Enhancing the NOx and soot emissions reduction benefits of a low compression ratio light duty diesel engine by optimization of piston bowl design

Abstract

In the present work, the benefits of the low compression ratio (LCR) concept on simultaneous reduction of oxides of nitrogen (NOx) and soot emissions could be further enhanced by optimizing the piston bowl design using a 3-dimensional computational fluid dynamics (CFD) tool. A single-cylinder, direct-injection diesel engine was used for the present investigations. In its mass-production version, the engine had a compression ratio (CR) of 18:1 that was considered as the stock CR from which the LCR variants were derived. The initial and unoptimized piston design of the LCR variant with CR of 14:1 was arrived at using an offset bowl geometry from the stock compression ratio variant, and the corresponding benefits were quantified. The NO (nitric oxide) and soot reduction potential of the LCR variant could be further enhanced by adopting a step-by-step optimization procedure focused on the principal parameters of the bowl viz., piston bowl diameter, centre pip depth, reentrancy and bottom profile. Based on numerical investigations, an optimal piston bowl design could be arrived at that can significantly enhance the benefits of the LCR approach on NO and soot emissions. At a reference operating point of 2000 rpm and 40 N-m, by adopting the optimized profile, the NO reduction potential of the LCR variant could be improved from 10.3% to 40.2%. Moreover, the soot reduction potential could also be improved from 79.1% to 84.2%. The benefits of the optimized bowl design were also confirmed by engine dynamometer measurements across the engine’s operating speed range. Thus, it can be concluded that optimization of the piston bowl design specific to the LCR variant could enhance the NOx and soot emission benefits that can help diesel engines comply with the stringent emission regulations.