Droplet Characteristics in Two Cylinders of a Firing Spark-Ignition Engine

Previous measurements of the velocity, size and number density of droplets have been reported in one cylinder of a production two-valve engine as a function of position, crank angle, injection timing, rotational speed, load and cooling water temperature. In this paper, similar measurements are reported in two cylinders of the same engine, this time with four cylinders firing, and with two manifolds and injectors. 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 air and fuel flow rates and temperatures.

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 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 decreased with manifold depression but the droplet sizes were little affected. These results are in accord with those obtained with one cylinder firing but the number of small droplets was reduced with injection against a closed valve by the higher temperatures associated with four cylinders firing and the pressure waves in the manifolds gave rise to higher velocities, particularly at the low load. The results in the two cylinders were similar when allowance was made for the different orientation of valves. In general terms, the results confirm that single-cylinder engine can provide conditions typical of multi-cylinder engine at least for droplet characteristics.