Basic Configuration
The self-starter (commonly referred to as the starter motor) is a necessity for internal-combustion engines, because the Otto cycle requires the pistons already to be in motion before the ignition phase of the cycle. This means that the engine must be started in motion by an outside force, before it can power itself.
Originally, a hand crank was used to start the engine, but it was inconvenient and rather hard work to crank the engine up to speed. It was also highly dangerous; when the engine did finally start up, the crank would begin to spin on its own at high speed. The operator had to pull away immediately or else risk a broken wrist.
The modern self-starter is not much different than the early ones. It still consists of an electric motor, called a starter motor or sometimes just plain starter that is used in place of the early hand crank to turn the engine over.
The modern starter motor is an electric motor with a solenoid switch, similar to a relay, bolted to its side. When low-current power from the lead-acid battery is applied to the solenoid (the thin blue wire in the image), usually through a key switch, it pulls out a small pinion gear on the starter motor's shaft and meshes it with the ring gear on the flywheel of the engine. The solenoid also closes high-current contacts (powered through the thick red cable in the image) for the starter motor and it starts to run. If the engine starts the key switch is released, the solenoid pulls the small gear back off the starter gear and the starter motor stops running. Modern starter motors have a special Bendix gear and freewheel that enables the flywheel to automatically disengage the pinion gear from the flywheel when the engine starts.
Larger, high power engines such as those used in heavy equipment sometimes have a smaller petrol powered engine attached to the side as a starter motor.
Principle of operation
The starting system consists of the battery, cables, starter motor, flywheel ring-gear, and the ignition switch.
During starting, two actions occur. The pinion of the starter motor engages with the flywheel ring gear, and the starter motor then operates to “crank” the engine.
The starter motor is an electric motor mounted on the engine block, and operated from the battery. It is designed to have high turning effort at low speeds.
The starter cables are the thickest on the vehicle, as a high current must be delivered to the starter motor, to turn the crankshaft from rest, and keep it turning until the engine fires, and runs on its own.
Source: CDX Global & Wikipedia - en.wikipedia.org
