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Gasoline FAQ - Part 2 of 4

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4.12 Are brands different?

Yes. The above specifications are intended to ensure minimal quality 
standards are maintained, however as well as the fuel hydrocarbons, the 
manufacturers add their own special ingredients to provide additional 
benefits. A quality gasoline additive package would include:-
* octane-enhancing additives ( improve octane ratings )  
* anti-oxidants ( inhibit gum formation, improve stability ) 
* metal deactivators ( inhibit gum formation, improve stability )
* deposit modifiers ( reduce deposits, spark-plug fouling and 
  preignition )
* surfactants ( prevent icing, improve vaporisation, inhibit deposits,
  reduce NOx emissions ) 
* freezing point depressants ( prevent icing )
* corrosion inhibitors ( prevent gasoline corroding storage tanks ) 
* dyes ( product colour for safety or regulatory purposes ).
During the 1980s significant problems with deposits accumulating on intake 
valve surfaces occurred as new fuel injection systems were introduced. 
These intake valve deposits (IVD) were different than the injector deposits,
in part because the valve can reach 300C. Engine design changes that prevent 
deposits usually consist of ensuring the valve is flushed with liquid 
gasoline, and provision of adequate valve rotation. Gasoline factors that 
cause deposits are the presence of alcohols or olefins [46]. Gasoline 
manufacturers now routinely use additives that prevent IVD and also maintain 
the cleanliness of injectors. These usually include a surfactant and light 
oil to maintain the wetting of important surfaces. Intake valve deposits have 
also been shown to have significant adverse effects on emissions [47], and
deposit control additives will be required to both reduce emissions and 
provide clean engine operation [48]. A slighty more detailed description 
of additives is provided in Section 9.1.

Texaco demonstrated that a well-formulated package could improve fuel 
economy, reduce NOx emissions, and restore engine performance because, as 
well as the traditional liquid-phase deposit removal, some additives can 
work in the vapour phase to remove existing engine deposits without adversely 
affecting performance ( as happens when water is poured into a running engine 
to remove carbon deposits :-) )[49]. Chevron have also published data on the 
effectiveness of their additives [50], and successfully litigated to get 
Texaco to modify some of their claims [51]. Most suppliers of quality 
gasolines will formulate similar additives into their products, and cheaper 
product lines are less likely to have such additives added. As different 
brands of gasoline use different additives and oxygenates, it is probable 
that important fuel parameters, such as octane distribution, are slightly
different, even though the pump octane ratings are the same. 

So, if you know your car is well-tuned, and in good condition, but the
driveability is pathetic on the correct octane, try another brand. Remember 
that the composition will change with the season, so if you lose 
driveability, try yet another brand. As various Clean Air Act changes are 
introduced over the next few years, gasoline will continue to change.

4.13 What is a typical composition?

There seems to be a perception that all gasolines of one octane grade are
chemically similar, and thus general rules can be promulgated about "energy 
content ", "flame speed", "combustion temperature" etc. etc.. Nothing is 
further from the truth. The behaviour of manufactured gasolines in octane 
rating engines can be predicted, using previous octane ratings of special
blends intended to determine how a particular refinery stream responds to 
an octane-enhancing additive. Refiners can design and reconfigure refineries 
to efficiently produce a wide range of gasolines feedstocks, depending on
market and regulatory requirements. There is a worldwide trend to move to
unleaded gasolines, followed by the introduction of exhaust catalysts and 
sophisticated engine management systems. 

It is important to note that "oxygenated gasolines" have a hydrocarbon
fraction that is not too different to traditional gasolines, but that the
hydrocarbon fraction of "reformulated gasolines" ( which also contain 
oxygenates ) are significantly different to traditional gasolines.

The last 10 years of various compositional changes to gasolines for
environmental and health reasons have resulted in fuels that do not follow 
historical rules, and the regulations mapped out for the next decade also 
ensure the composition will remain in a state of flux. The reformulated
gasoline specifications, especially the 1/Jan/1998 Complex model, will
probably introduce major reductions in the distillation range, as well as
changing the various limits on composition and emissions.

I'm not going to list all 500+ HCs in gasolines, but the following are 
representative of the various classes typically present in a gasoline. The 
numbers after each chemical are:- Research Blending Octane : Motor Blending 
Octane : Boiling Point (C): Density (g/ml @ 15C) : Minimum Autoignition 
Temperature (C). It is important to realise that the Blending Octanes are 
derived from a 20% mix of the HC with a 60:40 iC8:nC7 ( 60 Octane Number )
base fuel, and the extrapolation of this 20% to 100%. These numbers result
from API Project 45, and are readily available. As modern refinery streams 
have higher base octanes, these Blending Octanes are higher than those 
typically used in modern refineries. For example, modern Blending Octane
ratings can be much lower ( toluene = 111RON and 94MON, 2-methyl-2-butene 
= 113RON and 81MON ), but detailed compilations are difficult to obtain. 

The technique for obtaining Blending Octanes is different from rating the 
pure fuel, which often requires adjustment of the test engine conditions 
outside the acceptable limits of the rating methods. Generally, the actual 
octanes of the pure fuel are similar for the alkanes, but are up to 30 
octane numbers lower than the API Project 45 Blending Octanes for the 
aromatics and olefins [52].   

A traditional composition I have dreamed up would be like the following, 
whereas newer oxygenated fuels reduce the aromatics and olefins, narrow the
boiling range, and add oxygenates up to about 12-15% to provide the octane.
The amount of aromatics in super unleaded fuels will vary greatly from
country to country, depending on the configuration of the oil refineries 
and the use of oxygenates as octane enhancers. The US is reducing the levels 
of aromatics to 25% or lower for environmental and human health reasons.

Some countries are increasing the level of aromatics to 50% or higher in 
super unleaded grades, usually to avoid refinery reconfiguration costs or
the introduction of oxygenates as they phase out the toxic lead octane
enhancers. An upper limit is usually placed on the amount of benzene
permitted, as it is known human carcinogen.

15% n-paraffins                       RON   MON    BP      d     AIT  
        n-butane                      113 : 114 :  -0.5:  gas  : 370
        n-pentane                      62 :  66 :  35  : 0.626 : 260
        n-hexane                       19 :  22 :  69  : 0.659 : 225
        n-heptane (0:0 by definition)   0 :   0 :  98  : 0.684 : 225
        n-octane                      -18 : -16 : 126  : 0.703 : 220
     ( you would not want to have the following alkanes in gasoline, 
       so you would never blend kerosine with gasoline )
        n-decane                      -41 : -38 : 174  : 0.730 : 210
        n-dodecane                    -88 : -90 : 216  : 0.750 : 204
        n-tetradecane                 -90 : -99 : 253  : 0.763 : 200
30%  iso-paraffins  
        2-methylpropane               122 : 120 : -12  :  gas  : 460
        2-methylbutane                100 : 104 :  28  : 0.620 : 420
        2-methylpentane                82 :  78 :  62  : 0.653 : 306
        3-methylpentane                86 :  80 :  64  : 0.664 :  -
        2-methylhexane                 40 :  42 :  90  : 0.679 : 
        3-methylhexane                 56 :  57 :  91  : 0.687 :
        2,2-dimethylpentane            89 :  93 :  79  : 0.674 :
        2,2,3-trimethylbutane         112 : 112 :  81  : 0.690 : 420
        2,2,4-trimethylpentane        100 : 100 :  98  : 0.692 : 415
          ( 100:100 by definition )
12% cycloparaffins 
        cyclopentane                  141 : 141 :  50  : 0.751 : 380
        methylcyclopentane            107 :  99 :  72  : 0.749 : 
        cyclohexane                   110 :  97 :  81  : 0.779 : 245
        methylcyclohexane             104 :  84 : 101  : 0.770 : 250
35% aromatics        
        benzene                        98 :  91 :  80  : 0.874 : 560
        toluene                       124 : 112 : 111  : 0.867 : 480
        ethyl benzene                 124 : 107 : 136  : 0.867 : 430
        meta-xylene                   162 : 124 : 138  : 0.868 : 463
        para-xylene                   155 : 126 : 138  : 0.866 : 530
        ortho-xylene                  126 : 102 : 144  : 0.870 : 530
        3-ethyltoluene                162 : 138 : 158  : 0.865 : 
        1,3,5-trimethylbenzene        170 : 136 : 163  : 0.864 : 
        1,2,4-trimethylbenzene        148 : 124 : 168  : 0.889 :
8% olefins               
        2-pentene                     154 : 138 :  37  : 0.649 :
        2-methylbutene-2              176 : 140 :  36  : 0.662 :
        2-methylpentene-2             159 : 148 :  67  : 0.690 :
        cyclopentene                  171 : 126 :  44  : 0.774 :
    ( the following olefins are not present in significant amounts
      in gasoline, but have some of the highest blending octanes )   
        1-methylcyclopentene          184 : 146 :  75  : 0.780 :
        1,3 cyclopentadiene           218 : 149 :  42  : 0.805 :
        dicyclopentadiene             229 : 167 : 170  : 1.071 :     

Published octane values vary a lot because the rating conditions are 
significantly different to standard conditions, for example the API Project 
45 numbers used above for the hydrocarbons, reported in 1957, gave MTBE 
blending RON as 148 and MON as 146, however that was partly based on the 
lead response, whereas today we use MTBE in place of lead.
        methanol                      133 : 105 :  65  : 0.796 : 385
        ethanol                       129 : 102 :  78  : 0.794 : 365
        iso propyl alcohol            118 :  98 :  82  : 0.790 : 399
        methyl tertiary butyl ether   116 : 103 :  55  : 0.745 : 
        ethyl tertiary butyl ether    118 : 102 :  72  : 0.745 :
        tertiary amyl methyl ether    111 :  98 :  86  : 0.776 : 
There are some other properties of oxygenates that have to be considered
when they are going to be used as fuels, particularly their ability to
form very volatile azeotropes that cause the fuel's vapour pressure to
increase, the chemical nature of the emissions, and their tendency to 
separate into a separate water-oxygenate phase when water is present. 
The reformulated gasolines address these problems more successfully than 
the original oxygenated gasolines.

Before you rush out to make a highly aromatic or olefinic gasoline to 
produce a high octane fuel, remember they have other adverse properties, 
eg the aromatics attack elastomers, may generate smoke, and result in
increased emissions of toxic benzene. The olefins are unstable ( besides 
smelling foul ) and form gums. The art of correctly formulating a gasoline 
that does not cause engines to knock apart, does not cause vapour lock in 
summer - but is easy to start in winter, does not form gums and deposits, 
burns cleanly without soot or residues, and does not dissolve or poison the 
car catalyst or owner, is based on knowledge of the gasoline composition.

4.14 Is gasoline toxic or carcinogenic? 

There are several known toxins in gasoline, some of which are confirmed
human carcinogens. The most famous of these toxins are lead and benzene, and 
both are regulated. The other aromatics and some toxic olefins are also 
controlled. Lead alkyls also require ethylene dibromide and/or ethylene 
dichloride scavengers to be added to the gasoline, both of which are 
suspected human carcinogens. In 1993 an International Symposium on the Health
Effects of Gasoline was held [53]. Major review papers on the carcinogenic,
neurotoxic, reproductive and developmental toxicity of gasoline, additives,
and oxygenates were presented, and interested readers should obtain the
proceedings. The oxygenates are also being evaluated for carcinogenicity, and 
even ethanol and ETBE may be carcinogens. The introduction of oxygenated
gasoline to Alaska and some other areas of the USA resulted in a range of
complaints. Recent research has been unable to identify additional toxicity,
but has detected increased levels of offensive smell [54]. It should be noted 
that the oxygenated gasolines were not initially intended to reduce the 
toxicity of emissions. The reformulated gasolines will produce different 
emissions, and specific toxins must initially be reduced by 15% all year.

The removal of alkyl lead compounds certainly reduces the toxicity of 
exhaust gas emissions when used on engines with modern engine management
systems and 3-way exhaust catalysts. If unleaded gasolines are not 
accompanied by the introduction of catalysts, some other toxic emissions
may increase. Engines without catalysts will produce increased levels of
toxic carbonyls such as formaldehyde and acrolein when using oxygenated
fuels, and increased levels of toxic benzene when using highly aromatic 

There is little doubt that gasoline is full of toxic chemicals, and should
therefore be treated with respect. However the biggest danger remains the 
flammability, and the relative hazards should always be kept in perspective. 
The major toxic risk from gasolines comes from breathing the tailpipe, 
evaporative, and refuelling emissions, rather than occasional skin contact 
from spills. Breathing vapours and skin contact should always be minimised.

4.15 Is unleaded gasoline more toxic than leaded?

The short answer is no. However that answer is not global, as some countries 
have replaced the lead compound octane-improvers with aromatic or olefin
octane-improvers without introducing exhaust catalysts. The aromatics
contents may increase to around 40%, with high octane unleaded fuels reaching 
50% in countries where oxygenates are not being used, and the producers have 
not reconfigured refineries to produce high octane paraffins. In general, 
aromatics are significantly more toxic than paraffins. Exhaust catalysts  
have a limited operational life, and will be immediately poisoned if 
misfuelled with leaded fuel. Catalyst failure can result in higher levels of
toxic emissions if catalysts or engine management systems are not replaced or
repaired when defective. Maximum benefit of the switch to unleaded are
obtained when the introduction of unleaded is accompanied by the introduction
of exhaust catalysts and sophisticated engine management systems.

Unfortunately, the manufacturers of alkyl lead compounds have embarked on a 
worldwide misinformation campaign in countries considering emulating the 
lead-free US. The use of lead precludes the use of exhaust catalysts, thus 
the emissions of aromatics are only slightly diminished, as leaded fuels
typically contain around 30-40% aromatics. Other toxins and pollutants that 
are usually reduced by exhaust catalysts will be emitted at significantly 
higher levels if leaded fuels are used [55]. 

The use of unleaded on modern vehicles with engine management systems and 
catalysts can reduce aromatic emissions to 10% of the level of vehicles 
without catalysts [55]. Alkyl lead additives can only substitute for some of 
the aromatics in gasoline, consequently they do not eliminate aromatics,
which will produce benzene emissions [56]. Alkyl lead additives also require 
toxic organohalogen scavengers, which also react in the engine to form and 
emit other organohalogens, including highly toxic dioxin [57]. Leaded fuels 
emit lead, organohalogens, and much higher levels of regulated toxins 
because they preclude the use of exhaust catalysts. In the USA the gasoline
composition is being changed to reduce fuel toxins ( olefins, aromatics ) 
as well as emissions of specific toxins. 

4.16 Is reformulated gasoline more toxic than unleaded?

The evidence so far indicates that the components of reformulated gasolines
( RFGs ) are more benign than unleaded, and that the tailpipe emissions of 
hydrocarbons are significantly reduced for cars without catalysts, and 
slightly reduced for cars with catalysts and engine management systems. The
emissions of toxic carbonyls such as formaldehyde, acetaldehyde and acrolein 
are increased slightly on all vehicles, and the emission of MTBE is increased
about 10x on cars without catalysts and 4x on cars with catalysts [55].
When all the emissions ( evaporative and tailpipe ) are considered, RFGs
significantly reduce emissions of hydrocarbons, however the emissions of
carbonyls and MTBE may increase [55]. There has been an extensive series
of reports on the emissions from RFGs, produced by the Auto/Oil Air Quality 
Improvement Research Program, who measured and calculated the likely
effects of RFG [18,19,20,58,59,60,61]. More research is required before 
a definitive answer on toxicity is available.  

The major question about RFGs is not the toxicity of the emissions, but 
whether they actually meet their objective of reducing urban pollution.
This is a more complex issue, and most experts agree the benefits will only
be modest [18,19,20,61,62]. 

4.17 Are all oxygenated gasolines also reformulated gasolines?

No. Oxygenates were initially introduced as alternative octane-enhancers in 
the 1930s, and are still used in some countries for that purpose. 
In the US the original "oxygenated gasolines" usually had a slightly-
modified gasoline as the hydrocarbon fraction. The US EPA also mandated 
their use to reduce pollution, mainly via the "enleanment" effect on engines 
without sophisticated management systems, but also because of the "aromatics 
substitution" effect. As vehicles with fuel injection and sophisticated 
engine management systems became pervasive, reformulated gasolines could be 
introduced to further reduce pollution. The hydrocarbon component of RFGs is 
significantly different to the hydrocarbon fraction in earlier oxygenated 
gasolines, having lower aromatics contents, reduced vapour pressure, and a 
narrower boiling range. RFGs do contain oxygenates as the octane-enhancer, 
but have different hydrocarbon composition profiles [34,41,42,43,44].

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