White paper

Development of test cycle conversion factors among worldwide light-duty vehicle CO2 emission standards

Comparing the light-duty vehicle fuel efficiency standards in effect in the world’s largest auto markets is not a simple, straightforward process, because any fair, apples-to-apples comparison must account for differences in the test procedures that determine whether vehicles meet the standards or not. Of particular importance, the driving cycles specified by various regulations produce dissimilar engine loads and emissions.

This paper compares the dynamics of the four most important driving cycles and their impacts on fuel consumption and CO2 emissions. The driving schedules include those specified in the three most relevant national regulations—the U.S. CAFE standards (a composite of FTP75 and HWFET), the New European Driving Cycle (NEDC), and Japan’s JC08—plus the recently developed Worldwide Harmonized Light-Duty Test Cycle (WLTC). CO2 emissions were simulated over the test cycles for a variety of vehicle and technology packages using a vehicle emission model developed by Ricardo Engineering. Model runs based on the speed courses of the driving cycles were resolved on a second-by-second basis. Current vehicle architectures and advanced innovative technologies focusing on the 2020/2025 horizon were covered.

The outcome is a set of usable conversion factors to translate distance-based CO2 emissions among the different driving cycles. These factors were determined on distinct levels of detail, characterized by technology parameters such as share of diesel engines in the fleet, vehicle size, share of hybrid systems, aerodynamic drag, and others.

This study updates and refines an earlier analysis completed in 2007. The table below summarizes key differences between the earlier and later analyses.

2007 2014
Use of vehicle model data from the Modal Energy and Emissions Model (MEEM) Use of vehicle model data from Ricardo’s Data Visualization Tool (DVT) based on MSC.Easy
Simulation results for 12 gasoline LDV (internal combustion engine only; no hybrids, no diesels) Simulation results for a large variety of innovative technologies, including hybrids and advanced diesel technologies
2015 projection 2020/2025 projection
Multiplier logarithmic regression method Different regression approaches were evaluated, using a higher level of technical details
Resulting algorithms converting CAFE (mpg), NEDC (g CO2/km) and JC08 (l/km) Resulting CO2-based algorithms converting U.S. CAFE, NEDC, JC08 and WLTC