Cars need transmissions because of the physics of the gasoline engine. In this article, we won’t get into the complexities of how an engine works. Instead, we’ll start by showing you the basics of how the most common transmissions work:
Manual transmissions require that the driver change gears by depressing a clutch pedal and using a stick shift. The clutch pedal connects the transmission to the engine, and the input shaft of the transmission turns at the same rpm as the engine. When a driver changes gears, the clutch pulls away from the transmission so that you can shift into a different gear and produce a different rpm value. A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft.
Cars with an automatic transmission have no clutch that disconnects the transmission from the engine. Instead, they use a device called a torque converter. Using the torque converter, clutches, and sets of planetary gears, the automatics do all of the shifting work for drivers.
Ideally, a car’s transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the continuously variable transmission (CVT). A CVT has a nearly infinite range of gear ratios. This transmission uses rubber bands which can imitate any gear in a car. Originally, CVTs produced an unpleasant driving experience. They couldn’t compete with four-speed and five-speed transmissions in terms of cost, size and reliability, so you didn’t see them in production automobiles. Since then, they have since been improved for excellent fuel economy appearing in several models, like the Toyota Prius.
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