The challenges of using detailed chemistry model for simulating direct injection spark ignition engine combustion during cold-start

Abstract

Computational Fluid Dynamics (CFD) modeling of gasoline spark-ignited engine combustion has been extensively discussed using both detailed chemistry mechanisms (e.g., SAGE) and flamelet models (e.g., the G-equation). The models have been extensively validated under normal operating conditions; however, few studies have discussed the capability of these models in capturing DISI combustion under cold-start conditions. A cold-start differs from normal operating conditions in various respects, such as (1) having highly retarded spark timing to help generate a high heat flux in the exhaust for a rapid catalyst light-off; (2) having split-injection strategies to ensure a favorable stratification at the vicinity of the spark plug and reduced film formation; and (3) having optimized valve timings for reduced NOx emissions via increased internal residuals and reduced hydrocarbon (HC) emissions via prolonged oxidation of the combustion products. The retarded spark timing introduces the adverse effect of a decaying turbulence field, which results in a reduced turbulent flame speed. The analysis of all these factors happening inside the cylinder appears complicated at first glance; however, it could be made possible by efficient use of the existing CFD models. The current study explored the capability of the SAGE detailed chemistry model in capturing cold-start flame travel in a DISI engine. The results were then compared against the G-equation-based GLR model, which has been validated for excellent predictions of the DISI cold-start combustion as shown by Ravindran et al. The flame travel was captured on a Borghi-Peters diagram to find that the flame travels through corrugated, wrinkled, and laminar regimes. In order to fully evaluate the capability of the detailed chemistry model in predicting such changing turbulence-chemistry interactions, it will need to be studied individually in each regime; however, the scope of the current paper is limited to the study of the model behavior in the laminar regime, which will be shown to be important for DISI engine cold-start. The SAGE detailed chemistry model, with a toluene reference fuel (TRF) mechanism validated for gasoline laminar flame speeds, was found to significantly under-predict the flame propagation speeds because of the effects of numerical viscosity and discrepancies in capturing molecular diffusion. The causes and effects of this under-prediction and the ways in which this can be improved are presented in the paper.

Development of a fractal engine simulation model in a multidimensional simulation for the cold start process of a gasoline direct injection engine

“…When a relatively high turbulent Schmidt number, 0.78 for example, was used in the SAGE model for cylinder 3, the combustion was weak and the pressure trace was under-predicted, which was similar to what Ravindran and Kokjohn 10 found for the laminar regime, stating: “The SAGE detailed chemistry model … under-predict the flame propagation speeds.” In this study, the turbulent Schmidt number was tuned and eventually set to 0.3 for cylinder 3 to match the experimental pressure traces, and then the same settings were applied in the simulations for the other three cylinders. The final results from the SAGE model are presented as the blue solid curves in Figure A1.…”

“…The problem possibly arose due to the effects on the flame of different turbulence levels for different cylinders and even for the same cylinder at different crank angles (due to the rapidly changing engine speed within a cycle). Ravindran and Kokjohn 10 extensively explored the use of the SAGE model in the laminar regime for cold-start conditions, and found that it presented challenges, under-predicting the flame propagation speeds, also noting that further study was needed to evaluate the turbulence-chemistry interactions in other regimes. Consequently, we decided to explore the implementation of the FES model – a model physically predicting the turbulence-flame interaction and also a model we have been working on for years – into the multi-dimensional simulations for cold start.…”

“…Consequently, we decided to explore the implementation of the FES model – a model physically predicting the turbulence-flame interaction and also a model we have been working on for years – into the multi-dimensional simulations for cold start. It should be noted that the G-equation model, as presented by Ravindran et al, 10,11 appropriately modified, has been demonstrated to simulate cold-start conditions well for the portion of the cold start following the initial transient period where the engine is operating at constant speed in catalyst heating mode. However, it is not the focus of this paper.…”

A priori test of perfectly stirred reactor approach to evaluating mean fuel consumption and heat release rates in highly turbulent premixed flames

“…Besides, the PSR model was also promoted [18][19][20] for Reynolds-Averaged Navier-Stokes (RANS) simulations of premixed turbulent burning in combustion chambers of piston engines by assuming 19 weak temperature fluctuations in such chambers. Accordingly, the PSR model is widely used [21][22][23][24][25][26][27][28][29][30] in applied Computational Fluid Dynamics (CFD) research into turbulent burning in piston engines and other combustors. In particular, the PSR model is a default complex-chemistry model of flame-turbulence interaction in a popular CFD software such as Converge.…”