More for the Tankful

Fuel efficiency is an equation with many variables. Knowing how to gauge and use these variables to your benefit will go a long way in increasing your fuel mileage and your bottom line.

First and foremost is how fast you drive. Many fleets govern their trucks at 69 mph or slower, much to the dismay of drivers plying their trade in high-mph states. Many of those same fleets entice drivers toward lower fuel consumption with fuel bonuses. Many experts see excessive speed as a factor in rising maintenance costs as well. Gary Ziebell, fuel efficiency expert at Kenworth’s Advanced Concepts Engineering lab in Renton, Wash., believes, “The only way to get drivers to be more fuel-efficient is to give them positive feedback. Money in their pockets is one way. The other is to provide them with enough electronic information to show them exactly how their behavior affects consumption.”

Modern electronic engines and in-dash driver information centers give drivers tools to improve fuel economy. Cruise control is perhaps the most obvious and useful instrument for drivers seeking fuel nirvana. Grant Sheldon, an owner-operator with a DOT number, recently got 5.75 mpg on a trip of 2,425 miles between Lancaster, Ky., and Portland, Ore. Sheldon says he did 1,200 of those miles at the higher speed limits of the Western states, making his mileage look awfully good. Sheldon was pulling 78,600 pounds in his 1998 T2000 Kenworth with a 500 Detroit and 3:55 rears. Asked what techniques he used to get this mileage, he says, “I put it in cruise and left it there.”

Fleet drivers will also recognize the escalating use of optimized idle in their trucks. Lou Markowitz, fleet manager for Crete Carrier’s Allentown, Pa., terminal, says fleet managers there are looking for an average of no more than 25 percent idle time. Markowitz notes, “We’ve found more fuel is wasted idling than any other cause.” Optimized idling is a system designed to shut down and fire up the motor as needed to maintain battery charge, oil temperature and cab climate. If drivers do not select cab climate, optimized idle will automatically regulate battery charge and oil temperature. Sheldon uses optimized idle, but complains, “The truck shakes when it shuts down and when it kicks in, and can wake a man up.”

Despite such problems, fleet bonus programs will depend on optimized idle as a significant means of controlling fuel consumption. In addition, “Drivers need to shut down their trucks when at all possible,” Crete’s Markowitz says. He offers these numbers as an example: A driver whose idle time was 49.9 percent averaged 5.56 mpg, while a driver averaging 19.2 percent idle time managed 7.71 mpg. Markowitz also says Crete’s Allentown fleet of 140 trucks had an average idle time of 31.74 percent and 6.61 mpg over the last 12 months. Crete’s fleet of Freightliner Centurys is powered by Detroit 434/70s with 3:55 rears. Idle time, however, is not always calculated the same way. Detroit Diesel used a straight idle system until late 1997, when it converted to a system that also recorded high idles, according to Sheldon. Some idle recording systems also do not record if a driver tempts fate and uses only his trailer brake to park.

In addition, Crete’s fleet is governed at 65 mph. Tom Liethen, senior project engineer at Kenworth’s Advanced Concepts Engineering lab, notes that, “Every mile per hour increase over 50 increases fuel consumption by one tenth of a mile per gallon.” Thus, “If Crete governed its trucks at 68 mph, it would cost $4.8 million a year more in fuel,” according to Markowitz. In addition, “Insurance costs would rise if trucks were governed at higher speeds. At $40 net profit per load, Crete will keep its trucks governed at 65,” he says. If Crete’s evolving fuel efficiency policy is any indication, fleets not already factoring fuel efficiency into safety bonuses might well begin to do so. The smart driver will minimize idle time and optimize use of the cruise.

Tom Liethen, left, and Gary Ziebel, transfer their gear from one truck to another in The Dalles. Trailers are dropped here and hooked to the other truck to insure any differences in the trailer do not affect fuel mileage.

In the old days, conscientious drivers got up to speed, found the “sweet spot,” as it was called, and held throttle pressure even to maintain efficient fuel use. By finding the point
at which performance is optimized without wasting fuel through excessive throttle pressure, drivers can increase their fuel mileage. This “feathering” technique has been replaced by cruise control in most situations. However, certain traditional fuel-saving techniques have yet to be replaced by technological advances.

Having recently taken part in a fuel efficiency test conducted by Kenworth’s research and development facility in Renton, this writer can affirm the continued use of several techniques in addition to cruise control. In most situations cruise finds the sweet spot and maintains it. But moving through the gears to get up to speed, a driver can exercise some control of fuel consumption with progressive shifting and conscientious use of the tach to avoid exceeding 1,500 rpm. According to Gary Ziebell, “Modern engines give full power at 1,500 rpm. Running the tach above that mark is a waste of fuel.” Many drivers tend to shift based on engine sound, a sometimes subjective method. Engine sound – particularly in today’s quieter engines – no longer matches an optimal top-end rpm. Perhaps it never did. Progressive shifting, progressively using more rpm in each successive gear up to 1,500 rpm, also remains a way to conserve.

Pulling about 74,000 gross out of Renton headed south for The Dalles, Ore., both the Cummins ISX 500 and the Cat C-15 500 I drove on two different 500-mile rounds responded well to progressive shifting. Of course drivers must be aware that uphill big road get-ons and similar situations require patience and discipline to accelerate safely into traffic.

The trip south to The Dalles included only one grade on which it was necessary to grab gears. Both T2000s were equipped with 13-speed Eaton Fuller transmissions, engine brakes and the now-standard cruise control. Ziebell, a real pro with 35 years of experience, advised me to drop out of cruise on this particular hill, grab gears as needed and keep my rpm no higher than 1,350. Using this method I was able to maintain a higher instant mpg than when I experimentally tried to accelerate and went far over the turbo boost maximum. As the incline steepened, I cut back on the throttle, knowing that both these magnificent engines would pull very well as far down in the rpm range as an even 1,000. Perhaps this procedure sacrifices a little performance but the fuel efficiency benefit is substantial, especially on longer grades. Watching the turbo boost, rpm and instant fuel readouts gives the driver a real sense of what is happening as he competes with the information to find the right mix of throttle pressure and gear selection to get the highest possible mileage. This is what Ziebell is talking about when he remarks that drivers who get positive feedback have both more incentive to perform and the tools to achieve fuel gains and bonus money.

Certainly it is a constant challenge to drive with fuel efficiency in mind. In one sense, cruise makes everything easier; in another, conscientious drivers realize that cruise can put a truck far over 20 psi turbo boost even headed up rolling hills. Ziebell and I agreed I might increase mileage by dropping out of cruise at 20 psi turbo pressure before cresting small hills in rolling terrain and letting the truck coast over the summit, relying on the engine brake rather than the air brake to increase mpg even further.

Fuel-saving procedures like the ones above require much diligence. I tried to boost mileage in the last 50 miles of each trip by dropping out of cruise and using only traditional techniques not dependent on technology. I found the readouts went up considerably if I paid constant attention to mileage, but I sacrificed acceleration, hill climbing power and speed to do so. Most drivers will have neither the patience nor the incentive to look for tiny mileage gains when they know that cruise and optimized idle, and attention to details like proper tire inflation pressure and shutting down whenever possible, will probably get them their fuel bonus without making them late for delivery.

The aerodynamic T2000s I drove both got well over 7 mpg and were very close to each other in actual mpg. We spent the great majority of the time in cruise, effectively negating the driver’s influence. Over the very long haul the conscientious drivers can save fuel with technique, but they should recognize that the most influential choices they make are about using technology.

Fleet drivers will likely make more money in safety bonuses and will lead safer work lives by running against the governor on cruise and using the technology at their disposal to lift fuel mileage into the bonus bracket.


Fuel Efficiency Tips

  1. Maintain proper tire pressure. Even a small pressure loss can cut fuel mileage by as much as 10 percent.
  2. Track your mileage. You will probably notice that cold weather and wet roads, as well as wind, have significant impact on mpg. At 55 mph, it takes half an engine’s power to push a truck through the wind and half to pull the load. Cold, wet roads increase drag on tires.
  3. You can’t always tell how aerodynamic a truck is by its looks. Cabovers are generally more aerodynamic than big hoods with flat windshields, for instance. Don’t blame your coe’s poor mph on its aerodynamics.
  4. Remember that anytime your turbo boost rises substantially you are using excess fuel. Dropping out of cruise and matching turbo boost with rpm to find a comfortable speed at which to climb grades without excessive boost or rpm will save fuel.
  5. Getting out of cruise and allowing the truck to crest a grade may tempt some drivers to rely exclusively on the engine brake on the down side. This is fine unless your engine brake decides not to work. Be careful.
  6. Fuel gets hotter as you draw down below about 60 gallons. Fuel hotter than 150 degrees will cause today’s engines to de-rate themselves, and fuel mileage will plummet. Performance may also suffer. Fill up before getting down to a quarter full.
  7. Be judicious in your use of idling fuel. Don’t roast, don’t freeze, but when conditions permit open a window or throw on a blanket rather than burn fuel.
  8. Shut it down when you will be out of the truck for more than three minutes.
  9. Practice progressive shifting.
  10. There is no more obvious place where fuel economy and safety intersect than at speed. Every mile per hour over 55 costs you a tenth of a mpg. And the faster you go the greater would be the severity of any crash. Drive safely.
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