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.
1.2 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.
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 is 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.
2.0 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.
2.1 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.
2.2 BASIC DESIGN OF DUAL CLUTCH TRNSMISSION
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:
Â¢ A car travelling in second gear is controlled by the inner clutch .Power is sent to second gear along the outer transmission shaft
Â¢ As the car increases speed, the computer detects the next gearshift point and the third gear is pre-selected.
Â¢ When the driver changes gears, the inner clutch disengages and the outer clutch is activated.
Â¢ 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.
2.3 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.
2.4 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.
In principle, the DCT behaves just like a standard manual transmission:
Â¢ 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.
Â¢ 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.
Â¢ 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
Â¢ 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.
Â¢ 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.
Â¢ 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.
2.7 CONTROLLING OF DCT
A method of controlling the clutches of a dual clutch transmission during a two-gear positive downshift, wherein the first clutch drives an initial gear and the final gear and the second clutch drives an intermediate gear. The torque transfer across each clutch is controlled so that the torque output of the transmission will be linearly changed over from the first clutch to the second clutch to cause the engine to track a target engine speed profile. The method changes over the gears driven by the first clutch from the initial gear to the final gear as the engine continues to tracks the target speed. The torque transfer across each clutch is controlled so that the torque output will be linearly changed back from the second clutch to the first clutch in an inversely proportional rate to continue to cause the engine to track the target engine speed profile.
2.8 Dual-clutch Transmissions: Past, Present and Future
The man who invented the dual-clutch gearbox was a pioneer in automotive engineering. Adolphe KÃƒÂ©gresse is best known for developing the half-track, a type of vehicle equipped with endless rubber treads allowing it to drive off-road over various forms of terrain. In 1939, KÃƒÂ©gresse conceived the idea for a dual-clutch gearbox, which he hoped to use on the legendary CitroÃƒÂ«n "Traction" vehicle. Unfortunately, adverse business circumstances prevented further development.Ã‚Â¬
Both Audi and Porsche picked up on the dual-clutch concept, although its use was limited at first to racecars. The 956 and 962C racecars included the Porsche Dual Klutch, or PDK. In 1986, a Porsche 962 won the Monza 1000 Kilometer World Sports Prototype Championship race -- the first win for a car equipped with the PDK semi-automatic paddle-shifted transmission. Audi also made history in 1985 when a Sport quattro S1 rally car equipped with dual-clutch transmission won the Pikes Peak hill climb, a race up the 4,300-meter-high mountain.
Commercialization of the dual-clutch transmission, however, has not been feasible until recently. Volkswagen has been a pioneer in dual-clutch transmissions, licensing BorgWarner's DualTronic technology. European automobiles equipped with DCTs include the Volkswagen Beetle, Golf, Touran, and Jetta as well as the Audi TT and A3; the Skoda Octavia; and the Seat Altea, Toledo and Leon.
Photo courtesy VM Media Room
Volkswagon Jetta 2.0
Ford is the second major manufacturer to commit to dual-clutch transmissions, made by Ford of Europe and its 50/50 joint venture transmission manufacturer, GETRAG-Ford. It demonstrated the Powershift System, a six-speed dual-clutch transmission, at the 2005 Frankfurt International Motor Show. However, production vehicles using a first generation Powershift are approximately two years away.
Trucks and buses
Semi-automatic transmissions have also made its way into the truck and bus market in the early 2000s. Volvo offers its I-shift on its heavier trucks and buses, while ZF markets its ASTronic system for buses and coaches. These gearboxes have a place in public transport as they have been shown to significantly reduce fuel consumption.
In the UK though, semi-automatic transmission has been very popular on buses for some time, from the 1950s right through to the 1980s, an example being the well known London Routemaster, although the latter could also be driven as a fully automatic in the 3 highest gears. Leyland manufactured many buses with semi-automatic transmission, including its Leopard and Tiger coaches. Fully automatic transmission became popular with increasing numbers of continental buses being bought in the UK, and more and more British manufacturers began offering automatic options, mostly using imported gearboxes, and semi-automatic transmission lost favour. These days, very few buses with semi-automatic transmission remain in service, although many are still on the roads with private owners. Modern types of semi-automatic transmission though are becoming more common, mostly replacing manual gearboxes in coaches and small buses.
In addition to the Hondamatic system noted above, Yamaha Motor Company introduced a semi-automatic transmission on its 2007 model year FJR1300 sport-touring motorcycle in 2006. Notably, this system can be shifted either with the lever in the traditional position near the left foot, or with a switch accessible to the left hand where the clutch lever would go on traditional motorcycles.
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 dual clutch transmission (DCT).
1. Automobile engineering by R.B. Gupta
2. Automobile engineering by kirpal sing