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

摘要

In order to meet increasingly stringent emissions standards and lower the fuel consumption of heavy-duty (HD) vehicles, significant efforts have been made to develop high efficiency and clean diesel engines and aftertreatment systems. However, a trade-off between the actual engine efficiency and nitrogen oxides (NO_x) emission remains to minimize the operational costs. In addition, the conversion efficiency of the diesel aftertreatment system decreases rapidly with lower exhaust gas temperatures (EGT), which occurs at low load operations. Thus, it is necessary to investigate the optimum combustion and engine control strategies that can lower the vehicle's running costs by maintaining low engine-out NO_x emissions while increasing the conversion efficiency of the NO_x aftertreament system through higher EGTs. In this work, an experimental investigation has been performed on a HD diesel engine using external exhaust gas recirculation (EGR) and Miller cycle, which was achieved by delaying the intake valve closing (IVC) timing via a variable valve actuation (VVA) device. The study was carried out at two different loads of 6 bar and 12 bar indicated mean effective pressure (IMEP) at fixed boost pressures and constant engine speed of 1250rpm. The averaged in-cylinder gas temperature and burned zone gas temperature were calculated with a one-dimensional engine simulation model based on the experimental pressure measurement. The results revealed that the engine operation with EGR and Miller cycle effectively reduced the levels of NO_x with minimum impact on the fuel efficiency and smoke level at 6 bar IMEP. In addition, EGT was increased by up to 60°C for a 70% NO_x emissions reduction when combining the 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 NO_x emissions at the expense of increased smoke and higher fuel consumption. Nevertheless, a higher diesel injection pressure was found to be effective in reducing soot emissions and improved engine efficiency while maintaining a NO_x reduction benefit compared to the baseline case. Overall, the results demonstrated that the combination of the EGR and Miller cycle strategies can lead to minimum impact on the smoke emission and fuel economy while achieving lower engine-out NO_x emissions and higher EGTs for efficient exhaust aftertreatment systems.