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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 –  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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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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