In Focus: Operating in sync

As part of Daimler Trucks North America, Detroit Diesel worked with Freightliner, Sterling and Western Star to optimize the new DD15 engine to improve fuel economy.

Many powertrains on modern trucks have variable or multi-torque arrangements that provide more torque for unprecedented hill climbing and acceleration in top gear at cruise speeds without requiring heavier drivetrain components, enhancing fuel economy without more weight and higher component cost.

The common electronic architecture used by Detroit Diesel and Daimler Trucks North America provides early access to new electronic features and innovations in order to make the overall truck smarter for the driver and the owner, says Miguel Soetart, Detroit Diesel group leader of component optimization. Vertical integration allows the company to couple proprietary features together in an HIL-test (“Hardware In the Loop”) long before they make it to market, Soetart says; among other benefits, this will capture and remedy torsional vibration issues in the powertrain early on.

“Common electronics open the door to optimized torque differentiation,” Soetart says. For Detroit Diesel’s DD15 and trucks from Freightliner, Sterling and Western Star, an engine that could provide one torque curve per gear is not a dream, he says – it is a reality that allows more torque in top gears to keep the truck in top gear longer, therefore improving fuel economy. “It also allows the ability to limit the torque where needed to protect the drivetrain, not only when using the engine to power but also to brake the truck,” Soetart says.

Volvo Trucks of North America also has carried drivetrain integration further, according to Ed Saxman, drivetrain product manager. Volvo still offers the same standard components as most American trucks. But when the vehicle has an I-shift transmission coupled to a Volvo D13, the result is a degree of integration that can protect trucks from driver error and abuse in ways fleet maintenance managers could only dream of in the past, says Saxman. An I-shift transmission can make engine-related decisions to protect itself and its integral clutch if necessary, he says. “Consider what happens if the driver forgets the transmission is in gear and shuts off the engine. The transmission will keep the engine running, slip itself into neutral and then shut down the engine.”

What happens if the driver puts his foot heavily into the throttle, realizes he has forgotten to put the truck in gear, and then absentmindedly shifts the transmission into drive? “When the driver revs the engine and then engages ‘Drive,’ the engine will slow to idle before the transmission engages, even if the throttle pedal is floored,” Saxman says. “If the driver keeps the throttle floored even after this happens, the integrated system will modulate engine power and give a smooth clutch engagement. The driver cannot abuse it. That’s what integration can do.”

The big picture
Geopolitical changes have made for a challenging business environment, says John Beering, director of global marketing for Eaton Corp.’s truck components group. “But globalization also drives increased cooperation, and the possibilities are staggering,” says Beering, who allows that Eaton recognizes and accepts the desire of many truck manufacturers to provide their own components.

“As a global supplier, we know that we manufacture on a scale that gives us a competitive advantage,” he says. Eaton is handling the situation by staying customer-focused. “It’s all right for the customer to be making some things in-house. We respect their point of view that this can create advantages for their customer. But there may be many situations where an Eaton solution can enhance what is offered. If that is the case, we’ll gladly get involved.” Roadranger service is one of the ways the company’s economies of scale and long experience can enhance the total package offered to the customer, Beering says.

Steve Slesinski, director of global product planning at Dana Holding Co., says his company has been working closely with Eaton to make sure drivetrain components match perfectly. “This is particularly important where there has been a trend to having the engine develop more and more power and torque,” Slesinski says. The drivetrain needs to handle the increased stress, yet it’s not practical to keep escalating cost and weight, he says.

Slesinski underscored the benefits of integrating the engine and transmission in order to use that integration to vary torque output with gear position. “This is a nice feature, because providing that torque only in the top two gears helps prevent some abuse that can occur at low speeds and high torque conditions,” he says. “High engine torque output is nowhere near as damaging at higher speeds.” Thus, components optimized for 1,550 or 1,650 lb-ft can be utilized with a 1,650 or 1,750 lb-ft engine that gives full torque only in the top two gears without causing drivetrain trouble, Slesinski says.

The system protects the drivetrain from damage because the engine torque is multiplied by the transmission ratio by the time it gets to the driveshafts. Thus, a 1,550 lb-ft engine working through a 12:1 first gear could put as much as 18,600 lb-ft on the driveshafts at the torque peak, while with a 1,650 lb-ft engine, the figure would rise to 19,800 lb-ft. In a .74 top gear, the driveshafts will handle at most 1,221 lb-ft even with 1,650 lb-ft output.

Still, over time, the extra torque and horsepower can take a toll. “We are always looking at ways to design gears and formulate lubes for high horsepower and torque,” Slesinski says.

The higher rating for multi-torque engines typically maxes out at 1,650, 1,750 or 1,850 lb-ft. Slesinski says Dana is designing the standard product at about 1,850 lb-ft in order to load-balance the design for typical needs. “However, we do have components for higher performance applications,” he says. The end result is that many applications can be accommodated without adding extra weight or cost, even though the company does not take a “one-size-fits-all” approach but has components specifically designed to meet key applications, Slesinski says; this approach applies to both rear axles and driveshafts.

Drivetrain spec’ing
Specifying a properly integrated drivetrain that will be durable in an actual application is a traditional task, but still quite complex. For example, the torque that the driveshaft and axle pinions must handle depends not only on engine output torque, but also on the top gear ratio. Direct drive means significantly higher torque. That’s why direct drive transmissions get more fuel efficiency, as much as 3 percent better than overdrive, says Charles Allen, ArvinMeritor’s director of national service. “Yet, direct drive is limited by a lower input torque limit,” Allen says.

Drivetrain stress even varies with the duty cycle and GCW. “0 to 1 mph is the worst condition,” Slesinski says; that’s because the output torque is more damaging if it takes longer to get the load moving.

“Think right equipment to do the exact job,” Allen says. “What is right varies by engine, and with the loads.”

Matching components is not just about torque, Slesinski says; the entire system must be matched to the application. “When in doubt, specify the higher torque-capacity component because when you are starting to exceed the limits of the components, you could get component failure – or at least downtime,” he says.

Another critical factor is cruise speed rpm; with the best engine fuel consumption islands occurring at increasingly lower rpms, trucks run increasingly faster axles. Driveshaft and axle input shafts must handle more torque to produce the same cruise power, so the right clutch damper must be used for the increased torsionals.

A truck could be geared with a 10-speed to have the ideal rpm curve for the most economical cruise conditions with adequate gradeability for hill climbing, yet the needed startability might not be available. An adequate compromise could be a higher numerical axle ratio, but that might sacrifice good fuel consumption at 65 mph, Slesinski says. Here, the best answer could turn out to be a multi-speed transmission or one having a “Low-Low” starting gear.