B839 automatic transmission

Automatic Transmission System

1. INTRODUCTION

Automatic 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.

The 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

2.1 INTRODUCTION:

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.

2.2 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:
• Impeller or pump (driving element)
• Turbine (driven element)
• Stator (reaction member)
• The one way clutch

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

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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.

2.3 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. 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.

2.4 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.

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2.5 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.

2.6 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

The basic planetary gear set as shown in fig. 3 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 Gear Ratio

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

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

Sun (S) Ring (R) Planet Carrier (C) -R/S

Table 1

4. COMPOUND PLANETARY GEAR SET

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4.1 INTRODUCTION:

The 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.

4.2 CONSTRUCTION:

Fig. 4 shows the exploded view of the compound planetary gear set. The fig.
5 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.

4.3 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.

First Gear:

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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.

5. HYDRAULIC SYSTEM

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5.1 INTRODUCTION:

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.

5.2 FUNCTIONS OF THE HYDRAULIC SYSTEM:

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.

5.3 TRANSMISSION FLUID:

Transmission fluid serves a number of purposes including:
• Shift control.
• General lubrication
• Transmission cooling.

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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 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.

5.4 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

6.1 INTRODUCTION:

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.

6.2 CLUTCHES:

A clutch consists of alternating
disks that fit inside a clutch drum.
As shown in fig. 6, 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.

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6.3 BANDS:

A band is a steel strap with friction material
bonded to the inside surface. Fig. 7 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.

6.4 OIL 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 pump is 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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6.5 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.

6.6 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.

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6.7 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.

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

7.1 INTRODUCTION:

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 1 st gear to 2nd gear up-shift.

7.2 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 1 st gear or 2nd gear position,
the up-shift and down-shift are inhibited by the manual valve.

7.3 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.

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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.

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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

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