How does an electric starter motor start a diesel engine?

An electric starter motor starts a diesel engine by providing the necessary external power to rotate the engine's flywheel.1

Because diesel engines are compression-ignition systems that rely on air heated by high compression to ignite fuel, they cannot start on their own from a standstill. The electric starter motor is geared to the flywheel, and when electric current is supplied, it turns the engine over until it reaches the conditions—specifically the required air temperature and pressure—necessary to run under its own power.1

At start-up, the air temperature within the cylinders is determined by the engine's compression ratio and its current operating temperature. To facilitate this process, supplemental heating is sometimes employed, such as the use of a glow plug to preheat the air inside the cylinder.12