The Effect of Cycle-to-Cycle Variations on the NOx-SFC tradeoff in Diesel Engines under Long Ignition Delay Conditions

Paper #:
  • 2017-24-0100

Published:
  • 2017-09-04
Abstract:
Cycle-to-cycle variations in internal combustion engines are known to lead to limitations in engine load and efficiency, as well as increases in emissions. Cyclic variations are typically encountered in premixed, externally ignited engines as a result of the effect of turbulence on combustion, particularly during the early flame development. In compression ignition engines combustion is primarily spray-driven, and thus low cyclic variations are commonly expected. Recent research [1-3] has led to the identification of the source of cyclic variations of pressure, soot and NO emissions in direct injection common rail diesel engines, when employing a single block injection and operating under long ignition delay conditions. The variations in peak pressure arise from changes in the diffusion combustion rate, caused by randomly occurring in-cylinder pressure fluctuations. These fluctuations appear on the indication diagram as super-imposed pressure waves, and result from the excitation of the first radial mode of vibration of the cylinder gases. The excitation arises from the rapid premixed combustion after the long ignition delay period. Cycles with high-intensity fluctuations present faster diffusion combustion, resulting in higher cycle peak pressure, as well as higher measured exhaust NO concentrations. The aim of this study is to determine the effects of pressure fluctuations on the NOx-specific fuel consumption (SFC) tradeoff. The investigation is performed on a single-cylinder heavy-duty diesel engine, and focuses on the variation of injection parameters (injection timing and pressure) under constant, long ignition delay conditions achieved using Miller valve timing. Single-cycle NO concentrations are measured using a fast-NO analyzer placed near the exhaust valve, whereas the indicated SFC is determined from the measured cylinder pressure. The results show that the presence of pressure fluctuations is not beneficial for the NOx-SFC tradeoff, with the cycles showing higher fluctuation intensities leading to higher emission levels for constant indicated SFC. [1] P. Kyrtatos, C. Brückner, K. Boulouchos, Cycle-to-cycle variations in diesel engines, Applied Energy 171 (2016) 120-132. [2] P. Kyrtatos, A. Zivolic, C. Brückner, K. Boulouchos, Cycle-to-cycle variations of NO emissions in diesel engines under long ignition delay conditions, Combustion And Flame, doi:http://dx.doi.org/10.1016/j.combustflame.2016.12.025(2016). [3] P. Kyrtatos, K. Hoyer, P. Obrecht, K. Boulouchos, Apparent effects of in-cylinder pressure oscillations and cycle-to-cycle variability on heat release rate and soot concentration under long ignition delay conditions in diesel engines, International Journal of Engine Research 15 (2014) 325-337.
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