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Higher compression ratios in internal combustion engines generally improve the engine's fuel efficiency. But diesel engines don't seem to go above a ratio of 25 or so. What is the limiting factor for the compression ratio in diesel engines?

(For anyone who's wondering about gasoline engines, there the compression ratio is limited by the undesired auto ignition of the fuel, which does not apply to diesel engines.)

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    $\begingroup$ So, if you have found that for petrol engines, what reasons have you found for compression ignition engines? $\endgroup$
    – Solar Mike
    Apr 6 '18 at 16:33
  • $\begingroup$ I haven't, that's why I'm asking. $\endgroup$
    – JanKanis
    Apr 6 '18 at 16:38
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    $\begingroup$ So a bit of research on your part won’t go amiss then... A few names to help : Ricardo, Judge, Cummins, Otto... $\endgroup$
    – Solar Mike
    Apr 6 '18 at 16:40
  • $\begingroup$ Why do you think that auto-ignition of diesel fuel at the wrong phase of the stroke is not a problem? $\endgroup$ Apr 6 '18 at 17:42
  • $\begingroup$ @CarlWitthoft because in a diesel the fuel is only injected at the moment you want to burn it, i.e. at top dead center $\endgroup$
    – JanKanis
    Apr 6 '18 at 18:09
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A redditor states,

Now for the down sides, as you guessed higher compression gives higher delta T therefore higher peak pressures and this means higher loads on all the components. There is one other major downside, as we increase compression our cylinder temperature increases just before our combustion process from the compression doing work on the gas. So at some point mechanically it’s not practical to go higher compression. (This increase in temperature is used to ignite the diesel fuel, but as incorrectly noted earlier this ignition temperature is reached far before TDC (top dead center) or before the fuel is in the cylinder. Fuel ignites instantly upon injection into the cylinder, so timing is controlled from the injector and when it injects fuel) It’s also interesting to note high pressure injectors are needed for direct injection and seen on all diesels.

A NASA employee offers thusly:

Typically, about 25:1 for production type 4 stroke engines using diesel fuel. Materials, and thus costs, are probably the main limitation. Emissions are another factor, NOx goes up with compression ratio.

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  • $\begingroup$ What also needs to be taken into account is the ignition and burn time of the fuel - it does not burn instantaneously... $\endgroup$
    – Solar Mike
    Apr 6 '18 at 20:26

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