Combustion and emission characteristics of a gasoline/ammonia fueled SI engine and chemical kinetic analysis of NOx emissions

Mitigating climate change involves the greater adoption of carbon–neutral and renewable energy sources within the transportation sector. Ammonia (NH3), as a carbon-free and sustainable fuel, has garnered growing interest in recent years. The present study aims to investigate the impact of NH3 blending on combustion and emission characteristics of a stoichiometric spark-ignition gasoline engine, with a particular emphasis on nitrogen-based emissions. The experimental investigation was complemented by chemical kinetic calculations. The results showed that NH3 blending could effectively suppress engine knock, optimize combustion phase and improve thermal efficiency. For pure gasoline, advancing the spark timing resulted in increased NOx emissions. However, when NH3 was blended, NOx emissions decreased with advancing spark timing, indicating a negative correlation with pressure. The NH3 emission was attributed to the ‘crevice mechanism’ as well as the absorption/desorption in the lubricant oil film on the cylinder wall. Chemical kinetic analysis revealed that the NOx emission from NH3 blended combustion is closely related to reactive radicals such as OH, H and O. The reduction in NOx emissions under high-pressure conditions was primarily attributed to the consumption of these reactive radicals via three-body reactions. Interestingly, NOx emissions initially increased with increasing NH3 blending ratio but eventually followed a decreasing trend. This can be attributed to the lower combustion temperature, lower concentration of reactive radicals, and enhanced de-NOx reactions.