Influence of water injection on combustion identified through spectroscopy in an optical direct injection spark ignition engine

Within the context of increasing boost pressure and application of alternative methods for controlling knock, the present study scrutinized the effects of water injection on combustion characteristics and emissions. Conditions of wide open throttle and fixed spark timing were investigated in an optically accessible unit, that featured a UV-enhanced window-mirror combination fitted in an elongated piston configuration. Commercial gasoline was used in the direct injection fuel system, while water was delivered to the intake manifold. Engine performance was found to feature a slightly decreasing trend when increasing the injected water quantity in three steps, at 10, 20 and 30% of the mass flow of gasoline. Nonetheless, the maximum drop in indicated mean effective pressure was below 2%, even though the spark timing was kept constant. Changes in combustion stability were negligible. Thermodynamic analysis was coupled with cycle-resolved flame visualization and spectroscopy. The latter technique allowed the radiative emission related to sodium to be directly associated with the injected water quantity, thus allowing further detailed evaluations correlated with evaporation effects. Flame front characteristics confirmed minor effects on combustion evolution, with a slight increase of wrinkling when water flow was augmented. Finally, quasi-dimensional modelling allowed evaporated water quantities to be estimated at the time of ignition and during combustion.