The addition of a spark plug in place of the original fuel injector and fumigating natural gas (NG) inside the intake manifold is an economical way to convert heavy-duty diesel engines to NG spark-ignition (SI) operation. The literature shows that, when compared to a conventional SI engine combustion chamber, the different in-cylinder flow motion, turbulence intensity distribution, and interaction of propagating flame with chamber boundaries in these converted engines produce distinctive combustion stages. As the current understanding of how these combustion stages affect the engine performance is limited, this study used a triple-Wiebe combustion model to determine the effect of spark timing (ST) on the phasing and mass fraction of each combustion stage, at lean operation (ϕ = 0.73) and low engine speed (N = 900 rpm). Specifically, the first Wiebe function was associated with the fast burn inside the piston bowl, the second Wiebe function was related to the slower burning process inside the squish region, and the third Wiebe function described the late oxidation of the fuel trapped inside the various crevices. The results show that advancing the ST advanced the combustion phasing of the bowl and squish burn stages, but not the end of the squish burn. Further, there was an overlap between the bowl burn and squish burn, irrespective of the ST, but advancing the ST reduced the overlap and increased the fuel fraction consumed inside the squish. As a larger fraction of fuel burning inside the squish under less optimal conditions would affect the engine efficiency and emissions, a combustion model, such as the one presented in this work, can provide key information for heavy-duty NG engine development and optimization if the phasing and the fraction of the total energy released during each combustion stage are known.
