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pseudopad t1_j6io30s wrote

Reply to comment by dr_xenon in Eli5 Why gas turbine can rev at >10000 rpm but diesel engine red lone at 3000-4000 rpm? by sepientr34

I have a very rudimentary understanding of engines and this laid out very clearly why engines revolve at the rates they do. Thank you.

As a side note, do you know why that one diesel le mans car from Audi could still rev as high as it does (8k iirc)? Is it kind of the same reason as why high performance gas engine also rev much higher than regular gas engines.

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RampantRooster t1_j6iu5uy wrote

Generally an engine will encounter issues with reciprocating weight and getting enough fuel and air before it starts seeing issues with reaching the speed of the flame front. Racing engines have different requirements than road engines that allow different construction methods to reach higher RPM.

They can use lighter and possibly more expensive internals. This is bad for road engines because they have a lower lifetime and are more expensive to fix, but fine for racing cause they rebuild engines often. There's also more advanced valves operated pneumatically or electrically that maximize their open duration and minimize the time it takes to open or close them. Again these are more expensive to maintain, which is fine for racing but bad for road vehicles.

ELI5 summary: throwing more money at specific engine components can increase redline, but doing so is bad for regular car engines due to maintenance costs and reliability

I'm not familiar with that particular engine, but an example of a road diesel engine that has a redline above 5k RPM is the Mercedes OM606. It has dual overhead cams and pistons with a larger bore than stroke, which isn't the norm for diesels.

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