April 2013, TruckingInfo.com - WebXclusive
Poor fuel mileage can be a make-or-break proposition for owner-operators, and a lot of whether you get 6 mpg or 8 mpg depends on your skills and habits as a driver.
Two drivers on two identical trucks can see a difference of up to 30% in fuel economy. Much of that chasm stems from good or bad pedal and stick management, and the driver's willingness to use available technology to his best advantage.
The best illustration of this is probably automated transmissions.
Manufacturers claim there are fuel economy benefits to automated gearboxes, but how can that be? Automated transmissions – 10-, 13-, or 18-speed, it doesn't matter – are nothing more than a standard manual transmissions. They're just a box full of gears.
The difference is that the computer controls the shifting, not the driver.
"I call automated transmissions the great equalizer," says Itamar Levine, Bison Transport's director of maintenance. "They do wonders with the worst drivers, and our best drivers simply enjoy the convenience of not having to shift."
There's more to it that just being more precise in getting the stick into the right slot more often. Engine and transmission ECMs are calibrated to optimize fuel economy by hitting the right shift points more often than poor driver can seem to manage.
But a good driver can easily out-drive an automated transmission. He can see, the ECM cannot. He can manage, the ECM can only respond.
The idea is to use the least amount of fuel possible to get the job done. That means accelerating gently up to speed and flipping cruise control on as soon as you get there. But cruise control isn't perfect.
"Full cruise is designed to maintain road speed, and once you loose ground, cruise is going power right up to try to make roadspeed back," says Detroit Diesel application engineer Chuck Blake. "A really good driver can always out-perform cruise control, but it's tough and it's tiring because you're always working."
While the ECM might be able to finesse fuel flow better than you can on level ground, it can't tell the difference between a hill and a headwind. All it knows is some external force is conspiring to slow the truck down and it's going to feed as much fuel to the engine as needs to maintain the set roadspeed.
A good driver can make a difference by backing off the throttle pedal when full power isn't needed -- like when cresting a hill.
"Cruise would sense the load, and keep the power on trying to reach the set point, even has the hill flattens out at the top," Blake says. "A good driver can <it>see</it> the top of the hill, and would back off the throttle as he went over the top. Sure he'd give up a few mph, but the fuel savings is phenomenal."
What's the point, Blake asks, of coming over the top and full throttle, when you'll be on the brakes in a moment as you roll down the other side? "Kick it out of cruise as you near the top, and let gravity help you. You'll make up the lost roadspeed quickly enough on the way down."
If you can see the top of the hill, and you can see what your engine speed is, you can estimate whether or not you'll need to drop another gear before topping the hill. Cruise control can't see the top of the hill. All it wants to do is get back to the set speed, and that acceleration at the top of a hill costs -- big time.
Blake notes that all engine makers offer some variation on the "fuzzy" cruise theme where there's a gray zone between on and off. That allows for softer acceleration and a bit of roll-out before the engine brake kicks in when the set speed is exceeded. Blake says it's an ECM option worth activating. Engine makers have different names for the parameter, like soft cruise, elastic cruise, upper and lower droop, etc. Ask about it.
Newer cruise control technology, such as Freightliner's RunSmart Predictive Cruise, uses GPS and map data to evaluate upcoming changes in road terrain and adjusts the throttle accordingly, resulting in fuel savings.
A momentum bank
Ed Saxman, drivetrain product marketing manager with Volvo Trucks, suggests drivers think of the truck as a momentum bank.
"The object is to keep as much momentum in the bank as possible, and let the vehicle's momentum, or gravity when appropriate, work for you," he says.
Another common bad driving habit Saxman says can be easily remedied is the convoy configuration.
"Drivers have a tendency to stay too close to a vehicle they're following. If you pay attention to your throttle, you'll notice that you're on and off it a lot. You get up close, then back out of it, and then throttle up to close the distance, etc. Every time you accelerate to catch up you're wasting fuel," Saxman notes. "Why not back right out of it and let the other guy get way ahead, and then set your cruise and just motor along?"
Still with the momentum theme: Why power down an exit ramp when you're going to stop at the bottom? Back off the throttle half a mile ahead of time out on the interstate before the exit, Saxman suggests. "You'll save your brakes on the exit ramps too."
Everything offered or suggested here comes in the form of a decision a driver has to make. It would be easy to keep the cruise control on going over a hill, but sacrificing a few mph by "lugging" it over will save you mpg. Constantly adjusting speed, and maybe even braking because you're in too close to the guy in front of you to see the big picture, is an expensive way to keep friends. You could decide to the decidedly un-macho thing and be the first to back out of it so you can better manage your roadspeed.
It's up to you: mph or mpg?