Thread: Octane?
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Old 08-14-2004, 11:06 PM
Duke2.6 Duke2.6 is offline
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There is no difference in "burn rate" between premium and regular. Likewise, one doesn't burn "hotter" or "colder" relative to the other in normal (detonation free) combustion. Both are consumed at about the same rate as long as there is no detonation, which is abnormal combustion.

Detonation is a phenomenon that causes the remaining portion of unburned mixture to spontaneously react and literally "explode" like a stick of dynamite, before being consumed by normal flame front propagation. As the flame front advances, the unburned mixture is subjected to higher temperature and pressure. All gasoline blends have an autoigntion temperature. Compress them enough and they will spontaneously explode ("detonate"). Higher octane simply means that the gasoline has a higher autoignition temperature/pressure or lag time, so it is more resistant to detonation.

The shock waves from this abnormal combustion "ring" the engine structure, which is the "knock" or "ping" we hear. These shock waves also increase heat transfer to the combustion chamber boundaries by up to and order of magnitude, so sustained detonation can rapidly overheat pistons and valves, which can cause them to fail.

Commercial grade gasoline octanes are achieved by varying the blend of hydrocarbon species to attain the advertised rating. Oxygenates such as ethanol and MTBE are octane enhancers, but if oxygnates are blended in, the amount of high octane hydrocarbon species are reduced so the same PON is achieved as non-oxygenated gasoline.

Duke
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