Pelican Technical Article:
More Than You Ever Wanted to
Know About Gasoline
from usenet rec.autos.tech
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||This article is
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accompanying extensive step-by-step procedures, this book should be a staple
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||4. What is
Subject: 4. What is Gasoline?
4.1 Where does crude oil come from?
The generally-accepted origin of crude oil is from plant life up to 3
billion years ago, but predominantly from 100 to 600 million years ago .
"Dead vegetarian dino dinner" is more correct than "dead dinos".
The molecular structure of the hydrocarbons and other compounds present
in fossil fuels can be linked to the leaf waxes and other plant molecules of
marine and terrestrial plants believed to exist during that era. There are
various biogenic marker chemicals ( such as isoprenoids from terpenes,
porphyrins and aromatics from natural pigments, pristane and phytane from
the hydrolysis of chlorophyll, and normal alkanes from waxes ), whose size
and shape can not be explained by known geological processes . The
presence of optical activity and the carbon isotopic ratios also indicate a
biological origin . There is another hypothesis that suggests crude oil
is derived from methane from the earth's interior. The current main
proponent of this abiotic theory is Thomas Gold, however abiotic and
extraterrestrial origins for fossil fuels were also considered at the turn
of the century, and were discarded then. A large amount of additional
evidence for the biological origin of crude oil has accumulated since then.
4.2 When will we run out of crude oil?
It has been estimated that the planet contains over 6.4 x 10^15 tonnes of
organic carbon that is cycled through two major cycles, but only about 18%
of that contributes to petroleum production. The primary cycle ( turnover of
2.7-3.0 x 10^12 tonnes of organic carbon ) has a half-life of days to
decades, whereas the large secondary cycle ( turnover 6.4 x 10^15 tonnes of
organic carbon ) has a half-life of several million years . Much of this
organic carbon is too dilute or inaccessible for current technology to
recover, however the estimates represent centuries to millenia of fossil
fuels, even with continued consumption at current or increased rates .
The concern about "running out of oil" arises from misunderstanding the
significance of a petroleum industry measure called the Reserves/Production
ratio (R/P). This monitors the production and exploration interactions.
The R/P is based on the concept of "proved" reserves of fossil fuels.
Proved reserves are those quantities of fossil fuels that geological and
engineering information indicate with reasonable certainty can be recovered
in the future from known reservoirs under existing economic and operating
conditions. The Reserves/Production ratio is the proved reserves quantity
divided by the production in the last year, and the result will be the
length of time that those remaining proved reserves would last if production
were to continue at the current level . It is important to note the
economic and technology component of the definitions, as the price of oil
increases ( or new technology becomes available ), marginal fields become
"proved reserves". We are unlikely to "run out" of oil, as more fields
become economic. Note that investment in exploration is also linked to the
R/P ratio, and the world crude oil R/P ratio typically moves between
20-40 years, however specific national incentives to discover oil can
extend that range upward.
Concerned people often refer to the " Hubbert curves" that predict fossil
fuel discovery rates would peak and decline rapidly. M. King Hubbert
calculated in 1982 that the ultimate resource base of the lower 48 states of
the USA was 163+-2 billion barrels of oil, and the ultimate production of
natural gas to be 24.6+-0.8 trillion cubic metres, with some additional
qualifiers. As production and proved resources were 147 billion barrels of
oil and 22.5 trillion cubic metres of gas, Hubbert was implying that volumes
yet to be developed could only be 16-49 billion barrels of oil and 2.1-4.5
trillion cubic metres. Technology has confounded those predictions for
natural gas [6a].
The US Geological Survey has also just increased their assessment of US
( not just the lower 48 states ), inferred reserves crude oil by 60 billion
barrels, and doubled the size of gas reserves to 9.1 trillion cubic metres.
When combined with the estimate of undiscovered oil and gas, the totals
reach 110 billion barrels of oil and 30 trillion cubic metres of gas .
When the 1995 USGS estimates of undiscovered and inferred crude oil are
calculated for just the lower 48 states, they totalled ( in 1995 ) 68.9
billion barrels of oil, well above Hubbert's highest estimate made in 1982.
The current price for Brent Crude is approx. $22/bbl. The world R/P ratio
has increased from 27 years (1979) to 43.1 years (1993). The 1995 BP
Statistical Review of World Energy provides the following data [6,7].
Crude Oil Proved Reserves R/P Ratio
Middle East 89.4 billion tonnes 93.4 year
USA 3.8 9.8 years
USA - 1995 USGS data 10.9 33.0 years
Total World 137.3 43.0 years
Coal Proved Reserves R/P Ratio
USA 240.56 billion tonnes 247 years
Total World 1,043.864 235 years
Natural Gas Proved Reserves R/P Ratio
USA 4.6 trillion cubic metres 8.6 years
USA - 1995 USGS data 9.1 17.0 years
Total World 141.0 66.4 years.
One billion = 1 x 10^9. One trillion = 1 x 10^12.
One barrel of Arabian Light crude oil = 0.158987 m3 and 0.136 tonnes.
If the crude oil price exceeds $30/bbl then alternative fuels may become
competitive, and at $50-60/bbl coal-derived liquid fuels are economic, as
are many biomass-derived fuels and other energy sources .
4.3 What is the history of gasoline?
In the late 19th Century the most suitable fuels for the automobile
were coal tar distillates and the lighter fractions from the distillation
of crude oil. During the early 20th Century the oil companies were
producing gasoline as a simple distillate from petroleum, but the
automotive engines were rapidly being improved and required a more
suitable fuel. During the 1910s, laws prohibited the storage of gasolines
on residential properties, so Charles F. Kettering ( yes - he of ignition
system fame ) modified an IC engine to run on kerosine. However the
kerosine-fuelled engine would "knock" and crack the cylinder head and
pistons. He assigned Thomas Midgley Jr. to confirm that the cause was
from the kerosine droplets vaporising on combustion as they presumed.
Midgley demonstrated that the knock was caused by a rapid rise in
pressure after ignition, not during preignition as believed . This
then lead to the long search for antiknock agents, culminating in
tetra ethyl lead . Typical mid-1920s gasolines were 40 - 60 Octane .
Because sulfur in gasoline inhibited the octane-enhancing effect
of the alkyl lead, the sulfur content of the thermally-cracked refinery
streams for gasolines was restricted. By the 1930s, the petroleum
industry had determined that the larger hydrocarbon molecules (kerosine)
had major adverse effects on the octane of gasoline, and were developing
consistent specifications for desired properties. By the 1940s catalytic
cracking was introduced, and gasoline compositions became fairly consistent
between brands during the various seasons.
The 1950s saw the start of the increase of the compression ratio, requiring
higher octane fuels. Octane ratings, lead levels, and vapour pressure
increased, whereas sulfur content and olefins decreased. Some new refining
processes ( such as hydrocracking ), specifically designed to provide
hydrocarbons components with good lead response and octane, were introduced.
Minor improvements were made to gasoline formulations to improve yields and
octane until the 1970s - when unleaded fuels were introduced to protect
the exhaust catalysts that were also being introduced for environmental
reasons. From 1970 until 1990 gasolines were slowly changed as lead was
phased out, lead levels plummetted, octanes initially decreased, and then
remained 2-5 numbers lower, vapour pressures continued to increase, and
sulfur and olefins remained constant, while aromatics increased. In 1990,
the US Clean Air Act started forcing major compositional changes on
resulting in plummeting vapour pressure and increaing oxygenate levels.
These changes will continue into the 21st Century, because gasoline use
in SI engines is a major pollution source. Comprehensive descriptions of the
changes to gasolines this century have been provided by L.M.Gibbs [12,13].
The move to unleaded fuels continues worldwide, however several countries
have increased the aromatics content ( up to 50% ) to replace the alkyl
lead octane enhancers. These highly aromatic gasolines can result in
in damage to elastomers and increased levels of toxic aromatic emissions
if used without exhaust catalysts.
4.4 What are the hydrocarbons in gasoline?
Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and
carbon, both of which are fuel molecules that can be burnt ( oxidised )
to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is
not complete, carbon monoxide ( CO ) may be formed. As CO can be burnt
to produce CO2, it is also a fuel.
The way the hydrogen and carbons hold hands determines which hydrocarbon
family they belong to. If they only hold one hand they are called
"saturated hydrocarbons" because they can not absorb additional hydrogen.
If the carbons hold two hands they are called "unsaturated hydrocarbons"
because they can be converted into "saturated hydrocarbons" by the
addition of hydrogen to the double bond. Hydrogens are omitted from the
following, but if you remember C = 4 hands, H = 1 hand, and O = 2 hands,
you can draw the full structures of most HCs.
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. Detailed descriptions of structures
can be found in any chemical or petroleum text discussing gasolines .
4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes )
- stable, the major component of leaded gasolines.
- tend to burn in air with a clean flame.
- octane ratings depend on branching and number of carbon atoms.
normal = continuous chain of carbons ( Cn H2n+2 )
- low octane ratings, decreasing with carbon chain length.
normal heptane C-C-C-C-C-C-C C7H16
iso = branched chain of carbons ( Cn H2n+2 )
- higher octane ratings, increasing with carbon chain branching.
iso octane = C C
( aka 2,2,4-trimethylpentane ) | |
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
- high octane ratings.
cyclohexane = C
| | C6H12
4.4.2 Unsaturated Hydrocarbons
- Unstable, are the remaining component of gasoline.
- Tend to burn in air with a smoky flame.
Alkenes ( aka olefins, have carbon=carbon double bonds )
- These are unstable, and are usually limited to a few %.
- tend to be reactive and toxic, but have desirable octane ratings.
Alkynes ( aka acetylenes, have carbon-carbon triple bonds )
- These are even more unstable, are only present in
trace amounts, and only in some poorly-refined gasolines.
Acetylene C=C C2H2
Arenes ( aka aromatics )
- Used to be up to 40%, gradually being reduced to <20% in the US. tend to
be more toxic, but have desirable octane ratings. Some countries are
increasing the aromatic content ( up to 50% in some super unleaded fuels )
to replace the alkyl lead octane enhancers. C C // \ // \ C C C-C C Benzene
| || Toluene | || C C C C \\ / \\ / C C C6H6 C7H8 Polynuclear Aromatics (
aka PNAs or PAHs ) These are high boiling, and are only present in small
amounts in gasoline. They contain benzene rings joined together. The
simplest, and least toxic, is Naphthalene, which is only present in trace
amounts in traditional gasolines, and even lower levels are found in
reformulated gasolines. The larger multi-ringed PNAs are highly toxic, and
are not present in gasoline. C C // \ / \\ C C C Naphthalene | || | C10H8 C
C C \\ / \ // C C 4.5 What are oxygenates? Oxygenates are just preused
hydrocarbons :-). They contain oxygen, which can not provide energy, but
their structure provides a reasonable antiknock value, thus they are good
substitutes for aromatics, and they may also reduce the smog-forming
tendencies of the exhaust gases . Most oxygenates used in gasolines are
either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 1 to 6 carbons.
Alcohols have been used in gasolines since the 1930s, and MTBE was first
used in commercial gasolines in Italy in 1973, and was first used in the US
by ARCO in 1979. The relative advantages of aromatics and oxygenates as
environmentally-friendly and low toxicity octane-enhancers are still being
researched. Ethanol C-C-O-H C2H5OH C | Methyl tertiary butyl ether C-C-O-C
C4H9OCH3 (aka tertiary butyl methyl ether ) | C They can be produced from
fossil fuels eg methanol (MeOH), methyl tertiary butyl ether (MTBE),
tertiary amyl methyl ether (TAME), or from biomass, eg ethanol(EtOH), ethyl
tertiary butyl ether (ETBE)). MTBE is produced by reacting methanol ( from
natural gas ) with isobutylene in the liquid phase over an acidic
ion-exchange resin catalyst at 100C. The isobutylene was initially from
refinery catalytic crackers or petrochemical olefin plants, but these days
larger plants produce it from butanes. MTBE production has increased at the
rate of 10 to 20% per year, and the spot market price in June 1993 was
around $270/tonne . The "ether" starting fluids for vehicles are usually
diethyl ether (liquid) or dimethyl ether (aerosol). Note that " petroleum
ethers " are volatile alkane hydrocarbon fractions, they are not a Cx-O-Cy
compound. Oxygenates are added to gasolines to reduce the reactivity of
emissions, but they are only effective if the hydrocarbon fractions are
carefully modified to utilise the octane and volatility properties of the
oxygenates. If the hydrocarbon fraction is not correctly modified,
oxygenates can increase the undesirable smog-forming and toxic emissions.
Oxygenates do not necessarily reduce all exhaust toxins, nor are they
intended to. Oxygenates have significantly different physical properties to
hydrocarbons, and the levels that can be added to gasolines are controlled
by the 1977 Clean Air Act amendments in the US, with the laws prohibiting
the increase or introduction of a fuel or fuel additive that is not
substantially similar to any fuel or fuel additive used to certify 1975 or
subsequent years vehicles. Waivers can granted if the product does not cause
or contribute to emission device failures, and if the EPA does not
specifically decline the application after 180 days, it is taken as granted.
In 1978 the EPA granted 10% by volume of ethanol a waiver, and have
subsequently issued waivers for <10 vol% ethanol (1982), 7 vol% tertiary
butyl alcohol (1979), 5.5 vol% 1:1 MeOH/TBA (1979), 3.5 mass% oxygen derived
from 1:1 MeOH/TBA="~9.5" vol% of the alcohols (1981), 3.7 mass% oxygen
derived from methanol and cosolvents="5" vol% max MeOH and 2.5 vol% min
cosolvent with some cosolvents requiring additional corrosion inhibitor
(1985,1988), 7.0 vol% MTBE (1979), and 15.0 vol% MTBE (1988). Only the
ethanol waiver was exempted from the requirement to still meet ASTM
volatility requirements . In 1981 the EPA ruled that fuels could contain
aliphatic alcohols ( except MeOH ) and/or ethers at concentrations until the
oxygen content is 2.0 mass%. It also permitted 5.5 vol% of 1:1 MeOH/TBA. In
1991 the maximum oxygen content was increased to 2.7 mass%. To ensure
sufficient gasoline base was available for ethanol blending, the EPA also
ruled that gasoline containing up to 2 vol% of MTBE could subsequently be
blended with 10 vol% of ethanol . Initially, the oxygenates were added
to hydrocarbon fractions that were slightly-modified unleaded gasoline
fractions, and these were known as "oxygenated" gasolines. In 1995, the
hydrocarbon fraction was significantly modified, and these gasolines are
called "reformulated gasolines" ( RFGs ), and there are differing
specifications for California ( Phase 2 ) and Federal ( simple model ) RFGs,
however both require oxygenates to provide Octane. The California RFG
requires the hydrocarbon composition of the RFG to be significantly more
modified than the existing oxygenated gasolines to reduce unsaturates,
volatility, benzene, and the reactivity of emissions. Federal regulations
only reduce vapour pressure and benzene directly, however aromatics are also
reduced to meet emissions criteria . Oxygenates that are added to
gasoline function in two ways. Firstly they have high blending octane, and
so can replace high octane aromatics in the fuel. These aromatics are
responsible for disproportionate amounts of CO and HC exhaust emissions.
This is called the "aromatic substitution effect". Oxygenates also cause
engines without sophisticated engine management systems to move to the lean
side of stoichiometry, thus reducing emissions of CO ( 2% oxygen can reduce
CO by 16% ) and HC ( 2% oxygen can reduce HC by 10%) , and other
researchers have observed similar reductions also occur when oxygenates are
added to reformulated gasolines on older and newer vehicles, but have also
shown that NOx levels may increase, as also may some regulated toxins
[18,19,20]. However, on vehicles with engine management systems, the fuel
volume will be increased to bring the stoichiometry back to the preferred
optimum setting. Oxygen in the fuel can not contribute energy, consequently
the fuel has less energy content. For the same efficiency and power output,
more fuel has to be burnt, and the slight improvements in combustion
efficiency that oxygenates provide on some engines usually do not completely
compensate for the oxygen. There are huge number of chemical mechanisms
involved in the pre-flame reactions of gasoline combustion. Although both
alkyl leads and oxygenates are effective at suppressing knock, the chemical
modes through which they act are entirely different. MTBE works by retarding
the progress of the low temperature or cool-flame reactions, consuming
radical species, particularly OH radicals and producing isobutene. The
isobutene in turn consumes additional OH radicals and produces unreactive,
resonantly stabilised radicals such as allyl and methyl allyl, as well as
stable species such as allene, which resist further oxidation [21,22]. 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 . 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 . 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 . 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 . A survey
in 1995 indicated that over 50 countries ( 20 in Africa ) still permit
leaded fuels containing 0.8g Pb/l, whereas the European maximum is 0.15 g Pb/l
[29a]. 4.7 Why not use other organometallic compounds? As the toxicity of
the alkyl lead and the halogenated scavengers became of concern,
alternatives were considered. The most famous of these is
methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the
USA until banned by the EPA from 27 Oct 1978 , but is approved for use
in Canada and Australia. Recently the EPA ban was overturned, and MMT can be
used up to 0.031gMn/US Gal in all states except California ( where it
remains banned ). The EPA has stated it intends to review the whole MMT
siuation and , if evidence supports removing MMT, they will revisit banning
MMT. Automobile manufacturers believe MMT reduces the effectiveness of the
latest emission control systems . Canada also contemplated banning MMT
because of the same concerns, as well as achieving fuel supply uniformity
with the lower 48 states of the USA . MMT is more expensive than alkyl
leads and has been reported to increase unburned hydrocarbon emissions and
block exhaust catalysts . Other compounds that enhance octane have been
suggested, but usually have significant problems such as toxicity, cost,
increased engine wear etc.. Examples include dicyclopentadienyl iron and
nickel carbonyl. Germany used iron pentacarbonyl (Fe(CO)5) at levels of 0.5%
or less in gasoline during the 1930s. While its cost was low, one of its
major drawbacks was that the carbonyl decomposed rapidly when the gasoline
was exposed to light. Iron oxide (Fe3O4) also deposited on the spark plug
insulator causing short circuits, and the precipitation of iron oxides in
the lubricating oil also led to excessive wear rates . 4.8 What do the
refining processes do? Crude oil contains a wide range of hydrocarbons,
organometallics and other compounds containing sulfur, nitrogen etc. The HCs
contain between 1 and 60 carbon atoms. Gasoline contains hydrocarbons with
carbon atoms between 3 and 12, arranged in specific ways to provide the
desirable properties. Obviously, a refinery has to either sell the remainder
as marketable products, or convert the larger molecules into smaller
gasoline molecules. A refinery will distill crude oil into various fractions
and, depending on the desired final products, will further process and blend
those fractions. Typical final products could be:- gases for chemical
synthesis and fuel (CNG), liquified gases (LPG), butane, aviation and
automotive gasolines, aviation and lighting kerosines, diesels, distillate
and residual fuel oils, lubricating oil base grades, paraffin oils and
waxes. Many of the common processes are intended to increase the yield of
blending feedstocks for gasolines. Typical modern refinery processes for
gasoline components include * Catalytic cracking breaks larger,
higher-boiling, hydrocarbons into gasoline range product that contains 30%
aromatics and 20-30% olefins. * Hydrocracking cracks and adds hydrogen to
molecules, producing a more saturated, stable, gasoline fraction. *
Isomerisation raises gasoline fraction octane by converting straight chain
hydrocarbons into branched isomers. * Reforming converts saturated, low
octane, hydrocarbons into higher octane product containing about 60%
aromatics. * Alkylation reacts gaseous olefin streams with isobutane to
produce liquid high octane iso-alkanes. The changes to the US Clean Air Act
and other legislation ensures that the refineries will continue to modify
their processes to produce a less volatile gasoline with fewer toxins and
toxic emissions. Options include:- * Reducing the "severity" of reforming to
reduce aromatic production. * Distilling the C5/C6 fraction ( containing
benzene and benzene precusers ) from reformer feeds and treating that stream
to produce non-aromatic high octane components. * Distilling the higher
boiling fraction ( which contains 80-100% of aromatics that can be
hydrocracked ) from catalytic cracker product . * Convert butane to
isobutane or isobutylene for alkylation or MTBE feed. Some other countries
are removing the alkyl lead compounds for health reasons, and replacing them
with aromatics and oxygenates. If the vehicle fleet does not have exhaust
catalysts, the emissions of some toxic aromatic hydrocarbons can increase.
If maximum environmental and health gains are to be achieved, the removal of
lead from gasoline should be accompanied by the immediate introduction of
exhaust catalysts and sophisticated engine management systems, 4.9 What
energy is released when gasoline is burned? It is important to note that the
theoretical energy content of gasoline when burned in air is only related to
the hydrogen and carbon contents. The energy is released when the hydrogen
and carbon are oxidised (burnt), to form water and carbon dioxide. Octane
rating is not fundamentally related to the energy content, and the actual
hydrocarbon and oxygenate components used in the gasoline will determine
both the energy release and the antiknock rating. Two important reactions
are:- C + O2="CO2" H + O2="H2O" The mass or volume of air required to
provide sufficient oxygen to achieve this complete combustion is the "stoichiometric"
mass or volume of air. Insufficient air="rich" , and excess air="lean" , and
the stoichiometric mass of air is related to the carbon:hydrogen ratio of
the fuel. The procedures for calculation of stoichiometric air-fuel ratios
are fully documented in an SAE standard . Atomic masses used are:-
Hydrogen="1.00794," Carbon="12.011," Oxygen="15.994," Nitrogen="14.0067,"
and Sulfur="32.066." The composition of sea level air ( 1976 data, hence low
CO2 value ) is Gas Fractional Molecular Weight Relative Species Volume
kg/mole Mass N2 0.78084 28.0134 21.873983 O2 0.209476 31.9988 6.702981 Ar
0.00934 39.948 0.373114 CO2 0.000314 44.0098 0.013919 Ne 0.00001818 20.179
0.000365 He 0.00000524 4.002602 0.000021 Kr 0.00000114 83.80 0.000092 Xe
0.000000087 131.29 0.000011 CH4 0.000002 16.04276 0.000032 H2 0.0000005
2.01588 0.000001 Air 28.964419 For normal heptane C7H16 with a molecular
weight="100.204" C7H16 + 11O2="7CO2" + 8H2O thus 1.000 kg of C7H16 requires
3.513 kg of O2="15.179" kg of air. The chemical stoichiometric combustion of
hydrocarbons with oxygen can be written as:- CxHy + (x + (y/4))O2> xCO2 +
Often, for simplicity, the remainder of air is assumed to be nitrogen,
which can be added to the equation when exhaust compositions are required.
As a general rule, maximum power is achieved at slightly rich, whereas
maximum fuel economy is achieved at slightly lean.
The energy content of the gasoline is measured by burning all the fuel
inside a bomb calorimeter and measuring the temperature increase.
The energy available depends on what happens to the water produced from the
combustion of the hydrogen. If the water remains as a gas, then it cannot
release the heat of vaporisation, thus producing the Nett Calorific Value.
If the water were condensed back to the original fuel temperature, then
Gross Calorific Value of the fuel, which will be larger, is obtained.
The calorific values are fairly constant for families of HCs, which is not
surprising, given their fairly consistent carbon:hydrogen ratios. For liquid
( l ) or gaseous ( g ) fuel converted to gaseous products - except for the
2-methylbutene-2, where only gaseous is reported. * = Blending Octane Number
as reported by API Project 45 using 60 octane base fuel, and the numbers
in brackets are Blending Octane Numbers currently used for modern fuels.
Typical Heats of Combustion are :-
Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
i-octane l 44.374 100 100
toluene l 40.554 124* (111) 112* (94)
2-methylbutene-2 44.720 176* (113) 141* (81)
Because all the data is available, the calorific value of fuels can be
estimated quite accurately from hydrocarbon fuel properties such as the
density, sulfur content, and aniline point ( which indicates the aromatics
It should be noted that because oxygenates contain oxygen that can
not provide energy, they will have significantly lower energy contents.
They are added to provide octane, not energy. For an engine that can be
optimised for oxygenates, more fuel is required to obtain the same power,
but they can burn slightly more efficiently, thus the power ratio is not
identical to the energy content ratio. They also require more energy to
Energy Content Heat of Vaporisation Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0
Typical values for commercial fuels in megajoules/kilogram are :-
Hydrogen 141.9 120.0
Carbon to Carbon monoxide 10.2 -
Carbon to Carbon dioxide 32.8 -
Sulfur to sulfur dioxide 9.16 -
Natural Gas 53.1 48.0
Liquified petroleum gas 49.8 46.1
Aviation gasoline 46.0 44.0
Automotive gasoline 45.8 43.8
Kerosine 46.3 43.3
Diesel 45.3 42.5
Obviously, for automobiles, the nett calorific value is appropriate, as the
water is emitted as vapour. The engine can not utilise the additional energy
available when the steam is condensed back to water. The calorific value is
the maximum energy that can be obtained from the fuel by combustion, but the
reality of modern SI engines is that thermal efficiencies of only 20-40% may
be obtained, this limit being due to engineering and material constraints
that prevent optimum thermal conditions being used. CI engines can achieve
higher thermal efficiencies, usually over a wider operating range as well.
Note that combustion efficiencies are high, it is the thermal efficiency of
the engine is low due to losses. For a water-cooled SI engine with 25%
useful work at the crankshaft, the losses may consist of 35% (coolant),
33% (exhaust), and 12% (surroundings).
4.10 What are the gasoline specifications?
Gasolines are usually defined by government regulation, where properties and
test methods are clearly defined. In the US, several government and state
bodies can specify gasoline properties, and they may choose to use or modify
consensus minimum quality standards, such as American Society for Testing
Materials (ASTM). The US gasoline specifications and test methods are listed
in several readily available publications, including the Society of
Automotive Engineers (SAE) , and the Annual Book of ASTM Standards .
The 1995 ASTM edition includes:-
D4814-94d Specification for Automotive Spark-Ignition Engine Fuel.
This specification lists various properties that all fuels have to comply
with, and may be updated throughout the year. Typical properties are:-
4.10.1 Vapour Pressure and Distillation Classes.
6 different classes according to location and/or season.
As gasoline is distilled, the temperatures at which various fractions are
evaporated are calculated. Specifications define the temperatures at which
various percentages of the fuel are evaporated. Distillation limits
include maximum temperatures that 10% is evaporated (50-70C), 50% is
evaporated (110-121C), 90% is evaporated (185-190C), and the final boiling
point (225C). A minimum temperature for 50% evaporated (77C), and a maximum
amount of Residue (2%) after distillation. Vapour pressure limits for
each class ( 54, 62, 69, 79, 93, 103 kPa ) are also specified. Note that the
EPA has issued a waiver that does not require gasoline with 9-10% ethanol to
meet the required specifications between 1st May - 15 September.
4.10.2 Vapour Lock Protection Classes
5 classes for vapour lock protection, according to location and/or season.
The limit for each class is a maximum Vapour-Liquid ratio of 20 at one of
the specified testing temperatures of 41, 47, 51, 56, 60C.
4.10.3 Antiknock Index ( aka (RON+MON)/2, "Pump Octane" )
The ( Research Octane Number + Motor Octane Number ) divided by two. Limits
are not specified, but changes in engine requirements according season and
location are discussed. Fuels with an Antiknock index of 87, 89, 91
( Unleaded), and 88 ( Leaded ) are listed as typical for the US at sea
however higher altitudes will specify lower octane numbers.
4.10.4 Lead Content
Leaded = 1.1 g Pb / L maximum, and Unleaded = 0.013 g Pb / L maximum.
4.10.5 Copper strip corrosion
Ability to tarnish clean copper, indicating the presence of any corrosive
4.10.6 Maximum Sulfur content
Sulfur adversely affects exhaust catalysts and fuel hydrocarbon lead
response, and also may be emitted as polluting sulfur oxides.
Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum.
Typical US gasoline levels are 0.03 %mass.
4.10.7 Maximum Solvent Washed Gum ( aka Existent Gum )
Limits the amount of gums present in fuel at the time of testing to
5 mg/100mls. The results do not correlate well with actual engine deposits
caused by fuel vaporisation .
4.10.8 Minimum Oxidation Stability
This ensures the fuel remains chemically stable, and does not form
gums during periods in distribution systems, which can be up to 3-6 months.
The sample is heated with oxygen inside a pressure vessel, and the delay
until significant oxygen uptake is measured.
4.10.9 Water Tolerance
Highest temperature that causes phase separation of oxygenated fuels.
The limits vary according to location and month. For Alaska - North of 62
latitude, it changes from -41C in Dec-Jan to 9C in July, but remains 10C all
year in Hawaii.
Because phosphorus adversely affects exhaust catalysts, the EPA limits
phosphorus in all gasolines to 0.0013g P/L.
As well as the above, there are various restrictions introduced by the Clean
Air Act and state bodies such as California's Air Resources Board (CARB)
that often have more stringent limits for the above properties, as well as
additional limits. More detailed descriptions of the complex regulations
can be found elsewhere [16,41,42] - I've just included some of the major
changes, as some properties are determined by levels of permitted emissions,
eg the toxics reduction required for fuel that has the maximum permitted
benzene (1.0%), means total aromatics are limited to around 27%. There have
been some changes in early 1996 to the implementation timetable, and the
following timetable has not yet been changed.
The Clean Air Act also specifies some regions that exceed air quality
standards have to use reformulated gasolines (RFGs) all year, starting
January 1995. Other regions are required to use oxygenated gasolines for
four winter months, beginning November 1992. The RFGs also contain
oxygenates. Metropolitan regions with severe ozone air quality problems must
use reformulated gasoline in 1995 that;- contain at least 2.0 wt% oxygen,
reduce 1990 volatile organic carbon compounds by 15%, and reduce specified
toxic emissions by 15% (1995) and 25% (2000). Metropolitan regions that
exceeded carbon monoxide limits were required to use gasolines with 2.7 wt%
oxygen during winter months, starting in 1992.
The 1990 Clean Air Act (CAA) amendments and CARB Phase 2 (1996)
specifications for reformulated gasoline establish the following limits,
compared with typical 1990 gasoline. Because of a lack of data, the EPA
were unable to define the CAA required parameters, so they instituted
a two-stage system. The first stage, the "Simple Model" is an interim
stage that run from 1/Jan/1995 to 31/Dec/1997. The second stage, the
"Complex Model" has two phases, Phase I (1995-1999) and Phase II (2000+),
and there are different limits for EPA Control Region 1 (south) and Control
Region 2 (north). Each refiner must have their RFG recertified to the
Complex model prior to the 1/Jan/1998 implementation date. The following
are some of the criteria for RFG when complying on a per gallon basis, more
details are available elsewhere, including the details of the baseline fuel
compositions to be used for testing [16,41,42,43,43a].
1990 Clean Air Act CARB
Simple Complex Phase 2
I II Limit Average
benzene (max.vol.%) 2 1.00 1.00 1.00 1.00 0.8
oxygen (min.mass %) 0.2 2.0 2.0 2.0 1.8 -
(max.mass %) - 2.7 - - 2.2 -
sulfur (max.mass ppm) 150 no increase - - 40 30
aromatics (max.vol.%) 32 toxics reduction - - 25 22
olefins (max.vol.%) 9.9 no increase - - 6.0 4.0
reid vapour pressure (kPa) 60 55.8 (north) - - 48.3 -
(during VOC Control Period) 49.6 (south)
50% evaporated (max.C) - - - - 98.9 93
90% evaporated (max.C) 170 - - - 148.9 143
VOC Reductions - Region I (min.%) 35.1 27.5 - -
(VOC Control Period only) - Region II (min.%) 15.6 25.9 - -
NOx Reductions - VOC Control Period (min.%) 0 5.5 - -
- Non-VOC Control Period (min.%) 0 0 - -
Toxics Reductions (min.%) 15.0 20.0 - -
These regulations also specify emissions criteria. e.g. CAA specifies no
increase in nitric oxides (NOx) emissions, reductions in VOC by 15% during
the ozone season, and specified toxins by 15% all year. These criteria
indirectly establish vapor pressure and composition limits that refiners
have to meet. Note that the EPA also can issue CAA Section 211 waivers that
allow refiners to choose which oxygenates they use. In 1981, the EPA also
decided that fuels with up to 2% weight of oxygen ( from alcohols and ethers
(except methanol)) were "substantially similar" to 1974 unleaded gasoline,
and thus were not "new" gasoline additives. That level was increased to
2.7 wt% in 1991. Some other oxygenates have also been granted waivers, eg
ethanol to 10% volume ( approximately 3.5 wt% ) in 1979 and 1982, and
tert-butyl alcohol to 3.5 wt% in 1981. In 1987 and 1988 further waivers
were issued for mixture of alcohols representing 3.7% wt of oxygen.
4.11 What are the effects of the specified fuel properties?
This affects evaporative emissions and drivability, it is the property that
must change with location and season. Fuel for mid-summer Arizona would be
difficult to use in mid-winter Alaska. The US is divided into zones,
according to altitude and seasonal temperatures, and the fuel volatility is
adjusted accordingly. Incorrect fuel may result in difficult starting in
cold weather, carburetor icing, vapor lock in hot weather, and crankcase
oil dilution. Volatility is controlled by distillation and vapor pressure
specifications. The higher boiling fractions of the gasoline have
effects on the emission levels of undesirable hydrocarbons and aldehydes,
and a reduction of 40C in the final boiling point will reduce the levels of
benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce
HC emissions by 20% .
As gasolines contain mainly hydrocarbons, the only significant variable
between different grades is the octane rating of the fuel, as most other
properties are similar. Octane is discussed in detail in Section 6. There
are only slight differences in combustion temperatures ( most are around
2000C in isobaric adiabatic combustion ). Note that the actual
temperature in the combustion chamber is also determined by other factors,
such as load and engine design. The addition of oxygenates changes the
pre-flame reaction pathways, and also reduces the energy content of the
The levels of oxygen in the fuel is regulated according to regional air
Motor gasolines may be stored up to six months, consequently they must not
form gums which may precipitate. Reactions of the unsaturated HCs may
produce gums ( these reactions can be catalyzed by metals such as copper ),
so antioxidants and metal deactivators are added. Existent Gum is used to
measure the gum in the fuel at the time tested, whereas the Oxidation
Stability measures the time it takes for the gasoline to break down at 100C
with 100psi of oxygen. A 240 minute test period has been found to be
sufficient for most storage and distribution systems.
Sulfur in the fuel creates corrosion, and when combusted will form corrosive
gases that attack the engine, exhaust and environment. Sulfur also adversely
affects the alkyl lead octane response, and will adversely affect exhaust
catalysts, but monolithic catalysts will recover when the sulfur content of
the fuel is reduced, so sulfur is considered an inhibitor, rather than a
catalyst poison. The copper strip corrosion test and the sulfur content
specification are used to ensure fuel quality. The copper strip test
active sulfur, whereas the sulfur content reports the total sulfur present.
Manufacturers many also add additional tests, such as filterability, to
ensure no distribution problems are encountered.
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