system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why does combustion engine power taper off at a certain point while the crankshaft continues to speed up?

Why wouldn't a faster engine speed give more power?
gpt
Power = torque x RPM, so if RPM is going up but power is dropping you know that torque must be dropping (more than RPM increasing).

So your question boils down to "why does torque drop off at higher RPM?".

And that's because two things are working against you as RPM gets high...time available to burn fuel (to make pressure to make torque) is dropping, and loses (energy you spend to make the engine work) are going up.

Even if the engine was taking in the same charge of air & fuel per stroke (it's not), the faster it goes the less time there is to burn the fuel. As you get really fast, you don't get peak burn until relatively late in the power stroke and so your maximum pressure and maximum torque falls off.

And as you go faster, you need to move air through the system faster. You get more pressure drop in the intake, so less air & fuel enter the cylinder per stroke, and you do more work to push the exhaust out, which steals torque from the crankshaft. And the friction in the bearings and valve train and everything else is continuously going up with RPM and starts to eat more an more into your total available power.