Drivers

NACFE Shows Downspeeding Good for 2-3% Fuel Savings

October 29, 2015

By Jim Park

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The latest Confidence Report released by the North American Council for Freight Efficiency and Carbon War Room reveals that downsped drivelines in Class 8 tractors can produce fuel savings of 2-3% when used in appropriate applications.

The report, released Oct. 28, explores the benefits and challenges of downspeeding for Class 8 tractors. It was the eighth Confidence Report released by the group, and the fifth this year. Other reports covered automated transmissions, resetting engine parameters, low-rolling-resistance tires and lightweighting as fuel saving strategies.

Downspeeding is a term given to the use of fast rear-axle gear ratios to produce a lower engine rpm at vehicle cruise speed. It is one of the primary powertrain-focused strategies for improving fuel economy on road tractors and regional-haul day-cab trucks with high rates of steady-state highway driving.

Downspeeding is usually accomplished by using either a very fast axle ratio combined with a direct-drive transmission or a slightly slower axle ratio combined with an overdrive transmission. The former is more commonly used and better suited to linehaul applications on fairly flat terrain, while the latter is better suited to regional and city applications with more stop/start driving.

While the downspeeding concept – sometimes referred to as "gear fast, run slow" – has been around for decades, recently introduced complementary technologies have made the practice more attractive.

"Downspeeding is seeing tremendous growth among fleets whose operation profiles allow for it," said NACFE Executive Director Mike Roeth. "The OEMs we consulted for this report say about 25% of their total on-highway real-axle build is now 2.47:1 or below. They are typically between 2.24 and 2.47. They have told us that really aggressive downspeeding configurations of 2.07:1 are just around the corner."

NACFE gives downspeeding a high level of confidence in its matrix chart.
NACFE gives downspeeding a high level of confidence in its matrix chart.
On top of the reported fuel economy benefits, the NACFE report also credits downspeeding with improved drivability and much quieter operation at highway speeds, which is a big plus with drivers.

The report also contains a confidence matrix that expresses the study team’s confidence in downspeeding, and offers recommendations for fleets interested in using this technology to improve fuel economy.

Key Findings of the Downspeeding Confidence Report

  • When optimally applied, downspeeding will improve fuel efficiency and lower the operating rpm of the engine under cruise conditions, while helping in other areas as well, such as noise reduction and improved drivability.
  • Downspeeding alone can save 2–3% off the fuel bill. However, specifying a downsped engine without looking at the whole of the powertrain can have negative consequences, such as increased risk of driveline failure or insufficient horsepower.
  • Optimal truck design will see downsped powertrains in either of the two configurations spec’d with other technologies, including automated manual transmissions (AMTs), certain rear-axle ratios, modified engine torque levels that may be restricted to certain gears, carefully chosen electronic engine parameters, and reinforced drivelines.
  • This package of multiple fuel efficiency technologies results in about 3–6% fuel savings overall and reduces the negatives posed by adopting downspeeding exclusively.
  • Downspeeding is at a tipping point, with rear-axle ratios of 2.47:1, and engine rpms of 1,100–1,300 now common offerings among powertrain manufacturers. And "aggressive downspeeding" is just around the corner, with manufacturers poised to offer rear axle ratios of around 2.08:1, and even lower engine cruise rpms of just 900–1,000.

 According to Roeth, the ROI on a downspeed driveline is pretty good.

"Depending on the OE, the cost of upgrading to a downsped driveline, including more robust components recommended to mitigate possible driveline damage due to increased high-torque operation can be as low as $500 to $1000." he said. "With fuel at $3.75 per gallon and 1% savings, fleets can save about $700 in fuel per year. At 2%, about $1500. With today's sub-$2.50 fuel, the payback is going to take a little longer."

Increased use of faster rear‐axle ratios. (Source: Meritor)
Increased use of faster rear‐axle ratios. (Source: Meritor)

The report notes that downspeeding can also be used as a driver attraction and/or retention tool.

"They said what they liked best these drivetrains was the quiet operation and the drivability," said Roeth. "But I should say that some fleets we surveyed reported those same attributes as negatives. Drivers who had not been properly trained on what to expect found the performance sluggish due to operating in the high-high-torque, low-horsepower end of the engines power band."

That could be the result of operating a downspeed driveline in less than optimal conditions, like in the mountains or where there's a lot of stop/start driving.

"Downspeeding is ideally suited to rolling terrain, not mountains, where the truck would typically shift between the top two gears," Roeth told HDT. "The really tall gearing, especially with an overdrive transmission isn't conducive to mountain driving, or city driving. A direct drive would be better in those situations, or where mountains are a large part of the fleet's geographical profile, a standard driveline might be more suitable."

The Confidence Report also discusses the challenges of downspeeding, at the forefront the greater potential for driveline failure if improperly spec’d.

Drivetrain vulnerability is a critical concern for fleets, and faster axle ratios increase the potential for damage. This is primarily because lower rpm means more torque overall, and also means that torque spikes are applied to the driveline parts at a lower frequency, which can translate into gear chatter or wear issues.

However, it notes, "our findings indicate that vehicle and component manufacturers are actively addressing this torque issue by developing heavier-duty components like driveshafts and axle housings, bearings, and gears that can handle the increased torque produced when engines turn at reduced rpm."

NACFE plans to release one more Confidence report in early December covering best maintenance practices for optimum fuel efficiency.

The next NACFE Fuel Economy Workshop is scheduled for Dec. 9 at the ACR wind tunnel facility in Indianapolis. The Workshop agenda includes a tour of the facility when the wind tunnel in action. More information is available at NACFE.org or truckingefficiency.org.

The report can be downloaded here.

Comments

  1. 1. Robert Abrams [ October 30, 2015 @ 03:56AM ]

    Makes great sense. I own a 2008. Lexus ls460l. a 4400 lb. car that has an 8 speed double overdrive transmission that consistently gets over 26 m.p.g running rte. 95 between southern Fl. And Boston on regular gas.( 118,000 miles).In the late 1980's our Freightliners were also geared very high and delivered great m/p/g.
    Bob Abrams
    United Truck Leasing Corp.
    Braintree,Mass. ( Sold to Ryder in 1990)
    P.S. Miss the industry, but don't miss increasing regulations my old friends tell me about

  2. 2. Cliff Downing [ October 30, 2015 @ 01:48PM ]

    Key to the entire article was the first sentence. Appropriate applications. Many times, fleets and individuals spec this sort of thing, which is generally a good thing for interstate type of running, for all of their operation, and it doesn't quite play out as one would hope. And when driver has to deal with speed limited, low operating RPM spec'd trucks when pulling heavy in rolling hills, the opposite effect is more true.. reducing driver retention and satisfaction. Either way, the vehicle buyer has to do all the appropriate research, as dealers are woefully ignorant on how to spec out anything more than the mats in the dealership bathroom. It is amazing the look of confusion one gets from dealers when trying this sort of thing. But also one thing has to be considered, on trade off or resale, value will be reduced as the available market for this type of spec is very limited outside of major fleet operations. Many used truck buyers are doing other types of operational demands that a down speed spec'd truck is not appropriate for. Same can be said of spec'ing condos instead of mid roofs. More demand for the midroof than the condo.

  3. 3. Jesse [ November 03, 2015 @ 09:18PM ]

    Trying to run a loaded big truck at 1000rpm's down the interstate at posted speeds is in my opinion dumb. Really dumb if your not running big power, which big power in fleets is a thing of the past. And not many O/Ops are going to put their big power equipment in a constant 1000rpm situation. Very limited application is the only place I see this nonsense applied. Which brings me to, how about content for the rest of us.... :/

 

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