Quote:
Originally Posted by TheGrunch
Given that as far as I am aware the 100 octane fuel was mixed by mixing the iso-octane with existing 87 octane fuel I cannot imagine that a tiny remnant of 87 octane would be something to get distressed about, however I will admit that the possibility of Mod 154 being performed incorrectly might be something they would test, agreed. However the since local mod would be performed on existing engines that had already seen use I am unsure as to what this would mean in terms of the treatment of the engine immediately after the mod.
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Isooctane is a specific chemical isomer. It was used as a reference fuel by Ricardo when he produced the Octane scale. The other reference fuel was
n-Heptane.
So
0 octane = 100% n-Heptane
50 octane = 50% n-Heptane + 50% iso-octane
100 octane = 100% iso-octane
Fuels are evaluated by testing them on a variable compression ratio engine and then mixing a reference fuel to match their performance.
Note that this scale cannot measure any fuel outside of the range 0-100 octane. There is quite a lot of casual incorrect use of the octane scale for values >100, but this is simply incorrect.
A different scale (Performance number) is used to rate fuels superior to 100 octane; the performance number being the ratio of the power produced by the reference fuel running on the test engine without detonation to that produced by 100 octane running on the test engine.
This naturally varies as a function of mixture strength, so for example modern 100 LL will give approximately 100/130 performance number (weak/rich), whilst late war purple fuel could give 115/145 Performance number.
Care must be taken when comparing fuels to check exactly which rating scale is in use; the power output achievable from a fuel is not necessarily a linear function of its octane number, so you can't make a direct comparison between an octane number and a performance number. Additionally, there were several different test procedures in use which would produce different results from identical fuels (ie MON, RON etc).
Production fuels were not made in a chemistry lab by mixing individual chemical isomers together; this would have been impractical. The Germans were somewhat closer to this approach with their use of Fischer-Tropsch derived synthetic oils, but even then they didn't go to the lengths of blending up their fuels isomer by isomer, because it just wouldn't be worth the effort. About the closest that anybody has ever come to this kind of "chemistry lab" approach was JP-7 for the SR-71, and that was phenomenally expensive stuff...
Instead, production fuels were made by distillation of crude and the judicious use of additives such as TEL. AFAIK, 100 octane used a slightly different distillation range from 87, and there may have been some catalytic processing (eg cracking etc) to improve yields, but without making direct reference to a contemporary reference on the subject I couldn't be certain off the top of my head.
However, I'm pretty certain that the main difference between 87 octane and 100 octane was the amount of TEL used. This would have been a variable, because different wells produce different crudes which will produce different products at the refinery, so the amount of TEL required to meet the 100 octane standard would have been varied from batch to batch.
Even today, the specification therefore just includes an upper limit for TEL content.
To get an idea of just how many chemicals are actually contained in real fuel, you can read the
safety sheet.
In any case, if you don't clean the tanks when swapping from 87 octane to 100 octane then you'll end up with a mixture which will have some undetermined intermediate octane rating. Running at +12 on this mixture is not the same as running at +12 on 100 octane, and might cause pre-ignition or detonation.
It would therefore be advisable to flush the fuel system thoroughly with 100 octane before embarking on +12 running, and the obvious way to do that without taking the whole aeroplane & engine to bits would just be to ground run through say a tank full of 100 octane. Then you can be pretty certain that all the 87 octane is out of the system and it should be safe to start testing at +12.
You would then probably conduct a few minutes of running at high boost to ensure that the boost control cutout was really delivering +12.