Shifting for fuel economy

With fuel prices going through the roof, improving fuel economy is rapidly becoming a focal point of transmission design. According to the American Trucking Assns. (ATA), the industry spent $87.7 billion on fuel in 2005, a whopping $25.1 billion more than in 2004. For many motor carriers, fuel represents the second-highest operating expense, accounting for as much as 25% of total operating costs,

With fuel prices going through the roof, improving fuel economy is rapidly becoming a focal point of transmission design. According to the American Trucking Assns. (ATA), the industry spent $87.7 billion on fuel in 2005, a whopping $25.1 billion more than in 2004. “For many motor carriers, fuel represents the second-highest operating expense, accounting for as much as 25% of total operating costs,” points out Bill Graves, ATA's president and CEO.

A number of factors have combined to amplify fleet concerns about fuel. Clarence Werner, chairman, president and CEO of TL carrier Werner Enterprises, cites the price spike experienced in the wake of Hurricanes Katrina and Rita, the lower fuel economy of '02 engines, and the expected higher cost and reduced energy content of ultra- low-sulfur diesel (ULSD) fuel, which should hit the market this October.

“We continue to experience approximately 5% lower mpg with [the portion of our fleet spec'd with '02 engines],” he says. “When truckload carriers are required to use ULSD for all of their existing trucks, preliminary estimates are the new fuel may decrease mpg by approximately 1% to 3%.”

Even without factoring in the higher costs associated with the switch to ULSD later in the year, the price of diesel is projected to rise in 2006. The Energy Information Administration predicts that diesel prices will average $2.54, compared with $2.41 in 2004.


Manufacturers say that advances in transmission technology have already resulted in better fuel economy numbers. Some of these changes also make it easier for drivers to operate their vehicles at optimum efficiency, leading to even greater improvements in fuel economy.

“The variability from driver to driver in terms of getting the best fuel economy is a big issue,” says William “Bill” Gross, product manager for Eaton's medium-duty transmissions. According to Gross, research by TMC has demonstrated that fuel economy can differ by as much as 35%, depending on the skill of the driver.

“That's why we're seeing a shift to more automatic and automated transmissions. The electronics decide when to shift and they do it properly every time,” adds Charlie Allen, national service manager for ArvinMeritor. “They don't get tired or distracted like human drivers do after long days on the road.”

But manual transmissions are definitely not going the way of the dinosaur. Just the contrary, in fact. Many experts believe that in the Class 8 longhaul segment, the market share for automatic and automated transmissions will be limited to at most 20% to 30% because they are so much more expensive than manual models as the price differential can range between $4,000 and $10,000.

“Pricing is the main reason I see automatic and automated transmissions plateauing at around 20% or so [for Class 8],” says David McKenna, marketing manager for engines, transmissions and axles at Mack Trucks.

Manufacturers of manual transmissions know that they, too, have to pay attention to fuel economy. “We're trying to stay away from multi-speed [manual] transmissions and go to simple 6- and 10-speed formats,” McKenna says. “The more gears you have, the narrower the shift range becomes; that can lead to horrendous fuel economy.”

“The manual 10-speed should be the [over-the-road] standard transmission for the future,” says Bob Weber, chief engineer-heavy truck for International Truck & Engine Corp. He notes that day in and day out, it's much easier for drivers to navigate 10 gears than 13 or 18.

“There will still be some 18-speed users, but 90% to 95% of all future Class 8 manuals are going to be 10-speeds,” he adds. “You can't really go lower than 10 speeds because you need that many gears to get the proper amount of acceleration; you want to keep the power differential between the gears at less than 10% to get the best fuel economy.”

ArvinMeritor's Allen explains that today's engines have much wider operating ranges, with almost constant horsepower throughout each range. “For applications that needed the close-spaced gears of a 13, 15, or 18-speed transmission to ensure proper power, acceleration and maximum fuel economy, today's 10-speed may be the perfect fit, without the cost premium or operational complexity.”


Truck OEMs see the operating environment and '07 emissions regulations as key factors for fleets when deciding which transmission package will give them the best fuel economy.

“It's important to select a transmission and rear axle to match the engine and load requirements for the specific truck application, taking into account the terrain over which the vehicle operates,” says Mike Dozier, chief engineer for Kenworth Truck Co.

Mitch Murray, manager of tactical marketing for Allison Transmission, explains the impact of tighter emissions rules. “Every time we see a drop in emissions, how you shift gears to tap into engine power becomes a bigger part of the fuel economy equation,” he says.

If you look at the engine power curve and shift points after emissions regulations have been tightened, you'll see that the window of opportunity for shifting to obtain the best fuel economy narrows, he explains. “That's also when the human characteristic of not wanting to shift becomes much more damaging to the vehicle.”

Murray believes that with the exception of the Class 8 OTR segment — and no doubt, that's a big exception — most vehicles will be equipped with automatic or automated transmissions by 2010. “Just in terms of getting more fuel efficiency from less skilled drivers in stop-and-go environments, automatics and automated transmissions shine.”

Murray explains that manuals and, in many cases, automated transmissions, interrupt power to the wheels during shifting, resulting in lower average wheel horsepower. “Smooth, uninterrupted power from an automatic delivers higher average wheel horsepower — meaning you don't require as much engine horsepower — so you can reduce the engine's size and fuel consumption, yet still have the performance you need.”

On the automated side, Eaton is touting fuel savings as a big reason to switch to its Fuller UltraShift HV (Highway Value) medium-duty transmission. Designed for Class 6 and 7 vehicles with diesel engines in the 195 to 260 hp. range, the HV is capable of handling torque capacities up to 660 lb.-ft. and loads up to 33,000 lb. GVW.

When tested according to SAE protocol in an urban setting requiring numerous stops and starts, the UltraShift HV delivered fuel economy savings of up to 19%, according to Eaton's Gross. In the on-highway operation protocol, it demonstrated a 7.5% improvement in fuel economy.

“The key is allowing lock-up between the clutch and transmission at approximately 3 mph, as compared to 24 mph with a conventional automatic,” says Gross. “Until lock-up is achieved, the truck is wasting power and fuel, which is why earlier lock-up provides a significant improvement in fuel economy, particularly in stop-and-go operations.”

In Class 8 operations, however, the tipping point in favor of automatic or automated transmissions is how much shifting the driver must do every day, says International's Weber. “Fleets in high shift cycle operations…will switch to automatic and automated packages,” he says. “Those that do a lot of longhaul highway driving [will] stay with manuals.”

“Although experience in the longhaul driver segment is dwindling, there's still a big knowledge pool that understands good shifting practices,” adds Scott Steurer, Eaton's product line manager for heavy-duty transmissions. “You have to drive like the accelerator is an eggshell to get the best fuel economy. And the shift points are only going to get more critical as the emissions rules get tighter.”


Finally, to get the best fuel economy possible, we have to realize that transmissions don't operate in a vacuum. Engine horsepower, rear axle ratios, and even tire selection can all have an impact.

“You have to select components that are sized for the job,” says Kenworth's Dozier. “For example, larger gear sets generally have lower efficiencies. Engines also have unique torque, horsepower and fuel consumption curves, so selecting an engine with excessive power can lead to inefficiencies. Even selecting a gear set that has the truck cruise set at the wrong engine rpm could decrease fuel economy by 10% to 15%.”

Choosing the wrong engine can also nullify a transmission's ability to save fuel, adds Allison's Murray. “Too often we get all revved up about horsepower and torque, forgetting that what's under the hood may not be getting to the wheels,” he says. “Buyers end up purchasing way too much engine for the operating ranges their trucks are in 90% of the time. That's a waste of horsepower, fuel and purchase price money.”

Proper axle ratios are also critical to making sure the transmission delivers the engine's power to the wheels in the most fuel-efficient manner possible, says International's Weber. “The key is to make sure the axles can handle the engine speed.”

Put it all together and it's no wonder fleets are paying more attention to fuel economy gains from truck transmissions, says Eaton's Steurer. “It used to be well down the list of top ten concerns for most fleets,” he says. “Now we're finding it's in the top five.”

Talking points

Things were a lot simpler when fleets only had to differentiate between manual and automatic transmissions. While everyone agrees that the industry has certainly benefited from all the advances in transmission technology, the terminology can be confusing. Here's some clarification, courtesy of Allison Transmission:

MANUAL: A constant mesh transmission with shifting done by the driver through manipulation of the shift lever and foot clutch.

SEMI-AUTOMATED MANUAL: A constant mesh transmission with gear selection done by the driver through manipulation of an electronic shift module. Either pneumatic or electronic systems can be used to move shift forks and rails; a foot clutch is used for starts and stops only.

AUTOMATED MANUAL: A constant mesh transmission with gear selection done automatically. Pneumatic or electronic actuators move shift forks and rails, yet no foot clutch is needed. Available in two-pedal (no clutch) or three-pedal format; the latter allows the driver to disengage the automated system and drive manually.

AUTOMATIC: Shifts are made via electronic controls. Engine torque is not interrupted during the shift, but is controlled to a smooth shift to maintain output torque.

View more Fleet Owner news relating to idle reduction, fuel conservation, fuel economy and diesel fuel prices.

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