A Comparison of Direct Vehicle Fuel Consumption Measurements with Simultaneous CVS Carbon-Balance Fuel Economy 2008-36-0274

A chassis dynamometer test cell is employed along with a CVS system to test for both vehicle mass emissions and for fuel economy. In addition to the standard test equipment for gasoline vehicles, a highly accurate fuel flow meter is installed that measures the mass of fuel consumed by the engine during chassis dynamometer tests. A 3.8 liter V6 vehicle was tested over standard United States E.P.A. FTP and Highway Fuel Economy protocols, where it is found that the fuel flow meter mass measurements correlate with the carbon-balance fuel consumption results measured with the CVS. However, there are significant differences between the fuel flow meter and the CVS measured fuel consumption during vehicle cold-start tests. This is a concern because a large fraction of gasoline engine emissions are generated only during a cold-start. It is important to be able to relate mass emissions to mass fuel consumption in order to understand and control cold-start gasoline engine performance.

Experiments are performed using two different mass fuel flow metering systems where equivalent results are obtained. The carbon-balance fuel economy performance of the CVS is compared to that obtained with a partial-flow dilution sample system, namely a Bag Mini-Diluter paired with an ultrasonic Exhaust Flow Meter. The CVS and Bag Mini-Diluter systems correlate well over all test protocols, with the largest differences observed only during vehicle cold-start tests. The Exhaust Flow Meter is operated in series with the CVS, and the ultrasonic flowmeter and the CVS calculated tailpipe exhaust volume are statistically equivalent. The conclusion from this series of experiments is that the CVS and Mini-diluter sampling systems may have limitations under cold-start vehicle test conditions that are related to condensation in the vehicle exhaust system. This experiment suggests that significant errors may be associated with cold-start tests using vehicle exhaust-based fuel consumption measurement techniques.