Effects of altitude on combustion and emission characteristics of an opposed rotary piston engine fueled with hydrogen-ammonia blends

The opposed rotary piston (ORP) engine has the advantages of compact size and high power-density without connecting rod mechanism, which makes it an ideal power source for long-endurance and heavy-duty unmanned aerial vehicles (UAVs). Ammonia-hydrogen blends serve as carbon–neutral energy carriers with significant potential for decarbonizing ORP engines. This study systematically examines the effects of altitude on combustion and emissions of an ammonia-hydrogen ORP engine. The results demonstrate that elevated altitude conditions lead to a systematic reduction in both peak in-cylinder pressure and heat release. As a result, the indicated thermal efficiency decreases from 36.63 % to 34.45 % when the altitude increases from 0 m to 5000 m, and the emission of NO decreased from 0.763 mg to 0.374 mg. However, N₂O emission increases with the elevation. This study further investigates the optimization of combustion characteristics in high-altitude (5000 m) through systematic adjustment of the ignition timing parameters, the results demonstrated that advancing the ignition angle can reduce residual hydrogen and ammonia concentrations in the cylinder, consequently elevating both peak cylinder pressure and heat release rate during the combustion process. A shorter combustion duration is observed after enlarging ignition advancing angle, indicating enhanced combustion velocity under modified ignition timing conditions. Consequently, the indicated thermal efficiency increased from 30.97 % to 37.23 % with the ignition advance angle advancing from 12° to 18°. Furthermore, the optimized ignition timing strategy exhibits an additional benefit of reducing NO_x emissions.