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Stratified combustion in gasoline engines constitutes a promising means of achieving higher thermal efficiency for low to medium engine loads than that achieved with combustion under standard homogeneous conditions. However, creating a charge that leads to a stable efficient low-emission stratified combustion process remains challenging. Combustion through a stratified charge depends strongly on the dynamics of the turbulent fuel-air mixing process and the flame propagation. Predictive simulation tools are required to elucidate this complex mixing and combustion process under stratified conditions. For the simulation of mixing processes, combustion models based on large-eddy turbulence modeling have typically outperformed the standard Reynolds averaged Navier-Stokes methods.

A Spray-Guided (SG) Direct-Injection (DI) Spark-Ignition (SI) engine is widely recognized to be a promising technology capable for substantially reducing fuel consumption and carbon dioxide emissions. Accordingly, there is a strong need for developing models of some effects specific to stratified turbulent burning under conditions of elevated and rapidly varying pressure. Two such effects were addressed in the present work by performing unsteady three-dimensional URANS simulations of stratified turbulent combustion in a SG DISI engine. First, a simple method of evaluation equilibrium combustion temperature, implemented into the CFD code OpenFOAM®, was improved in order to take into account the dissociation of the combustion products. Second, stratified turbulent combustion is affected by fluctuations in mixture composition. A widely used approach to modeling this effect consists of invoking a presumed Probability Density Function (PDF) for mixture fraction f.

In recent years, a free, open source CFD software package called OpenFOAM has been attracting increasing amounts of attention as a promising, inexpensive, and efficient CFD tool for the numerical simulation of processes such as fuel injection and evaporation, turbulent mixing and burning. Here, we describe the further development of OpenFOAM to enable its use in simulating stratified turbulent combustion in DI SI engines. Advanced models of various phenomena relevant to partially premixed turbulent flames were implemented into the code, and the effects of these implementations were investigated by performing unsteady 3D RANS simulations of stratified turbulent burning in a DI SI engine. First, the Flame Speed Closure (FSC) model of premixed turbulent combustion was implemented. Second, a method for evaluating the mean density in premixed turbulent flames that is available in the standard OpenFOAM library was improved.

A semi-detailed chemical mechanism for combustion of gasoline-ethanol blends, which is based on sub-mechanisms of gasoline surrogate and for ethanol is developed and validated aiming at CFD engine modeling. The gasoline surrogate is composed of iso-octane, toluene, and n-heptane in volumetric proportions of 55%:35%:10%, respectively. In this way, the hydrogen-carbon atomic ratio H/C, which is around 1.87 for real gasoline, is accurately reproduced as well as a mixture equivalence ratio that is important for Gasoline Direct Injection engine applications. The integrated mechanism for gasoline-ethanol blends includes 120 species participating in 677 reactions. The mechanism is tested against experimental data on ignition delay times and laminar flame speeds, obtained for various n-heptane/iso-octane/toluene/ethanol-air mixtures under various equivalence ratios, initial temperatures, and pressures. Chemical, thermodynamic and transport properties used in the calculations are discussed.

In the first part of the paper, a generalization of the turbulent diffusivity concept is considered and a generalized diffusion coefficient is introduced to account for the development of turbulent diffusivity, pressure-driven countergradient transport, and effects of chemical reactions on turbulent scalar flux. The behavior of the generalized diffusivity is numerically studied in the 1-D statistically planar case and the contributions of the aforementioned processes to the diffusivity are assessed. In the second part of the paper, the generalized diffusivity is incorporated into the Flame Speed Closure (FSC) model of premixed turbulent combustion and the extended FSC model is applied to simulate recent experiments performed using the Leeds fan-stirred bomb. The extended FSC model well predicts the speed, thickness and structure of statistically spherical, premixed, turbulent flames that expand in the bomb after spark ignition.

Simulations of a Direct Injection Spark Ignition (DISI) engine have been performed for both early injection with homogeneous charge combustion and for late injection with stratified charge combustion. The purpose has been to study flow characteristics, fuel/air mixing, combustion, and NOx and soot formation. Focus is put on the combustion modeling. Two different full load cases with early injection are simulated, 2000 rpm and 6000 rpm. One load point with late injection is simulated, 2000 rpm and 2.8 bar net MEP. Three different injection timings are simulated at the low load point: 77, 82, and 87 CAD bTDC. The spray simulations are tuned to match measured spray penetrations and droplet size distributions at both atmospheric and elevated pressure. Boundary conditions for the engine simulations are taken from 1-D gas exchange simulations that are tuned to match engine tests.

Published experimental data obtained in well-defined simple cases are discussed in order to qualitatively test various models of premixed turbulent combustion, utilized in multi-dimensional numerical simulations of SI engines. An analysis of such data indicates that there exist several unresolved issues important for flame propagation in SI engines. Two of them, pressure dependence of turbulent flame speed St and turbulent flame development, are discussed in the paper. First, existing experimental data indicate an increase in St by pressure despite the marked decrease in the laminar burning velocity SL by P. Although this well established trend appears to be of substantial importance for SI engine applications, many combustion models utilize SL as the sole mixture characteristic and, hence, predict similar dependencies both of St and of SL on P, contrary to the aforementioned experimental results.