Operating Range and Emissions From Ammonia-Hydrogen Mixtures in Spark-Ignited Engines

Abstract Ammonia combustion in engines is a promising approach for decarbonization of the energy sector. If made from renewable energy sources, anhydrous “green” ammonia combustion results in zero net carbon dioxide emissions. As a renewable energy storage medium, green ammonia is superior to both compressed hydrogen and batteries for long term energy storage. Combustion of pure ammonia in spark-ignited engines is challenged by low laminar flame speeds and high required ignition energy. Partially converting ammonia to hydrogen and nitrogen through local decomposition is a practical method for enhancing ammonia flame speed and reactivity. This work experientially examines anhydrous ammonia/hydrogen combustion and emissions using a single-cylinder variable compression ratio engine over a range of operating conditions. The study finds that high combustion stability (CoV < 2%) and high efficiency (gITE > 40%) are both possible using only 5% volumetric H2 addition. Experimental results show that rich operation is favorable due to low NOx (< 100ppm) and N2O (∼20 ppm). Roughly 1% unburned NH3 was found in the exhaust, which increased during rich operation. Hydrogen addition decreased NH3 emissions and aided smooth operation while worsening NOx emissions. While ammonia displayed a high knock tolerance like natural gas, heat release analysis indicated that spark-assisted compression ignition combustion was achieved at advanced spark timing and elevated intake temperature. Initial range finding experiments and modeling presented in this work provide a basis for further investigation into the role of rich engine operation and in-cylinder turbulence to enhance ammonia combustion at low hydrogen blending levels.