automatic transmission sys

8/11/2019 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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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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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.

automatic transmission sys

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