The Global Trend toward Engine Downspeeding

Emerging Challenges from Low-RPM, High-Torque Powertrains

Two events last month on opposite sides of the Atlantic have made the subject of engine downspeeding top of mind for commercial-vehicle manufacturers and fleets alike.

At the IAA Commercial Vehicles tradeshow in Hanover, Germany, numerous exhibitors presented their latest solutions for supporting engine downspeeding.  For example, MAN presented its new six-cylinder, Euro 6-compliant D3876 engine, which supplies maximum torque in the 930- to 1,350-rpm range, enabling lower engine speeds at highway cruise speeds.

Meanwhile in the U.S., fleets presented many direct questions about the torque management strategies needed for low-rpm engines at the Fall Meeting of the Technology & Maintenance Council of the American Trucking Associations in Orlando.

The movement toward engine downspeeding is gaining momentum, and numerous engine downspeeding packages from OEMs and Tier 1 suppliers have been tailored to take advantage of the increased efficiencies associated with running engines at lower rpms.

The advantages of doing so are readily apparent.  Current technology allows downsped engines to reduce fuel consumption by about 550 gallons and carbon dioxide emissions by about 12,000 lbs. per truck annually.  Translated into dollars and cents, this increased efficiency delivers a fuel savings of nearly $2,200 per truck every year.

Lower RPMs, Higher Torques

With most engineering advances, though, there are also challenges that must be overcome, and engine downspeeding is no different. 

An engine running at lower rpms requires faster axle ratios to maintain the same vehicle speed and performance in all driving conditions, and it generates significantly higher torque stresses in the driveline. As shown below, by decreasing an engine's rpm at cruise speed from 1,450 rpm for the typical engine to 1,125 rpm for a downsped engine, torque loads in the driveline increase by 29 percent. These higher torques place added stress on the axle, driveshaft, and inter-axle shaft, greatly reducing the life of these components, especially U-joints.

AxleRatioVsDrivelineTorque_Chart_Rev 12.19.jpg

Strategies for Managing Increased Torque

Today, there are two prevailing approaches for handling the problem of increased torques in the driveline. The first is to de-rate the engine by adjusting its electronic controls. 

De-rating is generally performed by recalibrating the software that controls the engine.  Engineers set a calibration that only allows so much torque to go through the engine at certain speeds or under particular circumstances, predominantly while the truck is in first gear.

De-rating the engine is a common, reasonable strategy for addressing certain issues, and one that Dana engineers recommend in select circumstances or in combination with other measures.  When addressing engine downspeeding, it is an effective short-term workaround for reducing the likelihood of a catastrophic driveshaft failure during vehicle launches from zero speed and sudden acceleration.

However, engine de-rating alone does nothing to address the high-cycle fatigue that results from long-term torque stresses on the driveline.  No matter what is done to re-program the engine to reduce spikes in torque, a downsped engine at cruise speeds still increases torque in the driveline.

A second method for handling increased torques is the systems approach, which acknowledges that when engine characteristics change, the other parts of the powertrain need to be upgraded to match.

This solution requires an axle and driveshaft system engineered from the start to work together to support engine downspeeding, including an efficient, lighter weight tandem axle that offers the fast ratios required to fully leverage the efficiency benefits and a driveshaft that can withstand higher torques – both in the short and long term.

Spec'ing the Right Way

We've always held that the best way to spec a driveline is by selecting components that have been carefully engineered, thoroughly tested, and proven to deliver the performance and durability you need, mile after mile.

The tandem axle selected for an engine downspeeding package should incorporate numerous innovations to improve performance in a high-torque environment, including:

  • more capable primary gear, including:
    • wider face gearing with longer tooth length for lower contact and bend stress; and
    • rigid ring gear mounting that eliminates joint loosening.
  • more capable input shaft and pinion splines;
  • more capable input and pinion bearings; and
  • faster ratios to match intended operating speeds.

The other piece of a driveline solution for engine downspeeding is a driveshaft and inter-axle shaft engineered to maximize the efficiency and durability of the tandem axle.  The driveshaft should feature numerous engineering innovations designed to manage the higher torque stresses associated with engine downspeeding, including:

  • a larger U-joint cross engineered for high strength;
  • larger bearings with greater capacity; and
  • a high-density package to fit within the existing design envelope.

The chart below illustrates how a properly engineered driveline – including an advanced axle, driveshaft and inner-axle shaft – can manage increased torque stresses better than a traditional driveline system. 

Download our White Paper

We have more details on Dana's approach for addressing engine downspeeding in our white paper: The Right Solution for Downsped Engines; Managing Higher Driveline Torques to Maximize Fuel Economy.  Here, you can learn more about the effects of high torques on the driveline and detailed charts showing the effectiveness of spec'ing the right driveline.

We’d like to hear your thoughts:

  • How effectively are manufacturers addressing the high-torque challenges of engine downspeeding?
  • Besides higher driveline torques, what are your concerns as engine rpms are reduced?
  • What would convince you to purchase a truck with a low-rpm engine?
  • Are the efficiency paybacks associated with engine downspeeding worth the perceived risks?

Published by Andy Nieman


Published on .

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