by
Paul Abelson
There's no need to tell anyone reading Land Line that fuel costs have
climbed dramatically, and that any change in what is already the
highest cost item in trucking (other than personnel costs) will
have an equally dramatic effect on annual profits. So perhaps now
is the best time to make those improvements to get the greatest
return on your fuel dollar.
To
put things in perspective, let's review some basics about diesel
fuel, how it works, and how you can best manage its use. We'll start
with the fuel itself. Diesel is a mixture of hydrocarbons (see the
article on additives in this issue) that can be looked at as stored
energy waiting to be released. Diesel, which is refined from crude
oil, is the result of the decomposition of organic matter, liquefied
under tremendous pressure over millions of years. As a refined fuel,
it has the potential energy of about 135,000 British Thermal Units
(BTUs) per gallon. A BTU is the energy it takes to heat one pound
of water one degree F. The values are approximate because diesel
can vary in its makeup.
Not
all the energy is captured to do useful work when diesel is burned.
First of all, not all the fuel is completely combusted. If it were,
we'd have only carbon dioxide (CO2) and water vapor (H2O) in the
exhaust; no soot and no hydrocarbons or particulates. Second, there
are heat losses into the cooling system and out the exhaust. Some
of the exhaust heat is captured and reused in the turbocharger,
but in a modern engine, barely 40 percent of the energy in the fuel
is harnessed to do useful work. In the '80s, this was only about
33 percent, so engines have made significant progress in the last
15 years or so.
The
important thing to remember is that every bit of energy your truck
uses comes from just one place: the fuel in your tanks. Unless you're
hooked up to 110V shore power, diesel provides all your energy needs.
Even generator sets or fuel-fired heaters run on the same diesel
your engine uses. And anything that uses more energy than is absolutely
necessary means that you burn more diesel. Let's look at driving
habits as they relate to energy.
Rolling
resistance
Energy
is consumed by overcoming aerodynamic drag, internal friction, rolling
resistance and by accelerating and decelerating (braking). Internal
rolling resistance is largely a function of truck design, and other
than using low-friction lubricants, there isn't much you can do
to manage it. Rolling resistance is a mix of controllable and non-controllable
factors. Tread design and tire carcass construction play a large
role in rolling resistance, and they, too, are outside a driver's
control. But air pressure is within your control, and you can check
alignment.
Tires
flex when they roll and flexing absorbs energy. Air is a structural
part of a tire. The more air that is in the tire, the stiffer it
is and the less it will flex. Tires 20 psi low on air can lose more
than 3 percent fuel economy. Tire alignment is important because
if a tire is not rolling absolutely straight, it is being dragged
sideways, and rubber is being rubbed off the tire. It takes energy
to do that. The more the tires are out of line, the more energy
is wasted.
Aerodynamics
Aerodynamic
drag is a function of three things, two of which are outside your
control. Frontal area, coefficient of drag and speed make up total
drag. The shape of the truck dictates coefficient of friction. Once
you buy your truck, you're stuck with your choice, for better or
worse. The differences between long-nosed, classic conventionals
and the new, aerodynamic trucks could make more than a one mpg difference,
all other things being equal. Frontal area is also outside your
control, unless you're loading flatbeds (but that's a subject for
another time). We'll assume frontal area, the size of the hole your
truck punches through the air, to be constant. But closely related
to frontal area, the gap between tractor and trailer can affect
aerodynamic drag. The closer the two are together, the less the
air will fill in behind the tractor and impact the trailer's front
wall. Spread them out more than 36 inches and it starts to be like
pulling a second front through the air.
The
greatest variable is speed, because aerodynamic drag varies, not
directly with speed, but with the square of the speed. Sixty-five
miles an hour is only 18 percent faster than 55, but drag increases
by 40 percent. Controlled tests run by TMC demonstrated that increasing
maximum speed from 55 to 65 added between five and eight miles an
hour to average speed, depending on terrain and traffic, but took
as much as 18 percent more fuel. So holding speed down is important.
I'm not advocating going back to the days of 55 mph limits. Far
from it. But if you know you'll have time to make your delivery,
why push the speed limit? There are reasons the big, profitable
fleets limit their trucks to 65 or 70. It saves fuel, and it's easier
on equipment. If you have time to drive 68 instead of 75, do it.
The difference in aerodynamic drag is 21 percent, and you'll save
more than 10 percent in fuel mileage.
Potential energy savers
Acceleration
takes energy. Driving a truck isn't drag racing, so you don't need
to be the first to get up to speed. Acceleration (the rate at which
speed changes) is a significant contributor to energy consumption.
The faster you accelerate, the more fuel you burn. It's that simple.
Progressive
shifting is being taught in drivers' schools these days, and progressive
fleets are teaching it to their drivers. Many savvy owner-operators
already do it. Instead of running up to red line in every gear,
progressive shifting lets you take advantage of torque multiplication
through the gears. The lower the gear, the greater torque is multiplied,
so you don't need to run the engine up as much. Remember, just turning
the engine over, moving the pistons up and down and spinning the
crankshaft, take energy. The fewer revolutions the engine has to
turn, the less fuel it will burn.
Braking
wastes a great deal of energy. If that seems like a contradiction,
consider that when the brakes reduce your truck's momentum, its
kinetic energy (by converting it into heat) the energy used to hold
the truck at speed is being absorbed and cast away as waste. Fuel
was burned to bring you up to speed and to hold you there, so fuel
is what is wasted. You can reduce the amount of energy lost as heat
by anticipating stops and coasting to exits. Jim Booth, former trainer-driver
for Caterpillar, would often coast in top gear for a mile or more
approaching his exit. Climbing hills, he would ease-off approaching
to top, and let his momentum carry him over the top. He let gravity
work for him, not against him. Cresting hills slowly, he could more
easily get into the right gear to take him safely down the next
grade. He'd barely touch his brake on the way down.
If
you have cruise control, use it. The engine computers are set to
keep you at a smooth, steady speed. Even when climbing hills, stay
in gear and in cruise as long as you can. Don't try to help the
engine by tromping the pedal. Overall, you won't lose much time.
Jim used to tell about the drivers he'd see passing him on the highway.
They'd speed to where they were going, but often be more tired than
he. Whether it was to stop for fuel, for personal needs or to rest,
Jim couldn't know, but cruising at or under the speed limit, he'd
see the same trucks pass him as much as three or four times a day.
Next issue, we'll look at fuel saving equipment, and how to specify and maintain trucks for maximum fuel and operating economy. To sum up, just drive as if there are raw eggs between your feet and the pedals, and there's a bonus if you get to your destination with the eggs unbroken. Your gentle touch will assure the eggs stay safe. There is such a bonus. It's your fuel savings.
| Idling ...one of the biggest killers of fuel mileage There
are times when you'll absolutely have to idle, when the
temperature drops way down, and you don't have a heater,
or when your accessories have run your batteries down
and you need to recharge them. But how many times do we
see trucks idling on a beautiful evening, with the temperature
somewhere between 65 and 70. The drivers sure don't need
air conditioning, and they shouldn't have any problems
starting. Yet they idle, probably out of habit. Let's
see what idling does to fuel mileage. Assume a driver
runs 13 hours in a 24-hour period, covering 650 miles.
That's an average of 50 mph - a reasonable speed. To keep
the math simple, he gets 6.5 mpg, so he uses 100 gallons
on the road. But for the remaining 11 hours, he idles,
using 1.25 gallons of fuel an hour. That day, he burns
113.75 gallons, so his overall mileage drops from 6.5
mpg to 5.7 mpg. That's almost a 14 percent drop. Controlling
idling saves wear and tear on the engine, and it also
saves money. At the recent national average price of $1.46
per gallon, the driver in our example would save $20 a
day. Driving 250 days a year, that could total $5,000. |
