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An axle that disconnects when you don’t need it

By on July 1, 2015

The latest trend in engine design is “downspeeding.” It has nothing to do with speed limiting, but rather, relates to trucks doing the same work at significantly lower rpm. For every 100 rpm reduction in engine speed at any given vehicle speed, engineers tell us there is a one percent savings in fuel consumption. If your current truck’s engine cruises at 1,300 rpm and your next truck’s engine maintains that speed at only 1,100 rpm, all other things being equal, it will be two percent more fuel efficient. Drop that engine speed to 1,000 rpm and the engine will be three percent better.

That’s because every time the engine makes a revolution, there is mass being put in motion. The pistons move up and down and the valves open and close, and must also be stopped and reversed. Crankshaft and camshafts have to rotate, accessories such as power steering pumps and alternators must be driven. All this work requires energy, which comes from the fuel in your tank. Reducing engine revolutions each minute reduces the work requirement, and that saves fuel.

Preparing for 2017

Engineers are exploring other ways to reduce fuel use in advance of the next round of greenhouse gas regulations for 2017 trucks, due out soon. Oils and other lubricants will have less viscosity (resistance to flow), making them easier to pump and easier for crankshafts and gears to go through them.

Improved aerodynamics will play a big role in lowering fuel use, as will reducing the number of drive axles. Currently, the vast majority of long haul trucks in North America have a 6X4 configuration. That means the tractor has six wheel-end locations: two steer axle ends and four driving axle ends, two ends on each of two drive axles. In Europe and elsewhere 6X2 and 4X2 tractors are far more common.

Having only two driving wheel ends (one drive axle), the truck is lighter by several hundred pounds because the weight of differential gear sets, drive shafts, inter-axle shafts and universal joints is not needed. By eliminating them, you eliminate the friction involved in driving them.

The most popular configuration for a five-axle combination truck elsewhere in the world is a 4X2 tractor pulling a 3-axle trailer. In Europe, there are many twisty roads, and city streets are tighter than ours so maneuverability is a major consideration, as is fuel economy.

One would think, then, that the layout for an ideal truck would be a downsped engine producing maximum torque at 1,000 rpm, pulling through a 6X2 axle configuration with drive gears around a 2.2 to 1 ratio. But, as with so much else in trucking, there are disadvantages to such a layout, and tradeoffs must be made.

Downspeeding requires more torque to be put through the drive train. That, in turn, requires stronger, and thus heavier, drive shafts, U-joints and gears.

Engineers at Dana Holding Corporation, makers of Spicer drive train components, report that downspeeding an engine at cruise speed increases driveline torque by between 55 and 60 percent. That requires drivelines to be fortified to handle the resulting long-term stresses.

Operating at lower engine rpm using fast axle ratios limits low speed acceleration, a definite detriment in stop-and-go traffic. Reducing internal friction by having only one drive axle reduces traction in marginal weather conditions. It can also affect handling. Some drivers have described 6X2 trucks as being downright squirrely.

Finding a solution

Change-As-Needed-2That prompted Dana to produce one of the most innovative new products seen at MATS this year, the Spicer AdvanTEK Dual Range Disconnect Concept for tandem axles in Class 8 linehaul applications. It overcomes virtually all the problems with downspeeding and going to a single drive axle with few, if any, tradeoffs. It’s actually a simple concept: If each configuration works well in a particular set of conditions, keep it for those conditions.

For start-up and city traffic, traditional axle ratios between 3.10 and 3.25 to 1 work well. They do not need the extra heft in the drive shaft and U-joints because the axle provides adequate torque multiplication. Downspeeding works best when cruising at speeds above 45 or 50 mph, and in the transmission’s high range. You don’t need as much drivetrain strength because you don’t need as much torque multiplication. The axle can then be shifted to a downspeed ratio, perhaps as low (nominally) as 2.10 to 2.30 to 1.

The Spicer AdvanTEK shifts axle ratios between normal ratios for startup, hill climbing and off-road operations and downspeed ratios for economical cruising to provide each one when it is needed.

The number of drive axles is even simpler to control. AdvanTEK just disconnects the inter-axle drive shaft in order to convert to 6X2. When added traction or stability is needed, the trailing axle is again engaged and driven. Instead of rolling along in “neutral,” it resumes being a drive axle.

The AdvanTEK system will be controlled by its own electronic control unit (ECU) and will have its own sensors to measure operating conditions such as tire slip, lateral acceleration, torque and speed. Engagement and disengagement settings can be dealer-programmed according to the truck owner’s preferences. They can be based on torque, vehicle speed, grade and terrain and more. Operation will be seamless to the driver.

The AdvanTEK Dual Range Disconnect Technology Concept is still that, a concept. Final details are being worked out, but perhaps we will see it offered as soon as the end of this year.

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