Natural gas–ammonia dual-fuel combustion in spark-ignited engine with various air–fuel ratios and split ratios of ammonia under part load condition

•28% of CO2 is reduced with 50% volume fraction of ammonia in a natural gas engine.•The amount of fuel required increases as increasing ammonia ratio at a certain operation condition.•Fuel NOx formation leads to high NOx emissions when using ammonia.•The initial burn duration dominantly affects the burning speed of natural gas-ammonia fuel. Considering the revised standards pertaining to the regulations on the carbon dioxide emissions of marine engines by international maritime organizations, the use of low- or zero-carbon fuel is necessitated to reduce greenhouse gas emissions from the engine-out mixture. In this study, a natural gas–ammonia dual-fuel spark-ignited engine is experimentally investigated, focusing on its feasibility for marine engine operation. An 11 L 6-cylinder turbocharged spark-ignited engine is used to operate a dual-fuel combustion with various air–fuel ratios and split ratios of ammonia. A relatively low speed (840 rpm) with low-load operation is selected for this study because of its similarity with the operating point of typical marine engines. In addition, simulations are conducted to obtain the laminar flame speed of each point corresponding to the real experimental operating points. It is observed that increasing the air–fuel ratio and split ratio of ammonia retarded the laminar flame speed. By substituting natural gas with ammonia (by more than 50% of the volume fraction), specific carbon dioxide emissions reduced by approximately 28%. It is discovered that the air–fuel ratio is limited to a lambda value of 1.5, resulting in the deterioration of combustion efficiency and emission characteristics. As for nitrogen oxide emissions, the fuel-bound emissions dominated the total emission level of nitrogen oxides because combustion phasing retarded owing to the slow flame speed and large amount of minimum ignition energy when the ammonia fraction increased. The slow laminar flame speed of ammonia substantially affected the initial flame propagation.