ICCT study compares leading global driving cycles and provides usable conversion factors for CO2 emissions

8 December 2014

A team at the International Council on Clean Transportation (ICCT) has compared the dynamics of the four leading driving cycles worldwide—the US CAFE standards (a composite of FTP75 and HWFET); the New European Driving Cycle (NEDC); Japan’s JC08; and the recently developed Worldwide Harmonized Light-Duty Test Cycle (WLTC)—and their impacts on fuel consumption and CO₂ emissions on an equal basis. (WLTC will be replacing NEDC in a few years.)

The result is a set of usable conversion factors for distance-based CO₂ emissions among the different driving cycles. The ICCT team determined these factors 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. (Earlier post.) The new study uses a different methodology with different mathematical approaches.

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For the study, the ICCT team simulated CO₂ and efficiency results 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. The researchers focused on current vehicle architectures and advanced innovative technologies in 2020/2025 timeframe.

The direct comparison of CO₂ or FC standards’ stringencies from different regions not only depends on the different driving cycles applied but also on the technical characteristics of the regional vehicle fleets. The use of comprehensive adjustment factors therefore requires simplifying assumptions concerning the assessment of averaged fleet compositions.

Cycle deviations of CO₂ emissions averaged over all gasoline and diesel technologies resulting from basic single regression approach with zero intercept. The error bars here represent the standard deviations caused by individual vehicle technology packages. Gasoline vehicles emit strongest under the WLTC schedule. CO₂ emissions under the JC08, US CAFE and NEDC regimes are 18%, 15% and 13% lower.

The cycle-specific deviations of diesel vehicles are generally lower and show a different pattern. For example, averaged WLTC and JC08 emissions (the highest deviations for the gasoline vehicles) are almost equal for diesel, while CAFE emissions are lower than JC08 (the opposite of the gasoline vehicles). These results reflect the fundamental differences in the structures of gasoline and diesel engine maps. Source: ICCT. Click to enlarge.

Ricardo developed a complete, physics-based vehicle and drivetrain system model and implemented it in MSC.Easy5, a commercially available software package for vehicle system analysis that models the physics in the vehicle drivetrain during a drive cycle. Torque reactions are simulated from the engine through the transmission and driveline to the wheels. The model reacts to simulated driver inputs to the accelerator and brake pedals, thus enabling the actual vehicle acceleration to be determined based on a realistic control strategy.

The model determines key component outputs such as torque, engine speeds, and heat rejection. The combination of these engine load output data with fuel or CO₂ engine maps results in integral emission data for specific driving cycles.

Ricardo parameterized the CO₂ model for the predefined driving cycles and vehicle technologies and developed a user-friendly application tool, Data Visualization Tool (DVT) or Complex System Tool.

ICCT also commissioned Ricardo to assess likely technology developments occurring until 2020/2025, taking into account six different LDV classes: B, C, D, small CUV, small and large N1.

CO₂ savings was evaluated separately for gasoline and diesel concepts. The most promising technologies in terms of both reduction potential and market penetration in 2020/2025 were identified and explored further. The most relevant developments related to:

• improvements in transmissions and clutches (automatic, dual clutch transmission [DCT] continuously variable transmission [CVT]);
• advanced engines (valve controls, lean combustion, exhaust gas recirculation [EGR], direct injection, Atkinson);
• system electrification (parallel and powersplit hybrids); and
• efficient operation strategies such as stop/start systems.

The ICCT team applied different types of regression analyses to the modeled emission data to describe the dependencies for each pair of the different driving cycles. The regression types differ by the mathematical nature (linear vs. nonlinear approaches), the inclusion or exclusion of the y-intercept, the differentiation into different vehicle technologies and the inclusion of additional independent variables (multiple regression analyses). Therefore, the level of complexity and the achievable quality of the regression results vary among the different types, the ICCT authors noted.

They developed a general pattern to assist users in determining which conversion approach is most appropriate in each case and which regression coefficients should
be applied.

Resources

• Jörg Kühlwein, John German, and Anup Bandivadekar (2014) “Development of test cycle conversion factors among worldwide light-duty vehicle CO₂ emission standards”