Exploring the NOx reduction potential of Miller cycle and EGR on a HD diesel engine operating at full load

Paper #:
  • 2018-01-0243

  • 2018-04-03
The increasing demand for nitrogen oxides (NOx) reduction of diesel engines without sacrificing the total cost of ownership (TCO) which including the diesel fuel consumption and the aqueous urea solution consumption in the selective catalytic reduction (SCR) aftertreatment system, requires the development of more advanced combustion solutions. One promising strategy that can curb NOx emissions with a low fuel consumption penalty is to simultaneously reduce the combustion temperatures and pressures using Miller cycle and decrease the in-cylinder oxygen concentration by means of exhaust gas recirculation (EGR). As such, this strategy can minimize both diesel fuel and urea consumptions, enabling a low TCO. In this work, Miller cycle with late intake valve closing (IVC) and EGR technology were investigated on a single cylinder common rail heavy-duty diesel engine at a 24 bar indicated mean effective pressure (IMEP). The experiments were performed with a constant boost pressure of 3.0 bar and the fuel injection timings were optimized up against the peak in-cylinder pressure limit of 180 bar. The results revealed that the use of a Miller cycle strategy without EGR lowered combustion temperatures and pressures, reducing NOx emissions by 34% and the engine efficiency by 3% in comparison with the baseline IVC timing. The introduction of 8% EGR at the baseline IVC timing decreased the levels of NOx by 54% while maintaining a similar engine efficiency. The combination of Miller cycle and EGR achieved the highest NOx reduction, decreasing the NOx levels of the baseline case by 65% and the engine efficiency by 3.5%. Soot and carbon monoxide (CO) emissions increased and could be significantly reduced by advancing the diesel injection timing. Unburnt hydrocarbon (HC) emissions were decreased with EGR and late IVC timings. To demonstrate the effectiveness of the strategies in terms of TCO, the aqueous urea solution consumption in the SCR aftertreatment system was estimated. The calculation was based on the engine-out NOx emissions and the Euro VI NOx limit. The analysis showed that the combination of Miller cycle and EGR achieved the lowest TCO via lower the engine-out NOx emissions and a relatively high engine efficiency.
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