Subject: Gasoline FAQ - Part 1 of 4
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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

This FAQ is posted monthly to the Usenet groups news.answers, rec.answers,
and The latest copy should be available by FTP from in the pub/usenet-by-hierarchy/rec/autos/tech directory, and 
also available on the WWW from sites that automatically convert those FAQs.

- 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 and 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.

  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   

  cyclic = circle of carbons  ( Cn H2n )
  ( aka Naphthenes )       
  - high octane ratings.
    cyclohexane  =                 C
                                  / \
                                 C   C
                                 |   |                   C6H12
                                 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.

                                 |                       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 

                                  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
    Methyl tertiary butyl ether              C-C-O-C      C4H9OCH3
    (aka tertiary butyl methyl ether )         |

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 

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
            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? 

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.

Motor gasolines may be stored up to six months, consequently they must not 
form gums which may precipitate. Reactions of the unsaturated HCs may 
produce gums ( these reactions can be catalysed by metals such as copper ), 
so antioxidants and metal deactivators are added. Existent Gum is used to 
measure the gum in the fuel at the time tested, whereas the Oxidation 
Stability measures the time it takes for the gasoline to break down at 100C 
with 100psi of oxygen. A 240 minute test period has been found to be 
sufficient for most storage and distribution systems.

Sulfur in the fuel creates corrosion, and when combusted will form corrosive
gases that attack the engine, exhaust and environment. Sulfur also adversely
affects the alkyl lead octane response, and will adversely affect exhaust 
catalysts, but monolithic catalysts will recover when the sulfur content of 
the fuel is reduced, so sulfur is considered an inhibitor, rather than a 
catalyst poison. The copper strip corrosion test and the sulfur content 
specification are used to ensure fuel quality. The copper strip test measures 
active sulfur, whereas the sulfur content reports the total sulfur present.

Manufacturers many also add additional tests, such as filterability, to
ensure no distribution problems are encountered.


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