Modeling and Simulation of Gasoline Auto-Ignition Engines

Both the customer demand for increasing mobility and the emission legislation lead to a challenge for engine researchers and developers in order to reduce emissions and fuel consumption. One approach that is presently under extensive investigation is to implement auto-ignition combustion in gasoline engines. This combustion mode offers the possibility to reduce emissions and fuel consumption during part load operation. Furthermore it offers the advantage that it does not need an expensive exhaust gas after treatment due to nearly zero NOx-emissions in contrast to stratified direct injection operation. The auto-ignition depends strongly on stratification of air, residual gas and fuel. Furthermore, the thermodynamic state of the charge is of major importance to control the combustion process. Detailed knowledge of ignition and its dependency on operating conditions is necessary to develop efficient control strategies.This paper gives a summary on modeling strategies for gasoline auto-ignition developed within the collaborative research centre “SFB 686 – Modellbasierte Regelung der homogenisierten Niedertemperatur-Verbrennung” [1]. The auto-ignition process is simulated with two different approaches. 3D CFD calculation of flow, injection and mixture formation, which is bi-directional coupled to a multi-zone reaction kinetics solver. This 3D approach enables to analyze the thermodynamic conditions in the combustion chamber that lead to the auto-ignition. Thus, the temporal and spatial occurrence of exothermic reactions and their influence on the engine process are specified in detail.To reduce the computational costs and enable multi-cycle calculations, a second simulation approach was developed to analyze the process under steady state and transient operating conditions. The approach uses 1D gas exchange calculation with embedded burn function calculations based on reaction kinetics. The simulation shows good correlation to the test bench results, but requires a computational time of approximately 5 min per cycle. The calculation time can be further reduced with an approach based on a polynomial combustion model. Multi-cycle calculations are performed and compared to test bench results. Due to the small computational effort, this approach offers the possibility of a coupling to a controller design environment for synchronous simulation and control.