For all the strides made in tractor-trailer aerodynamic research, a fundamental design trait still bedevils experts, and it’s a design feature crucial to the truck’s efficiency: The trailer gap.
Any vehicle moving through an air mass faces two impediments: drag and a vacuum. Drag is simple enough to define: any object jutting out into the air that impedes a smooth flow of air streaming around, over and past the vehicle. Rearview mirrors and bug screens are excellent examples of drag-inflicting surfaces on a tractor-tractor.
A vacuum is a little harder to identify. Basically, it’s an area of lower air pressure around a vehicle that sucks in air that ought to be flowing smoothly past the vehicle. As this low-pressure area robs air streaming around the truck, it does two things: First, it causes the vehicle to use more energy to move through the air mass; and secondly, it can create turbulence in the airflow around the vehicle, making it more difficult to move efficiently forward while forcing the engine to generate more power to maintain forward motion. The undersides of both a truck and a trailer, as well as the trailer gap, are prime examples of low-pressure areas commonly found on big-rig trucks.
While bumper-mounted air dams, wheel coverings and trailer side skirts and tails have improved airflow underneath and around tractor-trailers dramatically, the trailer gap remains an inherently difficult area for engineers to address.
“It is difficult to improve the aerodynamics of the tractor-trailer gap area because the trailer needs to articulate,” says Mario Bravo, director of marketing for aerodynamic device manufacturer FlowBelow. “This challenge is simply due to the amount of clearance required between the two. In addition, because there are many trailer and tractor configurations, having to design and build many variations of any solution makes it difficult for a manufacturer to reach economies of scale and offer their product at an attractive price.”
While the gap area is not the largest component of tractor-trailer drag, it is made more challenging by several difficult-to-resolve geometric issues, says Mike Henderson, chief scientist for SmartTruck. Computation fluid dynamics and testing has shown that gap drag results from low pressure in the gap reacting on the rear of the tractor and its fairing (drag) and the front of the trailer (thrust), Henderson says.
“The reason they don’t completely cancel is because the low pressure in the gap pulls air coming off the tractor fairing partially into the gap, causing high-velocity air to hit the flat face of the trailer face top and sides,” he says.
The solution, Henderson says, is to close the gap or place fairings on the trailer’s face top and sides, keeping in mind that the larger the gap, the larger the fairings needed to be effective.