Fuel economy
Nov 1, 2006 12:00 PM
As manufacturers of heavy-duty truck engines roll out their new lineup equipped with technologies to meet the 2007 federal mandate for emissions, truck fleets are concerned with the cost of ownership of these new engines, especially in the area of fuel efficiency — one of the largest variable costs in a trucking business.
A study by J D Powers and Associates found that customers are least satisfied with the cost of engine ownership, particularly in the areas of routine engine maintenance costs and fuel efficiency. Reported fuel efficiency for heavy-duty engines declined to 5.72 mpg in 2006 — down from 5.91 mpg in 2005 and 6.04 mpg in 2004. The study is based on the responses of 2,529 primary maintainers of two-year-old Class 8 trucks.
With rising fuel prices, maximizing fuel economy is more important than ever to trucking companies. Original equipment manufacturers say their engines with new technology for 2007 emission standards are built to deliver outstanding fuel economy and performance, but they need to be driven differently than previous models to achieve their potential. Although trucking companies can't determine the price of fuel, they do have some control over the rate of consumption.
“The biggest factor in fuel economy — good or bad — is the driver,” says Russ Siegel, a test and demonstration driver for Caterpillar. “Driving habits have huge impact on fuel usage. It's critical that companies educate their drivers on fuel-saving driving techniques, as well as specifying their vehicles to achieve the perfect balance of fuel economy and performance.”
Excessive speed, excessive idling, operating the vehicle in the wrong gear, and accelerating and decelerating rapidly all consume extra fuel. Simple habits, such as coasting to a stop instead of pushing the accelerator until the last moment and braking hard, add up to significant fuel savings after thousands of miles.
The first step is to specify the vehicle correctly. Gear ratios should be lowered to operate at highway speeds with lower rpm — for example 3.42 or 3.25 instead of the 3.70 or 3.55 that was used in the past. “This lower gear ratio improves fuel economy and enables the driver to work in the preferred torque range for excellent performance,” Siegel says.
Every engine is somewhat different, so the ideal rpm for one engine isn't the same for others. Transmission gearing, axle gearing, and tire size are factors used in calculating the engine rpm for a given cruising speed. While the transmission's top gear ratio determines cruise rpm, startability — the maximum grade on which the vehicle can be put into motion when loaded to a specified vehicle weight — is determined by the transmission's first or low gear ratio. This may not be important for an over-the-road tractor on mostly flat interstates, but pulling away from a loading dock can test the startability of a tractor.
The required power to move the vehicle increases depending on its weight and the steepness of the grade. If the engine has too much horsepower, drivers may be tempted to accelerate rapidly and drive faster. Spec'ing too little horsepower reduces the vehicle's ability to climb a grade at a given speed and creates driver dissatisfaction.
Drivers need to be aware that peak torque is around 1,100 to 1,200 rpm, but the horsepower peak is above 1,500 rpm. “By expanding the torque range, drivers can operate their engines more efficiently and profitably,” he says. “Torque starts to fall off around 1,400 rpm — anything above 1,500 delivers more horsepower but eats fuel. Heavy haulers may need to operate that high at times but should try to avoid it when possible.”
When climbing a grade, Siegel recommends bringing the rpm down to 1,100 rpm if the truck can top the hill. If the driver determines that a downshift is necessary, doing it around 1,250 or 1,200 rpm keeps up momentum and reduces the amount of downshifts for that hill. This method keeps the vehicle in the sweet spot for all available torque and helps save fuel.
“It's important to remember to keep vehicle speed down,” he says. “Fuel economy drops by a tenth of a mile per gallon for every one mpg over 55. You'll also maximize fuel economy if you operate in the highest gear possible, so cruise in top gear. At the same time, keep the engine below 1,500 rpm, downshifting at around 1,100 rpm. Use progressive shifting techniques by upshifting around 1,400-1,500 rpm in the upper gears and 1,100 to 1,300 rpm in the lower gears. And use cruise control whenever possible to maintain average speed and good fuel economy.”
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