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There is an excellent Q&A about why diesel engines have lower redline RPMs (about 4500 RPM) than gasoline engines (about 6500 RPM). The reason is related to the combustion of diesel fuel.

However, what would happen if you manage to over-rev a diesel engine to, say, 5500 RPM by downshifting a manual transmission on a freeway? Surely, the engine management system would detect an over-RPM condition and would stop diesel fuel injection. Then there is no combustion happening, and the reason for diesel engines having a lower redline does not seem valid anymore.

My understanding of gasoline engines is that they generally break when managing to over-rev them. Does a diesel engine break when running at 5500 RPM? If so, why does this happen if gasoline engines can take 6500 RPM and if diesel engines are generally more sturdy than gasoline engines?

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  • Looking forward to seeing answers to this question :)
    – Ppoggio
    Dec 26, 2015 at 18:19
  • What kind of diesel engine. Two stroke or four stroke?
    – DucatiKiller
    Dec 27, 2015 at 22:39

2 Answers 2

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The rev limit of a petrol (gasoline) engine is generally set to protect the internals of the engine. Less sporty engines are designed to a cost and their components may not have the strength or manufacturing tolerances to cope with the higher forces experienced at higher engine speeds.

Diesel engines, as you say, are more often limited by the speed of combustion but that doesn't mean the engine manufacturer hasn't 'designed down' (sorry about the clunky phrase) the quality of the internals as they only need to cope with forces generated at engine speeds up to 5000 RPM or so. If you manage to over rev an engine designed like this, the same problems as over revving a petrol would apply.

You are probably correct about any electronically controlled engines and the fuel being cut off, it will essentially be in an overrun condition and that is how a petrol engine's ECU would respond. Mechanical injection pumps rotate to match engine speed and have a mechanical governor which will cut off fuel if the engine speed is too high.

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    One of the things that can easily be designed down is the valve spring stiffness. Valve bounce is one of the things that tends to dictate the upper limit on engine speed and stiffer springs will increase the engine speed you can reach before the valves start bouncing. However, stiffer valve springs mean the cams have to apply more force to open the valves, increasing valvetrain drag which reduces the engine's efficiency. The manufacturer will most likely put in the weakest valves that will do the job in order to maximise engine efficiency.
    – squigbobble
    Jan 28, 2016 at 15:34
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    I would also wonder if a diesel engine has heavier pistons to deal with the higher compression. This type of strengthening would limit the RPM due to higher loads on the connecting rod and crankshaft.
    – Tom Penny
    Mar 7, 2016 at 17:41
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Piston speed is the limiting factor. Diesels have a longer stroke. Even though they aren't reving fast the piston speed still gets as high as a gasoline engine.

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