Reduction of NOx Emissions from a Light Duty DI Diesel in Medium Load Conditions with High EGR Rates

The expansion of current driving cycles for emission regulations to higher load operation in the near future (such as the US06 supplement to the FTP-75 driving cycle) requires attention to low emission combustion concepts in medium to high load regimes. One possibility to reduce NO emissions is to increase the EGR rate. The combustion-temperature reducing effects of high EGR rates can significantly reduce NO formation, to the point where engine-out NOx emissions approach zero levels. However, engine-out soot and CO emissions typically increase at high EGR levels, due to the reduced soot and CO oxidation rates at reduced combustion temperatures and oxygen concentrations. The work presented in this paper focuses on different strategies to reduce soot and CO emissions associated with EGR rates of up to 50%, at which NO formation is largely avoided, but combustion temperatures are not low enough to consider the process as Low- Temperature Combustion (LTC). The studied strategies include use of high injection pressures (up to 1800 bar), increased swirl and increased boost pressures. Using a combination of these measures, soot emission levels could be reduced to 0.04 g/kWh and NOx emissions to 0.34 g/kWh at a medium engine load of 10 bar IMEP in a single cylinder research engine