Taking the Aero Route

Several winters back I picked up a Volvo V-something tractor for a test drive. It was an ugly day in the Toronto area, the weather a mix of snow and sleet and the roads mushy at best.

If I’d been smart I would have stayed at home but I thought, if the truck is good in this mess, I’ll have learned something worth reporting. So with box van trailing out back I left the dealer’s yard and somewhat gingerly headed for the highway, my tape recorder riding shotgun.

Not many minutes later I was amazed by the effectiveness of a tiny little aerodynamic trick, a triangular piece of rubber not much more than an inch long on each of its three sides. Attached to the lower outside corner of each mirror housing, and standard equipment, it was intended to keep the mirrors clear in just the sort of weather I was facing. And it worked! My mirrors stayed perfectly clean all day despite the muck everywhere else.

Another such example is the little Airtab vortex generators that are typically arrayed around the trailing edges of cab fairings and trailer rear sides. Inexpensive but effective by all accounts, they’re said to control the wind vortex at the back of tractor or trailer and minimize buffeting. Doors are said to stay cleaner and some drivers even report better trailer stability.

The lesson in there is that even the tiniest things can have a major impact on how air flows over and under and around a tractor and its trailer. It’s no coincidence that the latest “aero” tractors employ similar small touches like rounded mirror housings and flush-mounted marker lamps, even wheel covers. They make a difference, when every two-percent of drag reduction represents about a one-percent gain in fuel economy.

THE MIRROR STUDY

In fact, back in 2004 Freightliner engineers launched a two-year study looking largely at mirror design and mirror-mounting systems in their newly built, full-scale wind tunnel. Such a wind tunnel allows the replication of real-world scenarios in a controlled environment to evaluate even the slightest effects on aerodynamics. It also means the variables of driver influence, weather conditions, road surfaces, and ­traffic, to name a few, can be overcome.

And what did they find? The lowly, simple mirror — tested in 11 different iterations on a Century Class tractor — can affect vehicle aerodynamics by as much as six percent or more, depending on design and placement on the truck. That’s an awful lot. It’s not just the mirror’s shape that matters here but its placement on the truck and even the shape of the truck in its vicinity.

So, smooth things out, remove the obstructions that create drag, ensure as much as you can that ram air runs right past the tractor and its trailer without falling into the gaps and swirling around in there, and you’ll win a big advantage in fuel efficiency. Very big.

Even if you don’t have roof fairings and extension skirts on the trailing edges of your cabs, for example, moving the fifth wheel forward to close the gap between tractor and trailer can save money. Provided, of course, that you don’t muck up weight distribution over your axles. Beyond about 30 in., every 10-in. increase in tractor-trailer air gap increases aerodynamic drag by approximately two percent.

For the most part, though, you’ll have to open your wallet to buy an aerodynamic advantage, and these days various sorts of trailer skirts and tails seem to be hitting the spot with many truck operators, assuming on-highway work.

It may also be time to pony up for the latest “slippery” tractors and finally say goodbye to those lovely flat-nose, long-hood conventionals. They represent a very small part of the market nowadays anyway, less than 10 percent, and the trend will only be downward in the future.

Some time in the next few years, given the coming U.S. fuel-economy and greenhouse-gas emissions rules (to be matched in Canada, more or less), the classic North American ‘large car’ seems bound to be forced off the showroom floor because it likely won’t be SmartWay-approved.

THE SMARTWAY ERA

So what is SmartWay? Launched in 2004 by the U.S. Environmental Protection Agency (EPA), it was first seen as just a “brand”representing “environmentally cleaner, more fuel efficient transportation options.”

In its simplest form, says the EPA, the SmartWay brand identifies products and services that reduce transportation-related emissions. But it’s gone way beyond that to be the hub of what the EPA likes to call “a partnership between government and business” with a view to protecting the environment and reducing fuel consumption.
 

German truckmaker MAN showed its Concept S tractor at last year’s
IAA show in Germany. Paired with a matching, “slippery” trailer
that tapers in at the rear, it’s claimed to represent a
whopping 25-percent saving in fuel consumption.

At this point it’s really a rating regime, a means of judging whole trucks as well as individual components like tires and, yes, aerodynamic devices. And it’s about to become, by way of the joint EPA/DOT proposed rulemaking on fuel economy and greenhouse-gas emissions, the sole arbiter as to which trucks will be allowed to run on U.S. roads — effectively on Canadian highways too — and how they’ll be spec’d.

It’s very near to that sole arbiter status now, for all intents and purposes, as truck and component manufacturers strive to have their products labelled “SmartWay-approved”. And as shippers increasingly demand that carriers run environmentally “responsible” equipment, there are more and more SmartWay decals emblazoned on tractor sides.

Some of those decals, we’ve been hearing from legitimate SmartWay carriers, aren’t actually deserved. Fakes? Counterfeits? Not sure yet, but the accusations are out there.

Now, with the fuel-efficiency/GHG mandate starting with model-year 2014 and running through 2018, trucks from class 3 to 8 will have to meet new standards that are essentially SmartWay standards. If you’re already in the habit of wearing that decal on your tractors now, you’ll be good to go in 2014. If not, your buying process will change as you work with a thinner databook populated increasingly by only SmartWay-approved truck models and componentry. That collection of verified products will bring you to the fuel-efficiency level required in 2014.

And of all the bits of paraphernalia in that collection, by far the most effective are those that reduce aerodynamic drag. For the most part they’ll be trailer add-ons, even though trailers aren’t covered by the EPA/DOT rulemaking. Truth is, there’s been a growing interest in trailer aerodynamics for a few years now anyway, since the price of fuel began getting uncomfortable.

TRAILER TRICKS

It seems like no-brainer stuff, this business of making the 53-foot box a contributor to efficiency instead of a cement block you drag along.

You’ve got three main areas of drag to contend with — the gap between tractor and trailer, the underside of the trailer with its suspension hanging out in the breeze, and the back. You fill the first, cover over the second… and then there’s the back.

On the face of it, the back of a van trailer doesn’t seem a likely candidate for air-flow ugliness, but some experts say it actually represents about a third of all the forces conspiring to slow the truck down. And in the process soaks up something like 130 of the horses your engine is producing, according to one estimate. Reduce that load and you’ll clearly make a difference.

What happens at the back of a van trailer at speed is the creation of a vacuum that occurs when air rushes back into the space it was pushed out of as the rig rolled ahead. Because air can’t re-occupy that space instantly, a vacuum or low-pressure area is created until it does — along with a ton of turbulence. That’s effectively pulling the truck backwards.

So nowadays we have so-called “boat tails” that extend the trailer top and sides by a little or a lot, defeating that vacuum effect, but may in the process fall afoul of length laws. It’s an issue in several jurisdictions, even though the benefit is real.

You’ll see numbers all over the map concerning the amount of drag represented here and the effect of various devices, as with all such things. One of the main players in the boat-tail game – ATDynamics — claims a 6.6-percent fuel-efficiency gain with its latest product, closer to 12 percent if the tail is combined with a trailer side skirt.

TESTING AT BLAINVILLE

A couple of years ago some serious testing was done over the course of three days at Transport Canada’s test track in Blainville, Que., where researchers from various organizations aimed to pin down some of those numbers. They looked at trailer skirts, boat tails, and other such devices.

Spurred on by Robert Transport and Cascades Transport, who originally wanted to do some testing on their own, the project grew and management of the proceedings was turned over to FPInnovations-Feric, which ensured scientific validity. A dozen suppliers of trailer add-ons accepted the invitation to submit their technologies for scrutiny, and the team of about 30 researchers tested 16 configurations of devices and techniques.

Among the devices tested by Transport Canada in Quebec was
the Laydon Composites skirt. It managed a 6.8-percent fuel saving.

Results showed that the savings possible with the particular trailer aerodynamic devices on trial ranged from 1.4 to 7.2 ­percent. Most effective were the trailer side skirts from Freight Wing and Laydon Composites, each around seven percent. An earlier version of the ATDynamics tail managed just over 5 percent.

With a much less dramatic tail — though still long enough to run afoul of some European length restrictions — Swedish truckmaker Scania recently did some real-world, on-the-road testing of its own. The company has a very active research and development subsidiary called the Scania Transport Laboratory, which tests and evaluates all manner of things. Running a small fleet of 20 tractors and 70 trailers that haul freight between its European production and assembly plants, the company has a rolling test bed in active service.

After extensive testing of a boat-tail air deflector fitted to the rear of an ordinary three-axle van trailer, Scania says it reduced fuel consumption by up to two percent. The length of the vehicle increases by about a foot, 30 cm to be exact, which isn‘t actually allowed by European Union regulations.

APPLES & ORANGES

While the Blainville and Scania tests appear to be sufficiently rigorous and transparent, it’s hard to find reliable bottom lines in the aerodynamic world.

Yes, no doubt there are exaggerated claims here and there to muddy the waters, but they’ll be hard to spot because there are so few reference points. The real issue is that it verges on the impossible to see apples compared to apples every time. Or even to see all the apples examined in the first place.

The experts don’t seem to agree either. Take the back of a trailer, for example. You’ll have no trouble finding sizeable discrepancies in estimates as to the percentage of total drag represented by that nasty vacuum. The number might be as little as five percent, as high as 25.

This may not be a matter of disagreement or error, rather one of confused terms. Sometimes oranges get mixed in with those apples, but the would-be buyer of aerodynamic devices won’t know when.

Which brings us back to SmartWay. It’s resources are infinitely larger than yours and those good folks at the EPA have made themselves judge and jury when it comes to rating just about everything that contributes to reducing fuel consumption and GHG emissions. So why not look for the SmartWay seal of approval and save yourself some heartache?

However it is that you find a comfort level here, you’re going to need it. Aerodynamics is now the key target of virtually everyone involved in reducing the amount of fuel our trucks use, as it should be, so truck operators are going to be swarmed with options in the coming years.

Intelligent driving still has the greatest potential for trimming your diesel bill — there’s a 30-to-35-percent potential difference in fuel efficiency between the worst and the best driver — but the aerodynamic performance of the complete tractor-trailer combination is next in line. Best you dive in there and save a buck or three ASAP.