Transient Measurements of Discharge Coefficients of Diesel Nozzles

The discharge coefficient is an important functional parameter of an injector characterising the nozzle flow, in terms of cavitation and hydraulic flip, which subsequently play a crucial role in the spray formation and development. Thus it is important to have the possibility of measuring instantaneously the value of the discharge coefficient. The method proposed is based on the measurement of force developed during the impingement of the fuel jet on a normal target.

In this study the method was verified experimentally and also the variation of a diesel nozzle discharge coefficient over the entire injection time was studied. The impingement results were in good agreement, when compared with the results from mass flow measurements both at high and low injection pressures.

Strong variations of the discharge coefficient during the injector needle opening and closing periods were seen. During the main injection period when the needle was fully lifted, the discharge coefficient variations were modest and the average value of the discharge coefficient was about 0.75. The above value when compared with low pressure injection for which the discharge coefficient was about unity characterise the effects of cavitation which are thus shown to be substantial.