Investigation of In-Cylinder Cycle-to-Cycle Variation Using PIV, LIF and RANS Simulation

To elucidate the cycle-to-cycle variations (CCV) in gas flow and combustion within gasoline engine cylinders, simultaneous two-section PIV and LIF measurements were performed using an optical engine, coupled with RANS simulations. The results revealed the following: The variation in the equivalence ratio per cycle has little effect on initial combustion but does influence IMEP. Evaluating the laminar combustion velocity (SL) and turbulent combustion velocity (ST) as factors determining initial combustion revealed almost no correlation with SL, while moderate correlations were observed between ST and CA10. The position of the tumble vortex center at ignition timing was found to be critical; the vortex center position most favorable for advancing combustion timing was located to the right and below the spark plug. Under nitrogen-enriched conditions with a dilution ratio of approximately 40%, cycle fluctuations increased. Concentration characteristics of THC and CO, obtained from high-speed exhaust gas analysis, showed an inverse proportional trend. The flame propagation pattern under nitrogen-enriched conditions followed the vortex center distribution within the three-dimensional tumble vortex structure, riding on a strong tumble. During this process, flame breakup was observed. A model predicting cycle fluctuations during non-combustion was developed and verified against experiments. To investigate whether improving the accuracy of velocity calculations in CFD leads to improved CCV prediction accuracy, the trend of correlation coefficients at each crank angle was examined. Results showed that at several points where flow conditions changed, the velocity values and CCV exhibited the same trend, confirming the importance of improving velocity calculation accuracy.