Direct-Shift Gearbox (DSG) - Seminar Paper

INTRODUCTION

Thanks to the double multi-plate clutch design and different automatic gear selection  programmes, DSG  is well capable of meeting the high demands in comfort from drivers who favour automatic gearboxes. Furthermore, with direct selection and lightning fast, jolt-free gear changes, it also offers a high level of driving enjoyment to drivers who favour manual gearboxes. In both cases, fuel consumption is at a par with economical vehicles fitted with manual gearboxes. Currently, the world of transmission is dominated in Europe by manual gearboxes and in the USA and Japan by automatic gearboxes. Both types of gearboxes have specific advantages and disadvantages.

The advantages of a manual gearbox are, for example,

• high degree of efficiency

• robust and sporty characteristic.

The advantages of an automatic gearbox are, for example,

• a high level of comfort, above all in gear changes, as there is no interruption in tractive power.

This formed the framework for Volkswagen to combine both transmission concepts into one completely.

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, Advantages of the twin-clutch/DSG transmission. The Direct-Shift Gearbox electronically controlled dual clutch multiple-shaft manual gearbox, in a transaxle design - without a conventional clutch pedal, and with full automatic, or semi-manual control. The first actual Dual Clutch transmissions derived from Porsche in-house development for 962 racing cars in the 1980s. By using two independent clutches, a DSG can achieve faster shift times, and eliminates the torque converter of a conventional epicyclic automatic transmission.

The direct shift gearbox is distinguished by

• Six forward gears and one reverse gear

• Normal driving program "D", sports program "S" as well as Tiptronic selector lever and      Tiptronic steering wheel levers (optional)

• Mechatronics, electronic and electro-hydraulic control unit form one unit and are housed in the gearbox

• Hillholder function – if the vehicle begins to move when stationary, with just light brake application, the clutch pressure is increased and the vehicle is held in position

• Creep regulation – allows creeping of the vehicle, when parking for example, without accelerator pedal application

• Emergency mode - in the event of a fault, the vehicle can still be driven, with emergency mode activated, in 1st and 3^rd gear or just in 2nd gear.

The direct shift gearbox is already available for Golf R32 and Touran models. Future plans are to make it available for the New Beetle and Golf 2004.

Operation

The selector lever is actuated in the vehicle in the same way as an automatic gearbox. The direct

shift gearbox also offers the option of Tiptronic gear selection. As in vehicles with automatic gearboxes, the selector lever features lever locks and an ignition key lock. The function of the locks remains unchanged. The design, however, is new.

The gear selector lever positions are:

P - Park

To move the selector lever out of this position, the ignition must be "on" and the brakes applied.

Furthermore, the release button on the selectorlever must be pressed.

R - Reverse gear

To engage reverse gear, the release button must be pressed.

N - Neutral position

In this position, the gearbox is at idle. If, for a length of time, the gear selector lever is left in this position, the brake pedal must be pressed for it to be moved.

D - Drive

In this position, the forward gears are selected automatically.

S – Sport

Gears are selected automatically using a "sporty" program stored in the control unit.+ and –

The Tiptronic functions can be used with the selector lever in the right gate and with the

steering wheel gear selectors.

TRANSMISSION 

Transmission is the mechanism through which the driving torque of the engine is transmitted                 to the driving wheel of the vehicle so that the motor vehicle can move on the road. The   reciprocating motion of the piston turns a crankshaft rotating a flywheel through the connecting rod .The circular motion of the crankshaft is to be now transmitted to the rear wheels .It is transmitted through the clutch, gear box, universal joints, propeller shaft or the drive shaft, differential and axles extending to the wheels .The application of the engine power to the driving wheels through all these parts is called POWER TRANSMISSION .The power system is usually the same on all modern passenger cars and trucks, but its arrangement may vary according to the method of drive and type of transmission units. 

PURPOSE OF TRANSMISSION: 

It enables the engine to be disconnected from the driving wheels.

It enables the running engine to be connected to driving wheel smoothly and without shock.

It enables the leverage between the engine and the driving wheels to be varied.

It enables the reduction of engine speed in the ratio of 4:1 in case of passenger cars and in greater ratio in case of Lorries.

It enables the driving wheels to be driven at different speeds.

It enables turning the driving through 90 degrees. It enables the relative movement between the engine and the driving wheel. 

CLUTCH

In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and anytime the vehicle stopped the engine would perforce stall. Without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force, and selecting a gear requires the revolution speed of the engine to be held at a very precise value which depends on the vehicle speed and desired gear. In a car the clutch is usually operated by a pedal; on a motorcycle, a lever on the left handlebar serves the purpose. 

1. When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque transferred from the engine to the transmission (and by extension to the drive wheels). In this uncoupled state it is possible to select gears or to stop the car without stopping the engine. 

2. When the clutch pedal is fully released, the clutch is fully engaged, and practically all of the engine's torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid coupling, and power is transmitted to the wheels with minimal practical waste heat. 

3. Between these extremes of engagement and disengagement the clutch slips to varying degrees. When the clutch slips it still transmits torque despite the difference in speeds between the engine crankshaft and the transmission input. Because this torque is transmitted by means of friction rather than direct mechanical contact. 

4. Considerable power is wasted as heat (which is dissipated by the clutch). Properly applied, slip allows the vehicle to be started from a standstill, and when it is already moving, allows the engine rotation to gradually adjust to a newly selected gear ratio. 

5. Learning to use the clutch efficiently requires the development of muscle memory and a level of coordination analogous to that required to learn a musical instrument or to play a sport. 

6. A rider of a highly-tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when exiting corners to assist the engine in revving to the point where it delivers the most power. 

DUAL CLUTCH TRANSMISSION

A semi-automatic transmission (also known as clutchless manual transmission, dual-clutch transmission, automated manual transmission, e-gear, shift-tronic, flappy paddle gearbox, or direct shift gearbox) is a system which uses electronic sensors, processors and actuators to do gear shifts on the command of the driver. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronise the timing and torque required to make gear shifts quick and smooth. The system was designed by European automobile manufacturers to provide a better driving experience, especially in cities where congestion frequently causes stop-and-go traffic patterns 

Elaborated form of manual transmission in which two internal shafts, each connected to the input via an electronically controlled clutch, are coordinated such as to achieve an uniterrupted flow of torque to the driven wheels during gear changes. As well as reducing acceleration times, a dual clutch transmission also enchances refinement over a convectional manual or manual gearbox. 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, and automatics, which do all of the shifting work for drivers using clutches, a torque converter and sets of planetary gears. But there's also something in between that offers the best of both worlds -- the dual-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 the sequential 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.

Dual-clutch gearbox:

M: Motor

A: Primary drive

B: Double Clutch

C: shaft

D: main shaft, even gears

E: main shaft, odd gears

F: Output

OPERATION OF DCT

In standard mass-production automobiles, the gear lever appears similar to manual shifts, except that the gear stick only moves forward and backward to shift into higher and lower gears, instead of the traditional H-pattern. The Bugatti Veyron uses this approach for its 7-speed transmission. In Formula One, the system is adapted to fit onto the steering wheel in the form of two paddles; depressing the right paddle shifts into a higher gear, while depressing the left paddle shifts into a lower one. Numerous road cars have inherited the same mechanism.

Hall Effect sensors sense the direction of requested shift, and this input, together with a sensor in the gear box which senses the current speed and gear selected, feeds into a central processing unit. This unit then determines the optimal timing and torque required for a smooth clutch engagement, based on input from these two sensors as well as other factors, such as engine rotation, the Electronic Stability Program, air conditioner and dashboard instruments.

The central processing unit powers a hydro-mechanical unit to either engage or  disengage the clutch, which is kept in close synchronization with the gear-shifting action the driver has started. The hydro-mechanical unit contains a servomotor coupled to a gear arrangement for a linear actuator, which uses brake fluid from the braking system to impel a hydraulic cylinder to move the main clutch actuator. The power of the system lies in the fact that electronic equipment can react much faster and more precisely than a human, and takes advantage of the precision of electronic signals to allow a complete clutch operation without the intervention of the driver. For the needs of parking, reversing and neutralizing the transmission, the driver must engage both paddles at once, after this has been accomplished the car will prompt for one of the three options. The clutch is really only needed to start the car. For a quicker upshift, the engine power can be cut, and the collar disengaged until the engine drops to the correct speed for the next gear. For the teeth of the collar to slide into the teeth of the rings not only the speed, but also the position must match. This needs sensors to measure not only the speed, but the positions of the teeth, and the throttle may need to opened softer or harder. The even faster shifting techniques like powershifting require a heavier gearbox or clutch or even a twin-clutch gearbox. 

BASIC DESIGN OF DUAL CLUTCH TRANSMISSION

A 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 called synchronizers match 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 from on to off to on during 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: 

 A car travelling in second gear is controlled by the inner clutch .Power is sent to second gear along the outer transmission shaft 

 As the car increases speed, the computer detects the next gearshift point and the third gear is pre-selected. 

 When the driver changes gears, the inner clutch disengages and the outer clutch is activated. 

 The power is transferred along the inner transmission shafts to the pre-selected gear. 

Drivers can also choose a fully automatic mode that relinquishes all gear-changing duties to the computer. In this mode, the driving experience is very similar to that delivered by a conventional automatic. Because a DCT transmission can "phase out" one gear and "phase in" a second gear, shift shock is reduced. More importantly, the gear change takes place under load so that a permanent flow of power is maintained. An ingenious two-shaft construction separating the odd and even gears makes all of this possible.  

DUAL CLUTCH TRANSMISSION SHAFTS

A two-part transmission shaft is at the heart of a DCT. Unlike a conventional manual gearbox, this houses all of its gears on a single input shaft, the DCT splits up odd and even gears on two input shafts. 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 CLUTCH

Since a dual-clutch transmission is similar to an automatic, one might think that it requires a torque 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 with splines on 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. 

ADVANTAGES

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. It doesn't have 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 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. 

Audi TT Roadster

One of several Audi models available with a dual-shift transmission 

¢ 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. 

DISADVANTAGES

¢ 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 the continuously 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.

 

This is how it works:

Selector lever locked in "P":

When the selector lever is in the "P" position, the locking pin is in the locking pin hole for "P". This way, the selector lever is stopped from being moved inadvertently out of position.

Selector lever released:

When the ignition is switched on and the brake pedal is pressed, the selector lever sensors

control unit J587 energises the solenoid N110. In this way, the locking pin is pulled out of the

locking pin hole "P". The selector lever can now be moved into the drive position.

Emergency release:

If the power supply fails to the selector lever lock solenoid N110, the selector lever can no longer

be moved because selector lever lock "P" remains activated in the event of power failure.

By pressing in the locking pin mechanically using a thin object, the lock can be released and the

selector lever can be moved out of the "N" position for emergency purposes. The vehicle can then be driven again.

Ignition key withdrawal lock:

The ignition key withdrawal lock prevents the ignition key from being turned to the withdrawal

position unless the parking lock is engaged. It works on an electro-mechanical principle and

is actuated by the steering column electronics control unit J527.

Selector lever in "park position", ignition switched off. When the selector lever is in the park position, the "selector lever locked in position P" F319 is open. The steering column electronics control unit J527 detects the open switch.

The ignition key withdrawal lock solenoid N376 is not supplied with power.

The spring in the solenoid pushes the locking pin to the release position.

"Selector lever in drive position", ignition on. In the drive position, the "selector lever locked in

position P" switch F319 is closed. The steering column electronics control unit then

energises the ignition key withdrawal locksolenoid N376. The locking pin is pushed into the lock position against spring pressure by the solenoid. In the lock position, the locking pin prevents the ignition key from being turned back and withdrawn. Not until the selector lever is placed in the park position is the "selector lever locked in position P" switch opened. The control unit then isolates the power supply to the solenoid. The locking pin is then pushed back by the spring. The ignition key can be turned further and withdrawn.

CONSTRUCTION OF DSG

Basic principle

The direct shift gearbox comprises in essence of two transmission units that are independent of

each other. Each transmission unit is constructed in the same way as a manual gearbox. Allocated to each transmission unit is a multi-plate clutch. Both multi-plate clutches are of the wet type and work in DSG oil. They are regulated, opened and closed by the mechatronics system, depending on the gear to be selected. 1st, 3rd, 5th and reverse gear are selected via multi-plate clutch K1. 2nd, 4th and 6th gear are selected via multiplate clutch K2. One transmission unit is always in gear and the other transmission unit has the next gear selected

in preparation but with the clutch still in the open position. Every gear is allocated a conventional manual gearbox synchronization and selector element.

Torque input

The torque is transmitted from the crankshaft to the dual mass flywheel. The splines of the dual mass flywheel on the input hub of the double clutch transmit the torque to the drive plate of the multi-plate clutch. This is joined to the outer plate carrier of clutch K1 with the main hub of the multi-plate clutch. The outer plate carrier of clutch K2 is also positively joined to the main hub. In this way, plunger 1 is pushed along its axis and the plates of clutch K1 are pressed together. Torque is transmitted via the plates of the inner plate carrier to input shaft 1. When the clutch opens, a diaphragm spring pushes plunger 1 back to its start position.

Multi-plate clutch K1

Clutch K1 is of the multi-plate type. It is the outer clutch and transmits torque to input shaft 1 for 1st, 3rd, 5th and reverse gear. To close the clutch, oil is forced into the oil pressure chamber of clutch K1. The torque is transmitted to the relevant clutch through the outer plate carrier. When the clutch closes, the torque is transmitted further to the inner plate carrier and then to the relevant input shaft. One multi-plate clutch is always engaged.

Multi-plate clutch K2

Clutch K2 is of the multi-plate type. It is the inner clutch and transmits torque to input shaft 2 for

2nd, 4th and 6th gear. To close the clutch, oil is forced into the oil pressure chamber of clutch K2. Plunger K2 then joins the drive via the plates to input shaft 2. The coil springs press plunger 2 back to its start position when the clutch is opened.

Input shafts

The engine torque is transmitted to the input shafts from multi-plate clutches K1 and K2. Input shaft 2 is shown in relation to the installation position of input shaft 1. Input shaft 2 is a hollow construction and is joined via splines to multi-plate clutch K2. The helical gear wheels for 6th, 4th and 2^nd gear can be found on input shaft 2. For 6th and 4th gear, a common gear wheel is used. To measure the speed of this input shaft there is a pulse wheel for input shaft 2 speed sender G502 adjacent to the gear wheel for 2nd gear.

Input shaft 1 rotates inside input shaft 2, which is hollow. It is joined to multi-plate clutch K1 via splines. Located on input shaft 1 are the helical gear wheels for 5th gear, the common gear wheel for 1st and reverse gear and the gear wheel for 3rd gear. To measure the speed of this input shaft there is a pulse wheel for input shaft 1 speed sender G501 between the gear wheels for 1st/reverse gear and 3rd gear.

Located on input shaft 1 are – the three-fold synchronised selector gears for 1st, 2nd, 3rd gears,– the single synchronised selector gear for 4th gear and – the output shaft gear for meshing into the differential. The output shaft meshes into the final drive gear wheel of the differential.

Output shafts

In line with the two input shafts, the direct shift gearbox also features two output shafts. Thanks to the common use of gear wheels for 1st and reverse gear and 4th and 6th gear on the input shafts, it was possible to reduce the length of the gearbox. Output shaft 1 located on input shaft 2 are – the pulse wheel for gearbox output speed – the selector gears for 5th, 6th and reverse gears and – the output shaft gear for meshing into the differential. Both output shafts transmit the torque further to the differential via their output shaft gears.

Reverse shaft

The reverse shaft changes the direction of rotation of output shaft 2 and thereby also the direction of rotation of the final drive in the differential. It engages in the common gear wheel for 1st gear and reverse gear on input shaft 1 and the selector gear for reverse gear on output shaft 2. Both output shafts transmit the torque to the input shaft of the differential. The differential transmits the torque via the drive shafts to the road wheels. Integrated in the differential is the parking lock gear. Engagement of the locking pawl is purely by mechanical means via a cable between the selector lever and the parking brake lever on the gearbox. The cable is used exclusively to actuate the parking lock.

Parking lock

A parking brake is integrated in the differential to secure the vehicle in the parked position and to prevent the vehicle from creeping forwards or backwards unintentionally when the handbrake is not applied. When the selector lever is moved to the "P" position, the parking lock is engaged. To do this, a locking pawl engages in the teeth of the parking lock gear. The locking spring engages in the lever and holds the pawl in position. When the pawl engages in one of the teeth of  the parking lock gear, spring 1 is tensioned. If the vehicle begins to move, the pawl is pushed into the next gap on the parking lock gear by spring 1 as it releases its tension. When the selector lever is moved out of position "P", the parking lock is deactivated. The slide is pushed to the right back into its start position and spring 2 pushes the locking pawl out of the gap in the parking lock gear. Balancing of the large speed differences between different selector gears is faster in the low gears. Less effort is required to engage the gears. 4th, 5th and 6th gears are equipped with a simple cone system. The speed differences here are not as great when gears are selected. The balancing of speed is therefore faster. Little effort is required for synchronization. Reverse gear is equipped with dual cone synchronization.

Synchronization

To engage a gear, the locking collar must be pushed onto the selector teeth of the selector gear. The task of synchronization is to balance the speed between the engaging gear wheels and the locking collar. Molybdenum coated brass synchro-rings form the basis of synchronization. 1st, 2nd and 3rd gears are equipped with threefold synchronization. Compared with a simple cone system, a considerably larger friction area is provided. Synchronization efficiency is increased as there is a greater surface area to transfer heat. Three-fold synchronization comprises of

– an outer ring (synchro-ring)

– an intermediate ring

– an inner ring (2nd synchro-ring) and

– a friction cone on the selector gear/gear wheel.

Simple synchronization comprises of

– a synchro-ring and

– a friction cone on the selector gear/gear wheel.

Torque transmission in the vehicle

The engine torque is transmitted via the dual mass flywheel to the direct shift gearbox. On front-wheel drive vehicles, the drive shafts transmit the torque to the front road wheels. On four-wheel drive vehicles, the torque is also transmitted to the rear axle via a bevel box. A propshaft transmits the torque to a Haldex coupling. Integrated in this rear final drive is a differential for the rear axle.

Transmission route through gears

The torque in the gearbox is transmitted either via the outer clutch K1 or the inner clutch K2. Each clutch drives an input shaft. Input shaft 1 (inner) is driven by clutch K1 and input shaft 2 (outer) is driven by clutch K2. Power is transmitted further to the differential via

– output shaft 1 for 1st, 2nd, 3rd, 4th gears and

– output shaft 2 for 5th, 6th and reverse gears.

For reasons of clarity, the flow of power is shown stretched in the diagram. The change in direction of rotation for reverse gear is carried out via the reverse shaft.

Mechatronics module

The mechatronics are housed in the gearbox, surrounded by DSG oil. They comprise of an electronic control unit and an electro-hydraulic control unit. The mechatronics form the central control unit in the gearbox. All sensor signals and all signals from other control units come together at this point and all actions are initiated and monitored from here. Housed in this compact unit are twelve sensors. Only two sensors are located outside the mechatronics system. By hydraulic means, it controls or regulates eight gear actuators via six pressure modulation valves and five selector valves and it also controls the pressure and flow of cooling oil from both clutches. The mechatronics control unit learns (adapts) the position of the clutches, the positions of the gear actuators when a gear is engaged and the main pressure.

The advantages of this compact unit are:

– The majority of sensors are integrated within.

– The electric actuators are located directly on the mechatronics.

– The electrical interfaces required on the vehicle side are joined at one central connector.

As a result of these measures, the number of connectors and amount of wiring has been reduced. That means there is greater electrical efficiency and lower weight. It also means that a high degree of thermal and mechanical stress is placed on the control unit. Temperatures of –40 °C to +150 °C and mechanical vibrations of up to 33 g should not be allowed to impair the operability of the vehicle. g = acceleration of gravity, acceleration of an object influenced by the gravitational pull of the earth towards the earth's core

Operational introduction

The internal combustion engine drives two clutch packs. The outer clutch pack drives gears 1, 3, 5 (and 7 when fitted), and reverse — the outer clutch pack has a larger diameter compared to the inner clutch, and can therefore handle greater torque loadings. The inner clutch pack drives gears 2, 4, and 6. Instead of a standard large dry single-plate clutch, each clutch pack for the six-speed DSG is a collection of four small wet interleaved clutch plates (similar to a motorcycle wet multi-plate clutch). Due to space constraints, the two clutch assemblies are concentric, and the shafts within the gearbox are hollow and also concentric. Because the alternate clutch pack's gear-sets can be pre-selected (predictive shifts enabled via the 'unused' section of the gearbox), un-powered time while shifting is avoided because the transmission of torque is simply switched from one clutch-pack to the other. This means that the DSG takes only about 8 milliseconds to upshift. In comparison, the sequential manual transmission (SMT) in the Ferrari F430 Scuderia takes 60 milliseconds to shift, or 150 milliseconds in the Ferrari Enzo. The quoted time for upshifts is the time the wheels are completely non-powered.

DSG controls

The Direct-Shift Gearbox utilises a floor-mounted transmission shift lever, very similar to that of a conventional automatic transmission.The lever is operated in a straight 'fore and aft' plane (without any 'dog-leg' offset movements), and utilises an additional button to help prevent an inadvertent selection of an inappropriate shift lever position.

"P"

P position of the floor-mounted gear shift lever means that the transmission is set in "Park". Both clutch packs are fully disengaged, all gear-sets are disengaged, and a solid mechanical transmission 'lock' is applied to the crown wheel of the DSG's internal differential. This position must only be used when the motor vehicle is stationary. Furthermore, this is the position which must be set on the shift lever before the vehicle ignition key can be removed.

"N"

N position of the floor-mounted shift lever means that the transmission is in "neutral". Similar to P above, both clutch packs and all gear-sets are fully disengaged, however the parking lock is disengaged. This position should be used when the motor vehicle is stationary for a period of time, such as at red traffic lights, or waiting in a queue of stationary traffic. The DSG should not be held in any of the active gear modes while stationary using the footbrake for other than brief periods — due to the clutches being held on the bite point, as this can overheat the clutches and transmission fluid. This position also allows the engine to be restarted (in some cars needing the key to be partially disengaged) which cannot be done in any of the active modes.

"D" mode

Whilst the motor vehicle is stationary and in neutral (N), the driver can select D for "drive" (after first pressing the foot brake

pedal). The transmission's first gear is selected on the first shaft, and the outer clutch engages at the start of the 'bite point'. At the same time, on the alternate gear shaft, the second gear is also selected (pre-selected), but the clutch pack for second gear remains fully disengaged. When the driver releases the foot brake pedal, the outer clutch pack increases the clamping force, allowing the first gear to take up the drive through an increase of the 'bite point', and therefore transferring the torque from the engine through the transmission to the driveshafts and roadwheels — and the vehicle moves forward. Pressing the throttle / accelerator pedal will fully engage the clutch, and causes an increase of forward vehicle speed. As the vehicle accelerates, the transmission's computer determines when the second gear (which is connected to the second clutch) should be fully utilised. Depending on the vehicle speed, and amount of engine power being requested by the driver (full throttle, or part-throttle normal driving), the DSG then upshifts. During this sequence, the DSG disengages the first outer clutch whilst simultaneously engaging the second inner clutch (all power from the engine is now going through the second shaft), thus completing the shift sequence. This sequence happens in 8 milliseconds (aided by pre-selection), and can happen even with full throttle opening, and as a result, there is virtually no power loss.

Once the vehicle has completed the shift to second gear, the first gear is immediately de-selected, and third gear (being on the same shaft as 1st and 5th) is pre-selected, and is pending. Once the time comes to shift into 3rd, the second clutch disengages and the first clutch re-engages. This method of operation continues in the same manner up to 6th (or top) gear.

Downshifting is similar to upshifting but in reverse order, and is slower, at 600 milliseconds, due to the engine's Electronic Control Unit, or ECU, needing to 'blip' the throttle, so that the engine crankshaft speed can match the appropriate gear shaft speed. The car's computer senses the car slowing down, or more power required (during acceleration), and thus engages a lower gear on the shaft not in use, and then completes the downshift.

The actual shift points are determined by the DSG's transmission ECU, which commands a hydro-mechanical unit. The transmission ECU, combined with the hydro-mechanical unit, are collectively called a "mechatronics"unit or module. Because the DSG's ECU uses "fuzzy logic", the operation of the DSG is said to be "adaptive"; that is, the DSG will "learn" how the user drives the car, and will progressively tailor the shift points accordingly to suit the habits of the driver.

In the vehicle instrument display, between the speedometer and tachometer, the available shift-lever positions are shown, the current position of the shift-lever is highlighted (emboldend), and the current gear ratio in use is also displayed as a number.

Under "normal", progressive and linear acceleration and deceleration, the DSG shifts in a "sequential" manner, i.e. under acceleration: 1st > 2nd > 3rd > 4th > 5th > 6th; and the same sequence reversed for deceleration. However, the DSG can also skip the normal sequential method, by 'missing out' adjacent gears, and shift two or more gears. This is most apparent if the car is being driven at sedate speeds in one of the higher gears with a light throttle opening, and the accelerator pedal is then pressed down, engaging the "kick-down" function. During kick-down, the DSG will skip gears, shifting directly to the most appropriate gear depending on speed and throttle opening. (This kick-down may be engaged by any increased accelerator pedal opening, and is completely independent of the additional resistance to be found when the pedal is pressed fully to the floor, which will activate a similar kick-down function when in Manual operation mode).

When the floor-mounted gear selector lever is in position D, the DSG works in fully automatic mode, with emphasis placed on gear shifts programmed to deliver maximum fuel economy. That means that shifts will change up and down very early in the rev-range. As an example, on the Volkswagen Golf Mk5 GTI, sixth gear will be engaged around 52 km/h (32 mph), when initially using the DSG transmission with the 'default' ECU adaptation - although with an "aggressive" or "sporty" driving style, the adaptive shift pattern will increase the vehicle speed at which sixth gear engages.

"S" mode

The floor selector lever also has an S position. When S is selected, "sport" modeis activated in the DSG. Sport mode still functions as a fully automatic mode,  identical in operation to "D" mode, but upshifts and downshifts are made much higher up the engine rev-range. This aids a more sporty driving manner, by utilising considerably more of the available engine power, and also maximising engine braking. However, this mode does have a detrimental effect on the vehicle fuel consumption, when compared to D mode. This mode may not be ideal to use when wanting to drive in a 'sedate' manner; nor when road conditions are very slippery, due to ice, snow or torrential rain — because loss of tyre traction may be experienced (wheel spin during acceleration, and may also result in roadwheel locking during downshifts at high engine rpms under closed throttle). On 4motion or quattro-equipped vehicles this may be partially offset by the drivetrain maintaining full-time engagement of the rear differential in 'S' mode, so power distribution under loss of front-wheel traction may be marginally improved. On the six-speed unit in the 2010 Volkswagen GTI, "S" mode will not automatically shift to 6th gear... maxing out at 5th to keep power available at high RPM while cruising.

S is highlighted in the instrument display, and like D mode, the currently used gear ratio is also displayed as a number.

"R" Mode

R position of the floor-mounted shift lever means that the transmission is in "reverse". This functions in a similar way to D, but there is just one 'reverse gear'. When selected, R is highlighted in the instrument display.

Manual mode

Additionally, the floor shift lever also has another plane of operation, for manual mode, with spring-loaded "+" and "−" positions. This plane is selected by moving the stick away from the driver (in vehicles with the driver's seat on the right, the lever is pushed to the left, and in left-hand drive cars, the stick is pushed to the right) when in "D" mode only. When this plane is selected, the DSG can now be controlled like a manual gearbox, albeit only under a sequential shift pattern.

In most (VW) applications, the readout in the instrument display changes to 6 5 4 3 2 1, and just like the automatic modes, the currently used gear ratio is highlighted or emboldened. In other versions (e.g. on the Audi TT) the display shows just M followed by the gear currently selected, e.g. M1, M2 etc.

To change up a gear, the lever is pushed forward (against a spring pressure) towards the "+", and to change down, the lever is pulled rearward towards the "−". The DSG transmission can now be operated with the gear changes being (primarily) determined by the driver. This method of operation is commonly called "tiptronic". In the interests of engine preservation, when accelerating in Manual/tiptronic mode, the DSG will still automatically change up just before the redline, and when decelerating, it will change down automatically at very low revs, just before the engine idle speed (tickover). Furthermore, if the driver calls for a gear when it is not appropriate (e.g.: requesting a downshift when engine speed is near the redline) the DSG will not change to the driver's requested gear.

Current variants of the DSG will still downshift to the lowest possible gear ratio when the kick-down button is activated during full throttle whilst in manual mode. In Manual mode this kick-down is only activated by an additional button at the bottom of the accelerator pedal travel; unless this is pressed the DSG will not downshift, and will simply perform a full-throttle acceleration in whatever gear was previously being utilised.

Paddle  shifters

Initially available on certain high-powered cars, and those with a "sporty" trim level — such as those using the 2.0 TFSI and 3.2/3.6 VR6 engines— steering wheel-mounted paddle shifterswere available. However, these are now being offered (either as a standard inclusive fitment, or as a factory optional extra) on virtually all DSG-equipped cars, throughout all model ranges, including lesser power output applications, such as the 105 PS Volkswagen Golf Plus.

These operate in an identical manner as the floor mounted shift lever when it is placed across the gate in manual mode. The paddle shifters have two distinct advantages: the driver can safely keep both hands on the steering wheel when using the Manual/tiptronic mode; and the driver can immediately manually override either of the automatic programmes (D or S) on a temporary basis, and gain instant manual control of the DSG transmission (within the above described constraints).

If the paddle-shift activated manual override of one of the automatic modes (D or S) is utilised intermittently, the DSG transmission will "default" back to the previously selected automatic mode after a predetermined duration of inactivity of the paddles, or when the vehicle becomes stationary. Alternatively, should the driver wish to immediately revert to fully automatic control, this can be done by activating and holding the "+" paddle for at least two seconds.

ADVANTAGES AND DISADVANTAGES

Advantages

• Better fuel economy (up to 15% improvement) than conventional planetary geared automatic transmission (due to lower parasitic losses from oil churning) and for some models with manual transmissions;
• No loss of torque transmission from the engine to the driving wheels during gear shifts;
• Short up-shift time of 8 milliseconds when shifting to a gear the alternate gear shaft has preselected;
• Smooth gear-shift operations;
• Consistent shift time of 600 milliseconds, regardless of throttle or operational mode;

Disadvantages

• Achieving maximum acceleration or hill climbing, while avoiding engine speeds higher than a certain limit (e.g. 3000 or 4000 RPM), is difficult since it requires avoiding triggering the kick-down-switch. Avoiding triggering the kick-down-switch requires a good feel of the throttle pedal, but use of full throttle can still be achieved with a little sensitivity as the kick-down button is only activated beyond the normal full opening of the accelerator pedal.
• Marginally worse overall mechanical efficiency compared to a conventional manual transmission, especially on wet-clutch variants (due to electronics and hydraulic systems);
• Expensive specialist transmission fluids/lubricants with dedicated additives are required, which need regular changes;
• Relatively expensive to manufacture, and therefore increases new vehicle purchase price;
• Relatively lengthy shift time when shifting to a gear ratio which the transmission ECU did not anticipate (around 1100 ms, depending on the situation)
• Torque handling capability constraints perceive a limit on after-market engine tuning modifications (though many tuners and users have now greatly exceeded the official torque limits. Later variants have been fitted to more powerful cars, such as the 300bhp/350Nm VW R36 and the 272 bp/350 Nm Audi TTS.
• Heavier than a comparable Getrag conventional manual transmission (75 kg (170 lb) vs. 47.5 kg (105 lb));
• Mechatronic units in earlier models are prone to problems and requires replacement units

 Problems and Recall of DSG-equipped vehicles

In August 2009, Volkswagen of America issued two recalls of DSG-equipped vehicles. The first involved 13,500 vehicles, and was to address unplanned shifts to the neutral gear, while the second involved similar problems (by then attributed to faulty temperature sensors) and applied to 53,300 vehicles. These recalls arose as a result of investigations carried out by the US National Highway Traffic Safety Administration (NHTSA)where owners reported to the NHTSA a loss of power whilst driving. This investigation preliminary found only 2008 and 2009 model year vehicles as being affected. Other markets, such as Volkswagen group Australia, are yet to admit this being a wide spread issue and refuse to offer similar recall programs as their US counter parts, even though multiple reports of similar incidents and failures have occurred. 

CONCLUSIONS

New environmental and fuel efficiency legislation coupled with advances in electronics and manufacturing techniques have triggered new automated transmission technologies. The most likely winner that will replace traditional automatics and boost market penetration of automated transmissions will be the direct shift gearbox (DSG).