Is there any relationship between the autoignition temperature and octane number of fuel? Is higher autoignition temperature of fuel corresponding to a larger octane number?
Thank you for your attention and help.
The octane number is a measure of how the fuel resists auto-ignition (knock) in an internal combustion engine, so there may be some correlation, but it's also tied to the flame propagation rate. I recommend investigating this further.
Yes, it's true. For petrol (gasoline) autoignition temperature: 230°C to low-octane fuel and 480°C high-octane fuel.
Auto-ignition is the starting of ignition before the spark released from spark plug, that's means the ignition starts before spark.. But the knock is the "auto-explosion" for separate focuses of fuel inside the combustion chamber during the combustion process, under the effect of both pressure and temperature, but especially the pressure effect.
The octane number increase means resistance increase of both auto-ignition and knock.
We can think auto-ignition is knock for gasoline engine, the octane number of fuel is a measure of preventing auto-ignition. It seemly that gasoline with fixed octane number more tends to auto-ignition when higher tempreture in cylinder, but the higher tempreture must be accompanied by higher pressure, if can control the pressure is constant, then I think there is a fixe relationship between the autoignition temperature and octane number of fuel. The problem is that cylinder pressure is coupled with temperature, but they are not always keep synchronization, the pressure change is more intensity than temperature, and pressure effect on the temperature again, so it make the relationship is not simple.
To avoid knocking in gasoline engines under certain conditions you need to use higher octane number fuel
The higher the octane rating of gasoline (petrol), the less likely the occurrence of knocking (pre-ignition) in a conventional gasoline engine.
Thank you for all your helpful answers. In summary, the octane number or cetane number is alway used to measure the reactivity of a fuel, and it is not only associated with the autoignition temperature, but also related to the flame propagation rate and the sensitivity of fuel to the in-cylinder pressure. Therefore, I think a fuel with low autoignition temperature should be a high-reactivity fuel with low cetane number or high octane number. On the contrary, a high-reactivity fuel may be with a low autoignition temperature.
Looking forward for your supplyments and correction.
Higher cetane number fuel (lower octane value) => higher reactivity or easier to auto-ignite or lower autoignition temperature.
It depends on knock happens or not!! if lowering the octane number leads to the knock occurrence then auto ignition temperature goes high naturally. But, if lowering or increasing the octane number at a limited range which doesn't affect the knock at a special working condition, since the nature of combustion is premixed, then more experimental investigation should be done to study this parametric study.
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