Experimental study of wall-impinging diesel spray ignition and combustion characteristics under critical conditions

The ignition stability of diesel engines during cold starting is a key issue limiting the further reduction of compression ratio. To investigate the effect of diesel spray and wall interaction on ignition stability at the critical conditions with a low ambient density and different ambient temperatures, the spray characteristics, ignition process, and flame propagation process were quantitatively analyzed using the Mie-scattering and shadowgraph method, and direct photography method of simultaneous photography based on a constant volume combustion chamber. It was discovered that the spray penetration length spray cone angle, spread radius, and vapor-phase floatation height were insensitive to ambient temperature, but liquid-phase floatation height decreased as the ambient temperature increased. Ignition delay time decreased as the ambient temperature increased, but ignition positions all occurred near the wall surface and far from the axis of the nozzle. However, the distance between ignition position and axis of nozzle, and the distance between ignition position and wall both decreased as the ambient temperature increased. Higher equivalent ratio zones are more susceptible to low temperature reaction than lower equivalent ratio zones. However, the threshold for high temperature reaction of higher equivalent zones was higher than that of lower equivalent ratio zones. Yellow flames were usually accompanied by the production of a blue flame around them, and blue flame propagates faster than yellow flame under critical conditions.