Assessing Hydrogen–Ammonia Ratios to Achieve Rapid Kernel Inception in Spark-Ignition Engines

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Abstract In the quest for decarbonizing internal combustion engines, ammonia (NH3) is recognized as a viable alternative fuel due to its zero-carbon emission profile, positioning it as a potential substitute for conventional petroleum fuels. However, the suboptimal combustion characteristics of ammonia pose challenges for its direct application in engines. The introduction of hydrogen (H2) as a combustion enhancer shows promise in improving ammonia viability for engine use. While previous studies have confirmed the benefits of hydrogen addition to ammonia for enhanced engine performance, comprehensive analysis of the precise ammonia-to-hydrogen ratio for optimal efficacy remains scarce. This research aims to bridge this gap by evaluating hydrogen–ammonia mixtures for achieving methane-equivalent laminar flame speeds under typical engine conditions, with a focus on the kernel inception process primarily driven by laminar flames. The findings indicate that a minimum of 20% hydrogen mixed with ammonia is necessary to facilitate rapid spark inception, although it does not reach the laminar flame speed of methane. Additionally, employing a high compression ratio and operating near stoichiometry could lower the required hydrogen–ammonia ratio. Considering the challenges in generating ample hydrogen with NH3 dissociators and the need for operational conditions like full-load and low-speed to lessen hydrogen demand, ammonia–hydrogen fuel blends are deemed most suitable for stationary engine applications in the near term.