Automatic Transmission Sys
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Transcript of Automatic Transmission Sys
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Automatic Transmission systems
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Automatic Transmission
An automatic transmission is an automobile
gear box that can change gear ratios
automatically as the automobile moves under
varying conditions, thus freeing the driver
from having to shift gears manually.
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Automatic Transmission
The main components of an automatic
transmission are the converter housing case,
oil pan and the extension housing.
The converter housing encloses the torque
converter and may be integral with the case or
seperately bolted to the case.
The case contains the epicyclic gear train
while the extension housing encloses the
output shaft.
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Automatic Transmission
The oil pan is bolted to the case.
The entire transmission unit is attached to the
engine block by means of bolts through holes
in the converter housing flange.
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Borg Warner Automatic Transmission
It is a combination of the torque converterand epicyclic gear train.
The turbine of the torque converter drives
the ring gear of the first gear train through a
free wheel.
The drive to the ring gear of the second gear
train is then taken from the planet carrier of
the first train so that the two act in series.
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This arrangement gives three forward and
one reverse speeds.
For direct gear, clutch A is engaged.
The second gear is obtained by engaging
Clutch B and applying brake BS.
The application of both brakes BS and BF
gives the first i.e the lower most gear.
For reverse gear only brake BR is applied.
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The selection of the particular gear andapplication of corresponding clutch and brake
is done hydraulically. The hydraulic pressure is regulated by two
factors
1) Car speed that controls oil pressure on oneside of the shift valve and
2) The throttle opening controlled by thedriver through accelerator pedal, which
controls oil pressure on other side of a theshift valve.
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The selector lever for the fully automatictransmission generally has 5 positions PRNDL.
Position P is for parking and employs amechanical pawl system for locking, N stands for
neutral position. For safety, the car can be started only in position
p or N.
For reverse the lever has to be brought toposition R and position L is to hold onto a lowgear, ex: when travelling uphill.
PRNDL
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Position D is for fully automatic operation up
or down.
In some models positions 2 and 1 are
substituted for L making it as PRND 2L.
In position 2, automatic up and down
changes are available between first and
second gears only and the top gear is
excluded.
With the control in position 1, automatic
operation is restricted to gear 1 only.
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Comparasion with manual
transmission
MeritsMore precise control of shifting results in better
driveability.
No skill required for operating.
Demerits
Higher fuel consumption due to lower mechanicalefficiency.
Higher initial cost.More complex construction which results in
expensive repairs.
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Over drives
It is a device to step up the gear ratio in thecar.
It is fitted in between transmission and thepropeller shaft.
Over drives have become very popular inUSA.
It enables a high cruising speed to be
attained with a comparatively low enginespeed( upto about 20-25% less) on long
journeys.
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This results in less wear of the engine parts
and decreases vibration and noise.
As the friction losses at lower speeds are less,
there is a saving of fuel also with overdrive.
When overdrive is fitted on top, third and
second gear, seven forward speeds or torque
rations are available.
It may be operated manually or automatically
at a predetermined speed.
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Construction of over drive
It consists of an epicyclic gear train in whichthe sun gear is free to rotate on the input
shaft, while the carrier can move on splines
on the input shafts.
A freewheel clutch is also fitted on the input
shaft splines.
The output shaft is connected to the ring.
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Working of over drive
When the sun gear is locked with the casing, i.e.It becomes stationary, the speed of the output
shaft is increased i.e., overdrive is engaged.
When, however, the sungear is locked to thecarrier or to the ring, solid drive through the
gear train is obtained.
Thus depending on the locking of the sun gear
with the casing or with carrier, the overdrive or
the normal direct drive is obtained.
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Continuously Variable Transmissions
CVT is an automatic transmission that canselect any desired drive ratio with in its
operating range.
Unlike a conventional four or five speed
transmission, CVT is an infinite speed ratio
transmission.
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Development of CVT
In 1490, Leonardo da vinci made a sketch of astepless CVT, it was only in 1958 that Daf in
Netherlands produced CVT for a car, which
was developed and improved over the years. Honda Multimatic used in the Toyota hybrid
Insight and Audi Multitronic fitted in their A6
model are recent examples.