Reduction of perturbations on spray force measurements for quantification of Diesel sprays

Abstract

Over the last two years the diesel engine has lost the better part of its reputation in the public opinion. In spite of legitimate criticism, it still is an indispensable drive solution for all applications where reliability, flexibility and efficiency are key values. While state of the art exhaust aftertreatment can fulfil the demand for ultra-low emissions ‘under real driving conditions, the legal CO2 target values cannot be reached without a further development of the combustion.

The injection system is one of the key components to an efficient combustion. The better its potential influencing factors are understood, the more possibilities on enhancing future combustion processes are available. Conventional methods of examining injectors are high-speed imagery and hydraulic determination of the injection rate. While hydraulic measurements can only describe the mass flow coming out the whole nozzle, high-speed imagery can detect unequal distribution between the individual holes but only in a qualitative manner.

Thus, spray force or momentum flux measurement is an important link between those two techniques as it provides information related to mass flux on a hole to hole basis which helps to understand the processes within the combustion and helps to identify their leverage. The momentum flux can also be a valuable boundary constraint for computational fluid dynamics (CFD) simulations. The measurement principle consists in determining the force component the spray applies onto a rotationally symmetric transducer in direction of flow.

However, the value in use depends on the accuracy of the obtained data. In order to determine the momentum flux to be correct, the assumption of a rectangular deflection of the spray on the transducer’s surface has to be met. Previous investigations have shown two effects which interfere with this idealistic consideration. One is the formation of a moving eddy on the surface of the transducer, the other is the rebound of spray droplets especially under conditions of low backpressure, which lead to an overestimation of the spray’s momentum.

This paper introduces an alternative geometry for the transducer’s shape which helps to minimize or eliminate the mentioned effects.