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Dec 17, 2010
- Relieving driver stress, reducing power train wear, enhancing efficiency
- The EPS automated gearshift mechanism introduced in 1985
- Clutch automation followed in 1996
Stuttgart. It was two things mainly which in 1985 set Mercedes-Benz on course towards the truck of the future. They were the new OM 442 A and OM 442 LA engines and the new EPS gearshift mechanism. The abbreviation initially stood for ‘ electro-pneumatic gearshift system’ and subsequently for ‘ Electronic Power Shift’. These two elements, the engines and EPS, were closely interrelated because the Stuttgart plant installed this automated gearshift mechanism as standard in those trucks which were fitted with these new and particularly high-torque engines, both with a cubic capacity of 14.6 litres.
Europe’s most powerful truck
When introduced, the V8 engine, officially designated as the OM 442 LA, was nothing less than the most powerful standard truck engine produced in Europe. Its rated output of 320 kW (435 bhp) and its maximum torque of 1765 Nm were unique for their time and remained so for the next two years.
Even the more modest variant, the OM 442 A, which was turbocharged but, unlike the OM 442 LA, was not charge air cooled, still easily outstripped the maximum torque generated by the previous Mercedes-Benz truck flagship. This 14.6 litre, 260 kW (354 bhp) V8 engine placed a torque force of up to 1600 Nm at the crankshaft’s disposal. In comparison, the previous top-range engine had to be content with a maximum of 1550 Nm.
Torque in abundance
A completely new feature of these engines was the way in which they released their power potential. In the mid-1980s, ‘table mountain’ was the term for maximum torque applied across a broad range rather than culminating around a closely defined peak (mostly between 1200 and 1300 rpm). The OM 442 LA covered the spectrum between 1100 and 1600 rpm while the range of the OM 442 A extended from 1100 to 1500 rpm.
This produced a significant increase in power reserves in the lower engine speed range which meant that changing down when tackling inclines could be left until later than was previously the case with fewer gear changes overall. Altogether, thanks to this engine concept, the driver could take account of an engine speed collective, significantly lower than was previously available, but could still fully exploit the engine’s potential and thus complete his journey particularly economically but equally quickly.
Permanent relief for drivers
There’s no question, then, but that under these circumstances the clutch and transmission had to cope with significantly greater demands than previously. One way of getting power to flow from these vigorous V8 engines to the transmission with the least possible wear was the new EPS automated gearshift mechanism. This also produced benefits for the production and logistics departments at the Stuttgart plant; simply using plug-in cables as the link between the gearshift knob and the transmission was considerably easier than installing shift linkages, a wide variety of which needed to be held in stock. This applied especially to buses where the distance between shift lever and transmission was often more than 10 metres. “Even more important,” according to ‘Scheinwerfer’ [‘Headlight’], the in-house magazine, when referring to a further benefit of automation, was the “relief this afforded drivers from constantly having to change gear”. In fact, changing gears now required virtually no effort at all from drivers because the EPS had made the shift linkage unnecessary. Instead, the EPS employed servo units which used pneumatic power to carry out gear changes in the transmission; solenoid valves and shift cylinders achieved what had previously tested the strength of the driver’s biceps. These servo units were actuated by a control unit which accepted the gearshift commands coming from the driver’s shift lever.
The gearshift lever no longer protruded wantonly into the interior of the cab; rather, it was a stubby knob which functioned like a shift stick. The well-known shift patterns like the double-H or range gearshifts with a simple H shift pattern were outmoded. EPS reduced their complexity to a simple forwards-backwards logic; pushing the stick forward meant shifting up, pulling it back meant shifting down.
Simplified gearshift pattern
The splitter box was operated via a rocker switch located, as with the previous manual transmissions, on the front of the gearshift knob. However, in a departure from the usual gearshift mechanisms, there was a so-called skip shift button on top of the splitter switch which had to be activated if gears were to be skipped or reverse gear brought into operation. The centre position was the standard position for the gearshift knob. The transmission would move to the idling position (neutral) if tapped briefly so as to displace the gear stick from its central position over to the left.
The driver needed to ensure that the clutch was depressed carefully and far enough to fully disengage it. A sensor on the clutch pedal would prevent a gearshift from taking place if the clutch was not fully disengaged. This arrangement protected both the clutch and the transmission’s synchro device from any unnecessary excess wear but it also demanded a certain degree of patience from drivers who were often used to more lively gearshifting if they had mastered the manual transmission technique of immediately disengaging the gear at a load change point and then of quickly engaging the desired gear with an additional touch on the accelerator if required.
Very well received by drivers
Despite this, EPS became very popular with drivers in its early years. “The drivers were the greatest supporters of EPS,” concluded Ernst Göhring, its developer, two years after the system was introduced. The high-torque engines were easily able to compensate for the slightly longer interruptions to tractive power caused by the automatic transmission. It was, overall, a blessing for drivers who now only had to smoothly slide a knob instead of having to directly influence the transmission via the shift linkage and by exerting manual power.
A typical EPS gearshift occurred as follows: the driver would operate the clutch and pull the gearshift knob in the desired gear change direction as far as a cut-off point. This would activate the so-called shift lever unit, thus initiating a ‘ request’ to the computer which carried out an intended/actual comparison to determine whether the gear step requested by the driver was permissible.
If the request lay within the bounds of the possible, without damaging the transmission or the clutch, there would be a prompt reply. In this event, it would consist of the knob being released, together with the typical EPS ‘click’, and being allowed to move the short distance beyond the cut-off point as far as the endstop; the new gear step would be selected in the transmission and the driver would only have to let the clutch in again for tractive power to be restored. The gearshift knob would then automatically return to the centre position. EPS made it impossible to shift gears into engine speed ranges which exceeded the permitted maximum and which would pose a potential risk of damage to the engine or clutch, because the gear selected would not be authorised if such a risk existed. In this event, the currently engaged gear would be retained and an acoustic warning signal would sound as the inappropriate gear was selected.
Gear indicator on the display
A rectangular indicator, fitted centrally above the instrument panel, let the driver know which gear was currently engaged. The eight main gears of the 16-speed transmission were shown horizontally in numerical sequence; information about the splitter box was displayed above and below as an arrow symbol. If, for example, the Number 6 and the upper arrow were indicated, this meant that the sixth gear in the upper splitter box was engaged.
If the worst came to the worst, there was also small box at bottom centre which could be regarded as the forerunner of the fault code display which is commonly found today. It showed workshop staff the circuit path in which a malfunction had occurred. The little display indicated a possible malfunction by activating a warning symbol in the form of vertical dashes.
Search function for the appropriate gear step
Although still relatively limited from today’ s perspective, the range of automated gearshift variants available was astonishingly flexible for the time. For example, the system essentially selected the first main gear for moving off if the gearshift knob was pushed forward from the neutral position. If the driver pressed the skip shift button at the same time, he would move directly from neutral to the second main gear.
In general, the gearshift logic also permitted gears to be skipped by pressing the skip shift button. However, this was conditional on there being a guaranteed target engine speed of more than 750 rpm when shifting up and, when shifting down, on the maximum permissible engine speed not being exceeded. Also part of the package, even with the first generation EPS, was an extremely convenient search function; if the truck was freewheeling in neutral up to traffic lights, a touch on the gearshift knob was sufficient to get the computer to find the most appropriate gear to match the vehicle speed.
Of course, a sort of emergency program was also part of this early electronics package. Trucks which were still equipped with transmissions supplied by Zahnradfabrik Friedrichshafen (ZF) when EPS was first introduced could be given an additional switch to allow a pneumatically-operated shift either into neutral or into second gear. Buses which were already equipped with the Mercedes-Benz produced new G4 transmissions, could also engage reverse gear using a pneumatically-operated emergency switch.
Electronics generate greater efficiency
According to ‘Scheinwerfer’, the in-house magazine, EPS was designed to be “an effective contribution not only to an economical driving style but to active driving safety”. The electro-pneumatic gearshift system was also one of several modules in a whole package of efficiency-enhancing measures which the then
Daimler-Benz AG was presenting as the ‘integrated power train’.
Daimler-Benz AG was presenting as the ‘integrated power train’.
Among the most important elements of the package were the characteristics of the new OM 442 A and OM 442 LA engines which supplied maximum torque across a particularly broad engine speed range. The feature which allowed these engines to run particularly economically over the whole useable engine speed range was a refined injection system, the two-spring boost pressure compensator. This made it possible to adjust the injection quantity precisely to the power requirements and to the excess air across the whole engine speed range.
With its top-range OM 442 LA engine, Daimler-Benz was also the world’s first truck manufacturer to supply an electronic diesel control (EDC) system. Improved starting behaviour, further reductions in fuel consumption, an integral engine speed and speed limiter option, better performance characteristics at greater altitudes and not least lower emissions: these were the arguments put forward by the Stuttgart plant in favour of EDC.
Emission limits easily met
Electronic management was being introduced at that time not only into gearshift mechanisms and transmissions, but also into engines. But even without EDC, both these top-performance engines were approx. 20 percent below the future ECE R 49 exhaust emission limits which were then under discussion; they became mandatory for production vehicles from 1990 and are known today as Euro 0. Other electronics, designed to optimise an integrated transport system, were in preparation at the plant in the form of the new ‘Mercedes-Benz Assistance System’; this technology, the forerunner of today’s FleetBoard telematics, facilitated computer-assisted trip and fleet optimisation.
But these early electronics were not entirely without pitfalls. Operating the system was anything but intuitive. Among the more complicated EPS manoeuvres was, for example, engaging reverse gear. That only worked if neutral had first been selected, if the vehicle was stationary and if the clutch had been disengaged for at least five seconds.
Any driver who failed to follow this sequence point by point would move the gearshift knob to the rear with the skip shift button pressed to no effect and would have to learn his lesson all over again. Job advertisements would sometimes say: “Driver with experience of EPS wanted”.
EPS came under a hail of criticism when a fuel tanker and trailer went out of control on a downhill stretch in Hessen in 1987 and exploded in front of the ‘Rialto’ ice-cream parlour in the centre of the town of Herborn. Defective brakes were quickly identified as the cause of the accident. But the driver also stated that the EPS had not carried out his command to change down and he had therefore been unable to use the engine brake. So, discussion of the accident turned to a great extent on whether the disaster could have been averted if it had been possible to change down. There was never a clear answer to this question. In the end, EPS played no further part in the legal investigation into the cause of the accident. Nonetheless, an account of the defects of the system had been raised and there was eloquent testimony to the susceptibility of the electronics in the 1980s: of 29,401 vehicles fitted with EPS, some 8,300 reported a total of 17,000 defects to Daimler-Benz within the space of two years; and of 400 buses with the electro-pneumatic gearshift system, 200 reported a total of 500 defects.
EPS widely available as standard from 1988
With the introduction by Mercedes-Benz of the SK-series (Heavy Class) in 1988, EPS was supplied as standard for all trucks from 213 kW (290 bhp) and the teething troubles of the semi-automated gearshift mechanism were a thing of the past. Mercedes-Benz became the world’s first vehicle manufacturer to provide its customers with an automated gearshift mechanism at no extra cost. The most important innovation in conjunction with EPS was that transmissions were no longer bought in from ZF; instead, in-house products were installed including shift cylinders for the EPS, integrated into the transmission housing.
The housing itself was made of aluminium, making the new transmission around 100 kilograms lighter than its predecessor. They were available with either 9 or 16 gearshift steps. The steps and spacing between the gears were hardly any different from the previously installed ZF Ecosplit transmissions.
The EPS gearshift knob in the SK vehicles was of a slimmer design than before and was seated more elegantly in a flat-surfaced mount on the engine tunnel. The gear indicator was no longer located above the instruments but was now fully integrated into the instrument panel and positioned above the rev counter. Essentially, no changes were made to the gearshift logic. But the shift times were shorter and the gears engaged more gently and smoothly than before.
The CAN-Bus brought huge benefits for the automatic transmission
The next fundamental change occurred with the appearance in 1996 of the Actros, the successor to the SK. Apart from the transmission, which was still modelled on the basic four-speed transmission with range and splitter box, practically everything about this vehicle was new. Even the automated gearshift mechanism was given a completely new and thoroughly modern image. EPS was rebranded as the ‘Telligent gearshift system’, benefiting enormously from the unimagined possibilities resulting from the CAN bus network onboard the Actros.
The gearshift mechanism now communicated via the CAN bus with every other electronic system in the vehicle. The shift lever was replaced by a new rocker switch, more a joystick than a gear lever. A small lip protruded from in front, used by the driver for the split gearshifts. The general principle of pushing the rocker button forwards for shifting up and pressing it to the rear for shifting down was retained.
Not for nothing was the name ‘Telligent’ adopted: whereas the EPS engaged the next gear up or down each time the shift lever was operated, the same action carried out in the Actros resulted in the higher or lower gear most appropriate to the traffic situation being selected. The vehicle’s speed, acceleration and rolling resistance were now all included in the calculation made by the electronics. In addition, the computer also calculated the position of the most effective torque for the correct engagement speed or the maximum permissible engine speed in engine brake mode.
Easily-digested gearshift recommendations
The gearshift computer also offered the driver a gearshift recommendation on the instrument panel display which was fitted as usual centrally above the rev counter. Nothing was easier for the driver than to accept this suggestion. He was shown the currently activated gear and the splitter box in operation at any given moment; and the gearshift recommendation was shown flashing. To accept it, he had only to step on the clutch. If he wanted to correct the suggestions made by the gearshift computer, the old routine applied: complete gears via the joystick, half gears via the splitter switch which was still located on the joystick. Applying both controls simultaneously shifted three half gears in a single operation.
Essentially, the gear changes were carried out more rapidly than previously without placing any greater strain on the transmission synchromesh. In practice, the real improvement was that the gear change from fourth to fifth gear and vice versa, always a time-critical operation, became significantly quicker. In general terms, the design engineers had succeeded in reducing the interruption to tractive power to such a small extent that it was even possible with the Telligent gearshift system to switch rapidly between forwards and reverse gear and to free vehicles which had become stuck by rocking them back and forwards.
The automated clutch offered total comfort
The new Telligent automatic gearshift system, introduced together with the Actros, also assumed the clutch function. Based on the same variable speed manual transmissions as the Telligent gearshift system, the Telligent automatic gearshift system was a fully-automated transmission in which not just the gearshift mechanism but also the clutch was automated.
Generally speaking, classic fully-automated, converter-based transmissions did not provide sufficient gear steps to make long-distance journeys, for example, economical. Fully-automated transmissions are in any case more expensive than standard variable speed manual transmissions. They have always had a role, which they retain to this day, in special applications such as municipal refuse disposal or similar tasks involving frequent stopping and starting which can be hard on traditional clutches.
The pedal was now only for emergencies
There was now no need for a clutch pedal in an Actros equipped with the Telligent automatic gearshift system and it was no longer visible. Nonetheless, Mercedes-Benz still installed a foldaway clutch pedal in these fully-automated trucks for a few more years so that, in the event of a system failure, the truck could make it to the nearest workshop using a mechanical clutch.
Instead of a clutch pedal operated by the driver’s foot pressure, an Actros equipped with the Telligent automatic gearshift system had engine clutch control and electromotive clutch control. Both systems were integrated into the CAN data bus and communicated via the data bus with all the other onboard computers.
All the driver had to do to start off was to move the joystick forwards out of neutral and simply depress the accelerator. The Telligent automatic gearshift system engaged whichever gear it considered most appropriate and let in the clutch as soon as the driver had increased the engine speed sufficiently. The onboard computer automatically calculated and prepared the gear changes during the journey; the truck’s internal communication system coordinated and controlled the interaction between engine, clutch and electro-pneumatic gearshift system.
A variety of intervention options available to the driver
The driver could intervene manually in the Telligent automatic gearshift system at any time by operating the joystick or the split gearshift rocker button (other options were kickdown or releasing the accelerator pedal early). If required, the driver could also change to fully manual mode and, again using either the joystick or the split rocker, exercise complete control over gear changes. But he would not have to engage or disengage the clutch; that would still be done by the automated clutch.
The clutch was let in and out with minimal slip, thus reducing clutch wear. The main objectives of the gearshift strategy were: the least possible fuel consumption, avoidance of repeated shifting between two gears and preventing the engine from over-revving or being driven at too low revs. The calculation also always took account of the current vehicle weight. The Telligent automatic gearshift system always calculated the current gross weight from the acceleration data obtained when driving off.
Just how splendidly the Telligent automatic gearshift system performed in practice is illustrated by a comparative test undertaken by the ‘lastauto omnibus’ trade magazine in 1999. According to the magazine, the system “is very quickly aware of the precise weight of the truck plus its cargo”. This not only results in the ideal gearshift strategy but also applies to empty runs. “Only the Telligent automatic gearshift system is able to immediately recognise how light the vehicle is; it assesses the correct response to the circumstances, selects a high gear for starting off and manages low rev driving.”
As for the gearshift strategy and fuel consumption for a gross weight of 40 tonnes, the author comments: “The system continuously registers the nature of the route being followed, assessing gradients with incredible resourcefulness and calculating the necessary gearshift strategy with versatility and almost mind-blowing accuracy.” The summary concludes that “ in order to beat the Mercedes automatic transmission as regards consumption, you need to be an excellent driver at the top of your game.”
There was no question about it, then: the Telligent automatic gearshift system was the haulier’s means of choice for reducing fleet consumption. Even the best driver cannot help being occasionally off form. In contrast, the electronics exercised constant control over the driving style, saving fuel and avoiding drive train wear; they never succumbed to tiredness and could not be distracted.
The benchmark for the competition
Mainly thanks to the extensive electronic network in the Actros, the Telligent automatic gearshift system was way ahead of the competition. The emissions standard applicable at that time was Euro 2, and Euro 3 was imminent. Engine performance maps had for a long time presented a complicated picture and human drivers were barely able any longer to estimate how to drive fuel-efficiently at what load and at what engine speed. This was a much easier task for the computer which had all the engine’s characteristic maps stored in its memory.
But there were even more subtle features. An automated transmission gearshift took just 0.3 to 1.0 seconds; also available was an extra-rapid shift mode which automatically came into operation on gradients steeper than 7 per cent, ensuring minimal interruption to tractive power. If the driver had set the cruise control, the shift points in an Actros equipped with Telligent automatic gearshift system were moved a little higher so as to keep as closely as possible to the desired speed. The system would then automatically shift up again and accelerate back to the set cruise speed if the gradient had led to a loss of speed and had forced in a gear change.
If, on the other hand, the driver operated the engine brake when going downhill, then the Telligent automatic gearshift system would initially change down so as to set an engine speed of around 1500 rpm. If, by further pressure on the service brake, the driver indicated a need for greater deceleration, this would result in an additional gear downshift and thus an increase in engine speed and enhanced engine brake performance.
One of the lessons from the Herborn accident was the incorporation of a safety feature in the system logic. As long as the driver did not manually intervene to change anything, the gear currently engaged when decelerating would remain engaged to the bitter end. The Telligent automatic gearshift system was also the only automated variable speed transmission which allowed the engine to be switched off while in gear.
Automatic transmissions for the construction sector
The construction vehicle variants of the new Heavy Class followed in 1997, just one year after the introduction of the Actros road vehicles. They were fitted as standard with a hydraulic gearshift mechanism, replacing the shift linkage which had so far been the usual practice. If particularly requested, there was also an Actros construction vehicle with a special variant of the Telligent gearshift system. Its semi-automated transmission (the clutch still needed to be operated) was able to cope with particularly harsh construction site conditions by always shifting between the first four gears especially rapidly if the inter-axle or inter-wheel differential lock was activated.
The inventors of the Telligent gearshift system for construction industry use also increased the time for pre-selecting individual gears from the usual ten seconds in the case of road vehicles to 30 seconds to give drivers more flexibility when driving off-road. Not least, it was now also possible with the Telligent gearshift system to move directly back and forth between forwards and reverse without going via neutral. This allowed a truck which had got bogged down to be rocked free in an emergency. The only essential requirement for using rocking mode was to begin with reverse gear.
It would still take until 2004 and the appearance of Actros 2 before the Telligent automatic gearshift system was sufficiently well-developed for construction industry use. By then, the extra shift console had disappeared, as it had from the Actros 2 road vehicles introduced in 2002; this console had always been an obstacle, particularly in trucks which had an engine tunnel, to through-cab access between the driver’s and front passenger’s side. The more elegant solution in the Actros 2 consisted of a shift paddle, located to the front on a second extension arm below the armrest and which, if required, could be easily folded away to the rear like the armrest.
The renaissance of the unsynchronised transmissions
Meanwhile, a whole new generation of transmissions for road vehicles was on its way. It was christened ‘ PowerShift’ and did away with synchromesh for the main transmission. This could either be used to reduce the weight of the transmission or to increase the maximum possible input torque by permitting components to be of a more rugged design.
Not only automated transmissions but engines, too, underwent rapid development. Instead of a maximum of 1765 Nm, as was still the case in 1985, the maximum torque of the most powerful Mercedes-Benz V8 engines was now 2800 Nm. The previous 1600 Nm attained by the basic engine class had now risen to maximum torque levels in the 2000 to 2200 Nm range. And that is probably by no means the end of the story.
Twelve gears are enough
The consequence of this was the insertion of up to three step-up gears in the overdrive transmissions which, because of their design, could never run as economically as direct gear transmissions with their direct through-drive in the highest gear. A further consequence of the trend towards increasingly high-torque engines was that the 16 gears, used for decades, were gradually becoming superfluous and only made sense for special applications. Twelve gears, it was acknowledged, could now cope just as well and were, in any case, easier to manage in production terms.
In 2006, Mercedes-Benz introduced the new PowerShift transmission generation entirely in accordance with this thinking. The central feature was unsynchronised, but fully-automated manual transmissions with twelve gear steps, designed for maximum input torques of 2100, 2800 and 3300 Nm. Only the most powerful representative of this trio (called the G 330-12) had an overdrive fitted which was not initially revealed because it was mainly intended for construction vehicles and heavy-duty haulage tasks. It would still be some time before PowerShift actually became a reality in a special construction variant.
Direct through-drive in top gear
Consequently, the presentation in 2006 focused on the two direct gear variants, the G 211-12 and G 281-12 engines, which were available in the 235 to 324 kW and 338 to 441 kW trucks, i.e. 320 to 440 bhp and 460 to 600 bhp. These transmissions were produced in particularly significant numbers, earning them the name of ‘large-volume transmissions’. They were developed based exactly on their predecessors with 16 synchronised gears. While remaining in step with these well-known transmissions, they also followed the range splitting design principle of multiplying the number of main gears (and these were the only ones which were now unsynchronised). And they each had a directly stepped up top gear.
However, instead of four gears in the main transmission, there were now only three, forming the basic one to three gear ratio. Gears one to six followed by employing a synchronised rear-mounted range-change group, constructed on the planetary gear design (also called a range group). The front-mounted splitter box then subdivided these six gearshift steps yet again into a rapid and a slow gear ratio, giving a total of twelve gears.
PowerShift doubled the number of reverse gears from two to four. This now included two rapid reverse gears which have proved themselves very helpful when negotiating longer stretches crabwise (motorway construction sites or in tunnels, for example).
A comprehensive range of extras
Compared with the Telligent automatic gearshift system, the range of additional functions has also been greatly extended. Eco-roll, power mode, cruise control with flexible hysteresis, plus rock free mode and manoeuvring mode are the extras with which PowerShift pampers drivers. Manoeuvring mode, intended for tight parking close to loading ramps or similar displays of slow motion skill, works like this: with the engine speed restricted to around 100 rpm at most, the clutch can be delicately let out and in along the whole length of the accelerator pedal travel. This allows amazingly sensitive manoeuvring with minimal clutch wear.
If you are stuck in snow or other soft ground, you just need to activate rock free mode. Once this switch is pressed, the clutch disengages as soon as you take your foot off the accelerator; and re-engages just as quickly at the next touch on the accelerator. If this fails to solve the problem, reverse gear will hurry to your aid. With PowerShift, reverse can be selected directly from first gear without having to go via neutral as before.
PowerShift provides greater freedom than ever before when making fine adjustments to cruise control and automatic downhill brake control. The adjustment range within which peak engine revs can be freely specified is between 2 to 15 km/h above the set cruise speed, with just one restriction: beyond 94 km/h, the system will no longer cooperate. The same window exactly also applies to the new freewheel Eco-roll which Mercedes-Benz believes will deliver additional fuel savings, depending on the terrain.
Freewheeling reduced fuel consumption
Eco-roll is activated every time the vehicle is started up and, in certain operating modes, exploits the kinetic energy of the freewheeling truck/trailer combination as far as possible by activating the transmission’s neutral position when freewheeling down a gentle slope. Braking commands, whether initiated by the driver or automatically issued by the cruise and downhill brake control, will cancel the freewheeling function; the same applies if a specified maximum speed is exceeded.
Power mode moves the gearshift points upwards so as to place the greatest possible engine power at the driver’s disposal. This increases the vehicle’s responsiveness and can be an advantage on tough mountain routes or when joining motorway traffic. However, so that the benefits of the PowerShift gearshift strategy, which is designed for economy and comfort, can be fully enjoyed, power mode switches itself off again automatically after 10 minutes if the driver forgets to do so.
The driver can activate the new PowerShift functions using the six dedicated rocker switches on a control panel, specially installed for this purpose, located to the right of the steering wheel within arm’s reach. The well-proven shift paddle on an extension arm below the armrest has been retained as the means for actually operating the functions.
Well-proven options are retained
PowerShift has also remained faithful to features which have proven their worth in the Telligent automatic gearshift system for years, features such as the emergency driving program (two forward gears, one reverse gear), the roll-start and tow-start options and the automatic engaging of neutral when the vehicle comes to a stop.
However, one new feature is that even if the driver has been using manual mode for the whole day, he will always be presented with the automatic mode the following morning. PowerShift also no longer includes the option of switching off the engine while still in gear. And the twelve-speed PowerShift variants, at least, are around 500 euros gross cheaper than the automated 16 gear transmissions with the Telligent automatic gearshift system.
However, even with PowerShift, 16 gears have not yet come to the end of their usefulness. For example, the G 241-16 transmission, designed for a maximum input torque of 2400 Nm, is available as a 16-speed direct-gear transmission. There is also the G 281-16 transmission, designed for a maximum input torque of 2800 Nm, which operates with double overdrive and, like the G 241-16 transmission, is mainly intended for special applications such as heavy-duty haulage tasks.
Automation widely available
Now, 20 years after the introduction of EPS, it is a long time since automated transmissions were only to be found in the flagship Actros truck. From 2002 onwards, the Telligent gearshift system has also been available in Atego trucks from 18 tonnes gross weight. And since 2003, an automated transmission on the lines of the Telligent automatic gearshift system has also been at work as an optional extra in the medium class Atego. It is based on the G 85-6 transmission, i.e. it has only 6 rather than 16 or 12 gears, and is a real blessing particularly for short-haul transport. However, it demands significantly more gearshifting work due to frequent stops and a high proportion of inner-city journeys.
The Axor model series also came not only to enjoy the benefits of the Telligent gearshift system (from 2004) but, in 2007, it also caught up with the option of having the fully-automated G 211-12 PowerShift transmission which weighed 50 kilograms less than the 16-speed Telligent transmission and 40 kilograms less than the 9-speed manual transmission. However, the PowerShift variant installed in the Axor was not as lavishly provided with additional functions as the version in the Actros flagship. The manoeuvring mode was the only extra which PowerShift could provide in the Axor.
And for many years now, the Stuttgart plant has even been equipping the Unimog as standard with the semi-automated Telligent gearshift system. For a modest additional cost, a fully-automated version, called AutomaticShift, can also be supplied for the Unimog. In conjunction with the standard Electronic Quick Reverse (EQR) shuttle gear, this even allows rapid switching between forwards and reverse without having to stop.
PowerShift Mark 2
Just one year later, in March 2008, Mercedes-Benz introduced the second generation of the PowerShift transmission into the third generation of the Actros and, as a further novelty, launched the truck with this technology fitted as standard. The new PowerShift 2 features were integral gradient sensors and a more refined gearshift strategy. The sensors continuously measured the gradient of the road, constantly incorporating, more accurately than before, both the vehicle speed and the accelerator pedal position into their calculations so as to determine the optimum gearshift strategy. Great attention was also paid to ease of shifting. The more sophisticated sensors on the transmission’s countershaft and main shaft made it possible for the electronics to change gears more smoothly but still more quickly. The control software was optimised so that it was able, to a certain extent, to anticipate the initial vibrations in the drive train when moving off and to offset them by controlling torque build-up. This feature, which considerably reduces the pitching and rolling motion experienced in the cab, is known as ‘anti-jerk control’. Also, the manoeuvring mode functions with even greater delicacy.
New off-road mode full of finesse
The off-road variant of PowerShift followed hard on the heels of the 2008 International Motor Show (IAA). The off-road mode is activated by engaging the inter-axle differential lock, very much as it was with off-road EPS. One of the particularly brilliant features of off-road PowerShift is known at the Stuttgart plant as ‘load-dependent engine speed increase’, a technique by which the clutch can be said to sound out the perfect engine speed for it to fully engage so that the whole load can be set in motion even against high ground resistance. Whereas the starting-off engine speed range for road vehicles only extends as far as 750 rpm, it has been extended to 1100 rpm in construction vehicles with off-road mode.
If there is insufficient tractive power for the intended manoeuvre, then the clutch will not engage. Before there is a chance of the clutch suffering thermal overload, the driver must use the crabwise driving technique. The manoeuvring mode has also been further improved for the off-road version of PowerShift. “The dump truck responds fantastically delicately to the controls in both forwards and reverse,” enthused ‘lastauto omnibus’ magazine after its first test drive.
Moreover, gear changes in off-road mode take place more quickly than in road vehicles with PowerShift transmission. The integral anti-jerk control not only makes for generally more comfortable gearshifting, it also actually speeds up the task by precisely determining the exact moment for shifting gears. And, in off-road mode, PowerShift changes gear a little more vigorously, as is appropriate for off-road work; it changes down earlier and changes up later than usual.
A simple overdrive differentiates the G 330-12 transmission installed in construction vehicles from the large-volume transmissions installed in road vehicles. As regards the hardware, the main difference from all other PowerShift
transmissions is that the mounts from which they are suspended are made of steel rather than aluminium.
As a consequence, the G 330-12, with its maximum possible input torque of 3300 Nm, is not only able to cope with the toughest construction site conditions but can also work well in the heavy load range. The bar is set high; the G 330-12 must be able handle a total combined road-train weight of up to 250 tonnes.
This most powerful of the PowerShift transmission variants is available for all Mercedes-Benz 6x4 and 8x4 construction vehicle variants. The main reason why the all-wheel version is still excluded is that more time is needed to design a satisfactory method of integrating the electronics into the large range of differential locks which are a feature of an all-wheel drive vehicle.
The equipping rate is fine
Unlike road vehicles, PowerShift is not fitted as standard to construction vehicles. Generally speaking, new features take considerably longer to penetrate the construction sector than is the case with the road-haulage sector where owners retain their vehicles for much shorter periods. However, product manager Vincent Angellier is confident that the equipping rate of less than 10 percent when PowerShift was introduced at the beginning of 2009 will soon change: “I’m expecting this to rise to between 40 and 50 percent over the next few years,” he is quoted as saying.
In any case, just one IAA later, PowerShift had already taken the Axor series road vehicles as well by storm. At the 2010 IAA in Hannover, Mercedes-Benz announced that PowerShift transmissions would no longer be available as standard in the Actros flagship alone but would in future also be fitted as standard in Axos series road vehicles.