4.6 Why were alkyl lead compounds added?
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This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
4.6 Why were alkyl lead compounds added?
The efficiency of a spark-ignited gasoline engine can be related to the
compression ratio up to at least compression ratio 17:1 [23]. However any
"knock" caused by the fuel will rapidly mechanically destroy an engine, and
General Motors was having major problems trying to improve engines without
inducing knock. The problem was to identify economic additives that could
be added to gasoline or kerosine to prevent knock, as it was apparent that
engine development was being hindered. The kerosine for home fuels soon
became a secondary issue, as the magnitude of the automotive knock problem
increased throughout the 1910s, and so more resources were poured into the
quest for an effective "antiknock". A higher octane aviation gasoline was
required urgently once the US entered WWI, and almost every possible
chemical ( including melted butter ) was tested for antiknock ability [24].
Originally, iodine was the best antiknock available, but was not a practical
gasoline additive, and was used as the benchmark. In 1919 aniline was found
to have superior antiknock ability to iodine, but also was not a practical
additive, however aniline became the benchmark antiknock, and various
compounds were compared to it. The discovery of tetra ethyl lead, and the
scavengers required to remove it from the engine were made by teams lead by
Thomas Midgley Jr. in 1922 [9,10,24]. They tried selenium oxychloride which
was an excellent antiknock, however it reacted with iron and "dissolved" the
engine. Midgley was able to predict that other organometallics would work,
and slowly focused on organoleads. They then had to remove the lead, which
would otherwise accumulate and coat the engine and exhaust system with lead.
They discovered and developed the halogenated lead scavengers that are still
used in leaded fuels. The scavengers, ( ethylene dibromide and ethylene
dichloride ), function by providing halogen atoms that react with the lead
to form volatile lead halide salts that can escape out the exhaust. The
quantity of scavengers added to the alkyl lead concentrate is calculated
according to the amount of lead present. If sufficient scavenger is added
to theoretically react with all the lead present, the amount is called one
"theory". Typically, 1.0 to 1.5 theories are used, but aviation gasolines
must only use one theory. This ensures there is no excess bromine that could
react with the engine.
The alkyl leads rapidly became the most cost-effective method of enhancing
octane. The introduction was not universally acclaimed, as the toxicity
of TEL soon became apparent, and several eminent public health officials
campaigned against the widespread introduction of alkyl leads [25].
Their cause was assisted by some major disasters at TEL manufacturing
plants, and although these incidents were mainly attributable to a failure
of management and/or staff to follow instructions, they resulted in a
protracted dispute in the chemical and public health literature that even
involved Midgley [25,26]. We should be careful retrospectively
applying judgement to the 1920s, as the increased octane of leaded gasoline
provided major gains in engine efficiency and lower gasoline prices.
The development of the alkyl leads ( tetra methyl lead, tetra ethyl lead )
and the toxic halogenated scavengers meant that petroleum refiners could
then configure refineries to produce hydrocarbon streams that would
increase octane with small quantities of alkyl lead. If you keep adding
alkyl lead compounds, the lead response of the gasoline decreases, and so
there are economic limits to how much lead should be added.
Up until the late 1960s, alkyl leads were added to gasolines in increasing
concentrations to obtain octane. The limit was 1.14g Pb/l, which is well
above the diminishing returns part of the lead response curve for most
refinery streams, thus it is unlikely that much fuel was ever made at that
level. I believe 1.05 was about the maximum, and articles suggest that 1970
100 RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g
Pb/l, which matches published lead response data [27,28] eg.
For Catalytic Reformate Straight Run Naphtha. Lead g/l Research Octane Number 0 96 72 0.1 98 79 0.2 99 83 0.3 100 85 0.4 101 87 0.5 101.5 88 0.6 102 89 0.7 102.5 89.5 0.8 102.75 90
The alkyl lead antiknocks work in a different stage of the pre-combustion
reaction to oxygenates. In contrast to oxygenates, the alkyl lead interferes
with hydrocarbon chain branching in the intermediate temperature range
where HO2 is the most important radical species. Lead oxide, either as
solid particles, or in the gas phase, reacts with HO2 and removes it from
the available radical pool, thereby deactivating the major chain branching
reaction sequence that results in undesirable, easily-autoignitable
hydrocarbons [21,22].
By the 1960s, the nature the toxicity of the emissions from gasoline-powered
engines was becoming of increasing concern and extensive comparisons of the
costs and benefits were being performed. By the 1970s, the failure to find
durable, lead-tolerant exhaust catalysts would hasten the departure of lead,
as the proposed regulated emissions levels could not be economically
achieved without exhaust catalysts [29]. A survey in 1995 indicated that
over 50 countries ( 20 in Africa ) still permit leaded fuels containing
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