Lets talk about Gasoline.

[RedHawk]

I have seen a lot of racers say that 87 octane burns faster than 93 octane and will perform better if you can run the timing without detonation. True or False? I don`t know but this is what I believe about gasoline.####, we all use it everyday and should know how the "system and chemical works".

 

The first thing to realize is "gasoline" is a mixture of a number of different kinds of hydrocarbons each one of which if isolated and burned in a test engine by itself would have its own octane rating--some would be well over 100 and some would be much lower than 93 or 87--and its own rate of combustion (which I think would affect its "power"--the amount of energy released per second).

 

The "octane number" of a fuel refers to how that fuel burns compared to pure octane (a hydrocarbon with eight carbons), which researchers long ago arbitrarily assigned a rating of 100. Octane rating is affected mainly by two factors: how many carbon atoms are in the molecule and the shape of the molecule itself. Lighter molecules generally have higher octane (that's why certain vehicles can run on propane or butane, with three and four carbons, respectively), and "aromatic" molecules usually have much higher octane ratings than either straight-chain or branched molecules. The aromatics are ring-compounds--they are shaped in the form of a ring (actually its a hexagon) with any number of side chains. Benzene is the simplest of these; it has six carbons arranged in a hexagon with no side chains. In principle, you could boost your racing octane by adding some benzene to your tank. The other thing that affected octane was tetra-ethyl lead (TEL). As you know, this is no longer added to gasoline (because we don't want lead compounds in the air), but it is the cheapest way to improve octane, because the aromatic compounds are the most expensive of all these molecules to make--they require some very intensive processing--while TEL is (or was) relatively cheap.

 

There are actually two different octane ratings: Research Octane Number (RON), which, if I remember correctly, is more of a theoretical rating; it is the one we see at the pump. The more accurate rating--the one we don't see, but which is the more realistic one of the two--is the Motor Octane Number (MON), which is gotten by running a sample of fuel in a test engine (and again comparing it to pure octane itself).

 

Besides the octane rating, there are at least two other specifications that would affect the answer to your questions: front-end volatility (the amount of "light ends," i.e., light hydrocarbons such as propane, butane, etc. that affect how quickly the engine will start, depending on the time of the year the gasoline is made); and the "back-end" of the gasoline blend, which is a measure of how much of the heavier (low-octane hydrocarbons) are present. Both front-end and back-end are measured by what fraction (percentage) of the mixture is distilled off at different temperatures. I no longer remember what those are, but the front-end spec ensures that a minimum amount of light-ends are present, while the back-end spec ensures that there are not too many of the heavier ones.

 

By now it must be clear that gasoline is not gasoline is not gasoline, for there are a jillion ways for each refinery to meet these specifications, which is a good thing, because no two crude oils are alike, so the natural mixtures of these different molecules is not constant. Thus, how refineries process each kind of crude oil varies accordingly. Not only that, but the specs on gasoline change throughout the year. Oil companies want to make sure that there are not too many light ends in the summer time (because they are what cause "vapor lock"), nor too few in the winter time (otherwise cars will not start as well), so they adjust their blends throughout the year, while holding octane number constant for each grade. You probably know that, besides octane, the other difference between grades is additives.

 

I know almost nothing about the relationship between timing and octane number. I could be wrong, but I would think--based on the above--that higher octane generally would burn faster and thus would generate more power (which is energy per time). Likewise, front-end volatility would affect this, because it is a measure of how many light-ends (which are the fastest and cleanest burning, because they are smaller, simpler molecules) are present in the fuel. Also, I would think that the "burning profile" (a term I just made up to refer to what percentage of the fuel burns during each milli-second) might be different for each mixture of molecules, and thus in principle there would be a perfect timing for every tank of fuel--although I cannot say whether or not in practice you could ever see the difference.

 

Let the debate begin .

[CityTruck]

I've always heard that higher octane gas was harder to ignite?

[mikeb]

I've always heard that higher octane gas was harder to ignite?

You're right.  The higher octane gas burns 'slower' so that you don't get preignition, pinging.  That's why you use it in higher compression engines.  I still remember when you could get Chevron Custom Supreme 102 octane!

[tinman]

The advatage to a slow burning fuel is the same princpal that you use in gunpowder. with a pistol you use a fast burn power. Not a lot of power . You dont have alot of time to built pressure in a pistol. In a rifle you use very slow burning power. You have a lot of time to built pressure ie long barrel. In a engine you push the time ahead to take advatage of the slower burning gas . As you can raise the compression at the same time . that is how a slow burning gas has more power.

Im not sure that I explain this well but i try :sideways: