What Is A Starter Motor And How Does It Work?

Think back to the last time you drove your car (or were driven in one). In order to start the car, all that’s required is a simple turn of a key – or a push of a button, if you’re driving something especially up-to-date – following which, the engine will give off a reassuring purr, and you’ll be able to get on with the business of driving to wherever it was that you wanted to go.

As you might expect, there’s a lot of complex engineering going on behind the scenes which allows this to happen. The most important device when it comes to starting your engine is the starter motor, which is what first gets the crank spinning. If the starter motor in your car should fail, then you simply won’t be able to get the car going via traditional means – and the car will be barely useable if you don’t have a handy hill available every time you’re looking to get started.

What Is A Starter Motor And How Does It Work?

Let’s look at this incredibly useful technology, and see if we can determine just how it came to be.

Why is a starter motor required?

To start an engine, it must be turned sufficiently that oxygen can be drawn into the cylinders in order to get the fuel burning. After this occurs, the force of the combustion will be enough to draw in new oxygen steadily.

In the bad old days, this role would be performed by hand – the operator would turn a crank at the front of the engine to get it going. But this was, of course, risk business; when the engine caught, the crank might suddenly jerk back on itself. This was a frequent cause of serious injury and even death among early adopters of this technology.

Nowadays, things are different, and the role is performed by an electric motor, which draws current from the car’s battery. The motor is attached to the engine flywheel via a small gear-wheel called a pinon, which is moved back and forth between the engine itself. It’s typically situated somewhere near the rear-end of the engine.


If you were to manually start the motor using a switch, you’d be risking serious trouble; as the contacts neared one another, a dangerous spark would arc from one to the other. At currents this high, a special sort of switch is called for. This is called a solenoid switch, which uses an electromagnet to pull the switch into place faster than an arc can heat the surrounding air. Once the starter motor is active, the switch is then pushed out of the way using a spring.

So how is the motor switched off again? This step is important, because the car’s engine will spin far more quickly than the starter motor could manage – and it’s a waste of power to have the starter motor spinning constantly. This is usually achieved using a device called a Bendix gear, which automatically gets pushed out of position when the correct speed has been achieved. The bolt has a spiral-shaped groove wrapped around the exterior, which helps to push it out when it doesn’t match the speed of the car. This places stress on the pinion, however, which means that such gears tend to enjoy short lifespans. They’re a common source of starter motor error.

An alternative is to use another solenoid magnet used to slide the gear into place. Rather than coming with a spiral-shaped gear, these devices are equipped with straight grooves. This means that the pinion will always need to be matched with the speed of the engine.

This arrangement requires some complex engineering, and so solenoid-actuated starter motors tend to be far more expensive than their more traditional counterparts. However, they also tend to last far longer, for the reasons we’ve already touched upon, and thus they make a more sensible purchase in the short-term.

In conclusion

When you’re shopping for your car, it’s unlikely to that the exact design of the starter motor will feature high on your list of concerns. Starter motors, as far as most of us are concerned, come in two forms: those which work and those which are faulty. You’ll find a range of replacement car components available online, including Volkswagen, Ford and Audi starter motors. Your mechanic will be able to order them easily – or, if you’re a motoring enthusiast looking to get your hands dirty, you might try the installation yourself, at your own risk!

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