Against the Wind

Trailer aerodynamics wasn’t much of an issue when diesel fuel cost only a dollar a gallon. That 53-ft box you drag around the country is about as aerodynamic as a brick. With fuel teetering around three bucks a gallon, engineers are looking more closely at trailers for potential fuel savings because now there’s a payback.

Research has been conducted over the years, on mostly small-scale projects. It takes a lot of time and money to mount full-scale wind tunnel and real-world tests. Consequently, improvements to aero engineering have been slow. The devices available today are only slightly better than designs from years ago. The push for cost reductions, coupled with the recent advent of computer aided design (CAD) is really moving this field of research forward. What we’ll see coming from the labs in the coming years will be leaps and bounds ahead of what we’ve seen in the past.

The science of aerodynamics is both intuitive and test dependent. You’d expect the smooth rounded shape of a football to have better aerodynamic properties than a brick of the same size. But since a football-shaped trailer isn’t practical, engineers are trying to emulate the aero properties of a football by tweaking the structure of the basic trailer to reshape the flow of air over, under, and around it.

What a Drag:

Aerodynamicists refer to drag as positive or negative air pressure. One can visualize positive air pressure easily enough: put your hand out in the wind at even 50 km/h and you can feel the resistance or drag. You know intuitively that pushing through the air requires effort. That’s why in recent years we’ve taken to putting round-nose tractors out in front of our brick-shaped trailers. But that takes care of only one end of the problem.

A less obvious form of drag is the vacuum that occurs behind the trailer caused by the air rushing back into the space it was pushed out of.
Because air can’t reoccupy that space instantly, a vacuum or low-pressure area is created until it does — along with a ton of turbulence. That area of rough, irregular airflow and the partial vacuum behind the trailer is pulling the truck backwards.

Jason Leushen of the National Research Council in Ottawa (NRC) estimates that 130 hp is consumed in the effort. He and his team test truck aerodynamics in a full scale wind tunnel. All day long, you have a 130 hp engine pulling backwards, all day, hour by hour while traveling at 100 km/h.

“Your engine works hard enough to keep you moving forward without having to overcome a 130-hp pull from behind,” Leushen says.

In real life there is wind to contend with too. Wind can hit the truck from any angle. If you are driving in a crosswind, the air is passing at an angle to the truck. In fact, in this condition some of the air is passing under and over the trailer and exiting along the side of the trailer. The low-pressure problem occurs along the whole length of the trailer, in effect pulling the trailer to the side. It also creates drag because of the truck’s forward speed.

Bob Englar, a research engineer at Georgia Tech says there are three main areas of drag other then the front of the truck — the tractor/trailer gap, the underside of trailer which includes the trailer suspension and the cross members of the floor, and the back, or as researchers call it, the “base” of the trailer.

“Various wind tunnel tests have come up with coefficient-of-drag (Cd) numbers for these three areas,” Englar says. “The important number to look at here is the back of the trailer, at .25, it represents one-third of the total drag. That’s a big chunk.” (See sidebar).

The Cd number is significant if you’re out shopping for aerodynamic add-ons for your equipment, but it’s an apples-to-apples comparative number — assuming the configurations you’re considering are all the same height, width and length.

To illustrate the significance of the number, consider the following example.

A rough sphere has a Cd of .4 and a smooth sphere of the same diameter has a Cd of .1. The smooth sphere is 4 times more efficient, or 400 percent more efficient. That’s a huge difference. In the world of truck and trailer aerodynamics we’re talking comparatively smaller numbers of Cd improvement.

If a Cd of .75 can be reduced by .1 to .65 you have a 13 percent reduction in aerodynamic drag, which translates to about six percent in fuel savings. You might go from 5.75 mpg to 6.09 mpg. That .25-mpg improvement over 400,000 km at .90 cents liter would save you $2,600.

In terms of payback, it would take about two years to recover the cost of an aero add-on costing $2,600. In two more years, you’d be $2,600 ahead of the game, and so on. By comparing the cost saving from buying and installing the device against the projected savings, you can see that gains in trailer aerodynamics are a tight margin game. Still, as fuel gets dearer and dearer, research into trailer aerodynamics will become more compelling.