Transmission is a device that transmits power from the engine of a vehicle to other parts, from which it is eventually delivered to the wheels. The transmission is a major part of a system called the drive train, a series of devices that sends engine power to the wheels.
The drive power of a motor vehicle is a combination of rotational speed and torque (twisting effort). The transmission sets an engine speed that provides the wheel speed and other performance characteristics desired by the driver. The transmission can also reverse torque so the vehicle can back up. In addition, when the transmission is set in neutral, it delivers no power—even when the engine is running.
Automobiles, trucks, buses, bulldozers, and other types of motor vehicles have transmissions. This article discusses the most common automobile transmissions.
Why automobiles have transmissions
A transmission adjusts an automobile engine’s speed to provide such desirable characteristics as engine efficiency and smooth, swift acceleration. An engine can vary its speed and can produce a certain range of torque. However, engine efficiency varies greatly under different combinations of speed and torque. If a car did not have a transmission, the engine alone would have to produce all the combinations of speed and torque required in the normal operation of the car. The engine would therefore accelerate the vehicle poorly and would operate inefficiently most of the time.
The speed and torque of an automobile’s wheels require adjustment under a variety of conditions. For example, a car requires a large amount of torque to start moving after it has stopped. At this time, both torque and speed increase. When a car that has been traveling along a level road starts up a hill, it needs more torque merely to maintain its speed.
Transmissions and drives
An automobile’s transmission is connected to the engine so that the transmission can receive power directly from the engine. The arrangement of the engine and transmission depends upon the kind of drive the car has.
Most cars have front-wheel drive. This means that the front pair of wheels moves the car. Other automobiles have rear-wheel drive. In a four-wheel drive vehicle, the drive train can be adjusted to send power to the front two wheels, for ordinary driving, or to all four wheels, for driving on rough terrain or providing extra traction. In an all-wheel drive, power is provided to all four wheels.
In the most common arrangement, the engine and transmission are mounted transversely (across the car) between the front wheels. In this arrangement, the transmission is a part of a unit called a transaxle. This unit also includes a final drive and differential, a device that adjusts speed and torque. The transaxle drives the front wheels.
In most rear-wheel drive cars, the engine and transmission are mounted in front. A rotating cylindrical tube called a drive shaft delivers power from the transmission to a final drive and differential in the rear of the car.
What a transmission does
A transmission receives a speed and torque input from the engine and delivers a different speed and torque to the final drive and differential. The engine delivers its power to the transmission by means of a rotating crankshaft. The transmission receives this power at its flywheel, a heavy disk connected to the crankshaft. The flywheel turns whenever the engine is running. The transmission delivers its output power by means of a rotating shaft. Most transmissions adjust speed and torque by means of rotating gears.
Power.
A transmission can deliver almost the same quantity of power it receives. All transmissions lose a small amount of power due to friction between metal parts. Transmissions that use a liquid to transmit torque also lose power due to inefficiencies in devices that pump the fluid.
Power is the rate at which work is done. In the case of a rotating part, such as a shaft or a gear, power equals rotational speed times torque. In the inch-pound system of units that is customarily used in the United States, power is measured in horsepower. In the metric system, power is measured in watts. One horsepower equals 746 watts.
If the transmission receives a constant amount of power from the engine, the transmission cannot adjust its own output torque without also adjusting its output speed. This is so because power must always equal speed times torque. For example, if a transmission doubles the torque that it delivers, it must also decrease the speed of its output shaft by half.
Gears.
A pair of rotating gears whose teeth mesh (fit together) adjusts speed and torque in proportion to the numbers of teeth in the gears. One gear drives (applies torque to) the other. The relationship between the speed of the driver gear and the speed of the driven gear is called a reduction. Reduction is expressed numerically by gear ratio. The gear ratio of a pair of gears is equal to the number of teeth in the driven gear divided by the number of teeth in the driver gear. For example, if a gear with 9 teeth drives a gear with 27 teeth, the gear ratio is 3 to 1. This means that, for every three revolutions of the driver gear, the driven gear rotates once. In other words, the driven gear rotates at one-third the speed of the driver gear. The relationship between the torques of the two gears is numerically the same as the gear ratio—with the first number representing the torque of the driven gear. In the example above, the driven gear rotates with three times the torque of the driver gear.
Most transmissions use different sizes of gears to produce various gear ratios and thus various proportions of speed and torque. Gear ratios are often called simply gears or speeds.
Most transmissions have three to six forward gears, one reverse gear, and a neutral setting. In a typical three-speed transmission, first gear has a ratio of about 3 to 1; second gear, about 2 to 1; and third gear, about 1 to 1. Most transmissions with more than three forward gears have a high gear called overdrive. The gear ratio of overdrive is less than 1 to 1–in other words, the speed of the transmission’s output shaft is higher than the speed of the crankshaft.
There are two types of transmissions that differ in how they shift gears (change from one gear ratio to another). An automatic transmission has special devices that automatically shift gears as needed. In a car with a manual transmission, the driver uses a hand-operated lever called a gearshift.
How an automatic transmission works
An automatic transmission provides various gear ratios as they are needed. Cars with automatic transmissions have a lever called a selector. The driver can move the selector to positions for park, neutral, drive, low, and reverse. The engine can start only when the selector is in park or neutral.
To put a car into forward motion, the driver moves the selector to the drive position. Initially, the transmission will be in first, or low, gear. This gear provides the highest torque and the lowest speed. As the car picks up speed, the transmission will automatically shift into higher gears. In higher gears, speed is higher and torque is lower.
The driver may move the selector to low when going up or down hills or driving through snow or mud. With the selector in low, the transmission will remain in low gear, rather than shift automatically into higher gears. Some cars with automatic transmissions have one or more selector positions between low and drive. These positions prevent shifting above the selected gear.
The major parts of an automatic transmission are a torque converter, one or more sets of planetary gears, three kinds of clutches, and a controller.
A torque converter
transmits power from the engine to the gears. It also increases torque.
A torque converter resembles a large doughnut sliced in half. One half, called the pump, or impeller, is bolted to the flywheel. The other half, the turbine, is connected to the input shaft, which transmits power to the gears. The pump and turbine face each other in a case filled with a liquid known as transmission fluid. Both are lined with blades called vanes. A bladed wheel called a stator lies between them.
The engine causes the pump to rotate and throw transmission fluid against the vanes of the turbine. The moving fluid applies torque to the turbine. After hitting the turbine, the fluid passes into the stator and returns to the pump. The energy in the returning fluid helps the engine turn the pump. The addition of this energy makes the torque applied to the turbine higher than it would be if there were no stator.
When the engine is running slowly, the transmission fluid may not transmit enough torque to rotate the turbine. But when the driver presses the accelerator pedal, the engine runs faster and so does the pump. The torque gradually becomes high enough to rotate the turbine, and thus move the car.
As the automobile gains speed, the speed and torque of the turbine gradually approach the speed and torque of the pump. Eventually, when the automobile is cruising in third gear or overdrive, the torques become equal. However, the turbine always tends to run more slowly than the pump. The difference in speed is due to an unavoidable inefficiency in the torque converter. To overcome this inefficiency, many transmissions have a lock-up clutch. When the vehicle begins to cruise in a higher gear, the clutch locks the pump and turbine together. The pump and turbine then rotate as if they were a solid unit.
The torque converter transmits power to an assembly of planetary gears and clutches. The arrangement of the torque converter and this assembly depends on the location of the engine and transmission and the type of drive.
In a car with front-wheel drive and a transversely mounted engine and transmission, the torque converter and the assembly are offset from each other. The torque converter delivers power indirectly. A shaft leading from the torque converter drives one sprocket of a chain drive. The other sprocket drives a shaft that leads to the assembly.
In a car with a rear-wheel drive and a front-mounted engine, the assembly is lined up with the torque converter. The torque converter transmits the power directly, by means of a shaft leading to the assembly.
Planetary gears.
Most automatic transmissions have two sets of planetary gears arranged in a row. The transmission controls the parts, or elements, of these gear sets to produce various gear ratios and reverse.
Each planetary gear set consists of three elements. The first element, the sun gear, is mounted at the center of the set. The second element, the carrier, surrounds the sun gear. The carrier holds two, three, or four smaller planet gears. The planet gears are equally spaced around the sun gear and mesh with it. The third element, the internal gear, is a ring with teeth inside it. The internal gear surrounds, and meshes with, the planet gears.
Clutches.
Any element of a gear set can be held stationary or locked to one of the other elements to produce different gear ratios. In an automatic transmission, clutches perform these tasks.
A clutch is a device that can connect or disconnect two components (parts) of a transmission, one or both of which are rotating. An automatic transmission has three kinds of clutches: (1) plate clutches, (2) band clutches, and (3) one-way clutches.
A plate clutch
has a number of washer-shaped plates stacked next to one another. Alternating plates are permanently attached to the two components. With the clutch disengaged, the plates are separated so that neither component can drive or hold the other. When the clutch is engaged, the plates are pushed together. One component can now drive or hold the other.
A band clutch
can stop and hold one element of a planetary gear set. This clutch has a band mounted around the outside of a drum. The drum, in turn, is attached to the planetary element. With the clutch disengaged, the drum is free to rotate. When the clutch is engaged, the band tightens around the drum, stopping and holding the drum.
A one-way clutch
drives a component in one direction only. The clutch also enables another part of the transmission to rotate the component more rapidly in the same direction.
The controller
is an assembly of valves and passages through which a liquid called hydraulic oil flows. The controller is part of the transmission’s hydraulic system, which engages and disengages the plate and band clutches. A one-way clutch operates automatically and does not require hydraulic oil to work.
Other parts of the hydraulic system include a pump and various passages leading from the controller to pistons that control the clutches. The pump pushes oil into the controller. The valves shift to direct oil into the appropriate passages. The valves shift automatically, in response to the selector setting, the driver’s pressure on the accelerator pedal, and other factors.
How a manual transmission works
The driver shifts the gears of a manual transmission by means of a gearshift. The driver puts the transmission into the neutral position when starting the engine or when stopping the car with the engine running.
To put a car into forward motion, the driver shifts into first, or low, gear. As the car picks up speed, the driver shifts into second gear, then into third gear, and so on, until the transmission is in the highest gear desired. If extra torque is needed, the driver may downshift from a higher gear to a lower one. Such a situation might occur when the car goes up a steep hill.
A clutch connects the engine to the transmission. The driver controls the clutch by pressing and releasing a pedal. The driver must operate the clutch along with the gearshift. When the driver presses the pedal, the clutch is disengaged, and no power is sent to the transmission. When the driver releases the pedal, the clutch is engaged, sending power to the transmission. The driver must disengage the clutch when shifting gears.
The clutch.
The main parts of the clutch are three disks: the flywheel, the clutch plate, and the pressure plate. The flywheel and pressure plate are connected to the crankshaft. They therefore turn whenever the engine is running. The clutch plate is mounted between the flywheel and the pressure plate, and is connected to the input shaft of the transmission.
When the clutch is engaged, springs in the pressure plate press the clutch plate against the flywheel. The three disks therefore turn at the same speed. When the clutch is disengaged, the springs are released, so the disks separate.
The gears.
Power travels from the clutch plate to the transmission’s input shaft. A gear at the end of the input shaft drives a gear on another shaft called the countershaft. The countershaft holds several gears of different sizes. These gears drive other gears on a third shaft, the output shaft. The output shaft leads to the drive shaft.
The transmission produces various gear ratios by engaging different combinations of gears. For reverse, an extra gear called an idler operates between the countershaft and the output shaft. This gear turns the output shaft in the opposite direction of the input shaft, thus making the car go backward.
The final drive and differential
An automatic or manual transmission transmits power by means of a rotating shaft to a final drive and differential unit. This unit, in turn, delivers power by means of two output shafts–one leading to each drive wheel.
The final drive
provides an additional reduction. In a transaxle, the final drive is usually a set of planetary gears. In a car with rear-wheel drive and a front-mounted engine and transmission, the final drive is a set of bevel gears. The bevel gears mesh at a right angle. One gear is connected to the drive shaft; the other, to the differential. See Gear .
The differential
is a complex gear set that divides torque evenly between the two drive wheels. The differential also enables one wheel to rotate faster than the other. For example, when a car goes around a corner, the outside wheel must travel farther than the inside one. The differential enables the outside wheel to rotate faster than the inside wheel to cover the longer distance in the same time.
How a continuously variable transmission works
Unlike automatic and manual transmissions, which have set gear ratios between the input and output shafts, continuously variable transmissions (CVT’s) provide an infinite number of gear ratios between an upper and lower limit. A CVT achieves this using wheels, pulleys, cones, or some combination of these three components. There are many types of CVT’s.
A relatively simple example of a CVT is the disk and wheel. In this design, a disk is mounted on a shaft driven by the engine. The disk is in contact with a wheel that can slide on an axle mounted at a right angle to the shaft being driven by the engine. The wheel and axle rotate faster when the wheel meets the disk near its edge, and more slowly when the contact point is nearer the disk’s center. The gear ratio changes continuously as the wheel moves along the axle.
Other CVT designs have some other device between the driving shaft and the driven shaft. For example, in a variable diameter pulley CVT, two cones face each other with a belt riding between them. The belt is V-shaped so that it rides the “V” formed between the cones. If the cones of the pulley move closer together, the belt rides farther from the axis of the pulley. As the cones move farther apart, the belt comes closer to the axis. Both the driving shaft and the driven shaft have this pulley configuration. Constant tension on the belt is maintained by reducing the distance between the pulley cones on one shaft while increasing the distance between the cones on the other shaft.
