The accuracy of low-level emission measurements has become increasingly important, due to the development and implementation of ULEV, SULEV, and PZEV vehicles. Measurement of these decreasing levels of automotive emissions requires new sampling and measuring techniques. Several alternative emission sampling techniques have been investigated to minimize measurement variability and maximize system repeatability. An alternative technique to obtain accurate low-level emissions measurement from SULEV vehicles is the Bag Mini-Diluter, which uses a proportional signal from an Exhaust Volume Measurement Device to sample vehicle exhaust. Crucial to successful proportional sampling of vehicle exhaust flow is the performance of the Exhaust Flow Measurement Device. This study evaluates an Exhaust Volume Measurement Device commonly used with a Bag Mini-Diluter. A Bag Mini-Diluter with Exhaust Flowmeter sample system was installed in series with a CVS, so that mass emission results from an individual test could be directly compared between the Bag Mini-Diluter and the CVS. Difficulties are encountered in operating the Bag Mini-Diluter and CVS in series, and the criteria required to produce repeatable low-level emissions measurements will be addressed.
A test program using different methods of exhaust flow measurement was developed to compare the performance of different Exhaust Volume Measurement Devices. This device must meet several performance requirements to produce accurate test results. The test program covers the performance criteria of the flow measurement devices including but not limited to (1) the absolute accuracy of flow measurement during transient flow conditions, (2) the back-pressure on the vehicle minimized to less than 0.25 kPa during an EPA-FTP75 test cycle, and (3) be able to withstand the exhaust temperatures from various engine types. The tested Exhaust Volume Measurement Devices include ultrasonic flowmeters with different designs, including (1) full-radius and mid-radius ultrasonic signal paths and (2) vertical and horizontal orientation of the flowmeter body. Data from the test program and conclusions relating to the performance of the different Exhaust Flow measurement techniques is presented.