Cycle-to-cycle variations in diesel engines

Cyclic variations in diesel engines are undesirable since they are understood to lead to lower efficiency and higher emissions, as well as power output limitations. This work aims to improve the understanding of the source of cycle-to-cycle variations of in-cylinder pressure in conventional diesel engines and the implications these have for measurement and simulation best practices. Measurements in a single cylinder diesel engine employing single injection, under low temperature end-of-compression conditions and variable charge 02 concentration have shown that in-cylinder pressure fluctuations- i.e. excitation of the first radial mode of vibration of the cylinder gases, caused in single cycles by the rapid premixed combustion - result in increases in diffusion heat release rate. The average intensity of the pressure fluctuations was shown to increase with increasing amount and reactivity of the premixed combustion. This results in higher cycle-to-cycle variations under these conditions, revealed by greater cyclic deviations of maximum pressure. 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. (C) 2016 Elsevier Ltd. All rights reserved.