[AT] unleaded gas

[DAVIESW739 at aol.com]

TEL is produced by reacting ethyl chloride with  a sodium-lead alloy.
Pb + 4 Na + 4 CH3CH2Cl → (CH3CH2)4Pb + 4 NaCl 
The  product, TEL, is a viscous colorless liquid. Because TEL is charge 
neutral (vs.  a salt) and contains an exterior of alkyl groups, it is highly 
lipophilic and  soluble in petrol.
The most noteworthy feature of TEL is the weakness of its  four C-Pb bonds. 
At the temperatures found in internal combustion engines  (CH3CH2)4Pb 
decomposes, first into (CH3CH2)3Pb and an ethyl radicals. These  radicals help to 
propagating the combustion, which itself is a radical reaction.  When (CH3CH2)4Pb 
burns, it produces, not only carbon dioxide and water, but also  lead:
(CH3CH2)4Pb + 13 O2 → 8 CO2 + 10 H2O + Pb 
This lead can oxidize  further to give species such as lead oxide)
Pb + 0.5 O2 → PbO 
The Pb and  PbO would quickly accumulate and destroy an engine. For this 
reason, lead  scavengers such as ethylene dibromide and 1,2-dichloroethane are 
used in  conjunction with TEL - these agents form volatile lead(II) bromide and 
lead(II)  chloride, respectively, which are exhausted from the engine (and into 
the  air).
[edit]Uses of TEL as an antiknock agent
TEL was once used  extensively as an additive in gasoline (petrol) for its 
ability to increase the  fuel's octane rating (that is, to prevent its premature 
detonation ("knocking")  in the engine) thus allowing the use of higher 
compression ratios for greater  efficiency and power. In addition some of the lead 
was deposited on the valve  seats and helped protect them against wear.
The use of TEL in gasoline was  mostly dominant in the US while in Europe 
alcohol was used instead. However the  dominance of the US oil companies 
eventually led to a switch to leaded fuel.  When used as an antiknock agent alcohol 
would make the fuel hydroscopic (it  would absorb water from the air) over time 
fuel humidity could lead to rusting  and corrosion, as well as issues of 
biological contamination.
In most Western  countries this additive went out of use in the late 20th 
century, chiefly  because of the realization that most of its lead—which is toxic 
to humans and  other organisms—ended up in the exhaust fumes and became a 
major health and  environmental problem. The need for that additive was also 
lessened by the  introduction of harder metals for valves and valve-seats, a 
general reduction in  engine compression ratios and the introduction of other 
anti-knocking additives.  The deployment of the catalytic converter (which lead 
oxides from TEL would  foul) further reduced TEL use. TEL remains an ingredient 
of aviation gasoline  and is also still available from a limited number of 
outlets as a fuel additive,  mostly for owners of classic and vintage cars and 
motorcycles. In addition, TEL  is still in use as an ingredient in the fuels used 
by NASCAR in its top three  racing series. A more environmental friendly 
additive is composed of 10 ppm  potassium (sometimes sodium), solvents and 75% 
diesel, and is added to unleaded  fuel to boost the octane rating by 2-3 points.
In earlier times many vehicles  produced before TEL's phase-out required 
modification to a greater or lesser  extent to run successfully on unleaded 
gasoline. The installation of new  hardened valve seats can be done by a competent 
automotive machine shop. A major  engine rebuild, generally by the use of 
dished pistons, is required to reduce  the compression ratio of some older 
high-performance engines (which required  100-octane fuel) to a ratio that is 
compatible with currently available gasoline  ratings and this reform necessarily 
entails a decrease in engine power. However  by the 21st century additives were 
available to allow continued use of even  these sensitive engines, more or less 
to their normal function. TEL is still in  use today as a component of 100 
octane aviation fuel, as a suitable replacement  for it in the aviation industry 
has not yet been found.

Walt  Davies
Cooper Hollow Farm
Monmouth, OR 97361
503 623-0460