The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 6000 revolutions per minute (though this varies from design to design and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 2500 rpm.
Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or travelling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.
Manual transmissions come in two basic types: a simple unsynchronized system where gears are spinning freely and must be synchronized by the operator to avoid noisy and damaging "gear clash", and synchronized systems that will automatically "mesh" while changing gears. The former type is only used on some rally cars nowadays.
Most modern cars in the United States have an automatic transmission that will select an appropriate gear ratio without any operator intervention. They are primarily using hydraulics to select gears, depending on pressure exerted by fluid within the transmission assembly. Rather than using a clutch to engage the transmission, a torque converter is put in between the engine and transmission. It is possible for the driver to control the number of gears in use or select reverse, though precise control of which gear is in use is not possible.
Automatic transmissions are easy to use. In the past, automatic transmissions of this type have had a number of problems, they were complex and expensive, and sometimes had reliability problems (which sometimes caused more expense in repair), and often have been less fuel-efficient than their manual counterparts. With the advancement of modern automatic transmissions this has changed. With computer technology, considerable effort has been put into designing gearboxes based on the simpler manual systems that use electronically-controlled actuators to shift gears and manipulate the clutch, resolving many of the drawbacks of a hydraulic automatic transmission.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a common modification is to increase the stall speed of the transmission. This is even more advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that occurs when the engine is idling and the throttle is suddenly opened.
Semi-automatic transmission, or clutchless manual transmission, is a system which uses electronic sensors, processors and actuators to do gear shifts on the command of the driver. This removes the need for a clutch pedal which the driver needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronise the timing and torque required to make gear shifts quick and smooth.
Sensons sense the direction of requested shift, and this input, together with a sensor in the gear box which senses the current speed and gear selected, feeds into a central processing unit. This unit then determines the optimal timing and torque required for a smooth clutch engagement, based on input from these two sensors as well as other factors, such as engine rotation, the Electronic Stability Program, air conditioner and dashboard instruments.
The central processing unit powers a hydro-mechanical unit to either engage or disengage the clutch, which is kept in close synchronization with the gear-shifting action the driver has started. The hydro-mechanical unit contains a servomotor coupled to a gear arrangement for a linear actuator, which uses brake fluid from the braking system to impel a hydraulic cylinder to move the main clutch actuator.
A continuously variable transmission (CVT) is a type of automatic transmission that can change the "gear ratio" (gears are not generally involved) to any arbitrary setting within the limits. The CVT is not constrained to a small number of gear ratios, such as the 4 to 6 forward ratios in typical automotive transmissions. CVT control computers often emulate the traditional abrupt gear changes, especially at low speeds, because most drivers expect the sudden jerks and will reject a perfectly smooth transmission as lacking in apparent power. CVTs can smoothly compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. They also avoid torque converter losses. This improves both fuel economy and exhaust emissions. CVTs have much smoother operation. This can give a perception of low power, because many drivers expect a jerk when they begin to move the vehicle. The satisfying jerk of a non-CVT transmission can be emulated by CVT control software though, eliminating this marketing problem.