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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.

Emissions from heavy-duty vehicles in real operation on the road often differ greatly from those that would be projected from laboratory testing. Reasons for this difference include variations between laboratory and real-world driving conditions, wear and deterioration that are not effectively modeled by laboratory tests, inadequate or inappropriate in-use maintenance, and the use of “cycle-beating” strategies and “defeat devices” by engine manufacturers. This paper analyzes data showing in-use emissions from heavy-duty diesel and natural gas vehicles tested using various driving cycles on chassis dynamometers. It is shown that average in-use emissions of particulate matter (PM) and oxides of nitrogen (NOx) from late model heavy-duty diesel engines are much higher than predicted by current emission models, and greatly exceed the emission standards to which these engines were certified.