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DSG, DCT & Co.: double clutchin’

The newest gearbox techniques

09/04/2013 05:30 |  Comments: 

Editor

As typical of a long-serving automotive journalist he worked at nearly all of the major printed automotive magazines in Hungary before ending up on the internet. More of the new cartester type, he’s also an automotive engineer by profession. Although he’s the editor in chief
of the mother magazine, totalcar.hu, he loves doubling as a photographer – we sometimes
think he made a mistake when choosing titles. Has a wife and a small daughter.

Beware auto gearbox, your days are numbered! No more oil swirling around, no more hesitant downshifts, no more excessive fuel consumption. Double clutch transmissions are delivering your coup de grâce - au revoir!

More and more brands will offer you double clutch transmissions instead of regular automatics. Best known of them all is DSG by Volkswagen, but BMW and Ferrari have also jumped the bandwagon. DSG stands for the German expression Doppelkupplungsgetriebe, but the system is better known internationally by the acronym of DCT (double clutch transmission). Their name is actually indifferent. The point is these gearboxes come with two clutches; they can function automatically but can also be shifted manually like a sequential gearbox. Except, they are faster.

Double clutch or not

Before we take a plunge into the ghastly pool of hollow shafts and wet clutches, let's set one thing straight. Double clutch and double-disk clutch are two completely different things. The latter are used in high power vehicles where they are necessary for transmitting the immense torque of the engine. Take the Lotus Omega for example. It had a three-litre engine with two turbos yielding 377 PS. Peak torque was 577 Nm which the standard Opel clutch just could not handle. Normally you'd just increase the diameter of the clutch but space was tight in this case. So engineers chose to use two friction disks instead, yet that is not a double clutch unit. Same difference with the Lamborghini Gallardo which sports a single unit of double disk dry clutch.

What is the point of a clutch anyway? If you can recall your driver ed lessons you'll remember that they are necessary for shifting gears: without a clutch you could not disengage and re-engage the coupling claws of the individual gears. Well, alright you can theoretically shift gears without declutching but you need to be pretty talented – and doing so would expose the gear wheels to forces so high they would crack in short order.

Alright then, so you do need a clutch, but why two? Here's a little secret: double clutch units don't just have two clutches: they are actually two self-contained transmissions in one. If you take a detailed look at a DCT unit, you'll realise that it is in fact two three-speed sets paired most cunningly – one serves the odd numbered gears (first, third, fifth; possibly seventh); the other the even numbered ones (second, fourth, sixth).

Originally the DCT system was conceived to make gearshifts faster. In a regular gearbox the flow of driving torque is interrupted while you are shifting gears which means the car starts to slow down for a few tenths of a second, being held up by drag and rolling resistance. This makes absolutely no difference in everyday traffic but it is of utmost importance on the race track, which is why the technology debuted in Group C Porsche cars. But the concept itself dates back all the way to the beginning of the interwar period.

Double clutch transmissions can in fact have two gears engaged at the same time (with of course at least one of the clutches disconnected , or else there'd be double trouble). What's the point, you may ask? Let's say you have just set off, rolling in first gear. The DCT unit will engage second gear and when it's time to actually shift gears all it has to do is disengage the odd clutch and engage the even one. This can happen completely simultaneously, with both clutches slipping. The entire process is completed in a few hundreds of a second without the car losing momentum. Downshifts follow the same pattern. DCT units shift gears as smoothly as automatic gearboxes, there is no loss of drive power as opposed to single clutch units. Instead of slowing down, the car may even accelerate while shifting gears. Even the fastest of single clutch automated gearboxes (such as the F1 gearbox used on the Ferrari F430) cannot shift gears faster than 0.10-0.15 seconds.

In real life these gearboxes serve a different purpose than on the race track. Gear shifting can be automated neatly; shifts are accomplished swiftly and smoothly: the gearbox does what the customer wants and that makes them happy. Customer satisfaction is exactly why single-clutch automated gearboxes (such as Alfa Selespeed, Toyota MMT, Opel Easytronic, BMW SMG an so forth) have not been met with overwhelming success: drivers were dissatisfied with many aspects of their operation, especially the slow shifting.

DCT on the other hand is an automated gearbox which is just as comfortable to use as an A/T but consumes 5-10% less fuel, in fact it competes with manual cars in that respect. Double clutch units are, on the other hand, no more affordable than traditional automated gearboxes where all you need to install are a couple of slave cylinders, transmitters and new control electronics. DSG systems in fact require the construction of a completely different structure, and they also require a different type of clutch (initially at least they did).

These days DCT units utilise two types of clutches. More affordable variants use traditional dry friction discs, but first generation models were equipped with multi-plate wet clutches, similar to those used on motorcycles. These have disks which virtually never wear out, since the oil bath both lubricates and cools them.

Times have changed though, and today there are several double clutch systems available with traditional dry-plate clutches. Volkswagen offers such a gearbox – a seven-speed unit – with its smaller (1.6 TDI, 1.4 TSI) engines.

DCT gearboxes also have a different internal structure, as they are practically two gearboxes combined into a single system. Each clutch connects to a dedicated shaft – one of them is hollow and houses the other, solid shaft.

The next video shows the Ford Powershift system, developed by Getrag for front-wheel-drive vehicles and formerly used in certain Volvo models (S40/V50). First, you can see the two input shafts, placed coaxially, the two other, connected shafts with the even and odd number gear sets, the enclosed differential gear, the reverse gear, and the shaft lock for park (P). Next up there are the coupling forks with the slave cylinders, the housing, then the control unit containing the position senders and the electromagnetic valves. After these are assembled there is the double clutch looking like a flying saucer. The gearbox shifts through all the gears, and before the end of the film you can take a peak inside the multi-plate clutch structures. Do watch the film.

Double clutch systems are more complex than traditional manual gearboxes; they also weight more: while the above mentioned Powershift, rated at 450 Nm, weighs 91 kilos, a similarly rated six-speed Volvo manual gearbox is only 54 kg. Volkswagen's seven-speed dry clutch DSG (250 Nm) is in between the two with a weight of 70 kg. As regards maintenance some manufacturers require no oil change (BMW), others specify an interval of 120 000 km, while Audi's new seven-speed, 550 Nm S-tronic has in fact two isolated oil circuits. One contains traditional ATF, working the slave cylinders of clutches and coupling forks. This oil has a change interval of 60K km, while the traditional transmission oil used within the gear housing is maintenance free.

Double clutch transmissions were first used by Volkswagen. Their system dubbed DSG debuted in 2003 (Golf R32; six forward speeds) and lately it was also used by Audi as S-Tronic. The technology is a joint development of VW and BorgWarner of America. This latter is now the number one OEM supplier on the market, manufacturing a complete, enclosed system (known as DualTronic) which contains the double clutch as well as the coupling forks, slave cylinders, position senders and control unit for the gears. BorgWarner even supplies competitors such as Getrag or Ricardo. The special gearbox unit of the Nissan GT-R is also equipped with BorgWarner internals. In the meanwhile though the German specialist ZF has also begun engineering clutch sets, while Valeo and LuK offer the cheaper dry clutch alternative to manufacturers, so BW no longer holds a monopoly in the marketplace.

But why on earth should they be supplying their own competition, you may ask yourself. Well, a control unit by itself won't drive a car, someone has to manufacture the actual gearbox to go with it, even if it's the clutch and the control which are the tricky parts. This setup now allows the installation of a double clutch gearbox on virtually any engine layout. There are DCT units installed longitudinally, transversally and even in the rear; DCT for front- and rear-engined vehicles, for Porsche, Ferrari or BMW.

Of course these are developments of utmost importance, meaning even OEM partners (i.e. Volkswagen) try to gain a market advantage and delay competitors. That is why the technology is only reaching Ford now, with other companies (SAIC of China, General Motors, as well as French manufacturers) also planning to release their version. Most modern supercars also get DCT gearboxes, from the Bugatti Veyron through Ferraris to the Mercedes SLS AMG. Both Porsche (911, ZF) and BMW (M3, BW+Getrag) offer DCT units, despite their previous claims that nothing beats their SMG /Tiptronic system.

With mass market producers entering the arena, DCT may indeed achieve a market penetration of 25% (i.e.one out of every four new cars sold) in Europe within five years. The cheaper dry clutch system is now used not only in the Alfa Romeo Mito but also in various other models, from the Kia Cee'd to the Renault Mégane. Honda in fact also uses DCT on motorcycles, such as the VFR1200 or the NC700.

There is certainly market demand for DCT here in Europe, given that CVT (continuously variable) transmissions are not well accepted here, and that single-clutch automated gearboxes only have a future with small cars, and even so for the sole reason of being overly affordable. Current focus on CO2 reduction also favours DCT as they can serve as a more affordable but fully functional alternative to traditional A/T systems.

Who makes what?

BMW
M3, M5, M6 – 7-speed, wet clutch – M DCT

Fiat group
Alfa Mito, Fiat 500L - TCT

Ford
Focus, S-Max – 6-speed, wet clutch – Powershift

Hyundai/Kia
Veloster, Cee'd – 6-speed, dry clutch - DCT

Mitsubishi
Lancer Evolution – 6-speed, wet clutch – Twin Clutch SST

Nissan
GT-R, 6-speed, wet clutch

Renault
Mégane – 6-speed dry clutch - EDC (Efficient Dual Clutch)

VW group
Seat, Skoda, Volkswagen – 6- and 7-speed, dry and wet clutch – DSG
Audi – 6- and 7-speed, dry and wet clutch – S-Tronic
Bugatti Veyron 7-speed, wet clutch – DSG

Porsche
911, Cayman, Boxster, Panamera – 7-speed, wet clutch – PDK

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