twin clutch

8/11/2019 Twin Clutch

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Twin Clutch / Direct Shift Gearbox (DSG)

VW/Audi DSG transmission cutaway. Photo Volkswagen

The twin-clutch transmission, also known as the Direct Shift Gearbox (DSG) or dual-clutch

transmission, is an automated transmission that can change gears faster than any other geared

transmission. Twin-clutch transmissions deliver more power and better control than a traditional

automatic transmission and faster performance than a manual transmission. Originally marketed

by Volkswagen as the DSG and Audi as the S-Tronic, twin-clutch transmissions are now being

offered by several automakers, including Nissan, Mitsubishi, BMW and Porsche.

Before DSG: The SMT

The twin clutch/DSG is a development of the sequential manual transmission (SMT), which is

essentially a fully-automated manual transmission with a computer-controlled clutch, intended to

deliver stick-shift performance with automatic convenience. The advantage of an SMT is that it

uses a solid coupling (the clutch), which provides a direct connection between engine and

transmission and allows 100% of the engine's power to be transmitted to the wheels. (Traditional

automatics use a fluid coupling called a torque converter, which allows some slippage.) The chief

drawback of the SMT is the same as that of a manual: In order to change gears, the engine and

transmission must be disconnected, interrupting the flow of power.

Twin-clutch: Solving the SMT's problems

The twin-clutch transmission was designed to eliminate the lag inherent in SMTs and manuals. The

twin-clutch transmission is essentially two separate transmissions with a pair of clutches between

them. One transmission provides odd-numbered speeds (ie first, third and fifth gear), the other


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provides even-numbered speeds (second, fourth and sixth). When the car starts out, the "odd"

gearbox is in first gear and the "even" gearbox is in second gear. The clutch engages the odd

gearbox and the car starts out in first gear. When it's time to change gears, the transmission simply

uses the clutches to switch from the odd gearbox to the even gearbox, for a near-instant change to

second gear. The odd gearbox immediately pre-selects third gear. At the next change the

transmission swaps gearboxes again, engaging third gear, and the even gearbox pre-selects fourth

gear. The twin-clutch transmission's computerized controller calculates the next likely gearchange

based on speed and driver behavoior and has the "idle" gearbox pre-select that gear.

One advantage to both SMTs and twin-clutch/DSG transmissions is the ability to perform matched-

rev downshifts. When a driver selects a lower gear, both types of transmission disengage the

clutch(es) and rev the engine to the exact speed required by the selected gear. Not only does this

make for a smoother downshift, but in the case of the twin-clutch transmission, it allows plenty of

time for the proper gear to be pre-selected. Most (though not all) twin-clutch transmissions canskip gears when downshifting, i.e. shifting from 6th gear directly down to 3rd gear -- and because

of their ability to match revs, they can do so without the lurching or surging typical of traditional

automatic and manual transmissions.

Driving a car with a twin-clutch/DSG transmission

Twin-clutch equipped cars do not have a clutch pedal; the clutch is engaged and disengaged

automatically. Most twin-clutch transmissions use an automatic-style shift selector with atraditional P-R-N-D or P-R-N-D-S (Sport) shift pattern. In Drive or Sport mode, the twin-clutch

transmission operates like a regular automatic. In Drive, the transmission shifts to higher gears

early in order to minimize engine noise and maximize fuel economy, while Sport mode holds the

lower gears longer in order to keep the engine in its powerband. Sport mode also provides more

aggressive downshifts with less accelerator pedal pressure, and in some cars, including most DSG-

equipped VWs and Audis, engaging Sport mode also causes the car to react more aggressively to

the accelerator pedal.

Most twin-clutch transmissions have a manual mode which allows manual shifting via the shift

lever or paddles mounted on the steering wheel. When driven in manual mode, the clutch is still

operated automatically, but the driver controls which gears are selected and when. The

transmission will follow the driver's commands unless the selected gear would over-rev the engine

(i.e. commanding first gear while driving 80 MPH).

Advantages of the twin-clutch/DSG transmission


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The primary advantage of the twin-clutch/DSG is that it provides the same driving characteristics

of a manual transmission (i.e. quicker throttle response and no drop in engine speed when the

driver lifts off the accelerator) with the convenience of an automatic. However, the ability to

perform near-instantaneous gearshifts gives the twin-clutch advantages over both manuals and

SMTs. Volkswagen's DSG takes about 8 milliseconds to upshift. Compare that to the SMT in the

Ferrari Enzo, which takes 150 ms to upshift. Instant gear shifts mean faster acceleration: According

to Audi, the A3 runs 0-60 in 6.9 seconds with a 6-speed manual and 6.7 seconds with the 6-speed

DSG.

Disadvantages of the twin-clutch transmission

The main limitation of the twin-clutch/DSG is the same as all geared transmissions: Because there

are a fixed number of gears, and the transmission cannot always keep the engine at its best speed

for maximum power or maximum fuel economy. Because of this, twin-clutch transmissions

generally cannot extract as much power or fuel economy from an engine as acontinuously-

variable automatic transmissions (CVTs).But because twin-clutch transmissions provide a more

familiar driving experience than CVTs, most drivers prefer them. And while the twin-clutch

provides superior performance compared to a manual, some drivers prefer the interaction that a

manual clutch pedal and gearshift provide.
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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 -- the dual-clutchtransmission, 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(orSMG), 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-Off

A dual-clutch transmission offers the function of two manual gearboxes in one. To understand whatthis 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 clutchpedal. This operates a single clutch, which disconnects the engine from the gearbox and interruptspower 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 called synchronizersmatch 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 engineto the wheels. Instead, power delivery changes from onto offto onduring 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. Sequentially, it works like this:
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