Awise man once advised "spend your energy taking action on the things you can control not on complaining about the things you can't." As escalating prices at diesel pumps threaten to take yet another ...
Awise man once advised “spend your energy taking action on the things you can control not on complaining about the things you can’t.” As escalating prices at diesel pumps threaten to take yet another chunk out of fleet owners’ profits, that’s advice well-worth heeding.
Reality is the price at the pump is controlled by international forces beyond any government the industry can lobby. The only part of the fuel price equation the fleet does have direct control over is how much fuel its vehicles consume.
A number of factors, ranging from tire rolling resistance and road speed to gear train losses and aerodynamic drag, have a direct impact on fuel savings. Let’s start with aerodynamic drag.
Any vehicular body moving through the air is subject to aerodynamic drag, but trucks are impacted mainly by pressure drag, one of the three types of aerodynamic drag. As a rig punches through the air, it’s fighting a great deal of pressure, but as Randolph Baerg, of Innovative Vehicle Testing, explains, it’s the flow of air as it moves over the sides and back of the truck that’s a greater concern.
“What’s happening is a separation of the air flow coming down the side of the truck. It’s shedding off vortices and swirling around back there,” Baerg says. Picture yourself standing at the side of a highway when a tractor and trailer whiz by and you can understand what Baerg is talking about. For several seconds afterwards you are buffeted by wind, dirt and leaves kicked up and swirled about by the truck’s passing. Now imagine how big a fan would be required to create the same effect and how much fuel would be required to fuel that fan and you get an appreciation of how much fuel your rig must spend to punch through the air.
There are several variables that determine the degree of drag on your truck: the shape and size of your vehicle, the speed you are travelling at, and the air density and temperature. You can do something about the first two.
Several tests – ranging from wind tunnel setups to computer models – are used to determine which truck designs slip through the air with the least amount of drag. The process is fairly complicated but to provide you with a rough idea, consider the figures provided by Baerg. A flat plate (similar to the design of long nose conventionals) has a drag coefficient of 1.95. Change that shape to a half-hemisphere and the drag coefficient is reduced to 1.42. Change it to a typical wing design and the drag coefficient is reduced to 0.007.
What’s the impact on fuel consumption? “Take a 100-inch by 100-inch area, which amounts to a 10,000-square-inch frontal area on your truck, for example,” explains Baerg. “Say the resulting one hundredth of a psi pressure on the front creates 100 lb of drag. At 100/kmh that adds up to 17 hp. That’s quite a bit of fuel when your truck is doing 100,000 miles a year.”
The other variable you have control over is speed. Unlike rolling resistance, which is directly proportional to the vehicle weight (in other words, doubling the weight, doubles the rolling resistance) speed is proportional to the square of the speed. Without getting into complicated mathematics that means simply that if you increase speed by a factor of two you increase the drag on the rig by a factor of four. You can see the instant impact on fuel costs.
“Aerodynamic drag is not a big deal under 30 mph. If you’re operating an inner city fleet and you’re not going over 50 mph, aerodynamic drag is not really going to affect you. But at 60 mph the drag is twice the rolling resistance for a particular vehicle,” says Baerg. “If you are driving a semi across the prairies at high speed day in and day out you have to really look at the aerodynamic drag business because that’s where all your money is going. Your foot to the floor is really costing you money.”
Gear train loss is another factor that has a direct impact on fuel savings and may require some rethinking of your spec’ing preferences, particularly as they relate to direct drive versus overdrive transmissions.
“Contrary to popular belief, overdrive transmissions don’t give you something for nothing. There is no such thing as a free lunch,” says Charles Allen Jr., director of sales and marketing for ZF Meritor, a 50/50 joint venture between ZF and ArvinMeritor. “Direct drive can offer you fuel savings.”
Allen Jr. explains that the countershafts on direct drive transmissions run slower than they do on overdrives. A 10-speed direct drive runs at 75 percent of engine speed; a 10-speed overdrive runs at 102 percent of engine speed. And direct drives experience less churning loss than overdrives.
Even more important than the lower churning loss, however, is the fact that there are no active gear meshes with direct drive.
“With overdrive there are two active gear meshes in high gear. Any time you transmit power across a gear mesh you are losing efficiency,” he says. The loss of efficiency amounts to 0.75 percent per mesh, according to Allen Jr.
He adds that fuel economy testing shows a two to five percent improvement with a direct drive drivetrain. This is the impact of that to your fuel consumption: If you’re running 125,000 miles at 6.5 mpg, a two percent improvement will cut your fuel consumption by 385 gallons per year. Bump that fuel saving improvement to five percent with the same amount of mileage and you would be reducing your fuel consumption by 962 gallons per year.
The rolling resistance of a rig also affects the fuel bill. Rolling resistance is influenced by a myriad of factors from the ambient temperature to the composition of the road’s surface, but there are three tire design elements which play a significant role: tread pattern, rubber compounding and internal structure.
It’s generally accepted among those in the tire business that a three percent reduction in rolling resistance results in a one percent gain in fuel efficiency. The obvious lesson here is that you need to consider more than the sticker price when spec’ing tires. A lower-priced tire may not be such a bargain if it increases your fuel costs.
The next step beyond understanding the factors impacting fuel economy is putting together a realistic plan that allows you to reduce your fuel costs without downgrading the service you are able to provide. The FleetSmart program set up by Natural Resources Canada can help. The program has been created to help fleets conserve energy and is building a bank of information to help you along the way. See their Website at fleetsmart.nrcan.ga.ca.