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. In addition, the aerosol mixing tunnel was designed with the aid of CFD simulations to provide a portion of the dilution through the walls of its mixing zone to reduce sample loss from thermophoretic and diffusion deposition.
This paper will explain the engineering design considerations and calculations, as well as the results of flow calibrations and in-use measurements that confirm them. The ultrasonic flowmeter calibrations are verified with a subsonic orifice flowmeter traceable to a standard at the National Institute of Standards and Technology. Pulsation measurements are presented for verification of the dampener performance. Data from the usual propane stratification checks verify the static performance of mixing.
The viability of the use of alternative flowmeters and modified mixing schemes to lower the cost and improve the performance of a large full flow emissions sampling system is confirmed.