Experimental study on combustion and emission characteristics of a pure ammonia engine with spark ignition

Ammonia has the potential to replace traditional hydrocarbon fuels in engines, even being used as the sole fuel. The use of a single-fuel supply system can effectively simplify the system structure, thus reducing costs and enhancing safety. However, previous investigations have primarily been conducted over a limited range of engine conditions with lean-burn limits and emission trends poorly analyzed. This study aims to bridge these gaps by investigating an experiemnt in a heavy-duty engine with a high-compression ratio (17.3). The effects of load, engine speed, and excess air coefficient on the combustion and emission characteristics were analyzed. The operations of 1000 rpm improved indicated thermal efficiency and reduced heat transfer losses by 6–7 % compared to 600 rpm. An appropriate increase in λ to 1.09 further raised indicated thermal efficiency to 41 %. However, high turbulence at 1000 rpm reduced combustion stability and narrowed lean combustion limits. In contrast, the operations of 600 rpm exhibited weaker turbulence and prolonged combustion duration, enhancing combustion stability. The lean combustion limit could reach λ = 1.15 at IMEP = 1.04 MPa. Increasing λ under low-speed conditions further reduced reactivity, raised NH3 emissions and increased unburned losses, consequently lowering indicated thermal efficiency. High in-cylinder pressure and weak turbulence contributed to elevated NH3 emissions. NOx emissions generally increased with λ. Higher combustion temperatures and longer combustion durations facilitated N2O decomposition, thereby reducing N2O emissions. This study demonstrates the broad application prospects of pure ammonia engines in fields such as vehicle power, marine propulsion, and stationary power generation.