Effects of Low Pressure EGR on Transient Air System Performance and Emissions for Low Temperature Diesel Combustion

Low pressure EGR offers greater effectiveness and flexibility for turbocharging and improved heat transfer compared to high pressure EGR systems. These characteristics have been shown to provide potential for further NO_x, soot, and fuel consumption reductions in modern diesel engines. One of the drawbacks is reduced transient response capability due to the long EGR path. This can be largely mitigated by combining low pressure and high pressure loops in a hybrid EGR system, but the changes in transient response must be considered in the design of an effective control strategy.

The effect of low pressure EGR on transient emissions was evaluated using two different combustion strategies over a variety of transient events. Low pressure EGR was found to significantly lengthen the response time of intake oxygen concentration following a transient event, which can have a substantial effect on emissions formation. The difference in response time between the two EGR systems has important implications for prediction of transient emissions based on steady state mode points since the correlation between transient and steady state emissions may change substantially when low pressure EGR is implemented. NO emissions were found to be the most sensitive to intake oxygen concentration settling time, particularly for the kinetically controlled early injection combustion mode, resulting in large increases in transient emissions relative to steady state levels.