Effects of Injection Timing and Duration on Fuel-Spray Collapse and Wall-Wetting in a Stratified Charge SI Engine 2021-01-0544

Fuel-lean combustion using late injection during the compression stroke can result in increased soot emissions due to excessive wall-wetting and locally unfavorable air-fuel mixtures due to spray collapse. Multi-hole injectors, most commonly used, experiencing spray collapse, can worsen both problems. Hence, it is of interest to study the contribution of spray collapse to wall-wetting to understand how it can be avoided. This optical-engine study reveals spray characteristics and the associated wall-wetting for collapsing and non-collapsing sprays, when systematically changing the intake pressure, injection duration and timing. High-speed imaging of Mie-scattered light was used to observe changes in the spray structure, and a refractive index matching (RIM) technique was utilized to detect and quantify the area of fuel-film patterns on bottom of the piston bowl. E30 (gasoline blended with 30% ethanol by volume) was used throughout the experiments. E30 is known to be more susceptible to spray collapse and the high heat of vaporization of ethanol tends to exacerbate fuel-film formation.

These experimental results highlight the impact of in-cylinder ambient conditions on spray morphology and the influence of spray behavior on fuel-films. Analysis of the spray images reveals that spray collapse is a strong function of in-cylinder density and its evolution in spite of the changes in in-cylinder pressure, temperature, and flow at the operating condition used in this study. This explains similarities in the degree of spray collapse and resultant wall-wetting from various injection timings and intake pressures. It is also found that at operating conditions where the spray undergoes transition from non-collapsing to collapsing spray during an injection event, both fuel-film area and variability in fuel-film pattern increased.