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The Sweet Spot

By on June 30, 2011
RoadKing Mag

The higher fuel prices go, the greater their effect on your profitability in trucking. And obviously, the better your miles per gallon, the more profitable you will be.

Yet it never ceases to amaze me how many drivers ignore the driving techniques that affect fuel savings. They wind the engine at every up-shift, trying to get to speed as quickly, rather than as economically, as possible.

That is a characteristic separating wasteful drivers from fuel-efficient ones. Consider this: Some fleets have observed a 35 percent difference in fuel economy between their best and worst drivers, in miles per gallon achieved.

This considers fuel economy only, not safety record, claims or on-time deliveries. It is based only on miles per gallon. But it’s a fact that the most fuel conscious drivers are also usually the safest and most effective drivers.

That 35 percent spread was first reported at a Technology and Maintenance Council (TMC) meeting. A large fleet received delivery of more than 100 trucks, all with identical specifications. After a suitable break-in period, the fleet noticed a significant difference in fuel economy between the best and the worst, with most falling in between.

To determine if the difference came from the trucks or the drivers, the manager did some shuffling. He put the driver with the best recorded miles per gallon into the truck with the worst and went on down the line until the driver with the poorest fuel economy was in the truck with the best.

After several months of very similar operations, he checked again. The difference was still close to 35 percent, with the drivers who had achieved the higher mpg still the highest, and those with the lowest, still the lowest.

He naturally concluded that it was driver variability, not the equipment. Other fleets reported similar findings.

Technically speaking
So, what can a driver do to improve miles per gallon? For an owner-operator, the question is, what can I do to improve my bottom line? When I ask my engine-builder friends, they usually reply, “Keep it in the ‘sweet spot’!”

“The what?” I ask. How am I supposed to tell them to keep it in the sweet spot when I’m not sure what it is? My friends, being engineers, were all too happy to instruct me.

“The sweet spot is the engine speed measured in revolutions per minute (rpm) at which brake specific fuel consumption is at a minimum.”

“Then what is at a minimum?” I asked. “What is brake specific, whatever that is, and how am I supposed to know what the minimum is?”

They gave me that look that engineers have when educating us mere mortals and proceeded to explain. I won’t bore you with the details of our fascinating conversation nor the number of questions I had to have answered, but I did find out that brake specific fuel consumption (BSFC) is a calculation of the fuel consumption rate divided by the power produced. Lower numbers are better.

Naturally I asked how I find the BSFC. “You don’t. We do,” was the reply.

The engineers test, tweak, measure and test again until they certify the engine for release. Then they determine the rpm range the engine should be driven. For some, it’s 1,200-1,400 rpm. For others it’s 1,350-1,550, and so on. Your owner’s manual should list the ideal range for your engine. Roughly at mid-range is the sweet spot, where you’ll get your best fuel economy.

At the wheel
That’s the engineers’ world. In the real world, you may spec your truck for best fuel economy at 65 mph, then operate in a state with a speed limit of 55, 60, 70 or more. You’ll be off a bit, driving above or below the sweet spot much of the time. It won’t matter that much if you rarely hit the sweet spot exactly. What’s important is how you get from a lower speed to a higher speed.

Back in the day, we had engines that ran at 2,100 rpm with 300 rpm torque bands. You had to wind the engine for each up-shift or you wouldn’t have enough power in the next gear.

Today we have low speed engines with broad, flat torque curves starting at 1,200, 1,100 or even 1,000 and running for 500 rpm or more. Most have enough torque at 800 to keep you moving because of torque multiplication in low-range gears.

Progressive shifting is the modern way to drive. Shift out of first at about 1,100 rpm to catch second at about 850. You’ll have enough torque to bring you to 1,200. Keep progressing up through the gears, shifting out of third at 1,300, fourth at 1,400 and so on. Once you’re in high range, your engine will be at peak torque if you up-shift at 1,650. The progression of shift points is where the technique gets its name.

And technology has meant that operating in the sweet spot is really easier than you’d think. The degree of computerization in vehicles today would have been unthinkable just a decade ago. Now, when you press down on the accelerator pedal, you no longer have a direct mechanical link to the engine. The “drive by wire” controller determines throttle position and rate-of-change of position. From that, it tells the engine controller when to pulse fuel to each cylinder — timing as many as five variable pulses per stroke.

As a driver operating one of these intelligent vehicles, you can help maintain the sweet spot by staying in cruise control as much as possible.

If you have an automated transmission, the computers are programmed to stay close, even selecting the optimal gears.

Here are a few more tips: Drive as if you have a raw egg between your foot and the pedals. Coast as much as possible, and stay off the brakes. Braking wastes the fuel it took to get to speed.

Try that and then watch what happens to profitability.

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  1. Tatjana

    July 8, 2011 at 2:28 pm

    I have to agree with this blog. I see a lot of trip reports cross my desk for fuel taxes and it always amazes me how similar trucks doing the same runs get so different fuel mileage.

    I will have to remember to mention “specing” the trucks more often when asked how to save money on fuel and ultimately, fuel taxes along with other state and federal taxes.

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