The influence of eccentric needle movement on internal flow and injection characteristics of a multi-hole diesel nozzle

Abstract

The internal flow characteristics and injection characteristics of each nozzle hole in a multi-hole diesel nozzle are significantly affected by transient needle movement which studies have shown to be unconventional (eccentric). This eccentric needle movement (unconventional movement of needle within nozzle) is considered as one of the main causes responsible for injection start delay and also variations in fuel injection quantities and spray patterns from symmetric multi-hole nozzles. Needle eccentricity movement creating conditions (causes) and the effect of the eccentric behavior on flow in a symmetric multi-hole diesel nozzle, were computationally studied by examining the in-nozzle geometrical flow area of each nozzle hole during the transient movement of the needle. Based on experimentally determined needle displacement, moving mesh technique was employed in modeling the eccentric needle movement in the computational domain during the simulation. After which the effects of eccentricity within nozzle on in-nozzle hole flow characteristics (cavitation distribution, fuel flow velocity distribution) and injection characteristics (fuel injection rate) were investigated. With a direct effect on flow distribution within nozzle holes, eccentric needle movement has a direct effect on injection and can therefore be considered as one of the main causes of non-homogeneity during fuel injection process. At low needle lift, cavitation distribution, fuel flow velocity and fuel injection rate of each nozzle hole vary considerably and are highly dependent on the needle position. The progressive variations in fuel injection rate non-uniformity from the nozzle are caused to some extent, by the rate of change in in-nozzle geometrical flow area for each nozzle during injection process. The nozzle hole close to the needle displacement shows high level of sensitivity with regards to eccentric needle movement. This nozzle hole therefore exhibits the most variations of injection characteristics under all the cases of eccentricities. Also, with varying degree of needle eccentricity, rate of non-uniform coefficient of cycle fuel injection quantity within the nozzle varies disproportionately.