How does an Automatic Transmission work? All I know is you put it in Drive and it goes… Somehow it disengages when you stop, and switches gears when you go fast enough. I can understand a manual transmission, because it’s obvious how the gears are switching and you can feel it happen. The
Automatic one is like Magic… Help me Auto buffs…
Oh, and be simple… I’m, not mechanically inclined. <sob>
Fill a large washtub with water. Stick your arm down into the water and start running your arm through the water in a large circle.
At first it takes a bit of work to get all the water moving, but in just a few seconds, the water has motion and keeping it spinning is a lot easier.
An automatic transmission works in much the same way. When the transmission (and torque conconverter) determine that things are moving too easily, it shifts to the next gear.
Simple in theory, but there are hundreds of parts, several dozen of them moving at any given time, all working in concert to make things happen in an orderly and graceful fashion.
Just the opposite of a two stroke engine which shouldn’t work at all because there simply aren’t ENOUGH moving parts.
PS: there is an arm in my car :-)?
What SouthernStyle said.
Or get a garden hose and an old electric fan. Spray the water directly into the face of the fan and you’ll soon see the fan spinning around, driven by the water. Now con your innocent sibling into holding the blades stationary and THEN spray the water directly into the face of the fan. Fan doesn’t move, water splashes back towards you and all over your sib 
Now imagine that you hook up a tailshaft to the fan, plus some extra gears to the fan to multiply or reduce the revolutions per minute of the tailshaft, and on the other end of the tailshaft you hook a differential and a pair of wheels. Unlike a manual tranny, where the gears only shift when someone specifically tells them to, these gears are held in position by water pressure. Let’s pretend we can catch all the water that splashes back from the fan, which is the part that doesn’t go through the fan blades, and use that water to hold the gears in place. When the fan is still and we suddenly squirt water at the fan, lots of water splashes back and very little goes through the fan blades, and this backsplash water holds the gears in “first gear” formation. But the fan quickly starts turning, and more and more water goes THROUGH the fan, driving it faster as it does. If we can also catch the water that runs THROUGH the fan and use THAT water pressure to attempt to switch the gears to “second gear” formation, you can see that as the fan picks up speed, the ratio of water pressure pushing for first gear to the water pressure pushing for second gear shifts in favor of second gear until the gears move. In a loose sloppy way that’s how auto transmission hydraulics work.
Now the gears themselves aren’t like manual transmission gears either (imagine the results in a manual trans if you had some kind of pressure suddenly pull the first gear out of mesh and try to cram the second gear into the moving drive teeth!). Imagine a bunch of ball bearings arranged on the outside of a metal band (called a race) and another metal band running around the outside of the race. Each ball bearing rolls against both races. If you hold the outside race still with your left hand and spin the inside race, the inside race spins around in a circle. The ball bearings invidually spin backwards as it does, but they also as a group roll slowly in the same direction as the inner race. Now switch: hold the inner race still and spin the outer race. Again the ball bearings follow the outer race but at a slower pace. Now suppose you had a way of holding the ball bearings so that they could spin individually against the races, but can’t “walk” around the circle as a group, and you don’t hold EITHER of the races. Spin the inner one and the outer one backtracks in reverse! Now, hold the inner race, let go of the outer, and push the ball bearings around the inner race directly. The outer race speeds ahead, moving faster than the ball bearings. Hold the outer and let the inner race free and spin the bearings directly and the inner race speeds ahead of the ball bearings even faster yet. The tailshaft and the driven shaft can receive or deliver torque to inner race, outer race, or the bearings carrier. That is a crude description of how the gears of an auto tranny, called “planetary gears”, work.
The hydraulic pressure that changes the gears actually moves devices that hold or release the different parts of the planetary gear assembly. If they do this suddenly, gear changes can occur with a bang and a jerk and a squeal of rubber, but it isn’t like trying to slam a manual transmission’s second gear into the teeth of a moving drive gear. With the exception of suddenly popping it into reverse, it just works fluidly and efficiently to swap from one gear to another.
Thanks Southern and AHunter3…
Everything seems to make much more sense now. Finally I think I understand the concept behind the automatic tranny… The fan analogy really helped.
Thanks again.
TTYL
J.C.