Abstract
Decarbonising the legacy fleet of internal combustion engines requires practical, cost-effective retrofit strategies. This study investigated utilising ammonia as a supplementary fuel in a 1.6-litre port-fuel injection spark-ignition engine, representative of the vehicle population in Thailand. The engine operated on Gasohol 91 with ammonia energy shares of 10%–30% at 2,500 rpm under varying loads (25%–75%). The investigation quantified trade-offs between performance, combustion stability, and exhaust emissions. Experimental results indicated a clear thermodynamic trade-off: while pure Gasohol 91 demonstrated superior brake thermal efficiency (BTE) due to its higher energy density, the efficiency penalty from ammonia significantly narrowed at high loads (75%) as elevated temperatures overcame ammonia's low reactivity. Environmentally, the 30% ammonia blend achieved the lowest levels of Carbon Monoxide (CO) and Unburned Hydrocarbons (HC). However, this benefit was counterbalanced by a surge in Nitrogen Oxides (NOx), primarily driven by the fuel-NOx mechanism. Furthermore, statistical analysis revealed a strong inverse correlation, where a 1% decrease in thermal efficiency corresponded to an approximate 3.16 ppm increase in NOx. The study concludes that ammonia fumigation is a viable decarbonisation strategy for legacy engines, particularly at medium-to-high loads, provided that the efficiency-NOx trade-off is managed through optimised operating strategies or after-treatment systems. • Separate ammonia injection (10–30% energy share) tested in a 1.6 L SI engine • Case study evaluates performance, combustion, and emissions under practical loads • Ammonia lowered CO and HC emissions, confirming the benefits of carbon-free fuel • Combustion slowed at low load but improved at medium and high engine loads • NOx increased with ammonia share, highlighting the need for control strategies