Development of Continuous Dilution Factor for CVS Emissions Sampling and Calculation

During the last three decades, the emissions measurement system for heavy duty vehicle testing has employed a Constant Volume Sampler (CVS) system to continuously measure the pollutant concentrations in the dilution tunnel. Subsequent gaseous emissions calculation methods were based on Code of Federal Regulations, Title 40 (CFR-40) in which a formula for calculating dilution factor (DF) was specified to account for background pollutants. However, it is recognized that due to the mechanism of the CVS system, the dilution factor varies from a constant during a test cycle. The DF calculation technique can introduce error in the emissions data, but the magnitude of potential error is small relative to the current emissions standards. However, as the engine technologies improve and cleaner burning fuels are adopted in the near future, the pollutant concentrations from engines will approach those in ambient air. In that case, the current calculation method in CFR-40 can result in errors that are relatively large. In this study, a new approach was developed to calculate instantaneous DF during a test cycle for natural gas vehicles equipped with oxygen sensors. This new method has improved the accuracy of emissions measurement and calculation, which would be desirable when testing vehicles with near zero emissions. The instantaneous dilution factor is also used to calculate the emissions concentrations in the raw engine exhaust prior to being diluted. The emissions characteristics at different engine operation modes were analyzed for natural gas engines and the measures for reducing emissions were also discussed.