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Vehicle chassis dynamometer tests were performed to compare predicted and measured total feedgas emissions and fuel economy for dynamic operation of an engine. In general, these tests showed that predictions based on steady-state mapping data agreed well with measured values except for NOx emissions. Subsequent engine-dynamometer tests indicated that the discrepancy between predicted and measured NOx emissions was due to competing effects of combustion chamber wall temperature and dynamic EGR time response. A technique was developed which utilized the results of a simple transient test to improve the accuracy of predicting NOx emissions when EGR time response was not a factor. The effect of degraded EGR time response on both instantaneous and total NOx emissions was also explored.

Quantitative measurements were made of NOx feedgas emissions during transient engine operation as the response time of an EGR system was progressively-degraded. For a simple acceleration-cruise-deceleration engine speed/torque versus time trajectory, it was found that the NOx emissions were higher during acceleration and lower during deceleration than corresponding values predicted from steady-state mapping data. The magnitude of the differences, as well as the total mass of NOx integrated over the speed/torque trajectory, all increased as the EGR response time was increased. Using a simple dynamic EGR model, NOx feedgas emissions were predicted for engine operation with a production EGR system over a 128 second portion of the FTP CVS cycle. The NOx feedgas predictions were shown to be in excellent agreement with actual emission measurements.