Posted by KEB1 on December 28, 2007 at 07:54:49 from (63.227.113.109):
In Reply to: 87 or 92 Octane Gas posted by Minnesotan H on December 27, 2007 at 19:49:50:
Did a little research a while back regarding use of high octane fuel in my old Willys pickup. Thought poeple might be interested in the basic physics.
The idea is that when the fuel/air mixture is ignited in the cylinder, it takes a finite amount of time for the flame front to propagate through the mixture. If the flame moves through the mixture too fast, it'll cause knock. If the flame moves too slow, you'll lose power because maximum pressure occurs after the piston is already partway down.
Higher compression provides greater pressure and therefore more power for a given displacement, however, higher compression also increases the speed at which the flame front propagates through the mixture. This increased flame propagation speed also makes the engine want to knock, meaning we can't advance the spark far enough to obtain maximum power.
That's also why an engine is more likely to knock on heavy acceleration - more charge in the cylinder equal higher pressure equal faster flame propagation.
To counter this effect, octane boosters (originally tetra-ethyl lead, hence the term "ethyl" for high test gas) actually slow down the rate at which the flame propagates through the mixture, essentially offsetting the increased propagation speed from higher compression and allowing ignition timing to be set for maximum power.
However, in a low compression engine, the flame front already propagates relatively slowly due to the low compression, and higher octance reduces the speed even more. The net result is that the mixture takes too long to burn resulting in reduced performance.
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