Vaporization of Individual Fuel Drops on a Heated Surface: A Study of Fuel-Wall Interactions within Direct-Injected Gasoline (DIG) Engines

The impingement of liquid fuel onto the surfaces of the combustion chamber (wall-wetting) has been shown to be an important source of HC emissions from direct-injected SI engines, and can even result in pool fires and diffusion flames. Some degree of wall wetting, particularly on the piston top, is believed to occur in every current DIG engine design, but the behavior of the wall-bound fuel throughout the engine cycle is poorly understood. The goal of this study was to gain a better understanding of the fundamental interaction between liquid fuel droplets and the piston under engine-like conditions, by observing the vaporization of individual fuel drops as the surface temperature and ambient pressures were varied in a controlled environment. The vaporization of several single-component fuels, binary mixtures, and multi-component fuels was examined in the range of surface temperatures between 50 and 300 °C and ambient pressures between 50 and 1270 kPa (abs). The results of this work indicate that the vaporization of gasoline drops on the piston is reduced by the Leidenfrost effect when the pressure in the cylinder is low (intake and early compression stroke), but that the gasoline drop vaporization rates increase quickly as the pressure rises later in the compression stroke. A further analysis was performed to investigate what happens to the wall-bound fuel once it vaporizes. Results from a one-dimensional model show that, regardless of when the drop vaporization occurs, the diffusion of fuel vapor away from the wall is inhibited until after peak pressure. The results of this work are consistent with the observed trends for HC emissions due to wall wetting in SI engines, and elucidate the behavior of fuel droplets that impact the piston in DIG engines.