Cycle-to-cycle variations in diesel engines

A study of the single cycle pressure fluctuation intensity under long ignition delay conditions revealed that despite the increase of average intensity of fluctuations with increasing premixed reactivity, even under these conditions some cycles exhibited no pressure fluctuations. This indicates that the high premixed combustion rate is not in itself a sole prerequisite for the onset of the resonance of cylinder gases, with other, “random” effects also required to induce the pressure oscillations. Finally, under low temperature charge conditions the difference in peak pressure between the average cycle and cycles showing low fluctuation intensity was measured to be >3 bar (>3% of the peak). In all, these results underline the need for further understanding of the source of the pressure fluctuations and the effect of these cyclic variations on single-cycle and average emissions. The effect of the highly fluctuating cycles on the average cycle should lead the scientific community conducting measurements and simulations on such engines to reconsider the best practices for acquiring, evaluating and interpreting measured data. In addition, the community should acknowledge the contribution of extreme cycles when averaged data is used for simulation validation.