The downspeeding genie is out of the bottle and there’s no putting it back, so truck and engine makers, component manufacturers and fleets need to learn how to harness the power low of rpm drivetrains or suffer the consequences.
The premise and benefits of downspeeding are straightforward. Lowering the number of engine revolutions at highway cruising speed saves fuel. At lower rpm, there are fewer individual combustion events per mile, and each one consumes less fuel. Current experience indicates about 1% fuel savings for every 100-rpm drop in engine speed at highway cruising speed.
While engine speeds have been trending lower for years (the gear-fast, run-slow principle), Volvo Trucks gave the trend a name in 2011, and introduced the first downsped powertrain to North America, dropping engine revs at 65 mph from 1,350 to an astonishing 1,150. And engineers are already eyeing 900-rpm cruising speed.
Engine revs of 900 at cruising speed look very attractive from a fuel savings perspective, but there’s much more to downspeeding than simply changing a few gear ratios.
In a nutshell, we still need about 200 horsepower to keep a loaded truck at 65 mph on flat level ground. But we need torque to keep the truck moving on hills and, to the extent possible, to prevent downshifting. With peak torque in modern engines sliding toward the lower end of the rpm band, we now have rated (peak) torque across a range of perhaps 250 to 400 rpm, somewhere between 1,000 and 1,400 rpm, depending on the engine.
Cruising at 65 mph, a hair’s breadth above peak torque, gives the engine a few hundred rpm of latitude before a downshift would be required in a pull. The thinking is, if the rpm can be allowed to drop close to the bottom of the peak torque curve on most of the terrain likely to be encountered by an over-the-road linehaul truck, the number of downshifts would be reduced and the engine will be operating in its most efficient range a majority of the time.
You need gobs of torque at low engine rpm to make downspeeding work, so torque is both an enabler of downspeeding — and the source of much of the concern about delivering peak torque at very low engine speed. It’s not a big problem at cruise speed when the drivetrain is operating at roughly a 2:1 or 3:1 ratio compared to engine speed. However, it can wreak havoc on a driveline at launch and at low speeds, where the transmission’s lower gears greatly multiply the torque.
Inside the drivetrain
Each time a cylinder fires, it sends a shockwave through the driveline. The force of the fuel exploding in the cylinder is transmitted through the crankshaft, through the transmission and on through the driveshaft, U-joints and finally into the crown and pinion gears of the differential. Because of the multiplying effect of the transmission’s lower gear ratios, the amount of force that spikes through the driveline is much higher than many suppose.
According to a whitepaper produced by Meritor, “Understanding the Effects of Engine Downspeeding on Drivetrain Components,” drivetrain components can be subjected to torque spikes of close to 25,000 lb-ft when the transmission is in lower gears. The effects of those massive torque loads are worsened by the speed at which those stresses are introduced to the drivetrain.
“Changes in engine architecture since EPA 2010 — higher peak cylinder pressures, vastly more responsive turbochargers, etc. — really changed the engines’ power curves,” explains Karl Mayer, Meritor’s director of product line management for axles. “The time it takes an engine to go from idle at 700 rpm to peak torque at around 1,000 rpm is now measured in tenths of a second, whereas it may have taken a second or two in earlier, pre-2010 engines.”
That has a profound effect on the drivetrain. Because of a second multiplier, the rear axle ratio, torque on the driveline itself is much greater than the advertised torque rating of the engine would lead us to believe. According to Steve Slesinski, Dana’s director of product planning, as the rotational speed of the driveshaft goes down, the torque goes up.
“If you were operating at a driveshaft speed of about 1,450 rpm with an overdrive transmission, and you went to a direct drive transmission to slow the engine further, you might be at, say, 1,125 rpm at the driveshaft,” he says. “If you want to maintain the same road speed with a slower driveshaft speed, the torque on the driveshaft actually goes up by 57%.”
Slesinski says increased torque load can have a long-term effect on component life, especially U-joints, but the biggest threat to the drivetrain can come when the truck is hardly moving — such as when backing under a trailer or when the drive wheels slip on icy surfaces then regain traction.
Extensive testing into drivetrain shock loading conducted by Meritor suggests the combination of fast torque ramp-up times and fast rear axle gearing can dramatically overstress components in the driveline. Here’s a quote from Meritor’s white paper:
“A test truck equipped with an EPA 2010 engine rated at 1550 lb-ft, 410 hp, a direct drive transmission second gear ratio at 10.95 and a rear axle ratio of 2.47 has a calculated maximum torque of 13,700 lb-ft. However, during a second-gear aggressive start up, the measured drive shaft torque, using an instrumented drive shaft, was 21,600 lb-ft. The measured drive shaft torque value was 58 percent greater than the calculated torque. As a result, the test truck fractured a driveshaft universal joint cross.”
Dana, too, has conducted tests on the impact of torque on drivetrain components with similar results. The findings from one test are presented in a whitepaper called “The Right Solution for Downsped Engines.”
Dana tested a truck with a derated low-rpm, 400-hp engine in a simulated wintertime scenario where a driver operates the truck in first gear to hook up a trailer that has frozen brakes and wheels. Engine rpm was elevated at launch, with the driver loading the throttle while releasing the clutch. “The clutch was not fully engaged, but the engine rpm dropped to a point where it stalled. The maximum torque value recorded was 18,953 lb-ft, which is more than enough to break a traditional truck driveshaft at idle speed.”
Putting it all into perspective, in the days since the EPA-2010 generation of engines appeared, three factors have conspired to pose imminent (but avoidable) threats to vehicle drivetrains:
1. Engines produce more torque at lower rpm and deliver it much more quickly to the driveline than ever before;
2. The torque multiplying effect of the transmission’s lower gears, particularly at launch, coupled with the numerically lower, faster rear axles, put the entire driveline at greater risk from shock loading;
3. The push to downspeed engines to save fuel has increased torque on the driveline at cruising speed and at low speed, particularly during launch, when hooking to a trailer and when a wheel-slip occurs.
It would follow that fleets embracing downspeeding as a fuel-saving measure are exposing themselves to potentially greater problems resulting from drivetrain failures. That may be true on the surface, but there are ways to mitigate the potential for damage while still reaping the benefits of lower engine speeds.
While downspeeding presents a challenge to the driveline, solutions are at hand that remove much of the risk associated with such a spec — but not all.
How to avoid damage
First in the order of priorities has to be the customer getting the spec right for the intended application.
All the OEs currently offering downsped powertrains say upfront that they will not work in all applications, and indeed, they aren’t intended as a one-size-fits-all approach to powertrain spec’ing. The application best suited to this spec is an 80,000-pound gross combination weight truck running the vast majority of its miles on Interstate highway in lightly rolling terrain.
“The wider powerbands in today’s engines means the truck can spend more time in top gear at cruise speed before it requires a downshift,” says Brad Williamson, powertrain marketing manager for Daimler Trucks North America. “With more torque, more usable power at lower rpm, the transmission does not have to drop a gear at the first sign of a hill.”
Joe Puff, vice president of Truck Technology and Maintenance at NationaLease, cautions fleets that like to dual-purpose their trucks, from say a P&D truck in daytime to a linehaul truck at night, won’t see a benefit from the direct-drive, downsped powertrain.
“Direct drive has very limited, if any, fuel economy benefits in P&D operation,” he says. “The added torque stress on the axle and drivetrain can cause damage that would far outweigh any fuel efficiency, yet I see some fleets trying to run the same spec in these two vastly different applications.”
The challenge, according to Puff, is balancing the need to provide enough torque for adequate gradeability and startability along with the ability to navigate hilly terrain under full load without risking damage to the drivetrain in the lower gears.
“We don’t necessarily need all the torque that we have in those lower gears,” he says. “There’s certainly a good argument to be made for limiting torque and rpm in the lower gears, and that’s an easy thing to do now.”
Engineers can set a calibration that only allows so much torque to go through the engine at certain speeds or under particular circumstances, especially while the truck is in lower gears.
Another option is beefing up the driveline to handle the torque.
For instance, Dana recently released a new line of drive shafts and axles optimized for downsped, direct-drive powertrains, featuring the SPL 350 driveshaft with the SPL 250 inter-axle shaft and the Spicer AdvanTek 40 tandem axle.
“We have increased the torque-bearing capacity of the axle by 33% using upsized splines and bearing systems, while reducing the overall weight by 21 pounds,” Slesinski explains. “It’s really a whole new axle system designed expressly for the emerging lower-rpm, higher-torque trend.”
Meritor has been doing a lot of work on its existing 40,000-pound tandem axle, the 14X. Mayer says early in 2015 we’ll see a much faster ratio than the current 2.47:1 to meet customer demands for downsped drivetrains with direct-drive transmission.
“We saw downspeeding coming back in 2010, and we designed the 14X with 2050-lb-ft torque capacity, so it’s a very capable axle,” he says. “And with the faster ratios we have developed, it’s well suited for the emerging market.”
Meyer also hinted that a new axle will be unveiled soon with even higher torque bearing capacity, called the 17X Evo. It’s currently in wide service in Europe, and is being readied for North American installations.
Of course, in addition to multiplying the torque from the engine, the transmission is subject to that torque as well, along with the clutch. It, though, has a role to play in dampening some of the firing oscillations from the engine.
“Without any changes to the clutch damper, these oscillations could cause damage to the transmission, U-joints or axle,” says Ryan Trzybinski, product planning manager for Eaton Vehicle Group. “To ensure that adequate dampening is still performed by the clutch system, the clutch damper system needs to have a softer damper rate, with longer travel to still handle the high torque. In addition, there are other vehicle specification factors such as direct drive versus overdrive or single axles versus tandem axles that are critical considerations that can affect the specific clutch damper needed.”
From the clutch all the way back to the pinion shaft in the differential, all driveline components are subject to a tremendous amount of dynamic stress resulting from the push to lower engine rpm. And despite the reported driveline failures, it’s still a viable way of saving fuel. Indeed, it’s becoming the driveline spec of the future. With advances in engine control, torque limiting in the lower gears is one way to mitigate stress on the driveline. Beefing up the driveline is another. Maybe using both strategies is the way to go.
Reports from component manufacturers and OEs seem to suggest the equipment is up to the task, and getting better, so the damage we’ve seen may be more related to certain events, like wheel slippage and overloading the drivetrain when hooking up a trailer. So even with all that technology in place, there’s probably still room for a little old fashioned driver training — like teaching them how to be nice to their drivetrain.