Synergies between High EGR Operation and GDI Systems

A gasoline direct injection engine was operated at elevated EGR levels over a significant portion of the performance map. The engine was modified to use both cooled and un-cooled EGR in high pressure loop and low pressure loop configurations. The addition of EGR at low and part load was shown to decrease NO and CO emissions and to reduce fuel consumption by up to 4%, primarily through the reduction in pumping losses. At high loads, the addition of EGR resulted in higher fuel consumption benefits of 10-20% as well as the expected NO and CO reductions. The fuel economy benefit at high loads resulted from a decrease in knock tendency and a subsequent improvement in combustion phasing as well as reductions in exhaust temperatures that eliminated the requirement for over-fuelling. The results presented in this paper show that a strategy of cooled EGR at high loads and hot EGR at low loads can enable radical downsizing and high BMEP operation, which can in turn significantly reduce vehicle fuel consumption. A split injection strategy was investigated at low load conditions to determine if a semi-stratified charge would result in an improvement in EGR tolerance. The results show that, within a narrow window of both the amount of fuel injected and the injection timing, the addition of a second injection event late in the compression stroke leads to a reduction in initial burn duration. The improvement in burn duration leads to an increase in combustion stability and a reduction in fuel consumption and emissions.