Modal-Based Intermediate Soak-Time Emissions Modeling

A comprehensive modal emissions model for light-duty cars and trucks is being developed under the sponsorship of NCHRP Project 25-11. Model development has been described previously for vehicles operating under hot-stabilized conditions. A modal emissions model is presented for vehicles operated under incremental soak-time conditions. The Federal Test Procedure (FTP) measures vehicle emissions after a 24-h soak time during Bag 1 testing and vehicle emissions after a 10-min soak time during Bag 3 testing. Vehicle incremental soak-time emissions refer to vehicle emissions after intermediate variable soak times of between 10 min and 24 h. Recent research shows that most on-road vehicles experience soak times of between 10 min and 24 h during daily driving; thus, there is strong desire to model vehicle emissions under such circumstances. An intermediate soak emission model has been developed on the basis of second-by-second emissions measurements generated at the College of Engineering Center for Environmental Research and Technology, University of California–Riverside vehicle testing facility by using the FTP Bag 1 and Bag 3 test cycles. The modeling results are based on a composite vehicle concept in which more than 300 tested vehicles are composited into two dozen vehicle technology groups. The modeling approach is a fuel-based physical modal emissions model in which vehicles’ fuel use, engine-out emissions, air/fuel equivalence ratio, catalyst efficiencies, and tailpipe emissions are modeled individually as a function of variable soak time. Since the developed model is based on modeled vehicle fuel consumption under any given driving cycle, it not only is capable of predicting vehicle emissions under variable soak time for any given test cycle but also is capable of predicting emissions under different starting test cycles.