LP EGR Calibration Strategies

Low Pressure EGR Calibration Strategies for Reliable Diesel Particulate Filter Regeneration on HDD Engines

SAE Reference No 2002-01-2780
Soren Andersson, Claes Akerlund, and Micael Blomquist, STT Emtec AB

Low pressure Exhaust Gas Re-circulation (EGR) can be used along with Diesel Particulate Filter (DPF) systems to reduce NOx emissions from heavy-duty diesel engines. Normally the use of EGR decreases NOx emissions and increases soot emissions.

The effect of low pressure EGR on DPF regeneration has been investigated in connection with two passive DPF systems. One system contained a catalyst followed by an uncatalyzed DPF and one system was a Catalyzed DPF (CDPF). The objective was to obtain a better understanding of soot oxidation and to develop EGR calibration strategies that would ensure reliable DPF regeneration.

Formation of NO2 and the subsequent reaction of NO2 with soot in the DPF, was studied as a function of temperature, EGR rate, and exhaust gas flow rate under steady state conditions for a range of DPF soot preloads.

For the catalyst plus uncatalyzed DPF, NO2 emissions after the catalyst reached their highest levels at intermediate loads. Soot oxidation started at 200 °C and the rate increased exponentially with temperature. The reaction 2NO2+C? CO2+2NO prevailed over the reaction NO2+C? CO+NO. The influence of NO2 concentration on soot oxidation became greater as temperatures increased. Soot oxidation rate was directly proportional to soot loading.

For the CDPF, soot loading had only a minor effect on gaseous activity. Soot oxidation rates at different temperatures were similar for the two systems. The CDPF was less dependent on changes in soot loadings and NOx concentrations.

The results obtained were used to predict the effects of insulating the exhaust pipe, catalyst size, DPF size, and different EGR rates. EGR calibration strategies for passive STT Emtec AB DPF systems will be limited by the engine-out soot emissions and engine NOx/soot trade-off.

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