LAUSR

Abstract A solution to increase the use of natural gas in U.S. is to convert existing diesel engines to natural-gas spark-ignition operation through the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. This study presents experimental and numerical simulation results of flame propagation inside a such converted engine. Compared to the conventional theory of flame propagation inside a spark ignition engine, the results showed that the combination of strong radial inward movement and tumble resulted in a strong turbulence inside the bowl compared to the low turbulence intensity inside the squish region. After ignition, the characteristic turbulence distribution partitioned the combustion into two separated events: a fast, thick flame inside the bowl but a slow, thinner, and delayed flame inside the squish region, a feature unique to the lean-burn natural-gas spark-ignition combustion inside a bowl-in-piston geometry. Specifically, the high turbulence inside the bowl increased the local flame speed. By contrast, the squish experienced a much lower turbulence, which, combined with the higher surface/volume ratio, reduced the turbulent flame speed. In addition, the overlap between the fast- and the slow-burn events, which determined the phasing and the amount of fuel inside the squish, was controlled by the combustion phasing. For example, advancing the burning inside the bowl increased the fraction of fuel burning in the late combustion stage, which also increased the phasing difference between the fast- and the slow-burn events and produced a secondary peak in the apparent heat release rate. As the less-favorable combustion conditions inside the squish region would increase carbon monoxide and unburned hydrocarbons emissions, this study suggests that combustion strategies in diesel engines converted to lean NG SI operation should minimize the amount of fuel burning in the squish region.