Drivetrain

The drivetrain, also frequently spelled as drive train, or sometimes drive-train, is the group of components of a motor vehicle that deliver power to the driving wheels. This excludes the engine or motor that generates the power. In contrast, the "powertrain" is considered to include both the engine and/or motor(s), "as well as" the drivetrain.
Function.
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 long propeller shaft or drive shaft. The operating speed of the engine and wheels are also different and must be matched by the correct gear 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 automatic continuous variation.
Components.
The precise components of the drivetrain vary, according to the type of vehicle.
Some typical examples:
Final drive.
The final drive is the last in the set of components which delivers torque to the drive wheels. In a road vehicle, it incorporates the differential. In a railway vehicle, it sometimes incorporates the reversing gear. Examples include the Self-Changing Gears RF 28 (used in many first-generation diesel multiple units of British Railways) and RF 11 used in the British Rail Class 03 and British Rail Class 04 diesel shunting locomotives.
In a motor vehicle, the powertrain 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.
Powertrain.
Definition.
The powertrain consists of the power source (e.g. the engine) and all of the components that convert the engine's power into movement of the vehicle (e.g. the transmission, driveshafts, differential and axles); whereas the drivetrain does not include the power source and consists of the transmission, driveshafts, differential and axles.
Power sources.
Most passenger cars and commercial vehicles are powered by either an internal combustion engine, electric motor(s) or a combination of the two.
The most common types of internal combustion engines are:
Most purely electric vehicles use batteries for energy storage (although several prototypes have used fuel cells instead) and are referred to as battery electric vehicles.
Vehicles with both internal combustion engines and electric motors are called "hybrid vehicles". If a hybrid vehicle includes a charging socket, it is considered to be a plug-in hybrid, while vehicles that do not include a charging socket (therefore relying on the engine or regenerative braking to charge the batteries) are considered to be mild hybrids.