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The effect of real-world driving conditions on lightweight vehicles

Clare Goldsberry

May 19, 2016

5 Min Read
The effect of real-world driving conditions on lightweight vehicles

The Corporate Average Fuel Economy (CAFE) standard mandated by the federal government continues to cause controversy among engineers, OEMs and their suppliers. Can the 54.5-mpg goal truly be reached by 2025? What will it take to achieve that? Smaller vehicles? Lighter vehicles? Was the 54.5-mpg number the result of a scientific calculation or was it just a guess?

However you look at it, the standard is doing good things for the plastics industry by encouraging innovation in carbon-fiber-reinforced plastics, metal to plastic conversions and other amazing material technologies that are giving the industry a boost while helping to lightweight cars and trucks. But I keep coming back to the same question: Is the CAFE standard of 54.5 mpg realistic under real-world driving conditions?

Image courtesy Jaime Kopchinski/flickr.

Obviously not, because the rules don’t mean that cars and light trucks actually will achieve this mileage in 13 years. It’s actually closer to 40 mpg in real-world driving conditions, say various reports on the CAFE standard. So why won’t cars and light trucks actually achieve the 54.5 mpg?

First of all, the testing is done under optimal conditions on a test track in brand new cars. Government regulations only require that new cars coming off the production floor achieve the 54.5 mpg. There are no regulations that require cars and light trucks to maintain this mileage after five years on the highways. Or even after one year! Once the cars are out of the manufacturing plant, sold to consumers and driven under real-world road conditions, it’s quite likely that these vehicles will not meet the CAFE standard.

One of the reasons for this is that multiple factors contribute to improving fuel economy, said Rich Jordan, Vice President of Engineering for Corvac Composites LLC (Morgantown, KY), an automotive Tier 1 supplier of lightweight exterior shielding, in his presentation at the recent Plastics-In-Motion conference in Charleston, SC. Some of these factors include engine displacement (size), induction system (i.e., turbo charger), rolling resistance, altitude, temperature, mechanical resistance, mass and aerodynamics.

For example, Jordan showed a diagram of an older vehicle with the underbody completely exposed from front to rear. Another graphic showed how the vehicle’s undercarriage is now almost completely shielded, including front and rear fender liners; engine undercover; mid rocker covers, both left and right; fuel tank covers, left and right; and rear diffusers (right, left and center). The benefit of these automotive underbody panels is primarily to help control airflow under the vehicle and reduce resistance; improve performance, including management of water and debris to protect more delicate or critical components; and improve acoustics by sound absorption or sound blocking.

Jordan then presented benchmarking testing and case studies to explain why vehicles do not maintain the mpg that original factory testing achieved. “It will be demonstrated that a number of adopted lightweight components do not maintain their lightweight attribute through normal usage,” Jordan explained. “Consequently, the loss of lightweight performance also detracts from one of the perceived benefits of acoustic absorption.”

Corvac analyzed new and used parts, measuring and quantifying the changes in mass and acoustic absorption. Products tested were allowed to dry for a sufficient period of time, such that a constant mass was achieved over a 24-hour time period, and the vehicles undergoing analysis had all loose debris removed prior to analysis.

All of the case studies involved rear fender liners constructed using a nonwoven, compression-molded PET textile. The first study involved liners with an internal barrier film from a four-door SUV with 9,000 miles on the odometer from OEM A. When the rear fender liners were new, they had a mass of 1.0 kg. When they were tested used/wet, the mass increased 183% to 2.8 kg; when tested used/dry, the mass measured 1.9 kg, an increase of 89%. The part also had a 70% acoustic performance loss.

A second case study from OEM B involved liners with assembled acoustic insulator treatment from a four-door SUV with 18,000 miles on it. New, the rear fender liners had a mass of 1.5 kg. Used liners tested wet had a mass of 5.7 kg, a 283% increase. Used liners tested dry had a mass of 4.1 kg, a 174% increase.

A third case study looked at liners from a four-door passenger car from OEM C with 7,600 miles on it. Testing and analysis showed that when new the mass was 1.5 kg; tested used/wet, the mass increased to 6.5 kg, an increase of 340%; and tested used/dry, its mass was 5.01 kg, an increase of 243%.

“The unintended consequence of utilizing materials that improved the weight of the vehicle when new .  . .  was that they became heavier when used,” explained Jordan. “There’s no way to maintain 54.4 mpg as a vehicle is used in real-world driving conditions.”

Laboratory material studies measured for water mass gain by Corvac proved that hard plastic parts—those without various bio-fillers or textiles, which tend to absorb water—were optimal in terms of maintaining their original mass. Corvac also found that the inclusion of water eradicates all the acoustic absorption. “As the water carries other foreign matter such as dirt, salt, oil and other impurities into the components, these materials become permeated within the fibrous matrix, much like a filtration device," noted Jordan.

“There is no guarantee that the statistics for the car you buy will stay that way over the years you drive it, and therefore OEMs need to evaluate lightweighting technologies over the long term. They need to ask, ‘Is the material chosen suitable for the environment in which the vehicle operates?’ In efforts to improve fuel economy through aerodynamics and lightweighting techniques, it is incumbent upon the engineering community to ascertain that our solutions are in the best interest of overall vehicle performance in both the short and long term,” Jordan concluded.

About the Author(s)

Clare Goldsberry

Until she retired in September 2021, Clare Goldsberry reported on the plastics industry for more than 30 years. In addition to the 10,000+ articles she has written, by her own estimation, she is the author of several books, including The Business of Injection Molding: How to succeed as a custom molder and Purchasing Injection Molds: A buyers guide. Goldsberry is a member of the Plastics Pioneers Association. She reflected on her long career in "Time to Say Good-Bye."

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