Polar Tank has built an aerodynamically smoother trailer at the request of a hauler of bulk products in South Dakota. In regular service, the trailer is saving a third of a mile per gallon compared to older tankers.

Cliff Viessman Inc., in Gary, S.D., runs 70 pneumatic bulk trailers that haul refined sugar, flour and starches from shippers in the Midwest. The fleet's managers know that side skirts and other fairings that work on vans generally aren't practical for tankers. So they asked Polar for help.

Responding was Duane Plumski, Polar's research & development engineer.

"A pneumatic trailer is more streamlined than a flat-fronted trailer, but there are elements like external rings, handholds, and piping that disrupt the airflow," he said. "We analyzed virtually every aspect of the trailer to see what we could do to reduce the aerodynamic drag and potentially improve fuel economy without affecting capacity or operations."

Using computer 3D modeling tools, Plumski and his colleagues began on-screen exploration. They first focused on the shape of the trailer's front face, which accounts for approximately 30% of its aerodynamic drag. Engineers lowered the tank's front-end cone and tipped it forward slightly to reduce the profile and soften the impact of air coming over the tractor.

Then they looked at the trailer's sides. One obvious source of drag: the external rings, which wrap like ribs around the aluminum tank. The benefit of external-ring bracing is a smooth interior that's easy to clean. A trailer that's smooth on the outside will have struts inside the tank for structural support.

"In our experience, cleaning trailers with internal ribs is a major issue," says Joey Viessman, who manages the company's fleet in Renville. "Product collects around the bracing and makes it hard to unload or clean out." So engineers shaped transition points between the internal ribs so the trailer empties completely and clean-out goes smoothly.

"Our priority was to ensure that no change would alter the capacity of the tank, the distribution of the payload, or the ease of loading or off-loading product," Plumski says. "We preserved the round sloping interior surfaces and designed the [interior] ribs so that the trailer will empty cleanly, but you get the aerodynamic benefit of the smooth exterior."

CVI now has two aerodynamic pneumatic trailers and has ordered four more. They haul an average 55,000-pound payload on round trips of 500 miles a day behind day cab tractors.

The result: All things being equal, the new Polar trailers average roughly 0.3 mpg better than the externally ringed trailers.