How A Starter Motor Works

An engine cannot start itself. Something has to turn the crankshaft first — and on a freezing morning that takes more torque than you would think.

In the earlier articles on basic engine operation it was explained how the four stroke cycle works, but how do you initiate the process? With the engine running, the up and down movement of the pistons is transferred to the crankshaft to produce rotation. To start an engine we need to rotate the crankshaft by external means, which will then allow the engine run cycle to take over. The most basic method would be to use a suitably cranked handle temporarily engaged in the end of the crankshaft and turned by some athletic individual. Better still is the fitting of an electric motor under driver control: a starter motor.

Different engagement types

The most common starter motor found on modern vehicles is the pre-engaged type, which uses an electromagnet to operate switching and engagement functions. This differs from the older inertia type, which requires a separate heavy current switch and engages by throwing a drive gear into engagement. Engagement with the engine is achieved by the use of a toothed ring secured to the outer rim of the flywheel — the ring gear. When the starter is engaged, drive is transmitted from the motor drive gear onto the ring gear and the engine turns over.

The cranking phase

For the average engine to start, a minimum speed of 100 rpm is required; less than this will not produce the levels of compression and fuel vapour mixing required for proper combustion. During the cranking phase the starter motor will demand around 150 to 200 amps from the battery. However, under extreme climatic conditions even an average two litre family hatchback can ask for 450 to 500 amps to overcome the initial resistance to rotation and the friction found within the engine.

To give some idea of the torque required for that freezing cold morning scenario: the average wheel nut is tightened to around 90 to 110 Nm, while the amount required to start the engine moving can be as much as 450 to 500 Nm, dropping to 160 to 200 Nm to maintain the 100 rpm minimum crank speed.

Motor and solenoid

The two main parts of the starter motor are the motor itself and the solenoid, usually mounted on top of the motor casing. The motor is devoted purely to rotating, like most other electric motors. The solenoid has two functions: to handle the high current switching, and to push the drive gear into mesh with the ring gear.

It would be impractical to run large high current cables from the battery to a switch mounted in the cab and then down to the starter, so the pre-engaged system uses the solenoid to handle the heavy current switching required. This is done under the control of a much lower current circuit using far smaller wires from the starter switch, which can now be incorporated into a combined ignition and start switch assembly.

The high current feed from the battery is connected to one of the large terminals of the solenoid; the other large terminal feeds the motor itself. In the at-rest position this circuit is open, with no current flowing. When the solenoid is operated by a low current feed from the starter switch, the centre plunger pulls the drive gear into mesh with the ring gear and closes the high current circuit, powering the motor. As the engine starts, the driver releases the ignition key from the crank position, cutting the low current feed to the solenoid, which in turn breaks the supply to the motor and brings the drive gear out of mesh, assisted by a return spring.