Dual Clutch

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Abstract:-Adual-clutch transmission, (DCT) (sometimes referred to as atwin-clutch transmissionordouble-clutch transmission), is a type ofautomatic transmissionorautomatedautomotivetransmission. It uses two separateclutches[1]for odd and evengear sets. It can fundamentally be described as two separatemanual transmissions(with their respective clutches) contained within one housing, and working as one unit.[2][3]They are usually operated in a fully automatic mode, and many also have the ability to allow the driver tomanually shift gears in semi-automatic mode,[1]albeit still carried out by the transmission's electro-hydraulics.

History[edit]This type of transmission was invented by FrenchmanAdolphe Kgressejust beforeWorld War IIbut he never developed a working model. The first development of the Twin Clutch or Dual-clutch transmission started in the early part of 1980 under the guidance ofHarry WebsteratAutomotive Products(AP),Leamington Spawith prototypes built into theFord Fiesta Mk1,Ford Ranger&Peugeot 205. Initially, the control systems were based on purely analogue/discrete digital circuitry with patents filed in July 1981.[4]All of these early AP Twin Clutch installations featured a single dry clutch and multi-plate wet clutch. Following discussions with VW/Porsche, DCT work continued from Porsche in-house development, for Audi and Porsche racing cars later in the 1980s,[2]when computers to control the transmission became compact enough: the Porsche Doppelkupplungsgetriebe (English: dual-clutch gearbox) (PDK)[2]used in thePorsche 956[2]and962[2]Le Mansrace cars from 1983,[2]and theAudi Sport Quattro S1rallycar.[5][6]A dual-clutch transmission eliminates thetorque converteras used in conventional epicyclic-gearedautomatic transmissions.[1]Instead, dual-clutch transmissions that are currently on the market primarily use twooil-bathed wet multi-plate clutches, similar to the clutches used in most motorcycles, though dry clutch versions are also available.[7]The first series productionroad carto be fitted with a DCT was the 2003Volkswagen Golf Mk4 R32.[2][8][9]As of 2009, the largest number of sales of DCTs in Western Europe are by various marques of the GermanVolkswagen Group,[10]though this is anticipated to lessen as other transmission makers and vehicle manufacturers make DCTs available in series production automobiles.[2][11]In 2010, on BMW Canada's website for the 3 Series Coupe, it is described both as a 7-speed double-clutch transmission and as a 7-speed automatic transmission. It is actually a dual-clutch semi-automatic.[12][13]Overview[edit]In DCTs where the two clutches are arrangedconcentrically, the larger outer clutch drives the odd numbered gears,[1][2][8]while the smaller inner clutch drives the even numbered gears.[1][2][8]Shifts can be accomplished without interruptingtorquedistribution to the driven roadwheels,[1][2][3][5][8]by applying the engine's torque to one clutch at the same time as it is being disconnected from the other clutch.[2][5]Since alternate gear ratios can pre-select[1][2][3][5]an odd gear on one gear shaft while the vehicle is being driven in an even gear,[3](and vice versa), DCTs are able to shift more quickly than cars equipped with single-clutch automated-manual transmissions (AMTs), a.k.a. single-clutch semi-automatics. Also, with a DCT, shifts can be made more smoothly than with a single-clutch AMT, making a DCT more suitable for conventional road cars.[14][not in citation given]Clutch types[edit]There are two fundamental types of clutches used in dual-clutch transmissions: either two wet multi-plate clutches which are bathed in oil (for cooling), or two dry single-plate clutches.[9]The wet clutch design is generally used for higher torque engines which can generate 350newton metres(258lbfft) and more (the wet multi-plate clutch DCT in theBugatti Veyronis designed to cope with 1,250Nm (922lbfft)[2]), whereas the dry clutch design is generally suitable for smaller vehicles with lower torque outputs up to 250Nm (184lbfft).[2][7]However, while the dry clutch variants may be limited in torque compared to their wet clutch counterparts, the dry clutch variants offer an increase in fuel efficiency,[2]due to the lack of pumping losses of the transmission fluid in the clutch housing.[2]Clutch installation[edit]There are now three variations of clutch installation. The original design used a concentric arrangement, where both clutches shared the same plane when viewed perpendicularly from the transmission input shaft, along the same centre line as the enginecrankshaft; when viewed head-on along the length of the input shaft, this makes one clutch noticeably larger than the other.The second implementation utilised two single-plate dry clutches which are side-by-side from the perpendicular view, but again sharing the centre line of the crankshaft.A latest variation uses two separate but identical sized clutches; these are arranged side-by-side when viewed head-on (along the length of the input shaft and crankshaft centre line), and also share the same plane when viewed perpendicularly. This latter clutch arrangement (unlike the other two variations) is driven via a gear from the engine crankshaft

Most people know that cars come with two basic transmission types:manuals, which require that the driver change gears by depressing a clutch pedal and using a stick shift, andautomatics, which do all of the shifting work for drivers usingclutches, atorque converterand sets of planetary gears. But there's also something in between that offers the best of both worlds -- thedual-clutch transmission, also called the semi-automatic transmission, the "clutchless" manual transmission and the automated manual transmission.In the world of racecars, semi-automatic transmissions, such as thesequential manual gearbox(or SMG), have been a staple for years. But in the world of production vehicles, it's a relatively new technology -- one that is being defined by a very specific design known as the dual-clutch, or direct-shift, gearbox.This article will explore how a dual-clutch transmission works, how it compares to other types of transmissions and why some predict that it is the transmission of the future.Hands-On or Hands-OffA dual-clutch transmission offers the function of two manual gearboxes in one. To understand what this means, it's helpful to review how a conventional manual gearbox works. When a driver wants to change from one gear to another in a standard stick-shift car, he first presses down the clutch pedal. This operates a single clutch, which disconnects the engine from the gearbox and interrupts power flow to the transmission. Then the driver uses the stick shift to select a new gear, a process that involves moving a toothed collar from one gear wheel to another gear wheel of a different size. Devices calledsynchronizersmatch the gears before they are engaged to prevent grinding. Once the new gear is engaged, the driver releases the clutch pedal, which re-connects the engine to the gearbox and transmits power to the wheels.So, in a conventional manual transmission, there is not a continuous flow of power from the engine to the wheels. Instead, power delivery changes fromontoofftoonduring gearshift, causing a phenomenon known as "shift shock" or "torque interrupt." For an unskilled driver, this can result in passengers being thrown forward and back again as gears are changed.A dual-clutch gearbox, by contrast, uses two clutches, but has no clutch pedal. Sophisticated electronics and hydraulics control the clutches, just as they do in a standard automatic transmission. In a DCT, however, the clutches operate independently. One clutch controls the odd gears (first, third, fifth and reverse), while the other controls the even gears (second, fourth and sixth). Using this arrangement, gears can be changed without interrupting the power flow from the engine to the transmission.

Dual-clutch Transmission ShaftsA two-part transmission shaft is at the heart of a DCT. Unlike a conventional manual gearbox, which houses all of its gears on a single input shaft, the DCT splits up odd and even gears on two input shafts. How is this possible? The outer shaft is hollowed out, making room for an inner shaft, which is nested inside. The outer hollow shaft feeds second and fourth gears, while the inner shaft feeds first, third and fifth.The diagram below shows this arrangement for a typical five-speed DCT. Notice that one clutch controls second and fourth gears, while another, independent clutch controls first, third and fifth gears. That's the trick that allows lightning-fast gear changes and keeps power delivery constant. A standard manual transmission can't do this because it must use one clutch for all odd and even gears.

Multi-plate ClutchesBecause a dual-clutch transmission is similar to an automatic, you might think that it requires atorque converter, which is how an automatic transfers engine torque from the engine to the transmission. DCTs, however, don't require torque converters. Instead, DCTs currently on the market use wet multi-plate clutches. A "wet" clutch is one that bathes the clutch components in lubricating fluid to reduce friction and limit the production of heat. Several manufacturers are developing DCTs that use dry clutches, like those usually associated with manual transmissions, but all production vehicles equipped with DCTs today use the wet version. Many motorcycles have single multi-plate clutches.

Like torque converters, wet multi-plate clutches use hydraulic pressure to drive the gears. The fluid does its work inside the clutch piston, seen in the diagram above. When the clutch is engaged, hydraulic pressure inside the piston forces a set of coil springs part, which pushes a series of stacked clutch plates and friction discs against a fixed pressure plate. The friction discs have internal teeth that are sized and shaped to mesh withsplineson the clutch drum. In turn, the drum is connected to the gearset that will receive the transfer force. Audi's dual-clutch transmission has both a small coil spring and a large diaphragm spring in its wet multi-plate clutches.To disengage the clutch, fluid pressure inside the piston is reduced. This allows the piston springs to relax, which eases pressure on the clutch pack and pressure plate.

Pros and Cons of Dual-clutch TransmissionsHopefully it's becoming clear why the DCT is classified as an automated manual transmission. In principle, the DCT behaves just like a standard manual transmission: It's got input and auxiliary shafts to house gears, synchronizers and a clutch. What it doesn't have is a clutch pedal, because computers, solenoids and hydraulics do the actual shifting. Even without a clutch pedal, the driver can still "tell" the computer when to take action through paddles, buttons or a gearshift.Driver experience, then, is just one of the many advantages of a DCT. With upshifts taking a mere 8 milliseconds, many feel that the DCT offers the most dynamic acceleration of any vehicle on the market. It certainly offers smooth acceleration by eliminating the shift shock that accompanies gearshifts in manual transmissions and even some automatics. Best of all, it affords drivers the luxury of choosing whether they prefer to control the shifting or let the computer do all of the work.Perhaps the most compelling advantage of a DCT is improved fuel economy. Because power flow from the engine to the transmission is not interrupted, fuel efficiency increases dramatically. Some experts say that a six-speed DCT can deliver up to a 10 percent increase in relative fuel efficiency when compared to a conventional five-speed automatic.Many car manufacturers are interested in DCT technology. However, some automakers are wary of the additional costs associated with modifying production lines to accommodate a new type of transmission. This could initially drive up the costs of cars outfitted with DCTs, which might discourage cost-conscious consumers.In addition, manufacturers are already investing heavily in alternate transmission technologies. One of the most notable is thecontinuously variable transmission, or CVT. A CVT is a type of automatic transmission that uses a moving pulley system and a belt or chain to infinitely adjust the gear ratio across a wide range. CVTs also reduce shift shock and increase fuel efficiency significantly. But CVTs can't handle the high torque demands of performance cars.DCTs don't have such issues and are ideal for high-performance vehicles. In Europe, where manual transmissions are preferred because of their performance and fuel efficiency, some predict that DCTs will capture 25 percent of the market. Just one percent of cars produced in Western Europe will be fitted with a CVT by 2012.

Dual-clutch Transmissions: Past, Present and FutureThe man who invented the dual-clutch gearbox was a pioneer in automotive engineering. Adolphe Kgresse is best known for developing the half-track, a type of vehicle equipped with endless rubber treads allowing it to drive off-road over various forms of terrain. In 1939, Kgresse conceived the idea for a dual-clutch gearbox, which he hoped to use on the legendary Citron "Traction" vehicle. Unfortunately, adverse business circumstances prevented further development. Both Audi and Porsche picked up on the dual-clutch concept, although its use was limited at first to racecars. The 956 and 962C racecars included thePorsche Dual Klutch, or PDK. In 1986, a Porsche 962 won the Monza 1000 Kilometer World Sports Prototype Championship race -- the first win for a car equipped with the PDK semi-automatic paddle-shifted transmission. Audi also made history in 1985 when a Sport quattro S1 rally car equipped with dual-clutch transmission won the Pikes Peak hill climb, a race up the 4,300-meter-high mountain.Commercialization of the dual-clutch transmission, however, has not been feasible until recently. Volkswagen has been a pioneer in dual-clutch transmissions, licensing BorgWarner's DualTronic technology. European automobiles equipped with DCTs include the Volkswagen Beetle, Golf, Touran, and Jetta as well as the Audi TT and A3; the Skoda Octavia; and the Seat Altea, Toledo and Leon.

Ford is the second major manufacturer to commit to dual-clutch transmissions, made by Ford of Europe and its 50/50 joint venture transmission manufacturer, GETRAG-Ford. It demonstrated the Powershift System, a six-speed dual-clutch transmission, at the 2005 Frankfurt International Motor Show. However, production vehicles using a first generation Powershift are approximately two years away.To help it get from A to B a cars engine sucks in air and some fuel, combusting the two to generate rotating motion, which is then transferred to the wheels via a transmission.Not that long ago choosing between transmissions was easy: manual or automatic. Those who prioritised value, fuel economy, performance or driving pleasure went for manuals, while everyone else chose automatics. Simple.Now, though, there are Lineartronics, DSGs, X-Tronics and tonnes of other marketing names to choose between. What are they? Are they any different to the automatics and manuals of yore? And what are their benefits?

BackgroundBefore all that, though, why do we need transmissions? After all, the electricNissan LeafandTeslaModel S seem to do fine without.Internal combustion engines cant be directly connected to a cars wheels as engines typically rotate a lot faster. For instance, petrol motors generally operate between 600 and 6,500rpm, while a cars wheels usually rotate between zero and 1,500 times per minute.By introducing a gear ratio, the engines output speed can be reduced to match that of the wheels. Mind you, having just one gear ratio is rather limiting. Imagine a single gear petrol car thats been engineered for a top speed of 120km/h. In effect its solitary gear ratio would be the equivalent of third gear in most of todays cars.If youve ever tried accelerating from standstill in third gear youll know what a torturously slow and painful exercise it is. Not only that, but when the car finally hits 110km/h, the engine will be screaming at around 5,500rpm. Neither is great on ones patience, ear drums nor the longevity of the car.Multiple gear ratio transmissions, then, allow cars to strike an acceptable balance between acceleration, top speed and fuel economy.

ManualThis is the simplest and lightest type of transmission available, as gear selection and gear changing is solely the domain of the driver.The gears contained inside the transmissions metal housing typically five or six forward gears, although thePorsche 911andChevrolet Corvetteboth sport seven are accessed via a shifter in the cabin.To change gears, the driver depresses the clutch pedal, which detaches the clutch from the engine. This allows the engine to spin at one speed, and the transmission and wheels at another. After selecting a new gear ratio via the stick shift, the driver connects the engine, transmission and wheels together again by releasing the clutch pedal.Two pieces of tech have made it easier to shift gears over the years: synchromeshed gears and hill start assistance.Synchromesh gears have not only removed much of the gear grinding that used to plague manuals, but has also eliminated the need to double declutch, where drivers had to declutch once to enter neutral and declutch again to select the next gear. Hill start assist is a more recent development that tries to prevent manual cars from rolling backwards downhill during standing starts. It does this by keeping the brakes active for a few seconds after the brake pedal is released, giving the driver a bit of grace to operate the clutch and accelerator.For the greater part of automotive history, manual transmissions have rewarded drivers efforts with more control, more gears, faster acceleration, higher top speeds, lower fuel consumption and, if driven well, smoother gear shifts. But with the computer-led evolution of automotive transmissions, some cars are reporting better performance and fuel economy figures with automated transmissions.This is good news, especially in Australia and the USA, where the vast majority of cars are sold without a clutch pedal.

Automated manualWhile enthusiasts revel in mastering the art of perfectly executed shifts and heel-toe manoeuvres, for many the act of declutching, selecting a gear and clutching is a burdensome chore. So, why not stick some hydraulics or servos onto a manual gearbox to operate the clutch for us?Numerous car makers have dabbled with clutch-pedal-free manual transmissions over the years but, despite the simplicity of the concept, its an idea has still yet to be executed well in road-going cars.Automated manuals go back as far as the iconic Citroen DS, and more recent attempts have come fromAlfa Romeo(Selespeed),Audi(R-Tronic),BMW(Sequential Manual Gearbox),Citroen(Servotronic),Ferrari,Lexus,Maserati,PeugeotandSmart(pictured above).Jerky gear changes are the main problem with automated manual transmissions. In semi-automatic mode, where the driver changes gears via a +/- gate or paddles behind the steering wheel, this can be partially alleviated by lifting off the throttle when changing gears. As the driver is unable to finesse the clutch, it usually only serves to minimise the amount of lurching.Many systems also offer a fully automatic mode, but these are plagued with either brutally aggressive or overly slurred gear shifts. Again lifting off the throttle during gear changes helps, but its often less taxing to shift gears yourself rather than playing clairvoyant to the transmissions electronic brain.image: http://2-ps.googleusercontent.com/hk/QfFbG2TN5yVZ-d0l4wVSEEmYC_/www.caradvice.com.au/thumb/770/382/wp-content/uploads/2014/03/xjaguar-xk-auto.jpg.pagespeed.ic.iF6AgYkT0nHhQIxbT4IX.jpg

AutomaticGiven the failings weve noted above with automated manual transmissions, its easy to see why traditional automatics differ quite markedly from their manual brethren.By having a torque converter, which permanently connects the engine and the transmission, automatics avoid having to disengage and re-engage a clutch whenever gears are changed or the car comes to a stop.Whereas a clutch physically connects engine and transmission, a torque converter makes an indirect link through a bath of transmission fluid. Whenever the engines running it spins this oil-like hydraulic liquid, and if the car isnt stopped with the brakes engaged, the fluid then spins the transmissions impeller that in time turns the wheels.Because the engine spins the driveshaft indirectly, a certain amount of power (generally less than 10 percent) is lost. Modern autos compensate for this with a lock-up clutch that engages at cruising speeds and effectively transforms the entire torque converter assembly into one giant clutch.While a manual gearbox has a physical gear for each of its gear ratios, automatics have a much more complex series of interconnected planetary gearsets. Each gearset is comprised sun, planet and ring gears, and any of these smaller gears can be held still, spun by the engine or left to rotate freely. The transmissions gear ratios are produced by altering the parts in each gearset that are fixed, powered or free to spin.The elements of each planetary gearset are manipulated via a intricate chain of hydraulics and small clutches. In the past the logic behind when to change and hold gears was controlled by another hydraulic system. Improvements in computing technology have allowed modern autos to move to electronic control, allowing for smoother and more logical gear shifts.Although automatics are still heavier and more expensive to purchase or repair, nowadays they give up little ground to manual transmissions in terms of performance, efficiency and gear count five- and six-speed transmissions are the norm, and seven-, eight- and nine-speed models are becoming increasingly common.image: http://1-ps.googleusercontent.com/hk/QfFbG2TN5yVZ-d0l4wVSEEmYC_/www.caradvice.com.au/thumb/770/382/wp-content/uploads/2014/03/xporsche-911-tiptronic.jpg.pagespeed.ic.PnQXQWyFm-qMTKtU4GyQ.jpg

TiptronicIn the late 1990s Tiptronic transmissions became all the rage. They supposedly combined the best of both automatic (look ma, no clutch pedal!) and manual transmissions (driver selectable gears). In reality, Tiptronics werent really a different type of transmission, but rather an automatic gearbox that allowed for easy gear selection via a +/- gate and, sometimes, paddles or buttons around the steering wheel.Once relatively rare, Tiptronic features are now available in most automatic vehicles.image: http://2-ps.googleusercontent.com/hk/QfFbG2TN5yVZ-d0l4wVSEEmYC_/www.caradvice.com.au/thumb/770/382/wp-content/uploads/2014/03/xnissan-gt-r-dct.jpg.pagespeed.ic.RubHFlNahwI2zdqOYbFv.jpg

Dual clutchAlthough dual clutch transmissions (DCTs) are generally employed as replacements for traditional automatic gearboxes, their mechanical workings actually bear more in common with the humble manual transmission. In fact its probably easiest to think of a DCT as housing two manual transmissions one for even numbered gears and the other for odd numbered ones each with their own clutch (hence the name).Say, for example, youre accelerating along in third gear. In this situation the even numbered gearbox will have fourth gear pre-selected and primed to go. When the transmissions computer or the driver thinks the time is right to change up, the clutch for the odd numbered gears is disengaged and the even clutch engaged.Changing from one clutch to another takes anywhere between eight and 200 milliseconds; thats considerably faster than the half second or more required by most manual drivers to change gears. By wasting less time between gears, DCTs are often able to outsprint their manual equivalents.Some transmissions, particularly those from the Volkswagen Group (VW, Audi, Skoda, et al), have lightning quick gear changes, which are a delight to see in action. The downside with fast-shifting DCTs is that at lower speeds they tend to jerk and lurch around. In tight parking spaces, it can be a little frightening to suddenly lunge forward, even if its just a few centimetres, when youre feathering the throttle with the greatest of care.There are two types of dual-clutch transmission on the market: dry and wet clutch. Wet clutch models are so called because the clutch is bathed in a sea of oil and this type is often found in high-power cars. Dry clutch versions are more efficient, but restricted in the amount of power and torque they can handle.As with many new technologies, most car makers have decided to market DCTs under their own trademarked brand names: DSG (Volkswagen,Skoda, Seat), EcoShift (Hyundai), PDK (Porsche), PowerShift (Ford,Volvo), S-Tronic (Audi), SpeedShift (Mercedes-Benz), and TC-SST (Mitsubishi). For some companies, most notably Volkswagen and Ford, DCTs are available on mainstream vehicles. For others, like Nissan, BMW and Mitsubishi, usage is restricted to high-performance models.

Continuously variable transmissionInternal combustion engines deliver their maximum power and torque over a narrow rev range. For example, non-turbocharged petrol motors typically deliver peak power around 5500rpm and maximum torque at 4000rpm.With the transmission systems weve detailed above, the engine is often operating outside of its sweet spot for either power, torque or efficiency. A continuously variable transmission (CVT) seeks to overcome this by offering an infinite number of gear ratios between a transmissions upper and lower ratio limit.Most CVTs feature two pulleys connected via a V-shaped belt: one pulley is driven by the engine, and the other is connected to the wheels. To change gear ratios the transmission manipulates the ride height of the belt across the pulleys.Unlike cars with other transmissions, giving a CVT car a little bit more gas doesnt necessarily cause engine revs to rise unless youre really flooring it.For undemanding drivers CVTs are both elegantly simple and efficient as the engine is almost always operating at maximum efficiency. For drivers who harbour dreams of becoming a Senna-like deity, CVTs arent really much fun. Instead of the (hopefully) pleasant sound of an engine surging up and down through the rev range, CVTs holds the engine at high revs, commonly resulting in a dull drone when youre pressing on.All CVTs have a driver selectable low ratio or engine braking mode for steep hills, while some go a step further in offering a number of artificial gears or fixed gear ratios for drivers to cycle through.As with DCTs car makers are prone to use different marketing names for their CVT systems: Lineartronic (Subaru), Multitronic (Audi) and X-Tronic (Nissan).