Investigations on the impact of port water injection on soot formation in a DISI engine through CFD simulations and optical methods

With the introduction of innovative solutions aimed at reducing environmental impact and increasing internal combustion engine efficiency, water injection is considered an interesting solution to decrease the risk of knock, reduce charge temperature and employ dilute combustion operation. It allows to use extreme downsizing for engines with higher compression ratio as well as increased margins for optimal spark timing. The present work aims to explore the effects of applying water injection on combustion and emissions in an optically accessible spark ignition engine fueled with commercial gasoline. For this study, the water mass is equal to 30% of the gasoline mass, and is delivered through two injectors placed in the intake manifold. A 3D-CFD study is carried out using the G-equation turbulent combustion model in a RANS framework, properly calibrated against available experimental data. Experimental and numerical results are compared in terms of OH radical concentration obtained by UV emission spectroscopy and by CFD simulations, around the spark plug and in two regions of interest near the intake and exhaust valves. Furthermore, the 3D-CFD model allows to have more in-depth information on the processes inside the combustion chamber: in this context, an investigation on the mechanisms of soot formation was carried out, focusing on the effect of water injection on this phenomenon. The water injection produces a less homogenous distribution of the charge in the combustion chamber to such an extent that the flame struggles to reach the area close to the intake valves, leading to the production of a greater quantity of soot than in the case without water injection.