Correct, the combustion is caused by the high temperature (resulting from the compression of the air), not from the pressure itself. You will not get combustion by pressurizing liquid fuel, which actually does not require very much energy and does not add much energy (heat) to the liquid fuel.

Yes, what he said...that's why bicycle pumps and air compressors get so darn hot, it is the energy of compressing the air which makes it hot enough to burn the fuel, not the act of compression per-se. Technically liquids are considered incompressible too (though they do compress ever so slightly) which is why it takes so little energy to spike the pressure in a liquid filled space (like your brake system) but so much more to compress a gas like air.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz