Flat Spots while driving issues

[lotusflasherman]

so the Octane rating is a guide to how that is delayed. Interesting to note that the higher the octane rating the less flammable is the petrol...

I don't think that is correct - Octane rating is a measure of to resistance to pinking. at the bottom of the sacle is n- heptane (0 octane) and the top of the scale iso-octane (100 octane) so iso-octane is more resistant to pinking than n-Heptane.

Flammability of a fuel is defined by its flash point which is due to the volatile part evaporating enough at lower temperature - iso-octane has a flash point of 10 oF whereas n-Heptane has a flash point of 25 oF so iso-octane has a higher octane rating and is more flammable. Petrol on the other hand has an octane rating of 95/101 but needs to ignite easily so has a flash point of -45 oF. (clearly I have had to look up all these numbers )

The important part to octane rating is the temperature at which the fuel spontaneously ignites ie burns uncontrollably. If we link this back to the gas law PV=nRT then for a lower spontaneous T and an instantaneously fixed V, P will rise in an uncontrolled manner. ie pinking

Now back to the application in our engine. A higher octane allows a higher instantaneous pressure. Since the mixture is burning in a controlled manner at greater pressure, you can have a higher compression ratio or more boost or advanced ignition, thereby getting more bang for your buck.

The MPG of a fuel is also related to its calorific value, and this is where we are being conned as ethanol is an octane booster but has a lower calorific value.

An interesting discussion (for me anyway) and I'm happy to be corrected.

How are you getting on Stef?!

I'll try again, maybe it's syntax.

I was using 'flammable' in layman's terms,- or is it inflammable? and put in simple terms - if we go back - Leaded fuel Octane Rating was controlled by addition of Tetraethyllead (CH₃CH₂)₄Pb and the higher the octane the more 'pb' was taking the place of hydrocarbons. Hydrocarbons burn, lead doesn't. For today, if you go back to Octane Rating on Wikipedia and look at Effects it says "Higher octane ratings correlate to higher activation energies: the amount of applied energy required to initiate combustion." In other words it takes more energy to make a higher octane fuel burn, so 'less flammable' might not have been the right expression but I expect Rambo got the gist...

Glad you posted though... I went looking for what's in the bottles of Octane Booster and found something that told me the ECU in my Subaru is programmed for Japanese 100 Octane Fuel so not surprising it ran better when on holiday in Austria. Probably let my Knock Sensor have a holiday too..

Don't really want to hijack this thread with a continuing debate about thermodynamics, octane ratings, pre-ignition, auto-ignition and detonation but if it interests you suggest you put 'the bible' on your Christmas List ...
The-Design-and-Tuning-of-Competition-Engines-by-H-Philip-Smith

How's it going Stef? Cured those flat spots yet? ..

[Simon_P]

- Leaded fuel Octane Rating was controlled by addition of Tetraethyllead (CH₃CH₂)₄Pb and the higher the octane the more 'pb' was taking the place of hydrocarbons. Hydrocarbons burn, lead doesn't.

There is a tiny amount of Tetraethyl lead in Leaded fuel and it's effect on hydrocarbons is at the molecular level rather than due to the equally tiny amount of displaced hydrocarbons. The lead content of 2 star and 4 star was the same.