Abstract: A dual-piston hydraulic free piston engine (DHFPE) is a compact unit integrated by IC engines and a hydraulic pump. As a special IC engine, which realizes non-stiffness power transmission utilizing hydraulic pressure energy, it differs dramatically from conventional IC engines. To deeply explore its working mechanism, a special structure DHFPE prototype is presented and developed. Based on the studied DHFPE prototype, three possible typical mount types are especially discussed and the corresponding compression ratio change range is induced. Additionally，the relationship between its compression ratio and design parameters, mount type, working variables is concluded. Besides above, for a certain DHFPE prototype, the dependence between its compression ratio and the compression energy is achieved and the approaches to con-trol the real compression ratio are presented. The results are helpful for DHFPE prototype development from the structure design and compression ratio control as well as selecting of mounts type.

Key words: Structure design, Compression ratio, Dual piston hydraulic free piston engine, Frequency domain, Metro, Rail wear rate, Short-pitch rail corrugation, Track receptance