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The CMS system commissioned by EPA and built by AVL, is a “start from a clean sheet of paper” approach to a full flow sampling system for aerosol matter from engine exhaust. The challenge of measuring 2007 level post DPF type particulate matter and polyaromatic hydrocarbons led to this re-thinking of sampler design. Previously used CVS designs had evolved to include elements that were not ideally suited for scaling up to large flow rates, and had mixing tunnels that were less than ideal for the sampling of complicated aerosols. The solution presented in this paper used ultrasonic time-of-flight flowmeters in place of the usual Venturi flow tubes, reducing the size and cost of air handling components. Acoustically designed dampeners were used to reduce pulsation disturbances to the flow measurement.

When exhaust emissions from a vehicle are measured, a flow rate is needed in addition to pollutant concentrations in order to calculate the mass emitted. The highly unsteady flow from an internal combustion engine presents measurement challenges to exhaust flowmeters, especially at idle. Mass measurement methods used in the past have gotten around this problem by a variable dilution scheme (CVS) that measures a different, more favorable flow, but reduces and can contaminate exhaust gas concentration levels. The flow measurement system described here makes possible a more accurate measure of the vehicle exhaust flow by means of a number of design features. This improves considerably the cost effectiveness and accuracy of emissions measurement techniques such as the Bag-Minidiluter sampling system and raw modal analysis.