Combined Experimental and Numerical Analysis of the Influence of Air-to-Fuel Ratio on Cyclic Variation of High Performance Engines

The Cycle by Cycle Variation (CCV) of a SI engine is analyzed by combining experimental tests and numerical investigations. The quantification of CCV is based on the evaluation of the Coefficient of Variance (COV) of IMEP. The analysis of the experimental pressure data shows an increase in CCV towards leaner mixture conditions. The evaluation of the Heat Release Rate from the in-cylinder pressure traces reveals the strong influence of the early stages of combustion on the variability of the flame evolution. In order to evaluate the influence on CCV of local air equivalence ratio cycle-to-cycle variability and mixture homogeneity in the chamber, a numerical CFD methodology for the simulation of the combustion process has been proposed. The results reproduce with reasonable accuracy the increase in CCV with leaner combustions and put the basis for a deeper insight into the complex phenomena involved in the combustion process by the use of parametric analysis.