why do hot engines “click” when turned off?


After driving for a little while, you turn into a parking lot, park your car, turn it off and get out. Then you hear the engine making clicking sounds. It only does it when it’s hot, does it have something to do with metals expanding when they’re hot or?

In: 414


It might be the 12V clutch on the A/C compressor? Try it with the A/C off. Drive a block and then park and turn off car.

Unfortunately, this isn’t very eli5, but what’s going on is something called twinning. When the engine starts cooling, it contracts from the temperature change, which stresses the material since it’s constrained in certain shapes. The clicking is actually due to twinning of the atoms, where the atoms suddenly arrange in a mirror image pattern across a plane in order to relieve stress, and this transition makes the clicking noise.

Source: materials science phd student



Answered [here](https://www.reddit.com/r/askscience/comments/71d8o9/why_do_car_engines_tick_after_being_turned_off/):

>When the engine is running, it will heat up itself and the components near it. Heating something causes it to expand. Once the engine is turned off, it will cool down again which causes the materials to shrink back to their original state. The different parts of the engine are made of different materials, so the heating, cooling, expanding and shrinking happens at different rates. This can cause some friction and snapping, which causes the ticking sound you hear.

To expand on this:

Imagine a chunky metal box with no top, and a thin metal lid. The lid is attached to the box with bolts.

Now, heat the whole thing up, then let it cool down. The metal lid, being thinner, cools down faster than the chunky box beneath it, meaning the lid shrinks at a faster rate than the box.

This will cause the surface of the lid (that meets the box) to rub against the surface of the box.

That’s what’s causing the ticking sound. A car engine is comprised of several metal parts, and each may have a different size. When the engine is turned off, each engine part loses heat at a different rate, and some parts will shrink faster than others.

Different metals/materials expand and contract at different rates when heated or cooled. The clicking is just the parts doing their thing.