Double Clutch Transmission



Dual Clutch Transmission. With our automatic DCTs, one clutch is engaged with the current gear while sensors predict whether the next gear change will be an upshift or downshift. The second clutch prepares the next gear ratio for instant engagement with a negligible loss of torque. Our 8-Speed DCT includes a torque converter to smooth low-speed. History of the 7-speed M Double-Clutch Transmission (DCT) DCT comes from BMW’s Motorsports, where on the race track, every tenth of a second is critical for the win. Debuted first in the M3 Coupe, Sedan, and Convertible, the Performance Option 7-Speed Double-Clutch Transmission is currently priced at $2,900 and is available for the new M. Double-clutching (also called double de-clutching in countries outside of the United States) is a method of shifting gears used primarily for vehicles with an unsynchronized manual transmission, such as commercial trucks and specialty vehicles.

  1. Double Clutch Transmission Diagram
  2. Double Clutch Transmission In German
  3. Double Clutch Transmission Fluid

Rocketbook evernote. Despite what is undoubtably very clever and complex engineering, the Double (or Dual) Clutch Transmission (DCT), has a mixed reputation with drivers and has not achieved the level of ubiquity many thought it would.

Sitting in the middle ground between an automatic and a manual transmission, the simplest way of describing their operation, is that they’re an automatically operated manual transmission – achieved by using two clutches.

Doing away with the torque converter of a normal automatic transmission, they utilise the traditional gearing methods of a manual - but then split that gearing across two output shafts to allow power flow to shift between them incredibly quickly.

ORIGINS

Also referred to as a Direct Shift Gearbox (DSG), most car manufacturers have their own version or versions, but use them selectively across their ranges of vehicles – with the Volkswagen group the most invested in their implementation.

Like so many ‘new’ developments for automobiles, the double clutch transmission is an idea that had been around for a significant amount of time before finally being put to use, having been experimented with pre-World War two.

Its realisation had to wait until the 80’s, and it was Porsche who eventually became the first major adopter having worked on their own version since the 60’s.

The Porsche 956 released in 1983 received the distinction of being the first to feature one, and their complexity and expense kept them in the realm of motor racing and high-end super cars for the most part.

Space was also a considerable restriction. The electronics needed for the computerised control module had not yet reached the miniaturisation levels of modern times and so were quite sizeable and took up valuable engine bay space.

It wasn’t until 2003 that a production road car featured a DCT, in Volkswagen’s Mk4 Golf R32.

ADVANTAGES

Double

The purpose behind a DCT is to reduce the gap between gear shifts to a minimum, with the benefits being improved performance and fuel efficiency.

The less time spent off the power changing gear during acceleration the faster the vehicle can accelerate - and by reducing the time spent off the power during gear changes in general, the better fuel economy will be as engine power isn’t wasted.

Gear changes on DCTs can now be measured in the milliseconds, faster than any driver is capable of, and still retain the advantages a manual gearbox has over an automatic.

How it achieves these lightning fast changes is by anticipating what the next gear needed will be, based on factors such as rate of acceleration or deceleration, current speed of travel and engine speed.

This allows the computer to have the predicted gear already engaged before the change is made.

OPERATION

Unless you’re already familiar with how a DCT works, you’re probably already questioning how this type of transmission can have two different gears selected simultaneously without exploding into a shower of shrapnel.

The clue is in the name but that’s only describing part of the process. Yes, there are two clutches, but simply having an extra clutch doesn’t really explain how a transmission can select more than one gear at a time.

The twin clutches are arranged concentrically – meaning one sits within the circumference of the other, crucially enabling them to both work on the same input shaft.

However, the input shaft connected to both the clutches is not one shaft, but two - with one of them sitting within the other and allowing each clutch to contact and act upon a different one.

The gears are split across the split input shaft by odds and evens, allowing the transmission to switch back and forth between them as the car accelerates and decelerates – pre-selecting each gear on the opposite input shaft to the one engaged.

The gears themselves are no longer engaged purely mechanically by the gear level either and are instead actuated by shift forks powered by pistons, engaging and disengaging as necessary.

When you shift gears, the gear level is signalling for these pistons to actuate and engage the synchronisers onto the chosen gear, and not mechanically engaging them itself.

In this simplified diagram you can see how the input shaft is divided in two via a hollow shaft, with one running through the other and both controlled by the double clutch switching between them.

The gears work as a normal synchromesh transmission would, by utilising synchronisers to connect the gears to the output shafts.

Tracing a line from the blue clutch on the left, along the input shaft, up onto first gear on the first output shaft, and then continuing along and down onto the transmission output gives you first gear.

While this is happening, the computer is deciding which gear you’ll be wanting next. If you are accelerating it will almost certainly opt for second gear and connect this gear on other half of the input shaft so that it is ready as soon as you want to shift up into second.

On shifting, the clutches switch over and power transfers to the grey section of the input shaft, tracing a line along and down onto second gear and out onto the transmission output as before.

There’s a slight overlap between the two as the clutches engage and disengage, but the change is incredibly fast with almost no gap between the two speeds, giving the impression of a near seamless gear change.

It’s a very sophisticated system - but not one without its problems or detractors.

PROBLEMS

The system does run into problems when going from stationary to very slow speeds, where there’s a delay in engaging first gear but the driver is still depressing the accelerator waiting for power delivery. Once the system catches up and the gear is engaged, the car will then accelerate according to how fast the engine speed is - and if the driver has given it too much power it can cause the car to shoot forwards unexpectedly.

This problem can be largely attributed to unfamiliarity with the system, but other complaints are more directly related to its mechanical processes - although this is usually relevant for earlier model DCT/DSGs.

These include having to wait for the system to shift down gears when decelerating rapidly, as it is unable to skip gears due to the alternating method of operation between input shafts; and problems with reversing up hills where the system can get confused by which gear it should be engaging next.

Like any system, continued refinement and improvement eliminates problems and improves operation, and this could be the case here if given the development time. However, the writing could already be on the wall as some automotive manufacturers have already cast doubt over the future of the complex transmissions.

Advances made in automatic transmission technology have closed the gap to DCT/DSGs, and with automatics already selling in increasing numbers over manuals we could well see a time in the near future where manual transmissions phase out completely – leaving them to the enthusiasts.

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At their core, dual clutch transmissions are a bit of a hybrid of a manual and automatic transmission. They’re more akin to a manual transmission, however, use two clutches (hence dual clutch) to synchronize the gear change.

To better understand how a DCT transmission works, it’s best to understand how a manual transmission works. When using a manual transmission, the driver needs to frequently disengage the clutch to be able to change gears. The clutch works by disconnecting the engine’s drive from the driveline briefly so that the gears can be changed smoothly. A DCT works by using two clutches instead of one, and both are computer controlled, so there’s no need for a clutch pedal.

The dual clutch transmission operates via several in-built computers. These computers eliminate the need for the driver to manually change gears and the entire process is automated. In this respect, a DCT can be looked upon as being similar to an automatic transmission. The main difference is that the DCT operates the odd and even gear numbers separately, which prevents the engine from being disengaged from interrupted power flow when the gears are changed. The core difference between a DCT transmission and an traditional automatic transmission is that the DCT transmission doesn’t use a torque converter.

How Is a DCT Different to an Automatic Transmission?

While a dual clutch transmission looks very similar to an automatic transmission cockpit, this is where the similarities end. In fact, a DCT has more in common with a manual transmission than it does with an automatic transmission. One of the main advantages of a dual-clutch transmission is its economical fuel saving benefits. As the flow of power from the engine is not disrupted, the rate of fuel efficiency increases.

Transmission

It is estimated that a 6-speed dual-clutch transmission is able to increase fuel efficiency by approximately 10% compared with a standard 5 speed automatic transmission. Generally speaking, this is because a torque converter in a typical automatic transmission is designed to slip, so not all of the engine’s power is being transferred to the driveline all of the time, particularly while accelerating.

How Is a DCT Different to a Manual Transmission?

When a driver changes gears using a manual transmission, it take approximately half a second to complete the action. While this may not sound like much, when compared to the 8 milliseconds some DCT vehicles offer the efficiency becomes apparent. The increased speed when changing gears makes a DCT considerably faster than their manual transmission counterparts. In reality, a dual clutch transmission works in the same way that a standard manual transmission does.

It has both auxiliary and input shafts bat house the gears. It also has a clutch and synchronizers. The main difference is that a DCT does not have a clutch pedal. The need for the clutch pedal is eliminated due to the fact that Hydraulics, solenoids, and computers perform the shifting operation. The driver is still able to instruct the computer system when to perform certain actions through buttons, paddles or gear shifts. This ultimately improves the overall driver experience and is considered to be one of the most dynamic types of acceleration available.

How Is a DCT Different to a Continuously Variable Transmission?

Many of today’s cars feature CVT. Continuously variable transmission works by a belt that rotates between two pulleys. As the pulley diameter changes, it allows for many different ratios of the gears. This is where it gets the continuously variable name from. Much like DCT, a CVT eliminates shift shock due to the fact that there are no shifting of the gears required by the driver. As you increase or decrease your speed, the continuously variable transmission will adjust itself accordingly for maximum performance and efficiency.

The main difference between DCT and CVT lie in the type of vehicle that it is installed in. Typically, continuously variable transmission tends to be used in lower performance cars that are manufactured in a higher volume. DCT is more commonly found in high-performance vehicles that are manufactured in a lower volume. Another similarity between DCT and CVT their calls is that they operate with maximum efficiency, especially when it comes to fuel-saving and acceleration.

Is a Dual Clutch Transmission Right for Me?

There are plenty of benefits from opting for a dual clutch transmission. Of course, your own preferences will be a major deciding factor but don’t rule out a DCT without knowing how it can improve your driving experience.

As dual clutch transmission is still relatively new, many car manufacturers are using their own trademarked names. For Seat, Skoda and Volkswagen it is known as DSG, Hyundai call it EcoShift, Mercedes Benz call it SpeedShift. Ford has named it PowerShift, Porsche calls it PDK, and Audi has branded it as S-tronic. If you see these names associated with any car you are interested in, it means they have a Dual Clutch Transmission.

Improved Acceleration

Double Clutch Transmission Diagram

It takes approximately one-tenth of a second for a dual-clutch transmission to change gears, meaning that the driver experiences improved acceleration. This improved acceleration makes it a popular choice for high-performance cars. While DCT gearboxes have been around for many decades, their use has been reserved mainly for high-performance vehicles involved in motorsport. The superior power and speed offered by dual clutch transmission are now rapidly becoming a popular choice in many new makes and models of car.

Double Clutch Transmission In German

Smoother Gear Changes

Dual clutch transmission is perfect for performance driving. The computers make changing gears extremely fast and direct. These smooth gear changes eliminate many of the clunking or knocking sounds experienced with manual transmissions.

Shift shock is a common occurrence with manual transmission vehicles and a DCT completely eradicates this. One of the main advantages that many drivers appreciate is the ability to be able to choose whether they want to let the computer perform the shifts on their behalf or whether they would like to control it themselves.

Power and Efficiency

Double Clutch Transmission Fluid

When you compare a dual-clutch transmission with a standard automatic transmission, there is approximately 6% improved efficiency of both fuel and acceleration. Transitioning from automatic to manual is smooth and allows the driver to have more control over the driving experience. For those who appreciate improved power, efficiency, flexibility and fuel saving options, a DCT vehicle will easily provide all of these features.