1. Torque Converter and Planetary Automatic Transmission

2. Torque Converter
A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission.
If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal.
There are four components inside the very strong housing of the torque converter:
1. Pump
2. Turbine
3. Stator
4. Transmission fluid 2

3. Pump
The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at.
The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine.
The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center. 3

4. Turbine
The fluid then enters the blades of the turbine, which is connected to the transmission.
The turbine causes the transmission to spin, which basically moves your car. The blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is this directional change that causes the turbine to spin.
The fluid exits the turbine at the center, moving in a different direction than when it entered. If you look at the arrows in the figure, you can see that the fluid exits the turbine moving opposite the direction that the pump (and engine) are turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a torque converter has a stator. 4

5. Stator
The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission (the direction that the clutch allows the stator to spin is noted in the figure above).
Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades. 5

6. Lock up Clutch
Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.
At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileages than cars with manual transmissions.
To counter this effect, some cars have a torque converter with a lockup clutch. When the two halves of the torque converter get up to speed, this clutch locks them together, eliminating the slippage and improving efficiency. 6

7. Automatic Transmission
A hydraulic automatic transmission consists of the following parts:
Torque Converter which is a type of fluid coupling hydraulically connecting the engine to the transmission.
An ingenious planetary gearset.
A set of bands to lock parts of a gearset.
A set of three wet-plate clutches to lock other parts of the gearset.
An incredibly odd hydraulic system that controls the clutches and bands.
A large gear pump to move transmission fluid around. 7

8. Automatic Transmission
Two clutches, two brake bands and a one-way clutch. The transmission has planetary gearing for three forward gears and reverse gear. The clutches are engaged to connect and drive parts of the gearing. The bands and the one-way clutch are used to hold other parts of the planetary system stationary. 8

9. Planetary Gearset Any planetary gearset has three main components:
·         - The sun gear
·         - The planet gears and the planet gears' carrier
·         - The ring gear
Each of these 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.
From left to right: the ring gear, planet carrier, and two sun gears 9

10. Single Planetary Gearset
Let's take a look at a single planetary gearset:
One of the planetary gearsets from a transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.
The first gear ratio listed above is a reduction -- the output speed is slower than the input speed.
The second is an overdrive -- the output speed is faster than the input speed.
The last is a reduction again, but the output direction is reversed.
There are several other ratios that can be gotten out of this planetary gear set. 10

11. Operation
This one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs. 11

12. Compound planetary gears
Pinion operate in 3 different ways:
1.They can operate as idlers.When the carrier is held stationary, the pinions rotate on their own shaft and can be used to transfer drive from either of the sun gears to the ring gear.
2. They can walk around a sun gear. When either sun gear is held stationary, its pinion will walk around the outside of the gear, taking the carrier with them.
3. The secondary pinions can walk around the inside the ring gear. This occurs when the ring gears is held stationary. The secondary pinion will walk around the inside of the ring gear, taking the carrier with them.

13. Compound planetary gears
Consists of:
Primary (small) sun gear
Secondary (large) sun gear
Ring or internal gear
Pinion carrier with set of long (secondary) pinion and set of short (primary) pinion
Long pinions connect secondary sun gear to ring gear, while short pinion connect primary sun gear to ring gear through secondary pinion