Reproducibility and Accuracy of On-Board Emission Measurements Using the RAVEM™ System

Portable systems capable of measuring mass exhaust emissions while riding on-board vehicles and mobile equipment are relatively new. Their advantages include lower measurement costs, the ability to measure emissions under realistic operating conditions (including conditions that are difficult to simulate in the laboratory), the ability to measure emissions where no traditional laboratory is available, and applicability to a broader range of engines and vehicles than those addressed by traditional laboratory methods (e.g. construction machinery). However, on-board methods have not yet been fully accepted in the air quality community. This is due in part to concerns over their reproducibility and accuracy, and the adequacy of the corresponding quality assurance and quality control measures. This paper draws on four years of experience with on-board emission measurements using the “Ride-Along Vehicle Emission Measurement” (RAVEM) system in vehicles ranging from diesel and natural gas trucks to diesel ferryboats and locomotives. Quality assurance methods such as carbon-balance and CO₂ recovery checks of emission measurement accuracy have been developed, together with driving techniques to generate reproducible on-road emissions data. Results are presented showing repeatability (based on the standard deviation of repeated emission measurements over the same driving route) generally within 6% for CO₂ and NOx (in the absence of NOx aftertreatment systems), and within 10% for PM. Comparison of fuel consumption calculated by carbon balance to the mass fuel consumption determined by weighing the fuel tank shows agreement generally within 1 to 4%.