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The accuracy of low-level emission measurements has become increasingly important, due to the development and implementation of SULEV and PZEV vehicles. One technique to test the low-level measurement performance of a CVS is to inject a known mass of a trace gas, such as propane, into the sample system and verify that substantially all of the mass injected is recovered, typically within 2% of the total injected mass. A Vehicle Exhaust Emission Simulator has been used to inject precise amounts of trace gases with a known accuracy in the range of 0.5% to 1.0%. Recoveries for propane, carbon monoxide, and carbon dioxide are typically 98% or higher, while recoveries for nitrogen oxide are sometimes as low as 95% to 96%. In other words, as much as 5% of the injected nitrogen oxide mass is not recovered by the CVS. This represents an unexpected loss of 3% to 4% of the injected nitrogen oxide.

A single vehicle (3.8L V6) underwent FTP75, HWFE, and US06 emission tests over a 6-month period. A bag mini-diluter and 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 CVS. At one-month intervals, the bag mini-diluter and exhaust flowmeter sampling system was replaced with new units, while the vehicle and CVS remained unchanged. Assuming that the vehicle and CVS produce constant results, the variability of emissions over the test period are highly correlated with the variability of the bag mini-diluters and exhaust flowmeters. The average CO2 mass comparison between the bag mini-diluter and CVS shows the separate sample systems match within 0.5% for an individual test. The established baseline determined from the CVS has a standard deviation of about 2%, which we believe is predominantly due to vehicle variability.