Improving emissions and performance characteristics of lean burn natural gas engines through partial stratification

Abstract

A partially stratified charge (PSC) engine concept has been developed to improve the combustion process in a spark ignition engine fuelled by natural gas, operating at lean air-fuel ratios. In general, this technique is effective at increasing fuel conversion efficiency and brake mean effective pressure at relative air-fuel ratios greater than λ = 1.5. Preliminary testing at 2500r/min showed that PSC reduced combustion duration by almost 10 per cent at a given air-fuel ratio. It was also effective in extending the lean limit of engine operation, which meant that the engine could be run at lower loads without throttling. PSC technology provides an alternative method of load control to throttling, and hence reduces associated pumping losses. Further investigation was conducted at 1500 r/min and a lean air-fuel ratio of λ = 1.65. PSC was found to significantly improve engine performance when compared with homogeneous charge operation at the same air-fuel ratio. The optimized data were compared to simple throttled and lean baseline data and a dramatic reduction in NO_x emissions was observed, with little efficiency penalty. These initial results demonstrate that the lean-burn, partially stratified charge strategy is a viable means of engine control, and has significant performance and emissions benefits.