Tech feature: truck transmissions

Between the flywheel of a truck's diesel engine and the scattering of worn rubber on the road's surface, there is a long line of components to transfer, redirect, distribute, and multiply the torque that gives a truck its life.

Central to this process is the transmission. Given the relatively limited rev range of a large diesel, and the contrasting wide range of speeds that most applications require, the primary function of a transmission is to keep the engine functioning at peak efficiency for as long as possible.

But the technology underpinning transmissions varies widely, with different technologies appealing to different segments of the market.

As ever, tradition and experience play key roles in customer choice and now, more than ever, driver considerations impact heavily on the specification chosen. For example, an operator who is having trouble finding drivers who are both experienced and competent in handling the venerable and generally bullet-proof Eaton Roadranger 18-speed constant-mesh gearbox will opt for something that does not require a clutch pedal.

The next decision will be to pick between an automated manual, which still has a clutch and pressure plate and therefore wearable friction surfaces, or a fully automatic transmission which uses a fluid coupling or torque converter.

Here's a quick look at the types of transmissions currently available…

>> SYNCHROMESH MANUAL
Most manufacturers build their own transmissions in this category. The technology is proven, easily exported across both corporate and geographical boundaries, and requires the least amount of driver training for the least money. It's not easy, however, to adjust or modify ratios to suit the task. The manufacturer tends to fix that with a series of assumptions about how the particular truck model will be used. The use of synchromesh to make the gearbox mesh the gears reduces the torque capacity of these gearboxes, so the weight range of a synchro 'box doesn't generally go past medium-duty.

>> CONSTANT-MESH MANUAL
Of all the transmissions, this is the most difficult to master. It is designed with high-torque, extreme conditions in mind, and is still the staple in Australia for road train and other multi-combination long-distance applications. Eaton is the provider of choice in this category, and the 18-speed extended H-pattern gear lever with direct/overdrive and range-change buttons is branded on the consciousness of thousands of truck drivers across the country. A good operator can keep the torque up as well as any automated manual, but it's a skill that can take a lot of (sometimes expensive) practise.

>> AUTOMATED MANUAL (WITH OR WITHOUT CLUTCH PEDAL)
Scania used to offer its automated manual in either two- or three-pedal set-ups. I have no idea why. If you're going to get a computer to manage the clutch, why not let it do its job from start to finish? But I was told that some fleets like to have their drivers use the clutch for starting and it seems it is most popular in vocational rigs.

The AMT itself is now a proven system but there is a wide difference in the speed of the change. One Japanese AMT which the factory insists on building itself is so slow between gears you can probably make a coffee while you're waiting. One of the Swedes is very good at this and the swift changes keep the torque interruptions to a minimum.

Fuso has a twist on the theme – its Duonic AMT has dual clutches, so the 'box preselects the next gear as you're accelerating. On my first drive I was hooked within 100 metres. There have been some reliability issues that the factory has dealt with, but it's mainly a light-duty 'box for the Canter. You can't get it in the medium- or heavy-duty models yet.

Technically, an AMT is a garden variety constant-mesh gearbox with an automated clutch system bolted on and plugged in. That brings established reliability and ease of servicing, as long as you have the plug-in diagnostics for the change management. But it also means that with the best versions (from Europe and the US, that is), you can customise the shift pattern, rev range and time in gear to suit the application.

As an example, on my first drive of a Hino 700 with the ZF AMT, it was immediately clear no-one had looked closely at the engine characteristics. It revved its nuts off, so I switched to manual mode, lugged the big Hino engine down to 1050rpm before changing, used much less fuel and enjoyed a quieter drive. Coming back up Mount Ousley, just north of Wollongong, I was no slower than the trucks that relied on the computer.

>> FULL AUTOMATIC
My favourite truck test was carried out in the US several years ago. On a closed test track, two identical trucks were hitched up to trailers with the same weights. One had an Allison auto 'box, and the other an Eaton Roadranger 18-speed manual.

The auto truck was driven by a college student and the manual by a professional driver. The drag race up a steep hill saw the student win handsomely. Then, after the race, was it revealed that the student had 75 less horsepower!

It was a perfect illustration of the difference that unbroken torque makes, and you can only get that with a fully automatic transmission. Allison's 'box uses road speed, torque being used, acceleration rate and angle of incline to calculate the gross weight of the rig. It then manages the shift mapping accordingly.

The 'box also spells optimal traction in off-road conditions. At the moment, full automatics are not seen in interstate or on-highway road train applications. There are some developments in process, but don't hold your breath.

Tippers, concrete agitators and pumpers, waste trucks and metro delivery units are where you'll find the most widespread use of full autos, in some cases because of a high driver turnover rate and the opportunity to eliminate expensive and regular clutch replacements.

>> HYBRID AUTOMATED MANUAL WITH TORQUE CONVERTER
Mercedes-Benz has a unique string to its bow in the transmission arena. The Actros SLT is an ultra-heavy-duty truck with an Actros profile and an Actros interior but a new set of underwear. Drive on the SLT goes through the Powershift AMT but only after going through a torque converter first.

The benefits are more relevant to the heavy haulage role than any other, where extreme-weight standing starts test even the best driveline. On conventional AMTs, when you select drive the start gear engages and even without any throttle you'll creep forward as the system engages the clutch. But push the D button on the SLT and you're still going nowhere. It's engaged the gear but the torque converter isn't converting any torque.

Only when you press the throttle does the torque converter process power into the transmission. So the clutch is fully engaged and the fluid coupling handles all the slip. It's a heavy solution because you effectively have two transmissions in line, but for a heavy task tare weight isn't the key criteria. SLTs are used across the world in the heavy haulage fleets of the biggest transport groups.

SPECIALIST MANUFACTURERS
Apart from the factories that design and build their own transmissions, there are specialist gearbox builders that supply their products at the factory and after initial sale.

>> EATON
The hard-as-nails Roadranger manual transmission is renowned as the transmission of choice for many fleets still. But Eaton couldn't ignore the industry-wide trend towards relieving the driver of having to use a clutch, and so the Ultrashift and UltraShift Plus were introduced. It is basically a Roadranger with a hydraulic clutch that is electronically managed, so the development cost has mainly been the smarts. Even so, after moving off, hotshots will go from a crawl to highway cruising without touching the clutch anyway.

>> ZF
This German company makes most types and configurations of gearboxes and axles, and is most popular with the European builders. It now markets a full automatic to compete with Allison and also enjoys widespread support from NATO military forces in their supply and light-duty troop transport requirements.

Its TraXon modular design is a pioneer. It's basically a set of gears with either a clutch, dual clutch, hybrid or torque converter modules on the front. Application will determine which system suits best but with the common core, costs are reduced.

>> AISIN
Owned by the Toyota group, the company builds manual and auto transmissions for group and non-group companies, and supplies some European car companies too. Interestingly, you can still buy a Hino truck with an Allison transmission, so the Aisin doesn't yet cover all the potential uses of the builder's truck range.

>> ALLISON
It was the military that kicked off Allison transmission development, although the company has designed and built aero engines as well. In fact, my first introduction to Allison required earplugs. The scream of four Allison turboprops howling around the cargo hold of a RAAF C130 Hercules is a sound permanently etched on my Army Air Dispatch memory.

Boral has a two-word reason for ticking the Allison option box when ordering new concrete or gravel trucks – 'zero downtime'. The company supplies heavy-duty autos to construction equipment companies and the US military, also providing drive units for some marine applications.

Allison's sales boss for Australia is Robert Cavagnino, a Top Fuel dragster team boss in his 'spare' time. Between the ear-splitting crackle of nitro engines at the last WA meet, he told me about the increase in auto take-up among manufacturers, driven largely by customer demand.

A key point is that the trend has now developed from one of necessity because of driver limitations, to one of efficiency because of the way that the auto 'box maximises the engine's performance characteristics.

"That's why there's a developing problem with the AMTs," he said.

"The European engines in particular are low-revving, high-torque six-cylinders that really require a torque converter to produce the best performance."

DOWN THE ROAD
There are things on the horizon that will expand the diversification of transmission technology in the trucks on Aussie roads. Both Allison and ZF are aiming at long-distance operators with features that will appeal to these applications.

Allison's TC10 twin-countershaft 10-speed automatic transmission is flexible enough for both city and highway prime mover duty cycles and provides blended architecture with full power shifts, a torque converter and a twin countershaft gearbox design.

The company has operated a fleet of test users that have achieved an average five per cent fuel economy improvement with the TC10 as compared to their current manual and automated manual models.

The gearbox is rated up to 600hp and 2305Nm of torque with 10 forward speeds and two reverse. It comes equipped with Allison's newest generation of electronic controls, which provide prognostics to eliminate unnecessary oil and filter changes, and enhanced shift selector functionality.

The push for innovation is taking transmission development into new fields. For example, ZF has developed an Innovation Truck package that features a 120kW electric motor built directly into the 12-speed transmission's bell housing.

It not only functions as a retarder that recovers energy, but has a classy trick up its sleeve when combined with steering servos, a highly complex software program and a computer tablet. In short, the driver arrives at the depot and pulls up close to the loading bay, hops out of the truck, hands over the tablet to the loading bay personnel and heads indoors for his snooze.

The depot people then use the tablet to reverse the truck into the bay without using the engine. The electric motor handles the manoeuvre while the diesel engine also takes a nap, and the software manages the steering.

The system is demonstrated on a DAF XF pulling a semi with a pig trailer behind (see lead picture). The double articulation seems to be no problem for the operator, who directs the truck with one finger dragging the trailer graphic on the tablet to the required position. I can think of a few B-double drivers who need a system like that...

THE BIG QUESTION: AUTO OR MANUAL?
The biggest question can only be answered by a close examination of application and driver usage. A manual, whether the clutch is operated by the driver or a computer, has limitations that are being magnified by the unstoppable trend in engine design and characteristics.

In layman's terms, it appears that the waves of torque, which are more like pulses when you reduce the number of cylinders and reduce the revs, can set up surges within the transmissions that are magnified along the driveline when it's connected by a hard coupling, such as a clutch. It can result in the ECM interpreting those pulses as driver input from the throttle, causing the engine to hunt, even in cruise mode.

One top manufacturer, with probably the best AMT on the market as far as useability is concerned, is looking seriously at full automatics right now because of the massive torque loadings going through its own AMTs.

Another case in point is a big Swede, whose 3500Nm V8 headliner has the torque limited by 200Nm from first to ninth gear. Developing the AMT 'box to handle that kind of torque in the lower gears would be a massive engineering effort. So although you pay the extra for the big torque figure, you've only got it from 10th gear on. That means a heavy haulage operator doesn't get the maximum rating when he needs it most – while manoeuvring the odd Manly Ferry around an industrial area, for example.

Ultimately, torque multiplying is done by a gear set in the AMTs but by a fluid coupling in the auto, reducing gear train wear drastically.

Electronics on an AMT can only do so much, as when the gear is engaged it's a hard coupling. So it's the change point that is the focus of the ECM. But an auto can control the passage of torque while a gear is working, increasing torque when required – sometimes without changing gear – as well as keeping the torque constant, even during a gear change.

Interstate work is likely to be the last bastion of manual or AMT transmissions. With a relatively low frequency of changes, the hard coupling loses nothing in efficiency to the auto. The savings in fuel from constant torque on acceleration are miniscule when a truck finally hits the highway and is cruising with around 30 per cent of available torque being used for a day or two.

As always, do your sums, and buy on whole-of-life cost. It may mean the difference between a healthy profit, or a crippling loss.