Optical Studies of Gasoline Sprays and In-cylinder Mixture Formation using a High Pressure Multi-hole Injector

Abstract

An on-going challenge with Gasoline engines is achieving rapid activation of the three-way catalyst during cold starts, in order to minimise pollutant emissions. Retarded combustion can help achieve rapid light-up of the three-way catalyst and can be facilitated by stratified charge using late injection. Injecting late in the compression stroke however, provides the fuel insufficient time for fuel entrainment, resulting in locally fuel rich diffusion combustion. Employing a split injection strategy can help tackle these issues. The effects of a split injection strategy on the spray characteristics and in-cylinder charge formation are investigated in the current study. Varying pulse width (PW) combinations, split ratios and dwell times are investigated, with pressures of up to 35MPa, using a state-of-the-art solenoid actuated high pressure gasoline injector. The experiments were performed in a constant volume spray chamber. The droplet velocities and sizes were measured using Phase Doppler Anemometry. Short and large PWs, in the range of 0.3ms to 1.5ms, were investigated. The results revealed that the highest injected quantity of fuel was measured with the shortest dwell time of 2ms, owing to increased interactions between the injection events, which led to larger drop sizes measured. The drop sizes from the short PW of 0.4ms were generally larger than 0.8ms PW, due to closely spaced opening and closing events of the solenoid valve. The high injection pressure had also resisted the timely closing of the Solenoid valve when short PWs operating in the ballistic zones were used. This led to larger overall duration of injection. The studies on the charge motion using split injections are performed inside an optical Gasoline engine using high-speed particle image velocimetry in the tumble and Omega-tumble planes, at a repetition rate of 10KHz. The engine’s conditions were representative of low-load operations. The results revealed that a small split ratio of 25%-75%, with both injections in the intake stroke, was effective at generating a flow field with high turbulence levels close to the spark plug, when compared to 75%-25% split ratio. The injection coupled with inlet valve opening formed strong tumble charge motion, which was preserved throughout the compression stroke. This provides favourable conditions for fast flame propagation. The fuel injection timings which maximised interactions with the piston surface were detrimental for mixture formation due to heavy surface impingement. The findings from the study helped determine the optimum split injection properties.