Modeling of Multicomponent Fuels with Application to Sprays and Simulation of Diesel Engine Cold Start

Lippert, A. M. Modeling of Multicomponent Fuels With Application to Sprays and Simulation of Diesel Engine Cold Start. University of Wisconsin-Madison, 1999.

A new approach to modeling the multicomponent nature of commercial fuels was implemented for use in spray and engine computations. The model is based on a continuous distribution using a probability density function, rather than the use of discrete components, to capture more accurately the entire range of composition in commercial fuels. Both low-pressure and high-pressure formulations to calculate vapor-liquid equilibrium are shown. The pressure transport equations and vaporization modeling are shown for both liquid drops and two-dimensional wall films. The model was applied to single droplet calculations to validate the predictions against experimental and previous modeling results. The direct injection gasoline spray results show noticeable composition stratification in the vapor plume of the spray. Good comparison of the predicted liquid lengths with experimental data for a diesel spray was obtained. Axial composition gradients were observed in the multicomponent diesel spray predictions. The continuous multicomponent model was used to analyze the cold-start behavior of a heavy-duty diesel engine. The predictions show the importance of including both the multicomponent nature of the fuel, as well as a detailed model of the wall-film and spray-wall interaction. Heating the intake air temperature was shown to result in reduced ignition delay and accelerated vaporization. Increasing the fuel temperature increases the ability of vaporization to occur prior to and away from the initial heat release. Increasing the injection pressure increased vaporization, but without much change in the ignition delay. Using split injection timings, the second pulse, containing 75% of the fuel, displayed a substantial reduction in ignition delay due to the effect of combustion of the pilot pulse. The spray targeting, by way of enhanced secondary atomization (splashing) due the spray impingement, was found to be a very powerful way of enhancing cold start.