Adrivetrain (also frequently spelled asdrive train or sometimesdrive-train) ortransmission system, is the group of components that deliver mechanical power from theprime mover to the driven components. Inautomotive engineering, the drivetrain is the components of a motor vehicle that deliver power to thedrive wheels.^[1] This excludes the engine or motor that generates the power. In marine applications, thedrive shaft will drive apropeller, thruster, or waterjet rather than adrive axle, while the actual engine might be similar to an automotive engine. Other machinery, equipment and vehicles may also use a drivetrain to deliver power from the engine(s) to the driven components.

In contrast, thepowertrain is considered to include both theengine and/or motor(s) as well as the drivetrain.

The function of the drivetrain is to couple the engine that produces the power to the driving wheels that use this mechanical power to rotate the axle. This connection involves physically linking the two components, which may be at opposite ends of the vehicle and so requiring a longpropeller shaft ordrive shaft. The operating speed of the engine and wheels are also different and must be matched by the correctgear ratio. As the vehicle speed changes, the ideal engine speed must remain approximately constant for efficient operation and so this gearbox ratio must also be changed, either manually, automatically or by an automaticcontinuous variation.

The precise components of the drivetrain vary, according to the type of vehicle.

Some typical examples:

• Flywheel
  • Dual mass flywheelstill rare
• Clutch
• Gearbox
  • OverdriveSince the adoption of 5 speeds has become standard
• Propeller shaft
• Rear axle
  • Final drive
  • Differential

• Torque converter
• Transmission
• Propeller shaft
• Rear axle
  • Spool
  • Differential

• Clutch
• Motor
  • Gearbox
  • Final drive
  • Differential
  • Drive shafts andconstant-velocity joints to each wheel

• Clutch
• Gearbox
• Transfer box
• Transmission brake
• Propeller shafts, to front and rear
• Front and rearaxles
  • Final drive
  • Locking differential
  • Portal gear

The final drive is the last in the set of components which delivers torque to thedrive wheels. In a road vehicle, it incorporates thedifferential. In a railway vehicle, it sometimes incorporates the reversing gear. Examples include theSelf-Changing Gears RF 28 (used in manyfirst-generation diesel multiple units ofBritish Railways)^[2] and RF 11 used in theBritish Rail Class 03 andBritish Rail Class 04 diesel shunting locomotives.In amotor vehicle, thepowertrain consists of the source of propulsion (e.g. the engine or electric motor) and the drivetrain system which transfers this energy into forward movement of the vehicle.^[3]

The powertrain consists of the prime mover (e.g. an internal combustion engine and/or one or more traction motors) and the drivetrain - all of the components that convert the prime mover's power into movement of the vehicle (e.g. thetransmission,driveshafts,differential andaxles);^[4][5] whereas the drivetrain does not include the power source and consists of the transmission, driveshafts, differential and axles.^[6][7]

Most passenger cars and commercial vehicles are powered by either aninternal combustion engine,electric motor(s) or a combination of the two.

The most common types of internal combustion engines are:

• petrol engines
• diesel engines
• ethanol blends (such asE85 andE10)
• liquefied petroleum gas

Most purely electric vehicles use batteries for energy storage and are referred to asbattery electric vehicles.

Vehicles with both internal combustion engines and electric motors are calledhybrid vehicles. If a hybrid vehicle includes a charging socket, it is considered to be aplug-in hybrid, while vehicles that do not include a charging socket (therefore relying on the engine orregenerative braking to charge the batteries) are considered to bemild hybrids.