Name : The 10th International Conference on Modeling and Diagnostics for Advanced Engine Systems (COMODIA 2022)
Date : July 05, 2022 - July 08, 2022
To further reduce the CO2 emissions of SI engines, maximum compression ratios must be realized in combination with high boost pressures. At high engine loads, knocking combustion then requires a retardation of the ignition timing at the cost of reduced efficiency and increased exhaust gas temperatures. Advanced computer-aided engineering tools enable prediction of the knock behavior and achievable peak efficiencies prior to actual thermodynamic testing, which significantly reduces development costs and times.
Current, well-calibrated 1D-models that include an estimate of the knock behavior can provide valuable insights into overall engine operation, even going so far as to optimize transient operation of hybrid powertrains for Real Driving Emissions. In contrast, 3D-CFD studies enable also the evaluation of the influence of geometric details in the early stages of development, as well as gaining physical understanding that can be applied to reduced, more cost-effective modeling approaches.
Reynolds-Averaged Navier Stokes (RANS) based turbulence models are able to represent a mean cycle since all turbulent structures are averaged. However, the mean cycle then typically does not exhibit significant knock intensities, so approaches must be found to nevertheless evaluate the knock tendency of the operating point. Depending on the combustion model, faster burning cycles can be represented by artificially enhancing the turbulent flame speed or by a virtual spark advance. These models provide both spatial location and knock onset timing information for a fast cycle.
On the other hand, Large-Eddy Simulation (LES) only models the turbulent structures smaller than the grid size. Therefore, LES is capable of also representing cyclic variations. The computational cost and the necessity of advanced combustion models due to insufficient spatial flame-resolution will be analyzed and discussed.
Finally, an outlook will be given on how the disadvantages of both RANS and LES can possibly be overcome to enable a cost-effective 3D-CFD evaluation of the knock tendency. Therefore, existing approaches based on Probability Density Functions (PDF) will be summarized and further developments based on the findings of the conducted research projects will be discussed.
