A Quasi-Dimensional SI Burn Rate Model for Predicting the Effects of Changing Fuel, Air-Fuel-Ratio, EGR and Water Injection 2020-01-0574

As a result of the R&D focus being shifted from internal combustion engines to electrified powertrains, resources for the development of internal combustion engines are restricted more and more. With that, the importance of highly efficient engine development tools is increased. In this context, 0D/1D engine simulation offers the advantage of low computational effort and fast engine model set-up. To ensure a high predictive ability of the engine simulation, a reliable burn rate model is needed. Considering the increasing interest in alternative fuels, the aspect of predicting the fuel influence on combustion is of special importance. To reach these targets, the change of engine combustion characteristics with changing fuels and changing air-fuel-ratios were investigated systematically in a first step. For this purpose, engine test bed data were compared with expected fuel-dependent flame wrinkling trends based on Markstein/Lewis number theory. Furthermore, the possibility of influences caused by the Darrieus-Landau instability was evaluated. Based on these comparisons, an existing burn rate model was improved by adapting the sub-models for laminar and turbulent flame speed as well as the approach to calculate a characteristic burn-up time. Reaction kinetics calculations were used as a basis to model the laminar flame speed. The influence of water injection was included, too. Concerning turbulent flame propagation, different models of the turbulent flame speed were compared qualitatively. The most promising one was implemented in the simulation environment. For this, a model for the laminar flame thickness was developed. Furthermore, the possibility to consider the local distribution of turbulent kinetic energy was implemented and evaluated. From test bed data comparison, an overestimation of the laminar flame speed influence on burn rate was observed for the baseline model. The approach to calculate the characteristic burn-up time was identified as the reason for this overestimation and was consequently adapted. The improved burn rate model has been validated against different engines running on different fuels (methane, methanol, ethanol, gasoline, methyl formate), air-fuel-ratios, EGR rates, engine speeds, engine loads and water injection rates. For most cases, the IMEP can be predicted with an error below ±1.5 %. The validation also proved that the burn rate model is able to predict the influence of a fuel variation on engine performance, without the need for recalibration. The model thus allows the investigation of the potential of alternative fuels and engine concepts like lean combustion, on the basis of a calibration for e.g. gasoline. In the outlook, the fuel influence on engine knock prediction and its relation to cycle-to-cycle variation is analyzed.