Automatic Transmission

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Automatic Transmission

            The concept of an automatic transmission is new in India. An automatic transmission is a motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears.  In this transmission system, the gears are never physically moved and are always engaged to the same gears. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems, and computer controls, all working together in perfect harmony manually.

Main Components of an Automatic Transmission:

1.      Planetary Gear Sets            

2.      Clutches and Bands

3.      Torque Converter

4.      Valve Body


1. Planetary Gear Sets

            The planetary gear set is the device that produces different gear ratios through the same set of gears. Any planetary gear set has three main components:

·         The sun gear           

·         The planet gears and the planet gears' carrier

·         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 gear set.

  • Compound Planetary gear set: 

            The automatic transmission uses a set of gears, called a compound planetary gear set that looks like a single planetary gear set but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.

The figure below shows a compound planetary gear set:


  • First Gear: 

            In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counter-clockwise but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so the gear ratio is:

Ratio = -R/S = - 72/30 = -2.4:1

            So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction -- this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counter-clockwise).

  • Second Gear: 

            The two planetary gear sets connected to each other with a common planet carrier.

            The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear).

            The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is:

1 + R/S = 1 + 36/30 = 2.2:1

            The planet carrier turns 2.2 times for each rotation of the small sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts like the sun, and the ring gear acts as the output, so the gear ratio is:

1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

            To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction.

 

  • Third Gear: 

            Most automatic transmissions have a 1:1 ratio in third gear. To achieve a ratio of 1:1 engage the clutches that lock each of the sun gears to the turbine. If both sun gears turn in the same direction, the planet gears lockup because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio.

  • Overdrive Gear: 

            Overdrive has a faster output speed than the input speed. When overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by the clutch to the planet carrier. The small sun gear freewheels and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. This time with the planet carrier for input, the sun gear fixed and the ring gear for output.

Ratio = 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

            So the output spins once for every two-thirds of rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.

 

  • Reverse Gear: 

            The reverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have:

Ratio = R/S = 72/36 = 2.0:1

So the ratio in reverse is a little less than the first gear in this transmission.

 

Summary of Gear Ratios:

             Transmission having four forward gears and one reverse gear.

                          

2. Clutches & Bands: 

            These are friction devices that drive or lock planetary gear sets members. They are used to cause the gear set to transfer power. In other words, they are used to hold a particular member of the planetary gear set motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body their sequence controlled by the transmission's internal programming.


3. Torque Converter: 

            It is a hydraulic device that connects the engine and the transmission. It takes the place of a mechanical clutch, allowing the transmission to stay 'in gear' and the engine to remain running whilst the vehicle is stationary, without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.


3. Valve Body: 

            The valve body is the control centre of the automatic transmission.  It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves.  Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift lever is in third gear, it feeds a circuit that prevents overdrive from engaging.  The valve body of the transmission contains several shift valves. Shift valves supply hydraulic pressure to the clutches and bands to engage each gear.  The shift valve determines when to shift from one gear to the next

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