The windmill and the motor car may seem unlikely cousins, but both transmit power through rotating shafts. Methods of connecting vertical and horizontal shafts, equipment of different weights, and shafts turning at different speeds were crucial obstacles to automotive pioneers. Steam carriages moved forward by simply opening a throttle, but the internal combustion engine must not be connected to the rear wheels (where the load is exerted), it must continue running while it is stationary, and it must accelerate from a low initial speed to a higher one. Consequently, a device is needed to apply loads gradually after the engine is started.
In the 1890s, Emile Levassor adapted a cone-shaped clutch and sliding gears he observed on woodworking machinery to the transmission of automotive power. Louis Renault followed in 1899 and created the general layout of the transmission that was to be adopted by the fledgling automotive industry; he furnished a propeller shaft with universal joints that drove a pinion and crown wheel linked to the differential on the rear axle. This speed-changing mechanism allows the power to be controlled at low speeds and increased at high speeds for maximum engine efficiency. A ratio or relationship exists between engine torque (turning capacity) and wheel speed which is the same factor as the increase of engine speed.
To select the best gear ratio (in the simplest type of transmission), the driver moves a shift lever to snag a spur gear that engages the forward speed gears or the reverse gear. This sliding gear transmission used clutches and a blocker ring to prevent the gear teeth from grinding. The driver has to select the appropriate gear for the speed of the car (and other conditions) and manipulate the shift lever while operating the accelerator and clutch pedals. The automatic transmission eliminated this complex action.
Clutches progressed through several evolutionary steps from the cone clutch to those constructed of combinations of interleaves and discs or multiple plates. Moulded friction materials improved simultaneously, and, by 1921, the complication of sticking plates was eliminated and replaced with a single plate, the operation of which was smoothed by helical springs and diaphragms.
The automatic transmission was invented by Gaston Fleischel of France in 1933. Automatic transmissions may use a fluid coupling, hydraulic torque converter, or mechanical linkage to adjust the torque over the gears without the driver's intervention. The driver still manually selects a low drive gear or reverse, but the automatic transmission then adjusts for grades on hills and the change of gears, although at a lower gear ratio than the manual method. By 1970, the hydraulic torque converter predominated. A governor in this transmission makes shifts depending on engine speed, and the driver can control the gear changes to a certain extent by adjusting the car's speed. Multiple-disk clutches and bands in oil are hydraulically operated by the governor. Automatic transmissions tend to produce the best performance because power is continuously applied. The opportunity for error and inefficiency exists when the driver can interrupt this power flow.
Hydraulic valves used to control transmissions have been replaced as of the mid-1970s by electronic systems. Sensors that can measure road and driving conditions improve the efficiency of the automatic transmission by adjusting the shift speed for hills, for example. While electronics may "read" conditions, they still actuate fluid pressure to engage brakes and clutches within the transmission.
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