Quote:
Originally posted by psfred
Ah, but higher octane fuel does ignite in buld less readily -- it also has slower flame probagation rate, so in an engine that does not require it, it CAN reduce power output and economy from poor combustion. Higher compression (the main reason for needing higher octane) generates higher heat, so the higher octane, less volatile and slower to ignite fuel is more volitalized, more easily ignited, etc.
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This is so full of myth and misinformation it's tough to know where to begin.
Flame propagation rate is primarily a function of turbulence, which is a function of combustion chamber design. That's why different design engines have differences in best power ignition timing. For engines of the same bore, lower optimum best power timing usually translates into better thermal efficiecy, which means more power for less fuel. Difference in fuel blends have little effect on flame propagation rate for normal combustion. Once an engine gets into detonation, which is "abnormal combustion" the rules change.
Higher compression engines don't generate higher heat. They convert more of the fuel's energy into usable work at the crankshaft. Therefore, as a rule, they have lower EGT, which transfers less heat to the cooling jacket as the exhaust flows out the port. The energy of commercially available gasolines is similar, but I've seen reports that show some regular blends actually have a little more energy than some premiums.
Fuel "volatility" is its willingness to evaporate as measured by Reid vapor pressure. In a modern fuel injected engine the fuel is invariably evaporated and mixed homogeneously with the air when the spark occurs, initiation combustion.
Duke