Diesel Engine Combustion Modeling Using the Coherent Flame Model in Kiva-II

A flamelet model is used to calculate combustion in a diesel engine, and the results are compared to experimental data available from an optically accessible, direct-injection diesel research engine. The 3∼D time-dependent Kiva-II code is used for the calculations, the standard Arrhenius combustion model being replaced by an ignition model and the coherent flame model for turbulent combustion. The ignition model is a four-step mechanism developed for heavy hydrocarbons which has been previously used for diesel combustion. The turbulent combustion model is a flamelet model developed from the basic ideas of Marble and Broadwell. This model considers local regions of the turbulent flame front as interfaces called flamelets which separate fuel and oxidizer in the case of a diffusion flame. These flamelets are accounted for by solving a transport equation for the flame surface density, i.e., the flame area per unit volume. The chemical source term is determined by calculating the local consumption rate of the reactants in the flamelets. This consumption rate is evaluated from analytical formulas or numerical fits to the results of laminar flamelet calculations. This formulation has been extended to account for the particular phenomena involved in diesel combustion. Specifically, the reactants mixed after the start of fuel injection burn rapidly when ignition occurs and combustion is controlled later in the cycle by large-scale mixing and diffusion processes at small scales. The characteristics of the fuel spray in the computation are initialized to correspond to available experimental injection characteristics.

The experimental diesel engine on which the computer simulations are based is derived from a Cummins N14 production engine, with a bore of 140 mm and a 152 mm stroke. Modifications were made to permit optical access for laser-based two-dimensional imaging diagnostics. Soot distributions were investigated using laser-induced incandescence and elastic/Mie scatter imaging. Fuel distributions were determined from laser-induced fluorescence and elastic/Mie scattering. The computational results are compared to experimental ignition delay, pressure histories and apparent heat release profiles deduced from the mean cylinder pressure. The spray characteristics are also investigated with regard to liquid and vapor fuel penetration in the combustion chamber of the engine.