2. BY ANIRUDH SARAIYA M.TECH AUTOMOBILE ENGINEERING
An automatic gearbox contains special devices that automatically provide
various gear ratios, as they are needed.
Most automatic gearboxes have three
or four forward gears and reverse.
Instead of a gearstick, the driver moves a
lever called a selector.
Most automatic gearboxes now have selector positions
for park, neutral, reverse, drive, 2 and 1
3. AUTOMATIC-GEAR-SELECTOR
4.
The engine will only start if the selector is in either the park or neutral
position.
In park, the drive shaft is locked so that the drive wheels cannot
move.
It is now quite common when the engine is running to be able to move
the selector out of park only if you are pressing the brake pedal.
This is a
very good safety feature as it prevents sudden movement of the vehicle.
5.
For ordinary driving, the driver moves the selector to the drive position.
The transmission starts out in the lowest gear and automatically shifts into
higher gears as the car picks up speed.
The driver can use the lower positions
of the gearbox for going up or down steep hills or driving through mud or snow
6. SPEED AUTOMATIC TRANSMISSION IN BMW
7.
The torque converter is a device that almost all automatic transmissions now
use.
It delivers power from the engine to the gearbox like a basic fluid
flywheel but also increases the torque when the car begins to move.
The
torque converter resembles a large doughnut sliced in half.
One half, called
the pump impeller, is bolted to the drive plate or flywheel.
8.
The other half, called the turbine, is connected to the gearbox-input shaft.
Each half is lined with vanes or blades.
The pump and the turbine face each
other in a case filled with oil.
A bladed wheel called a stator is fitted
between them.
The engine causes the pump to rotate and throw oil against the
vanes of the turbine.
9. TURBINE, STATOR, PUMP
10.
The force of the oil makes the turbine rotate and send power to the
transmission.
After striking the turbine vanes, the oil passes through the stator
and returns to the pump.
When the pump reaches a specific rate of rotation, a
reaction between the oil and the stator increases the torque.
While in a
fluid flywheel oil returning to the impeller tends to slow it down.
11. 11.
At zero output speed the output torque will be several times input torque thus
torque input to transmission will be twice that of engine torque when
transmission is stalled
As speed builds up and transmission input reaches
engine speed the torque ratio drops to unity
Thus torque amplification
provides more tractive effort speed performance
12.
The fluid flywheel action reduces efficiency because the pump tends to rotate
faster than the turbine.
To improve efficiency, many transmissions now
include a lock-up clutch.
When the pump reaches a specific rate of rotation,
this clutch locks the pump and turbine together, allowing them to rotate as
one.
13.
Epicyclic gears are a special set of gears that are part of automatic
gearboxes.
They consist of three elements:
a sun gear, located in the
centre;
the carrier that holds two, three, or four planet gears, which mesh
with the sun gear and revolve around it;
an internal gear or annulus is a
ring with internal teeth, which surrounds the planet gears and meshes with
them.
14. 14.
EPICYCLIC GEAR OPERATION
15. 15.
Any part of a set of planetary gears can be held stationary or locked to one of
the others and will produce different gear ratios.
Most of the automatic
gearboxes of the type shown earlier have two sets of planetary gears that are
arranged in line. This provides the necessary number of gear ratios. The
appropriate elements in the gear train are held stationary by a system of
hydraulically operated brake bands and clutches.
16. 16.
TWO EPICYCLIC GEAR SETS CAN PRODUCE THREE FORWARD GEARS AND REVERSE
17.
These are worked by a series of hydraulically operated valves in the lower part
of the gearbox.
Oil pressure to operate the clutches and brake bands is
supplied by a pump.
The supply for this is the oil in the sump of the
gearbox.
18.
The main aim of electronically controlled automatic transmission (ECAT) is to
improve on conventional automatic transmission:
gear changes should be
smoother and quieter; improved performance;
reduced fuel consumption;
reduction of characteristic changes over system life and increased
reliability.
19. ELECTRONIC CONTROL OF TRANSMISSION
20.
The important points to remember are that gear changes and lock-up of the
torque converter are controlled by hydraulic pressure.
• In an ECAT system
electrically controlled solenoid valves can influence this hydraulic pressure.
• Most ECAT systems now have a transmission ECU that is in communication with
the engine control ECU.
21.
•
With an ECAT system the actual point of gearshift is determined from
pre-programmed memory within the ECU. Data from other sensors is also taken
into consideration.
• Actual gearshifts are initiated by changes in hydraulic
pressure, which is controlled by solenoid valves.
• The two main control
functions of this system are hydraulic pressure and engine torque.
• A
temporary reduction in engine torque during gear shifting allows smooth
operation.
22.
•
This gives better fuel economy, quietness and improved driveability.
• Lock-up
is carried out using a hydraulic valve, which can be operated gradually to
produce a smooth transition.
• The timing of lockup is determined from ECU
memory in terms of the vehicle speed and acceleration.
23. ECAT BLOCK DIAGRAM SHOWING INPUTS AND OUTPUTS
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