What is exactly happening to the engine while you are breaking-in a new car?

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What is exactly happening to the engine while you are breaking-in a new car?

In: Engineering
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Mostly the piston rings and cylinder wall wear on each other to form a tight seal.

In this day and age the tolerances of machining are really tight and the breaking in is done at the factory as part of the test drive. But for some people old habits die hard.

It used to be more of thing, whereas you needed to change out your “break-in oil” after about five hundred miles. I think that it no longer necessary because of advances in design and technology.

Proper seating of piston rings, heat cycle/thermal expansion of the metal components.

You want to heat the metal up so it expands to its full extent within the cylinder chamber and works right without scraping the piston rings and causing breakage. It basically just makes everything smooth in action.

In a more modern car the Engine shouldn’t break in much if any. The Automatic Transmission however is controlled by a computer. It needs some time to learn how to best control the car and run efficiently. You normally don’t notice this too much because the computer has a basic program preloaded (this allows the test drives to go smoothly).

When the different parts of the engine are made, the dimensions and shapes between moving parts are slightly off from each other. So during the break in period of an engine some parts that should fit together tightly or slide past each other smoothly don’t. The engine is broken in by allowing parts to wear down a bit so they fit together better. Piston rings will wear small grooves into the cylinder wall, while themselves being rounded out a bit; this will help them seal better and create less friction in the long term.

It may also be worth noting that because of the extra friction during a break in period, a different oil is used from what you normally would put in the engine as well. Normal use oil has chemicals added to it to help clean your engine and stabilize old oil called additives. Additives in oil aren’t really useful in a brand new engine (there’s no dirt to clean yet), and additives don’t actually lubricate like the oil does, but they do displace oil. Without the additives, the oil lubricates a little better. Because of this the oil added to the engine by the factory will be ‘Break-In’ oil that doesn’t have any additives in it.

On newer cars, because of better manufacturing, the tolerances on new parts are closer so the engine will require less break-in than older cars did. New cars still require a break-in period though, and this is often done by the manufacturer before the car is shipped.