1. INTRODUCTIONAutomatic transmission system shifts the gears without assistance from the driver.

They start the car moving in first and then shift into higher gears as the car speed

increases and engine load decreases. The shifts are produced by hydraulic pressure

acting through the transmission fluid.

The control system takes into account the engine load and in general produce

changes up when the engine load is light and changes down when the engine load is

heavy.

Fig.1.1 cut-away model of automatic transmissionThe main components that make up an automatic transmission include:

The Torque Converter: This acts like a clutch to allow the vehicle to come to a

stop in gear while the engine is still running.

Planetary Gear Sets : They are the mechanical systems that provide the various

forward gear ratios as well as reverse.

The Hydraulic System: It uses a special transmission fluid sent under pressure to

control the transmission

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2. TORQUE CONVERTER

On automatic transmissions, the torque converter takes the place of the clutch found

on standard shift vehicles. It is there to allow the engine to continue running when

the vehicle comes to a stop.

2.1 CONSTRUCTION

A torque converter is a large doughnut shaped device that is mounted between the

engine and the transmission. As shown in the fig.1, there are four components

inside the very strong housing of the torque converter

Fig.2.1 Torque converter

Impeller or pump (driving element)

Turbine (driven element)

Stator (reaction member)

The one way clutch

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The housing of the torque converter is bolted to the flywheel of the engine, so it

turns at whatever speed the engine is running at. The pump of the torque converter

is attached to the housing, so it also turns at the same speed as the engine. The

pump inside the torque converter is a type of centrifugal pump. The pump has many

curved vanes, along with an inner ring, which form passages for the fluid to flow

through. The turbine is inside the housing and is connected directly to the input

shaft of the transmission providing power to move the vehicle. To get maximum

force on the turbine vanes when the moving fluid strikes them, the vanes are curved

to reverse the direction of flow. The stator is mounted on a one-way clutch so that it

can spin freely in one direction but not in the other.

2.2 WORKING:

As the pump spins, fluid is flung to the outside due to centrifugal force. As fluid is

flung to the outside, a vacuum is created that draws more fluid in at the center. The

fluid then enters the blades of the turbine. Since the blades of the turbine are

curved, the fluid, which enters the turbine from the outside, has to change direction

before it exits the center of the turbine. It is this directional change that causes the

turbine to spin. The fluid exits the turbine moving opposite the direction that the

pump (and engine) is turning. This is shown in fig.2.1 If the fluid were allowed to

hit the pump, it would slow the engine down, wasting power. The stator resides in

the very center of the torque converter. Its job is to redirect the fluid returning from

the turbine before it hits the pump again. The stator has a very aggressive blade

design that almost completely reverses the direction of the fluid. Because of the

one-way clutch, the stator cannot spin with the fluid (it can spin only in the opposite

direction), forcing the fluid to change direction as it hits the stator blades.

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2.3 FLUID COUPLING PHASE:

As the speed of the turbine catches up with the pump, the fluid exit the turbine in

the same direction as the pump is turning, so the stator is not needed. At these

speeds, the fluid actually strikes the back sides of the stator blades, causing the

stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving

through it. All three now elements begin to turn at approximately the same speed. 

 

2.4 TORQUE CONVERTER EFFICIENCY:

It is seen that the efficiency of the torque converter is reasonably good at only

narrow range of turbine speeds. The fall-off of efficiency at low speed end of the

range can be tolerated because those speeds are used for short periods. But the fall-

off of efficiency at high speeds cannot be tolerated and must be circumvented. The

efficiency can be increased, by substituting a direct drive for the torque converter at

higher speeds.

2.5 THE LOCKUP IN TORQUE CONVERTER (DIRECT DRIVE):

Because the only connection between two sides of a torque converter is a fluid

connection, there is always a little slippage, running from about 2-8%. To increase

efficiency and mileage, most modern automatic transmissions also have something

called a lockup clutch.

It works like this. As the two speed of the car reaches 50 to 60 kph, the highly

pressurized transmission fluid is channeled through the transmission shaft and

activates a clutch piston. This metal pin locks the turbine to the pump, in effect

bypassing the torque converter and giving a direct drive. It remains this way until

the vehicle slows below 50 kph, at which point the clutch piston disengages and the

torque converter kicks in again.

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3. SIMPLE PLANETARY GEAR SET

Fig.3.1 Planetary gear system

The basic planetary gear set as shown in fig3.1 consists of a sun gear, a ring gear

and two or more planet gears, all remaining in constant mesh.  The planet gears are

connected to each other through a common carrier. Each of these three components

can be the input, the output or can be held stationary. Choosing which piece plays

which role determines the gear ratio for the gear set. Following table shows the

different gear ratios possible:

Input Output Stationary Calculation Gear Ratio

A Sun (S) Planet Carrier (C) Ring (R) 1 + R/S 3.4:1

B Planet Carrier (C) Ring (R) Sun (S) 1 / (1 + S/R) 0.71:1

C Sun (S) Ring (R) Planet Carrier (C) -R/S -2.4:1

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Table-1 Different gear ratios

4. COMPOUND PLANETARY GEAR SETThe compound planetary gear set looks like a simple planetary gear set but actually

behaves like two planetary gear sets combined. It has one ring gear that is always

the output of the transmission, but it has two sun gears and two sets of planets.

4.1 CONSTRUCTION:

Fig.4.1 From left to right: the ring gear,planet carrier,and two sun gears

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Fig.4.2 Planet carrier: Note the two sets of planets.

Fig.4.1 shows the exploded view of the compound planetary gear set. The fig.4.2

shows the planets in the planet carrier. The planet on the right sits lower than the

planet on the left. The planet on the right does not engage the ring gear, it engages

the other planet. Only the planet on the left engages the ring gear. The shorter gears

are engaged only by the smaller sun gear. The longer planets are engaged by the

bigger sun gear and by the smaller planets.

In such gear sets, the sum of number of teeth on sun gear and ring gear divided by

the number of planets must be a whole number. Otherwise certain combination of

tooth numbers cannot be assembled because of need of equal spacing on the

planets.

4.2 GEAR RATIOS :

Gear Input Output Fixed Gear Ratio

1st 30-tooth sun 72-tooth ring Planet carrier 2.4:1

2nd

30-tooth sun Planet carrier 36-tooth ring 2.2:1

Planet carrier 72-tooth ring 36-tooth sun 0.67:1

Total 2nd 1.47:1

3rd 30- and 36-tooth suns 72-tooth ring 1.0:1

OD Planet carrier 72-tooth ring 36-tooth sun 0.67:1

Reverse 36-tooth sun 72-tooth ring Planet carrier -2.0:1

Table-2 Gear ratios

Consider a planetary gear set with ring gear having 72 teeth the smaller sun gear

having 30 teeth and the larger sun gear having 36 teeth.

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First Gear:

In first gear, the smaller sun gear is driven clockwise by the turbine in the torque

converter. The planet carrier tries to spin counterclockwise, but is held still by the

one-way clutch (which only allows rotation in the clockwise direction) and the ring

gear turns the output. Referring to table 1, the gear ratio is:

-R/S = - 72/30 = -2.4:1

So the rotation is negative 2.4:1. But the output direction is really the same as the

input direction. This is due to the two sets of planets. The first set of planets

engages the second set, and the second set turns the ring gear; this combination

reverses the direction. This would also cause the bigger sun gear to spin; but

because that clutch is released, the bigger sun gear is free to spin in the opposite

direction of the turbine (counterclockwise).

Second Gear:

This acts like two planetary gear sets connected to each other with a common planet

carrier. The first stage of the planet carrier actually uses the larger sun gear as the

ring gear. So the first stage consists of the sun (the smaller sun gear), the planet

carrier, and the ring (the larger sun gear). The input is the small sun gear; the ring

gear (large sun gear) is held stationary by the band, and the output is the planet

carrier. For this stage, with the sun as input, planet carrier as output, and the ring

gear fixed, referring to table 1 the gear ratio is:

1 + R/S = 1 + 36/30 = 2.2:1

The planet carrier turns 2.2 times for each rotation of the sun gear. At the second

stage, the planet carrier acts as the input for the second planetary gear set, the larger

sun gear (which is held stationary) acts as the sun, and the ring gear acts as the

output, so referring to table 1, the gear ratio is:

1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

To get the overall reduction for second gear, we multiply the first stage by the

second, 2.2 x 0.67, to get a 1.47:1 reduction.

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Third Gear:

Most automatic transmissions have a 1:1 ratio in third gear. All we have to do is

engage the clutches that lock each of the sun gears to the turbine. If both sun gears

turn in the same direction, the planet gears lockup because they can only spin in

opposite directions. This locks the ring gear to the planets and causes everything to

spin as a unit, producing a 1:1 ratio.

Overdrive:

By definition, an overdrive has a faster output speed than input speed. It's a speed

increase. When overdrive is engaged, a shaft that is attached to the housing of the

torque converter (which is bolted to the flywheel of the engine) is connected by

clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is

held by the overdrive band. Nothing is connected to the turbine; the only input

comes from the converter housing. With the planet carrier for input, the sun gear

fixed and the ring gear for output, referring to table 1 the gear ratio is:

1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

Reverse:

Reverse is very similar to first gear, except that instead of the small sun gear being

driven by the torque converter turbine, the bigger sun gear is driven, and the small

one freewheels in the opposite direction. The planet carrier is held by the reverse

band to the housing. So, referring to table 1, the gear ratio is:

-R/S = -72/36 = -2.0:1

So the ratio in reverse is a little less than first gear.

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5. HYDRAULIC SYSTEM

The hydraulic system provides the pressurized fluid to operate an automatic

transmission.Major components of the hydraulic system include the bands, clutches

and oil pump. Other major components are the governor, throttle valve, modulator

and the valve body.

The effective operation of an automatic transmission relies upon a hydraulic control

system to actuate the gear changes relative to vehicle’s road speed and acceleration

pedal demands with engine delivering power.

The system performs the following functions,

Supplies fluid to the torque converter.

Directs pressurized fluid to the bands and clutches.

Lubricates the internal parts.

5.2 TRANSMISSION FLUID:

Transmission fluid serves a number of purposes including:

Shift control.

General lubrication

Transmission cooling. 

Unlike the engine, which uses oil primarily for lubrication, every aspect of a

transmission's functions is dependant on a constant supply of fluid under pressure. 

This is not unlike the human circulatory system where even a few minutes of

operation when there is a lack of pressure can be harmful or even fatal to the life of

the transmission. A typical transmission has an average of ten litres of fluid

between the transmission, torque converter, and cooler tank.  In fact, most of the

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components of a transmission are constantly submerged in fluid including the

clutch packs and bands.  The friction surfaces on these parts are designed to operate

properly only when they are submerged in oil.

5.3 COOLING THE TRANSMISSION FLUID:

In order to keep the transmission at normal operating temperature, a portion of the

fluid is sent through one of two steel tubes to a special chamber that is submerged

in the radiator. Fluid passing through this chamber is cooled and then returned to

the transmission through the other steel tube. 

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6. PARTS OF THE HYDRAULIC SYSTEM

For the change of gears, lots of things have to be connected and disconnected. The

clutches connect different members to be driven and the bands hold the required

member stationary. The hydraulic system controls which clutches and bands are

energized at any given moment. The hydraulic system receives information from

the governor and throttle cable or vacuum modulator.

6.1 CLUTCHES:

Fig.6.1 Clutch

A clutch consists of alternating disks that fit inside a clutch drum. As shown in

fig.6.1 half of the disks are steel and have splines that lock on the inside of the

drum.  The other half have a friction material bonded to their surface and have

splines on the inside edge that lock onto one of the gears. There is a piston inside

the drum that is activated by oil pressure at the appropriate time to squeeze the

clutch pack together so that the two components become locked and turn as one.

In this transmission, when overdrive is engaged, a shaft that is attached to the

housing of the torque converter (which is bolted to the flywheel of the engine) is

connected by clutch to the planet carrier. The small sun gear freewheels, and the

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larger sun gear is held by the overdrive band. Nothing is connected to the turbine;

the only input comes from the converter housing.

6.2 BANDS:

Fig.6.2 Band

A band is a steel strap with friction material bonded to the inside surface. Fig.6.2

shows the band and its servo. One end of the band is anchored against the

transmission case while the other end is connected to a servo.  At the appropriate

time hydraulic oil is sent to the servo under pressure to tighten the band around the

drum to stop it from turning, thus locking that part of the gear train to the casing.

The band in a transmission are,literally,steel bands that wrap around

sections of the gear train and connect to the housing.They are actuated by hydraulic

cylinder inside the case of transmission.The metal rod is connected to the piston

which actuates the band.

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6.3 THE PUMP:

The automatic transmission systems use a gear pump. The gear pump is responsible

for producing all the oil pressure that is required in the transmission.  The oil pum is

Fig.6.3 Gear pump from an automatic transmission

mounted to the front of the transmission case and is directly connected to a flange

on the torque converter housing. Since the torque converter housing is directly

connected to the engine crankshaft, the pump will produce pressure whenever the

engine is running as long as there is a sufficient amount of transmission fluid

available. The oil enters the pump through a filter that is located at the bottom of

the transmission oil pan and travels up a pickup tube directly to the oil pump. The

oil is then sent, under pressure to the pressure regulator, the valve body and the rest

of the components, as required.

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6.4 PRESSURE REGULATOR:

The pump’s output pressure will increase roughly in proportion to the engine’s

speed. However, the pressure necessary to actuate the various valves and to

energise the clutch and band servo pistons will vary under different work

conditions. Therefore the fluid pressure generated by the pump, is unlikely to suit

the many operating requirements. To overcome these difficulties, a pressure

regulating valve is used which automatically adjusts the pump’s output pressure to

match the working requirements at any one time. The pressure regulating valve is

normally a spring-loaded spool type valve.

As pump pressure builds up with rising engine speed, line pressure is conveyed to

the rear face of the plunger and will progressively move the plunger forward against

a control spring, causing the exhaust port to be uncovered, which feeds back to the

pump intake. Hence as the pump output pressure tends to rise, more fluid is passed

back to the suction intake of the pump, thus regulating the fluid pressure. To enable

the line pressure to be varied to suit the operating conditions, a throttle pressure is

introduced to the spring end of the plunger, which opposes the line pressure.

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6.5 GOVERNOR:

Fig.6.4 The governor

The governor tells the transmission how fast the car is moving. The governor is

connected to the output shaft and regulates hydraulic pressure based on vehicle

speed. It accomplishes this using centrifugal force to spin a pair of weights against

pull-back springs.  As the weights pull further out against the springs, more oil

pressure is allowed past the governor to act on the shift valves that are in the valve

body which then signal the appropriate shifts.  

6.6 THROTTLE CABLE AND VACUUM MODULATOR:

Vehicle speed is not the only thing that controls when a transmission should shift,

the load that the engine is under is also important.  The more loads you place on the

engine, the longer the transmission will hold a gear before shifting to the next one.  

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The throttle valve and modulator serve the purpose of monitoring engine load. A

transmission will use one or the other but generally not both of these devices.  Each

works in a different way to monitor engine load. 

The Throttle Cable simply monitors the position of the accelerator pedal through a

cable that runs from the gas pedal to the throttle valve in the valve body. The

further the gas pedal is pressed, the more pressure is put on the throttle valve.

Engine vacuum reacts very accurately to engine load with high vacuum produced

when the engine is under light load and diminishing down to zero vacuum when the

engine is under a heavy load.  The vacuum modulator is attached to the outside of

the transmission case and has a shaft which passes through the case and attaches to

the throttle valve in the valve body. 

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7. VALVE BODY:

The valve body is the brain of the automatic transmission.  It contains a maze of

channels and passages that direct hydraulic fluid to the numerous valves which then

activate the appropriate clutch pack or band servo to smoothly shift to the

appropriate gear for each driving situation. Each of the many valves in the valve

body has a specific purpose and is named for that function. For example the 1-2

shift valve activates the 1st gear to 2nd gear up-shift.

7.1 THE MANUAL VALVE:

The manual valve is directly connected to the gear shift handle and covers and

uncovers various passages depending on what position the gear shift is placed in. If

the gear shift lever is moved in the 1st gear or 2nd gear position, the up-shift and

down-shift are inhibited by the manual valve.

7.2 SHIFT VALVES:

Shift valves, as shown in fig. 8 supply hydraulic pressure to the clutches and bands

to engage each gear. The valve body of the transmission contains several shift

valves. The shift valve determines when to shift from one gear to the next. The shift

valve is pressurized with fluid from the governor on one side, and the throttle valve

on the other. The shift valve will delay a shift if the car is accelerating quickly. If

the car accelerates gently, the shift will occur at a lower speed.

When the car accelerates gently, as car speed increases, the pressure from the

governor builds. This forces the shift valve over until the first gear circuit is closed,

and the second gear circuit opens. Since the car is accelerating at light throttle, the

throttle valve does not apply much pressure against the shift valve.

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Fig.7.1 Simple shift circuit

When the car accelerates gently, as car speed increases, the pressure from the

governor builds. This forces the shift valve over until the first gear circuit is closed,

and the second gear circuit opens. Since the car is accelerating at light throttle, the

throttle valve does not apply much pressure against the shift valve.

When the car accelerates quickly, the throttle valve applies more pressure against

the shift valve. This means that the pressure from the governor has to be higher

(and therefore the vehicle speed has to be faster) before the shift valve moves over

far enough to engage second gear.

Each shift valve responds to a particular pressure range; so when the car is going

faster, the 2-to-3 shift valve will take over, because the pressure from the governor

is high enough to trigger that valve.

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8. ADVANTAGES AND DISADVANTAGES:

8.1 ADVANTAGES:

1. It minimizes driver fatigue, especially in heavy traffic by eliminating the need

to operate the clutch pedal and gear lever for starting from rest and changing gear.

2. It contributes to safer driving because the concentration of the driver is not

disturbed by the need to change gear; also, both hands can remain on the steering

wheel.

3. Progress can be smoother under normal driving conditions, because gear

changes will occur at the theoretically correct moment in terms of road speed and

throttle opening.

8.2 DISADVANTAGES:

1. Cars with automatic transmission are costlier than those having manual

transmission.

2. Fuel economy of cars with automatic transmission is not very good.

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9. CONCLUSION: The automatic transmission with its torque converter and planetary gear set, can

successfully replace the clutch and the manual transmission gear box.

The planetary gear set gives the required gear ratios and the hydraulic system

controls the planetary gear set.

Automatic transmission system shifts the gears automatically, depending upon

both, speed of vehicle and load on the engine.

However in cases like, descending hills, when it is desirable to employ a lower

gear, though the load on the engine maybe nil or the engine maybe acting as a

brake, the human element has to be retained in control.

10. FUTURE SCOPE

Electronically controlled transmissions, which appear on some newer cars,

still use hydraulics to actuate the clutches and bands, but each hydraulic

circuit is controlled by an electric solenoid. This simplifies the plumbing on

the transmission and allows for more advanced control schemes.

However automatic transmission system is very heavy. So it is very

necessary to reduce it’s weight for improving overall car performance. Now

a days some company’s engineers keenly work on fiber or plastic body and

some parts of transmission system.

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11.REFERENCES

1. William H. Crouse & Donald L. Anglin, “Automotive Automatic

Transmissions”, Tata McGraw Hill Publishing Co., Sixth Edition, 1996.

2. John Fenton, “Handbook of Automotive Powertrains and Chassis Design”,

Professional Engineering Publishing Ltd., First Published 1998.

3. Heinz Heisler, “Advanced Vehicle Technology”, Butterworth-Heinemann

Publishers, Second Edition 2002.

4. www.familycar.com

5. www.howstuffworks.com

6. www.edmunds.com

7. Dr.kirapal singh “automatic transmission”,edition 2006

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