Cycle-to-cycle variations of NO emissions in diesel engines under long ignition delay conditions

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Cycle-to-cycle variations in combustion have been observed in diesel engines when operating under long ignition delay conditions. In this work, cyclic variations of NO emissions are investigated in a single-cylinder direct injection diesel engine employing a single-injection strategy under long ignition delay conditions. Two measurement sets are used to study the effect of ignition delay and amount of diffusion combustion on cyclic variations. Single-cycle NO concentration was measured using a fast NO analyzer sampling above the exhaust valve. Measurement results confirmed previous investigations reporting the presence of in-cylinder pressure fluctuations – i.e. excitation of the first radial mode of vibration of the cylinder gases by the rapid premixed combustion – which arise intermittently in cycles. The probabilities of cycles showing pressure fluctuations and the intensity of the fluctuations were shown to increase with increasing ignition delay. Pressure fluctuations were confirmed to enhance air–fuel mixing and increase diffusion combustion rate. In cases with significant diffusion combustion, cycles with pressure fluctuations showed higher-than average NO concentrations, with the intensity of fluctuation determining the increase in NO. At low charge temperature, single-cycle NO concentrations measured in cycles with the highest pressure fluctuations were ∼60% higher compared to the lowest values measured at unchanged conditions. On the contrary, measurement points where no diffusion combustion was present showed lower cyclic variation of NO, and no correlation between pressure fluctuation intensity and NO emissions. In cases with high amounts of diffusion combustion, the effect of fluctuations on average NO emissions correlated with increasing ignition delay. At conditions with short ignition delay, the discrepancy in NO concentration between cycles with low pressure fluctuation intensities and the average NO concentration of all cycles was below 0.5%, whereas at high ignition delay this increase was measured to be up to 10%. This indicates that at the latter conditions, the contribution of cycles with high pressure fluctuation intensities results in a significant increase in average NO emissions. In all, the results of this investigation underline the necessity for the determination of the contribution of single-cycles to observed emission trends in diesel engines, with significant implications on measurement and simulation best practices.