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Transmissions Drive Medium-Duty Truck Fuel Efficiency

A decade ago, a manual transmission always beat out an automatic, primarily because it was more efficient — but, times are changing.

May 2012, Work Truck - Feature

by Sean Lyden - Also by this author

In the wake of the first-ever fuel economy and greenhouse gas (GHG) emissions standards applied to medium- and heavy-duty trucks, which go into effect in 2014, engine and truck manufacturers are evaluating several options to comply with those regulations. 

One strategy manufacturers are pursuing — in addition to using lighter weight materials, improving aerodynamics, and enhancing engine performance — is designing more “intelligent” and efficient transmissions.  

Craig Renneker, Ford Motor Company’s chief engineer for new transmissions and Steve Spurlin, executive director of global application engineering and vehicle integration for Allison Transmission, shared their insights on the latest trends in transmission technologies being developed to equip medium-duty trucks with greater fuel-efficient technology — and comply with the upcoming regulations.

More Efficient power

The primary purpose of a transmission is to allow an engine to operate at its optimum speed, while “transmitting” the engine’s horsepower and torque, at appropriate levels, to the truck’s drive axle and then the wheels to propel the vehicle at the desired acceleration and top-end speed. Wasted energy, at any point in that process, results in diminished fuel economy. 

According to Renneker, the industry is focusing on the following three aspects of transmissions to minimize energy loss and ensure optimal operation:

1. Adding gears. “The objective is to do everything we can to keep an engine running in its ‘sweet spot.’ To that end, more gears typically help,” Renneker said. “That’s why you’re seeing the migration from 4-speeds to 5-speeds, and, now, 6-speeds. The more gears we have, the better chance we have to put an engine in its sweet spot. It’s through engine optimization that we can achieve fuel economy gains.” 

2Increasing the ratio span. “This is the comparison between the lowest ratio of the transmission to the highest ratio of the transmission,” Renneker said. “We want to make that span as big as we can because the deeper the low gear can be, the better we can accelerate a given vehicle with a smaller engine, which helps fuel economy. And, at the high-end, the higher we can make the top-gear ratio the lower we can get an engine’s speed to get a vehicle cruising on the highway, which also improves fuel economy.”

3. Reducing drag. “The automatic [transmission] uses an oil pump, which can create some drag, so we’re always looking at every opportunity to reduce the rotating drag of the transmission because every meter or foot-pound of drag takes away from the fuel economy,” Renneker said.

Automatics vs. Manuals

It’s this “drag,” traditionally associated with automatic transmissions, that has led to the perception that manual and automated manual transmissions (AMT) offer better fuel economy because there are fewer components involved in the gear-shifting process, minimizing the potential for energy loss. 

However, Spurlin of Allison Transmission, which supplies transmissions to most medium-duty truck OEMs, said the latest technology offered in automatic transmissions actually gives the fuel-efficiency edge to automatics over manuals.

“Since the fully automatic transmission never disconnects power to the wheels, unlike a manual or AMT transmission, the fully automatic transmission wastes the least amount of energy, making it the best transmission type for realizing the best fuel economy at an equivalent speed profile,” Spurlin said. “The automatic transmission can also support, in some cases, the use of a smaller power and/or displacement engine, which also can be better for fuel economy.”

Renneker of Ford agreed. “Ten years ago, manual transmissions always beat automatics, primarily because they were more efficient. There was less drag loss inside a manual transmission than an automatic. What’s happened over the years is that, as we add more speeds to both the manual and automatic, we’re now to a point where usually the automatic gets better fuel economy.”

But how? Renneker explained that, in addition to new technologies that have reduced drag in automatic transmissions, narrowing the efficiency gap with manuals, the increased number of speeds in the manual transmission have made the manual more complicated for a driver to operate a vehicle in the optimal gear. If the driver is highly skilled and knows how to pick the right gear with every shift, the manual is fuel efficient; however, as Renneker stated, “We have a broad customer base and a lot of times the operator won’t pick the right gear. The automatic is driven by a computer designed to always pick the most optimal gear at whatever point in the vehicle’s duty cycle.”

What about automated manual transmissions?

“With the automated manual, you do have the ability to have a computer-controlled ratio (like fully automatic transmissions), and that’s a good thing,” Renneker said. “The downside is that you still need some sort of mechanical device to shift gears. If it’s a hydraulic device, that means I have to add a pump to the device, which can cause a little bit of energy loss. If it requires an electrical device, then you’ll need a bigger alternator, which will add a little bit of loss. So, you’re gaining in the area of driver control over gear selection, but you’re losing a little bit in efficiency. You would have to do a very careful comparison to know whether you’re better off with the traditional torque converter type automatic or a manual or automated manual.”


  1. 1. Mike [ May 21, 2012 @ 08:00PM ]

    Take a look at fuso's AMT. This dual clutch computer controller manual shifts great, and offers increased power to the ground, increasing fuel mileage, and does not require any extra electrical power. Their 2012 model even came standard with a single battery to increase weight reduction.


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