I understand fuel is mostly octane, which is stable and a small percentage of heptane, which is highly unstable. I know 92 octane is 92% octane and 8% heptane.. so what is 110% octane gas composed of? logic says you can't really have anything more pure than 100% octane gas.. i'm confused.. help?

 

all i know its the fuel i used before at etown....and i got through 6 rounds of eliminations...gives me confidence

 

The way I understand it is this....
Octane, 87, 89, 93, 110, etc.. is a measure of how easy or hard it is to ignite the fuel. So 87 ignites easier, with less compression and heat and could cause preignition problems in a high compression/high boost application. That is why the higher grades are used in more performance oriented applications, without the high octane, a high compression motor would blow itself to bits. Hope that helps. Someone please correct me if I am wrong.

 

1flySi has it right the lower the octane the more uncontrolled the burn. That is why diesel burns so slow it has oil in it recducing the amount of hydrocarbons thus creating less of an octane rating. All an octane rating is, is a measurment of the hydrocarbons in the fuel that is easy for everybody to understand.

 

The octane rating is not directly related to the chemical composition of the fuel but rather an empiricle measurement of the fuel's resistance to burning. A fuel with a higher octane rating is harder to ignite.

The rating on a gas pump has (or had) the sticker for R+M/2 which is the abreviation for the method of determining the octane rating. It means they took the Theoretical Octane rating (R=Research) and Measured (M=Measured) and averaged the two to obtain the # on the pump.

 

Octane rating is a measure of a gasoline’s anti-knock properties in a liquid motor fuel. The rating represents iso-octane (octane rating of 100) volume in a fuel consisting the mixture of iso-octane and normal heptane (octane rating of 0).
A high-performance car is design to take advantage of higher-octane-rating fuel. However, higher-octane-rating fuel does not work efficiently on every car. Many people are willing to pay $0.20/gallon more for premium unleaded (octane rating of 92 or 93), believing that it is better for their cars. Again, nevertheless, higher octane fuel is only needed by high-compression, high-performance sports or luxury cars. Therefore, unless they have those kind of cars, their cars will run just fine on lower-octane-rating fuel (regular unleaded, octane rating of 87 or 89).

The octane number is an important measurement in evaluating the gasoline because it permits increases in engine compression ratio. During the engine compression cycle, knocking sound is produced by an internal combustion engine when fuel ignites spontaneously and prematurely (preignition) in an engine’s combustion chamber. Consequently, the piston will be forced down when it should be traveling upwards on its compression stroke. Low-octane fuel is often associated with this problem because it burns faster than high-octane fuel.

Knocking is determined largely by the chemical structure of fuel hydrocarbons. Straight-chain paraffins are recognized to be more prone to knocking than branched-chain paraffins, olefins, or cyclic hydrocarbons. Similarly, cycloparaffins (naphthenes) are more knock-prone than aromatics. To overcome the problems, negative catalysts and the antiknock compounds are often used to slow precombustion reactions that might convert fuel hydrocarbons to autoigniting, detonating compounds.

 

Great explanations from everyone but Teken gave me the answer i wanted to know

 

I always thought the octane was just the amount the fuel was refined. The higher the octane rating,, the more refined and cleaner/faster it will burn.

 

But it does not explain what is going on.

The octane number does not tell you the ratio of the chemical mixture in the fuel. A *rating* of 100 was given to the speed at which octane burns, and a rating of 0 was given to the speed at which heptane burns. Each fuel was tested and the rate at which it burns was compared to octane's and heptane's rating. A rating over 100 means that the fuel burns slower than octane. The RON and MON are two different ways of measuring the rating.

Grumpy and ITC gave the accurate answers. The explosion in your cylinders does not happen instantaneously. Many factors influence the rate, but the most important one is the fuel. Higher octane number fuels burn slower than lower octane number fuels so that the explosion is more controlled. For high compression n/a or turbo motors regular fuel burns too fast due to the high cylinder temps and pressures, a slower burning fuel is required to prevent detonation (cylinder pressure peaking before the piston reaches the top of the cylinder).

 

yeah i just wanted an explanation of how they "rate" the fuel.. thanks guys