SAE International Development of a Quasi-Dimensional Combustion Model for Stratified SI-Engines 2009-01-2659

The simulation of the combustion process is an essential part of the internal combustion engine development. For simulating whole engine maps quasi-dimensional models in combination with 1-D-flow simulations are widely used. This procedure is beneficial due to short computation times and accurate forecast capability of quasi-dimensional combustion models. For the simulation of homogeneous SI-engines the two-zone entrainment model is usually used, which is based on hemispherical flame propagation. In this work a new approach for the quasi-dimensional calculation of the stratified SI-engine combustion process is proposed, which is based on the two-zone entrainment model. This proven approach was extended with regard to the inhomogeneous air/fuel composition of stratified SI-engines that make a two-zone treatment not sufficient. Therefore, four unburnt zones are defined: a rich zone, a stoichiometrical zone, a lean zone and a remaining air zone. Furthermore in analogy to existing approaches a burnt zone is defined. These zones are connected to each other by mass flow rates which are calculated by a mixture model. This mixture model considers the current geometry of the zones. In order to model the combustion progress a complete new approach for the flame propagation was developed that meets the concerns of the stratified combustion process. The newly developed approach was validated on measurement data of a modern, stratified SI-engine with multiple injection. 158 operating points were validated, that are covering the whole stratified range of the engine. The selected operating points also include a detailed variation of the multiple injection capabilities. A high accuracy of forecast capability could be proven by the new combustion model.