Guide To Fuel Economy
Hands-On Tips For Buying Fuel, Driving & Spec'ing For Better Economy
By John Bendel
Technical Editor
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Tom Berg
Equipment Editor
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Deborah Whistler
Editor
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Fuel supplies and costs have become a permanent crisis in trucking. Commercial carriers that might have weathered the recent recession were unable to do that because of escalating fuel costs. More than a few went out of business.
Owner-operators fared worse. Literally thousands failed during the same period for the same reasons. Meanwhile, private fleet operators found fuel consuming ever larger pieces of limited budgets.
As if that weren't enough trouble, newly mandated low-emissions engines actually lower fuel performance by from 1% to more than 10%. Clearly, fuel economy is a major challenge.
Spec'ing For Economy
A vehicle's basic configuration has a lot to do with fuel economy. A properly spec'ed truck can achieve impressive miles-per-gallon performance. Spec poorly and the numbers will surely go down.
Tests show that gross vehicle weight is the largest factor in drag or rolling resistance. But weight is largely determined by the cargo, something fleets can rarely control. You have to haul what you have to haul.
The second largest factor is air resistance; fleets can do something about that.
There may be arguments for picking a classic, long-nosed conventional tractor. Style appeals to many drivers. The mystique of the classic cab makes it easy to sell, and it usually brings more money than an aero-type truck.
But even a classic tractor will usually have an air deflector on the roof. And choosing an aero model can save 4 cents per mile or more in fuel. Many drivers would change jobs to get a 4-cent-a-mile raise. Fleets should be willing to change specs to keep that much more revenue for every truck.
Aero-style trucks usually come with air deflectors included for your trucking application. If you pull a van or reefer, the most important deflector is the one on the cab's roof. It moves air up and over the roof of the trailer. Without it, air blasts straight into the trailer's nose, which is like pushing a barn door. It creates considerable drag and costs plenty of fuel.
According to the TMC, a full cab roof fairing - one that matches the height of the trailer - will save up to 15% compared to running with nothing at all (see accompanying story). If the tractor has a raised-roof sleeper, chances are it's a little shorter than the trailer's roof, so you'd need a small fairing to kick the air completely up. Without it, you'd lose 4% to 10% of the benefits of a full roof fairing.
If you pull a low-height trailer, like a tanker, grain hauler or flatbed, a full-height deflector would probably add resistance. You'd do better with a flat-roof or mid-roof sleeper that better matches your trailer. Aero tractors can be ordered with varying sleeper types or with no sleeper at all.
Other deflectors usually come as a package on an aero tractor. These include cab extenders, which cover some of the tractor/trailer gap; side skirts that cover the fuel tanks and battery box and smooth out the profile of the steps; and an air-dam front bumper. If you put any of these on a non-aero tractor, you might pick up 1% to 3% in fuel economy.
Aero devices on trailers are rare, but they can save as much or more as aero aids on tractors. Bubble-type nose and tail appendages and fin-type vortex generators are available for fitting to trailers. If your tractors are married to trailers, they would probably save you money. If yours is a hook-and-drop operation, you'd have to figure out how much time each trailer sits idle to see if aerodynamics would pay.
Medium duty trucks often run in cities where stop-and-go traffic pulls down average road speed and reduces the benefits of aerodynamics. That's why few city trucks have full fairing packages, or any fairings at all. But most have smooth styling to ease air flow. And the van boxes mounted on many midrange trucks have rounded roof edges to reduce wind resistance. Any truck that spends a lot of time on freeways where cruising speeds are high would save considerable fuel if fitted with a supplemental fairing on the body's nose or on the cab's roof.
Engine Selection
Adequate power for an over-the-road tractor generally means 430 or more horses. A 475 is enough to briskly climb most steep grades and will still get good fuel economy if driven right; 500 hp or more is probably excessive from a fuel-use standpoint. A good compromise is a "multi" model that boosts power and torque on upgrades and automatically cuts back for cruising. Of course most electronic controls can be "flashed" to uprate the engine, and that adds value when the truck's being sold or traded in.
If it's a regional tractor or dump truck, an 11- or 12-liter diesel is OK if it's got around 400 horses. Mixer fleets tend to run even smaller engines, and drivers understand the need for light weight.
You can take advantage of the speed control in many of today's electronically controlled engines. Fleet analysis shows that reducing road speed to 60 mph from 65 can lower fuel use by about 8%. A rule of thumb established by TMC is 1 mph equals 0.1 mpg. So a 5-mph reduction in average road speed would yield a 0.5 mpg improvement in fuel economy.
Unfortunately, cutting speed frustrates many drivers. Fleets may have to settle for a middle ground where speed control is concerned.
At the same horsepower output, an engine with electronic controls burns 7% to 15% less fuel than one with mechanical controls. If you buy used trucks, and older ones at that, you will almost certainly be better off with electronically controlled diesels. They are reliable, use less fuel, make less smoke and allow quick and easy diagnosing of problems.
Gear Ratios
Gear ratios in the transmission and rear axle(s), along with the tire/wheel size, determine how fast the engine will spin at a given road speed. Each must be chosen with the other in mind.
The best way to gear for fuel economy is to "gear fast, run slow." Spec the truck to run fairly fast, but drive it slowly so the engine loafs. Most trucks are set up so they can run as fast as 75 or 80 mph but cruise at about 65 mph, at which point the engine will turn over at 1,500 to 1,600 rpm. This is the engine's sweet spot, where it uses the least fuel; exactly where that spot is will vary with the engine make and model, so check out its specifications before picking or approving the gear specs. Of course, engine controls can be set to limit top road speed no matter what mechanical gearing allows.
If you want the truck to actually cruise at 75 mph, which is legal in some Western states, it can be done. Just be realistic when setting up the truck. If it runs cross-country and that speed varies from 55 to 75 mph, pick a multispeed transmission that lets drivers split up or down to lower or raise engine speed appropriately.
Axle ratios are said to be "fast" or "slow." All other specs being the same, the higher the axle's numerical ratio (for instance, a 4.11), the slower the truck can run. The lower the number (like a 2.90), the faster the truck can run. You can pick either ratio type, or something in between. Just be sure the transmission's top-gear ratio is something that works well with the axle. An overdrive top-gear ratio is usually paired with a slow (numerically high) axle ratio, and a direct-drive transmission is used with a fast (numerically low) axle.
A direct-drive top gear can save a bit of fuel. That's because it involves fewer gears in the transmission and less churning of lubricant. Controlled tests show the reduced friction of a direct-drive transmission saves up to 2% in fuel at cruising speed. Some fleet managers have measured some savings in tank mileage, and you may or may not see it at the pump. The more varied your cruising speed and operating conditions, the less actual savings you'd get from this or any other component that is most effective at relatively high speeds.
Automatic Transmissions
New engines are less forgiving and require more skill from the driver to keep the revs at optimum fuel conservation levels. An automated transmission can help less skilled, less economical shifters do a better job. According so some experts, automated transmissions typically save fleets 5% in fuel use - enough to pay for the transmissions in two years.
So while automatics only account for about 10% of the heavy truck market today (50% for mediums and 95% for light trucks), Class 8 fleet managers might seriously consider spec'ing automatics in their next truck order.
Tires
Tread type greatly affects fuel economy because it helps determine rolling resistance. The least resistance is offered by rib-style tires. Most fleets try to put these at all wheel positions, at least when weather allows. Switching the drive axles to lug-type tires in winter improves traction but adds resistance, and costs 2% to 4% in fuel economy. Installing shallow rib tires on the trailer can save 6% to 14% compared to a trailer with regular ribs.
Retreads usually offer more rolling resistance than original treads, according to TMC. Drive-axle tires still on their original tread can be up to 7% better on fuel than retreads. The shallower the tread - original or recap - the better the fuel economy. Of course, results vary in different operations and retreads save money in other ways.
Trailer tires generate most of the fuel saved, according to testing done by Bridgestone, whose tests also found that the contribution of individual axle positions to fuel economy varied with vehicle configuration.
Don't waste gains in rolling resistance by neglecting inflation maintenance. Maintaining the correct pressure is as important as the tires used. Tires under-inflated by 15% will cause fuel penalties of around 2.5%, and a 30% under-inflation will increase fuel consumption by 5%.
Other Components
Many fuel-saving components are standard equipment on nearly all trucks. Other items can be spec'ed. For example:
Low-restriction exhaust, including large-diameter pipes and dual exhausts, can save 0.3% to 2% over a more restrictive exhaust system.
On/off fan drives save varying amounts of fuel by turning off the power-robbing fan, depending on how much the fan has to run to cool the engine. Measured savings range from 0.5% to 18%.
Synthetic gear lubricant in axles and transmission can save up to 0.5% in summer and 2% in winter.
A tag tandem, where a single drive axle is paired with a non-powered tag or pusher axle, can save 2% to 3% over a live twin-screw tandem. A liftable dead axle can shift weight and traction to the drive axle, so there's little or no driving penalty. But a twin-screw tandem always brings more resale money.
Trailer types also affect economy by adding or reducing air resistance. Double trailers cost 6% to 10% more fuel to pull than a single semi; a smooth-sided van costs 2% to 4% more in fuel than one with exterior posts; and a 13-foot, 6-inch-high van costs 2% to 8% more to pull than one that's 12 feet, 6 inches high depending on the tractor's aerodynamics.
Bug deflectors can cost as much as 1.5% in fuel because they spoil air flow over the hood.
Idling
According to EPA, a truck consumes up to one gallon of diesel fuel for each hour idling. Idle for an average of 6 hours a day and you've burned up almost 2,200 gallons of fuel. At $1.70 per gallon, that amounts to more than $3,700. Clearly, one of the most immediate ways many fleets can save fuel is to curtail idling.
However, that could put some fleet managers on a collision course with driver preferences - and even traditions. Yet there are ways to encourage driver cooperation.
The traditional way to warm or cool the sleeper is idling the tractor's main engine to run the heater or air conditioner. With engines off, how can drivers stay warm or cool unless outside temps are in a comfortable range?
Here are some alternatives
Inverters.
An industrial-grade inverter - not a cheapie that plugs into the cigarette lighter - will cost about $1,500, plus installation, but it will provide the power to run electrical accessories without risk to the truck. It must be hard-wired to the battery bank, and wiring between there and the outlet must be robust. The inverter also should have a built-in low-voltage shut-down that will protect the truck's batteries so they can still crank the engine in the morning.
Isolated battery.
This idea, borrowed from motor homes, keeps one or more batteries dedicated to "house" power, using 12-volt DC lights and accessories, and the others strictly for engine cranking and chassis power. All batteries are charged by the engine-driven alternator as the truck goes down the road; the alternator must be strong enough for the extra work. A kit for an isolated battery circuit should cost a few hundred dollars.
Shorepower.
Available now as an option from several truck builders, shorepower works like it does in recreational vehicles: The driver plugs into an outside 110/120-volt AC power source that runs all on-board accessories and charges the truck's batteries. Shorepower usually includes an inverter, which can be used when no plug-in is available. Total cost might be $2,500 to $3,000. For the moment, there are few AC outlets available for trucks, but if the folks at IdleAire have it right, there are lots more on the way. IdleAire systems provides heat, air conditioning, cable TV and Internet access along with AC power. The company says it has contracts to install its systems at hundreds of truckstops around the country.
Cab heaters.
Some burn very small amounts of diesel fuel and keep cabs and sleepers as warm as drivers can stand. Installation instructions stipulate venting to the outside, and the devices come with necessary ducting and thermostats for on/off operation. Some battery power is needed to run a circulating fan. A good heater from Espar or Webasto costs $1,000 to $1,500 and takes a few hours to install. However, there are alternatives.
Cab coolers.
These operate chemically or mechanically, cooling the sleeper area without running the main engine. "Phase-change" units have chemicals that chill or freeze at relatively high temperatures, which is done by blowing cold air from the truck's A/C system across containers as the truck runs; air in the sleeper is blown across the containers while the truck is parked and the engine is off. Mechanical systems use battery or outside power to run motor-driven A/C compressors.
Auxiliary power units.
An APU has its own small diesel engine to run heating and air conditioning devices, and often an alternator to keep batteries charged, the engine warm and run electrical accessories. An APU costs $3,000 to $6,000, but can be moved from an old truck to a new one. It weighs 200 to 400 pounds, and is generally mounted along the frame where it's easy to service, but can be damaged by collision and road debris.
Integrated power units.
The Willis APU is best installed at the truck factory so its systems can be combined with and, even replace, equipment like the alternator and starter. The product can be set up to warm fuel and pre-lube the engine before cranking. The basic Willis APU can be retrofitted for $7,900 list; Installation by factory or its modification center should be about $1,000 less, and the company expects this to begin soon.
Caterpillar Electronics' MorElectric system must be installed at the truck factory. It replaces the engine-driven water pump and air compressor with electrically powered accessories, includes an APU, and will work with all truck engines, not just Cats. It is being field tested but won't be ready for general retail sales until late 2005, so the company has not yet set a price.
Extra insulation.
Cabs and sleepers can be ordered with "premium" or "Arctic" insulating packages to fight off extreme temps outside and retain the heat or cool air that you're paying to make. Ask exactly how the builder insulates the cab and to what "R" value - the same rating used for home insulation.
Drivers Are Key
You can spend many hours and dollars setting up trucks to attain good fuel economy, but that won't matter if drivers don't care. Their bad habits can negate almost every fuel saving effort, so you need to get them to strive to drive economically. Train drivers on proper techniques, and look into using an incentive program to reward them for saving fuel.
Offering to share dollar savings with drivers gives them a stake in the effort and greatly increases the likelihood they will act conscientiously. Most of them will like it because it amounts to a raise. Making it a contest can yield even better results. Some managers post fuel economy figures periodically, and drivers tend to cajole each other into getting better numbers. Some drivers have been known to actually buy fuel with their own money to qualify for cash bonuses that are higher than what they spent.
Economy figures can be captured from electronic engine controls and processed by your computer system. Or you can simply compare miles run with gallons of fuel burned. Truck and engine manufacturers have developed incentive programs that they will supply to you, usually for free. Many builders have driver trainers who will teach the techniques of economical driving to your people. And you can get materials - pamphlets, booklets, and audio and video tapes and CDs - to distribute to drivers, who can use them on trips to sharpen their skills. There's a lot of help available from manufacturers and dealers if you ask for it.
Sidebars
Top 10 Fuel Economy Factors
Tips For Frugal Buying
The Skinny On Fuel Networks
Tips For Drivers
