Comparison of flash boiling resistance of two injector designs and the consequences on downsized gasoline engine emissions

This paper presents a comparative study of two injectors designed for the same Gasoline Turbocharged Direct Injection engine, one featuring 5 holes and one with 6 holes. Hole diameter and circumferential spacing also differed between the two injectors in order to optimise targeting while maintaining flow rate and drop size distribution. By comparing the macroscopic spray characteristics of the two injectors, this study investigated possible design features which may better maintain a spray's intended morphology under severe flash boiling conditions. The sprays of each injector were firstly investigated by imaging in a quiescent pressure vessel before also being imaged in an endoscopically accessed version of the target 3-cylinder downsized engine to understand the impact of the spray morphology on performance and emissions. Near field images from the pressure vessel indicated that the 5-hole injector could tolerate a greater superheated degree before experiencing spray collapse, maintain its intended morphology better and exhibited a wider plume and shorter penetration length than the 6-hole injector for a given condition. Endoscopic images from the engine indicated that the spray area of the 5-hole injector was always wider under a range of start of injection timings, leading to a better air-fuel mixture and the observation of less diffusive combustion. The PN (particulate) emissions of the 5-hole injector was also consistently lower than the 6-hole injector under different injection timings due to better mixing and less piston impingement, whilst also being less sensitive to changes of injection timing due to its ability to maintain its spray morphology.