Improving Ammonia Combustion in Spark-Ignition Engines Coupling Hydrogen Enrichment with Multiple Spark Plugs

The use of ammonia as a fuel for internal combustion engines has attracted a growing amount of interest in recent years. However, its poor combustion properties have led to extensive research focused on identifying solutions to the challenges posed by ammonia combustion. In particular, several studies have examined the potential of hydrogen enrichment to improve combustion efficiency in spark-ignition engines that use ammonia as a fuel. Recently, the use of multiple spark plugs has also been proposed for the same purpose. The aim of the work is to assess the performance and emissions of a spark-ignition ammonia engine coupling hydrogen enrichment with multiple spark strategies (up to three spark plugs). A 3D-CFD model, coupled with a chemical kinetic approach, already validated based on experimental tests, has been used. Three different configurations have been analyzed: single spark, twin spark, and triple spark. In the analyzed conditions, the gross indicated mean effective pressure (IMEPH), moving from a single to twin spark strategy, increased by 9% and 11% and 6.5%, respectively, for neat ammonia and 5% and 15% of hydrogen by volume in the fuel mixture. The addition of a third spark plug led to negligible improvements of the combustion process considering hydrogen-enriched fuel blends. The multiple spark strategies allow an improvement in terms of gross indicated fuel conversion efficiency, with the maximum increase reached with 5% hydrogen by volume in the fuel blend, equal to about 12%, corresponding to a decrease of the gross indicated specific fuel consumption (GISFC) of about 11%. The emissions of unburned ammonia decreased by about 13%, 8%, and 3%, respectively, for the twin spark and by about 14%, 11%, and 5% for the triple spark, moving from neat ammonia fueling to 15% of hydrogen by volume in the fuel mixture. Considering multiple spark strategies, NOx emissions decreased even if only a slight reduction in terms of NO emissions has been obtained.