While powertrain spec’ing is the first step in achieving optimal fuel economy, there are other vehicle components and calibrations that can help you put your power to the ground and through the air as efficiently as possible.
If you’ve been through a powertrain-spec’ing exercise, you’ve probably chosen engines with enough horsepower for your application, without overkill (see “Savvy spec’ing 1: Understanding engine power needs,” CCJ, March 2002). You’ve probably also chosen the best transmission/rear axle combinations for your engines, loads, speeds and terrain (see “Savvy spec’ing 2: Gears ‘n rears,” page 36).
But, according to Recommended Practices from the Technology & Maintenance Council (TMC), there are other areas where spec’ing the right equipment and calibrations can contribute substantially to vehicle fuel efficiency. The three highest-payback areas are aerodynamics, idle limiting and road-speed limiting.
Be an aero-smith
By most industry estimates, overcoming air resistance can consume about the same amount of available power as rolling resistance – up to about 35 percent, depending on the shape of the truck.
So, if you’re neatly carving through the air with a clean, wedge-shaped vehicle, instead of bullying your way through with something resembling a barn door, your power, and fuel, will be spent on making miles, not wind.
According to TMC, a standard, roof-mounted wind deflector can provide up to a 6 percent improvement in fuel economy. A full-roof fairing (sides closed above the roof) can contribute up to a whopping 15 percent, with cab extenders contributing another one to 2 percent. And a front air dam and tractor side skirts can each contribute up to 3 percent.
The latter two devices serve to shield the high drag of axles, oil pans and other aerodynamically “dirty” under-cab components, says TMC. Side skirts also help get air flow attached to the sides of the tractor near the ground. Without them, near-ground air flow is very turbulent – a major contributor to aerodynamic drag.
“We use full fairings and side skirts on all our over-the-road equipment,” says Tom Newby, area maintenance manager, Old Dominion Freight Line, Greensboro, N.C. “They definitely pay back the investment in fuel savings.”
Of course, nothing comes without a price, and aero-enhancing add-ons, such as front air dams, frequently fall victim to impact damage. “We stopped using side fairings because the benefits didn’t justify the repair costs,” attests Les Hazen, director of maintenance, Prime Inc., Springfield, Mo. “And thank God for three-piece front bumpers/air dams, which allow replacing just the damaged section.”
“Drivers need to be aware that these components are vulnerable,” agrees Newby. “But ours are, so we really don’t have a problem.”
Finally, TMC advises that outboard components, such as external air cleaners, unshielded exhaust stacks and extra mirrors, make it harder for trucks to slip through the air, and should be minimized.
Don’t sit idly by
There are lots of good reasons to make sure your trucks spend as little time idling as possible. Foremost, an idling engine gets zero mpg. Using TMC/SAE findings that an idling big-bore diesel consumes one half gallon of fuel per hour, even a lunch break’s worth of idling can amount to hundreds of dollars’ worth of fuel per year, per vehicle, for no miles. Increase idling time, and there’s practically no limit to how much you can spend going nowhere.
Less apparent is the effect of idling on maintenance costs. Excessive idling hastens oil degradation, because a diesel’s temperature is lower when idling than when it’s under load. This results in less efficient combustion and more soot in the crankcase, depleting the engine oil’s detergents and dispersants.
The lower temperature also allows water vapor from combustion to condense in the crankcase. The water mixes with sulfurous combustion byproducts to form sulfuric acid, which reduces the oil’s alkalinity reserve, or total base number (TBN).
Once essential additives are gone, sludge accumulates, reducing lubrication and accelerating wear, and acid begins to attack valve stems, bearings, liners and piston rings.
For these reasons, engine manufacturers specify more frequent oil changes for engines that spend a lot of time idling – a condition they classify as severe service.
The easiest fix is to have the idle-time shutdown feature in your engines’ electronic control modules (ECMs) activated right off the bat. But that’s not going to fly for a driver trying to get some shuteye in a frosty sleeper.
“Idle shutdown is a two-edged sword,” notes Darry Stuart president, DWS Fleet Management, Wrentham, Mass., and a recipient of CCJ’s Career Leadership Award. “It can provide tremendous fuel-saving benefits, but there have to be exceptions to its use. Take a driver who’s just spent an hour unloading a reefer in the winter. When he’s done, he needs heat, right now. You have to look at it from the perspectives of both an equipment manager and driver manager, and weigh the benefits. We recommend starting with a 5-minute idle shutdown, and working from there.”
A more insightful solution than idle shutdown is to ask yourself why the engine is being idled. Usually it’s for heat or A/C, so a thermostatically controlled, engine start/stop function, which can be baked into some engine ECMs, makes more sense.
Add-on devices, such as auxiliary cab/bunk heaters, coolers and gen-sets can perform functions traditionally accomplished by idling, but can cost from a few hundred dollars to several thousand. Depending on how long you plan to keep a vehicle and how much fuel is saved by avoiding idling, it’s not too hard to figure out if and when these devices will pay for themselves.
Mo fasta, no betta
Spec’ing sensible road speed – which is really just a matter of having an engine’s ECM parameters set – is among the most effective ways to ensure optimal fuel efficiency, according to TMC.
OEM testing has shown that, for a loaded, aerodynamic rig, riding on fuel-efficient radials, about 213 hp is needed to maintain a steady 65 mph on a level road. The same vehicle requires about 297 hp to maintain 75 mph. That’s because air resistance increases, not in a linear fashion, but exponentially, with respect to an increase in vehicle speed.
Assuming the truck’s engine has a modest brake specific fuel consumption (BSFC) of .32 lb/bhp-hr, its fuel economy would be about 6.7 mpg at 65 mph, and about 5.5 mpg at 75. That’s around a 20 percent difference, or about a 2 percent fuel-economy drop for each 1 mph over 65.
Accelerated component wear at higher speeds is another factor to consider. An engine’s longevity is directly related to the amount of fuel burned over its lifetime. Better mpg, then, means lower engine stress and longer life.
Moreover, OEM testing has shown that driveshaft torque increases 20-25 percent at 75 mph, compared to 65, and U-joint life decreases more than 30 percent. While this can be offset by spec’ing higher-capacity components, these add to vehicle cost and weight.
It takes longer to stop a faster-moving vehicle, so tires and brakes run hotter and wear significantly faster. In addition, higher speeds generate more wind and road noise, and vibrations at the higher frequencies associated with increased speed may well adversely affect moving suspension components.
So, a very effective spec’ing decision, for all the right reasons, is the “gear-fast, run-slow” concept. This means the powertrain is spec’d for a high top speed, but the vehicle is limited to a slower speed, thereby keeping the engine running in an efficient, low-rpm range. It maximizes fuel economy and protects your investment in your equipment.
“We limit speed to 65 mph,” says Old Dominion’s Newby, “although our trucks are geared to go much faster. It really works.”
DWS’ Stuart, who was one of the pioneers of the gear-fast, run-slow concept, recently worked for a fleet, and spec’d trucks to run at 90 mph. “But we limited top speed to 69 mph,” he says. “Fully loaded, they were getting nearly 7.5 mpg.”
“As a leasing company, we’re not paying for the fuel,” notes Jerry Thrift, group manager, maintenance services, Ryder Transportation Services, Miami. “But we want to give our customers what they want. So we offer a performance-spec package and a gear-fast, run-slow package. The latter really is more fuel-efficient.”
The rest of the best
According to TMC’s Recommended Practice 1111, Relationships between truck components and fuel economy, there are a host of other spec’ing decisions that fleet managers can make in their quest for better mpg.
For example, cruise control can contribute up to a 6 percent improvement, especially where less experienced drivers are concerned. “It really does depend on the driver,” agrees Prime’s Hazen. “On an eight or 10-hour drive, that right foot gets moving. With cruise, you know road speed is locked in. We even counsel our drivers on percent of time spent in cruise, based on downloaded ECM data.”
Spec’ing rib-tread drive-axle tires instead of lug tread, advises TMC, can improve mpg 2 to 4 percent, while using ribs on the drive axles and shallow ribs on the trailer can brighten the picture from 6 to 14 percent.
And, do you really need tandem drive axles? If you go with a single drive, perhaps with a tag or pusher axle as is the norm in Europe, you could see a fuel economy gain of 2 to 3 percent. “We run single drive axles on all our trucks,” says Tom Newby, “and we’ve never had a situation where they didn’t perform as well as a tandem.”
What about transmission/drive-axle lubes? TMC notes that, in cold weather, synthetics can provide up to a 2 percent fuel economy improvement. “It really does provide a small increase in mpg,” attests Ryder’s Thrift, “although not all of our customers will notice. A side benefit for us is ultra-extended drain intervals.”
