Feasibility of Ammonia as a Spark-Ignition Engine Fuel: Strategies for Combustion Stability and Engine Efficiency

Ammonia (NH3) is a promising energy carrier and a potentially alternative fuel to selected sectors due to its carbon-free nature and its relatively high energy density. However, its low reactivity and slow flame propagation pose significant challenges for a direct use in an internal combustion engine, and stable operation at all engine’s conditions. This study investigates three combustion strategies for utilizing NH3 in an adapted four-cylinder 2L turbocharged, compression-ignition engine, adapted for spark-ignition (SI) operation. Initially, the engine was tested using pure ammonia as fuel, obtaining high efficiencies and acceptable stability at medium/high loads. Nevertheless, intense combustion instabilities could not be avoided below a minimum load level (which increases with engine speed), making engine operation unfeasible in approximately 30% of its operating map. To address these limitations, two enhancement strategies are explored: Firstly, hydrogen (H2) doping pre-mixed with NH3 to enhance the fuel properties and secondly the implementation of a passive Turbulent Jet Ignition (TJI) system, with a target to accelerate flame propagation. H2 on-board supply and storage is not part of this study and H2 could be generated through a downsized NH3 reformer of the types being currently developed for maritime and power generator applications. Experimental results indicate that the addition of small amounts of H2 (2 to 4% in mass) significantly improves both combustion stability and thermal efficiency due to increased flame speed. These findings highlight the feasibility of NH3 as a fuel for spark-ignited engines, provided that suitable combustion enhancement techniques are implemented.