Liquid fuel inflow into the cylinder is considered to be an important source of exhaust hydrocarbon (HC) emissions from automotive spark ignition engines. These liquid-fuel-caused emissions are increased significantly during the start up and subsequent warm-up period. This study analyzes the influence of several engine and injector design variables, and fuel parameters, on the in-cylinder liquid fuel behavior. The effect of the following parameters on the characteristics of the fuel droplets entering the cylinder was studied: Fuel volatility, injection timing, intake valve timing, injector type, spray geometry, and spray targeting in the intake port.
A Phase Doppler Particle Analyzer (PDPA) was used in a single-cylinder flow visualization combustion engine to assess this in-cylinder liquid fuel behavior. Measurements of fuel droplet characteristics (size and velocity) were taken in selected locations in the vicinity of the intake valve during starting and warm up. The measurements were performed with open and closed valve injection timing for each of the parameters listed above.
A previous paper by the authors [1] identified four different mechanisms that carry liquid fuel droplets into the cylinder (first forward flow atomization, high speed intake air transport, injection contribution, and fuel film squeezing). The present study shows substantial dependence of in-cylinder liquid fuel characteristics on the above parameters, both during starting and under warmed up conditions. It is shown that the droplet size distribution, the amount of liquid fuel, and the spatial distribution of droplet characteristics around the intake valve circumference were affected to different degrees. The observed trends in the droplet characteristics are explained in terms of changes in the previously identified mechanisms. The observed differences in in-cylinder liquid fuel behavior are significant enough to be reflected in engine out emissions behavior.