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GASOLINE & purpose of OCTANE ratings

Old 04-16-2007, 04:20 AM
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Default GASOLINE & purpose of OCTANE ratings

Originated from various sources

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine.

Sustained knocking can damage an engine, so it is not something you want to have on a long climb. Lower-octane gas can handle the least amount of compression before self igniting (knocking). The compression ratio of your engine determines the octane rating best for WOT of your car. {see FAQ on WOT)

One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the higher octane gasoline costs more.

Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking. (Note that it is the absolute pressure (compression) in the combustion chamber which is important - not the compression ratio. The compression ratio only governs the maximum compression that can be achieved).

It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion).

Using a fuel with a higher octane lets an engine run at a higher compression without having problems with knock .

It is only when the throttle is opened fully and the manifold pressure increases to atmospheric (or higher in the case of supercharged or turbo-charged engines) that the full octane requirement is achieved.

Using high octane fuel for an engine makes a difference when the engine is producing its maximum power. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. Depending on the engine design, this particular cir***stance can be anywhere along the RPM range, but is usually easy to pin-point if you can examine a print-out of the power-output (torque values) of an engine.

On a typical high-revving motorcycle engine, for example, the maximum power occurs at a point where the movements of the intake and exhaust valves are timed in such a way to maximize the compression loading of the cylinder; although the cylinder is already rising at the time the intake valve closes, the forward speed of the charge coming into the cylinder is high enough to continue to load the air-fuel mixture in.

When this occurs, if a fuel with below recommended octane is used, then the engine will knock. Modern engines have anti-knock provisions built into the ECM control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and RETARDING the spark.

When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more by adding tetraethyl lead (TEL). Airplanes are still allowed to use leaded gasoline (known as AvGas), and octane ratings of 100 or more are commonly used in super-high-performance piston airplane engines.


* NASCAR engines burn 110-octane leaded gasoline.
* Indy cars burn pure methanol (a.k.a. wood alcohol, CH3OH).
* Top Fuel dragsters and funny cars burn nitromethane (CH3NO2).

Notice that CH3OH has no NO2 ? Emissions SMOG test coming up ?

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