Dual clutch transmission

History

This type of transmission was invented by Frenchman Adolphe Kégresse just before World War II, although 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 of Harry Webster at Automotive Products (AP), Leamington Spa, with prototypes built into the Ford Fiesta Mk1, Ford Ranger, and Peugeot 205. Initially, the control systems were based on purely analogue/discrete digital circuitry with patents filed in July 1981. All of these early AP twin-clutch installations featured a single dry clutch and multiplate 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, when computers to control the transmission became compact enough: the Porsche Doppelkupplungsgetriebe (English: dual-clutch gearbox) (PDK) used in the Porsche 956 and 962 Le Mans race cars from 1983, and the Audi Sport Quattro S1.

A dual-clutch transmission eliminates the torque converter as used in conventional epicyclic-geared automatic transmissions. Instead, dual-clutch transmissions that are currently on the market primarily use two oil-bathed wet multiplate clutches, similar to the clutches used in most motorcycles, though dry-clutch versions are also available.

The first series production road car with a DCT was the 2003 Volkswagen Golf Mk4 R32.

As of 2009, the largest sales of DCTs in Western Europe are by various marques of the German Volkswagen Group, though this is anticipated to change as other transmission makers and vehicle manufacturers make DCTs available in series production automobiles. In 2010, on BMW Canada's website for the 3 Series Coupe, it is described both as a seven-speed double-clutch transmission and as a seven-speed automatic transmission. It is actually a dual-clutch semiautomatic.

Overview

In DCTs where the two clutches are arranged concentrically, the larger outer clutch drives the odd-numbered gears, while the smaller inner clutch drives the even-numbered gears. Shifts can be accomplished without interrupting torque distribution to the driven roadwheels, by applying the engine's torque to one clutch at the same time as it is being disconnected from the other clutch. Since alternate gear ratios can preselect an odd gear on one gear shaft while the vehicle is being driven in an even gear, (and vice versa). DCTs are the fastest-shifting road car transmission available and are even able to shift faster than a professional race car driver using a manual transmission. DCTs can even shift more quickly than cars equipped with single-clutch automated-manual transmissions (AMTs), also called single-clutch semiautomatics. 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.

Clutch types

Dual-clutch transmissions use two fundamentally different types of clutches: either two wet multiplate clutches, bathed in oil (for cooling)—or two dry single-plate clutches. The wet clutch design is generally used for higher torque engines that can generate 350 newton metres (258 lbf·ft) and more (the wet multiplate clutch DCT in the Bugatti Veyron is designed to cope with 1,250 N·m (922 lbf·ft)), whereas the dry-clutch design is generally suitable for smaller vehicles with lower torque outputs up to 250 N·m (184 lbf·ft). However, while the dry-clutch variants may be limited in torque compared to their wet-clutch counterparts, the dry-clutch versions offer an increase in fuel efficiency, due to the lack of pumping losses of the transmission fluid in the clutch housing.

Clutch installation

Currently, three variations of clutch installation are used. 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 engine crankshaft; when viewed head-on along the length of the input shaft, this makes one clutch noticeably larger than the other.

The second implementation uses two single-plate dry clutches—side-by-side from the perpendicular view, but again sharing the center line of the crankshaft.

A later 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 by a gear from the engine crankshaft.

BorgWarner

BorgWarner Inc. supplies wet dual clutches and electrohydraulic control modules (mechatronics) for these dual-clutch transmissions, along with complete dual-clutch transmission and transaxle assemblies. BorgWarner, which call their technology "DualTronic", entered series production (excludes Bugatti Veyron) for Volkswagen Group, which renamed Direct-Shift Gearbox (DSG) in 2003 in the Volkswagen Golf Mk4 R32. The company signed further agreements with three other (unnamed) European automotive manufacturers to incorporate their components in DCTs.

On 14 January 2009, BorgWarner announced a joint venture with the China Automobile Development United Investment Co., Ltd., which is owned by 12 Chinese automakers. This joint venture is known as the BorgWarner United Transmission Systems Co., Ltd., and is located in Dalian, China. The company has produced various dual-clutch transmission modules beginning in 2011.

The Nissan GT-R uses BorgWarner components, including the dual-clutch module, clutch-control, shift-actuation systems, and synchronizer assemblies.

EATON

EATON developed the first dual-clutch transmission for class 6-7 trucks in North America - Procision.

Fiat Powertrain Technologies

Fiat Powertrain Technologies developed a dual-clutch transmission with Magneti Marelli and BorgWarner called Euro Twin Clutch Transmission. Magneti Marelli produces the control system, which integrates BorgWarner's hydraulic actuation module into its own power and transmission control units. It can handle torque inputs of up to 350 N·m (260 lb·ft), making it the highest-torque dry-clutch application. Its weight of 81 kg (179 lb), including oil and transmission control unit, its three-shaft architecture, especially axially, means it can be installed in several types of B- and C-segment vehicles.

FEV GmbH

FEV developed a DCT with an integrated electric motor, providing hybrid capability. The 7H AMT Transmission provides seven forward speeds with multiple usage of gears (the electric drive can use unused gears from the combustion engine path): this aims to substantially reduce the complexity of the unit, as well as the package size and weight. Another spin-off from this design was the FEV xDCT that provides 10 forward speeds, but with the mechanical complexity of a standard six-speed DCT.

Getrag

Getrag has developed a range of DCT transaxles, including 7DCL750, a seven-speed transaxle for midengine longitudinal applications, capable of taking more than 750 N·m (553 lbf·ft). Getrag will provide its DCT in its first commercial applications, for the Dodge Journey and Volvo S40 and V50, from mid-2008. Getrag has developed the 6DCT250 dry clutch DCT for use in front wheel drive transverse applications. With use of electromechanical actuation, rather than electrohydraulic, the 6DCT250 transmission surpasses the conventional manual transmission in fuel consumption and CO₂ emission.

In the second quarter of 2008, Getrag had signed an agreement with Chrysler to supply its PowerShift DCTs for use in American markets. However, due to the global economic downturn, this was subsequently cancelled.

In 2006, Getrag was also working with Bosch to develop a DCT for use in hybrid vehicles.

In 2015, Getrag introduced a new seven-speed, wet-clutch DCT, the 7DCT300, with a maximum torque of 300 Nm. The first application was the Renault Espace with a 1.6 L, Turbo GDI engine.

Getrag dual-clutch transmissions are used in the BMW M3, BMW 335is, BMW Z4 Sdrive35i, Ferrari California, Ferrari 458 Italia,LaFerrari, Mercedes-Benz SLS AMG, Mitsubishi Lancer Evolution X, Mitsubishi Lancer Ralliart, Ford Fiesta, Ford Focus, Renault Megane, Renault Scenic, smart fortwo, Volvo V40, Volvo V60 Volvo V70 vehicles.

Graziano

Italian specialist Oerlikon Graziano was in the process of developing a wet-clutch DCT for original equipment manufacturer fitment into supercars. They planned to have it in production by 2010. Their DCT is aiming to have a torque-handling capacity of 750 N·m (553 lbf·ft), and they are also aiming to improve shift quality over their current AMTs. As of 2011, they supplied the seven-speed DCT for McLaren's MP4-12C supercar with a shifting time of 50ms.

LuK

LuK Clutch Systems, LLC. designed and manufacture the dual dry single-plate clutch system for the Volkswagen Group seven-speed DQ200 direct-shift gearbox (DSG) introduced in 2008. This DSG variant is used in smaller cars, with smaller-displacement engines with relatively low torque outputs. LuK are naming their technology "XSG" for their overview of automated shift gearbox systems, and "Parallel Shift Gearbox" for its own proprietary DCT.

Ricardo

English specialist consulting engineering company Ricardo plc designed and built the Bugatti Veyron's seven-speed dual wet multiplate dual-clutch transmission, specifically to cope with the 1,250 N·m (922 lbf·ft) of torque generated by W16 engine.

ZF

ZF Friedrichshafen AG, with Porsche, designed and now build the two different variants of the Porsche Doppelkupplungsgetriebe (PDK) seven-speed '7DT' dual wet multiplate clutch transmission.

Volkswagen Group

Volkswagen Group produces, so far, the fastest DCTs in the world (8ms) under the direct-shift gearbox name, from the German: Direkt-Schalt-Getriebe. It is used in all of their mainstream marques, including Volkswagen, Audi, SEAT, Škoda, and Volkswagen Commercial Vehicles, and also its top-tier marque Bugatti. Audi originally used the direct-shift gearbox name, but now uses the name "S tronic" for its DCTs.

The first ever worldwide series production DCT was the Volkswagen Group DQ250 six-speed dual-clutch transmission, with dual concentric wet multiplate clutches. It was produced at the Group's Kassel plant under exclusive license from BorgWarner for use in transverse powertrain installations, of either front-wheel drive or four-wheel drive (4WD) layouts. The 4WD versions are fundamentally identical to the FWD versions, but the 4WD versions use an additional bolt-on power take-off unit to direct engine torque to the Haldex Traction rear axle. This DQ250 variant is used in a wide range of models: Volkswagen Passenger Cars (Polo, Golf/Rabbit/Golf Plus, Scirocco, Jetta, Eos, Passat, and Touran); Audi cars (A3, and TT); SEAT cars (Ibiza, León, Altea, and Toledo); Škoda cars (Octavia and Superb); and Volkswagen commercial vehicles (Caddy and T5 Transporter).

A second variant of the DSG went into series production in 2008—the DQ200. This unit has seven forward ratios, but the notable difference over the original DQ250 is the change from wet to dry clutches. This variant uses two single-plate dry clutches, arranged in a tandem design (instead of concentrically) and therefore similar in size. The DQ200 is again for use in transverse applications, but is intended for use in smaller cars, with smaller-displacement engines that generate relatively low torque outputs. When used in the latest Golf with the 90 kilowatts (122 PS; 121 hp) engine, this new seven-speed DSG uses roughly 6% (5.9l/100 km for the seven-speed DSG compared to 6.3l/100 km with the six-speed manual gearbox) less fuel than the same engine with a manual transmission and up to 20% less than a conventional automatic transmission. The original DQ250 also remains available.

Volkswagen Group subsidiary Audi AG have also developed an all-new DCT, the DL501, for use in longitudinal powertrains. Like the original transverse DQ250, this DL501 uses dual wet multiplate clutches, but unlike the DQ250, this variant uses seven forward ratios. This DL501 variant made its debut in the Q5, and also is used in the latest versions of the A4 (B8) and S4 (B8). It is also being considered for use in an all new A6.

After the facelift in 2012, the Audi R8 uses a seven-speed S tronic.

Ricardo plc produces the seven-speed DCT for the 736 kilowatts (1,001 PS; 987 bhp) 2005 Bugatti Veyron EB 16.4.

Lamborghini (VW GROUP)

Lamborghini's first dual-clutch transmission is available in the Lamborghini Huracán LP610-4 (2014–present). The name of the transmission is 7-speed LDF Dual Clutch "Doppia Frizione" with shift characteristics variable via Drive Select Mode and all-wheel drive with electrohydraulic multiplate clutch.

The car has three modes changed using the steering-wheel-mounted "ANIMA" or mode selector. The three modes are strada, sport, and corsa, each with faster shifts. Launch control raises the rotation speed to 4200 rpm before dropping the clutch. The Huracán upshifts automatically at redline, but not before running into the limiter for a fraction of a second. Using the paddles to call for earlier shifts knocks 0.1 to 0.2 sec off the figures.

Kia

Kia Motors has announced application of the all-new seven-speed DCT to a production model for the first time, the new Kia Cee’d GT line, which made its global debut at the 85th Salon International de l’Automobile in Geneva on 3 March 2015..

Compared to the six-speed DCT currently used in the European-market Cee'd models, the new transmission is expected to deliver a fuel-economy improvement of 7% and a 5% improvement in 0–62 mph acceleration times. Like most new DCTs, the new transmission features dual dry clutches and two input shafts, allowing for seamless torque delivery between shifts, as well as gear-jumping to the optimal ratio, should the driver ask for it. If that were to apply to the current Forte K3 Koup or ProCee'd GT and GT line, with its 204-hp/265 nm 1.6L inline-four Gamma II T-GDI Turbo engine and also 133-ps/300 nm 1.6L CRDi diesel engine, for example, that would raise its fuel economy from 24 mpg city/36 highway to about 26/39 while bringing its 7.4-second 0–60 run to about 7.0 seconds.

The new transmission is the first of its type from the brand, and has been developed in-house by Kia’s research and development teams at its Namyang, Korea R&D centre. More fuel efficient than Kia’s existing six-speed automatic transmission, the new DCT has been engineered with low fuel consumption and a sporty driving feel when in manual mode, and with comfort and smoothness when left in automatic mode.

Innovative hollow double-gear input shaft allows quick shifts. The DCT is made up of two dry clutches, each fitted with electric motor-driven clutch actuator to improve responsiveness, and an innovative hollow double-gear input shaft. The hollow shaft itself allows the system to quickly engage even gears, while a solid shaft that runs through the middle operates odd gears. This crossed gear shifting enables continuous power delivery and more efficient packaging, two development cornerstones for the engineering teams behind the project.

The hollow shaft mechanism lets the DCT operate sequentially, the car anticipating the next gear that the driver is likely to need next under acceleration or braking, or jump immediately to any of its seven forward gears (and reverse). This is particularly useful under kickdown or heavy braking, allowing the driver to remain in control of the vehicle at all times.

The continuous power delivery of the DCT minimises the loss of torque and forward motion by the powertrain during gear shifts, resulting in more decisive acceleration and a smoother drive. This contributes towards the transmission’s boost to performance in all road conditions in its first application in the Cee’d GT line.

Combined with an increase in power from 128 to 133 ps, the DCT’s seven gears allow the GT Line’s diesel engine to operate at its most efficient speeds at all times. With the engine’s torque increased from 265 to 285 Nm, the new DCT is engineered to effectively manage outputs up to 300 Nm, achieving a better balance between the engine’s improved, more effortless performance and potential for greater fuel economy.

Reduction of noise, vibration and harshness has also been a focus for the team behind the DCT’s development, with an external damper ensuring a higher level of refinement for the new transmission. After the Cee’d GT line, which will go on sale across Europe in Q4 2015, the new seven-speed DCT will be made available on a range of other Kia models.

BMW

In January 2008, BMW introduced a dual-clutch transmission for their M3 model, manufactured by Getrag, embedding a DualTronic dual-clutch module from BorgWarner, and dubbed "M double-clutch transmission" (M DCT). The same transmission is used in the second-generation Z4 (the Sdrive35i and sDrive35is versions), and is an available option on the 335i (coupe, convertible), 335is and later 135i builds.

BMW released the new M3 (F80) and M4 (F82) with the seven-speed DCT in 2014.

BYD

BYD Auto introduced a dual-clutch transmission for its mid-sized family sedan BYD G6.

Mercedes-Benz

The Mercedes SLS AMG uses a Getrag AMG 'SpeedShift' seven-speed dual-clutch gearbox, mounted at the rear in a transaxle configuration, and connected to the engine by a carbon fiber driveshaft enclosed in an aluminium torque tube. According to Mercedes-Benz, it is capable of gear changes in as little as 100 ms. The SLS and the Ferrari California share the same DCT unit.

The Mercedes-Benz CLA 250 and GLA250 both use a seven-speed dual-clutch gearbox developed in-house.

Smart

The "Twinamic" dual-clutch transmission built by Getrag is offered in third-generation Smart Fortwo and Forfour from 2014 on.

Ferrari

Ferrari incorporated a Getrag-designed and -manufactured seven-speed dual wet-clutch transmission in their California and 458 Italia sports cars.

Fiat Chrysler

Chrysler received the Fiat Powertrain Technologies C635 dual dry-clutch transmission for use on Chrysler's turbo inline-four in the Dodge Dart in 2013.

Fiat Powertrain Technologies (FPT) started production of "TCT" dual dry-clutch transmission (DDCT) in early 2010 at the Verrone plant. The C635 DDCT gearbox is used for B (supermini), C (compact), and D segment (large family) cars, and can handle torque up to 350 N·m (258 lbf·ft). Fiat's C635 DDCT gearbox was introduced in Alfa Romeo MiTo in 2011.

Ford Motor Company

Ford Motor Company released a wet-clutch "PowerShift" transmission on the 2008 Ford Focus (international) and Ford C-MAX. This wet-clutch DCT was designed with gearbox specialist Getrag under the Getrag Ford Transmissions joint venture, founded in 2001, and is expected to feature in other Ford and Volvo models.

Ford announced the US-market version of the Mark VI Ford Fiesta for 2011 featured a dry-clutch PowerShift transmission. Ford also announced the introduction of PowerShift transmission to North American market by 2010.

The Volvo version will be built in Koping, Sweden, at Volvo's transmission plant. The PowerShift gearbox will be introduced on the second-generation Volvo S60, and then on to the V50 and C30 models. The XC60 is expected to get this Ford PowerShift gearbox along with other new models to make more sporty cars.

General Motors

In the company's restructuring plan, it revealed that dry dual-clutch transmission would have been available in the 2012 calendar year. However, this plan was cancelled. The GM dual-clutch gearbox was introduced on the GMC Granite, a concept car. Had it been released, the new front-wheel drive transmission would have incorporated the latest innovations for improving fuel economy and performance. The transmission alone would have provide upward of 10% improvement in fuel economy over today’s conventional six-speed automatic transmissions. The transmission co-developed between GM and Shanghai Automotive Industry Corporation features dry, dual-clutch technology. It would have provided shift comfort equal to a conventional fully automatic transmission, with superior quality, while reducing CO₂ emissions.

Honda

Honda implemented the eight-speed DCT in the Acura ILX and TLX. It is also the only company to pair their DCT with a torque converter to smooth low-speed crawling (a problem found in many traditional DCTs). The Acura NSX super car also uses a nine-speed DCT, which consists of launch gear and a cruising gear while seven gears in the middle are geared closer to each other.

Motorcycles

In October 2009, Honda launched the VFR1200F, a motorcycle with a 1,237 cc (75.5 cu in) V4 engine and optional DCT dubbed the Next Generation Transmission. In May 2009, Honda CEO Takeo Fukui mentioned that Honda was working on a new dual-clutch transmission system that could be matched with future hybrids.

2012 NC700 series have dual-clutch models available: NC700SD, NC700XD and NC700D Integra. It is the second generation from the six-speed dual-clutch transmission first used on the Honda VFR1200F.

2013 Honda introduced the CTX700DCT, a 670 cc dual clutch motorcycle selling as a 2014 model. [1]

The 2016 CRF1000L Africa Twin is available in both 3rd generation DCT and standard transmissions. The NC line has moved to the 3rd generation as well.

In 2016, Honda announced that they are studying a DCT for use in sport bikes as well.

Hyundai

The DCT transmissions used in some Hyundai and Kia passenger vehicles are supplied by Hyundai DYMOS. At the 2009 Geneva Motor Show, Hyundai unveiled ix-onic concept car, which was said to feature a six-speed dual-clutch transmission.

Hyundai released the Veloster with the 6-speed EcoShift Dual Clutch Transmission in 2011.

Hyundai released the Sonata and Veloster Turbo with the 7-speed EcoShift Dual Clutch Transmission in 2015.

Hyundai released the i30 and Tucson SUV with the 7-speed EcoShift Dual Clutch Transmission in 2016.

John Deere

John Deere Tractors introduced its DirectDrive dual-clutch transmission in 2012 in tractors. Currently the transmission is available in the 6R models, although it will likely be phased out in favour of the autopower transmission on the new 6230R and 6250R.

Lotus

Lotus Cars has submitted a patent application for a lighter, simpler and more compact dual-clutch transmission system. The gearbox shown in the application is an eight-speed unit, with seven forward gears and reverse.

McLaren Automotive

McLaren Automotive introduced a seven speed dual clutch on MP4-12C made in Italy by Oerlikon Graziano. The design comes with a feature called Pre-Cog that lets the driver preselect the next highest or lowest gear decreasing shift times.

Mitsubishi

The Mitsubishi Motors Lancer Evolution X and Lancer Ralliart are supplied with Mitsubishi's Twin Clutch SST (abbreviated TC-SST, for "Twin Clutch Sport Shift Transmission" or "Twin Clutch Sportronic Shift Transmission") dual-clutch transmission, controllable by steering wheel mounted magnesium paddle shifters. It is the Getrag-built, PowerShift 6DCT470 transmission in the Mitsubishi Lancer Evolution.

Mitsubishi Fuso Truck and Bus Corporation

The Mitsubishi Fuso Truck and Bus Corporation presented a world first with a double-clutch transmission for commercial vehicles. The new six-speed M038S6 "Duonic transmission" features wet clutches and incorporates the ability to creep in traffic for smoother operation. Although Duonic-equipped trucks will probably be driven mostly in fully automatic mode, the transmission can also be manually shifted.

Nissan

The Nissan GT-R's rear transaxle six-speed dual-clutch transmission also contains the differential for the car's all-wheel drive (AWD) system. Much of the dual-clutch system was engineered by BorgWarner, partnered with Nissan's gearbox supplier Aichi Machine Industry, and weighs a total installed mass of 117 kg (258 lb). Code named the GR6, the transmission is quite durable in that it is able to hold in excess of 600 lb·ft (810 N·m) of torque. Nissan reports shift times of 150 milliseconds.

PSA Peugeot Citroën

PSA has dual-clutch transmission, produced by Getrag, in its Peugeot 4007 and Citroën C-Crosser SUVs, both based on the Mitsubishi Outlander. However, dual-clutch transmissions are never offered in other models.

Porsche (VW GROUP)

Porsche AG offers a series production of two new longitudinally installed, ZF Friedrichshafen AG designed and built with Porsche "7DT" wet-clutch versions of its previously race-only Porsche Doppelkupplungsgetriebe (PDK) transmission.

The first variant, the 7DT-45, is used on its 2009 997 Carrera and Carrera S models. This version is also offered on the 2009 Cayman, Boxster It uses a ZF Sachs ND2015 clutch pack, and has a torque handling capacity of 500 N·m (369 lbf·ft). A higher torque version of the same transmission, the 7DT-70, is also available for the 2010 911 Turbo. This is rated at 780 N·m (575 lbf·ft), and uses a different ZF Sachs clutch—the ND2216.

The second PDK variant, the 7DT-75, is available on the 2009 Panamera. This is constructed fundamentally differently from the 7DT-45/7DT-70 versions; in that the internal shafts are mounted above the input shaft, so as to achieve a lower centre of gravity for the Panamera. It also uses just one oil circuit, whereas the 7DT-45/7DT-70 use two separate circuits; with very different specifications of fluids needed.

Both variants use seven forward speeds, and Porsche claim the Doppelkupplung PDK transmission will replace the outgoing conventional Tiptronic automatic transmissions. However, other reliable industry sources state that Porsche still intends to use conventional automatics; with the eight-speed ZF 8HP being cited. Like all DCTs, the Porsche PDK transmission is fundamentally two separate manual transmissions in one. With the 1st, 3rd, 5th, and 7th gears and reverse available on one shaft, and 2nd, 4th, and 6th gears available on the other shaft.

Porsche claims noteworthy improvements in CO₂ emissions of around 15% when comparing DCT installations against its former automatic transmission, of which half can be directly attributed to the DCT. When compared directly with a manual transmission, 16% improvement can be directly attributed to the DCT.

Qoros

Getrag received the first order for its dual clutch transmission 6DCT250 from Qoros Automotive Co., Ltd, an independent JV carmaker in China. Attributes of the transmission include easy shifting, reduced fuel consumption compared to conventional automatic transmissions, and flexibility and functionality in hybrid applications. Start of production is planned for 2013 in the Getrag plant Nanchang. In addition to the six speed manual transmissions Getrag will supply to Qoros, the dual clutch transmission 6DCT250 will be applied to the first car under Qoros brand, which will meet all European standards and be exported to western European markets.

Renault

In the early part of 2010 Renault introduced a new six-speed dual dry clutch DCT. This is available on the Mégane with the dCi 110 DPF engine. The DCT system was built by Getrag. These DCT-equipped Meganes are the marque's first automatic cars to qualify for the Renault eco² signature. Since 2012 facelift, Renault Mégane is the first French automobile with dual-clutch transmission.

Railcar use

A different type of dual-clutch transmission has been used in some railcars. The two clutches are placed one on the gearbox input shaft and the other on the gearbox output shaft. To make a gear change, both clutches disengage simultaneously and a brake inside the gearbox engages. The gearchange occurs with all gears stationary, so no synchronizing mechanism is needed. After the gear change, both clutches re-engage. There is a significant break in power transmission, so this system is unsuitable for shunting locomotives.