Investigation of ultra-high injection fuel sprays and their effect on in-cylinder mixing

Abstract

Retarded start of injection is a fuel injection strategy that can be implemented in supressing knocking combustion in the spark ignition (SI) engine, a major limiting factor in raising compression ratio to achieve higher thermal efficiency in such engine. Retarded start of injection requires a more rapid homogeneous fuel and air mixture formation within cylinder, which can be facilitated by the fast atomisation of liquid fuel at higher fuel injection pressure. Thus, in this study, systematic studies were carried out on the fuel injection process and in-cylinder mixture formation at ultra-high injection pressures well above those used in production SI engines. Characterisation of fuel sprays from a multi-hole injector was done by means of the high-speed imaging, Phase Doppler Particle Anemometry (PDPA) and in-cylinder mixture formation in an optical engine by the laser induced fluorescence (LIF) technique. High-Speed Imaging revealed that the momentum of the fuel spray leaving the injector and hence its penetration length increase rapidly at ultra-high injection pressures, which can be an issue using the multi-hole injector in a direct injection spark ignition engine because the injector was designed for compression ignition diesel engines. PDPA was used to assess the D10 and D32 droplet sizes and how diameters of the fuel droplets would be changed with pressures ranging from 20 to 100MPa. D32 droplets progressively reduced till roughly 70MPa beyond which the diameter did not reduce any further whereas the D10 continued to reduce with the increased injection pressure. It is likely that the pressure increase is not the sole cause for a decrease in droplet diameters but rather a turbulent effect brought on by the exiting fuels kinetic energy causing the perimeter of the fuel spray boundary to be increasingly interacting with the surrounding air leading to smaller droplet sizes. Normalised standard deviation in fuel distribution was used to quantify the effects of fuel pressure and end of Injection (EOI) on the in-cylinder fuel and air mixing process in an optical engine by LIF testing at fuel quantities equivalent to 4.5, 7 and 9 Bar BMEP at 1200rpm. The LIF results confirmed wall impingement with earlier EOI as was expected with the earlier high-speed spray imaging results although a reduced standard deviation in fuel distribution was found with later EOI. With the current injector design, the occurrence of wall impingement is a major limiting factor and the geometry, number and distribution of the injector holes need to be optimised in order to be able to implement effectively the retarded injection with ultra-high injection pressures.