Here's what I was getting at above.
When the piston compresses the air fuel mix, we create heat from compressing the gas. When the air fuel mix reaches a certain temp( by compression ) the fuel air mix ignites. If this temp is reached, and the mix ignites before the spark plug fires, its called preignition.
However, when the spark plug fires ( no Pre-ignition ) and the mix begins to burn, it expands greatly( addition of heat to a gas=increase in volume). When the flame front expands it compresses the unburned portion of the mix left in the combustion chamber. If the yet unburned portion of the mix is near the point of ignition( because of the initial compresstion ), it is possible for this second compression to bring the temperature up, and ignite the unburned mix. Now you have two flame fronts converging on each other. When they meet, a pressure spike is created. The spike is highly detrimental to engine parts. This is known as detonation.
If it is mild, possibly only bearings are harmed, if severe, anything is possible. I.e. new ports in your cylinder, piston, etc..
The temperature at which a fuel will begin to break down and ignite is refered to as the Autoignition Temperature . This temperature is different for different octane grades of gasoline. This is the most useful bit of information that you could have about a fuel, except that its often to difficult for people to comprehend when they buy gas. Also, unless you do the math of how much your compressing the mix, and how much temperature you are generating, its fairly unrelatable and usless.
When you have a higher octane fuel ( higher autoignition temp ), it is harder to ignite by deffinition. Temperature being the energy "measure", it takes more temperature to ignite, thus more energy. However, when ignited it burns with "approximately" the same flame speed and liberates roughly the same amount of energy.
Will you have problems starting your saw? NO. The difference in the energy required for ignition is much less than the overabundance of energy that the ignition system of your saw produces. Problems with igniting the mix only become a problem at higher compression ratios ( 14:1 and over ) and race gas.
If you understand, knod your head. If not, read it a couple of times, and read the FAQ, then post questions.
I know the FAQ is long, but you can read the table of contents and then skip to whatever part your interested in.
I hope no one jumped out of a window after reading this
Also, both pre-ignition and detonation are BAD. One is different from the other. Many other factors influence the the start of combustion "problems". But I'll save that for another day.
Specifically, check out part six in section 3 of the FAQ.
Later
Harley
