Automatic transmissions have become very common in American cars. While Asian or European vehicles have tended to stay with manual transmissions, American vehicles have moved almost exclusively to automatic, especially among non-performance machines. In fact, for the everyday American driver, the term €standard€ transmission, which refers to the manual transmission, is a misnomer, since the standard is now automatic.

Vehicles with automatic transmissions are arguably easier to drive, and can be done so using only one hand and one foot. Contrast this with the two hands and two feet needed to properly shift gears using a manual. To switch gears using a manual transmission, the driver needs to feel for where the clutch slips, and where it €sticks€, and develop skill at transitioning between the two. A skilled driver can shift gears using a manual as smoothly as an automatic.

But how does an automatic transmission shift gears? Instead of a clutch, an automatic transmission has a torque converter. A torque converter is a fluid coupling that transfers the rotational energy of the input shaft to the output shaft. Because fluid is used, the efficiency is never 100%. The best fluid couplings have an efficiency of about 94%, which means that for every 100 revolutions of the input shaft, the output shaft rotates 94 times.

While primitive hydraulic couplings were simply two rotors in a liquid-filled case, modern torque converters are capable of converting speed to torque, much like a reduction gearbox. This is done by a system of rotors and stators that control the speed and direction of the fluid in the case.

Torque converters need to operate at three distinct categories of speeds. The first category is stall speed. In this situation, the engine is running, but the vehicle is not moving, such as when the driver's foot is on the brake while waiting for a red light. In a manual transmission situation, the clutch would be fully depressed, and the engine entirely disconnected from the wheels. However, in the case of an automatic, there is no clutch, and so fluid flows out of the converter, and partially bypasses the output rotor.

At low speeds, the fluid flows from the input rotor to the output rotor, through the stator and back to the input rotor. At high speeds (greater than 40 mph), the torque converter locks. This means that instead of merely a fluid connection between the moving rotors, a pin connects them rigidly, bringing efficiency up to 100%.

Operating the torque converter with the rotors at vastly different speeds sends the highly pressurized fluid through the stator region of the case at high speeds, and generates heat. If done excessively, the increased pressure and heat can ruin the case. In such situations, a qualified auto mechanic would be necessary to replace the converter.