An experimental research on the combustion and heat release characteristics of a free-piston diesel engine generator

Free-piston engine generator (FPEG) is an unconventional engine; which is characterized by freely moving pistons without crankshaft system. This paper focused on an experimental analysis for the combustion and heat release characteristics of a free-piston diesel engine generator. An experimental prototype was established to test the operation performances of the new engine under different injection conditions. Experimental results show that the actual compression ratio of the FPEG is influenced by the injection advance position (IAP), which decreases with injection position advancing; but the peak in-cylinder gas pressure varies in positive correlation with the IAP. Moreover, the heat release characteristics of the unconventional engine were also analyzed using the experimental data. The results indicate that at the optimum injection position, the ignition delay of the FPEG prototype is 0.86 ms and generally shorter than a corresponding conventional crank engine due to its faster piston motion around the top dead center; and the heat release process has more significant pre-mixed combustion, which differs from the conventional engine situation. The heat release center of the FPEG at all injection position lags a stable time after injection; and the departure time of heat release center (DTHC) is increased significantly with the retarded injection. Meanwhile, a particular feature was found that the optimum efficiency of the FPEG does not occur in the lowest DTHC condition due to its variable compression ratio.