A gearbox is needed to help the engine
A gearbox is needed to help the engine apply more turning effort when necessary.
Car engines develop their maximum turning effort when the crankshaft reaches a certain number of revolutions per minute. This number is not the same for all cars. For example, a car designed for high-speed driving develops its maximum effort at a high number of engine revs, but a car designed to have pulling power develops it at a lower number of revs.
With any car, more effort is needed to get it moving than to keep it going once it has picked up speed. And more effort is needed to drive it uphill than on the flat. In bottom gear, the propeller shaft turns about three times slower than the crankshaft, in second gear about twice as slow, and in third gear just slightly slower.
This gives the engine more push, because the same power applied at a slower speed has greater force - just as it takes more effort and slower winding to raise a heavy bucket from a well than a light one.
When reverse gear is engaged, the propeller shaft turns about four times slower than the engine.
The exact difference in turning relationships between the engine and the propeller shaft depends on the design of the car, and is known as the gear ratio. In top gear, the propeller shaft turns at the same speed as the crankshaft, so thel.ear ratio is 1: 1. This is because by the time the car reaches top gear, the engine is near its maximum effort and the car has gained sufficient speed for extra effort to be unnecessary.
Inside the gearbox, the change in speeds between the two shafts is brought about by locking together different-sized gear wheels with toothed outer edges. For example, if one gear wheel is locked with another that has twice the number of teeth, the smaller wheel makes two revolutions for each revolution of the larger wheel. In a manual gearbox, the driver makes the adjustments by operating the gear lever.
In an automatic gearbox, the changes are brought about by oil pressure. The oil pressure is regulated by valves known as shift valves.
They work either by pressure from throttle when the driver pushes down the accelerator pedal, or by a device called a governor that is sensitive to road speed, and is located on the gearbox output shaft.
The governor takes control from the throttle pressure once the car reaches a predetermined road speed. When the car drops below the predetermined speed, the throttle pressure takes over again.
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