Spray stability of outwards opening pintle injectors for stratified direct injection spark ignition engine operation

Abstract

The spray characteristics and spray stability from three prototype piezoelectric pintle-type injectors were investigated under different operating conditions in an optical direct injection engine designed for stratified combustion. The pintle-type outwards opening injector has the potential to address and overcome many of the typical problems related to close-spacing, spray-guided configurations owing to its hollow cone spray, exhibiting better air utilization than multihole sprays, with good penetration during early injection, and a spray angle almost independent of cylinder backpressure. The three prototype injectors have different nozzle exit geometries for optimization of spray stability under all engine operating conditions. The emerging fuel sprays for both single- and double-injection operation were visualized using Mie scattering and a high-speed CCD camera. The performance of the injectors was assessed by constructing mean and RMS images at different operating conditions of injection pressure, backpressure, injector needle lift, and engine speed. From these images, a spray angle analysis was performed by comparing the mean, standard deviation, maximum, and minimum cone angle under different operating conditions; the spray stability was quantified by analysing the mean and RMS images and the mean and RMS spray cone angles. Evaluation of the three prototypes has revealed that the positive-step inward seal band design produces the most robust spray angle ideally suited for stratified fuel/air mixture formation and combustion in spray-guided direct injection spark ignition (DISI) gasoline engines.