A primer on the concept of downspeeding heavy duty trucks

 

Today, with tractor trailers so aerodynamic, it takes only about 220hp to cruise at 65 mph. With injection systems and other aspects of engine design tremendously improved, and the use of selective catalytic reduction (SCR) reducing firing pressures, diesels can generate a great deal of power and torque, plus run at maximum efficiency at less than 1,200 rpm. So, an industry that employed a 65-mph cruise speed at more than 1,500 rpm through much of the 1990s has now begun seeing cruise rpm of 1,150 at 65 mph.

A benefit of this engine “downspeeding” – having an engine operate at low rpm with high torque – is that it allows engines to spend more time operating in the load and speed where they yield better fuel economy and the resultant reduction in greenhouse gasses while maintaining safe, driver-pleasing performance, notes Steve Slesinski, director of global product planning for Dana Holding Corporation. Furthermore, higher torque at all speeds maintains performance.

Dana (www.dana.com) is a global leader in the supply of highly engineered driveline, sealing and thermal-management technologies for vehicles with both conventional and alternative-energy powertrains.

While engine downspeeding does offer benefits, the faster axle ratios needed to accommodate the lower rpms generate additional torque in the drivetrain place much higher stresses on the main driveshaft and interaxle shaft, he explains. As expected, the faster ratios and increased torque also reduce the life of driveshaft components.

A typical engine downspeeding reduction of 325 rpm decreases the life of standard U-joints by nearly 60 percent, he points out.

Slesinski says an alternative method for handling the high torque associated with engine downspeeding is to limit the performance of the engine. But this is not the optimal solution because it reduces performance, fuel efficiency and driving satisfaction.

A key enabler to heavy duty engine downspeeding is the transmission. The lower rpm and increased torque can be handled with an optimized transmission and axle ratios, an optimized shift strategy and engine calibration and reduction/elimination of torque interrupts.

The best results are achieved when downspeeding of the engine and corresponding drivetrain are optimized together to provide the required vehicle performance, says Slesinski.

 

THE TREND BEGINS

Volvo Trucks North America began the trend with its XE 13 powertrain in 2011, dubbing the concept “downspeeding.” The company upgraded this powertrain package from 425hp to 455hp in 2012.

Part of the way in which Volvo accomplished additional cruise horsepower and fuel efficiency was the coupling of its I-Shift transmission with the engine in the XE powertrain.

The Volvo I-Shift is a 12-speed, two-pedal, automated mechanical transmission that uses microprocessor technology to continuously monitors changes in grade, vehicle speed, acceleration, torque demand, weight and air resistance. With the engine and transmission in constant communication, the I-Shift automatically selects the best gear for the engine, “allowing every driver to shift like a fuel efficiency expert,” says the company.

Volvo Trucks is part of the Volvo Group, one of the world's leading manufacturers of trucks, buses and construction equipment, drive systems and of heavy diesel engines (9L to 16L).

 

FEWER BURDENS ON THE DRIVER

The biggest reason engines were cruising at 1,350 rpm up until 2011 or so was that it gave enough miles per hour between a 65-mph cruise and the point where a downshift was needed, explains Aaron Peterson, chief engineer, performance engineering, at Navistar. This enabled the truck to stay in top gear 85 to 90 percent of the time.

This way, the driver was not burdened with frequent downshifts, the source of driver fatigue and related complaints, he says. Incorporating an automated transmission into the drivetrain removed these concerns for drivers, even though frequent downshifts would have to occur more frequently with downspeeding.

Peterson says with downspeeding, the time in the top gear might now be as little as 65 to 75 percent. Before downspeeding, a downshift would occur after a drop in speed of 15 mph. Now it happens at about half that.

Navistar produces International brand commercial and military trucks, MaxxForce brand diesel engines and IC Bus brand school and commercial buses.

 

LESS FUEL WASTE

In Volvo’s case, making the transmission a 12-speed with relatively close ratios removed concerns about fuel waste due to running at too high an rpm after the downshift, according to Wade Long, the company’s director of product marketing. Downshifting at as low as 1,000 rpm and employing a closer ratio split kept the engine very close to the “sweet spot.”

The sweet spot is the engine speed range in rpm at which brake specific fuel consumption (BSFC) – a measure of fuel efficiency – is at a minimum.

Moreover, Volvo avoided power complaints by reprogramming the ECM so the engine torque would remain at peak levels down to 1,000 rpm. Previously, the lower limit had been 1,100 rpm.

 

THE BOTTOM LINE

The goal of downspeeding is to save fuel, and experts agree that it is quite effective.

The engine “likes” running so slowly because it reduces friction and parasitic losses, such as driving the water and oil pumps. Because the engine then runs under more load it effectively becomes “smaller.” This means less air needs to be ingested, less exhaust needs to be forced out, and the fuel and air in the cylinder get hotter and develop more pressure on the power stroke.

Estimates as to the amount of fuel saved are consistent. Navistar’s Peterson estimates the savings at 2 to 3 percent, which means $700 to $800 per year. Volvo Trucks’ Long puts fuel savings at 1.5 to 3 percent.

Dana’s Slesinski adds: “Each 100 rpm reduction saves 1 percent of your fuel bill, or $730 per year in fuel costs.”

 

WHO’S WHO

Volvo is no longer the only OEM providing downspeeding drivetrains. Many have jumped onto the downspeeding bandwagon, and many trucks are running down the road while still giving good on-road performance at an ultra-low cruise rpm, something that would have been thought almost as impractical as perpetual motion only a few years ago.

- Allison

Allison Transmission introduced its TC-10, range-type 10-speed, fully automatic, twin-countershaft transmission to the press right around the same time Volvo put the XE 13 powertrain on the market. The TC 10 was designed for downspeeding from the get-go, says Lou Gilbert, director of Allison’s North American marketing and global brand development.

“Between the hydraulic clutches allowing full power shifting and the torque converter’s assistance in starting, allowing close ratio steps in the upper ranges (7th through 10th gears), the TC10 enables users to choose rear axles that allow running at low engine speeds,” he explains. “Power shifting across very small ratio steps allows for optimal drivability and performance.”

Allison is the world’s largest manufacturer of fully automatic transmissions for medium and heavy duty commercial vehicles and hybrid-propulsion systems for city buses.

- Eaton and Cummins

Eaton jumped into the fray in March of 2013, announcing a version of its Advantage series transmission – a heavy duty 10-speed automated transmissions designed in part to reduce weight and increase efficiency for fuel economy improvements – called the SmartAdvantage, which is linked to a Cummins IXS 15L diesel engine.

This setup, the SmartAdvantage Powertrain, features Cummins’ SmartTorque2 technology, with multi-torque capability. SmartTorque2 automatically senses vehicle weight, grade and operating gear and then selects the optimum torque for the best fuel economy and performance in every gear.

Eaton is a power management company. Cummins is a global power leader that designs, manufactures and services diesel and natural gas engines and related technologies.   

Mario Sanchez, technical sales support director for Cummins, says one function of the SmartAdvantage Powertrain is to limit torque output appropriately, considering the driver’s commands.

“We also can leverage the 15L displacement on our ISX15 engine with the highest compression ratio to produce higher and prolonged peak torque at lower operating rpms to deliver the necessary horsepower,” he says. Because of the engine’s larger displacement, he continues, it can operate at the efficient higher compression ratio while actually enduring less stress.

- Paccar

A similar Eaton Advantage transmission is now available with the Paccar MX engine.

Paccar is a global technology leader that designs and manufactures light, medium and heavy duty trucks under the Kenworth, Peterbilt and DAF nameplates, as well diesel engines.

Kevin Baney, Kenworth’s chief engineer, calls this downspeeding version of the transmission “an automated transmission with a small-step overdrive ratio.”

As a 10-speed transmission, it has fairly wide gear steps in the range of 39 to 43 percent through most of the gears, but the 9th-10th gear step is just 20 percent with a 0.80:1 overdrive, he explains. This allows responsive performance at low rpm over a wide range of cruise conditions.

This arrangement (International uses the same transmission with the Navistar MaxxForce 13 diesel engine) “means faster, smoother shifts,” says Navistar’s Peterson. “It’s only a change of 200 to 300 rpm to get into the next gear.”

The MX engine has several design features, such as the use of compacted graphite iron in block and head, to allow it to handle operation at low rpms, and had them from the beginning. Baney of Kenworth explains that “to accommodate downspeeding, the peak torque was shifted to a lower rpm (1,000 rpm) beginning with engine model year 2013.”

He says testing was also done to make sure the oil pump and water pump would produce sufficient pressure and flow under the low rpms.

- Daimler Trucks

Daimler Trucks North America is also providing downspeeding using its Detroit powertrains – the D12, 12-speed automated manual transmission and Detroit drive axles, says Brad Williamson, Daimler Trucks’ manager of engine and components marketing. “Our engineers have selected the right components that work together to deliver 5 to 7 percent improved fuel economy.”

Daimler Trucks, the leading heavy duty truck manufacturer in North America, manufactures, sells, and services commercial vehicles under the Freightliner, Western Star, Detroit and Thomas Built Buses nameplates.

 

INTEGRATION

Putting a drivetrain together has always been a horizontally integrated activity in the U.S. and it pretty much remains so. Volvo’s Long says all XE drivetrains utilize an I-Shift transmission, but the clutch is a custom-designed, 17” design with an organic lining (for smoothness) from Volvo Sachs. Dana or Meritor driveshafts and rears of appropriate specification carry the drive on backward.

Sachs is an international manufacturer of car and truck clutch and suspension dampening products that develops the custom, co-branded clutches in cooperation with Volvo. Meritor is a leading global supplier of drivetrain, mobility, braking and aftermarket solutions for commercial vehicle and industrial markets

Roy Horton, Mack Trucks’ director of segmentation management, allows that various driveshafts and rears of adequate strength are available with the Mack mDRIVE 12-speed automated manual transmission in the company’s Super Econodyne drivetrain, Mack’s downspeeding arrangement, which is powered by the Mack MP Series Engines. A special heavy duty driveshaft comes with the package.

Mack also offers the Mack 125/126 axle series. These axles have gear ratios in the 2.50 to 2.80 range. The 2.66 is the recommended ratio for the Super Econodyne, he says.

The mDRIVE also gets a special data file for low-rpm running.

Mack is one of North America’s largest manufacturers of heavy duty Class 8 trucks, engines and transmissions.

Peterson of Navistar says International MaxxForce downspeeding drivetrains come with an Eaton clutch, the Advantage transmission and Meritor 14X tandem drive axles.

The MaxxForce 13 was designed to provide peak torque down to 1,000 rpm, in part practical because it has a stiff block, he says. Plus, the engine’s unusual twin-turbo setup includes a smaller-diameter turbo designed to spin up quickly at low rpms, thus making it more responsive at those low cruise rpms.

 

MARKET GROWTH

Market penetration of downspeeding drivetrains is becoming significant because they are now universally available.

Volvo’s Long says penetration of the XE drivetrain was around 24 percent in 2013 and will be higher this year. Horton of Mack says the percentage of Super Econodyne sales have also been increasing.

Kenworth’s Baney says approximately 20 percent of the company’s over-the-road customers have spec’d rear axle ratios that achieve 1,150 to 1,300 rpm at cruise speed. It expects that percentage to grow to more than 50 percent by 2017.

As Navistar’s Peterson concludes: “Downspeeding is here and it will be here to stay.”

 

THE CHALLENGES

With any change in a vehicle that works as hard as a heavy duty truck, there is a risk of durability and reliability problems. The potential for problems with downspeeding is accentuated by a fact of mechanics. It takes more torque to generate the same cruise horsepower at lower rpms.

“Torque has to come up to drive the truck down the road,” says Peterson of Navistar.  

How much? Slesinski of Dana says: “If you drop your drive ratio from 3.55:1 to 2.26:1, torque has to increase by 57 percent. You don’t need just a lower axle ratio, you need the right components to support that torque.”

He explains that this means the bearings and gears in the axle, as well as the bearings and yokes in the driveshafts, have to be designed to last a million miles with as much as 57 percent more torque passing through them.

But, it’s clear enough that the industry has stepped up with the right stuff to handle all this torque, Slesinski points out. By way of example, he says Dana’s Spicer AdvanTEK 40 tandem axle “has wider gear teeth, a thicker pinion stem and larger pinion bearings, yet it is lighter because CAD and finite element analysis was used to remove metal in the housing where it wasn’t contributing to strength. Its power density is actually greater.

“Our Spicer Life (SPL) Series 350 Driveshaft has bearing and journal sizes in the crosses that are much larger.”

Charlie Allen, Meritor’s general manager of North American service, says: “Our 14X tandem drive axle is designed for faster ratios. A typical change in ratios might be a drop from 3.55:1 down to 2.64.

“We anticipated that ratios would become a lot faster and designed the 14X with stronger gearing, bearings and shafts, even in parts like the interaxle differential. There was a substantial change as compared with the previous design, the 145.”

“Our downspeeding drivetrains are integrated with components such as the Dana SPL 250 driveline and both Meritor and Dana rear axles,” Navistar’s Peterson says. “We have not seen any warranty issues when using ratios such as 2.79:1 and 2.64:1 with cruise speeds from 62 to 65 mph.

“Meritor even has a 6x2 axle that we’ve supplied with ratios as low as 2.56:1.”

Williamson of Daimler Trucks says: “For axle ratios lower than 2.41, we use an axle that allows heavier weight ratings (i.e., 23,000 lbs) with a new carrier and more robust components.”

All the industry experts we spoke with insisted that they have seen no significant warranty issues with downspeeding drivetrains, and note that there are no additional maintenance requirements.

 

ENGINE-TRANSMISSION COMMUNICATIONS

Also, many of the systems include engine-transmission communications that limit the torque when the stresses are greatest on the drivetrain.

Cummins’ SmartTorque2, by way of example, drops the torque rating 200 ft/lbs when in any but the top two gears, while also adapting the torque output to the acceleration and grade conditions continuously.

“One of the key features of the Detroit DT12 transmission for downspeeding is that it has active driveline protection,” says Daimler Trucks’ Williamson, “so it knows exactly how much torque the driveline can accept and will manage it to protect the engine, axle and driveline. Because of the intelligence of the DT12, it manages the power and torque that goes through the rest of the driveline and axles.”

 

TORSIONAL VIBRATION

One other concern with downspeeding is torsional vibration. This enemy of smooth gear operation and long driveline life can produce damaging vibration – even separation of gear teeth under load – in any area of the drivetrain because of the torque pulses produced by the engine. Turning the engine slower increases the “amplitude,” or severity, of the spikes while also changing the frequency to a lower value that can show up as unexpected vibration and noise.

Kenworth’s Baney says: “For downspeed drivetrains that utilize 2.64:1 gearing or lower with a single rear axle or dead axle tandem, clutch suppliers have developed a new clutch that incorporates a different spring damping rate.”

Ryan Trzybinski, development and product planning manager for commercial vehicle transmissions at Eaton, says: “Eaton worked closely with engine makers to ensure clutch dampers are adequately isolating potential damaging vibrations from the driveline. This analysis revealed a need to release a new damper platform Eaton calls Optimized Vibration Control, which is designed to address single axle direct-drive applications with axle ratios below 2.64. The VCT (Vibration Control Technology) Plus family of Eaton dampers has acceptable spring rates for all applications, including single axle direct drive.”

He adds that it is essential to continue with such dampening at time of clutch overhaul.

“We’ve been working with Eaton, who supplies our clutches, to make sure engine torsionals will remain well within our damping capability,” Navistar’s Peterson says. “We are not pushing the limits of any of the components.”

All the suppliers mentioned such dampening, including Allison, since their torque converter is only there to smooth things out up to about 4 mph.

 

DRIVETRAIN COMPONENTS

Meritor’s Allen says those ordering a new truck are welcome to mention favored components or arrangements like direct drive. But, anyone spec’ing out a new vehicle with a downspeeding drivetrain needs to toe the line when it comes to allowing the OEMs to advise them on what is or is not going to be durable and practical.

Fortunately, the OEMs have built protections against bad specifications into their ordering systems.

“The key,” he says, “is to tell the OEM what your operation is like and everything about what the truck is actually doing. For example, are there multiple starts in part of most runs, or is it a linehaul vehicle that spends hours rolling along at the same speed on a highway? You need to use the engine manufacturer’s software to dial it all in.”

Furthermore, Allen says that direct drive works fine in some applications but it inevitably means a big increase in torque as the truck moves out from a dead stop. “A 3.36:1 axle being driven by a transmission with a 0.74 overdrive top gear produces a 2.41:1 effective drive ratio.”

A faster axle with a direct-drive transmission will need to handle much more torque at the pinion shaft and gear even though the engine torque and overall gearing are the same, he points out. Such a setup might not last in an LTL (less than truckload) operation that makes many stops daily.

 

SPEC’ING

Baney of Kenworth says the company’s “Prospector truck ordering tool insures that component specifications are compatible. The Rear Axle Ratio work screen in Prospector identifies the range of rear axle ratios that ensures the proper balance between fuel economy and drivability.”

“We are changing the way the industry looks at the specification process,” says Daimler Trucks’ Williamson. “In the past, customers would spec individual components and run the risk of spec’ing one that didn’t deliver the desired results. We’ve now made it easier by packaging components together to help our customers achieve maximum efficiency.”

Like other truck manufacturers, the company also provides recommendations on tire size, road speed, cruise speed, engine mounts, fuel maps, etc.

Navistar’s Peterson says International even analyzes the way the engine and cab mounts work together so the different frequencies won’t pass right through the chassis into the cab. He says the net result of downspeeding can actually be a quieter atmosphere in the cab.

So, it seems that downspeeding is not only here to stay, but that the industry has pulled out all the stops to make sure end users won’t be facing unexpected troubles.

John Baxter is a freelance truck journalist and serves as the technical director of Advance Diesel Concepts, a small venture that is developing a stratified-charge-compression ignition combustion system that will shortly be tested at Argonne National Laboratory. He is a full member of SAE International and a member of the Truck Writers of North America (TWNA) and serves on its Technical Achievement Award committee. Over his long career, he has been employed by a number of the major truck industry publications.

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