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The octane value of a fuel is an empirical measure of its ability to resist detonation and preignition. octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases.
Preignition occurs when the fuel/air mixture in a cylinder ignites before the spark plug fires. It can be caused by burning contaminates (such as carbon, or a spark plug of the wrong heat range) in the cylinder or by extreme overheating.
Detonation occurs when the flame-front in a cylinder does not proceed smoothly from the point of ignition (the spark plug) to the opposite side of the cylinder. It refers to the spontaneous ignition of the entire charge in the cylinder. This ignition is often caused by the extreme pressure rise in the cylinder that occurs when the charge is first ignited (by the spark plug).
if the fuel is too low octane, it may spontaneously ignite before the spark plug fires due to thermal rises from the heat of compression or from hot spots in the cylinder itself. This kind of ignition is called pre-ignition (as opposed to knocking) and is a pathological case which will just turn an engine to scrap.
What usually happens, and what we usually call knocking or pinging is that
the fuel/air mixture does not ignite before the spark plug fires but does
ignite spontaneously after that. The sparkplug fires and this causes an
immediate, rapid, rise in combustion chamber pressure. This causes fuel
on the other side of the flame-front to ignite before the flame-front reaches it. In turn, this causes combustion chamber pressure to rise even more rapidly. The result is an explosion inside the combustion chamber as opposed to the desired rapid burning.
A high octane rating ensures that it takes a REALLY hot ignition
source to ignite the fuel (such as a spark plug or the flame-front itself)
and not just the rise in pressure & temperature that's a result of normal
combustion. Note that the thermal rises in the cylinder are in direct
proportion to the compression ratio of the engine.
The higher the compression ratio, the higher the octane of the fuel that's needed.
Again, if the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence.
To reiterate, what we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug).
, Tue, 9 Jan 2001 09:41:05
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