Every engine has an ideal range of operating speeds, measured in revolutions per minute (RPM) at which the engine is at its peak efficiency. This is referred to as the power band. This peak efficiency range can be altered by changing several characteristics of the engine's operation, but it remains a relatively narrow band, compared to the speeds at which the car must be driven. The transmission allows the engine to remain within its designed operating RPM range while the car is driven at widely varying speeds. This is done through the use of rotating gears. If there were no transmission, the car could only be driven at speeds that would allow the tires to rotate at the same speed as the crankshaft (this is ignoring the gears that are within the differential). This would severly limit the speed range in which the car could be driven.
The gears in a transmission are preselected to allow a car to be accelerated from a dead stop to its highest speed, all the while keeping the engine in its most efficient operating range. This is acheived by using gears of different sizes between the input and output shafts of the transmission. For example, the transmission's lowest gear (commonly referred to as first gear) might have 3 times as many teeth (and 3 times the diameter) on the output shaft gear as on the input shaft gear. This means that the input shaft would rotate 3 times for each revolution of the output shaft (see gear math). The result is that the car can be driven at 1/3 the speed of what the car would be travelling if the engine's output went directly to the wheels, all the while keeping the engine within its power band.
Transmissions today, whether automatic or manual, generally have 3 to 5 forward gears, along with a reversing gear. Reversing gear simply changes the rotation of the output shaft relative to the input shaft, through the use of an additional idler gear.
The Manual Transmission
A manual transmission requires driver intervention not only to start from a standstill, but also to select the proper gear ratio for various driving conditions. When the car is first started, the driver disengages the transmission from the engine by means of a clutch. The clutch is a device that uses two friction surfaces, one attached to the output shaft of the engine and one attached to the input shaft of the transmission. When it is fully engaged, it forces the engine output and transmission input to rotate at the exact same speed. The transmission is geared directly to the transmission output shaft and hence the wheels of the car. If the clutch did not exist, the car would not be able to idle when stopped. The car would either have to continue moving, or the engine would stop running when the car stopped.
An experienced driver operating a manual transmission knows how to slip the clutch to gently bring the car's speed up to that of the engine when first starting out. This is acheived by gradually increasing the pressure between the rotating friction plates. If this seems backwards, keep in mind that there is spring pressure pushing the plates together. When the driver depresses the clutch pedal, it is that spring pressure that tries to push the pedal back up to engage the clutch.
The gears in the manual transmission are named numerically based on their ratio. First gear has the highest numerical ratio of output to input rotation, and is generally used at the lowest vehicle speeds. It offers the greatest amount of torque multiplication as well as higher engine RPM for a given vehicle speed. The higher numbered the gear is, the higher the gear (which can be confusing, since its ratio is numerically lower). In normal acceleration, the gears are changed in numerical order when accelerating until cruising speed is reached. That is, one would start from a dead stop in first gear, shifting to second gear based on engine speed, followed by third and so on.
The mechanism that changes gears in the manual transmission involves fork-shaped devices that slide dog clutches inside the main shaft so that different gears can be either engaged or allowed to spin freely independent of the main shaft, depending on the position of the gear shifter. It is a direct mechanical connection in that the gear shift lever is mechanically connected to the shifting forks inside the transmission (incidentally, the English refer to the transmission as the gearbox).
Modern manual transmissions use synchronizer rings to help match gear speeds to one another. Without synchronizers, the rotating gears would be more difficult to mesh or engage when shifting, since the teeth would be passing each other at different speeds. Synchronizers use low friction clutch-like action to gently nudge the counter-shaft's gear speed so that it matches the speed of the gear that it will be engaging with. As the synchronizers engage, through friction, they bring the speed of the counter shaft gear up or down to match that of the driving gear.
The Automatic Transmission
The most important part of the automatic transmission is the torque converter. This device replaces the clutch, and instead of mechanical friction, utilizes a fluid coupling between the engine's output shaft and the transmission's input shaft. The torque converter uses several vanes, not unlike fan blades except that they are turned almost parallel to the orientation of the shaft. When the engine's output shaft turns, it rotates the pump. The force of the spinning pump is transferred to the transmission fluid that resides in the torque converter. This spinning fluid then transfers the force to the turbine connected to the input shaft of the transmission. A third set of vanes called the stator spins freely, but redirects the spinning fluid back to the pump to allow for the maximum transfer of torque. This scheme allows enough slip at low engine speeds to provide minimal forward motion of the car. As the engine speed is increased, the amount of force applied to the fluid, and thus the turbine is increased, causing the car to accelerate. The fluid coupling allows enough slip to let a car idle when it is not moving, but will still provide the transfer of torque when the engine's RPM increases.
Transmission gearing in an automatic is somewhat different than in a manual transmission. The automatic uses planetary gearsets instead of counter-rotating shafts that are used in the manual transmission. A planetary gearset involves smaller gears that rotate around a center gear much like planets around the sun. An outher ring, with gear teeth, contains the planetary gears and keeps them meshed with the inner gear. The gear ratio in a planetary gear set is determined not only by the size and number of teeth in each gear, but also by which shaft is connected to which gear(s). If the input shaft is connected to the inner center gear, and the output shaft is connected to the rotating planet gears, there will be a gear reduction based on the fact that the inner gear will turn many times before the planet gears make one complete revolution within the ring. So, in this case, gear ratio changes can be acheived by the use of clutches that engage the input and output shafts to various gears within each planetary gearset.
In the automatic transmission, these clutches are controlled hydraulically. A pump in the torque converter provides the necessary pressure to engage each clutch as needed. In older automatic transmissions, this was acheived by check balls and springs in the transmission's valve body. The newest transmissions are controled electronically by the engine's computer (see electronics). In the case of the valve body, increasing pump pressure created by increased engine speed would overcome spring resistance, forcing a check ball out of the way to open a valve that would direct this pressurized fluid to the clutch actuator, causing the transmission to shift gears. The valve body is very complex, and is beyond the scope of this article.