The automatic transmission receives power (torque) created by the engine and sends it to the differential. Inside the transmission is a collection of gears that, based on vehicle speed and driving conditions, change automatically in order to keep the engine operating most efficiently.
The engine is a mechanical device designed to convert chemical energy (fuel) into kinetic energy (motion), which powers your vehicle. Almost all vehicles (including hybrids) use internal combustion engines that burn fuel. In an internal combustion engine, air and fuel are ignited inside a cylinder and the pressure that is created forces pistons to move up and down. This rotates the crankshaft, which delivers power to the wheels via the drivetrain.