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

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. Recent research 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 result from the excitation of the first radial mode of vibration of the cylinder gases which 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. In this study the effects of pressure fluctuations on the NOx-specific fuel consumption (SFC) tradeoff are determined. 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. 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.