----- Original Message -----
From: "Robert Barber" <rbarber41@hotmail.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Monday, November 13, 2000 10:05 PM
Subject: [MV] gasoline
>
> Ok we have beat this around a bit but lets see if
> we have the shell answer man (or woman) amongst us ..
>
Not me, but I have done a bit of research.
> is there anyone out there thats a perto chem eng or
> even a chem engineer that knows the current chemical
> makeup of todays automotive and aircraft gasolines ??
>
I doubt we'd understand the detailed composition and characteristics even if
anyone could scientifically define it.
> Now I know they call the avgas on the market 100 Low lead
> cause it still does contain some lead antiknock additives
> So with that in mind and talking to a number of in the know
> types that todays automotive fuels also still have a lead
> content to them..
>
The only way soluble lead, TEL (Tetra Ethyl Lead) gets into gasoline is by
man adding it, any lead compound in current US or UK pump fuel would be
below measurable limits and would destroy the standard CAT in short order
having you fail a smog test and me the MoT.
We've been here before a month or so back.
Octane rating is measured in a weird lab test engine with variable
compression, its title is a "Co-operative Fuels Research engine" known
simply as a CFR. Using the sample fuel the engine is adjusted until
knocking occurs, then a test fuel of iso-octane (cetane I think) which is
highly knock resistant is used with n-heptane which has a rating of about 10
and can be considered the zero reference because it always knocks; by
increasing the ratio of cetane to n-heptane the point of knocking is again
reached and the rating for the fuel is then the ratio of the two, ie., 20%
n-heptane to 80% iso-octane would be called 80 Octane. By this means you
cannot get numbers above 100 so after this figure the last two digits
represent the ratio of TEL added.
TEL was added in America initially to boost radial aero engines in the 20's
for (successful) carrier take-offs by allowing a useful power boost of the
then typical 75 oct AVGAS.
TEL works by burning to lead oxide and forming a reflective heat shield
around the burning mixture stopping the rest of the charge from simply going
bang. TEL itself does nothing for the valve seats, in fact its a problem as
it chokes the cylinders and shorts spark plugs but such are the gains that
something had to be found to alleviate the problem.
Initially ethylene dibromide was used, but proved expensive and subsequently
a chlorine eliminator was almost universally adopted, after combustion the
lead oxide reacts with the eliminator to make a white crystalline salt which
for the bromide eliminator melts at 370 deg C and with chlorine 500 deg C, a
lot lower than the melting point of the original lead oxide at some 900 deg
C.
Its this melted liquid salt that lubricates the valve seats and eats the
stems away, it also appears as the grey/white deposit in exhaust pipes and
the streaks down aircraft cowlings or over the wings although largely not
seen now with regular cleaning and 100LL but very evident in WWII
photographs. The other deduction is that as no problems with exhaust seats
were apparent, no one went looking for a non-existent problem so specific
engine powers climbed steadily and the lead salts went about their job
unknown, that is, until the TEL was removed. . . . . . . . . . . .
Note, soluble lead (TEL) is totally, utterly different from metallic lead,
the exhaust lead salts (halides) are not absorbed by the human body, do not
destroy brain cells and are water soluble. A significant point that
politicians, greenies and other do-gooders would not or could not
understand. Chemistry often does this of course, eg., NaCl, sodium
chloride. Sodium is a quite nasty and poisionous stuff, so is chlorine;
sodium chloride as a compound is of course table salt.
To conjoin with an earlier posting:
> Interesting since if you look at the original pre-war Piper Cub
> Aronca Chief, ect aircraft owners manuals they all state that if
> the 80 octane aviation fuel is not available then "tractor gas"
> read automotive pump gas was an acceptable alternative..
>
That might be so for the US at the time.
> So what happened between the BOB and latter in the war since the merlins
> that were stuffed into P-51 required 145 octane"purple"
> gas ? Both 145 purple and red 80 octane aviation gas were
> dropped somewhere around the mid 70's leaving only 100 Low lead
> blue left. Really upset the warbird crowd in that they all had to
> drop the power setting from 54 - 56 Hg on take off down to a max
> of about 44 Hg.. Lost lots of HP that way, but better to loose the
> HP than detonate the head off the motor ..
>
Britain at the time was introducing 100 oct AVGAS having found that simply
dumping more TEL into the gas allowed higher boost pressures and therefore
power, simply open the throttle more.
The problem was desperate shortage of the stuff so US imports were arranged,
however clerks are not chemists and the simple octane statement falls far
short of the full definition since the aromatics in gas have a marked effect
on performance. The initial US 100 oct AVGAS performed no better than the
80 oct at the time until the chemists realised the very different
hydrocarbon structures produced from the two different sources of original
crude base stock.
Once this was defined equivalent fuel could be formulated with the initial
batch arriving in the tanker "Bunker Hill" and reference samples still
available to this day are known as Bunker Hill 100.
The 145 oct you refer to means a refined gasoline to 100 oct with some 45
milligrams of TEL per gallon added, enough to have an army of high-pressure
greenies turn red with rage. . . . . . . . . .
Warbird racing is peculiar to the US where you have a more amenable FAA and
the infrastructure/demand to provide new-manufacture approved spares,
Merlins and Griffons are cosseted here to keep them flying by lightening the
airframe of guns and armour together with operation at zero boost typically,
thereby the effective sea level naturally aspirated power is maintained at
altitude and the use of 100LL is not a drawback.
Richard
Southampton - England
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