Why Is n-Heptane the Baseline for Octane Ratings?

[dRic2]

Is n-heptane really explosive? Why is it chosen to be the lowest value (zero) for the octane number? Is it because it is really explosive? and why should n-heptane be so explosive (if that is the case)?

Thanks
Ric

 

[Borek]

Please google for octane number, almost everything you wrote is incorrect.

 

[dRic2]

Octane number measure "the tendency of a fuel to resist self-ignition" (quoted from Wikipedia to avoid misunderstandings).

If a fuel has octane number $n$, it behaves like a mixture of n-heptane and iso-octane with $n$% of iso-octane. This means that a fuel with octane number 100 behaves like pure iso-octane and a fuel with 0 octane number behaves like pure n-heptane.

Also an octane number of 0 means the fuel is likely to self-detonate.

I don't understand what I wrote wrong in the previous post

 

[Borek]

You are mistaking being an explosive with tendency of an fuel/air mixture to ignite during an adiabatic compression, these are fundamentally different things.

 

[dRic2]

I'm sorry, may fault. Let me rephrase my question: Why is a mixture of n-heptane and iso-octane chosen to be the reference to establish the tendency of a fuel to resist self-ignition? Why n-heptane and not other substances ?

I hope the English is correct

 

[jbriggs444]

I'm sorry, may fault. Let me rephrase my question: Why is a mixture of n-heptane and iso-octane chosen to be the reference to establish the tendency of a fuel to resist self-ignition? Why n-heptane and not other substances ?

If you want to quantity how an atomized liquid fuel resists self-ignition then a scale with one pure liquid fuel on the one end and a different pure liquid fuel on the other seems a reasonable way to go. The choice of those standard fuels is more or less arbitrary, though you do want the range between to cover the fuels and fuel mixtures that you are interested in classifying.

 

[Asymptotic]

Why n-heptane and not other substances ?

High purity n-heptane was readily available when the heptane/octane rating system was conceived.

 

[dRic2]

Ok so I guess it's just an arbitrary decision based upon historical circumstances, there is not a scientific explanation for this.

 

[jbriggs444]

High purity n-heptane was readily available when the heptane/octane rating system was conceived.

[joke]And resublimated thiotimoline is finicky when dissolved in iso-octane.[/joke]

 

[dRic2]

I don't understand

 

[jbriggs444]

I don't understand

You can Google for "The Endochronic Properties of Resublimated Thiotimoline". It is an article written by some guy named Azimuth or @asymptotic or something in the 40's.

 

[dRic2]

I googled it and found out Resublimated Thiotimoline is an invented chemical compound that should "start dissolving before it makes contact with water" (Wikipedia), but I still don't get the joke. Anyway thank you all for the replies.

 

[Asymptotic]

Ok so I guess it's just an arbitrary decision based upon historical circumstances, there is not a scientific explanation for this.

n-heptane (C7) and iso-octane (C8) are both in the mix of what constitutes gasoline (C4 through C9). A major difference between the two is n-heptane has a straight chain structure while iso-octane is branched, and it turns out that straight chain HCs are more prone to causing detonation. You'd have to research this further, but my suspicion is these things in combination make n-heptane and iso-octane good proxies for how much 'knock' to expect from a particular gasoline formulation.

 

[dRic2]

You'd have to research this further

Ok thanks.

it turns out that straight chain HCs are more prone to causing detonation.

Yeah, I really want to know why is that