By Jason Rhyno

You navigated your way through a wicked whiteout and have just battled winter winds for 12 hours. The load was delivered safely, and on time, but now that you are back at the yard, you’re greeted with questions about your poor fuel economy.

Fingers get pointed, excuses are made, and a black, wintry cloud forms over everyone’s head.

It’s no secret that winter driving is hard on fuel economy: plowing through snow on the road, tires that have lower pressure when the temperature drops, extra time to warm up oil and tires, slush chilling the differentials and axles, carrying a ton of extra ice under the trailer, heater and wipers on, and, of course, fuel density.

While fuel density does have an effect on fuel economy, dropping from 860 or 850 to 830 kg per cubic meter—about a three-­percent decrease, depending on the location and time of year—may not be the primary reason for poor fuel numbers.

Air: that invisible gaseous substance that surrounds the earth can also kick the daylights out of fuel economy. Especially winter air.

“As humans, we really can’t sense the density of the air,” explains Ray Camball, head of Project Innovation Transport (PIT) Ontario division, which provides solutions for the transportation industry (see sidebar, pg. 25). “If you can imagine picking up a cubic meter of air, how much do you think that would weigh? It’s over a kilogram! That’s like picking up a liter of water! And it weighs more when it is cold.”

While the average human doesn’t have a problem pushing through air, a truck would tell a different story.

“As vehicle speed goes up four times, the force to push the air goes up 16 times, and the horsepower needed goes up 64 times,” explains Camball. “At 100 km/h, the force gets serious and if you are heading into a 40-km/h wind, the force needed to push it out of the way is almost double of what is needed at 100 km/h when there is no wind.”

Camball’s curiosity about winter air’s affect on fuel was first peaked while strapping snowboards to the bike rack on his van.
“I noticed my fuel economy was not very good, so on the way back home, I turned the boards 90 degrees so they cut into the wind, and tucked them down a bit more,” he explains. “It was dramatic in terms of fuel economy.”

Years later when Camball started at PIT, he came across a question from a fleet member regarding fuel consumption, and thought the issue needed closer inspection. Other team members at PIT had published a newsletter discussing cold air lowering fuel economy years prior for their members, but Camball wanted to add more information. (All PIT results are kept confidential for members for a given period of time to provide members with a competitive advantage. Results are released later to the ­public. Information from the 2006 newsletter was made available to help with this article.)

Camball made a computer simulation comparing the air drag of a highway truck in warm moist light summer air (90F, 32C) against going through cold dry air (0F, minus 18C). The difference was astounding and far beyond a typical three-percent spread between summer and ­winter fuel.

While winter winds can howl at high speeds and destroy fuel economy, explained Camball, the more dismal news for winter drivers is that cold, dry air is 20-percent heavier than what you get in the summer months. Who knew?

As Camball explains, at 100 km/h the truck would use 20-percent more force just to push the cold air aside, which means an approximately 14-percent increase in overall vehicle fuel consumption. (In windy conditions, this ­figure can jump to 20 percent overall when the wind is coming from the front or the side.)

Adding to the 14-percent increase in consumption is the three-percent loss for winter fuel as well as the three-percent loss for a spray chilled driveline, totaling 20-26 percent.

Parking the trucks for the winter and going to Florida is not an option for most people, so what does this mean for the driver and fleet?
Good driving habits, like looking far ahead and leaving a gap will still save fuel and help avoid accidents, but the driver will have to wait until summer to get fuel economy numbers that he can brag about.

Fleets, however, will have to adjust their fuel-economy expectations.

Strategy is important. Sheldon Hayes, one of the founding members of PIT and Director of Safety, Compliance and International for SLH Transport, has been addressing the problem head on.

“We have formed a permanent Fuel Economy Task Force whose primary role is to evaluate this and other fuel-saving research information to help our drivers squeeze out the best fuel efficiency, while doing it safely.

“We have tried to establish fair economy benchmarks with drivers by setting realistic seasonal, vehicle specific and regional fuel economy goals that recognize that the same good driving habits can result in very different results in good weather versus bad,” says Hayes, adding that the SLH Fuel Taskforce also works with PIT to develop training tools for remedial and new driver orientation training.

“We try to avoid running empty trailers on days when winter winds are severe,” says Al Thompson, Director of Maintenance at Meyers Transport. “In heavy crosswinds and bad winter weather, we leave it up to the discretion of the driver to decide if it is safe to proceed or pull off into a safe spot.”

Mark Irwin, Terminal/Regional Maintenance Manager for Bison Transport, (who is also an aircraft pilot) notes that “Air temperature is a big factor in the lift of an aircraft, the spread between 40C to -20C will affect lift by 30 percent. How will this affect our trucks running down the road in cold air?

“In the aircraft industry the principle of lift is improved immensely (30 percent) however drag will also increase as the air is much denser. In the trucking world drag is all we know, the cold air effects will improve the volumetric efficiencies of the engine but they are lost as the truck is being pushed through thick air. Can you ­imagine the effects of a cold head wind?

We are definitely heading in the right direction as many manufactures are doing their best to reduce drag. We as an industry need to embrace the changes as a new look emerges for trucks and trailer.”

Camball points out that while the fuel ­numbers are less in the winter, fleets shouldn’t underestimate the value in having smoother aerodynamics.

“When the wind is denser and the vehicle can hold a straighter and safer course, it can penetrate through the wind blasts better than a blunt front end that tends to drift more in diagonal and straight on winds,” he explains. “It is easier and safer for the driver to not have to fight and correct as much in gusts to keep in the lane on slippery roads.”

But winter wind can also provide a boost. “While a wind on your tail may help fuel consumption,” Hayes notes, “it poses additional challenges that require the driver to be more careful about speed and the need to be aware of leaving extra space to allow for increased stopping distances. As in any season, a key strategy is to drive according to road, traffic and weather conditions—even if that means that speeds must drop a bit to be safe.”

How To Calculate Air Drag

If you are using metric units for speed and area:

Air drag force ( in pounds) = 0.008678 x air density x coefficient of drag x frontal area x air speed x air speed

To get HP, multiply the air drag force by road speed (note that speed gets multiplied three times to get to HP and fuel consumption, so HP to push air goes up with speed x speed x speed.)

Example: Your truck has a 0.5 drag coefficient and a vehicle speed of 100km/h and a 10.7 square meter frontal area, and no oncoming wind on a hot, humid summer day (90F 23C). Air is weighing 1.1013 kg/cu metre, and you are pushing the air with 511lbs or 0.607psi or using 63.1 KW or 84.7 HP of power to do that.

Drag = 0.008678 x 1.1013 x 0.5 x 10.7 x 100 x 100 = 511.282 lbs

At the same speed in winter (0F, -18C), it would take 642 lb of push, or 106 HP.

If you have a 40km/h head wind on that winter day, the force jumps to 1259 lb and 208 HP just for air, not including rolling resistance.

This formula can be used for a truck, car, motorcycle or the wind blowing on you when you carry a 4 x 8 sheet of plywood on a windy day just by adjusting the measurements and the drag coefficient ( 1.1 for a flat plate, around 0.5 for many vehicles, 0.1 for a smooth air foil).

 — Courtesy of Ray Cambell