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A normal, working car, nothing wrong with it, makes a very distinctive sound when you turn the ignition. I have a vague understanding of a distributor spinning around real quick and firing the spark plugs, but I don't really grasp what's making the sound.

Could you explain it?

temporary_user_name
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2 Answers2

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The main noise you hear is the starter motor. This is a high torque motor whose purpose is to turn the engine at a rate sufficient to allow it to run.

If you run a starter motor with no load it is still quite noisy, but as you will hear when starting your car, it tries to get the engine up to speed, fuel is pumped in, the spark plugs fire, and if all is well the engine spins up. Otherwise you have to try again.

Because the cylinders are still compressing a fuel and air mix, the starter motor does not simply accelerate, but instead sounds quite variable as it passes points of high and low compression.

The load is so high that car manuals recommend taking a break after every few tries to avoid overheating the motor, and it is very easy to quickly drain a battery using the starter.

Rory Alsop
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  • It should probably be mentioned that the pulsing sound of the starter motor comes from extreme variation in the load on the motor (and thus the rate at which it turns) due to compression in the engine. If you operate the starter with all the spark plugs taken out (and thus no compression), it sounds a lot different. – R.. GitHub STOP HELPING ICE Jan 30 '13 at 17:29
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If you ask about the sound of starting the engine, it's mostly the starter motor's gear whine. Similar sound many gearboxes make when reversing.

Gear whine is the sound of gear teeth meshing and unmeshing. Most gears in a car have helical teeth, to avoid making that sound. But reverse and starter are used so infrequently and shortly that cheaper, spur gears (with straight teeth) are used. Old trams and subway often make this sound.

The main difference between starter and reverse is that reverse makes constant sound, while starter is modulated by cylinders presenting bigger and smaller resistance as the crankshaft rotates. This makes it so distinct.

Electric motors hardly make any sound at all, most of what we understand as "sound of electric motor" is the sound of the gears it's driving. E.g., in Formula E what you hear is mostly the gear whine (sometimes with a bit of coil whine created by electronic power modulation). Imagine this sound, but slightly lower and the driver pumps the accelerator many times per second and you get your starter sound.

Agent_L
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