Thread: What is octane? KR? Explained here.

I like what you've written Reptile but whenever I've read about the whole 'flame front' thing with regards to KR I'm a little foggy. Another way I think about KR and timing is that fuel takes time to burn in the cylinder. Once combustion begins and starts to spread you will reach a point of peak combustion. Ideally you want this to occur slightly after the cylinder reaches TDC. If peak combustion happens before TDC you get Knock. Imagine your piston racing towards the top of the cylinder and before it gets there it runs into the meat of the fuel burn explosion. All of a sudden the force of peak combustion is trying to force the cylinder down and yet the mechanical makeup of the engine keeps forcing the piston up. At some point things are going to break. Anyway, that is why timing is retarded as one way to combat Knock. You start the combustion later to allow peak combustion to happen after TDC.

That is a picture I can get in my head. It helps me understand timing with relation to what I want to achieve with tuning but also how I would want to use it to battle KR. KR is actually the PCM retarding timing to accomplish this.

I need to try to get my head around the part about higher octane not burning slower, though. My understanding has always been that higher octane fuel has a more controlled (hence slower) combustion. That's why it helps combat knock. It allows the peak combustion to occur after TDC. Just a different method to get to the same place as retarding timing. I'll need to re-read your section on octane and burn rates and why higher octane reduces Knock a few more times.

Thanks for doing this.

What is octane?
If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)
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. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. 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 gasoline for your engine costs more.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, methane, propane and butane are all hydrocarbons. Methane has a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

Originally Posted by IndeedSS

What is octane?

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

Not to call you out, but if octane rating were a percentage, It would be called octane percentage wouldn't it? and how would 115 octane gas fit into this theory.....

sorry if I stepped on any toes, just want to make sure the info in this sticky is accurate