Fuel Droplets Inside a Firing Spark-Ignition Engine

Experiments have been performed in one cylinder of a production two-valve engine under firing conditions and quantify the velocity, size and number density of droplets as a function of position, crank angle, injection timing, rotational speed, load and cooling water temperature. They were obtained with a phase-Doppler velocimeter with measurements ensembled in relation to an optical shaft encoder. The engine was also instrumented to provide pressure traces, air and fuel flow rates and temperatures. The injection timings included those with open and closed inlet valve.

The results show that most of the droplets emerge in a comparatively small region of the inlet valve and that the characteristics of the spray are important mainly when injection takes place with the inlet valve open. Injection against a closed valve can lead to few droplets when the injection takes place many crank-angle degrees before valve opening and to large droplets when there is insufficient time for evaporation but sufficient for the formation of liquid films on the surfaces of the port/valve assembly. Increase in the speed of the engine led to faster and larger droplets as the time for evaporation was reduced. The number of droplets increased with manifold pressure but the droplet sizes were little affected. The effect of the temperature of the cooling water was small.