Fuel Smarts

Engine 'Downspeeding' Can Cause Driveline Failures, TMC Attendees Told

September 24, 2014

By Tom Berg

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Driveline damage from engine torque “spikes” is among the many nuts-and-bolts subjects discussed by fleet managers and supplier representatives so far this week at a maintenance conference in Florida.

The Fall Meeting of the Technology & Maintenance Council of the American Trucking Associations began Monday morning and will run through mid-day Thursday at the Dolphin Hotel Conference Center at Disney World near Orlando. Scores of educational sessions are the main draw for the 1,317 attendees, and some are involved with the 10th annual SuperTech competition being held concurrently.

A trend toward fuel-saving “downspeeding” of engines, with highway cruise rpms of 1,300 and less, requires extra torque to compensate for loss of horsepower, and that can damage driveline components, a panel of experts said during a session Tuesday afternoon. “Fast” axle ratios further slow the driveline, exacerbating the problem, they said.

Axle ratios of 2.47 are commonly used with downspeeded engines and direct-drive transmissions – another method of saving fuel by reducing internal lube “churn” – and even lower ratios are coming, said Chuck Blake, applications engineer at the Detroit Diesel division of Daimler Trucks North America.

This means a need to “use torque appropriately” by electronically modifying engine output so torque is limited at clutch engagement. Thanks to electronics, many operating parameters can be set, and truck buyers should know what they are and work with truck builders to pick the correct ones, Blake said.

Another approach is to “fortify” the driveline, said Don Remboski, vice president of research and development and innovation at Dana Holding Corp. He noted that the trend toward lower revs could mean cruising at 1,150 rpms or less. So, along with the torque limiting, beefier components should be chosen.

Dana has the stronger components available, he said, but they should not be too strong that they pass on damage to transmissions or axles, which are more costly to repair. “Spec the driveline around the engine,” he advised.

Another source of driveline damage is torque spikes caused by sudden shocks to the system, said Bob Ostrander, chief driveline engineer for Meritor. Spikes happen when a driver backs his tractor under a trailer, drags trailer tires against curbs, or just “drops a clutch” when his foot slips off the clutch pedal. Tens of thousands of pounds-feet are momentarily sent through drivelines, causing fractures.

Meritor engineers began getting reports of driveline fractures and began analyzing parts that were sent in. They concluded that torque limiting, as Detroit’s Blake had suggested, was the solution. If done correctly, drivers won’t notice any loss of performance at very low engine speeds involved during clutch engagement, Ostrander said.

About 50 pounds might be added to a truck’s weight with stronger driveline components, Ostrander and Dana’s Remboski said. Truck makers set the monetary upcharge, but it might be $1,000 to $2,000 for the option, a Dana sales representative later said.

All three experts agreed that automated manual transmissions reduced incidences of driveline damage because correct gears are more likely to be used at any given time. But shock and torque spikes are still possible in some of the maneuvers Ostrander described.

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