-combustion and detonation phenomenon in homogeneous charge compression ignition combustion process

thus ensuring the stability of the maximum cylinder pressure and the maximum heat release rate

and reducing indicated work loss.