Improving the conversions between the various passenger vehicle fuel economy/CO2 emission standards around the world

Published Wed, 2014.12.03 | By

Zifei Yang

Most governments of countries (or, in the case of the European Union, regions) with large auto markets regulate passenger vehicle fuel efficiency or CO2 emissions (essentially the same thing, since CO2 emissions are directly proportional to the amount of fuel consumed). But they take differing approaches to designing regulations, and use different underlying drive cycles and test procedures to certify that a vehicle complies with the standards. Converting the standard values—that is, the fuel efficiency mandates or emissions limits—between different regulations involves not just converting physical units, but also accounting for the impacts of differences in test cycles.

For years, the ICCT has maintained a ready reference to worldwide passenger vehicle fuel efficiency standards, updating the charts and data as governments adopted or modified regulations, that aimed to compare the relative stringency of regulations as accurately and fairly as possible. It’s generally seen as the best such comparison available, and is widely used by governments, industry, NGOs, and media to understand and illustrate what’s happening with vehicle efficiency globally.

But we always knew it could be better—even fairer and more accurate. And we’ve just concluded a significant research effort to make it so, by improving the test cycle conversion factors used to translate the standard values among the different regulations. Full details are available here. The following paragraphs give the summary version.

Improved methodology

The study establishes CO2 emission conversion factors for four major test driving cycles: the Corporate Average Fuel Economy, or CAFE, test in the United States; the New European Driving Cycle (NEDC) in the European Union (EU); the JC08 in Japan; and the Worldwide Harmonized Light-duty Test Cycle (WLTC), which is not yet officially in use anywhere but will be (it is expected) adopted in the EU and Japan (and probably by other governments as well) beginning in 2017. Every government that has promulgated fuel efficiency standards uses one of these four test cycles for establishing compliance with the standards.

Compared with the previous conversion factors, the new approach reflects many improvements:

Old New
Vehicle model data from the Modal Energy and Emissions Model (MEEM) Vehicle model data from Ricardo’s ‘Data Visualization Tool’ (DVT), the same data used in developing US fuel economy standards and generating EU CO2 cost-benefit curves
Simulation results for 12 gasoline LDV (ICE only) Simulation results for a large number of current and advanced gasoline, hybrids and diesel vehicles with automatic and manual transmissions
2015 projection Vehicles simulated with both a 2008 baseline and with technologies projected for 2020
Multiplier logarithmic regression method Different linear and non-linear regression approaches evaluated, higher level of technical details
Resulting algorithms converting CAFE [mpg], NEDC [g CO2/km] and JC08 [l/km] Resulting g CO2/km based algorithms converting CAFE, NEDC, JC08 and WLTC

Different types of regression analyses were developed and evaluated using the modeled CO2 emission data in order to describe the dependencies for each pair of driving cycles. The new conversion factors can evaluate gasoline and diesel data separately. Different regressions can be used based on the availability of information related to vehicle architecture, aerodynamic drag, and engine technology.

We found that there is a trade-off between making conversions more precise and making this information useable for global policymakers. As the fleet average CO2 emission targets are technology agnostic, the study establishes a “universal approach” by merging linear regression lines for gasoline and diesel vehicles. At this regressions level, the complete data set was evaluated without taking into account possible technology specifics. The only additional input required is the share of diesel vehicles in the assumed fleet.

New conversion factors for global comparison

The new conversion equation for the “universal approach” for converting regional fleet average fuel economy/CO2 emission targets is:

C2 = (a1 * DS + a2) * C1 + d1 * DS + d2

The table below summarizes the results for the coefficients for each combination of cycle pairs.

(g CO2/km)
(g CO2/km)
a1 a2 d1
(g CO2/km)
(g CO2/km)
CAFE NEDC -0.0975 0.8658 9.852 14.076
NEDC CAFE 0.0884 1.1325 -7.480 -13.739
CAFE JC08 -0.1162 0.7212 7.602 36.736
JC08 CAFE 0.0941 1.2749 0.030 -38.423
CAFE WLTC -0.0348 0.9318 11.826 -8.827
WLTC CAFE 0.0587 1.0454 -14.600 12.590
NEDC JC08 -0.0227 0.8457 -2.891 24.840
JC08 NEDC 0.0290 1.1430 3.786 -24.907
NEDC WLTC 0.0486 1.0475 5.037 -22.727
WLTC NEDC -0.0494 0.8984 -3.752 28.059
JC08 WLTC 0.0722 1.1532 11.230 -45.172
WLTC JC08 -0.0653 0.7319 -6.170 53.293

DS: Fleet diesel share (0-1)

The figures illustrate the relationship between different cycles. Each pair of cycles has separated regression parameters for each direction. The figures merely plot one-direction relationship between each cycle pair (Cycle 1 → Cycle 2). The opposite-direction conversion (Cycle 2 → Cycle 1) is not identical to the inverse results of the original direction conversion. The solid lines reflect cycles relationships for 100% gasoline fleet, while the dashed lines reflect 50% gasoline and 50% diesel fleet. The dotted line is a hypothetical baseline of no CO2 emission difference between two cycles.

New global comparison charts

Using the new conversion factors, we have updated the global LDV CO2 emission standard comparison charts . The updates alter the global rankings, especially for passenger car standards. Some major changes in the new global comparison charts are:

  • Standards for cars and for commercial vehicle/light truck are now compared separately.
  • The EU 2021 CO2 emission target of 95 g/km becomes the current most stringent numerical standard, surpassing the US 2025 target of 97 g/km in NEDC terms.
  • Japan standards over the JC08 test cycle are much weaker than previous evaluations showed. With the 2020 CO2 emission target of 122 g/km, the Japanese standard appears lenient. In practice, due to the high market share achieved by hybrids in Japan , new passenger cars there are expected to significantly exceed the standards and to be on par with or surpass the EU’s 95g/km target.
  • South Korea’s standards are not significantly affected by the new conversion factors. However, its recent proposal to set a 2020 target makes it a global leader in setting passenger car CO2 emission standards.

Historical fleet CO2 emissions performance and current or proposed passenger vehicle standards:

Historical fleet CO2 emissions performance and current or proposed light commercial vehicle/light truck standard:s

Note that the standards comparison does not account for fleet differences such as average vehicle weight, size, and power.

ICCT will continue to update the global standards comparison on a regular basis as newer and better data become available. We will also soon publish a new summary report on global passenger vehicle fuel economy and GHG standards, so stay tuned for that as well!