Automobile

Once the pistons are firing and the crankshaft is spinning, this energy must be converted, or transmitted, to drive the wheels. But the crankshaft spins only within a limited range, usually between 1,000 to 6,000 revolutions per minute (rpm), and this is not enough power to cause the wheel to turn when applied directly. The transmission accomplishes the task of bringing the engine's torque (the amount of twisting force the crankshaft has as it spins) to a range that will turn the wheels. One way to experience the effect of torque is by using two wrenches, one with a short handle, the other with a long handle. It may be difficult to turn a nut with the shorter wrench; but the nut turns much more easily with the longer wrench, because it allows a more powerful twisting force. The transmission has two other functions: allowing reverse movement, and braking the engine.

There are two types of transmission: manual and automatic. With a manual transmission, the driver controls the shifting of the gears. In an automatic transmission, as its name implies, gears are engaged automatically. Both types of transmission make use of a clutch, which allows the gears to be engaged and disengaged.

Automobiles generally have at least three forward gears plus a reverse gear, although many manual transmissions have four or even five gears. Each gear provides a different ratio of the number of revolutions per minute (rpms) of the crankshaft (the power input) to the number of revolutions per minute of the output of the transmission, directed to the wheels. In first gear, for example, which is needed to move the automobile from a standstill, the ratio of input to output is 3.5 to one or even higher. The greater the ratio, the more torque will be achieved in the output. Each successively higher gear has a lower input to output ratio. This is because once the automobile is rolling, progressively less torque is needed to maintain its movement. The fourth and fifth gears found on most cars are used when the engine has achieved higher speeds; often called overdrive gears, these gears allow the input to output ratio to sink lower than one to one. In other words, the wheels are spinning faster than the crankshaft. This allows for higher speeds and greater fuel efficiency.