Peter Kelly-Zion

A computational study of the effect of fuel type on ignition time in homogenous charge compression ignition engines

The homogeneous charge, compression ignition (HCCI) engine has advantages in terms of efficiency and reduced emissions in comparison to conventional internal combustion engines. One of the distinguishing characteristics of an HCCI engine is that the ignition is controlled by the chemical kinetics, unlike the diesel or spark ignition engines, for which ignition time can be controlled externally by the fuel injection or spark time. As a consequence of being controlled by chemical kinetics, the HCCI ignition time can vary significantly with changes in the operating conditions, and this variation can limit the practical range of operation of the engine. Using a single-zone combustion model and established reaction rate mechanisms, the influences of the compression ratio, intake temperature, equivalence ratio, engine speed, and exhaust gas recirculation on the ignition time of two fuels, normal heptane and iso-octane, were studied. The model simulated the environment in the engine combustion chamber by assuming adiabatic compression and expansion. The sensitivity of the ignition time to changes in operating conditions was found to be dependent on the type of fuel. The results indicate that the use of fuels with a characteristics two-stage ignition (e.g., n -heptane) exacerbates the problem of ignition control compared with the use of fuels with a single-stage ignition (e.g., iso-octane).

APPLICATION OF LASER INTERFEROMETRY FOR TRANSIENT FILM THICKNESS MEASUREMENTS

A laser interferometry technique for making transient measurements of film thickness of the order of 10 to more than 1000 µm is described. The basis for these measurements was reported previously [1] but the technique was applied to solid glass slides and a slowly thinning silicone oil film. The current work describes an adaptation of the technique for the measurement of rapidly changing film thickness, as occurs with evaporating films. A beam from a helium-neon laser is focused on the film at an oblique angle. Some of the laser light is reflected off of the top surface of the film and some is reflected off of the bottom surface. The light reflected from the two surfaces forms a concentric interference fringe pattern which is projected onto a screen and recorded by a high-speed camera. The film thickness is directly related to the spacing of the fringes. To demonstrate the technique, measurements of the time-varying thickness of three evaporating films are presented and experimental considerations are discussed.

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.Copyright