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.”
An engine geared to run 1,450 rpm at 65 mph gets approximately 4% better mpg than one geared to run 1600 rpm at 65 mph, according to Secrets of Better Fuel Economy published by Cummins. If the truck is geared too high, it will force a driver to run one gear down. Doing so decreases mpg by about 3% because of gear mesh effect. The engine cruising rpm is about 400+ rpm higher, which decreases mpg by about 4%.
Mack also recommends against downshifting too early when climbing grades. “Inexperienced drivers often want to push on the throttle at the bottom of the hill,” says David McKenna, Mack powertrain products marketing manager. “That shows a major misunderstanding of how to use properly torque versus horsepower. The tendency can be for drivers to rely on horsepower to get out of trouble rather than torque. Drivers need to maximize the percentage of time in top gear and use the full operating range of the engine before gearing down.”
Minimizing accessory loads to the engine will improve operating efficiency and decrease fuel consumption, says Chuck Blake, senior manager in the technical support group for Detroit Diesel. “Something as simple as proper installation of a winter cover on the grill of the engine will impact air flow, which impacts fuel use,” he says. “And make sure that the cover is removed after outside temperatures have risen above zero.
“Low coolant, a faulty fan clutch — anything that makes the fan run longer can take a big bite out of fuel economy. Fleets should take advantage of the new engine technology. Don't give drivers the option of using a manual fan switch. Let the engine decide. And don't leave the defroster on all day long. That's an unnecessary waste.”
When used properly, aerodynamics can improve fuel consumption by as much as 10-15%, Blake says. Trailer types and loads hauled in addition to vehicle speed will determine the effectiveness of such features as full roof deflector, fairing, sloped hood, round corners, air dam, and skirts.
“The smaller the distance between the cab and the trailer, the better the aerodynamics,” he says. “If axle weights allow, slide the fifthwheel forward to minimize the tractor-trailer gap and reduce wind resistance.”
Engine manufacturers continue to cite improper use of tires and underinflation of tires as significant causes of poor fuel economy. Every 10 pounds per square inch of underinflation represents 1% penalty in fuel economy, the Cummins study says.
Running all tires underinflated causes significant effects on fuel economy. But the tires most likely to be ignored, poorly maintained, or underinflated are in the trailer position — and they have a greater effect on fuel economy than steer or drive tires.
Tire tread is another factor. “I find it phenomenal the number of fleets using lug tires,” says Mack's McKenna. “Certainly you may need them for snow and ice conditions or driving on loose earth. But why are 75% of gasoline transports going from the tank farm to the retail gas station with lugs on the drive axle? Using a rib tire for highway travel always will give you better fuel economy.”
- Every 2% reduction in aerodynamic drag results in approximately 1% improvement in fuel economy.
- Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by 0.1 mpg.
- Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.
- Lug drive tires that are broken in can get up to 0.4 mpg better than new lug tires.
- Ribbed tires on the drive axles provide 2-4% better fuel economy than lugged tires.
- Every 10 psi that a tire is underinflated reduces fuel economy by 1%.
- The break-in period for tires is between 35,000 and 50,000 miles.
- Tires make biggest difference in mpg below around 50 mph; aerodynamics is the most important factor over around 50 mph.
- The most efficient drivers get about 30% better fuel economy than the least efficient drivers.
- Idle time is costly. Every hour of idle time in a long-haul operation can decrease fuel efficiency by 1%.
source: Cummins MPG Guide
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