A car engine needs to be rotating at a significant speed in order for it to start. The starter motor is a powerful electric motor which provides the power to turn the engine. The starter motor works to provide enough power to get the car engine turning at the correct speed for it to start.
The starter motor is usually situated near the back of the engine, on the underside, where it can be bolted into position on the outer edge of the flywheel. The flywheel is a large cog, approximately 12" in diameter, which is connected to one end of the crankshaft, the main shaft inside the engine, which is turned by the movement of the pistons.
This powerful, electric motor is driven by the car's battery, and starts working when you turn the key in the ignition. It has a thick shaft with a small stubby sprung-loaded cog. When the ignition key is fully turned or starter button pressed, the power is transmitted via a solenoid switch to the motor. When the starter motor rotates, the sprung-loaded cog, due to its design, spirals out against its shaft spring and engages the cog of the flywheel. The starter motor then turns the flywheel, which rotates the engine via the crankshaft and the firing sequence is initiated and the engine starts. When the ignition key or button is released, the sprung-loaded cog on the starter motor disengages from the flywheel and pops back clear.
It is not uncommon for starter motors to malfunction. General wear and tear takes it toll on the starter motor and it will eventually need replacing. Symptoms of a faulty starter motor can include an engine which is really slow to turn over and no response at all or just a click or clunk from the starter motor when you turn the key in the ignition. It is advisable to have any problems looked at by a professional. When replacing the starter motor it should be compatible with the make and model of your car.