Investigation of EGR and Miller Cycle for NOx emissions and Exhaust Temperature Control of a Heavy-Duty Diesel Engine

Paper #:
  • 2017-01-2227

  • 2017-10-08
Abstract In order to meet increasingly stringent emissions standards and lower the fuel consumption, significant efforts are being made to develop high efficiency low emission internal combustion engines and after-treatment systems. In addition to the trade-off between thermal efficiency and nitrogen oxides (NOx) emissions, the conversion efficiency of the diesel aftertreatment system decreases rapidly at lower exhaust temperature, which occurs at low load operations. Thus, it is necessary to investigate and identify the appropriate combustion and engine control strategies which can lead to lower vehicle exhaust emission by keeping the engine-out NOx low and NOx aftertreament conversion efficiency high through elevated engine exhaust gas temperature (EGT). In this work, an experimental investigation has been performed using external EGR and Miller cycle achieved by delaying the intake valve closing (IVC) timing on a single cylinder heavy-duty diesel engine. The study was carried out at two different loads of 6 bar and 12 bar IMEP at fixed boost pressures. The averaged in-cylinder gas temperature was calculated by means of a one-dimensional engine simulation model. The results showed that the use of EGR attained lower NOx emissions than the use of Miller cycle. This was mainly attributed to the decreased in-cylinder combustion temperatures and oxygen concentration. At the lower load of 6 bar IMEP, the engine operation with EGR and Miller cycle effectively reduced the levels of NOx with minimum impact on the fuel efficiency and smoke level. In addition, EGT was increased by up to 60°C with 60% lower NOx emissions at this condition through the combined use of EGR and Miller cycle. At the medium load of 12 bar IMEP, the use of Miller cycle with relatively high EGR rate of 16% was effective in reducing the levels of NOx emissions but at the expense of increased smoke and higher fuel consumption. However, a higher diesel injection pressure was found to be effective in reducing soot emissions and improving engine efficiency while maintaining the NOx reduction benefit compared to the baseline. Therefore, the study has demonstrated that the introduction of EGR combined with Miller cycle has potential to reduce vehicle out NOx emissions by keeping higher EGT’s for efficient exhaust aftertreatment systems with little impact on the smoke emission and fuel economy.
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