POWER TRAIN

The parts of the automobile that transmit power from the engine to the driving wheels make up the power train. These are the clutch, transmission, drive shaft, and differential. In most cars power is delivered to the rear wheels (rear-wheel drive). Increasing numbers of car engines, however, transmit power to the front wheels (front-wheel drive). This improves traction because the engine's weight is centered over the wheels that power the car. It also eliminates the drive shaft and the hump in the car's floor to accommodate it. This, in turn, increases interior legroom. An alternative with similar advantages is the rear-engine, rear-wheel drive car. Four-wheel drive cars have even better traction and are generally used to travel rough country or to drive through snow. Some have a part-time feature in which a flick of a lever transmits power to a set of wheels to provide four-wheel drive.

 

Clutch

It is not practical to connect the wheels of an automobile directly to the engine. A device is needed to uncouple the wheels from the running engine so that the driver can warm the engine and keep it running while the vehicle is not moving. The engine must also be separated from the gearbox when the driver is shifting gears. Most automobiles provide for this uncoupling either with a friction (disk) clutch or with a fluid coupling.

 
   Friction clutch. In the operation of a friction clutch, the driver slowly releases the clutch pedal, and the disk slips against the flywheel. As the clutch is further let in, or engaged, springs hold the disk more firmly. Friction on the disk gradually increases until finally the disk and flywheel move together as a unit without slipping.

 
   Fluid coupling. The fluid coupling is also called a fluid flywheel or hydraulic coupling. In it there is no solid connection between the driving and driven members. Oil permits the fluid coupling to slip easily at low engine speeds. At high speeds the slippage is almost eliminated, and the fluid coupling functions like a solid connection. Sometimes both a fluid coupling and a friction clutch are used.

 

Transmission

The transmission is a device installed at some point between the engine and the driving wheels of a vehicle to change speed and power. Power from the engine is provided in the form of torque, or twisting force. The amount of this force varies a great deal, depending on the individual characteristics of the engine and the speed at which the engine is running. At high speeds the torque is greater. The amount of torque needed to move a car is not always directly related to speed. When a car is traveling at moderate speed on a level road, the engine does not need to supply much torque to keep it going. When the car is starting from a dead stop or moving up a hill, however, the engine must deliver enough torque to get or keep the car moving. Turning speed (revolutions per minute) from the engine may be reduced or increased by gears and thus converted to provide greater torque or greater speed.

   The gearshift lever of a car moves shifting forks, which engage or disengage various combinations of gears. These combinations provide more or less torque and speed, and they determine the direction in which the vehicle will move. Most modern cars are equipped with an automatic transmission, which alters the combinations of gears or of torque converters without the movement of a lever by hand.

 
   Standard gear transmission. The conversion of engine speed and the resulting torque may be accomplished by engaging various combinations of large and small gears. The so-called standard transmission for many years has been a gearbox with a combination of gears. The standard gearbox usually provides three forward speeds and one reverse speed. Four and five forward speeds are not uncommon, especially on sports cars, and some trucks have even more.

   Most standard transmissions manufactured after 1965 were equipped with synchromesh, a system in which all forward gears have a similar mesh design. This makes it easier for slow-moving gears to engage smoothly with fast-moving ones and eliminates clashing and grinding when the driver shifts.

 
   Planetary gear transmission. Another means of changing speed and torque is through the use of planetary transmission. Just as the planets in space move about the sun, so planetary gears move around a sun gear. Both of these types of gears move within a ring gear, which has its teeth on the inside. The same principles of torque conversion that govern the standard gear transmission apply to the planetary system. Small and large gears follow the same mechanical laws of speed and power. In the planetary transmission system the parts may be either driving or driven (that is, providing power or receiving power) depending on the torque conversion needed.

 
   Automatic transmission. Although all automatic transmissions essentially eliminate the driver's need to shift gears and use a clutch, the design and construction of automatic transmissions may vary with each manufacturer. All modern systems, however, incorporate one or more fluid couplings or a torque converter, one or more sets of planetary gears, and valves (with suitable controls) that direct the flow of automatic transmission fluid.

   One form of automatic transmission is a hydraulic torque converter, which achieves the same shifting effect as a gear transmission. This method somewhat resembles the fluid coupling. Oil transmits power in both. At lower speeds the blades of the pump, or impeller, force oil against the blades of a stator. These blades deflect the oil against a turbine, therefore increasing torque. At higher speeds, as in the case of the fluid coupling, the oil, pump, and turbine turn together as a unit.

   The oil moves in different directions in different parts of a hydraulic torque converter. The pump spins and throws the oil outward. The doughnut-shaped housing that encloses the pump and turbine forces the oil toward the turbine. There it strikes the turbine blades and slides inward toward the turbine hub and then returns back through the stator.

   The stator is equipped with an overrunning, or one-way, clutch. This device permits the stator to be used for deflection of oil at low speeds and to move with the pump and turbine at high speeds.

   There are many variations of the hydraulic torque converter. They involve complexities not described here. The number of elements to deflect and direct the oil vary with the type of unit. Some torque converters have as many as five elements. Others are combined with gear transmissions.

   Systems with hydraulic torque converters provide excellent smoothness in shifting gears. The torque ratio changes automatically to produce changes in car speed or to meet the need of extra torque, as in climbing hills.

 
   Other systems. Hydraulic transmissions may not be fully automatic. Some of them are semiautomatic. In the most common type of semiautomatic transmission, the driver moves a lever but does not use a clutch pedal. In this case the clutch operates automatically in response to a control such as engine oil pressure or intake manifold vacuum. The initial movement of the automobile's shift lever releases the clutch, and, when the shifting is completed, the clutch automatically reengages.

   A European development is the magnetic powder coupling. This has some of the characteristics of a fluid coupling. The system is electromechanical; no hydraulics or pneumatics are involved. The unit provides three automatically changing forward speeds and reverse gears. One magnetic powder coupling gives direct drive from the engine. A second powder coupling gives an indirect drive. The transmission for this system performs automatically in response to road speed and accelerator pedal position.

 

Drive Shaft

The drive shaft carries the torque from the transmission to the axle. The shaft must be equipped with several universal joints. These permit the axle to move freely up and down or from side to side as the wheels roll over road irregularities.

   A sliding, or telescoping, joint also is used on drive shafts. This joint permits the shaft to change its length slightly with the up-and-down movement of the rear axle.

   A flexible drive shaft with a minimum number of joints has been used on some cars. This small-diameter shaft is of alloy steel. Vibration and whip of the shaft as it turns are limited by two center bearings mounted around the shaft. A plastic-type coating prevents corrosion.

 

Differential

The rear end of the drive shaft leads into a bulge in the rear-axle housing where the differential is located. The differential applies power as needed to the wheels while they turn at different speeds on curves.

   The difference in speeds is necessary because the outer wheels must travel both farther and faster than the inner wheels when the vehicle is going around a turn. This could not occur if the two wheels were rigidly attached to a solid axle. The two front wheels of an automobile with rear-wheel drive present no problem, as each is mounted on its own spindle and turns independently. The rear wheels, however, drive the car, and they must be attached to a strong axle supplying torque from the engine. The rear axle, therefore, is in two pieces connected through the differential.

   In some ways the differential works like planetary gears, for, depending on need, some gears will move slower or faster than others or even remain stationary. The system turns the wheel that is easiest to turn. This aids in turning corners but has other disadvantages.

   One of these disadvantages, for example, if one wheel of an automobile is on dry pavement and the other is in slippery mud, the wheel in the mud will spin and the other will remain stationary. To eliminate this problem, a complex controlled slip differential has been developed. It prevents one wheel from slipping while the other is standing still.

   Some front-wheel drive and rear-engine cars use a transmission-axle combination called a transaxle. In this arrangement a gearbox or torque converter is positioned so that it will drive the axle directly, eliminating a drive shaft connection. Few cars of this type are currently manufactured.

 
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