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The Structure and principle of the hydraulic free piston engine (HFPE) is analyzed, and the mathematical model is established according to the feature of the working principle. The transient processes are analyzed through simulation. The result shows that: 1) the working cycle of the HFPE is unsymmetrical, the compression stroke time is 16ms and the expansion stroke time is 10ms. 2) The speed of the power piston is faster than the conventional diesel engine, the maximum speed in the compression stroke is 12m/s and the maximum speed in the expansion stroke is 17m/s. 3) The acceleration is bigger than the conventional engine, the maximum value is 16500m/s2, at the top dead centre and the smallest value is 3500m/s2 at the bottom dead centre.

In this paper the experiments of hydraulic free piston diesel engine is described. The experimental data were obtained from measurement instruments on the free piston diesel engine that has been developed by Beijing Institute of Technology [ 1 ]. This article discusses the influences of compression pressure, injection timing, and combustion process to the free piston diesel engine principle. The compression process experiment shows that the piston velocity, the compression ratio can be controlled by adjusting the compression pressure. With the increasing of the compression pressure, there is a growing a compression ratio and piston velocity. The study on injection timing shows that the injection timing impacts the cylinder pressure peak value and the pressure peak arrival time. The combustion process is quite different from the crankshaft engine because of the unique piston movement characteristics of the hydraulic free piston engine.