A Detailed Analysis of the Initiation of Abnormal Combustion with Reaction Kinetics and Multi-cycle Simulation

Abstract

For highly boosted gasoline engines with direct injection (DI) the operating conditions with lowest fuel consumption are restricted by irregular combustion like knocking. Therefore, the initiation mechanism for knocking was the subject of this research work. A 4-cylinder DI test engine that was provided by GM Europe was set up at the institute’s test bench. Experimental data at low speed at the knock limit (2–3 knock events per 100 cycles) were the basis for numerical investigations. A 1D multi-cycle simulation of gas-exchange and combustion was applied to calculate the in-cylinder properties and charge composition for the knocking cycles. Additionally, a CFD-simulation was carried out in order to obtain the spatial in cylinder distribution of temperature, mixture and residual gas. These results served to set up a stochastic reactor model including the full chemistry of the low and high temperature combustion. The model was initialised with the boundary conditions of the knocking cycle and the temperature and concentration distributions from the CFD-simulation. This method enabled the analysis of knocking combustion on the basis of chemical principles. The results of the multi-cycle analysis showed that important charge properties like the charge temperature at inlet valve closing or the internal residual gas fraction were within a narrow range over all cycles. Furthermore it could be shown that the burn duration for converting 2 % of the fuel mass correlated with cycles showing autoignition. All cycles with an accelerated early flame development showed an irregular heat release later on during the combustion phase. A detailed modelling of this behaviour was carried out for the stochastic reactor model. It could be shown that only with an initial reactor temperature distribution according to the CFD simulation, which included hot regions, the autoignition occurred as early as in the measurement. The formation of typical intermediate species for the low temperature oxidation leading to autoignition could be described. Finally, a possibility was shown to introduce quasi-spatial information of the flame propagation into the zero dimensional reactor model. In this way critical regions for knocking were identified in accordance with optical measurements of knock sources on the test bench. Most theoretical investigations on abnormal combustion are based on a simplified approach considering the mean cycle for a given operating point. However, abnormal combustion never occurred for the mean cycle but rather for a very early cycle. For that reason, this work focused especially on the detailed reconstruction of a measured knocking cycle with reaction kinetics. Considering the effect of temperature inhomogeneities and flame propagation on knock initiation in a stochastic reactor model is thereby a new and innovative approach.

F2012-A06-021