Dual Clutch Transmissions

Dual Clutch Transmissions are quickly becoming the standard for performance and efficiency in roadcars. This will be a brief review on how they work and how they compare to other transmission designs.

How They Work

Figure 1
Image via Wikimedia Commons

Dual clutch transmissions (DCT) are a type of automatic transmission that uses two clutches for odd and even and gear sets. Figure 1 is a diagram of a typical DCT. Essentially it is two separate manual transmissions that are combined as one in a single housing. Unlike manual transmissions, most DCTs have electro-hydraulically actuated clutches that are operated automatically. Some vehicles with DCTs offer modes in which the driver can control gear shifts manually via paddle shifters or other means.

Advantages

Shift times are very important in performance vehicles, because while a transmission is shifting gears the vehicle is moving without torque being sent to the wheels. Other benefits of faster shift times include a smoother/seamless acceleration, and increased fuel economy. As compared to traditional transmissions, DCTs are able to offer much faster shift times between gears. This is because alternate gears can be pre-selected. For example the vehicle could be propelled by the fourth gear with one clutch engaged and as soon as the vehicle upshifts to the fifth gear the even gear clutch disengages and the odd clutch which is already pre-selected to the fifth gear can then engage. These shifts can be as fast as 8 milliseconds; a typical manual transmission shift time averages around 500 milliseconds.

Disadvantages

The main disadvantage to DCTs is their high cost. This is attributed to their complex design, economies of scale (since they are not produced at the rate of other traditional transmission designs), and maintenance costs which are attributed to their complexity. DCTs rely heavily on computers to dictate gear shifts further adding to their cost. Although DCTS can provide smooth shifts while accelerating quickly, they sometimes have problems shifting to often at low speeds when the throttle is being applied intermittently. This can cause a jerky ride compared to traditional transmissions.

Applications

Due to the inherent disadvantages of DCTs they are usually reserved for higher end sports cars. Some examples of vehicles that use DCTs include:

Bugatti Veyron
Image by Brian Snelson via Flickr

BMW Z4
Image by Dein Nordrhein-Westfalen via Flickr

Nissan GTR
Image by Sebastien Cosse via Flickr

Some examples of inexpensive models available with DCTs include:

Volkswagen  GTI
Image by Justin Capolongo via Flickr

Dodge Dart
Image by Michael Gil via Flickr

Future of Transmission design

With ever increasing pressure on automakers to design more fuel efficient vehicles, transmission design is an obvious way to increase efficiency and performance. Another recent transmission design is the Continuously Variable Transmission (CVT). CVTs have no gears but instead use a belt that can be adjusted to an infinite number of effective gear ratios. This design can be more cost effective than 

DCTs while also providing increased fuel economy over traditional transmissions. The main disadvantage behind CVTs is they are not capable of handling the high amounts of torque that DCTs can. Thus they are not suitable for performance/heavy duty oriented vehicles, and are mostly limited to smaller economy vehicles. Transmission design will always be an important element in vehicle design, and with advances in technology and manufacturing techniques DCTs may become a mainstream component in consumer vehicles.