Choice of tuning parameters on 3D IC engine simulations

Paper #:
  • 2018-01-0183

  • 2018-04-03
Modern 3D CFD IC engine simulations are extremely complex for the regular user due to the use of complex phenomenological sub-models with solution-adaptive mesh refinement and coarsening, and improved chemistry solvers. This study used ANSYS® Forte, Version 17.2, an IC engine CFD software package, to investigate two tuning constants that influence flame propagation in 3D CFD SI engine simulations: the stretch factor coefficient, C_ms and the flame development coefficient, C_m2. After identifying several C_m2-C_ms pairs that matched experimental data at one operating conditions, simulation results showed that except for HC emissions, the engine models that used different C_m2-C_ms sets predicted similar combustion performance, when the spark timing, engine load, and engine speed were changed from the operating condition used to validate the CFD simulation. The differences in HC emissions were probably due to variations in local flame speed close to the walls or towards the end of combustion. A dramatic shift was observed when engine speed was doubled, which suggested that the flame stretch coefficient, C_ms, had a much larger influence at higher engine speeds compared to the flame development coefficient, C_m2. Therefore, the C_m2-C_ms sets that predicted a higher turbulent flame under higher in-cylinder pressure and temperature increased the peak pressure and efficiency. This suggest that the choice of the C_m2-C_ms will affect the simulation accuracy when engine speed increases from the one used to validate the model. Overall, the results suggest that finding the set of constants that can be used to predict different engine operating conditions without the need to modify the set is not trivial. In addition, the 3D CFD simulation needs enough experimental data for validation at very different conditions. As a result, the less-experimented CFD user should use the CFD tool for its original purpose, which is to guide and/or complement experimental investigations.
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