Knock Tendency Prediction in a High Performance Engine Using LES and Tabulated Chemistry

Abstract

The paper reports the application of a look-up table approach within a LES combustion modelling framework for the\ud prediction of knock limit in a highly downsized turbocharged DISI engine.\ud During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for\ud relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine\ud performance.\ud In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use\ud of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an\ud acceptable approximation of the actual chemical kinetics.\ud The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading\ud from detailed chemical kinetic schemes for gasoline. The look-up table access is provided by a multi-linear interpolating\ud routine internally developed at the “Gruppo Motori (GruMo)” of the University of Modena and Reggio Emilia. As the\ud experimental tests were conducted operating the engine at knock-limited spark advance, the tool is at first validated for\ud three different LES cycles in terms of knock tolerance, i.e. the safety margin to knock occurrence.\ud As a second stage, the validation of the methodology is performed for discrete spark advance increases in order to\ud assess the sensitivity of the modelling strategy to variations in engine operations. A detailed analysis of the unburnt gas\ud physical state is performed which confirms the knock-limited condition suggested by the experimental tests