How does an automatic transmission system in automobiles work?

Answer 1

What is an automatic transmission? What does it do, and why do we need it? First, a little history. The earliest automobiles offered only manual transmissions. Similar in principle to today's stick-shift vehicles, these cars, such as the Ford Model T, sported two forward gears and one reverse, coupled to the engine via a series of pedals. But as cars grew larger and traffic got worse, engineers began searching for a way to have the car "automatically" shift from one gear to another. Designers spent decades perfecting the modern automatic transmission. Although groundwork had been laid in the early 1900s by German manufacturers of marine engines, the first appearance of an automatic transmission in a car did not occur until 1938, when GM invented "Hydra-matic Drive." The first production automobile to offer this transmission was an Oldsmobile, which rolled off the line in 1940. This was followed in close succession by Chrysler (their spies must have been working overtime), which in 1941 introduced three separate models with "Vacamatic" (they changed the name to "Fluid Drive" in 1942). By 1948, most major American automakers offered passenger cars with optional fully automatic transmissions as an option. The technology came along at a propitious time in American history. The United States, rife with victory from World War II and building up steam for the post-war boom, produced scads of babies and cars (not necessarily in that order). Into those cars they dropped thousands of automatic transmissions. It could be fairly argued that the automatic transmission, with its simplicity and ease of use, offered up the automobile to the masses, fulfilling the promise of President Herbert Hoover, whom a generation earlier had promised "a car in every garage and a chicken in every pot." At the very least it widened the vistas of an increasingly mobile workforce, fed the flow of migration to the suburbs, and welcomed women back into the economy following the war effort. The automatic transmission did this by offering a "no-muss, no-fuss" form of shifting. No more missed shift gates. No engine lugging or racing. No torn-stocking, high-heel clutch-pedaling dramas. None of that. Just press the gas and go. Okay, enough with history. Let's take a look inside an automatic transmission. The Torque Converter The key to the modern automatic transmission is the torque converter. It takes the place of a clutch in a manual transmission. A direct descendant of an earlier component called a fluid coupling, the torque converter offers the advantage of multiplying the turning power provided by the engine. It is connected to the motor by means of a metal rod known as the transmission shaft (sometimes called the input shaft), which fits next to but does not touch the engine crankshaft. Repeat: there is no direct connection between the engine and the transmission. Instead, the engine turns the transmission by means of a process called hydraulic coupling. Think of two electric fans in a room. Line them up a few feet apart, one in front of the other, both facing the same direction. Now turn on the rear fan. What happens? If you've turned the fan up high enough, and if the fans are close enough together, the front fan will begin to turn as well. In this same manner does the engine crankshaft influence the transmission shaft, causing it to rotate. The identical process occurs in a torque converter, except that Transmission Fluid takes the place of air. But there's more. Inside the torque converter are several components that help multiply the power. These are the impeller (or pump), the turbine, and the stator (or guide wheel). A torque converter is like a giant doughnut. The impeller and the turbine (the two fans in the analogy above) face one another inside the round metal casing. The impeller is on the engine side, the turbine on the tranny side. Both of these components have blades that catch the transmission fluid and cause them to spin. Picture two halves of a chambered nautilus, and this will perhaps give you a better image. As one fan begins to spin, the other will spin as well. Through centrifugal force, the fluid moves to the outside of the blades, where it is redirected by a third fan, called a stator, back to the turbine side. This continual flow of fluid is what causes the power to be multiplied. The Lockup Torque Converter Because the only connection between two sides of a torque converter is a fluid connection, there is always a little slippage, running from about 2-8%. To increase efficiency and gas mileage, most modern automatic transmissions also have something called a lockup clutch (aka, torque converter clutch). It works like this. As the speed of the car nears 40 miles per hour, the highly pressurized transmission fluid is channeled through the transmission shaft and activates a clutch piston. This metal pin locks the turbine to the impeller, in effect bypassing the torque converter. It remains this way until the vehicle slows below 40 mph, at which point the clutch piston disengages and the torque converter kicks in again. Simple, right? Planetary Gearsets No, we're not talking Mars and Venus here. These are different-sized gears, just like the ones on the back of your mountain bike. They're called planetary because they're circular and revolve around a central gear known as a sun gear. If it sounds like our solar system, you're right -- it's designed on the same model. A planetary gearset system has three major elements: