An Investigation of SF6 Gas for Testing Instrumental Integrity of the Emerging SULEV/PZEV Measurement Technology

For the past several years, manufacturers have been developing emission measurement systems for Super Ultra Low Emission (SULEV) measurements. The Bag Mini-Diluter (BMD) with an advanced exhaust flow measurement device is designed as an alternative to the traditional method for sampling vehicle exhaust, the constant volume sampler (CVS).

Exhaust sampling instruments require system verification tests. The system verification test described and mandated for the CVS in the Code of Federal Regulations (CFR) §86.119-90(c) is a simulated test with propane. The very low concentration measurements required for SULEV regulations demand a more enhanced and accurate verification technique and procedure than the method described in the CFR.

This investigation focuses on the technique and necessary equipment for verifying system integrity of the entire emission sampling system, including the Bag Mini-Diluter and the exhaust flow measurement device in the test cell. The technique involves the precisely controlled injection of an inert check gas (SF₆) into the exhaust gas stream. This technique has significant advantages over the propane injection for the CVS check since the check gas (SF₆) is absent from the background measurement providing more accurate detection. Additionally, the check test is also run in situ. This test is a real verification of system integrity performed during an automotive test cycle, not just a simulation like the propane injection. Measurement of this test gas (SF₆) relies on a sensitive NDIR analyzer. In this investigation, practical operational suggestions are presented along with data to show the efficacy of the verification system.