Effect of ignition timing on engine performance of a linear range extender: An experimental study fueled with methanol

The Free Piston Engine Generator (FPEG), as a linear range extender, offers advantages such as adjustable compression ratios and multi-fuel adaptability. The application and promotion of methanol fuel and lean combustion technology contribute to achieving low-carbon and high-efficiency power systems. This study achieved stable operation of methanol fuel in FPEG under lean-burn conditions. Through experimental research, the effects of ignition timing under different load conditions on the combustion process, performance, and cyclic variation of FPEG were explored. The results show that adjusting ignition timing effectively regulates the peak magnitude and phase of in-cylinder pressure and heat release rate during methanol lean combustion in FPEG. When the ignition timing is set to ig24mm, thermal efficiency, power output, and IMEP reach optimal levels. Compared with traditional internal combustion engines and FPEG under stoichiometric conditions, the optimal ignition timing for methanol FPEG under lean-burn conditions is more advanced. Increasing intake flow rate accelerates combustion speed and enhances engine power but reduces thermal efficiency. When the intake flow rate increases from 250 L/min to 350 L/min, Indicated power increases by 20.8 %, while thermal efficiency decreases by 11.6 %. Both advancing ignition timing and increasing intake flow rate enhance combustion stability, thereby reducing cyclic combustion variation. When the ignition timing is 24 mm and the intake flow rate is 350 L/min, the corresponding COV-IMEP is 2.73 %, while it is 5.73 % at an intake flow rate of 250 L/min.