Octane, Detonation, Knocking - What is it all about? When it comes to getting an understanding of gasoline, octane levels, and engine performance, a clear comprehension of detonation and engine knock and it's effects are essential. Eliminating and preventing knock is the key to choosing the correct octane fuel and having the most efficient running engine. Knock
Knock is the side-effect of gas self-igniting due to compression instead of by the natural process of being burned by the spark. Knocking sounds like hammers hitting metal or rocks falling on rocks. It is extremely damaging to the piston & engine cylinder. It is not something that should happen and most times can be fixed by using the correct octane gasoline. Combustion
To get a better understanding of knock and how it relates to octane, a review of combustion will help. In the combustion chamber of the engine a mixture of fuel & air is sparked causing the build up of pressure and heat. As the gas is burned the pressure builds the piston is forced downward. When all the gas is gone and the pressure decreases, the circular motion of the crankshaft forces the piston back towards the top of the combustion chamber. Then more air and fuel mixture is let into the chamber and the process starts again.  | Now let's breakdown this process a bit farther. As the fuel & air mixture is sparked and burns, left over un burned gases are compressed in front of the flame at the same time. This compressed gas moves ahead of the spark and is hopefully burned before the piston is on it's upward swing. The air & fuel mixture needs to be a perfect combination to create a clean and efficient burn. Rich
When the mixture is rich, or has too much fuel and not enough air, this can cause a few different outcomes. One, the engine may get flooded and not even run. The engine may also run very smoky. With a rich mixture the engine may stall easily. And, at the very least a rich mixture will waste that precious fuel that costs so much. |
Lean
When the mixture is lean, or has too much air and not enough fuel, the fuel burns slower and the compressed gases are fewer. This leaves left over gas during the up swing. "End Gas" is the last part of the gas to burn. Detonation is caused by "end gas" fragments that self-ignite causing shock waves. These shock waves move at the speed of sound making the loud knocking noise. Detonation is what causes knock in the cylinder and damage to the engine. Detonation is not just caused with a lean gas mixture, but it is the most common reason. Too low of an octane of gasoline can have the same effect on the engine. Knocking is the first sign that you may be using the wrong octane gas for your vehicle. There are some tale-tale signs that there is detonation going on in the cylinder. The most obvious is to listen for knocking. Also if there is metal splash on the spark plug that looks like dull cigarette ash, there is damage to the cylinder caused by detonation. |  |
 | Supercharging/Turbocharging
Forced Induction greatly effects the fuel to air mixture pushed into the chamber. The job of the supercharger and turbocharger is to add more and cooler air to the combustion process, thus changing the fuel to air ratio. Since detonation is common with a lean condition, supercharging and turbocharging tend to make detonation more possible. This can be compensated for with larger injectors. Many supercharger companies recommend higher fuel injectors if detonation is a know problem with their kit. Higher octane gasoline can also play a role in decreasing the possibly of detonation on a forced induction vehicle. Since higher octane can take longer to burn, it will help to have the extra air for combustion. |
Octane
So how does octane rating play a part in all this? The octane rating (ie. 87, 93, etc.) reveals how much the fuel can be compressed before it ignites. Another way of saying that is that it is a measurement of the gasoline's resistance to detonation and knock. The octane rating of gasoline has to do with it's combination of heptane and iso-octane. This is a fancy way of saying that gasoline is a mixture of a more and a less combustible materials. An 87 Octane gas has 87% iso-octane and 13% heptane. Iso-Octane is less combustible than heptane. This has to do with their molecular structure and stability when compressed and sparked. 
The octane rating combines with conditions like temperature and timing to reduce the risk of detonation. Because every engine is different, it is best to follow the instruction manual to use the proper octane for your vehicle. A good rule of thumb if you don't know, is large bore and air-cooled engine usually require a higher octane gasoline where liquid cooled engines do not require higher octane. Using a higher level octane will not "treat" your engine or improve gas mileage. The only reason to upgrade above the recommended octane is if you are experiencing knocking or detonation as talked about above. In conclusion, the best way to protect the engine against detonation is keep an ear out for knocking. When knocking does occur, try the next higher octane gasoline. This will inject a fuel to air mixture that is less likely to self-ignite in the combustion chamber. If knocking persist, the vehicle should be taken in to a mechanic that can test that the compression ratio of air to fuel is correct.
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