Discovery Guides Areas


Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels
(Released January 2008)

  by Chao-Hsu Yao  


Key Citations





Atmospheric Engine: The atmospheric engine invented by Thomas Newcomen in 1712, today referred to as a Newcomen steam engine (or simply Newcomen engine), was the first practical device to harness the power of steam to produce mechanical work.

Automatic Transmission: An automatic transmission (commonly abbreviated as "AT") is an automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually.

Clutch: A clutch is a mechanism for transmitting rotation, which can be engaged and disengaged.

Continuously Variable Transmission (CVT): A continuously variable transmission (CVT) is a transmission which can change steplessly through an infinite number of effective gear ratios between maximum and minimum values.

Double Clutching: Double clutching (also called double declutching) is a driving procedure primarily used for vehicles with an unsynchronized manual transmission. Before the introduction of transmission synchronizers (in the 1920s) and helical cut gears, double clutching was required to prevent damage to an automobile's gear system.

External Combustion Engine: An external combustion engine (EC engine) is a heat engine where an internal working fluid is heated, often from an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, provides usable motion and usable work. The fluid is then cooled and reused (closed cycle), or dumped, and cool fluid pulled in (open cycle).

Gear Ratio: The gear ratio is the relationship between the number of teeth on two gears that are meshed or two sprockets connected with a common roller chain, or the circumferences of two pulleys connected with a drive belt.

Internal Combustion Engine: The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber.

Manual Transmission: A manual transmission (also known as a stick shift or just stick, straight drive, or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector. Most automobile manual transmissions allow the driver to select any gear at any time, but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow the driver to select the next-highest or next-lowest gear ratio.

Overdrive: Overdrive can refer to two different things. An overdrive is a device which was used to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. It also refers to a combination of gearing inside a transmission or transaxle which results in the output speed being greater than the input speed. In the latter case, "overdrive" does not refer to a physically separate identifiable part/assembly.

Planetary Gears: Planetary gearing is a gear system that consists of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear.

Semi-Automatic Transmission: A semi-automatic transmission (also known as clutchless manual transmission, dual-clutch transmission, automated manual transmission, e-gear, or paddle shift gearbox) is a system which uses electronic sensors, processors and actuators to do gear shifts on the command of the driver.

Steam Engine: A steam engine is an external combustion heat engine that makes use of the heat energy that exists in steam, converting it to mechanical work.

Synchronizer: In automobiles, a synchronizer is part of a synchromesh manual transmission that allows the smooth engagement of gears.

Tiptronic Transmission: Tiptronic is a type of discrete automatic transmission. A Tiptronic transmission can operate just as the common type of automatic transmission, but it also allows the driver to override the automatic mode by moving the shift lever into a second (Tiptronic) shift gate equipped with two spring-loaded positions:"upshift" and "downshift".

Torque Converter: A torque converter is a modified form of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load.