An experimental study on a hydraulic free piston engine（HFPE） was made

focusing on homogeneous charge compression ignition（HCCI） process. The effects of mixture equivalent ratio and compression ratio on reaction starting points at low and high temperatures

two-stage heat release ratio and indicated efficiency were analyzed. The result shows that when the compression ratio is constant

the lower the mixture equivalent ratio is

the earlier the reaction starting point is at low temperature

the later the reaction starting point is at high temperature

and the higher the proportion is of reaction heat release at low temperature to that at high temperature； when the mixture equivalent ratio is constant

with the increase of the compression ratio

the reaction starting points at low and high temperatures are both advanced

and the ratio of reaction heat release at low temperature to that at high temperature is lower； when the mixture equivalence ratio increases from 0.30 to 0.40

the maximum indicated thermal efficiency is slightly reduced

and the difference between the maximum and the minimum is 3%； when the mixture equivalence ratio is constant

there is a best compression ratio for the maximum efficiency

and a compression ratio that is too low or too high is not conducive to improving the efficiency of thermal power transformation.