4. What is Gasoline?
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 gasoline,
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 + (y/2)H2O
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 level,
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 additional
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 gasolines 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. eg 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 vapour 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 driveability, 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, carburetter icing, vapour lock in hot weather, and crankcase
oil dilution. Volatility is controlled by distillation and vapour pressure
specifications. The higher boiling fractions of the gasoline have significant
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 fuel.
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 catalysed 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 measures
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.