If there’s a problem with trucks and truck equipment in general, the ATA’s Technology & Maintenance Council is usually aware of it. Sometimes it takes a while for the murmuring and grumbling to percolate up through the bureaucracy, but if the complaints gain sufficient momentum, a study group or a technical session is often scheduled to explore or address the issue.
The surprise technical session at this year’s TMC Fall Meeting in Cleveland was about the impact of downsped drivelines on brakes. How could downspeeding be bad for brakes? It was a well-attended session, so apparently more than a few fleets believe downspeeding is responsible for increased or premature brake wear.
It’s fairly easy to understand why that perception exists, but I can’t see downspeeding, per se, as the source of the problem. In my opinion, the problem is more likely either the dearth of driver training today, poor vehicle spec’ing, poor programing of various engine and transmission parameters, or a misapplication of the technology.
Justin Worden, Nussbaum Transportation’s maintenance coordinator, was one of the panelists of the S.6 Chassis & Brake Subsystem study group session. He stated clearly that downspeeding wasn’t contributing to excessive brake wear at Nussbaum.
“We actually think with downspeeding and all the other systems, that we're getting longer brake life,” he told the audience. “If treated correctly, and with the right PM, we’re often not even [changing] one set of pads over the life of the truck.”
Other remarks he made later in his presentation made me think the problem isn’t so much with the equipment, but the way it’s managed.
Gear Fast, Run Slow
Downspeeding is the modern term for gear-fast-run-slow. Cruising at lower engine speed consumes less fuel. Because the engine runs slower, so do all the auxiliary components such as compressors, pumps and fans, all of which contribute to lower parasitic losses and reduced wear and tear on those components. Additionally, some OEMs now allow oil change intervals of up to 75,000 miles based on the resulting reduction in fuel consumption (Cummins allows 75,000-mile drains for operators achieving seven or more miles per gallon with the X12 and X15 Efficiency engines).
Not only does downspeeding conserve fuel (with a corresponding reduction in carbon dioxide), it can also reduce maintenance costs.
Downsped drivetrains offer much to be thankful for, but they’re not for everyone. It’s an application best suited to truckload operations on flat or rolling terrain, mostly on-highway long-haul. It provides fewer benefits for regional and local applications with higher ratios of stop-and-go driving and less time spent in top gear. And downspeeding is really not the best driveline spec for fleets with a lot of mountain exposure.
The Driveline Spec
The goal with a downsped driveline is to keep the engine at or close to the fuel economy sweet spot as much as possible while cruising at highway speed. We can do that today because the engine makers have delivered engine torque curves that are broad and flat, as opposed to the peaked curves of yesteryear.
Some engines deliver peak torque from 900 rpm all the way out to 1,400 rpm, giving the transmission a full gear range of high-torque, low-rpm operation. Spec’d properly, a truck should cruise somewhere close to the middle of the peak torque band, in this case maybe 1,100-1,200 rpm (this is engine make and model dependent). This provides some headroom below cruise speed — but still in peak torque — for climbing slight inclines before downshifting becomes necessary. It also provides a good rpm range for passing or higher speed operation without getting into the thirstier rpm ranges above peak torque.
But none of this speaks to the central concern of increased brake wear.
Weak Engine Brakes?
The obvious suspect is the engine brake. Conventional engine brakes don’t have much of an impact at 1,100 or 1,200 rpm, but I think there’s still more to it than that. Read on.
Cummins Technical Support Manager Kristopher Ptasznik, the second speaker on the panel, explained how engine brakes work, and described factors that affect engine brake performance. He mentioned displacement, turbo size and design, cam lobe design and valve lift duration, etc., all critical factors in engine brake performance. He also mentioned engine speed and illustrated the point with a chart showing retarding power relative to engine speed.
Ptasznik said Cummins had recently reconfigured its line-haul engine brakes to provide a bit more power at lower engine speed. Notice the area between 1,200 and 1,400 rpm. The line haul engine produces about 50 more horsepower at that speed than the heavy haul engine.
According to the chart above, the engine brake retarding capability at typical downsped engine speed (1,100-1,200 rpm) is a paltry 150-250 horsepower. Before you condemn the engine brake, consider the impact of dropping a single gear: the engine rpm bumps up by 300 rpm or so which increases the engine brake retarding power to between 375 and 425 horsepower. That’s more than enough to wrestle road speed back to the company limit on a modestly hilly bit of interstate.
If the driver needs a little more retarding capability, another downshift will push the engine up to 1,700 or 1,800 rpm, pushing retarding power close to 500 horsepower. That’s sufficient to hold an 80,000-pound truck to a reasonable speed on a 6% grade all day long -- without ever touching the service brakes.
At 2,100 rpm, the line haul engine brake in Ptasznik’s example delivers about 525 retarding horsepower. For all the Nervous Nellies reading this, that engine speed is published in Cummins’ own literature.
Ptasznik said the company made some hardware changes to increase the output at lower engine speed, but that resulted in a bit of a compromise at higher speeds.
At typical cruise speed (Zone A), 1,100-1,250 rpm, the engine brake produces about 25-50 additional horsepower than the older engine did. At higher engine speed (Zone C), the older engine produced nearly 100 horsepower more than the newer engines. Zone B is the area we should be concerned with.
“Zone B is really just to show that if your drivers are saying ‘the engine is revving too high, we think it's going to blow up,’ governed speed, the rpm the engine will achieve when fueling is not necessarily the rpm where stuff starts coming apart,” he explained. “Automated manual transmissions will not allow an engine to overspeed. Just make sure your drivers are aware that [engine speed] can go higher under braking.”
I think this brings us somewhat closer to the reason some fleets are experiencing accelerated brake wear with downsped powertrains. If drivers are accustomed to driving between 1,000 and 1,500 rpm, and believe there be dragons north of 1,500, they are robbing themselves of some serious retarding capability. And naturally, if the engine retarder isn’t — or can’t — do its job, guess what drivers are likely to do next? Correct. Hit the service brakes.
Check with a rep from your engine supplier to determine the top speed limit for the engine under engine braking. I’ll bet they’ll say 2,000 to 2,200 rpm.
Lack of Understanding
Nussbaum’s Worden spoke a bit about the importance of driver training. As concepts go, downspeeding is pretty easy to understand, but drivers often express dissatisfaction with such powertrains because they don’t “feel” like they have any power. In truth, low rpm is where all the torque is, so they do in fact have lots of power. It’s the feel of the horsepower they are missing.
That’s the get-up-and-go sensation you find in the higher horsepower ranges, usually north of 1,500 rpm — where fuel economy begins to suffer.
When used properly, with cruise control turned on, the engine and the transmission will make all the shifting decisions and do their best to keep the truck at optimum rpm for the set cruise speed. Drivers shouldn’t even have their foot on the throttle pedal — or the brake pedal.
That raises the question, if drivers are complaining about sluggish performance, are they using cruise control? If they are using cruise control, they shouldn’t be depressing the throttle pedal, and they shouldn’t notice that depressing the throttle pedal doesn’t push them back in the seat.
If they aren’t using cruise control, they are also missing out on engine brake functionality. It should apply in stages without driver command, beginning with a low setting to keep road speed in check on a small hill, and then going to the higher settings when holding the truck back on a longer, steeper grade.
When not using cruise control, the driver will have to manage the engine brake application. If they are afraid of what might happen if the revs get “too high,” that’s a failing of the training department.
But here’s the rub: cruise control is no longer a simple on or off proposition. There are so many different modes built into cruise control now, and so many fleet-programmable parameters for droop, engine-brake-on speed, coasting time and speed, etc., drivers might not get the performance they are expecting. And with some advanced cruise control systems, adaptive and predictive for example, the truck may perform differently depending on the mode it’s in.
If drivers don’t understand all the functionality baked into today’s advanced systems, they probably won’t be using it to its full advantage.
I’m bound to get some disagreement on this, but fleets can get carried away with warnings and alerts.
Drivers live in mortal fear of getting an overspeed warning on their driver profile. As a result, they may be inclined to use their service brakes even before the engine brake activates. In other words, if the cruise control and engine-brake on and off parameters are not set with drivability in mind, drivers will either not use cruise control, or over-ride it and drive as they see fit. That could result in service brake applications where an engine brake application would be more appropriate.
The other problem that arises is that some fleets lock out certain functionality on the automated transmission, such as the ability to temporarily hold the transmission in a certain gear or manually initiate a downshift. Those are particularly useful features when descending long grades.
Simply put, if a driver is expected to descend a long grade with the engine running no faster than 1,500 rpm, they will have to use the service brakes.
I produced a video series a couple of years ago outlining proper mountain driving techniques. One of the videos is about using only the engine brake to manage speed on long grades. In that video, we make two trips down a 7-mile 6-8% grade, the first loaded to 80,000 lbs and the second at 102,500 lbs to demonstrate how selecting the right gear will keep the truck at a safe and manageable speed without using the service brakes.
There are links to those videos in this commentary I wrote about mountain driving.
So, to wrap this up, I admit to being somewhat surprised that TMC had to do a study session to explain why some fleets that have embraced downspeeding might be experiencing excessive brake wear. Frankly, that puzzled me. The only connection between brake wear and downspeeding is low engine speed, and thus lackluster-engine brake performance.
But when you couple the reluctance to let the engine run at a higher rpm with the threat of overspeed warnings and write-ups, it’s easier to understand why drivers aren’t using the full capability of the engine brake.
In other words, excessive brake wear is more of a self-inflicted wound than a reason to eschew downspeeding.
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