Some frequently asked questions:
Credit to: faqs.org/faqs/autos/gasoline-faq/part1/
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 [1].
"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 [2]. The
presence of optical activity and the carbon isotopic ratios also indicate a
biological origin [3]. 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 [4]. 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 [5].
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 [6]. 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 [7].
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 [8].
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 [9]. This
then lead to the long search for antiknock agents, culminating in
tetra ethyl lead [10]. Typical mid-1920s gasolines were 40 - 60 Octane [11].
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 [14].
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.
alkanes
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 ) | |
C-C-C-C-C C8H18
|
C
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
- high octane ratings.
cyclohexane = C
/ \
C C
| | C6H12
C C
\ /
C
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.
C
| C5H10
2-methyl-2-butene C-C=C-C
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 [15]. 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 [15]. 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 [16].
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 [16].
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 [16].
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%) [17], 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 [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
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 [30], 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 [31]. 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 [31]. MMT is more expensive
than alkyl leads and has been reported to increase unburned hydrocarbon
emissions and block exhaust catalysts [32].
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 [33].
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 [34].
* 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 [35].
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 [36]:-
Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
g 44.955
i-octane l 44.374 100 100
g 44.682
toluene l 40.554 124* (111) 112* (94)
g 40.967
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
content ).
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
vaporise.
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 [37]:-
Gross Nett
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) [38], and the Annual Book of ASTM Standards [39].
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
sulfur compounds
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 [40].
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?
Volatility
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% [44].
Combustion Characteristics
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 [45]). 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
quality standards.
Stability
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.
Corrosiveness
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.
Gasoline has no kinds, but there are different kinds of gasoline for cars. We have the unleaded, premium, and super premium.
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