Duke:

Higher compression ratio equals more heat at the top of the compression stroke -- above 12:1 you are in the compression ignition range of 400 F or so, and in fact some early deisels run at 12:1 -- usually require some sort of pre-heat to get started, but once running will continue. Terribly ineffecient, but that's a different story.

Higer octane fuels would have to resist self-ignition of the mixture under compression heat, else they would pre-ignite.

High compression ratio engines convert the heat into work much better, so that less is wasted as hot exhaust gas and heat dumped into the cooling jacket, but the mixture and burn temps are higher --- again, this produces more useful work, but only if the fuel/air mix burns correctly at the proper time -- lighting off the mixture on the compression stroke makes for VERY hot combustion chambers!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!