Detailed Calculation of Heating, Evaporation, and Reaction Processes of a Thin Liquid Layer of Hydrocarbon Fuel 2000-01-0959
A one-dimensional model has been developed for the species and energy transfer over a thin (0.1-0.5 mm) layer of liquid fuel present on the wall of a spark-ignition engine. Time-varying boundary conditions during compression and flame passage were used to determine the rate of methanol vaporization and oxidation over a mid-speed, mid-load cycle, as a function of wall temperature. The heat of vaporization and the boiling point of the fuel were varied about a baseline to determine the effect of these characteristics, at a fixed operating point and lean conditions (ϕ = 0.9). The calculations show that the evaporation of fuels from layers on cold walls starts during flame passage, peaking a few milliseconds later, and continuing through the exhaust phase. The vaporized fuel diffuses off the film, reacts with excess oxygen in the neighborhood of the fuel film, forming a transient diffusion flame which releases enough heat to lead to a second peak in heat transfer to the liquid film and increased vaporization. The amount of fuel vaporized increases with wall temperature, fuel volatility (lower heat of vaporization and lower boiling point), and decreasing film thickness. In the case of methanol, most of the vaporized fuel oxidizes before the end of the cycle (97-98 %), a fraction that remains constant regardless of wall and fuel conditions for flame passage around peak pressure. Since the fraction of fuel oxidized under the current conditions does not vary with wall temperature or liquid fuel properties, the total potential for contributing to emissions is proportional to the total amount of liquid fuel initially present in the chamber.