Tech Briefs
Improving emissions test equipment accuracy

Modern limits for vehicle exhaust emissions have become so low that they challenge the precision of traditional laboratory equipment and procedures. Southwest Research Institute (SwRI) has formed a consortium to improve the ability of test equipment to measure vehicle exhaust emissions consistently at very low levels.

The consortium, called IMPROVE (Investigating Measurement Precision and Repeatability Of Vehicle Emissions), will examine technologies and procedures that can be quickly and cost-effectively added to existing vehicle emissions test equipment. It also will evaluate alternative or novel technologies and procedures that hold promise for significantly reducing measurement variability at very low pollutant levels.

"Cost-effective measurement of the very low pollutant levels from vehicles capable of meeting future emissions standards will require improvements to test equipment and procedures," said Bill DiSilverio, Research Engineer in SwRI's Automotive Products and Emissions Research Division.

Exhaust emissions test results have always been affected by several sources of variability such as the driver, emissions instrumentation, vehicle driveability, and ambient conditions. Meanwhile, vehicle exhaust emissions standards continue to become more stringent. The LEV II super-ultra-low emissions vehicle (SULEV) standards adopted by California include hydrocarbon and oxides of nitrogen requirements that are an order of magnitude lower than those of the early 1990s. These further reductions in tailpipe exhaust emissions concentrations mean that measurement errors with existing emissions test equipment become more significant.

Several studies have attempted to identify the variability present in exhaust emissions testing, but these were conducted in the 1970s prior to the emergence of multi-point fuel injected engines, high-speed data acquisition systems, and LEV II standards. Although much can be learned by reviewing previous studies, a thorough investigation of FTP test-to-test variability, and the reduction of such variability, has not been published in 20 years. However, a recent study of emissions instrumentation has shown the estimated error of conventional exhaust-gas analyzers to be 15% or more at a sample level equal to the LEV II SULEV standards (SAE Paper No. 982555).

Several techniques have been proposed for reducing measurement variability during vehicle exhaust emissions testing, including modal methods, mini diluters, and techniques for reducing dilution air requirements. In addition, the American Industry/Government Emissions Research (AIGER) Cooperative Research and Development Agreement (CRADA) has focused on identifying and developing new instrumentation and techniques for measuring exhaust emissions, with the goal of ensuring commercialization and availability of these new technologies.

Several manufacturers of emissions test equipment already have agreed to allow SwRI to evaluate their products, including emissions test benches and advanced technology concepts. Membership will allow companies to determine the steps needed to retrofit their equipment to minimize test variability, and to evaluate the next generation of emissions test equipment, in an open and cooperative forum.

Jean L. Broge