The Advantages of Automatic Transmissionsby Richard Rowe
Although they are less popular in other parts of the world, most cars sold in America since the 1950s have been automatics. America's fondness for this transmission type began as a matter of convenience, but over the years automatic transmission vehicles have shown themselves to be capable and efficient performers.
The main benefit of automatic transmissions is that they are simply easier to use. Driving an automatic transmission car is so easy, in fact, that many nations, including the United Kingdom, France, Germany, Austria and Poland, require that new drivers take their certification tests in manual transmission cars in order to get their drivers' licenses.
As convenient as they are to drive, the very simplicity of automatic transmissions puts them at the center of some heated controversy. There are many who contend (primarily in Europe, where 80 percent of all cars are manuals) that automatics make driving too easy, decreasing driver involvement in the driving process and increasing the likelihood of an accident. That assertion has not, to date (2010), been proven by any scientific studies.
Automatic transmissions are inherently more powerful than any equivalent manual transmission. Standard automatics use a "planetary" gear-set housed near the back of the transmission, which utilizes a set of small "planet" gears driven by a "sun" gear on the inside and a "ring" gear on the outside to transfer power. This arrangement increases the amount of surface contact between gears, which spreads the torque load out over a greater area and reduces breakage.
Generally speaking, automatics shift faster and more accurately than most manual transmission drivers can manage. Shifting a manual transmission involves several degrees of driver movement, all of which offer opportunity for mistakes and mis-timings. The distance between the engagement clutches on most automatic transmissions is a matter of nanometers, so any performance-calibrated automatic could theoretically shift faster than its driver can blink.
Almost all true automatics use a fluid coupling called a "torque converter," which is what allows the engine to idle. Torque converters transfer power using an engine-driven turbine blade that pushes fluid through the transmission-mounted turbine blade. When the engine-side turbine blade reaches a pre-set RPM (stall speed), it overwhelms the transmission turbine's ability to pass fluid without moving.
In the few moments between engine idle and full transmission engagement, the difference between the turbines' speed creates a sort of gearing effect similar to that of a Continuously Variable Transmission. This increases torque just off of idle, and with it the vehicle's acceleration. It is for this reason that automatics are preferred for most types of professional drag racing, as they will almost always run lower times than the same car with a manual transmission.
- Auto gear shift handle closeup image by George Dolgikh from Fotolia.com