Unraveling the Chemistry of Race Fuels: A Beginner's Inquiry

[evltwins]

Let me start with a little background. I have a no more of an understanding of chemistry above what was taught to me in high school, and even that was a day or two ago. I spend my free time making horsepower for use in drag racing. In becoming better at what I enjoy doing, I have taken up learning how to tune the ECM for the cars I work on to make best use of the modifications I apply. Well this has led to numerous questions, the latest of which being chemistry related. Someone told me about this site and it seems like the perfect place to get an answer to my questions regarding race fuels. My biggest questions are:

1. Does a fuel burn slower as octane increases? Is that how it resists knock?

2. If so, what causes the fuel to burn slower? Stronger bonds between molecules or greater number of bonds?

3. As the octane rating goes up, does the amount of energy, in this application heat, increase as well? Is this how we make more power with a race fuel, more heat results in more pressure on the piston top? Or does the amount of energy released depend more on the fuel itself than the octane

I would just like to get a basic understanding of how the race fuels work and benefit the higher compressions and its affect on timing. Again please forgive what may be very basic question to most of you but I would like to learn this portion of my hobby. Thanks in advance

 

[Mech_Engineer]

1. Does a fuel burn slower as octane increases? Is that how it resists knock?

An octane rating is basically related to a fuel's auto-ignition temperature. The higher the octane rating, the hotter the cylinder has to be before it autoignites. This allows higher compresison ratios in the engine before the engine begins to knock/detonate.

2. If so, what causes the fuel to burn slower? Stronger bonds between molecules or greater number of bonds?

The "slower burn" of a high-octane fuel is a bit of a misnomer, and not really true for all cases.

3. As the octane rating goes up, does the amount of energy, in this application heat, increase as well? Is this how we make more power with a race fuel, more heat results in more pressure on the piston top? Or does the amount of energy released depend more on the fuel itself than the octane

Actually, beased on what I have seen as octane rating goes up total stored energy tends to go down. However, because the octane rating is higher you can compress more air into the cylinder either through higher compression ratios or supercharger/turbochargers. Higher compression ratios increase the efficiency of an engine, and increase power output because you're burning more fuel with each power stroke.

 

[evltwins]

So let me see if I understand something. When we use a race fuel vs regular 93 pump gas, its not that the bonds are stonger or there are double bonds between elements of carbon and hydrogen but there are more of these stranded together. Is that why it could seem that higher octane fuels burn slower? So instead of C8H18, you would have say C16H24 or something to that affect for C16 race fuel.

 

[lightarrow]

So let me see if I understand something. When we use a race fuel vs regular 93 pump gas, its not that the bonds are stonger or there are double bonds between elements of carbon and hydrogen but there are more of these stranded together. Is that why it could seem that higher octane fuels burn slower? So instead of C8H18, you would have say C16H24 or something to that affect for C16 race fuel.

It really depends on the kind of compounds you are using. A simple way to increase octane rating with hydrocarbons is to use branched molecules. Comparing a linear chain with a branched one, you can generically say that the first burns faster, because it has a greater contact surface with air molecules. In this case only, you can say that the higher the octane rating, the lower the burning speed.
With different kinds of chemicals it could be the opposite.

 

[boknows]

you sound like a spec miata racer.