Fuel-stingy equipment
Looking for the best mpg from your equipment? Don’t stop at the powertrain: Consider aerodynamic additions,
idle-limiting systems and road-speed governors.
By Paul Richards
Powertrain spec’ing is the first step in achieving optimal fuel economy, and the best choices depend on your application. You’ve probably chosen engines with enough horsepower for your application, without wasteful overkill. You’ve probably also chosen the best transmission/rear axle combinations for your engines, loads, speeds and terrain.
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 to fuel efficiency. The highest-payback areas – especially in over-the-road applications – are aerodynamics, idle limiting and road-speed limiting.
Slip through the air
By most industry estimates, overcoming air resistance can consume about the same amount of available power as rolling resistance – up to about 40 percent, depending on the shape of the truck. So, if you’re neatly carving through the air with a clean, wedge-shaped vehicle, 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 1 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.
Old Dominion Freight Lines uses full fairings and fuel tank skirts on its over-the-road sleepers, says Tom Newby, director of field maintenance for the Thomasville, N.C. – based carrier. “And we use air shields and side fairings on our linehaul daycabs. We definitely see value in them.”
Nothing comes without a price, and aero-enhancing add-ons, such as front air dams, frequently fall victim to impact damage. “Drivers need to be aware that these components are vulnerable,” says Newby. “But ours are, so we really don’t have a problem.”
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.
Less apparent is the effect of idling on maintenance costs. Excessive idling hastens oil degradation, since 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.
Some degree of engine wear takes place whether an engine is idling or moving freight, which could mean reduced time to overhaul for high-idle-time engines. According to the Argonne National Laboratory, unnecessary idling can shorten a diesel’s life by six to 12 months.
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 of Quickway Transportation, based in Wrentham, Mass. “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,” says Stuart, a recipient of CCJ’s Career Leadership Award. “We recommend starting with a 5-minute idle shutdown, and working from there.”
Rather than just shutting down an idling engine, 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. TMC’s Recommended Practice 1105A, Idling limiting systems, provides a list of these devices and their associated capabilities and costs.
Ease up on the throttle
Spec’ing sensible road speed – 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 typically-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 approximately 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 to 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 faster. In addition, higher speeds generate more wind and road noise, and vibrations at the higher frequencies associated with increased speed may 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, keeping the engine running in an efficient, low-rpm range. This maximizes fuel economy and protects your investment.
“We limit speed to 68 miles per hour,” says Old Dominion’s Newby, “although our trucks are geared to go much faster. We used to limit them to 65 miles per hour, and we’ve lost a little fuel economy since we raised the limit, but we’ve made gains in service.”
DWS’ Stuart, who was one of the pioneers of the gear-fast, run-slow concept, spec’d trucks to run at 90 mph a couple of years ago. “But we limited top speed to 69 mph,” he says. “Fully loaded, they were getting nearly 7.5 miles per gallon with pre- ’02 engines. With the newer engines, we’ve lost an average of nearly 1 mile per gallon.”
“As a leasing company, we’re not paying for the fuel,” notes Jerry Thrift, senior manager of new product development for Miami-based Ryder Transportation Services. “But we want to give our customers what they want. So we offer three performance packages – a performance-spec package, a balanced performance package and a gear-fast, run-slow package. The latter package is the most fuel-efficient.”
The rest of the best
According to TMC’s Recommended Practice 1111, Relationships between truck components and fuel
economy, a host of other spec’ing decisions can help
fleet managers 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. “I tell our owner-operators to use cruise control,” says Bill Taylor, director of maintenance for Prime Inc., based in Springfield, Mo. “It meters the fuel far more effectively than your lead foot.”
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 our trucks,” says Newby, “and, at least in our application, they work just fine.”
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 miles per gallon,” attests Ryder’s Thrift, “although not all of our customers will notice. Another benefit is that we never have to drain the lube.”
For more on these and other fuel-saving spec’ing tips, order Recommended Practices 1105A or 1111 or the complete Recommended Practices Manual. Call TMC at 703-838-1763 or 800-ATA-LINE, or visit http://tmc.truckline.com.
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