Quantifying Vehicle Emission Factors for Various Ambient Conditions using an On-Road, Real-Time Emissions System

This paper demonstrates vehicle emission factor measurement using an on-board, on-road system and examines the effects of ambient temperature on those emission factors. Vehicle operating parameters, fuel consumption and emissions were measured on-road using a portable measurement system designed for ease of use with a range of vehicles, drivers and driving situations. The results reported here come from repeated trips over a 17.4 km urban / suburban route with a particular driver and vehicle. As such, the emission factors developed here do not represent the current on-road fleet. However, they show the strong influence of actual operating conditions (particularly ambient temperature) and of the vehicle control system's response to non-standard conditions. This leads to an appreciation for on-road testing as a means to illustrate vehicle emission behavior in real conditions and to highlight conditions which may require more detailed study.

A series of trips over a one year period, (with an ambient temperature range of -25 to +20°C), were analyzed to develop emission factor models with one, two or three emission factors. The results emphasize the inadequacy of using a single “grams-per-mile” emission factor to model emissions of vehicles operating over a range of trip lengths and ambient conditions. A two-emission-factor model provides adequate information to accommodate short trip lengths and a three-emission-factor model can fit the detailed behavior but is generally not required.