Effects of increased injection pressures of up to 1000 bar – opportunities in stratified operation in a direct-injection spark-ignition engine

The gasoline direct-injection engine with spray-guided combustion is one of the most promising strategies to reduce fuel consumption and CO2 emissions of spark-ignition engines. This benefit results primarily from lean and unthrottled operation, which is realized with a highly stratified mixture at part load. At the upper load limit of stratified charge operation, charge stratification is insufficient to realize substantial fuel economy benefits, especially when using multihole injectors. This can be attributed to a lower injector flowrate than is available from outward-opening piezo injectors. One measure to increase the flowrate is to increase the injection pressure. A higher stratification gradient thereby can be achieved, which leads to combustion at richer air–fuel ratios. As a result, combustion duration and hydrocarbon emissions decrease. The enhanced evaporation due to the increased injection pressure reduces soot emissions. This paper presents the results of thermodynamic and optical investigations at the upper load limit of stratified charge operation in a spray-guided direct-injection engine. To this end, variations of the injection pressure from 200 to 1000 bar are performed. The associated effects on mixture preparation and soot formation are investigated. The mixture preparation process and flame propagation information are recorded using a high-speed intensified complementary metal oxide semiconductor (CMOS) camera. In order to investigate soot formation and oxidation behaviour, soot concentrations are measured using the extended two-colour method.