The combination of gasoline direct injection and turbocharging is a promising method to reduce the fuel consumption of internal combustion engines through engine downsizing, which leads to increased engine efficiencies and a reduction of CO2 emissions at a comparable power output. Spray-guided direct injection allows overall lean and unthrottled operation, which is realized with a highly stratified mixture at part load. However, exhaust gas aftertreatment with conventional three-way catalysts is currently not possible. Furthermore, insufficient mixture preparation, especially at the upper load limit of stratified charge operation, causes increased particulate matter emissions. This paper discusses the advantages of engine downsizing, by gasoline direct injection in combination with turbocharging, to reduce fuel consumption and presents the results of experimental and numerical investigations of stratified exhaust gas recirculation in a single-cylinder gasoline engine to reduce nitrogen oxide emissions. The radial exhaust gas stratification was achieved by a spatial and temporal separated induction of exhaust gas and fresh air, performed by specially shaped baffles and impulse charge valves in the inlet port. The thermodynamic and optical investigations with injection pressures up to 1000 bar demonstrate the capability to reduce soot emissions in a spray-guided direct-injection engine.

The gasoline direct-injection engine with spray-guided combustion is one of the most promising strategies to reduce fuel consumption and CO 2 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.

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