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Current SAE practices for evaluating potential improvements in fuel economy on heavy-duty vehicles rely on gravimetric measurements of fuel tanks. However, the recent evolution of portable emissions measurement systems (PEMS) offers an alternative means of evaluating real-world fuel economy that may be faster and more cost effective. This paper provides a direct comparison of these two methods based on a recent EPA study conducted at Southwest Research Institute. More than 228 on-road tests were performed on two pairs of class 8 tractor-trailers according to SAE test procedure J1321 in an assessment of various chassis components designed to reduce drag losses on the vehicle. During these tests, SEMTECH-D™ portable emissions measurement systems from Sensor's, Incorporated were operating in each of the vehicles to evaluate emissions and to provide a redundant measure of fuel economy.

As part of the EPA's ongoing in-use vehicle emissions compliance program, two hybrid vehicles were tested. A 2001 Honda Insight and a 2001 Toyota Prius were tested on a chassis dynamometer at the National Vehicle Fuel and Emissions Laboratory. Both vehicles were run on the Federal Test Procedure drive cycle. Emissions were measured simultaneously using the SEMTECH-G Portable Emissions Measurement System (PEMS) for comparison to the chassis dynamometer bench analyzer. The correlation for cumulative emissions (bag) between dynamometer and on-board analyzer were adequate for CO2, CO, and NOx, though the PEMS HC measurements were low by comparison. The measurements were limited by the very low exhaust volume flow rates on both vehicles. A more sensitive flow meter will be required to measure emissions from vehicles certified to SULEV standards.