Emissions Correlation Between a Partial-Flow Diluter and The Full-Flow Constant Volume Sampler (CVS) for a Heavy-Duty Vehicle Under Steady-State Operation

The California Air Resources Board (CARB) examined the performance of a Partial Flow Sampling System (PFSS) against a reference Constant Volume Sampling (CVS) system in measuring emissions from a heavy-duty vehicle (HDV) during dynamometer testing at CARB's Stockton Heavy-Duty Emissions Laboratory (SL). The SL PFSS system is a Sierra BG-2 system that uses flow-based (rather than CO₂-based) dilution. The CVS system uses the University of California, Riverside's (UCR) Mobile Emissions Laboratory (MEL). The test vehicle was a 2000 model-year HD tractor powered by a CAT C-15 engine. Exhaust samples were collected simultaneously with the SL and MEL systems and analyzed for total particulate matter (PM), oxides of nitrogen (NO_x), carbon dioxide (CO₂), carbon monoxide (CO), and total hydrocarbons (THC). The samples were taken during steady-state vehicle operation. Each test mode was repeated seven times in each of two patterns: consecutive and sequential. The test protocols used by Southwest Research Institute (SwRI) in their benchmark assessment of the performance of PFSS relative to the CVS were followed. R² values of results for SL and the MEL were 0.92 or better. The comparison for PM measurements returned an R² of 0.98. A systematic bias high of PFSS results relative to CVS results was detected. Percent differences in SL average results relative to MEL were mode and pollutant dependent, and on the order of 15%. Some suspect parameters, such as flow rate measurements, will be the subject of further investigation. Most importantly, a simplified low-cost alternative to the CVS for examining PM emissions from existing in-use HDVs using a chassis dynamometer was demonstrated. Although, some differences were note during modal analysis, our findings add to previous evidence that suggests statistical consistency between the two approaches for steady state testing.