Abstract: To achieve a high efficiency and reliable compression process and satisfy the requirement for compression ignition of a single piston hydraulic free-piston engine, characteristics of the compression stroke are analyzed. The compression process is divided into different categories based on the system configuration. The main characteristics of the categories are presented. The prime control parameters in the compression process realized by hydraulic power are confirmed. A compression model describing the gas cylinder and hydraulic chamber states under different operation conditions is given. The characteristics of the compression stroke under different operation conditions are investigated by combining with simulation and test results. The results indicate that the requirement for compression ignition can be satisfied easily in the low pressure compression process. The pressure stabilizing effect of the compression accumulator should be eliminated and the effect of throttling action of the frequency control valve should be limited in the high pressure compression process in non-normal working state.

Key words: Compression ignition, Diesel engines, Free-piston engines, Hydraulic accumulators