Nov 1, 2006 12:00 PM

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%.

Every engine has a sweet spot where it delivers its best fuel economy. To increase fuel economy, the engine should run at this speed during normal highway cruising. When spec'ing the powertrain, consider two factors: the vehicle's performance (startability, gradeability, and cruise speed) and the fleet owner's desire to achieve fuel economy.

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.”

Saving fuel by shifting properly isn't just for the top one or two gears. It works all the way through the gears. Cummins' Load Based Speed Control (LBSC) allows the engine to adjust the speed available to the driver. The ECM looks at the load and sets the rpm limit accordingly. Fuel economy is assured when loads are light, but the driver still gets the engine speed and needed power when the loads are heavy. LBSC can reduce the variability between the least and most efficient drivers.

Cummins Gear Down Protection maximizes the percentage of distance in top gear. When the driver downshifts, the gear down protection cuts back on the top speed, encouraging the driver to stay in top gear. Gear down protection also senses engine load to ensure that it doesn't limit engine speed when the downshift is needed because of a heavy load. Fleets can calculate and select the settings, or allow the dealer to calculate the optimum settings.

Getting the most out of the engine means running it at the proper rpm. But optimizing other engine operating characteristics also will improve fuel economy. An engine that is starved for air or unable to expel exhaust will lose power and waste fuel.

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.”

Cat's Siegel offers the same advice. “Don't run the fan while driving unless you need it. The fan draws horsepower, which reduces fuel economy. Under normal operating circumstances, leaving the fan switch in automatic mode will activate it only when needed,” he says.

Fan horsepower increases with rpm. When cooling fans are running, they use between 1.5 and 3.5 gallons of fuel per hour, according to Cummins. Increasing fan run distance from 30% to 50% will decrease mpg by between 3% to 5%.

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. Using an air deflector doesn't necessarily mean you'll have improved fuel economy. In fact, it could hurt your mileage if the size doesn't properly match the vehicle.”

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. In addition, underinflation contributes to increased tire wear, irregular tread wear, and reduction of casing durability.

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.”

Rock-solid rules

• 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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