How does a starter motor work?

Although starter motors are fairly robust and reliable, they are not service items. This means that they are not inspected or serviced along with other critical components like the brakes, suspension, and steering systems. In practice, starters are out-of-sight, and out-of-mind parts that only receive attention when you start having starter motor problems.

A diagram showing the internal parts of a car starter motor

Although this simplified diagram appears to be complicated, we don't have to consider all the starter motor parts shown here to understand how a starter works.
For our purposes, the parts indicated by coloured circles or ovals are the most important, so let us start with the-

Solenoid spindle
This is the small part that is circled in red in this diagram. It is shown here relative to the body of the starter motor solenoid, which is the part indicated by the red arrow. When you turn the ignition key to the “START” position, the battery supplies the starter solenoid with a current. The current creates a magnetic field that pulls the solenoid spindle into the body of the solenoid against a spring, which then acts on the-

Bendix fork
This is the part that is circled in green. Since this part is connected to the solenoid spindle, the movement of the spindle causes the upper part of the bendix fork to move. In this case, the top part of the fork moves outward, which is to the left in this view. This movement acts on the-

The bendix fits on the front of the armature, and it is the part circled in blue in this diagram. This part is connected to the upper part of the bendix fork, and as a result, it follows the movement of the fork. Therefore, when the starter motor solenoid is activated, the bendix fork forces the small pinion gear on the bendix to engage with a gear on the engine’s flywheel. 

However, the starter motor solenoid has one other function, which is to close a set of contacts when it is activated. This completes a circuit that supplies the motors' armature (indicated by the black arrow) with battery current. This, in turn, creates a series of magnetic fields in the armatures' windings.This forces the armature to rotate to crank the engine.

The bendix contains a one-way mechanical clutch that transmits the motor’s rotation to the ring gear on the flywheel until the engine starts. However, the one-way clutch works only in one direction of rotation. In the opposite direction, the bendix can rotate freely to allow the bendix to disengage from the flywheel when the engine starts.

So when you turn the ignition key to the “START” position, a series of mechanical movements and actions happen before the engine will start. Assuming that all actions complete successfully and the engine starts, the series of actions are reversed when you release pressure on the ignition key. All mechanical components then return to their rest positions under spring tension until the key is again turned to “START”.

How does an air starter motor work?

Although air starters have several major advantages over electric starter motors, air starters are mainly used on large industrial engines.

In simple terms, air starters use highly compressed air to start large engines in one of two ways. In one way, the compressed air is used to spin a series of blades, which then act on a flywheel. The rotation of the flywheel is then applied to the engine through a reduction gearbox to crank the large engine until it starts.

In another way, highly compressed air is forced into one or more cylinders of very large engines. This forces the pistons downwards, which causes the engine to start rotating. By switching the air to alternate cylinders, the speed of the engine is increased to the point where the engine starts when fuel is added.

Air starters eliminate the need for heavy batteries and complicated electric motors with their associated wiring. However, air-start systems require so much highly compressed air that these systems are used only on very large engines. Two examples are large marine engines and heavy aircraft engines.

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