Further Investigation of Losses in NOx Recovery with a CVS

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 test program (Ref. 1) characterized the loss of nitrogen oxide in this type of CVS injection test. An analysis of the results indicated that the mass lost is simply related to the concentration of the diluted nitrogen oxide. A correction function based on this result correctly predicts the mass of NOx recovered in the CVS bags over a wide range of CVS operating conditions. Experiments indicated that the lost mass was localized to the flexible silicone rubber hose leading into the CVS. This paper describes further experiments which indicate that the loss of nitric oxide is primarily due to the diffusion of the gas into the polymer matrix of the flexible silicone rubber hose.