Air quality assessments for road schemes typically use average speed related emission rates, expressed in terms of vehicle type, Euro emission class, size of vehicle and fuel. These emission rates vary by average speed with higher emissions typically at low and high speeds. The use of average speed as a surrogate for emissions from modern vehicles is becoming less robust, and in addition many of the operational and management regimes currently being introduced or piloted for application to the high speed road network are designed to change the style of driving (that is the proportion of time spent accelerating or decelerating or the range of speeds encountered during a trip), rather than specifically the average speed.
Assessment tools that are able to incorporate the driving characteristic changes are therefore needed. The MODEM emissions database and model has been applied to these types of situation. However, the MODEM model is now some 10 years old, and whilst it has been modified to include current and future Euro class vehicles, the uncertainty associated with its output has increased. Therefore this project was aimed at comparing the results from MODEM and current emissions databases with emission measurements from existing vehicles using a variety of monitoring and modelling techniques.
Between 15 and 17 February 2005, three passenger cars were equipped with on-board exhaust measurement systems and repeatedly driven over the M42, between junction 3 and 7. Petrol vehicles were equipped with the TRL GasScan system and the Leeds University Horiba OBS system, whilst a diesel vehicle was equipped with the Imperial College VPEMS and Montana systems. This allowed the continuous measurement of CO, HC, NOX, CO2 and PM (the latter from the diesel vehicle only), and the derivation of fuel consumption. In addition to the direct measurement of exhaust emissions, each vehicle recorded the driving characteristics over each of the M42 trips. These driving characteristics were subsequently processed and used to derive emission estimates using a range of commercial and research emission modelling tools, including the DMRB, MODEM, PHEM and VERSIT+.
The investigation confirmed that all systems were capable of continuously measuring exhaust emissions and operational characteristics. These operational characteristics were subsequently used as input into emission modelling tools, allowing the derivation of average and continuous emission estimates. All the measured emissions showed some similarities and some differences with modelled emissions. Of particular interest was the observation of emission peaks during normal driving conditions. These short duration events appeared to contribute to the majority of the emissions over the specific trip. Some of the trips appeared to exhibit a partial cold-start like effect at the beginning of the run, even though the vehicles were nominally warm. Significantly, it was noted that all of the measurement systems and mod

Want to know more about this project?