| Further reductions in hydrocarbons (HCs) emitted from spark ignition (SI) engines are needed in order to meet increasingly stringent air pollution standards. Therefore, a research program was initiated to study the effects of engine operating variables on the fuel distribution in the cylinder and to identify both the characteristics of the fuel distribution which can cause a high level of HC emission and the operating parameters which most influence these characteristics.; An optical engine was built to provide access to the cylinder to visualize the entry of liquid fuel, to measure droplet sizes, and to qualitatively measure the vapor and liquid phase fuel distributions using planar laser induced exciplex fluorescence (PLIEF). A novel technique of simulating gasoline with a fuel blend that is appropriate for PLIEF measurements was developed. In conjunction with these measurements, estimates of droplet vaporization time were used to assess the potential impact of the presence of liquid fuel in the cylinder on HC emission.; Liquid fuel in the cylinder was found to be the primary cause of inhomogeneity in the fuel distribution and therefore a principal contributor to HC emission. Much more liquid enters the cylinder with open-valve injection than with closed-valve injection. For some conditions a significant amount of liquid wets a small region of the cylinder wall and the slow evaporation of this liquid causes a locally high fuel vapor concentration at the end of the compression stroke. Increasing intake air velocity reduces the rate of fuel accumulation in the intake port, the sizes of droplets passing into the cylinder, and the amount of wall impingement, providing a more uniform fuel distribution in the cylinder. In addition, inhomogeneities in the fuel distribution within the cylinder which are present during the intake event can be substantially reduced by the end of the compression stroke with in-cylinder swirl. Increasing engine speed from 200 to 1200 rpm was much less effective for enhancing mixing than was increased swirl. Finally, the injector spray quality was found to affect the droplet sizes passing into the cylinder only for an open-valve injection timing. |
