AUTOMATIC TRANSMISSION

AUTOMATIC TRANSMISSION
• Automatic transmission and transaxles have similar
components. Three basic parts include:
1. Torque Converter.
2. Gear train: planetary gear sets
3. Hydraulic system
• Torque converter connects to the crankshaft and transmits
engine power to the gear train.
• Hydraulic pressure acting through the automatic-transmission
fluid (ATF) in the transmission or transaxle produces the shifts.
Gear-Shift Mode
 A/ T OPERATION
• Most A/T have 3 or four forward speed.
• They also have PARK, NEUTRAL, and REVERSE.
• 4th speed is usually has overdrive ratio. Some
others have fifth gear which is an overdrive.
• A typical A/T move the vehicle in 1st gear. Then
it shifts to 2nd, 3rd & 4th without assistance
from driver . They happen automatically as
vehicle speed increases & the engine load or
throttle opening decreases.
• To slow and stop the vehicle the driver only
needs to release accelerator pedal or apply
brake.
• The A/T disengages the torque converter
clutch & automatically downshifts until 1st
gear when the vehicle stops.
• Slippage in the torque converter allows the
engine to idle with the transmission in gear.
 TORQUE CONVERTER
• Torque converter is a form of fluid coupling.
• It uses a fluid ( such as ATF) and vaned rotors to transmit power between shafts.
The torque converter is filled with ATF.
• When the engine runs, power flows from the crankshaft trough the fluid to the
transmission input shaft.
• Torque converter can be divided into 3 main members:
a. Impeller: The impeller, also known as the pump, is the driving member and
rotates with the engine.
b. Turbine: The impeller vanes pick up fluid in the converter housing and direct it
toward the turbine. Unless the torque converter is locked, the turbine is
normally turns slower than the impeller.
c. Stator: Fluid flow drives the turbine, and when the flow between the impeller
and turbine is adequate, the turbine rotates and turns the transmission input
shaft. A torque converter contains the stator, or reactor, a reaction member
mounted on a one-way clutch.
• The vanes used in each of the three elements of a torque converter are curved to
increase the diversion angle of the fluid. This also increases the force exerted by
the fluid and improves the hydraulic advantage.
• The Pump
Automatic transmissions have a neat pump, called a gear
pump. The pump is usually located in the cover of the
transmission. It draws fluid from a sump in the bottom of the
transmission and feeds it to the hydraulic system. It also feeds
the transmission cooler and the torque converter.
• The inner gear of the pump hooks up to the housing of the
torque converter, so it spins at the same speed as the engine.
The outer gear is turned by the inner gear, and as the gears
rotate, fluid is drawn up from the sump on one side of the
crescent and forced out into the hydraulic system on the
other side.
• When the engine runs, the fluid
between the vanes in the impeller
is thrown outward by the
centrifugal force.
• The fluid strikes the turbine vanes.
This action produce rotating force
on the turbine and the transmission
input shaft attached to it.
• The vanes then directs the fluid
toward the centre of the turbine
and back toward the centre of the
impeller.
• STATOR:
• To make the torque converter more efficient, a 3rd member or
stationary reactor called stator is placed between the impeller
& turbine.
• The stator have curve vanes that change the direction of the
fluid after it leaves the turbine. This causes the fluid to pass
trough the impeller & then push on the turbine vanes again
with a helping force that aid rotation. The result is torque
multiplication under certain condition.
• Torque multiplication: in many torque converter the torque is
more than double.e.g, 1.35N.m of torque entering the
impeller, the turbine delivers more than 2.7N.m of torque to
the transmission input shaft. Torque multiplication occurs only
when the impeller turns faster than the turbine.
Fluid flow within a torque converter. The stator redirects the
fluid that is thrown out by the turbine, thereby improving
efficiency.
 Fluid pumped into the turbine by the impeller not
only creates rotary fluid flow but also vortex flow that
increases the efficiency of the torque converter.
 STATOR ONE- WAY CLUTCH
• As the vehicle approaches cruising speed, the turbine
begins to catch up with the impeller (coupling point).
• The fluid leaving the turbine is moving at about the
same speed as the impeller. This fluid could pass
directly into the impeller without stator action. In
fact, the stator vane are now in the way. The fluid is
striking the back sides of the stator vanes.
• To allow the stator vanes to move out of the way, the
stator mounts on a one- way clutch (overrunning
clutch)
 One- way Clutch
• A mechanical device that tranmits torque in one direction &
permits free rotation in the opposite side.
• When the speed difference between the impeller & the
turbine is large, the fluid from the turbine tries to spin the
stator backward. This causes the one- way clutch to lock the
stator to its shaft. When the speed difference is small, the
clutch unlocks & allows the stator to freewheel. 2 types of one
way clutch are roller clutch & sprag type clutch.
 TORQUE CONVERTER CLUTCH
(TCC)
• Function:
A lockup torque converter eliminates the 10%
slip that takes place between the impeller and
turbine at the coupling stage. The engagement
of a clutch between the impeller and the turbine
assembly greatly improves fuel economy and
reduces operational heat and engine speed.
• The lock- up torque converter (TCC) has a lock-
up or clutch piston with a lining of friction
material.
• The piston or plate attaches to the turbine
hub.
• Isolator springs helps dampen the shock of
engagement as the torque converter locks.
They also dampen out the power pulse from
the engine while the torque converter is
locked.
TCC operation
• The clutch is controlled by hydraulic valves, which are
controlled by the PCM. The PCM monitors operating
conditions and controls lockup according to those conditions.
• When the converter is not locked (clutch disengage), fluid
enters the converter and moves to the front side of the
piston, keeping it away from the shell or cover. Fluid flow
continues around the piston to the rear side and exits
between the neck of the torque converter and the stator
support.
• During the lockup mode ( clutch engage), the switch valve
moves and reverses the fluid path. This causes the fluid to
move to the rear of the piston, pushing it forward to apply the
clutch to the shell and allowing for lockup. Fluid from the
front side of the piston exits through the turbine shaft that is
now vented at the switch valve.
• Reduction in engine speed
and the elimination of the
normal slippage in the
torque converter improves
fuel economy.
• The torque converter clutch
is released during rapid
acceleration for maximum
torque multiplication
through the torque
converter for best
acceleration.
HYDRAULIC CIRCUIT (HOMEWORK)
• Revise a basic hydraulic circuit components:-
a. Hydraulic pump
b. Control valves (directional control)
- Pressure relief valves
- Pressure regulators
- Shuttle valves
- Check valves
c. Actuators( Hydraulic cylinder, Swashplates, Hydraulic motor , hydrostatic
transmission, Brakes)
d. Reservoir
e. Accumulators
f. Hydraulic fluid
g. Filters
h. Tubes, pipes and hoses
i. Seals, fittings and connections
A/T GEAR- TRAIN
 A/T GEAR- TRAIN
• Gear-train consist of the following:-
a. Planetary gearset: A compound epicyclic planetary gearset,
whose bands and clutches are actuated by hydraulic servos
controlled by the valve body, providing two or more gear
ratios.
b. Clutches and bands: to effect gear changes, one of two types
of clutches or bands are used to hold a particular member of
the planetary gearset motionless, while allowing another
member to rotate, thereby transmitting torque and
producing gear reductions or overdrive ratios. These clutches
(overrunning clutch) are actuated by the valve body (see
below), their sequence controlled by the transmission's
internal programming.
C. Valve body: hydraulic control center that receives pressurized fluid from
the main pump operated by the fluid coupling/torque converter. The pressure
coming from this pump is regulated and used to run a network of spring-
loaded valves, check balls and servo pistons. The valves use the pump
pressure and the pressure from a centrifugal governor on the output side (as
well as hydraulic signals from the range selector valves and the throttle valve
or modulator) to control which ratio is selected on the gearset; as the vehicle
and engine change speed, the difference between the pressures changes,
causing different sets of valves to open and close. The hydraulic pressure
controlled by these valves drives the various clutch and brake band actuators,
thereby controlling the operation of the planetary gearset to select the
optimum gear ratio for the current operating conditions. However, in many
modern automatic transmissions, the valves are controlled by electro-
mechanical servos which are controlled by the electronic engine control unit
(ECU) or a separate transmission control unit (TCU, also known as
transmission control module (TCM).
 a. PLANETARY GEARS
• Any planetary gearset has three main components:
a. The sun gear
b. The planet gears and the planet gears' carrier
c. The ring gear
• 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 gearset.