Investigation of Flame Propagation Description in Quasi-Dimensional Spark Ignition Engine Modeling 2018-01-1655
The engine development process has been enhanced significantly by virtual engineering methods during the last decades. In terms of in-cylinder flow field, charge flow and combustion modelling, 3D-CFD (three dimensional) simulations enable detailed analysis and extended investigations in order to gain additional knowledge about design parameters. However, the computational time of the 3D-CFD is an obvious drawback that prevents a reasonable application for extensive analysis with varying speed, load and transient conditions. State-of-the-art 0D (zero dimensional) approaches close the gap between the demand of high computational efficiency and a satisfying accordance with experimental data. Recent improvements of phenomenological combustion approaches for gasoline spark ignition engines deal with the consideration of detailed flow parameters, the accuracy of the laminar flame speed calculation and the prediction of the knock limit. Little attention has been given to the influence of different combustion chamber designs on the prediction capability so far. This leads to an often used simplification consisting of a combustion chamber modeled as a disk and an acceptable inaccuracy of combustion modelling. With an increasing deviation of the surrogate combustion chamber from the investigated real chamber, the prediction capability becomes insufficient. This effect is intensified by the shift of the combustion process to a fast combustion nearby the top dead center (TDC), typical for high performance engines with advanced ignition timing for maximum brake torque.
In order to improve the model accuracy, this examination highlights the effect of different descriptions of the flame propagation in 0D combustion modeling. Two calculation paths are introduced. On the one hand the flame propagation description is determined by the combustion chamber geometry prior to the model calibration process, and on the other hand the flame data is derived from measured data after the model calibration. The forward path considers exemplary combustion chamber designs, e.g. through different piston cavities, their effect on the flame front as well as on the 0D model results. A deep analysis via 3D-CFD of the flame propagation reveals characteristic points, which are related to different geometrical aspects. Despite the consideration of the flame maps, the related changes of the charge motion are calculated through 3D-CFD and transferred to 0D. The improvement of the predictive capability through the flame data and flow parameters is investigated by experimental data of two different high performance engines. The backward path deals with the calculation of flame propagation from measured cylinder pressure data. On the one hand this gives the opportunity to analyze the combustion process with the knowledge gained by the previous introduced characteristic aspects, on the other hand it creates flame maps that are simple to use. Latter improve the 0D combustion model accuracy even without knowing the exact geometry.
Simon Malcher, Michael Bargende, Michael Grill, Ulrich Baretzky, Hartmut Diel, Sebastian Wohlgemuth, Gordon Röttger
Universitat Stuttgart, FKFS, Audi AG
International Powertrains, Fuels & Lubricants Meeting