Is Downspeeding Unhealthy for Brakes? – On the Street

Drivelines have modified to match engine torque profiles, chopping engine cruise velocity almost 500 rpm over the previous 15 years.

Picture: Jim Park

If there’s an issue with vans and truck gear typically, the ATA’s Expertise & Upkeep Council is often conscious of it. Generally it takes some time for the murmuring and grumbling to percolate up by the paperwork, but when the complaints achieve ample momentum, a research group or a technical session is commonly scheduled to discover or deal with the difficulty.

The shock technical session at this 12 months’s TMC Fall Assembly in Cleveland was concerning the impression of downsped drivelines on brakes. How may downspeeding be unhealthy for brakes? It was a well-attended session, so apparently quite a lot of fleets imagine downspeeding is accountable for elevated or untimely brake put on.

It’s pretty simple to know why that notion exists, however I can’t see downspeeding, per se, because the supply of the issue. In my view, the issue is extra seemingly both the dearth of driver coaching as we speak, poor automobile spec’ing, poor programing of varied engine and transmission parameters, or a misapplication of the expertise.

Justin Worden, Nussbaum Transportation’s upkeep coordinator, was one of many panelists of the S.6 Chassis & Brake Subsystem research group session. He said clearly that downspeeding wasn’t contributing to extreme brake put on at Nussbaum.

“We truly assume with downspeeding and all the opposite methods, that we’re getting longer brake life,” he instructed the viewers. “If handled appropriately, and with the precise PM, we’re usually not even [changing] one set of pads over the lifetime of the truck.”

Different remarks he made later in his presentation made me assume the issue isn’t a lot with the gear, however the best way it’s managed.

Gear Quick, Run Gradual

Downspeeding is the trendy time period for gear-fast-run-slow. Cruising at decrease engine velocity consumes much less gas. As a result of the engine runs slower, so do all of the auxiliary elements corresponding to compressors, pumps and followers, all of which contribute to decrease parasitic losses and lowered put on and tear on these elements. Moreover, some OEMs now permit oil change intervals of as much as 75,000 miles primarily based on the ensuing discount in gas consumption (Cummins permits 75,000-mile drains for operators reaching seven or extra miles per gallon with the X12 and X15 Effectivity engines).

Not solely does downspeeding preserve gas (with a corresponding discount in carbon dioxide), it may additionally scale back upkeep prices.

Graph: TMC

Downsped drivetrains provide a lot to be grateful for, however they’re not for everybody. It’s an utility finest suited to truckload operations on flat or rolling terrain, principally on-highway long-haul. It gives fewer advantages for regional and native functions with increased ratios of stop-and-go driving and fewer time spent in high gear. And downspeeding is actually not the most effective driveline spec for fleets with quite a lot of mountain publicity.

The Driveline Spec

The aim with a downsped driveline is to maintain the engine at or near the gas economic system candy spot as a lot as attainable whereas cruising at freeway velocity. We are able to try this as we speak as a result of the engine makers have delivered engine torque curves which are broad and flat, versus the peaked curves of yesteryear.

Some engines ship peak torque from 900 rpm all the best way out to 1,400 rpm, giving the transmission a full gear vary of high-torque, low-rpm operation. Spec’d correctly, a truck ought to cruise someplace near the center of the height torque band, on this case perhaps 1,100-1,200 rpm (that is engine make and mannequin dependent). This gives some headroom under cruise velocity — however nonetheless in peak torque — for climbing slight inclines earlier than downshifting turns into crucial. It additionally gives a superb rpm vary for passing or increased velocity operation with out moving into the thirstier rpm ranges above peak torque.

However none of this speaks to the central concern of elevated brake put on.

Weak Engine Brakes?

The plain suspect is the engine brake. Standard engine brakes don’t have a lot of an impression at 1,100 or 1,200 rpm, however I feel there’s nonetheless extra to it than that. Learn on.

Cummins Technical Assist Supervisor Kristopher Ptasznik, the second speaker on the panel, defined how engine brakes work, and described components that have an effect on engine brake efficiency. He talked about displacement, turbo dimension and design, cam lobe design and valve elevate period, and so forth., all vital components in engine brake efficiency. He additionally talked about engine velocity and illustrated the purpose with a chart displaying retarding energy relative to engine velocity.

The purple line represents a line-haul engine, whereas the inexperienced line represents a heavy-haul engine — presumably Cummins’ Effectivity and Efficiency sequence engines. That is Cummins-specific, clearly. Different OEMs’ engine brake efficiency curves can be related however not precisely the identical.

Graph: Cummins

Ptasznik mentioned Cummins had not too long ago reconfigured its line-haul engine brakes to offer a bit extra energy at decrease engine velocity. Discover the world between 1,200 and 1,400 rpm. The road haul engine produces about 50 extra horsepower at that velocity than the heavy haul engine.

In keeping with the chart above, the engine brake retarding functionality at typical downsped engine velocity (1,100-1,200 rpm) is a paltry 150-250 horsepower. Earlier than you condemn the engine brake, take into account the impression of dropping a single gear: the engine rpm bumps up by 300 rpm or so which will increase the engine brake retarding energy to between 375 and 425 horsepower. That’s greater than sufficient to wrestle highway velocity again to the corporate restrict on a modestly hilly little bit of interstate.

If the motive force wants a bit extra retarding functionality, one other downshift will push the engine as much as 1,700 or 1,800 rpm, pushing retarding energy near 500 horsepower. That’s ample to carry an 80,000-pound truck to an inexpensive velocity on a 6% grade all day lengthy — with out ever touching the service brakes.

At 2,100 rpm, the road haul engine brake in Ptasznik’s instance delivers about 525 retarding horsepower. For all of the Nervous Nellies studying this, that engine velocity is printed in Cummins’ personal literature.

This chart, additionally from Ptasznik’s presentation, exhibits how Cummins has tailored the engine brake for higher efficiency at decrease engine speeds, ie, downsped engines. The blue line represents a 2013-vintage engine, whereas the purple line displays the present era’s retarding profile.

Graph: Cummins

Ptasznik mentioned the corporate made some {hardware} adjustments to extend the output at decrease engine velocity, however that resulted in a little bit of a compromise at increased speeds.

At typical cruise velocity (Zone A), 1,100-1,250 rpm, the engine brake produces about 25-50 further horsepower than the older engine did. At increased engine velocity (Zone C), the older engine produced almost 100 horsepower greater than the newer engines. Zone B is the world we ought to be involved with.

“Zone B is actually simply to point out that in case your drivers are saying ‘the engine is revving too excessive, we predict it will blow up,’ ruled velocity, the rpm the engine will obtain when fueling shouldn’t be essentially the rpm the place stuff begins coming aside,” he defined. “Automated handbook transmissions is not going to permit an engine to overspeed. Simply make certain your drivers are conscious that [engine speed] can go increased underneath braking.”

I feel this brings us considerably nearer to the explanation some fleets are experiencing accelerated brake put on with downsped powertrains. If drivers are accustomed to driving between 1,000 and 1,500 rpm, and imagine there be dragons north of 1,500, they’re robbing themselves of some critical retarding functionality. And naturally, if the engine retarder isn’t — or can’t — do its job, guess what drivers are prone to do subsequent? Right. Hit the service brakes.

Verify with a rep out of your engine provider to find out the highest velocity restrict for the engine underneath engine braking. I’ll guess they’ll say 2,000 to 2,200 rpm.

Lack of Understanding

Nussbaum’s Worden spoke a bit concerning the significance of driver coaching. As ideas go, downspeeding is fairly simple to know, however drivers usually categorical dissatisfaction with such powertrains as a result of they don’t “really feel” like they’ve any energy. In fact, low rpm is the place all of the torque is, so that they do actually have numerous energy. It’s the texture of the horsepower they’re lacking.

That’s the get-up-and-go sensation you discover within the increased horsepower ranges, often north of 1,500 rpm — the place gas economic system begins to endure.

Fleets that lock-out sure AMT performance could also be including to the workload of the brakes.

Picture: Jim Park

When used correctly, with cruise management turned on, the engine and the transmission will make all of the shifting choices and do their finest to maintain the truck at optimum rpm for the set cruise velocity. Drivers shouldn’t even have their foot on the throttle pedal — or the brake pedal.

That raises the query, if drivers are complaining about sluggish efficiency, are they utilizing cruise management? If they’re utilizing cruise management, they shouldn’t be miserable the throttle pedal, and so they shouldn’t discover that miserable the throttle pedal doesn’t push them again within the seat.

In the event that they aren’t utilizing cruise management, they’re additionally lacking out on engine brake performance. It ought to apply in phases with out driver command, starting with a low setting to maintain highway velocity in test on a small hill, after which going to the upper settings when holding the truck again on an extended, steeper grade.

When not utilizing cruise management, the motive force should handle the engine brake utility. If they’re afraid of what would possibly occur if the revs get “too excessive,” that’s a failing of the coaching division.

However right here’s the rub: cruise management is now not a easy on or off proposition. There are such a lot of totally different modes constructed into cruise management now, and so many fleet-programmable parameters for droop, engine-brake-on velocity, coasting time and velocity, and so forth., drivers may not get the efficiency they’re anticipating. And with some superior cruise management methods, adaptive and predictive for instance, the truck could carry out in another way relying on the mode it’s in.

If drivers don’t perceive all of the performance baked into as we speak’s superior methods, they in all probability received’t be utilizing it to its full benefit.

Extreme Oversight

I’m sure to get some disagreement on this, however fleets can get carried away with warnings and alerts.

Drivers stay in mortal concern of getting an overspeed warning on their driver profile. Consequently, they could be inclined to make use of their service brakes even earlier than the engine brake prompts. In different phrases, if the cruise management and engine-brake on and off parameters usually are not set with drivability in thoughts, drivers will both not use cruise management, or over-ride it and drive as they see match. That might end in service brake functions the place an engine brake utility could be extra applicable.

The opposite drawback that arises is that some fleets lock out sure performance on the automated transmission, corresponding to the power to quickly maintain the transmission in a sure gear or manually provoke a downshift. These are significantly helpful options when descending lengthy grades.

There’s quite a lot of performance baked into cruise management as we speak. Drivers in all probability want coaching in learn how to get essentially the most from it. Performance varies throughout varied makes, fashions and software program generations.

Picture: Jim Park

Merely put, if a driver is anticipated to descend an extended grade with the engine operating no quicker than 1,500 rpm, they should use the service brakes.

I produced a video sequence a few years in the past outlining correct mountain driving strategies. One of many movies is about utilizing solely the engine brake to handle velocity on lengthy grades. In that video, we make two journeys down a 7-mile 6-8% grade, the primary loaded to 80,000 lbs and the second at 102,500 lbs to display how choosing the precise gear will maintain the truck at a secure and manageable velocity with out utilizing the service brakes.

There are hyperlinks to these movies on this commentary I wrote about mountain driving.

Abstract

So, to wrap this up, I admit to being considerably shocked that TMC needed to do a research session to clarify why some fleets which have embraced downspeeding may be experiencing extreme brake put on. Frankly, that puzzled me. The one connection between brake put on and downspeeding is low engine velocity, and thus lackluster-engine brake efficiency.

However whenever you couple the reluctance to let the engine run at a better rpm with the specter of overspeed warnings and write-ups, it’s simpler to know why drivers aren’t utilizing the complete functionality of the engine brake.

In different phrases, extreme brake put on is extra of a self-inflicted wound than a purpose to eschew downspeeding.

On longer, steeper grades, drivers will want their service brakes except they downshift to get the engine rpm as much as the place the engine brake is producing most retarding energy. They’ll range the output utilizing the excessive, medium, and low selector to range the retarding impact.

Picture: Jim Park