Corporate Average Fuel Economy (CAFE) Standard

What is CAFE?

Fuel efficiency standards for automobiles are crucial in regulating fuel consumption and reducing emissions of harmful greenhouse gases. Establishing fuel efficiency standards encourages automakers to develop fuel-efficient vehicles, leading to sustainable transportation. This paves the way for the large-scale adoption of electric vehicles and alternative fuels, which can significantly reduce reliance on fossil fuels and mitigate the effects of climate change.

The Corporate Average Fuel Economy (CAFE) standard is a fuel efficiency standard implemented by the National Highway Traffic Safety Administration (NHTSA) to enhance the average fuel economy of cars and light trucks sold in the U.S. The CAFE standard regulates the mileage or the distance a vehicle must cover on a gallon of fuel by calculating the average fuel consumption of an auto manufacturer's entire fleet rather than individual models.

The Environmental Protection Agency (EPA) provides the CAFE target that appears on the fuel economy label of all new cars and light trucks. To ensure accuracy, automakers must follow specific test procedures and submit the fuel economy data of all their models to the EPA annually.

The EPA is implementing new federal greenhouse gas emissions standards for passenger cars and light trucks for model years (MY) 2023 through 2026. The new rule sets a 55 miles per gallon CAFÉ target for model year 2026 vehicles and is expected to increase the sales of electric and plug-in hybrid vehicles from 7% in MY 2023 to 17% in MY 2026.

As a next step, the NHTSA has proposed new fuel economy standards to reduce greenhouse gas emissions and fuel use by requiring a fleet-wide average of 58 miles per gallon by 2032. This proposal would increase CAFE requirements by 2% per year for passenger cars and 4% per year for light trucks, covering the MY 2027 through 2032. In addition, the agency is proposing new fuel efficiency standards for heavy-duty pickup trucks and vans for 2030-2035, with an annual increase of 10%.

How Do OEMs Define CAFE Standards for Their Fleet?

The CAFE achieved for an original equipment manufacturer’s (OEM) fleet is calculated as the mean of the fuel economy (in miles per gallon) of current model year passenger cars, light trucks, and medium-duty passenger vehicles produced for sale in the United States.

If OEMs do not meet the CAFE target, they can buy credits from other manufacturers with excess credits, such as EV vendors. This incentivizes car manufacturers to invest in developing and producing electric and low-carbon-emitting cars, allowing them to comply with regulations and avoid penalties.

How are EPA Fuel Economy Measurements Performed to Establish the CAFE Target?

A vehicle's mileage or fuel economy is estimated for the CAFE target by conducting laboratory tests using a dynamometer. The dynamometer simulates the driving environment, and the energy required to move the rollers is adjusted for factors such as wind resistance and vehicle weight.

During the test, the driver runs the vehicle through standardized driving routines called cycles or schedules, which simulate typical city or highway driving. To measure the fuel burned during the test, a hose is connected to the tailpipe to collect the engine exhaust. The carbon content in the exhaust is measured to provide an accurate estimation of fuel consumed.

The vehicles are tested over five defined test cycles:

1. City Test: the heating, ventilation, and air-conditioning (HVAC) system will not be operational during this test.

2. Highway Test: the HVAC system will not be operational during this test.

3. High-Speed Test: the HVAC system will not be operational during this test.

4. Hot Test at 95°F: during this test, the HVAC system will be set to cool the cabin.

5. Cold Test at 20°F: during this test, the HVAC system will be set to specific settings for heating and defrosting.

For accurate results, the tests are conducted three times for repeatability, and the average value is reported to the EPA. The certification group is responsible for gathering and organizing the data and then proposing the label for the vehicle to the EPA. The EPA can audit the results at launch and post-launch to guarantee compliance.

How Can Computational Fluid Dynamics (CFD) Help Achieve the CAFE Target?

Automakers face the constant challenge of building vehicles with better performance than their previous models. This challenge is driven by various end-user demands, including more functionality, improved capability, cost-effectiveness, and government requirements for greater fuel efficiency, lower carbon emissions, and better safety measures. Conducting aerodynamic analysis is crucial for enhancing automotive performance and meeting the CAFÉ target.

The turbulence produced in the wake of a vehicle creates vortices that result in drag. This drag can negatively impact vehicle performance. The greater the drag, the greater the required thrust, resulting in higher fuel consumption, lower miles per gallon, and increased fuel costs.

Using CFD technology is an efficient and affordable way to study the aerodynamics of automobiles. Automobile manufacturers can shorten development time and save money on materials and parts by simulating designs before building physical prototypes. In addition, this approach allows testing multiple vehicle designs, resulting in an optimized design that improves aerodynamic performance and fuel economy.

Improving Automotive Fuel Efficiency with Cadence Products

Cadence has been at the forefront of computational software for over three decades. In 2021, Cadence entered the CFD market with its acquisitions of Pointwise and NUMECA International, marking a strategic move to broaden the company's computational expertise into additional high-growth areas. With the ability to simulate complex fluid flow and heat transfer problems, CFD software is becoming increasingly important in aerospace, automotive engineering, and energy production.

With its latest acquisition of Cascade Technologies—an integrated suite of multiphysics simulation software—Cadence has opened a new chapter of computational software and systems simulation. The large eddy simulation (LES) CFD solver Fidelity CharLES is at the software suite's core. Encompassing low-speed, high-speed, and reacting flows, Fidelity LES also tackles the challenging problem of accurate turbulent flow simulation with the advanced wall-modeled LES (WMLES) technique. Fidelity LES software also exploits GPU acceleration, leading to rapid turnaround time. This technology enables the virtual testing of automotive designs for fuel economy while considering aerodynamics. This reduces investment in physical testing while meeting CAFE targets.