Dual Clutch/Direct Shift Gearbox (DSG)

Dual Clutch Transmission (DCT) was invented by Frenchman Adolphe Kégresse just prior to World War II, but he never developed a working model. The first actual DCTs arrived from Porsche in-house development, for Porsche racing cars in the 1980s. The first series production road car to be fitted with a DCT was the Direct-Shift Gearbox (DSG) in the 2003 Volkswagen Golf Mk4 R32.

Sectional view of the Volkswagen Group dual clutch Direct-Shift Gearbox (DSG) transmission
IMG Ref: http://en.wikipedia.org/wiki/File:VW_DSG_transmission_DTMB.jpg

The revolutionary direct shift gearbox (DSG) combines the advantages of a conventional six-speed manual-shift gearbox with the qualities possessed by a modern automatic transmission. The driver enjoys immense agility and driving pleasure with, at the same time, smooth, dynamic acceleration with no interruption to the power flow.

The technical basis of the direct shift gearbox (DSG) is a double clutch. This consists of two wet plate-type clutches with hydraulically regulated contact pressure. One of the two clutches engages the odd-numbered, the other the even-numbered gears. This principle enables gear shifts to be made without interrupting the power flow and keeps the shift times extremely short. While the first clutch is transmitting the power, the second clutch is ready to engage the next gear, which is preselected. When the driver makes the gear shift, the first clutch is released and the second engages, so that the gear shift takes place in a fraction of a second.

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 multi-plate clutches, similar to the clutches used in most motorcycles, though dry clutch versions are also available.

Clutch Types

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

Clutch Installation

There are now three variations of clutch installation. 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 utilized two single-plate dry clutches which are side-by-side from the perpendicular view, but again sharing the centre line of the crankshaft.

A latest 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 via a gear from the engine crankshaft.

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The driver can operate the DSG manually or allow changes to take place automatically. In the automatic mode there is a choice between the well-balanced, comfortable standard shift settings and a program with greater sports emphasis. Manual shifts are made either at the gear lever or at shift paddles behind the steering wheel.

Reference:
http://en.wikipedia.org
Audi Glossary

Adaptive Air Suspension

Adaptive air suspension is an electronically controlled air suspension system at all four wheels with a continuously adaptive damping system. It unites sporty handling and a high level of ride comfort. In addition, the air suspension allows the speed-dependent lowering of the body – this change in ride height means a low centre of gravity and significantly increased directional stability as a result. The vehicle’s aerodynamics are improved at the same time.

The vehicle has air suspension struts on all four wheels. The data from sensors on the axles and acceleration sensors on the body is evaluated in the adaptive air suspension's central control unit. This computer controls the adjustment of the individual shock absorbers in milliseconds depending on driving situation. Provided no higher damping forces are required – for instance when driving straight ahead on good roads – the damper settings remain comfortably soft.

Controlled changes to the damping force at individual wheels help to eliminate body movements at any time which could reduce occupant comfort. The adaptive damping system automatically reduces rolling or pitching movements when cornering, braking or driving off. Adaptive air suspension moreover offers the advantages of a traditional self-levelling suspension system. The vehicle's suspension height remains constant irrespective of the load it is carrying.

The adaptive air suspension generally also allows the driver to influence the suspension characteristic – and thus the operating dynamics – as individually preferred. The damping characteristics and ride height can be adjusted in a single process via the MMI "CAR" menu system. Adaptive air suspension (sport), which is available as an option in the Audi A8, is the first Audi sports suspension system to be based on the principle of air suspension.

Adaptive Air Suspension Basic Principle

Adaptive air suspension operation

IMG Ref: http://moodle.student.cnwl.ac.uk/

When the ignition is switched on, or when the vehicle’s door is opened before ignition, the control system is activated. The height sensor uses the induction principle to constantly monitor the distance between the vehicle’s axle and its chassis.

When the vehicle is being loaded, unloaded, or lowered due to driver command or vehicle speed, the electronic readings from the height sensor monitor the change. This is picked up by the electronic control unit and compared to the stored reference values.

The ECU either activates the electric motor of the compressor, or the exhaust solenoid valve. This also requires the solenoid valve block to be actuated, in order to maintain the required level. The corner solenoid valves are subject to stringent leakage requirements to maintain the vehicle’s height even without system operation.

When the vehicle is being loaded, the compressor delivers air into the four air suspension bellows, until the normal level has once again been reached. For additional air delivery or rapid response, the reservoir solenoid valve is opened and air flows directly from the reservoir.

When the vehicle is being unloaded, the solenoid valve block is activated. This results in airflow from the air suspension bellows being removed via the air dryer solenoid valve in the compressor, then via the relay valve. The air is then exhausted into the atmosphere.

Reference:
http://moodle.student.cnwl.ac.uk/
Audi Glossary

Locking Differential

A locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds).

Locking Differential
Image courtesy Eaton Automotive Group's Torque Control Products Division

A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.

All the above apply to central differentials as well as to those in each axle: full-time four-wheel-drive vehicles have three differentials, one in each axle, and a central one between the front and rear axles.

There are two main types of lockers: automatic and selectable.

Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking differential designs ensure that engine power is always transmitted to both wheels, regardless of traction conditions, and will "unlock" only when one wheel is required to spin faster than the other during cornering.

• Pros: Automatic action, no driver interaction necessary, no stopping for (dis-) engagement necessary
• Cons: Intensified tire wear, noticeable impact on driving behaviour (most people often tend to understeer)
  

A "selectable" locker allows the driver to lock and unlock the differential at will from the driver's seat. This can be accomplished via compressed air (pneumatics) like ARB's "Air Locker" or vacuum, electronic solenoids (electromagnetics) like Eaton's "ELocker" and Nissan Corporations electric locker found as optional equipment on the Frontier (Navarra) & Xterra, or some type of cable operated mechanism as is employed on the "Ox Locker."

• Pros: Allows the differential to perform as an "open" differential for improved driveability, maneuverability, provides full locking capability when it is desirable or needed
• Cons: Mechanically complex with more parts to fail. Some lockers require vehicle to stop for engagement. Needs human interaction and forward-thinking regarding upcoming terrain. Un-skilled drivers often put massive stress on driveline components when leaving the differential in locked operation on terrain not requiring a locker.

HUMMER H3 Electronic Locking Rear Differential
IMG Ref: http://www.2405.com/press-library/Hummer-H3-2006/HUMMER-H3-Electronic-Locking-Rear-Differential.asp

GMC_09 Canyon automatic locking differential
IMG Ref: http://www.gmc.com/canyon/canyon/index.jsp

Applications

• Race cars often use locking differentials in order to maintain traction during high speed maneuvers or when accelerating at extreme rates.
• Some utility vehicles such as tow trucks, forklifts, tractors, and heavy equipment use locking differentials to maintain traction, especially when driving on soft, muddy, or uneven surfaces.
• Lockers are common in agricultural equipment and military trucks.
• Locking differentials are considered essential equipment for serious off-road driving Four-wheel drive vehicles.
• Differential locks are also used on some non-utility four-wheel-drive vehicles to compensate for a relative lack of axle articulation (vertical wheel movement).
• Used in the sport of drifting as an alternative to a limited slip differential.

Reference:
http://en.wikipedia.org

Electronic Differential Lock (EDL)

Limited slip differentials are considered a compromise between a standard differential and a locking differential because they operate more smoothly, and they do direct some extra torque to the wheel with the most traction compared to a standard differential, but they are not capable of 100% lockup.

Traction control systems are also used in many modern vehicles either in addition or as a replacement of locking differentials. One example is that offered by Volkswagen under the name of electronic differential lock (EDL).

This Electronic Differential Lock is a limited slip differential imitation which is an electronic system that detects wheel spin via ABS sensors and applies brakes to spinning wheels. This results in the torque being transferred via open differentials to another wheel that has more traction.

For example, the system brakes the wheel that is raised in the air and spinning freely, and engine power goes to another wheel that is on the ground.

Reference:
www.awdwiki.com

Audi quattro® permanent all wheel drive

IMG Ref: www.audi.com (Click on the picture for a high resolution image)

Permanent all-wheel drive offers an unusually high level of active safety. In terms of tractive force, acceleration and hill-climbing ability on a poor surface it is unbeatable. By distributing the power input from the engine between two axles, higher lateral locating forces can be absorbed when cornering. This enhances lateral acceleration and at the same time ensures the highest possible level of safety.

Asymmetric/dynamic torque distribution, with a rear-biased split, allows the exceptional driving forces produced by powerful engines to reach the road even more efficiently. It reacts to conditions on the road more responsively. And with more agility on tight bends, it delivers a more exhilarating performance than ever.

Better Traction

State 1: Ideal driving conditions

If more traction is required – when towing a trailer, for example – quattro offers real advantages by offering greater tractive force in proportion to the vehicle’s weight.

State 2: Only 50 percent grip

In conditions where tyres experience reduced grip – as on a wet road – the advantages of quattro immediately become apparent. Whenever one wheel loses traction the others can compensate, so the car remains stable and continues to grip the road.

State 3: Only the front wheels have grip

quattro continually adjusts to road conditions to permanently distribute power between the front and rear wheels precisely where and when required. It means the vehicle stays responsive even if only one axle has enough grip. By contrast, if front- or rear-wheel drive vehicles lose grip at the driven axle, they can no longer transmit the engine’s power onto the road.

The self locking centre differential

The self locking centre differential sits at the heart of quattro on models with the engine positioned lengthwise along the car’s centreline.

Operating entirely mechanically, it continually reacts to road conditions and responds to any differences in the rotational speeds between the wheels. This ensures more power is always transmitted to the wheels with a better grip.

In addition, the Electronic Differential Lock (EDL) can act when needed to prevent the wheels from spinning. Excess power at one wheel is diverted to the other wheels that have more grip, maintaining traction in virtually every situation.

IMG Ref: http://www.automobilesreview.com/uploads/2009/05/audi-quattro.jpg

quattro for cars with transverse engines

To ensure the optimal distribution of engine power for each model, Audi uses specially configured all-wheel drive systems that vary in design.

The Haldex clutch is an electronically-controlled multi-plate clutch. It performs the function of the Torsen centre differential in cars with transverse engines, such as the Audi A3, A3 Sportback and Audi TT.

It ensures that engine power is permanently distributed between the front and rear wheels as and when required.

The Haldex clutch works by reacting to differences in the rotating speed between the front and rear wheels. This causes variations in the system’s hydraulic pressure, which in turn compress the clutch plates together to balance the distribution of power between the front and rear wheels. So if the front wheels begin to lose traction, the Haldex clutch channels power to the rear. And the greater the difference in rotational speed, the higher the pressure applied to the plates – which means that more engine power can be transmitted to the rear wheel.

There are six generations (unofficial) of quattro Evolutions.

The latest one is the 6th generation of quattro in the 2010 RS5. The key change in generation VI is the replacement of the Torsen Type "C" centre differential with an Audi-developed "Crown Gear" differential. The net result of this advance in quattro is the ability of the vehicle electronics to fully manage the vehicle dynamics in all traction situations, whether in cornering, acceleration or braking or in any combination of these.

Self-locking crown-gear centre differential

IMG Ref: www.awdwiki.com

Reference:
http://www.audi.co.uk/home.html
http://www.awdwiki.com/quattro.html

Audi S tronic Gearbox

IMG Ref: www.audi.com (Click on the picture for a larger image)

Faster gearshifts, sportier driving: the S tronic combines the sporty characteristics of a manual gearbox with the advantages of an automatic. Gear-changes with the dual-clutch gearbox are performed easily and with no appreciable delay. Depending on the driver’s preference, the gears may be shifted in fully automatic mode or in manual mode using the gearshift paddles on the steering wheel.

This new S tronic transmission can also handle up to 550 Nm of torque, meaning it can be coupled to engines such as the 3.0 V6 TDI or the 4.2 V8 engine of the S5. Or the 4.2 V8 of the RS4, for that matter, since it can handle engines revving up to 9,000 rpm!

Double act: the dual clutch forms the technical basis of the S tronic. One of the two clutches engages the odd-numbered gears and reverse, the other engages the even-numbered gears. The benefit: a gearbox that shifts from one gear to the next in less than 0.2 seconds and with no interruption in power flow.

When in operation, one gear is always engaged and another is pre-selected: while the first clutch is transmitting power in one gear, the second clutch has the next gear selected and ready to engage as shown in the following figures.

Operating principle, acceleration in 1st gear

IMG Ref: www.audi.com (Click on the picture for a larger image)

Operating principle, acceleration in 2nd gear

IMG Ref: www.audi.com (Click on the picture for a larger image)

When the gearshift point is reached, one clutch segment opens in a flash while the other closes. The driver therefore hardly notices the gearshift process.

Highly precise management of both multidisk clutches was one of the most important development goals. This was achieved in part with a compact pressure cylinder, electronically controlled rotation speed compensation and the use of an optimized coil spring package. This package of technology provides maximum precision and comfort at startup and shifting.

The transmission is managed by the so-called mechatronic module. This module involves a compact group of control units and hydraulic control valves that is integrated on the left of the transmission when facing the direction of travel. Its control concept allows the speed of the gear shifting process to vary and extremely precise control of the power necessary for the process.

The required control pressure is provided by an efficiently operating oil pump that is located next to the mechatronic module and is driven by a gear section. The oil pump is supported by a vacuum booster for cooling the twin clutch during starting. This allows the amount of oil pumped to be roughly doubled as needed without increasing power.

A unique feature of the seven-speed S tronic is its two separate oil systems. While the twin clutch, mechatronic module and oil pump are supplied by their own oil circuit with seven liters of automatic transmission fluid (ATF) oil, the wheel sets and the central and front-axle differential are lubricated with about 4.5 liters of hypoid gear oil. This separation allowed the development engineers to position all of the components ideally, without being forced to compromise by using a single lubricant.

The consistently efficient power transmission ideally supports a sporty driving style – both in automatic mode and with manual shifting:

• The flexible automatic mode allows the driver to vary the gearshift behaviour between the sporty character of the shift programme S (Sport) and the comfort-oriented shift programme D (Drive).
• If the driver prefers manual gearshift, the gears can be changed in sporty style with the gearshift paddles on the steering wheel or with the selector lever.

Other advantages of S tronic: the gearbox’s high efficiency leads to low fuel consumption and reduced emissions.

Reference:
http://www.audiusa.com/us/brand/en/tools/advice/glossary/s_tronic.browser.html
http://www.eurocarblog.com/post/677/the-new-audi-s-tronic-7-speed-gearbox