Numerical investigation of the effect of wall wetting on hydrocarbon emissions in engines

The latest development of the direct injection strategy for small gasoline and diesel engines produces significant wall wetting. It has become necessary to develop a better understanding of the relationship between wall wetting and hydrocarbon (HC) emissions. To simulate these emissions from practical fuels, a multidimensional code for the spray and combustion was used for the study. It includes the spray/wall impingement model and the multicomponent droplet and film vaporization model for both gas- and liquid-phase transport processes. To extend these models to simulate combustion, a multicomponent fuel combustion model was developed. This combustion model combined the rate constants for each of the fuel components on the basis of mole fraction to form an effective fuel. The effective fuel was then used to calculate the reaction rates. The resulting models were then used to simulate the HC emissions resulting from wall-wetting in gasoline engines. Qualitative comparisons between the computed and measured results were made. The agreement of both the overall trends and the transient history were reasonable. The computed results also provide insight into the causes of HC emissions. It was revealed that wall-wetting location has a significant effect on HC emissions by allowing some of the unburned HCs an easy access to the exhaust, while making it nearly impossible for others to escape into the exhaust. The wetting of the cylinder liner underneath the exhaust valves and the piston top resulted in large increases in HC emissions. Injection timing affects the amount of film that survived past the combustion process. As film survived into the expansion and the exhaust stroke, the film's composition gradually shifted toward its heavier components, and this new composition controls the vaporization characteristic of the film.