Correlation Study of PM and NOx for Heavy-Duty Vehicles Across Multiple Drive Schedules 2004-01-3022

When heavy-duty truck emissions are expressed in distance-specific units (such as g/mile), the values may depend strongly on the nature of the test cycle or schedule. Prior studies have compared emissions gained using different schedules and have proposed techniques for translating emissions factor rates between schedules. This paper reviews emissions data from the 5-mode CARB HHDDT Schedule, UDDS Schedule, and a steady-state cycle (AC5080), with reference to each other. NO_X and PM emissions are the two components of emissions which are reviewed. A heavy-duty chassis dynamometer was used for emissions characterization along with a full scale dilution tunnel. The vehicle test weights were simulated at 30,000 lbs, 56,000 lbs, and 66,000 lbs. For each vehicle, average data from one mode or cycle have been compared with average data for a different mode or cycle. When the Cruise Mode and Transient Mode of the HHDDT Schedule were compared, it was evident that injection timing strategies affected the average Cruise Mode emissions of NO_X. As a result, there was substantial data scatter when mode-averaged Cruise emissions were plotted against mode-averaged transient emissions. Moreover, the relationship between Cruise and Transient NO_X was not the same for the various test weights. Correlations for PM varied widely in goodness of fit to the data. This is because the PM may increase substantially due to elemental carbon “puff” which occurs when the turbocharger has not reached full boost pressure at the onset of an increase in engine load. The AC5080, which is a short test originally developed for inspection and maintenance applications, showed reasonable correlation with the UDDS Schedule, although a best fit line still caused the AC5080 to mis-predict 16 out of 25 UDDS NOX values by over 20%. It was concluded generally that information is needed on both the transient and steady high speed emissions characteristics of a vehicle before an emissions factor can be estimated for a road link.