Emissions Reduction During Warm-Up Period by Incorporating a Wall-Wetting Fuel Model on the Fuel Injection Strategy During Engine Starting

The purpose of this paper is to quantify the improvements possible for ULEV emissions by improved air-fuel ratio control during starting by modifying conventional fuel injection strategy with a first order wall-wetting-fuel model. Measurements of emissions during first 30 starting cycles of a ULEV engine, made with a fast response flame ionization detector (FID) and conventional fuel injection strategy, show that these account for 17% of the overall FTP-75 mode HC emissions. The wall-wetting-fuel model is a two coefficient model: α, the ratio of the injected fuel mass to the fuel mass inducted into the cylinder during a given cycle, and β, the ratio of the total fuel mass accumulated on the intake port wall to the mass inducted into the cylinder from the accumulated fuel at a given cycle. Fast FID measurement of the transient HC emissions during starting permits, through curve fitting, the determination of the two coefficients a and b as 30.4% and 45.0% for an engine coolant temperature of 25°C: the increase of the coefficients with increasing engine coolant temperature has also been measured. Modification of the fuel injection strategy with this model, using engine speed and coolant temperature as inputs, resulted in a reduction of engine emissions by 80% during the first 30 cycles, which corresponds to a 14% reduction in the overall FTP-75 mode emissions.