Once the engine in your car is started, as long as it receives air, fuel, and spark it will effectively run indefinitely without any assistance. This is why generators and irrigation pumps can be used in remote areas without an engineer tending them constantly. There has even been talk of using internal combustion on satellites and space craft to provide heat and generate electricity (obviously, they would need to bring an oxygen supply with them).
But the engine needs to be started in the first place, which is where the starter motor comes into play! Can you imagine what life would be like if we still hand cranked our cars?
To turn the engine you need an electric motor that can provide lots of torque for a brief period of time, on demand, over and over again. That is exactly what the starter motor is, plus a mechanism to allow it to engage and disengage mechanically in an instant.
Charles Kettering invented the electric starting in 1911 for use on the 1912 Cadillacs, integrating an electric motor, generator, and spark ignition system, vastly modernizing cars of the time. Vincent Bendix engineered a drive system that allowed the starter gears to engage or disengage quickly and effectively, which is an important part of the starting system. The generating and ignition functions would soon be divorced from the starter and get their own dedicated systems, but ironically, many modern mild hybrids use an integrated alternator/starter system once again. The pinion gear then turns the flywheel and the engine starts. As soon as the engine starts (and you let go of the ignition key) the solenoid allows the pinion gear to retract and disengage from the flywheel, preventing damage to the starter.
But how does the starter motor work? It’s actually relatively simple – as you turn the key switch, power is sent to the ignition system to fire the spark plugs, and to a larger magnetic switch, which sends a rush of power direct from the battery to the starter.
That magnetic switch is called the solenoid, and is typically bolted to the starter itself, both switching high amperage power and causing the gears to mesh. When the electromagnet is engaged, the solenoid plunger connects the thick battery cable to windings within the starter to actually turn the electric motor, plus it pushes a rod, engaging a fork which in turn pushes a pinion gear (connected to the motor) to automatically engage with the flywheel.