Cooled EGR Rate Measurement with a Thermal Anemometer for EPA02 Heavy Duty Diesel Engine Emission Control

Cooled EGR is one of the engine technologies that has been certified by the EPA for on-highway heavy duty diesel engines to meet the EPA October 2002 2.5 g/bhp-hr NMHC + NO_x and 0.1 g/bhp-hr particulate matter exhaust emission regulation. Cooled EGR as the primary exhaust emission control reduction technology also minimizes the fuel economy penalty associated with this exhaust emission regulation. The cooled EGR system however requires precise EGR rate of flow control in a very unfriendly environment that includes acidic exhaust gas condensates, static pressures up to 4 Bar, temperatures over the entire range of -40 to 250° C, and high engine vibration levels. Several technologies have been proposed and evaluated to achieve a closed loop feedback signal for the EGR flow control valve and VGT (Variable Geometry Turbocharger) vane position. These technologies include flow derived from the ratios of temperatures of EGR gas and intake air, difference in calculated (speed density) total combustion air flow to measured fresh intake air flow, ratios of commanded A/F ratio to measured exhaust A/F ratio, and delta pressure measurements across a restriction in the EGR tube. This paper will describe a new and improved direct measurement of EGR rate of flow by means of a high temperature thermal anemometer directly installed in the EGR tube. The sensor principle of operation, application in an on-highway heavy duty diesel engine, and engine performance advantages will be included.