Detailed Simulation of Cycle-To-Cycle Fluctuations at Gasoline Auto Ignition Engines and Derivation of a Synchronous Simulation and Control Approach

Abstract For gasoline engines the controlled auto ignition (GCAI) operating mode provides the potential to enable the fuel consumption benefit of stratified lean combustion systems without the drawback of additional NOx aftertreatment. The auto-ignition process depends strongly on stratification of air, residual gas and fuel. Furthermore, the thermodynamic state of the charge is of major importance to control combustion. Due to the high amount of residual gas in the combustion chamber, the preceding cycle and its combustion has a high effect on the next cycle by directly affecting the thermodynamic state of the in-cylinder charge. In addition to RANS calculations with k-e turbulence modeling this paper investigates the effects leading to cycle-to-cycle combustion fluctuations in a GCAI engine by using LES with Smagorinsky turbulence modeling. The auto-ignition process is simulated with 3D CFD calculation of flow, injection and mixture formation, which is bi-directionally coupled to a multi-zone reaction kinetics solver. This 3D approach enables to analyze the thermodynamic conditions in the combustion chamber that lead to 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 effort of multi-cycle calculations which take cycle-to-cycle fluctuations into account, a reduced simulation approach is derived from the 3D CFD. This approach uses 1D gas exchange calculation with GT-Power coupled with online reaction kinetics simulation. Due to the small computational effort, this approach offers the possibility of a coupling to a controller design environment for synchronous simulation and control. Both simulation approaches are validated against test bench measured data. The 3D CFD approach shows a nearly exact agreement to the measured mean pressure curve. Also the cycle-to-cycle fluctuations are resolved. They can be described by flucutations in fuel mixture, residual gas and temperature stratification. The reduced simulation approach calculates the released heat for all cases with a maximum error of 5 %. This is sufficient to determine the control variable indicated mean effective pressure very well.