Experimental and numerical study on the performances of a free-piston engine generator using ammonia and methane fuel mixtures

In this paper, experimental and numerical studies of the methane-ammonia dual-fuel glow-ignition free-piston engine generator (FPEG) are carried out, with emphasis on its operation stability, performance and combustion characteristics. The ammonia in FPEG operates at different energy substitution rates (ESRs), including 100 % CH4, 90 %CH4+10 %NH3, 80 %CH4+20 %NH3, and 70 %CH4+30 %NH3. The experimental results show that the FPEG can achieve stable operation in the methane/ammonia dual-fuel combustion mode. However, the opera-tion stability of the FPEG becomes worse with the increase of the ESR. Especially when ESR = 30 %, engine shutdown accident will occur due to the high cycle-to-cycle variation caused by ignition delay. The peak pressure and heat release rate (HRR) decrease significantly because the additive ammonia weakens the combustion ac-tivity and reduces the combustion rate of mixture. In the case of pure methane, the indicated power and indi-cated thermal efficiency reach 319 W and 11.33 %, respectively. However, when ESR = 10 %, 20 % and 30 %, the corresponding values decrease significantly to 210 W, 7.48 %, 189 W, 6.72 % and 139 W, 4.95 %, respec-tively. The high-speed flame images show that with the increase of ESR, the flame structure becomes less wrinkled, because the addition of ammonia reduces the flame speed and weakens the combustion reaction, leading to the weakening of turbulent combustion intensity. In addition, the ammonia addition will also lead to post combustion phenomenon. The simulation results show that the start of combustion (SOC) is delayed and the crank angle (CA) 0-10 and CA10-50 are prolonged with the increase of ESR, because the ammonia addition reduces the amount of key radical, such as OH and H, which reduces the mixture reaction rate. The NOx emission decreases with the increase of ESR due to the low combustion temperature. Soot emissions from the pure methane remain extremely low, and the CO emissions gradually increase with the increase of ESR due to the incomplete combustion of mixture. In addition, the amount of unburned ammonia increases with the increase of ESR.