Modeling Vehicle Emissions in Hot-Stabilized Conditions Using a Simultaneous Equations Model

The purpose of this study is to incorporate the correlation between different types of emissions into microscale vehicle emissions models. Simultaneous equations models were calibrated for two vehicle categories classified in a National Cooperative Highway Research Program project. One represents normal emitting vehicles, while the other represents high emitting vehicles. Among various vehicle operation conditions, only those represented by Bag 2 of the Federal Test Procedure cycle were considered in this study. The developed models were evaluated based on the criteria of root mean square errors and correlation coefficients between the estimated and measured emissions. The results indicate that the correlation between emissions of CO and HC can be significantly identified; thus, these two emission types should be included in the simultaneous equations models. In addition, the results indicate that simultaneous equations models can achieve better and reliable emissions estimation results compared with other vehicle emissions models, which implies that it is worthwhile to apply simultaneous equations models to other operation conditions.