No silver bullet
There is not one end-all, be-all solution to achieving better fuel economy. It takes Zen-level attention from the driver and a constant evolution of the truck and trailer, the components, the lubricants, oils, and more – right down to the flex factor in tire sidewalls.

By Paul Abelson, senior technical editor

You see them on every street and highway. Big trucks have aerodynamic fairings and tails, wide-base tires and rounded shapes. Today, with fuel hovering near $4 a gallon, fuel mileage is king.

All the energy consumed on, in or by your trucks comes from just one place: the fuel you burn in your engine. Anything that lessens that energy demand saves you fuel.

Aerodynamics is a critical contributor to fuel savings for over-the-road trucks. As speeds increase above 50 mph, aerodynamic forces increase by the square of the speed. At 70 mph, just a 40 percent speed increase over 50 mph, the wind resistance is almost doubled. The engineering rule-of-thumb is that for every 2 to 3 percent drop in aerodynamic force, about a 1 percent fuel savings can be realized.

Rounded bumpers, side fairings, streamlined mirrors and brackets all help air flow smoothly. Today, emphasis is on the trailer. Skirts keep air flowing smoothly around the undercarriage where previously it created a great deal of turbulence. Similarly, boat tails smooth air behind the trailer. Each action by itself may not seem significant, but together they can lower fuel costs 10 to 15 percent or more.

Fuel mileage can also suffer if components and parts on the truck require more power to operate. For example, if a bearing seal on a wheel-end starts to go bad and allows contamination to get into the bearings, it will put a drag on the bearing. More power will be needed to turn the wheel. The added power means more fuel must be burned, not to mention increased future maintenance. Inspection and maintenance directly affect fuel and operating economy.

Tires are another example. For years, all truckers ran dual tires to increase carrying capacity.

All-season, all-terrain tires often had lug tread patterns and hard tread rubber. The first was for traction, the other for long life and damage resistance. The downside to this was that these tires needed a great deal of energy to overcome their inherent stiffness.

Newer fuel-efficient tires are constructed with ribs and shallow lugs to reduce rolling resistance while offering good traction. While traction has been, and remains, a concern in low-rolling-resistance tires, both tests and real world trucking have not revealed any significant falloff in traction.

Using two tires instead of one requires twice the energy to flex the sidewalls. All major tire makers now offer wide-base single tires for on-highway use. They can reduce fuel use 3 to 4 percent. Changing from deep-lug to shallow-lug tires can save another 2 percent or more. 

Engine lubricants have been improved over the past few years, especially synthetic oils made from polyalphaolefin base stocks. Today, 15W-40 is the standard viscosity for CMVs. Research has shown 10W-30 synthetic oils can stand up to the rigors of heavy-duty work cycles. The thinner oils cut internal engine drag, reducing fuel use about 1.5 percent. Synthetics in the gear box and drive axles can save an additional 1 percent. Amsoil, the pioneer in synthetic oils, has a heavy-duty, full synthetic 5W-40 in its line. It claims that in SAE/TMC tests conducted against conventional 15W-40, fuel savings were 8.2 percent.

Running the engine fan consumes as much as 50 to 60 horsepower, more in a 15- or 16-liter engine. Running waterless coolant and adjusting the fan-on temperature can save up to 3 percent fuel usage as tested in over-the-road trucks.

According to the Technology and Maintenance Council’s Recommended Practice RP1114, Driver’s Effect on Fuel Economy, “driver performance can account for as much as a 35 percent difference in fuel economy performance.” Variables that the driver controls include horsepower demand, vehicle speed and brake use. Horsepower demand is directly related to acceleration rates. The quicker a truck accelerates, the more fuel it burns in the process.

Braking also affects fuel consumption. Drivers who look to the next traffic light and time their approach, who coast to stop signs rather than braking hard, who ease up near the top of a hill so they can use gravity on the downhill side rather than burning off speed with the brakes use the least amount of fuel. These techniques account for much of the 35 percent difference between the best and worst drivers.

When I entered our industry, engines had relatively narrow power bands, sometimes as little as 14 percent of total available engine revolutions (from 1,800 to 2,100 rpm). Today, engines are designed to run much slower. Peak torque is as low as 1,000 to 1,100 rpm and peak horsepower at 1,800 rpm. Modern engines can be driven across 45 percent of the rpm range, so shifting can occur at lower engine speeds. It takes more energy to run the engine at higher rpm. Driving at lower rpm saves fuel.

Those who drive for fuel economy use progressive shifting. They take advantage of torque multiplication in lower gears to shift at lower rpm and often skip unneeded gears. To help drivers narrow the 35 percent gap improving both operating performance and fuel economy, a new breed of automated manual transmissions has evolved. Eaton’s UltraShift Plus, the latest improvement in the Eaton line, has variations programmed to optimize shift points for on-highway economy.

Engine makers Volvo (I-Shift), Mack (mDRIVE) and Detroit (DT12) all recently introduced automated 12-speed transmissions programmed to provide optimal shifting for the proprietary fuel mapping of their engines. For those who prefer the constant torque application from a full automatic, Allison announced a new 10-speed, the TC10. Initially designed for on-highway use, it features Load Based Shift Schedules.

These transmissions will help narrow the 35 percent gap and help any driver with a light touch achieve better fuel economy.

The technologies exist, each offering improvement in small increments, not big chunks. The greatest opportunity for improvement is still driver performance. LL

Aug/Sept Digital Edition