Influence of Exhaust Gas Composition on Measured Total and Exhaust Flow Using a CVS with Critical Flow Venturi and Smooth Approach Orifice

The Constant Volume Sampler (CVS) is often used to dilute automotive exhaust with ambient air for measurement of emissions from light duty vehicles. A CVS is traditionally equipped with Critical Flow Venturi (CFV) to control and measure total flow. If the CVS is equipped with a Smooth Approach Orifice (SAO) to measure dilution air flow, the exhaust flow of the vehicle can be calculated as the difference between dilution and total flow. Calibration of the CVS and SAO is routinely done using ambient air, but carbon dioxide (CO₂) and water vapor in diluted exhaust have an influence on the flow through the CFV. In current US emission legislation the provisions to include water vapor is added. However, if this is done then the effect of carbon dioxide (CO₂) in exhaust has not been considered. Further on, when using the CVS to measure exhaust flow, only the CFV will be affected by the diluted exhaust gas composition. The measurement of dilution air will still be unaffected and thus will the resulting exhaust flow be influenced by the unbalanced conditions of the CFV compared to the SAO. Even though the effect of gas composition on flow may be small for the CFV, the influence will be much larger on measured exhaust flow since this is calculated from the difference of two large flows. This paper presents how measurements on exhaust flow are influenced by diluted exhaust gas with different composition. The results are then compared with theoretical studies, and differences are discussed.