Direct Drive or Overdrive?

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Content below provided by Eaton. Links provided:

Jan, 2015

There’s no single answer to this age-old debate

The long-running debate over which type of transmission – direct drive or overdrive – provides the greatest efficiency is one that’s not easily settled and not likely to go away. Conventional wisdom suggests direct drive transmissions, with their ability to transmit power directly through the main shaft with minimal parasitic losses, should be more efficient – and sometimes that’s true. If only it were so simple.

In reality, there are many variables to consider when deciding whether your fleet will be best served by a direct drive or overdrive transmission.

“Direct drive and overdrive transmissions have existed side by side for some time, but there’s certainly a lot of confusion as to which gives you the greatest efficiency,” says Evan Vijithakumara, Eaton’s global product strategy manager for heavy duty transmissions.

It all depends on the application. Generally speaking, in ideal conditions running on-highway over relatively flat ground, direct drive is the more efficient option. But throw in some hills, higher average road speeds and some off-road requirements and overdrive becomes a better fit.

Direct drive transmissions are well suited to relatively flat on-highway applications.

Eaton advocates the “power of choice” and offers its full line of manual and automated transmissions in both direct drive and overdrive configurations. Vijithakumara warns that while it’s true that a direct drive transmission itself will operate more efficiently in top gear, that doesn’t always translate into greater fuel economy.

“While direct drive can be more efficient as a transmission, that doesn’t guarantee it will be more efficient for the system – the system being the engine, transmission, axle and tire combination,” Vijithakumara explains.

When in top gear, direct drive transmissions do not transmit torque down through the countershaft, so all the power generated by the engine is passed directly through the input shaft into the output shaft. “We consider that a ‘no-mesh’ condition and when you have no loaded gear meshes, the transmission has reduced parasitic loss, resulting in more efficient operation,” according to Vijithakumara.

On the other hand, the higher front box ratios inherent in direct drive transmissions have implications on downstream components that must be considered during the spec’ing process.

“With a direct drive transmission, the torque is passed straight through to the driveline, so an adverse event such as moving from an icy road to dry pavement can transmit a lot of force further downstream through the driveshaft and u-joints, which could be potentially damaging. On an overdrive transmission, you have torque being transmitted through the countershaft gears, so you’re not effectively bypassing the transmission. When a harsh event occurs, the additional backlash and elasticity within the transmission can prevent damage downstream.”

Routes that include harsh environments and hilly terrain are best suited to overdrive transmissions.

Interest in direct drive transmissions has increased in recent years, as fleets explore all available options to increase their efficiency. But the reality is that in North America, high average road speeds and hilly, even mountainous, terrain often make overdrive transmissions a more appropriate solution. It all depends on application and duty cycle, Vijithakumara says.

“I think there is a misperception that a single product can fit all applications,” he adds. “It really depends on what you’re trying to do.”

Overdrive transmissions, with their higher overall ratios, provide greater startability and gradeability, as well as lower transmission sump temperatures in hilly terrain. The contribution this makes towards total vehicle efficiency should not be overlooked. In addition, trucks equipped with overdrive transmissions have more application flexibility which can help with vehicle resale compared a direct drive configuration tailored for a specific route.

Overdrive transmissions also allow for the greater use of downspeeding (cruising at lower engine rpms), which can drive fuel savings. Eaton’s new Fuller Advantage™ Series automated transmission, available as part of the SmartAdvantage Powertrain with the Cummins ISX15, behind the PACCAR MX-13 engine in the APEX package at Peterbilt and the Kenworth T680 Advantage package, and at International paired to the N13 engine, marries the benefits of direct drive transmissions with those offered in an overdrive configuration.  Small-step technology (a 26-percent step between 9th and 10th gear) allows downspeeding in overdrive and efficiency in direct drive that keeps the engine in the “sweet spot” when paired with rear axle ratios in the 2.64 to 3.08 range (depending on tire size and desired cruise speed). This combination of specifications is delivering impressive fuel economy savings versus competitive powertrains.

Small step technology used in the Fuller Advantage series transmissions, as used in the SmartAdvantage Powertrain, keeps the engine in the “sweet spot” to maximize efficiency.

“The Fuller Advantage Series automated transmission is an overdrive which  provides the best of both worlds,” Vijithakumara says of the package that supports both direct drive and overdrive. “It enables the downspeeding capability and robustness you have with an overdrive platform as well as the efficiency of direct drive where it makes sense.”

The Fuller Advantage Series transmissions deliver further gains, thanks to a new Precision Lubrication system that reduces oil churning losses for greater efficiency across all gears.

In recent years, the interplay between the transmission, engine and axles has increased, meaning it’s more important than ever for fleets to take a holistic approach to spec’ing the powertrain as a complete system. “When evaluating transmissions for fit, all parts of the system do factor in,” Vijithakumara shares, and Eaton’s Roadranger network is available to consult with customers on the best configuration available for their application.

“Our Roadrangers assist fleets and dealers with the information they need to help with these decisions,” Vijithakumara says. To tap into their expertise, call 800-826-HELP (4357) or visit www.Roadranger.com.

Direct drive and overdrive Eaton transmissions are equally robust and reliable, but to ensure the best performance possible it’s important to consider every aspect of your application, so consult with the experts and then specify accordingly.

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