An Evaluation of the Conversion of Gasoline and Natural Gas Spark Ignition Engines to Ammonia/Hydrogen Operation From the Perspective of Laminar Flame Speed

Power generation systems will reduce carbon emissions primarily through the application of low or even zero carbon fuels under the global decarbonization trend. Ammonia is an ideal alternative fuel because it is cheap, readily available, and easy to store and transport. However, its mediocre combustion performance has raised concerns about its use in engines. The objective of this paper was to evaluate the amount of hydrogen that would need to be added to the ammonia from a laminar flame speed perspective if converting existing spark ignition engines to ammonia operation. The benchmark for determining the hydrogen blending ratio was to help ammonia achieve efficient combustion in the cylinder comparable to that of gasoline or natural gas. The results showed that hydrogen addition had the potential to greatly improve engine efficiency and emissions, although the combustion kinetics of ammonia-hydrogen mixtures were still dominated by ammonia with hydrogen addition levels below 60%. In addition, the hydrogen addition ratio was mainly determined by the kernel inception process, as this burning stage heavily influenced the repeatability of the combustion and the ease of combustion control. Also, at least 20% of hydrogen was required to be added to ammonia to adapt the engine to various operating conditions, while such a strategy still cannot help ammonia to obtain a rapid burning event compatible with gasoline or methane. Moreover, natural gas engines were more suitable for retrofitting to ammonia-hydrogen operation because they have a higher compression ratio and their combustion chambers are less demanding on the fuel laminar flame speed. Further, ammonia lean operation was recommended to be avoided in spark ignition configurations. Altogether, all of these findings support the need for additional efforts in ammonia engine optimizations and onboard ammonia dissociation system efficiency improvements.