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Maximum possible compression in a gasoline engine was originally determined by knocking in engines with carburetor. With fuel injection the fuel could be injected after compression and thus eliminate knocking. How high compression does fuel injection permit?

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If you want to turn the engine into a compression-ignition one, I suppose there isn't really a limit other than physical based on strength of materials. Your proposed solution is rather how diesel engines run. I believe diesel engines run up to the low 20's in compression ratio.

None of this has anything to do with whether or not it would be worthwhile to do so.

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Calculate the temperature of the air in the cylinder after compression - if that exceeds the self-ignition temperature of the fuel injected then that is a problem.

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  • $\begingroup$ You mention a factor yes but since the fuel is injected gradually the combustion will also be gradual and not knocking. I am also not convinced that explosive combustion is a problem if it occurs after maximum compression. Neither is production of nitrous oxides considered in the question, only mechanical power efficiency is important. $\endgroup$
    – David Jonsson
    Jun 3 at 8:45
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    $\begingroup$ @DavidJonsson "Neither is production of nitrous oxides considered in the question" nor in the answer... $\endgroup$
    – Solar Mike
    Jun 3 at 8:57
  • $\begingroup$ Basically, you need the piston rod to be smaller than the piston. You can use a scotch yoke to move the wrist pin out of the cylinder. Big marine diesels do this already because the wrist scales funny as cylinder sizes get big. And Honda made an engine with oval "cylinders" for their racing program that had a better form factor and gave the rod more room. Once the rod is taken care of, the piston rings and dead spaces become the problem. Going big helps here, but you can only go so big. $\endgroup$
    – Phil Sweet
    Jun 3 at 9:46
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    $\begingroup$ The whole autoignition relationship is kind of weirdly complicated. yumpu.com/en/document/view/18157279/… $\endgroup$
    – Phil Sweet
    Jun 3 at 9:47

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