Which vehicle emissions test cycle is most demanding?

After we published our new test cycle conversion factors, a question came up: which of the test cycles is the most demanding, in terms of its impacts on fuel efficiency or CO2 emissions, and how much more demanding is it? The question is specifically relevant only to the process of comparing different vehicle standards to see which is more stringent. But it’s useful and interesting to just have a rule of thumb: for any given value produced by a test, what would the CO2 or fuel consumption be on another cycle?
Even though it’s a simple enough question, it has no simple answer, since the new conversion factors vary with vehicle efficiency and are different for gasoline and diesel engines. Yet in comparing the cycles some patterns do emerge, which are described below.
Using our old conversion factors, CO2 emissions (or fuel consumption) were higher on the JC08 than on either the U.S. combined or the NEDC test cycles for vehicles emitting more than 50 g/km CO2 (Figure 1).
Figure 1. Cycle conversion factors used in previous assessments

Figure 1. Cycle conversion factors used in previous assessments

But the relationship between these test cycles changed significantly after we derived the new conversion factors. The change is attributable to a broader vehicle sample and the inclusion of models with more advanced technologies, such as improved transmissions and clutches, advanced engines, system electrification, and efficient operation strategies. An example of the effect of the change: The widespread use of stop-start systems provides considerably CO2 savings during idling time. That benefits vehicles in the JC08 and NEDC test cycles, which have much higher share of stop phases relative to the total duration of the cycle (JC08: 28.7%, NEDC: 23.7%) than either the Worldwide Harmonized Light-Duty Test Cycle (WLTC, 12.6%) or the U.S. CAFE test cycle (9.9%).
Figures 2 and 3 illustrate the relative stringency of the four test cycles (we added the WLTC to the comparison in our recent update) using the new conversion factors. 
Figure 2. New Gasoline conversion factors Figure 3. New Diesel conversion factors

Figure 2. New Gasoline conversion factors          Figure 3. New Diesel conversion factors

The main points to make about these charts:
  • The conversion factors change with the fuel economy/CO2 emissions value, and gasoline and diesel vehicles behave very differently under the four test cycles. 
  • The relative difficulty of the test cycles depends heavily on individual vehicle technology packages. Different vehicle and engine sizes show significant variations in cycle sensitivities. Engine stop/start systems have a major impact on the conversion factors because the proportion of test time spent idling time varies significantly among the driving cycles. Cold start emissions also contribute to differences in cycle difficulty, depending on the engine’s starting temperature and the cycle’s distance.
  • For gasoline vehicles, WLTC is most demanding on average for current technology vehicles, with emissions 18% higher than JC08, 15% higher than the US CAFE cycle, and 13% higher than the NEDC. The NEDC and JC08 are more demanding than the US cycles for less-efficient gasoline vehicles. As technologies improve (e.g., stop-start, advanced engine and transmission, hybrid), the differences decrease, and the relative stringency of the NEDC and JC08 cycles versus the US even reverse for vehicles emitting less than 130 g/km. (The WLTC maintains higher emissions than the other cycles for all vehicle efficiency levels.)
  • For diesel vehicles, the JC08 and NEDC are almost equal on average, regardless of vehicle efficiency. Both are about 10% more demanding than the US cycles for high-efficiency vehicles, and somewhat less demanding for low-efficiency vehicles. The WLTC matches the JC08 and NEDC for low-efficiency vehicles and matches the US combined cycles for high-efficiency vehicles.
To futher explain the relative changes in vehicle efficiency under the four test cycles, the table below compares the average CO2 emissions gaps between different test cycles based upon the old and new conversion factors. The significant differences between the two studies illustrate how the technological change and the capability of the chosen simulation model are likely to influence the relative difficulty of each test cycle. For instance, compare to the JC08, the NEDC was 13% less demanding using our old conversion factors. But the new conversion factors show that on average the NEDC is 5% more demanding for gasoline vehicles. Again, the numbers only summarize the simulation results and are too simplistic to represent the difficulty of the test cycles for individual vehicles. 
  Old (gasoline) New (gasoline) New (diesel)
NEDC/JC08 -13% 5% -2%
US combined/JC08 -22% 4% -7%
US combined/NEDC -11% -2% -6%
WLTC/JC08 18% 0%
WLTC/US combined 15% 9%
WLTC/NEDC 13% 4%

Table 1. Average vehicle CO2 emissions differences between test cycles in the old and new conversion factors studies (shares of technologies unweighted)

Note that all conclusions reflect only the effects of the drive cycles. The effects of other parameters associated with the test procedures, such as temperature, preconditioning, coast-down, and dynamometer calibration, are not addressed in the simulations. In a recently released ICCT working paper, my colleagues conducted an in-depth assessment of WLTP-NEDC conversion factors to translate the NEDC based EU 2020/21 fleet average CO2 target of 95 g/km. The study took into account two other key elements in the test procedures: vehicle test mass and the engine temperature at test start. All three elements together show that the WLTP test procedure is only 6%–8% more demanding than the current NEDC test procedure in relation to a projected EU fleet composition in 2020.