Demonstration of a single/dual cylinder free-piston engine generator prototype: Milestone achieved on system stability

Free-piston engine generator (FPEG) is a new type of energy conversion system directly coupled with the free piston engines and linear motors, and it is expected to be the next-generation substitution to traditional internal combustion engine because its high efficiency, high volumetric power density and multi-fuel adaptability potential. However, the continuous and stable operation of FPEG is one of the biggest challenges for its large-scale application. In this paper, a complete dual-cylinder type FPEG system test bench was established to experimentally investigate the dynamic and combustion fluctuations. The results of the experiments showed that the dead centre position increased from 16.1 mm to 26.0 mm, and the peak gas pressure increased from 2.4 bar to 7.5 bar when the starting current increased from iq/iqmax = 15% to iq/iqmax = 21%. The low-pressure cold start strategy is feasible, as the experimental results showed that the ignition was achieved at the starting pressure of 4 bar. Compared with the combustion process with linear motor thrust, the operation of the system tends to be more stable, and the coefficient of variation (COV) of the peak in-cylinder gas pressure is 22.78% and 10.12%, respectively. The COV results of the top dead centre (TDC) position, compression ratio, peak in-cylinder pressure and peak in-cylinder pressure position are 0.84%, 3.33%, 2.27% and 0.84%, respectively in the single-cylinder working mode. This means that there is still a large room for the improvement of the stable operation of the FPEG system.