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I read the following:

  1. Automatic transmission uses torque converter and planetary gears, while manual transmission uses clutch and a gear box.
  2. In automatic transmission, computer and hydraulics decide when and how to change gears.

What I don't understand is the connection between the two. Why, for example, manual transmission doesn't use torque converter or planetary gears?

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2 Answers 2

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A friction clutch and a gearbox makes a very efficient transmission (i.e. very little power is "lost" in the transmission). This system is relatively simple for a driver to control manually using a clutch pedal and a shift lever. A torque converter by comparison creates more power loss. This fact, along with the simplicity and robust nature of the design, are reasons that a manual transmission uses this arrangement.

Why then do automatic transmissions use torque converters and planetary gears if they're less efficient? They're a cheap-to-produce and simple means of achieving automatic shifting, and the shift action they produce can be very smooth (i.e. no "lurching" during gear changes). They typically operate on a hydraulic control system that selects gears based on vehicle speed and accelerator pedal (or throttle) position. Howstuffworks.com has a great, in-depth discussion of how this control system works. Basically, this arrangement was used because at the time it was developed, it was the cheapest and simplest way to acheive automatic shifting.

With advances in technology, it has become possible to create an "automatic" transmission that uses a friction clutch (or commonly more than one friction clutch) and a "conventional" gearbox (e.g. BMW's SMG and M DCT, VW/AUDI's DSG, Porsche's PDK). These are often referred to as "manu-matic" gearboxes or "dual clutch transmissions". These combine the efficiency of a manual transmission with the convenience of an automatic, but the control systems and hardware required are far more complex than a "conventional" torque converter automatic, so these transmissions have not yet largely replaced "conventional" automatics.

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Since OBD2 and not to mention the 1973 oil crisis torque converters have lockup to cancel the power loss. –  Allan Osborne Dec 5 '13 at 21:41
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The reason you have gears on any vehicle is that the gears are used to vary the torque applied to the driving wheels. To overcome its inertia a stationary vehicle requires a greater amount of torque to make it move then the torque required to keep the same vehicle in a state of motion. Extra gears allow the engine to power the vehicle at a lower engine speeds for the higher vehicle speeds. Lower engine speed means greater fuel economy. The clutch or torque converter allows you to have a 'neutral' gear so you do not have to stop the engine to bring the vehicle to rest, or change a gear. The decision to change gear on a manual is made by the driver, up or down, to maintain speed for differant engine loads, using the clutch. Autos have a torque converter which at engine idle speeds will not drive the transmission, as it is an hydraulic 'clutch'. On an auto, gear changing is the primary concern of line and governor positions. Line pressure from the throttle position, governor pressure form vehicle speed. Higher or lower pressures control the movement of spool valves, which engage or disengage clutches and brake bands. (Later auto vehicles have micro processors and solonoids to activate the spool valves.) The combination of clutch and brake bands changes the gearbox output speeds, and also direction for reverse. Autos generally use epicyclic geartrains which have the advantage of compact space when put together with their control components. Not the end of the story though. In Europe there is an increasing popularity for ManuMatics. These are manual gearboxes with electronic controls to change the gears.

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