Automatic Transmission

An automatic transmission is an automobile gearbox that can change gear ratios automatically as the car or truck moves, thus freeing the driver from having to shift gears manually. (Similar but larger devices are also used for railroad locomotives.)

Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe and much of the rest of the world. Automatic transmissions, particularly earlier ones, reduce fuel efficiency and power. Where fuel is expensive and, thus, engines generally smaller, these penalties were more burdensome. In recent years, automatic transmissions have significantly improved in their ability to support high fuel efficiency but manual transmissions are still generally more efficient. (This balance may finally shift with the introduction of practical continuously variable transmissions; see below.)

Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission. However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.

Recently manufacturers have begun to make continuously variable transmissions available. These designs can change the ratios over a range rather than between set gear ratios. Even though prototypes for CVT have been around for decades, it is just now reaching commercially practicability.

Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars. You've probably never tried driving a manual transmission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly un-driveable. So the transmission uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed.

The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission. When you take apart and look inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see:

An ingenious planetary gear set

A set of bands to lock parts of a gear set

A set of three wet-plate clutches to lock other parts of the gear set

An incredibly odd hydraulic system that controls the clutches and bands

A large gear pump to move transmission fluid around

The center of attention is the planetary gear set. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gear set do its thing. An automatic transmission contains two complete planetary gear sets folded together into one component.

Oldsmobile's 1940 models featured Hydra-Matic drive, the first mass-production fully automatic transmissions. Initially an Olds exclusive, Hydra-Matic had a fluid coupling (not a torque converter) and three planetary gearsets providing four speeds plus reverse. Hydra-Matic was subsequently adopted by Cadillac and Pontiac, and was sold to various other automakers, including Bentley, Hudson, Kaiser, Nash, and Rolls Royce. From 1950 to 1954 Lincoln cars were also available with GM Hydra-Matic. Mercedes-Benz subsequently devised a four-speed fluid coupling transmission that was similar in principle to Hydra-Matic, but did not share the same design.

The first torque converter automatic, Buick's Dynaflow, was introduced for the 1948 model year. It was followed by Chevrolet's Powerglide and Packard's Ultramatic for the 1950 model year. Each of these transmissions had only two forward speeds, relying on the torque converter for additional gear reduction.

In the early 1950s Borg-Warner developed a series of three-speed torque converter automatics for Ford Motor Company, Studebaker, and several foreign and independent makes.

Chrysler was late in developing its own true automatic, introducing the two-speed torque converter PowerFlite in 1953 and the three-speed TorqueFlite in 1956.

By the late 1960s most of the fluid-coupling four-speeds and two-speed transmissions had disappeared in favor of three-speed units with torque converters. By the early 1980s these were being supplemented and eventually replaced by overdrive-equipped transmissions providing four or more forward speeds. Many transmissions also adopted the lock-up torque converter (a mechanical clutch locking the torque converter impeller and turbine together to eliminate slip at cruising speed) to improve fuel economy.

As the engine computers became more and more capable, even more of the valve body's functionality was offloaded to them. These transmissions, introduced in the late 1980s and early 1990s, remove almost all of the control logic from the valve body, and place it in into the engine computer. (Some manufacturers use a separate computer dedicated to the transmission but sharing information with the engine management computer.) In this case, solenoids turned on and off by the computer control shift patterns and gear ratios, rather than the spring-loaded valves in the valve body. This allows for more precise control of shift points and shift quality, and (on some newer cars) also allows semi-automatic control, where the driver tells the computer when to shift. The result is an impressive combination of efficiency and smoothness. Some computers even identify the driver's style and adapt to suit it.

A hydraulic automatic transmission consists of the following parts:

Torque converter This device fits between the engine and the transmission, providing a torque boost at low speeds and during acceleration, while leaving the two loosely coupled at rest; this allows the engine and transmission to be mated full-time without the need for a clutch, consequently the car has only two pedals. The torque converter provides a small amount of continuously variable power transmission between the fixed gear ratios.

Planetary gear set The main gears in most hydraulic automatics are a compound planetary set. Gear ratios are selected by a system of brake bands and clutches, which are actuated by hydraulic servos controlled by the valve body. · Valve body This component is the control center of the system. The valve body receives pressurized fluid from a main pump connected to the transmission's input. The pressure coming from this pump is regulated and runs a network of spring-loaded valves, check balls and servo pistons. The valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as hydraulic signals from the range selector valves and the throttle valve or modulator) to control which ratio is selected on the gear set; as the car and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions. However, in many modern automatic transmissions, the valves are controlled by electro-mechanical servos which are controlled by the Engine Management System or a separate transmission controller microchip. (See History and improvements below.)

The multitude of parts, and the complex design of the valve body originally made hydraulic automatic transmissions much more complicated (and expensive) to build and repair than manual transmissions. In most cars (except US family, luxury, sport-utility vehicle, and minivan models) they have usually been extra-cost options for this reason. Mass manufacturing and decades of improvement have reduced this cost gap.

Tiptronic is a type of discrete automatic transmission developed by Porsche and used in its vehicles and those of its licensees. A Tiptronic transmission can operate just as the common type of automatic transmission, but it also allows the driver to override the automatic mode by moving the shift lever into a second (Tiptronic) shift gate and permits, for example, downshifting to increase the braking effect of the engine.

Though Tiptronic transmissions allow the driver a certain measure of discrete control, the Tiptronic design is implemented using a torque converter like other automatic transmissions. A Tiptronic is not a computer controlled clutch-manual transmission or semi-automatic transmission.

Most luxury vehicles with a Tiptronic transmission have two fully-automatic modes: One, identified as "Comfort" or similar, and another, usually called "Sport," which delays upshifts for a sportier driving style at the expense of fuel, wear, comfort, and noise. Shift quality has been improved due to better electronic controls; these electronics modify the shift points to adapt to a given operator's driving style. Some exotic marques such as Aston Martin and Ferrari offer paddle-shifters behind the steering wheel for controlling their similar transmissions. Tiptronicr is a registered trademark, and it is owned by German sports car maker Porsche, who licenses it for use by others such as Volkswagen and Audi.

Other manufacturers use Tiptronic-like systems under different names:

Acura: SportShift

BMW: Steptronic

Hyundai: Shiftronic

Infiniti: Manual Shift Mode

Jaguar: Boschr Mechatronic

Lexus: E-Shift

Lincoln: Select Shift Transmission

Mazda: Sport AT

Mercedes-Benz: TouchShift

Mitsubishi: Sportronic

Peugeot: 2Tronic

Saab : Sentronic

Smart : Softip

Volvo: Geartronic

A different type of automatic transmission is the continuously variable transmission or CVT, which can smoothly alter its gear ratio by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. Some continuously variable transmissions use a hydrostatic drive consisting of a variable displacement pump and a hydraulic motor to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, but early designs lose efficiency as engine speed increases.

A slightly different approach to CVT is the concept of toroidal CVT or IVT (from infinitely variable transmission). These concepts provide zero and reverse gear ratios.

Some current hybrid vehicles, notably those of Toyota, Lexus and Ford, have an "electronically-controlled CVT" (E-CVT). In this system, the transmission has fixed gears, but the ratio of wheel-speed to engine-speed can be continuously varied by controlling the speed of the third input to a differential using an electric motor-generator.

Some automatic transmissions have a manual valve body. These transmissions are generally limited to sports cars or race cars. A manual valve body allows the driver to shift gears at will, similar to a manual transmission, but with no clutch. This is both a blessing and a curse: a clutch offers additional control only to those drivers who are capable of using it; to most others, a clutch is simply a means of converting gasoline into heat or harmonic vibration. Some automatic transmissions modified or designed specifically for drag racing may also incorporate a transmission brake, or "trans-brake," as part of a manual valve body. Activated by electical solenoid control, a trans-brake simultaneously engages the first and reverse gears, locking the transmission and preventing the input shaft from turning. This allows the driver of the car to raise the engine RPM against the resistance of the torque converter, then launch the car by simply releasing the trans-brake switch.

Some manual valve bodies are also capable of semi-manual function. In this case the transmission can function as a standard automatic OR it may be shifted as a manual transmission.

With the advent of computer controls, it has become much simpler to build a hybrid system that allows fully automatic operation, fully manual operation, or a mix of the two. This is now a popular feature on sport cars from various manufacturers.

Semi-automatic transmission, or clutchless manual transmission, 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 needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronize 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.

In standard mass-production automobiles, the gear lever appears similar to manual shifts, except that the shift stick only moves forward and backward to shift into higher and lower gears respectively, instead of the traditional H-pattern. The Bugatti Veyron will use 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.

Historically, the first semi-automatic transmission which was marketed was the 1941 M4/Vacamatic Transmission by Chrysler. It was an early attempt at an automatic transmission that still required the use of a clutch, primarily to start and stop. Later, the Volkswagen Beetle came with an optional "Autostick", which was essentially a clutchless manual with three forward gears.

Drag Racers have their own type of clutchless manual transmissions. A Liberty is basically a manual transmission with no clutch, and is used in Pro Stock. The Lenco is a transmission also used in drag racers. A Lenco is different than a Liberty because a Lenco uses planetary gears and requires a torque converter, like an automatic. Both transmissions can be manually shifted, or use an air shifter. However, they require a clutch to use when leaving off the line for traction. The Lenco uses separate levers to shift while the Liberty uses a single shifter hooked up to several levers.

Some variants of the system, such as Citroën's Sensodrive, allow the driver to select automatic mode, in which the processor takes responsibility for gear changes. The car then drives much like a standard automatic, including features such as kickdown. Selecting a gear (using either stick or paddles) will restore manual control.

Some of the best known automatic transmission families include:

General Motors: Powerglide, Turbo-Hydramatic 350 and 400, 700R4, 200-4R (also 4L60-E, 4L80-E, 4L65-E)

Ford: Cruise-o-matic, FMX, C4, C6, AOD, E4OD, ATX, AXOD, AX4S, AX4N

Chrysler: Torqueflite 727 and 904, A500, A518, 45RFE, 545RFE

Borg-Warner

ZF Friedrichshafen AG

Voith Turbo

Aisin AW; Aisin AW is a Japanese automotive parts supplier, known for its automatic transmissions and navigation systems.

Automatic transmission families are usually based on Ravigneaux, Lepelletier, or Simpson planetary gearsets. Each uses some arrangement of one or two central sun gears, and a ring gear, with differing arrangements of planet gears that surround the sun and mesh with the ring. An exception to this is the Honda automatic transmission, which does not contain any planetary gearsets; It is essentially a robotized manual gearbox with a torque converter in the place of a conventional clutch.

Transmission problems fall into two very similar categories:
·Won't go.
·Won't go smoothly
These two groups of problems are caused by the same faults in your transmission, so whichever your car is doing, the following applies.

Is your fluid level correct?

It's important to check your transmission fluid at least twice a year. Not only can a low fluid level cause your car to shift poorly, it can eventually lead to transmission damage, and a costly repair. If your car seems to be losing fluid on a regular basis, you may have a leak.

Is your transmission leaking?Here are a few places to check for leaks:

·At the filler tube base.
·At the drain hole underneath the transmission.
·Between the transmission and the engine.
·At the selector shaft - the rod that connects your gear shift to the transmission.
·Speed sensor mounting point. This will either be your cable screwing into the transmission housing or an electronic sensor bolted to the housing.
·The radiator. If your car has a transmission cooler, it's possible that a leak will cause transmission fluid to leak into the radiator fluid. They don't mix well, so you'll see it floating around in there.

Is your filter clogged?

Your transmission's filer is vital to its performance. If you haven't replaced your filter in a while (or ever for a lot of us), be sure to do this before you start talking about rebuilds or replacements. Most transmission problems can't be fixed by the average do-it-yourselfer. There are just too many specialized tools and pieces of equipment you'll need, and buying this expensive gear just to screw up your first three tries at fixing the thing just doesn't make too much sense. Now that you're in front of the firing squad, it's time to drop some knowledge on your fix-it guy. Tell him what the car's doing. Then tell him what you found out when you inspected the transmission. If there's a leak, let him know where and how much is leaking.

Time for a rebuild?

When your transmission gets tired enough, you'll have to have it rebuilt. It's true. For some makes and models of car, it's true a little too often, but that's neither here nor there. The important thing is checking any other possible causes to your problem before you take the transmission apart, which is very expensive. If you haven't replaced your filter yet, do it! This fixes a nice percentage of transmission problems. If your filter is good, and nothing simple is out of adjustment, be prepared to drop some serious dough on the rebuild. The good news is that most shops warrantee a transmission rebuild for a nice amount of time. A small consolation as your handing over the credit card, but at least you know that it will get fixed and stay fixed.