AUTOMATIC TRANSMISSION IN CARS
CONTENTS: -
Page
No.
1.
ABSTRACT…………………………………………………………..03
2.
INTRODUCTION……………………………………………………04
3.
COMPONENTS OE AUTOMATIC TRANSMISSION…………….05
4.
SPOTTING PROBLEMS…………………………………………….10
5.
PRECAUTIONS AND MAINTENANCE…………………………...10
6.
ADVANTAGES OF AUTOMATIC TRANSMISSION..…...……....10
7.
LIMITATIONS OF AUTOMATIC TRANSMISSION ...…………...10
8.
CONCLUSIONS……………………………………………………..11
9.
REFERENCES……………………………………………………….11
ABSTRACT
The modern automatic transmission is by far, the most complicated
mechanical component in today's automobile. Automatic transmissions
contain mechanical systems, hydraulic systems, electrical systems and computer
controls, all working together in perfect harmony which goes virtually
unnoticed until there is a problem. This article will help you understand the
basic of the transmission, about automatic transmission, differences between
manual and automatic transmission, the concepts behind what goes on inside
these technological marvels and what goes into repairing them when they fail.
What is a
transmission?
The transmission is a
device that is connected to the back of the engine and sends the power from the
engine to the drive wheels. An automobile engine runs at its best at a
certain RPM (Revolutions Per Minute) range and it is the transmission's job to
make sure that the power is delivered to the wheels while keeping the engine
within that range. It does this through various gear combinations. In
first gear, the engine turns much faster in relation to the drive wheels, while
in high gear the engine is loafing even though the car may be going in excess
of 70 MPH. In addition to the various forward gears, a transmission also
has a neutral position, which disconnects the engine from the drive wheels, and
reverse, which causes the drive wheels to turn in the opposite direction
allowing you to back up.
INTRODUCTION
One of the biggest things
that has always separated drivers is whether they prefer an automatic
transmission (sometimes known as a "slushbox", because of the characteristic
"slushy" feel it creates when shifting between gears) or a standard
transmission (often called, for more obvious reasons, a "stick
shift").
There are a number of
benefits to the standard transmission, but the prevailing reason that most
people get them is because they prefer them. Car enthusiasts enjoy the feel of
being able to control what gear they're in; it makes them feel closer to the
vehicle. People who just want to drive and don't care much about how the car
works usually prefer the automatic trannie.
If you have ever
driven a car with an automatic transmission, then you know that there are two
big differences between an automatic transmission and manual transmission:
Ø There
is no clutch pedal in an automatic transmission car
Ø There
is no gear shift in an automatic transmission car. Once you put the
transmission into Drive, everything else is automatic!
The
need to shift from manual to automatic transmission
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. Driving
a manual transmission is especially painful in stop-and-go traffic, where you
constantly have to change your gear state. You might also make the case that
having to concentrate on shifting your transmission takes some of your
attention away from the actual driving, which is dangerous, because you might
be concentrating on your tachometer or your gearshift when you really should be
focusing on the traffic. An automatic
transmission bypasses these problems entirely.
COMPONENTS OF 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:
Ø Planetary gear sets
The
automatic system for current automobiles uses a planetary gear set instead of
the traditional manual transmission gear set. The planetary gear set contains
four parts: sun gear, planet gears, planet carrier, and ring gear (see Figure
1.) Based on this planetary set design, sun gear, planet carrier, and ring
gear spin centrifugally. By locking one of them, the planetary set can generate
three different gear ratios, including one reverse gear, without engaging and
disengaging the gear set.
Normally
an automatic transmission system has two planetary gear sets with different
sizes of sun gears with their planet gears inter-meshed (see Figure 2.)
Only one planet carrier is used to connect both sets of planet gears. This is
called a compound planetary gear set. This design can generate four different
gear ratios and one reverse gear.
Figure 1 : Planetary Gear Set Figure 2 : Compound
Planetary Gear set
Figure 3 : planetary Gear sets with Band and Clutch
It uses
a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets. Both
the band and clutch piston are pressurized by the hydraulic system. The part connecting the band or
clutches to the hydraulic system is called the shift valve, while the one
connecting the hydraulic system to the output shaft is called the governor (see
Figure 4). The governor is a centrifugal sensor with a spring loaded
valve. The faster the governor spins, the more the valve opens. The more the
valve opens, the more the fluid goes through and the higher the pressure
applied on the shift valve. Therefore, each band and clutch can be pushed to
lock the gear based on a specific spin speed detected by the governor from the
output shaft. To make the hydraulic system work efficiently, a complex maze of
passages was designed to replace a large number of tubes (see Figure 5).
Figure 4 Governor
Figure 5 Hydraulic
System with maze
·
·
Figure 4 :Governor Figure 5 :Hydraulic System with Maze
structure
Unlike a manual
transmission system, automatic transmission doesn’t use a clutch to disconnect
power from the engine temporarily when shifting gears. Instead, a device called
a torque converter was invented to prevent power from being temporarily
disconnected from the engine and also to prevent the vehicle from stalling when
the transmission is in gear. Consider two fans facing each other: when one of
them is turned on and starts spinning, the other one will also start spinning
at a lower speed (see Figure 6). That’s because the first fan moves the
air to drive the second fan to spin. This is the same idea as the torque
converter of an automatic transmission system, except that it uses fluid
instead of air as the transportation media.
A torque converter
consists of four parts, a pump (impeller), turbine, stator, and transmission
fluid (Figure7). The pump is
connected to the engine and transfers engine power to the transmission fluid.
The fluid causes the turbine, which is connected to the input shaft, to spin.
The stator is used to redirect fluid returning from the turbine before it hits
the pump, again to increase the efficiency. In this design, even when the
vehicle is in gear but not moving (the turbine is forced to stop), the pump can
still keep spinning without causing the engine to stall. When the vehicle speed
is slow, the turbine is always spinning more slowly than the pump. However,
when the vehicle moves at high speed, the turbine can spin at almost the same
speed as the pump. Therefore, for modern cars, a “lock up” will occur between
pump and turbine when the vehicle is at high speed in fourth gear, which makes
the torque converter work more efficiently.
F
Figure 6 Figure 7: The Structure of a Torque Converter.
Ø
Tiptronic
transmission is a special type of automatic transmission with a computer
controlled automatic shift. The driver can switch the transmission to manual
mode, which lets the driver to shift the gear at his wish sequentially up (+)
or down (-) (see Figure 8) without disengaging the clutch. This works
just like a manual transmission; however, it still uses a torque converter to
transfer power from the engine. Unfortunately, this is less efficient than a
manual transmission.
Figure 8 : Tiptronic transmission.
Spotting problems before they get worse
Ø Watch for leaks or stains under the car.
Ø
Check fluid for color
and odor.
Ø
Be sensitive to new
noises, vibrations and shift behavior.
Precautions and Maintenance
Ø
Transmission fluid should be changed
periodically.
Ø
According
to ‘B & M’, the optimum operating temperature for automatic transmission
fluid is 170 degrees. For every 10 degree increase in operating temperature,
the useable life of the fluid is reduced by half.
Advantages of
automatic transmission
Ø It's a lot less work because you
don't have to worry about juggling the gearshift with the clutch.
Ø You don't need to worry about
stripping your gears.
Ø Car will never stall during gear
shifting.
Limitations of automatic transmission
Ø
Indirect
contact of the torque converter causes the power loss during transmission.
Ø
The
‘killer’ of automatic transmissions is heat. Excessive heat causes the fluid to
lose its ability to lubricate, which leads to higher internal-component
friction, all resulting in rapid wear-and-tear and premature transmission
failure.
Ø
Complicated
planetary structure makes the transmission heavy and easily broken.
Conclusion
Being able to
drive a stick-shift means more than just understanding your gears, however.
That's because to shift gears, you usually don't just move the shift lever and
be done with it. (You can do this sometimes, but it's not recommended in
day-to-day driving, and we'll get to it later.) Instead, you have to press the
clutch pedal while you shift. . Keeping the transmission and its fluid cool,
and in the correct operating-temperature range is crucial for long life.
