Reduced soot emissions by piston bowl-shape guided late cycle oxidation in low-swirl heavy-duty diesel engine combustion

摘要

Late cycle soot oxidation in heavy-duty diesel engine low-swirl combustion was investigated using single-cylinder engine and spray-chamber experiments supported by engine combustion computational fluid dynamics (CFD) simulations. The in-cylinder flow situation during interaction between adjacent flames (flame-flame event) was shown to have a large impact on the combustion late in the cycle. In order to modify the flame-flame flow-situation, a new piston bowl shape having wave-shaped protrusions (waves) for guiding of the near-wall flow, was developed. This resulted in a significant reduction of soot emissions and also increased fuel efficiency. The waves were observed to enhance the late cycle mixing which manifested as an increased apparent rate of heat release after the end of injection. Combustion simulations were used to show that the increased mixing was driven by the enhanced intensity of side-vortices leading to the creation of a radial mixing zone. This radial mixing zone extended further into the centre of the piston bowl where unused ambient gas is available, which promotes oxidation. This flame-interaction effect was isolated and further studied using an optical spray-chamber with a two-hole fuel injector nozzle and a wall interaction. Finally, the influence of bowl geometry on the flow field was investigated and coupled to observed late cycle soot oxidation efficiency.