Experimental Analysis of the Influence of Water Injection Strategies on DISI Engine Particle Emissions

Increasing the efficiency of modern gasoline engines (with direct injection and spark-ignition - DISI) requires innovative approaches. The reduction of the engine displacement, accompanied by an increase of the mean pressure, is limited by the tendency of increasing combustion anomalies. Conventional methods for knock mitigation, on the contrary, have a negative effect on consumption and efficiency. A promising technology to solve these conflicting objectives is the injection of water. Both the indirect and the direct water injection achieve a significant reduction in the load temperature. The fuel enrichment can be reduced, whereby the operating range of the exhaust aftertreatment can be extended. In addition, water injection paves the way for an increase in the geometric compression ratio, which leads to an efficiency advantage even at partial load. The influence of water injection on the combustion process and raw emissions was analyzed experimentally on a single-cylinder research engine with direct and indirect water injection. Even though water injection initially slows down the combustion process, both injection concepts allow a clear shift in the knock limit and a reduction in the exhaust gas temperature. Since a further tightening of the limit values for gaseous and particulate emissions is to be expected, for the analysis of the raw emissions a mobility spectrometer was used in addition to a conventional exhaust gas measurement. Experimental investigations on particle size distribution in connection with different water injection concepts are not yet known. Water injection strategies are worked out with a variation of the injection parameters. Since the number and size of the particles were influenced by the injection parameters, a strategy recommendation for an engine efficiency improvement using water injection was developed.