Evaporation of In-Cylinder Liquid Fuel Droplets in an SI Engine: A Diagnostic-Based Modeling Study 1999-01-0567
Liquid fuel behavior in the cylinder impacts SI engine HC emissions particularly during engine start-up. Inflow of liquid fuel into the cylinder is largely determined by the flow and temperature environment in the intake port. Subsequent evaporation of fuel droplets in the cylinder prior to impact on the piston and cylinder liner reduces the amount of liquid fuel in the cylinder that is likely to contribute to HC emission and is therefore important.
In this study, measurements of liquid fuel droplet characteristics in the vicinity of the intake valve of a firing SI engine were analyzed to estimate the amount and spatial distribution of in-cylinder evaporation of liquid fuel prior to droplet impact on the cylinder liner or piston. A one-dimensional fuel droplet evaporation model was developed to predict the amount of fuel evaporation given measured fuel droplet sizes and velocities, intake port and valve temperatures during warm up, and cylinder geometry.
Measurements of the liquid fuel droplet distribution entering the cylinder were made around the circumference of the intake valve with open and closed valve port injection timing, during engine starting and warm up. Based on these measurements the dependence of in-cylinder fuel evaporation on changing temperatures and droplet size distributions during engine warm-up has been assessed. The fraction of the liquid fuel entering the cylinder that vaporizes before wall impact was found increase from about 20% at start-up to about 50% under warmed up conditions. This fraction proved to be most sensitive to the intake port and valve temperature.