Genuine question… why does an engine run in? I’ve seen it mentioned loads on many forums about new engines needing to be run in.
Like what happens to them to run them in that makes them more economical and perform better
Less of an issue these days with CNC machining but the issue still remains. When you build parts that fit together (especially moving ones) they have to made precisely. There will be exact measurements but there will still be tolerances so not every part will be exact. So when the car operates the inexact parts move together and over time wear ti become a much more precise pairing. But this bedding in needs to be done gently and you need to ensure that the correct lubrication has got to all the necessary places. If you wear inexact inadequately lubricated parts too quickly they will get too hot and may damage. Take it gently they will bed in smoothly.
Back in the day, when parts manufacture wasn’t so good you were told to take it easy for thousands of miles. It less of an issue these days but it is still wise to take it easy on the car for a while from new (or after any new significant moving part has been replaced).
Best example is cylinders and pistons, you really want them to fit perfectly. They will be engineered to very small tolerances but worst nightmare is a cylinder that is slightly to wide and a piston (ring) that is slightly too narrow - no amount of bedding in would every make them match. So the engineering is deliberately designed to make the piston (ring) too tight so that bedding in will make it perfect. Aggressive driving at the beginning could score the piston liner or generate host spots which distort the parts. It is a very complex piece of engineering because it also has to allow for how parts made of different metals expand and contract as the temperature changes.
Electric motors are a lot easier.
Cheers,
Nigel