Measuring the Changing Composition and Mass of Evaporating Fuel Films

When a fuel spray impinges on an interior surface of an engine, a thin liquid film can form. The relatively slow evaporation of the film has been shown to be a cause of increased pollutant emissions and reduced engine performance. To improve the understanding of how fuel films affect engine emissions and performance, a research program was initiated to study the physical processes involved in the evaporation of films composed of mixtures of hydrocarbons. The specific goal of the research reported here is to develop a method of simultaneously measuring the mass and composition of evaporating films. This method enables one to compute the evaporation rate of each component in the film. To our knowledge, these composition measurements are the first direct, time-resolved measurements of the changing composition of an evaporating liquid film composed of multiple volatile components. Mass and composition of evaporating liquid films were measured quantitatively using a Fourier transform infrared spectrometer (FT-IR). Evaporation rates for pure solvents and mixtures were determined through a calibration of the FT-IR measurements and these results were validated by measurements acquired with an analytical balance. The FT-IR also measured compositional changes for bi-component mixtures during the evaporation process. Three of the hydrocarbon solvents studied were hexane, cyclohexane, and 3-methylpentane. These were chosen for their similarities in molecular weight and physical properties as well as their comparatively unique infrared absorption spectra. Isooctane was also used because of its prevalence as a gasoline substitute in many engine studies and because of its slow evaporation rate compared to the smaller hydrocarbons. Solvents were studied individually and in various mixtures. Based on these preliminary results the method developed here is expected to be an important tool for studying the transport processes in an evaporating film.