The starter motor of a car works by engaging with a gear found on the flywheel of the engine to start the combustion process. A starter motor and a starter solenoid comprise the starting system of a car.
A car engine is started by turning it at some speed to suck in fuel and air into the cylinders for compression. The starter motor, which carries a small pinion on its shaft, engages with the large gear ring on the engine flywheel. This motion spins the engine a few revolutions and cranks it, making it fire. To spin a cold engine, the starter motor must be powerful enough to overcome the following.
- All of the internal friction induced by the piston rings.
- The compression pressure that is in the compression stroke of any cylinder.
- The required energy to open and close the valves using the camshaft.
- All other engine peripherals such as the oil pump, water pump and alternator.
If the starter motor stays engaged after the engine has been fired, it will not be capable of coping with the centrifugal forces produced by the engine that is now spinning the starter. Hence, one of the primary prerequisites in starter motors is that it disengages once the engine has fired.