Drivetrains
The engine can provide all the power in the world, but if it isn’t transferred to the wheels the car will not move. Automobiles need to perform equally well under a variety of loads. The transfer of power needs to accommodate the conditions on the highway, the weight of the vehicle and its passengers, and the performance desired to accelerate. Today’s automobile buyers can choose between rear-wheel, front-wheel, four-wheel, or all-wheel drivetrains. This section discusses the purpose of the drivetrain, drivetrain components, and types of drivetrains.

Purpose of the Drivetrain
The purpose of the drivetrain is to transfer power from the engine to the wheels in order to propel the vehicle. This transfer needs to be done smoothly and efficiently. Without smooth transitions, the automobile would not be very comfortable or easy to drive. The drivetrain also helps to control the speed and power through gears.

Drivetrain Components
Several components need to work together to transfer the power smoothly and efficiently from the engine to the wheels:

• Gears
  
• Transmissions
  
• Drive shafts
  
• Clutches
  

Gears
Gears are used in power transfer systems. Gears are always in sets. Gears can be used to change speed, torque, and direction of travel.

Transmissions
Gears are housed in a transmission. A transmission can be automatic, manual, or constant variable. In automatic transmissions the gear shifting is done automatically. All the operator has to select is the forward or reverse gear. Many transmissions today are computer controlled electronic transmissions that calculate the most efficient time to shift. In manual transmissions the operator of the vehicle does the gear shifting manually. Manual transmissions commonly come in 3, 4, or 5 speeds. Constant variable transmissions have recently begun to appear on some passenger cars. Instead of using gears they use two variable speed cone-shaped pulleys in conjunction with a belt. Transmissions change the power to the wheels for different applications. Maintaining a steady speed requires less power than accelerating to that speed.

Drive Shafts
On front-wheel drive vehicles two drive shafts, called constant velocity (CV) shafts, connect the transmission to the wheels. On rear-wheel drive vehicles, the drive shaft connects the transmission to the rear differential, which sends the power to the wheels. Four-wheel and all-wheel drive vehicles usually have a combination of constant velocity (CV) and standard drive shafts. Standard drive shafts have joints, called universal or U-joints, to accept the moving chassis. The shafts, whatever the type, are designed to transfer power from the transmission to the wheels.

Clutches
A clutch disc is used in conjunction with a pressure plate in a manual transmission to shift gears. The clutch is the connection between the transmission and engine. When the clutch is engaged, the engine is driving the transmission. When the clutch is disengaged (when the clutch pedal is depressed), the transmission is disconnected from the rotational motion of the engine’s crankshaft. A pressure plate works in conjunction with the clutch disc to aid in the engaging and disengaging process. This allows the operator the ability to shift from one gear to the next.

Drivetrain Systems
Drivetrain systems can be divided into the following categories:

• Rear-Wheel Drive
  
• Front-Wheel Drive
  
• Four-Wheel Drive
  
• All-Wheel Drive
  

Rear-Wheel Drive
On a rear-wheel drive vehicle the power from the transmission is transferred to the rear wheels. This was the standard on cars before the 1980s. This has been and still is the standard on most pickups.

Front-Wheel Drive
On a front-wheel drive vehicle the power from the transmission is transferred to the front wheels. This is the standard on most cars today. The weight of the engine on the front wheels gives front-wheel drive cars extremely good traction on diverse road conditions.

Four-Wheel Drive
On a four-wheel drive vehicle the power from the transmission is transferred to the rear and/or front wheels. On most four-wheel drive vehicles the operator has the choice of selecting either two-wheel drive or four-wheel drive. Many trucks have this option. Four-wheel drive vehicles have a tendency to get worse gas mileage than two-wheel drive vehicles. This is from the addition of friction from the drivetrain turning more components.

All-Wheel Drive
On an all-wheel drive vehicle the power from the transmission is transferred to the front wheels and rear wheels. All-wheel drive systems are different from four-wheel drive systems. The operator doesn’t have the control to select from two-wheel drive to four-wheel drive. All-wheel drive vehicles use electronics to control the power transfer to the wheels. Speed sensors are mounted on each wheel to monitor wheel traction. Under normal conditions the vehicle is two-wheel drive. The benefit of an all-wheel drive vehicle is that when traction is lost at any one wheel, the power is transferred to another. The result is superior traction and control. More and more automotive manufacturers are using all-wheel drivetrain systems on a broad range of vehicles.

Summary
Drivetrains transfer power from the engine to the wheels. Gears, transmissions, and drive shafts are used to transmit the power efficiently. Vehicles can be front-, rear-, four-, or all-wheel drive. All-wheel drive vehicles have become popular because they have superior traction on all types of road conditions.

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