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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?

David Jonsson
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    Check out the Mazda Skyactive-G engine at: https://newsroom.mazda.com/en/publicity/release/2010/201010/101020a.html#:~:text=SKYACTIV%2DG%3A%20a%20next%2D,compression%20ratio%20of%2014.0%3A1 – Takala Mar 15 '23 at 22:45

3 Answers3

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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.

Tiger Guy
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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.

Solar Mike
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  • 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. – David Jonsson Jun 03 '22 at 08:45
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    @DavidJonsson "Neither is production of nitrous oxides considered in the question" nor in the answer... – Solar Mike Jun 03 '22 at 08:57
  • 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. – Phil Sweet Jun 03 '22 at 09:46
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    The whole autoignition relationship is kind of weirdly complicated. https://www.yumpu.com/en/document/view/18157279/autoignition-temperatures-for-mixtures-of-flammable-liquids-with-/5 – Phil Sweet Jun 03 '22 at 09:47
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The only limit on compression would be the mechanical limits of the piston, cylinder, and valves.

So, a related and perhaps implied question is:

  • Why is it not more fuel efficient to mechanically inject gasoline into a high compression cylinder?

IE, no spark, but rather the high temperature created during compression causes the air to spontaneously combust with the fuel.

Would this not have higher fuel efficiency? (Assuming the amount of fuel injected can be controlled precisely enough)

seanhalle
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  • Formally you should not ask questions in an answer. Why not ask a new question regarding fuel efficiency? Fuel efficiency was also the underlying motive for my question. Also avoid hinting or implying any condition in an answer. You ask as if it is not more efficient to inject fuel into high compression. The reason for not doing it could be something else. I guess misdirected environmental concerns are the reasons, that is avoidance of nitrous oxides and carbon monoxide. – David Jonsson Jun 18 '23 at 12:39
  • See also my questions about oxygen enrichment for combustion engine efficiency. Those questions were also misunderstood. – David Jonsson Jun 18 '23 at 12:40