Investigation of the optimum operating condition of a dual piston type free piston engine generator during engine cold start-up process

The free-piston engine generator (FPEG) is a new type of energy conversion device that converts chemical energy of fossil fuel into electric energy and eliminates the limitation of crankshaft mechanism, and can be applied to range extenders in electric vehicles. This article investigated the optimal compression ratio of FPEG under successful ignition condition so as to explore the range of working conditions of FPEG engine during cold start. A numerical model was established in MATLAB/Simulink, and the simulation results were compared with the test data of stably running prototype. The difference between the simulated in-cylinder gas pressure and the experimental data could be controlled within 5%, indicating the accuracy of the model. The simulation results show that the target compression ratio is the most important parameter during the cold start of engine, and the operating frequency has the least influence on the system performance during cold start. During cold start of FPEF, it is recommended to control the excess air ratio between 0.8 and 0.9, the compression ratio range between 4 and 7, and the operating frequency of the system between 10 and 16 Hz.