Posted by RDavis on Monday, December 23, 2002 at 12:54PM :
In Reply to: Fuel posted by Dave on Monday, December 23, 2002 at 9:30AM :
Keep in mind that Octane is a rating or measure, not a substance.
Octane Rating.Fuel is blended to meet the compression requirements of various engines. Compression pressures vary as low as 6 to 1 in older engines to as high as 10.5 to 1 in some larger, more modern engines.
"Octane rating" is a measure,or index, of the detonation, or knock characteristics, of gasoline.It is obtained by burning and testing a sample of the refined fuel, in a test engine where it is possible to vary the compression pressure. After the point at which the tested fuel knocks is determined, it is compared with that of a known mixture of isooctane (pronouced: i-so-octane)plus heptane.
Isooctane is a hydrocarbon that has an extremely high resistance to knocking. Heptane is a hydrocarbon that knocks even at very low compression pressures.
These check fuels are mixed and burned in the same test engine. The percentage of isooctane to heptane is adjusted until the engine runs with the same knock characteristics as the tested fuel. The final percentage becomes the rating of the fuel.
The octane rating of a fuel depends on its blend and method of refining. The addition of Ethyl increases the octane rating of any fuel.
EXAMPLE:Octane 80 means it took 80% isooctane and 20% heptane to produce the same result obtained with the tested fuel.
If an engine knocks using 80-octane fuel, it is advisable to use a higher-test or higher-octane-rated fuel.
Ethyl Gasoline. An important additiveplaced in present-day fuel is a chemical mixture of tetraethyl lead, ethylene bromide,ethylene dichloride, and a small amount of soluble coloring.
It has been found through experimentation that a Ethyl mixture added to motor fuel reduces the the detonation at higher compressions. The Ethyl fluid does not speed up or slow down the burning process,but for some unexplainable reason, it lessens the detonation. Before a gasoline can be called Ethyl or high-test, it must perform satisfactorily with a minimum octane rating.
Higher Compression. When fuels are used that allow the engine to operate at its highest compression and most advanced spark position, the engine will run at a slightly higher temerature. Burning gases at their highest possible compression results in a more forceful and power-producing burn of fuel, and this greater amount of energy expended produces more heat.
It is poor fuel economy to burn a low-test gas in a high-compression engine with a retarded spark to reduce knocking. The vehicle will give better mileage and performance if the spark is advanced to its greatest possible extreme for the high-test fuel recommended for the engine.
On a personal note:Today we do not use tetraethyl lead as a anti-knock or octane rating booster due to the harm ful results of breathing the lead fumes (or so they say).Today we use other "more deadly" chemicals to accomplish the same thing But, it is (claimed) that they don't last as long in the atmosphere.