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- From: Bruce Hamilton <B.Hamilton@irl.cri.nz>
- Newsgroups: rec.autos.tech,rec.answers,news.answers
- Subject: Gasoline FAQ - Part 1 of 4
- Followup-To: rec.autos.tech
- Date: Thu, 15 Jan 2004 22:15:11 +1300
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- Archive-name: autos/gasoline-faq/part1
- Posting-Frequency: monthly
- Last-modified: 17 November 1996
- Version: 1.12
-
- FAQ: Automotive Gasoline
- Bruce Hamilton
- B.Hamilton@irl.cri.nz
-
- This FAQ is posted monthly to the Usenet groups news.answers, rec.answers,
- and rec.autos.tech. The latest copy should be available on the WWW from sites
- that automatically convert those FAQs, such as www.faqs.org.
-
- Changes:
- - added a little more data on US crude oil resources.
-
- ------------------------------
-
- Subject: 1. Introduction, Intent, Acknowledgements, and Abbreviations
-
- 1.1 Introduction and Intent.
-
- The intent of this FAQ is to provide some basic information on gasolines and
- other fuels for spark ignition engines used in automobiles. The toxicity and
- environmental reasons for recent and planned future changes to gasoline are
- discussed, along with recent and proposed changes in composition of gasoline.
- This FAQ is intended to help readers choose the most appropriate fuel for
- vehicles, assist with the diagnosis of fuel-related problems, and to
- understand the significance of most gasoline properties listed in fuel
- specifications. I make no apologies for the fairly heavy emphasis on
- chemistry; it is the only sensible way to describe the oxidation of
- hydrocarbon fuels to produce energy, water, and carbon dioxide.
-
- 1.2 Acknowledgements.
-
- Thanks go to all the posters in sci.energy and rec.autos.tech who spend
- valuable time responding to questions. I would also like to acknowledge
- the considerable effort of L.M.Gibbs of Chevron, who has twice spent his
- valuable time courteously detailing errors and providing references
- for his corrections. All remaining errors and omissions are mine.
-
- 1.3 Abbreviations.
-
- AKI = Antiknock Index of Gasoline ( (RON+MON)/2 )
- CI = Compression Ignition ( Diesel )
- Gasoline = Petrol ( Yes, complaints were received :-) )
- IC = Internal Combustion
- MON = Motor Octane Rating
- Octane = The Octane Rating of the Gasoline
- RFG = Reformulated Gasoline ( as defined by US Clean Air Act )
- RON = Research Octane Rating
- SI = Spark Ignition ( Gasoline )
-
- ------------------------------
-
- Subject: 2. Table of Contents
-
- 1. Introduction, Intent, Acknowledgements, and Abbreviations
- 1.1 Introduction and Intent.
- 1.2 Acknowledgements.
- 1.3 Abbreviations.
- 2. Table of Contents
- 3. What Advantage will I gain from reading this FAQ?
- 4. What is Gasoline?
- 4.1 Where does crude oil come from?.
- 4.2 When will we run out of crude oil?.
- 4.3 What is the history of gasoline?
- 4.4 What are the hydrocarbons in gasoline?
- 4.5 What are oxygenates?
- 4.6 Why were alkyl lead compounds added?
- 4.7 Why not use other organometallic compounds?
- 4.8 What do the refining processes do?
- 4.9 What energy is released when gasoline is burned?
- 4.10 What are the gasoline specifications?
- 4.11 What are the effects of the specified fuel properties?
- 4.12 Are brands different?
- 4.13 What is a typical composition?
- 4.14 Is gasoline toxic or carcinogenic?
- 4.15 Is unleaded gasoline more toxic than leaded?
- 4.16 Is reformulated gasoline more toxic than unleaded?
- 4.17 Are all oxygenated gasolines also reformulated gasolines?
- 5. Why is Gasoline Composition Changing?
- 5.1 Why pick on cars and gasoline?
- 5.2 Why are there seasonal changes?
- 5.3 Why were alkyl lead compounds removed?
- 5.4 Why are evaporative emissions a problem?
- 5.5 Why control tailpipe emissions?
- 5.6 Why do exhaust catalysts influence fuel composition?
- 5.7 Why are "cold start" emissions so important?
- 5.8 When will the emissions be "clean enough"?
- 5.9 Why are only some gasoline compounds restricted?
- 5.10 What does "renewable" fuel or oxygenate mean?
- 5.11 Will oxygenated gasoline damage my vehicle?
- 5.12 What does "reactivity" of emissions mean?
- 5.13 What are "carbonyl" compounds?
- 5.14 What are "gross polluters"?
- 6. What do Fuel Octane ratings really indicate?
- 6.1 Who invented Octane Ratings?
- 6.2 Why do we need Octane Ratings?
- 6.3 What fuel property does the Octane Rating measure?
- 6.4 Why are two ratings used to obtain the pump rating?
- 6.5 What does the Motor Octane rating measure?
- 6.6 What does the Research Octane rating measure?
- 6.7 Why is the difference called "sensitivity"?
- 6.8 What sort of engine is used to rate fuels?
- 6.9 How is the Octane rating determined?
- 6.10 What is the Octane Distribution of the fuel?
- 6.11 What is a "delta Research Octane number"?
- 6.12 How do other fuel properties affect octane?
- 6.13 Can higher octane fuels give me more power?
- 6.14 Does low octane fuel increase engine wear?
- 6.15 Can I mix different octane fuel grades?
- 6.16 What happens if I use the wrong octane fuel?
- 6.17 Can I tune the engine to use another octane fuel?
- 6.18 How can I increase the fuel octane?
- 6.19 Are aviation gasoline octane numbers comparable?
- 6.20 Can mothballs increase octane?
- 7. What parameters determine octane requirement?
- 7.1 What is the Octane Number Requirement of a Vehicle?
- 7.2 What is the effect of Compression ratio?
- 7.3 What is the effect of changing the air-fuel ratio?
- 7.4 What is the effect of changing the ignition timing
- 7.5 What is the effect of engine management systems?
- 7.6 What is the effect of temperature and Load?
- 7.7 What is the effect of engine speed?
- 7.8 What is the effect of engine deposits?
- 7.9 What is the Road Octane Number of a Fuel?
- 7.10 What is the effect of air temperature?.
- 7.11 What is the effect of altitude?.
- 7.12 What is the effect of humidity?.
- 7.13 What does water injection achieve?.
- 8. How can I identify and cure other fuel-related problems?
- 8.1 What causes an empty fuel tank?
- 8.2 Is knock the only abnormal combustion problem?
- 8.3 Can I prevent carburetter icing?
- 8.4 Should I store fuel to avoid the oxygenate season?
- 8.5 Can I improve fuel economy by using quality gasolines?
- 8.6 What is "stale" fuel, and should I use it?
- 8.7 How can I remove water in the fuel tank?
- 8.8 Can I use unleaded on older vehicles?
- 8.9 How serious is valve seat recession on older vehicles?
- 9. Alternative Fuels and Additives
- 9.1 Do fuel additives work?
- 9.2 Can a quality fuel help a sick engine?
- 9.3 What are the advantages of alcohols and ethers?
- 9.4 Why are CNG and LPG considered "cleaner" fuels.
- 9.5 Why are hydrogen-powered cars not available?
- 9.6 What are "fuel cells" ?
- 9.7 What is a "hybrid" vehicle?
- 9.8 What about other alternative fuels?
- 9.9 What about alternative oxidants?
- 10. Historical Legends
- 10.1 The myth of Triptane
- 10.2 From Honda Civic to Formula 1 winner.
- 11. References
- 11.1 Books and Research Papers
- 11.2 Suggested Further Reading
-
- ------------------------------
-
- Subject: 3. What Advantage will I gain from reading this FAQ?
-
- This FAQ is intended to provide a fairly technical description of what
- gasoline contains, how it is specified, and how the properties affect the
- performance of your vehicle. The regulations governing gasoline have
- changed, and are continuing to change. These changes have made much of the
- traditional lore about gasoline obsolete. Motorists may wish to understand
- a little more about gasoline to ensure they obtain the best value, and the
- most appropriate fuel for their vehicle. There is no point in prematurely
- destroying your second most expensive purchase by using unsuitable fuel,
- just as there is no point in wasting hard-earned money on higher octane
- fuel that your automobile can not utilize. Note that this FAQ does not
- discuss the relative advantages of specific brands of gasolines, it is
- only intended to discuss the generic properties of gasolines.
-
- ------------------------------
-
- 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 [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.
-