Every automotive innovation has had to win over the cynics, and very often it seems that enthusiastic drivers - who you might think would be eager early adopters of new technology - are actually the last to accept any sort of radical change. Anti-lock braking, fuel injection and even automatic control of ignition timing were all decried by enthusiasts, who clung to wire wheels and carburettors long after the average consumer had accepted innovative alternatives. It is those same enthusiasts who seem likely to be the last to let the manual gearbox fade into obsolescence, but that is what they will soon be facing. Despite the challenges that have been faced by transmission engineers over the years - and the numerous occasions when promising innovations have turned out to be conceptual dead-ends - automatic transmissions are finally reaching the level of development where they offer important advantages over manual transmissions without any significant drawbacks. Now we can seriously begin to consider how long manual gearboxes have might have left.
A conventional transmission - consisting of a coupling, a multi-ratio gearbox and a final-drive unit - is a vital partner for an internal combustion (IC) engine in a road vehicle, because IC engines have limited operating speed ranges. Unlike a steam engine or an electric traction motor, an IC engine cannot develop useful torque at a standstill, so a coupling has to be incorporated into the driveline to allow the engine to run while the vehicle is stationary. That coupling - a friction clutch or hydraulic torque convertor - must be capable of progressive operation so the vehicle can be accelerated smoothly from rest. Low gears are necessary for good acceleration, high gears for speed and economy, so a ratio-changing gearbox is a must. The final drive controls the overall gear ratio to give the vehicle gearing which is appropriate in all eventualities.
Multi-ratio gearboxes with manual engagement were standard by the early part of the 20th century, but the combination of a tricky ‘crash’ gearbox and rudimentary clutch made driving a task requiring considerable skill. Development of clutch operation and friction materials, and the arrival of synchromesh, made life easier for the driver, but still there were concerted efforts to automate the whole process.
Pre-selector gearboxes like the Wilson of the 1920s were a stepping stone to truly automated transmissions, popularizing the use of epicyclic geartrains in automotive gearboxes. GM then set the pattern for successful automatic transmission with the Hydra-Matic, a fully-automatic epicyclic, as early as 1939. Most of today's automatics follow the same pattern, with added layers of electronic control and a three-element hydraulic torque convertor coupling in place of the Hydra-Matic's simple 'fluid flywheel'.
Hydra-Matic and the epicyclic transmissions that would follow it provided the first workable solution to the problem of automatic ratio changing. Only much later, in the crisis-hit 1970s, would the disadvantages come into sharp focus - and then it was not the gearbox that was the issue, but the convertor. Fluid flywheels and torque convertors never quite ‘lock’ completely, allowing a small amount of slip even in steady-state highway motoring which translated into poor fuel consumption figures. The addition of a friction clutch which locked up when the transmission had shifted into top gear and the speed differential across the two halves of the convertor had dropped to the minimum made a big improvement. But even then, losses in the convertor at lower speeds still gave automatic transmissions a bad name for heavy fuel consumption, and performance was poor with small engines.
Van Doorne’s belt-drive Variomatic transmission side-stepped the convertor problem by adopting a centrifugal clutch, making it more suitable for low-powered cars. A heavily revised version of the same concept, using a thrust belt with V-shaped steel links in place of the original belt, was adopted by Fiat and Ford in the 1990s and has been further developed since. In theory, the continuously variable gearing it provides can select perfect gearing for any vehicle operating point, rather than the best compromise from a set of discrete ratios, resulting in improved performance and economy.
But the advantages of continuously variable transmissions (CVTs) are balanced by notable disadvantages. Reliability has been one concern, with some designs proving troublesome in the long term. Another issue, and one which probably puts off more customers, is the alien nature of the driving experience. When the driver demands hard acceleration a CVT will drop to a lower gear ratio to allow the engine to run at or near peak power, maximising the excess tractive effort available – in much the same way that a stepped-ratio automatic will ‘kick down’ to an intermediate gear. The difference is that with a CVT the engine continues to run at its peak power speed until the driver chooses to relax pressure on the accelerator pedal. Instead of a reassuring rise in engine speed as the vehicle speed increases, the engine of a CVT-equipped car thrashes away at high speed while the vehicle acclerates to ‘catch up’. It’s a strange sensation, and one which many drivers find unpleasant. But CVTs are being more widely used - with applications now ranging from twist-and-go scooters to Audi’s Multitronic transmission in the A4, A6 and A8, capable of handling up to 330Nm - so those perceptions may change over time.
Other approaches to automated gear selection stick with the principle of multiple, fixed gear ratios. Automated operation of a manual gearbox - a so-called ‘assisted manual transmission’ or AMT - retains the high efficiency and low cost of a conventional transmission but adds foolproof automatic or semi-automatic control. Jerky operation has often been a drawback in the past, but AMTs have been acceptable for low-cost applications in small, low-cost vehicles. Ricardo has recently revealed work on its electromagnetic linear actuation technology which promises further cost reductions. Demonstrated on a 1.2-litre Open Corsa with Easytronic AMT, the Ricardo electromagnetic actuator is capable of delivering 800N peak, 350N continuous force and has proved to be eight times faster than an equivalent hydraulic valve actuated system. Gearchanging is rapid, with a torque interruption during ratio changes of as little as 0.35 seconds, and the actuator system is simpler and potentially cheaper - an attractive option for A- and B-class vehicles and price sensitive markets.
Ricardo’s actuator technology is now being applied to twin-clutch transmissions, which are making serious inroads into the market. They were introduced to production cars by VAG in the 2003 Volkswagen Golf R32 and have since spread to other brands within the VAG group and to a growing number of other OEMs. The significance of the twin-clutch designs for the future of automatic gearboxes is two-fold: from the end user’s point of view they offer all the advantages of conventional automatics without the disadvantages, and from the OEMs point of view their manufacture is relatively simple and cost-effective because much of the componentry can be machined using existing plant designed for production of manual gearboxes.
VW's original twin-clutch transmission, dubbed DSG for ‘direct shift gearbox’, was a six-speed unit with twin wet clutches, capable of handling up to 350Nm. Recently that has been joined by a smaller, lighter and more efficient seven-speed unit with dry clutches, designed for use with lower torque outputs (up to 250Nm) in vehicles ranging from the supermini Polo to the D-segment Passat. Both transmissions are based on the same principles.
DSG has two input shafts, one carrying the odd-numbered gears (and reverse) and the other providing the even-numbered gears. The key innovation which makes the transmission possible is that the input shafts of the two geartrains are concentric, with the ‘odds’ shaft running inside the hollow ‘evens’ shaft (which, as an aside, is similar to the idea which made VAG brand Audi's quattro four-wheel drive system possible). The two clutches operate under the control of an ECU to determine which shaft is in use at any moment, and the ‘idling’ shaft can independently pre-select a gear ready for the next gearchange. Switching from one gear to another is simply a matter of engaging one clutch and opening the other to switch shafts. Upshifts are split-second fast and all but imperceptible. DSG’s default ‘D’ mode behaves just like a conventional automatic, while the ‘S’ mode delays gearchanges until the optimum engine speed for maximimum performance. In addition the driver can move the shift lever left to select a Tiptronic sequential-shift mode, or gearchanges can be activated using paddles behind the steering wheel.
Audi has also adopted the six-speed DSG for use in its transverse-engined vehicles - though the premium brand calls its version the 'S-tronic' transmission - and has its own seven-speed twin-clutch gearbox specifically designed for in-line applications.
Other early adopters of this technology include Porsche, which first used twin-clutch gearboxes in competition in the 1980s. The PDK (for Porsche-Doppelkupplungsgetriebe, Porsche Dual Clutch Gearbox) is now replacing the Tiptronic S transmission, a conventional epicyclic automatic, and is said to be able to deal with up to 700Nm from the company's most powerful turbocharged engines. Meanwhile BMW is introducing a twin-clutch gearbox, labelled M DCT, to replace its SMG assisted manual transmission. Mitsubishi's twin-clutch gearbox, the six-speed Sports Shift Transmission (SST), was unveiled in the new Evo X performance saloon in 2007. SST has three drive modes, Normal, Sport and S-Sport, which offer progressively faster gearchanges and higher engine speed shift points. Other OEMs, including Volvo and Chrysler, are working on transmissions of this type.
Because they are based, essentially, on manual transmission designs and use friction clutches these twin-clutch systems all have excellent mechanical efficiency. The result is the convenience of automatic operation without compromise in driving dynamics or fuel consumption. Porsche's PDK, for instance, is said to change gear up to 60% quicker than the outgoing Tiptronic transmission, while delivering significant improvements in fuel consumption and CO2 emissions. VAG claims its DSG transmission can cut emissions by 20 per cent compared to a conventional automatic, and by 12 per cent compared to a manual gearbox. Performance and fuel consumption figures for DSG-equipped vehicles better those of manual gearbox equivalents, and there is little doubt that in real-world road conditions a DSG-equipped VW is easier to use and more readily responsive. Only on a first-cost basis would a manual gearbox prove superior.
As a result, VAG's DSG transmissions are already finding favour with large numbers of customers. At the end of 2007, even before the introduction of the seven-speed unit, DSG claimed between nine and 12 per cent of sales of Golf-family vehicles, 13 per cent of Jetta sales and 14 per cent of Eos sales. Take-up of DSG in the Touran and Passat was even stronger, with 28 per cent of Passat Variant (estate) buyers specifying the twin-clutch gearbox. VAG claims that 400,000 DSG-equipped vehicles were sold in 2007. VAG's transmission plant at Kassel is now producing 2250 DSGs each day, with the capacity to increase daily production to 3000 units should there be sufficient demand. The plant built its millionth DSG in 2007.
Even if twin-clutch gearboxes take over as the most common form of transmission, manual gearboxes might still have a place in particularly cost-conscious applications and in developing markets (though even the Tata Nano will acquire a CVT in due course). In Europe and North America it will probably be enthusiasts who will be the last to give up on the manual gearbox, however convincing the advantages of new-age automatics might seem. But it might not be too long before these same demanding drivers have to face a more fundamental challenge to their equilibrium. If the car of tomorrow swaps the internal combustion engine for a hydrogen fuel cell and drives its wheels with an electric traction motor - and it might well do - then the multi-ratio gearbox as we know it will probably be redundant. So while the heyday of the automatic transmission might be just around the corner, spearheaded by the rapid adoption of twin-clutch units, that heyday may not last very long.
Published in European Automotive Design 2008