Strategies for High EGR Rates in a Diesel Engine

The potential to achieve future NOx targets for a heavy-duty diesel engine with low and high-pressure EGR solutions is controlled by the choice of boosting system. A cycle simulation model representing a current production 6-cylinder truck diesel engine with variable turbine geometry was used to investigate the impact of EGR while maintaining air excess ratio and BSFC.

The disadvantages of the high EGR rates necessary are discussed in terms of increased peak cylinder pressure, compressor work, pumping power and heat rejection requirements. The importance of choosing the correct compressor characteristics for the specific EGR system is demonstrated. Low-pressure systems can achieve EURO 4 limits of 3.5 g/kWh but the increased compressor inlet temperature becomes critical at the high EGR rates necessary for EURO 5. It is possible to reach EURO 5 limits of 2.0 g/kWh NOx with a high pressure EGR loop if the boost characteristics can be extended beyond the current single stage turbocharger to provide greater pressure ratios.