A Comparison of Tailpipe Gaseous Emissions for RDE and WLTC Using SI Passenger Cars

The drive characteristics and gaseous emissions of legislated Real Driving Emissions (RDE) test data from 8 different spark ignition vehicles were compared to data from corresponding Worldwide harmonized Light vehicles Test Cycle (WLTC) tests. The effect of the official RDE exclusion of cold start and idling on the RDE test, and the effect of the use of the moving averaging window (MAW) analysis technique, were simultaneously investigated. Specific attention was paid to differences in drive characteristics of the three different driving modes and the effect this had on the distance-based CO₂, CO and NO_x emission factors for each. The average velocity of the RDE tests was marginally greater than the WLTC tests, while the average acceleration was smaller. The CO₂ emission appeared on average 4% lower under the RDE tests compared to the WLTC tests, while the CO was 60% lower. The NO_x values were 34% lower under the RDE testing, and appeared to be linked to the average acceleration. No link was seen for the maximum acceleration or deceleration, indicating that this is not a good indicator for test cycle emissions. The exclusion of cold start and idling decreased all RDE emissions. RPA (Relative Positive Acceleration) had little correlation with CO₂, CO and NO_x distance-based emissions, and was shown to be uncorrelated with any mass-rate emissions. The range of RPA values seen was much greater for RDE tests than WLTC tests, with individual RDE tests having variable values for each drive mode. The application of the MAW technique had minimal effect on the CO₂ distance-based emission, but it appeared to decrease CO and NO_x emissions by 12% and 21% on average respectively. The MAW also decreased the variation in emissions across different modes.