Abstract
The present study targets numerical investigations of the effects leading to cycle-to-cycle combustion fluctuations in a gasoline controlled auto-ignition (GCAI) engine. The auto-ignition process depends strongly on the thermodynamic state of the in-cylinder charge. Therefore, the preceding cycle and its combustion characteristics have a high impact on the current cycle.
An approach is introduced which correlates the thermodynamic in-cylinder state at the end of combustion to the heat release of the next cycle. In a first step, a reduced 1D gas exchange model is used in which the detailed chemistry of the trapped gas mass is neglected. Subsequently, computational fluid dynamics (CFD) coupled to chemistry calculations is used to identify the sources of cycle-to-cycle fluctuations in detail.
It is shown that cycle-to-cycle fluctuations can be divided into two parts: An autocorrelation part and a superimposed stochastic part. Autocorrelations are the correlations of the thermodynamic state at the end of combustion to the state during gas exchange and finally to the state before ignition. From this ideal point of view a fast burning cycle should be followed by a slow burning one and vice versa. The CFD describes the stochastic deviations from this ideal case, since fluctuations of mixture preparation and temperature stratification have a superimposing effect on combustion.
KEYWORDS – gasoline controlled auto-ignition; GCAI; simulation; in-cylinder flow; combustion; combustion variability