Experimental investigation of a heavy-duty natural gas engine performance operated at stoichiometric and lean operations

•Performance and emissions differences compared to traditional spark ignition engine.•Equivalence ratio had negligible effect on end of the bulk burn.•Retarding spark timing increased combustion efficiency.•Nitrogen oxides emissions were higher at a leaner equivalence ratio than theory.•Nitrogen oxides emissions suggest the contribution of another formation mechanism. There is a need for decarbonization in power generation and transportation. Natural gas can replace conventional petroleum fuels due to its low carbon-to-hydrogen ratio, especially in existing diesel engines. This paper was based on the hypothesis that “ideal” natural gas engines would operate both stoichiometric and lean, as needed. Therefore, the need to identify the effect of leaning the mixture on several parameters not usually shown in the literature. This paper compared the performance and emissions of a heavy-duty diesel engine converted to natural gas spark ignition operation under stoichiometric (equivalence ratio, ϕ = 1.0) and lean (ϕ = 0.8) operation. The original cylinder head and piston were maintained, and no exhaust gas recirculation was used. The results showed that lean operation decreased peak cylinder pressure and maximum pressure rise rate by 10–15% and 1 bar/°CA, respectively, and increased the volumetric efficiency from ~72% to ~74%. While it also increased the ignition lag, which translated in up to 5°CA delay in the location of peak pressure and crank angle associated with 50% of energy release at the same spark timing and a ~ 5°CA advance in the location of maximum indicated mean effective pressure, it had a negligible effect on the combustion duration due to distinguishing characteristics of gas and flame motion in the bowl-in-piston chamber. Lean operation increased unburned hydrocarbon and nitrogen oxides emissions by up to 15% and 300%, respectively, and carbon monoxide emissions were ~ 20× lower. Lean operation improved indicated thermal efficiency by two percentage points due to a 15% decrease in the heat losses but decreased the exhaust temperature by ~ 50 °C, which would affect the aftertreatment performance. Stoichiometric operation reduced the combustion fluctuations. However, the increased turbulence inside the “fast-burn” bowl-in-piston chamber compensated for the lower natural gas flame speed, with a variation of the indicated mean effective pressure below 2.5% even at ϕ = 0.8. Finally, important differences were observed compared to a traditional spark i