Experimental Study On High Temperature/supercritical Spray Characteristics And Droplet Velocity And Particle Size Of A Direct Injection Gasoline Engine

Owing to spray atomization and other reasons,the particle emissions from Gasoline direct injection(GDI)engine is high.And fuel injected into the cylinder in high temperature or supercritical state has the potential to further improve the thermal efficiency of gasoline engine and to reduce the particle emissions.This paper mainly studies the characteristics of single-hole and multi-hole high temperature and supercritical spray.Gasoline and n-heptane spray experiments were carried out on supercritical spray device.By using high speed camera and PDA,spray images and the gasoline spray droplet velocity and particle size were obtained.As the fuel temperature rising,the spray-break mode gradually transitions from the primary and secondary crushing(25 ° C to 88 ° C)to the outlet swelled flash boiling spray(88 ° C to 261 ° C)and finally to the supercritical injection state.The penetration of gasoline single-hole spray decreases at first.Once the fuel temperature reaches 261 ℃ and keeps rising,the penetration would start to increase.While the cone angle enlarges first,and then reduces.The spray collapses and converges when gasoline multi-hole spray reaches the flash boiling state.The penetration of gasoline multi-hole spray decreases first with the increase of the fuel temperature.After 173 ℃,the converging occurs,and the penetration increases.Before the fuel temperature is 261 ℃,the penetration of multi-hole spray decreases first and then increases and then decreases with the increase of fuel temperature,while the single hole spray penetration always decreases.After 261 ℃,the fuel reaches the supercritical state,and the penetration of the single hole and the multi-hole spray increase with the increase of the fuel temperature.PDA measurement method doesn’t apply to gasoline in supercritical state.As the temperature rises,the spray droplet velocity increases first,and then significantly decreases.While the droplet size of spray droplet keeps decreasing.There’s a highly chance that large droplets(d> 70μm)will be formed when the temperature is below 88 ° C.But at higher temperature,it’s hard for them to generate and thus barely exist.With the fuel temperature rising,the n-heptane multi-hole spray undergoes three forms: ’maple leaf’,’umbrella’ and ’spindle’.The collapsing convergence of the multi-hole spray leads to ’spindle-shaped’,which is related to the ambient pressure,ambient temperature,fuel temperature and fuel injector structure.Fuel temperature and ambient temperature will contribute to the collapsing convergence of the spray,and the ambient pressure will inhibit it.In the same spray pattern,the penetration decreases with the ambient temperature rising.Under all spray patterns,the penetration decreases with the environmental pressure increasing.For ’Umbrella’ or ’maple leaf’ spray,the penetration decreases as the fuel temperature rising until it reaches the supercritical injection state.For ’Spindle’ spray,the penetration increases with the fuel temperature rising,but for several conditions of higher fuel temperature,there will be a slight reduction for penetration.When the fuel temperature rises,the penetration of n-heptane will significantly increase after reaching the supercritical injection state.