automotive transmission

automotive transmission

[¦ȯd·ə¦mōd·iv tranz′mish·ən]
(mechanical engineering)
A device for providing different gear or drive ratios between the engine and drive wheels of an automotive vehicle, a principal function being to enable the vehicle to accelerate from rest through a wide speed range while the engine operates within its most effective range.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Automotive transmission

The device in the power train of a motor vehicle that provides different gear ratios between the engine and drive wheels, as well as neutral and reverse. An internal combustion engine develops relatively low torque at low speed and maximum torque at only one speed, with the crankshaft always rotating in the same direction. To meet the tractive-power demand of the vehicle, the transmission converts the engine speed and torque into an output speed and torque in the selected direction for the final drive. This arrangement permits a smaller engine to provide acceptable performance and fuel economy while moving the vehicle from standstill to maximum speed. The transmission may be a separate unit as in front-engine rear-drive vehicles or may be combined with the drive axle to form a transaxle as in most front-drive vehicles. See Automobile, Automotive drive axle, Differential

The two general classifications are manual transmissions that the driver shifts by hand after disengaging the foot-operated clutch, and automatic transmissions that shift with no action by the driver. However, manual transmissions can have a clutch that is automatically disengaged by an actuator when the driver moves the shift lever, and automatic transmissions can have manual-shift capability which allows the driver to select the shift to the next lower or higher gear ratio by movement of the shift lever. See Clutch

The manual transmission is an assembly of gears, shafts, and related parts contained in a metal case or gearbox partially filled with lubricant. The transmission input shaft connects through the clutch and flywheel to the engine crankshaft (see illustration). The transmission output shaft connects through a driveshaft to the final-drive gearing in the drive axle. To get the vehicle into motion, reduction or underdrive gearing in the transmission allows the engine crankshaft to turn fast while the drive wheels turn much more slowly but with greatly increased torque. As the vehicle accelerates, and less torque and more speed are needed, the driver shifts the transmission into successively lower numerical gear ratios, known as higher gears. In a typical five-speed manual transmission, gear ratios are approximately 3.35:1 for first gear, 2:1 for second gear, 1.35:1 for third gear, 1:1 (direct drive) for fourth gear, and 0.75:1 (overdrive) for fifth gear. Most transmissions with four or more forward speeds are operated by a floor-mounted shift lever. See Gear

Six-speed manual transmission for a rear-drive carenlarge picture
Six-speed manual transmission for a rear-drive car

Automatic transmission provides automatic control of drive-away, gear-ratio selection, and gear shifting through four or five forward speeds. A typical automotive automatic transmission includes a hydrodynamic three-element torque converter with locking clutch, a planetary-gear system that provides overdrive in fourth or higher gear, and a hydraulic or electrohydraulic control system. Shifts are made without loss of tractive power. See Hydraulics, Torque converter

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.
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