Mixture formation in a partially stratified directly injected natural gas engine

A rapid compression machine was redesigned to allow the use of acetone UV laser diagnostics to investigate the mixture distribution that results from the injection of a methane partially stratified charge (PSC) and direct injection (DI) jet. A central composite test matrix was used to investigate the effect of relative injection timing and bulk charge air-fuel ratio upon the mixture distribution. Comparison was made between the distribution that resulted from a capillary injected PSC charge and a bespoke ‘sparkplug insert’ injected charge. The capillary injected PSC jet was found to preserve a jet-like structure despite its interaction with the direct injection jet, while the effect of the DI jet upon the insert injected fuel was to encourage coalescence of the jets to form a largely homogeneous mixture at the point of injection and near the leading edge of the DI jet. The DI jet, with a weak bulk charge, served to reduce the fluctuations in relative air-fuel ratio compared to PSC injection into air; while the insert injected PSC charge exhibited increased fluctuation levels with advanced relative injection timing. The improved ignition of a partially stratified charge from the introduction of a weak bulk charge had been presumed to work through reductions in fuel concentration gradients, however the findings of the this work suggest that this works in unison with a decrease in fuel concentration fluctuations that increases ignition efficacy. The insert injected PSC fuel demonstrates scalar dissipation rates that are potentially too low to provide robust enough combustion for a viable partially stratified charge approach. The PSC insert engenders more mixing than a capillary injected PSC, but penetrates the DI jet less well. In all cases, and throughout the region of the interaction, there exists a finite probability of encountering pure fuel or the bulk fuel concentration that suggests mixing driven by engulfment rather than entrainment. The PSC ‘sparkplug’ insert offers better opportunity for mixing than the capillary injection and using a stochastic design approach should be pursued further to improve the performance of partially stratified charge combustion for natural gas engines.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat