The effect of direct water injection on unsteady fuel spray combustion

'Direct water injection (DWI)' is a method in which water is injected directly into the combustion chamber under high injection pressure from dedicated injectors other than fuel injectors. DWI is supposed to have improving effects not only on NOx emission by decreasing flame temperature but also on PM by promoting combustion inside the diesel engines with a certain timing of water injection, which could be different from those for NOx reduction. In this paper, the potential of the DWI with the later injection timing (abbreviated to 'LDWI') was investigated in detail by using numerical simulations and several experiments. Unlike the earlier DWI usual in NOx reduction, LDWI around the end of fuel injection greatly improved the combustion during the after-burning period. In observation of flame propagation processes in a visual constant-volume combustion chamber, the water spray injected just before the end of fuel injection proved to reduce the soot formation during after-burning stage. This agreed well with prediction results by the spray model calculation. It was also confirmed in visual engine tests with LDWI that the after-burning was much shortened and heat release from residual spray cores was accelerated along with the reduction of soot formation.