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As automakers begin to develop and certify vehicles that meet the California Air Resources Board LEV II and Environmental Protection Agency Tier II Regulations, emissions test cells must be designed and implemented that are capable of accurate low-level measurements. A new test cell has been installed at Ford Motor Company for use in testing vehicles that meet the stringent Partial Zero Emission Vehicle tailpipe requirements (NMOG = 10 mg/mile, NOx = 20 mg/mile). This test cell includes a redesigned Bag Mini-Diluter (BMD), improved analytical benches, an ultrasonic exhaust flow meter with an integrated tailpipe pressure control system, a conventional constant volume sampler (CVS), and a moveable electric dynamometer. The Bag Mini-Diluter will be used as the primary sampling system for the tailpipe measurements. The moveable electric dynamometer enables the test cell to be configured so that the vehicle is moved to the test equipment rather than moving the test equipment to the vehicle.

Two test techniques are currently used for the measurement of evaporative hydrocarbon emissions during the running loss portion of the Enhanced Evaporative Emission Test Sequence: point source and enclosure. The point source technique uses a constant volume sampling (CVS) system to sample and measure running loss emissions at the vehicle's discrete vapor vents. The enclosure technique uses atmospheric sampling within a sealed housing for evaporative determination (SHED) to measure running loss emissions. A test program was carried out to compare the two measurement techniques and investigate the differences between the measurements. During the test program, point source emissions averaged 0.001 grams/mile (0.0006 grams/km), or 2% of the standard (0.05 grams/mile or 0.031 grams/km), while enclosure emissions averaged 0.021 grams/mile (0.013 grams/km), or 41% of the standard.

The on-going regulatory emphasis on reducing vehicle emission levels and increasing fuel economy has resulting in numerous efforts to improve the technology used to obtain these measurements. For example, raw exhaust flow meters are being implemented to provide improved tailpipe (i.e. modal) emission measurements as part of the vehicle development process. In addition, new sampling system technologies (e.g. partial flow dilution systems) are being implemented that use either synthetic air or catalyzed/dried ambient air to dilute exhaust samples. With these new technologies, however, often comes an increased sensitivity to leakage from both the test equipment and from the hoses/connectors used to route the vehicle's exhaust gas to the test equipment. In the case of the exhaust flow meter, a leak upstream of the measurement zone will provide an inaccurate flow reading. With an emissions sampling system, an exhaust leak will result in inaccurate emissions and fuel economy readings.