Hydraulic free piston internal combustion engine. Experimental investigation on the fundamental test apparatus with opposed piston.

The test results on the dynamic behavior of the free piston displacements, the hydraulic pressures and the gas pressure on the first cycle at the start of the engine at room temperature are reported in this paper. It becomes clear that the entire cycle consists of four processes, that is, the compression, expansion, bounce and damped oscillation processes. Dynamics of the free piston in these four processes are discussed in this paper. Furthermore, the value of the polytropic index on the compression process is obtained experimentally. The energy losses in the compression process are also discussed in this paper. It becomes clear that the main sources of energy losses are piston friction and pressure loss in the pilot-operated check valve. Finally, the firing operation is repeated every 0.5 or 1.0 second, and the free pistons stop after each operation.