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Rather a famous document on the internet, Bruce has given us permission to use it here. The original, can be found at http://www.faqs.org/faqs/autos/gasoline-faq/part2/
The FAQ is broken up into four parts for ease of use. This is part II
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 :
Oxygenates
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
fuels.
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].
Subject: 5. Why is Gasoline Composition Changing?
5.1 Why pick on cars and gasoline?
Cars emit several pollutants as combustion products out the tailpipe,
(tailpipe emissions), and as losses due to evaporation (evaporative
emissions, refuelling emissions). The volatile organic carbon (VOC)
emissions from these sources, along with nitrogen oxides (NOx) emissions
from the tailpipe, will react in the presence of ultraviolet (UV) light
(wavelengths of less than 430nm) to form ground-level (tropospheric) ozone,
which is one of the major components of photochemical smog [63]. Smog has
been a major pollution problem ever since coal-fired power stations were
developed in urban areas, but their emissions are being cleaned up. Now it's
the turn of the automobile.
Cars currently use gasoline that is derived from fossil fuels, thus when
gasoline is burned to completion, it produces additional CO2 that is added
to the atmospheric burden. The effect of the additional CO2 on the global
environment is not known, but the quantity of man-made emissions of fossil
fuels must cause the system to move to a new equilibrium. Even if current
research doubles the efficiency of the IC engine-gasoline combination, and
reduces HC, CO, NOx, SOx, VOCs, particulates, and carbonyls, the amount of
carbon dioxide from the use of fossil fuels may still cause global warming.
More and more scientific evidence is accumulating that warming is occurring
[64,65]. The issue is whether it is natural, or induced by human activities
and and a large panel of scientific experts continues to review scientific
data and models. Interested reader should seek out the various publications
of the Intergovernmental Panel on Climate Change (IPCC). There are
international agreements to limit CO2 emissions to 1990 levels, a target that
will require more efficient, lighter, or appropriately-sized vehicles, - if
we are to maintain the current usage. One option is to use "renewable" fuels
in place of fossil fuels. Consider the amount of energy-related CO2 emissions
for selected countries in 1990 [66].
CO2 Emissions
( tonnes/year/person )
USA 20.0
Canada 16.4
Australia 15.9
Germany 10.4
United Kingdom 8.6
Japan 7.7
New Zealand 7.6
The number of new vehicles provides an indication of the magnitude of the
problem. Although vehicle engines are becoming more efficient, the distance
travelled is increasing, resulting in a gradual increase of gasoline
consumption. The world production of vehicles (in thousands) over the last
few years was [67];-
Cars
Region 1990 1991 1992 1993 1994
Africa 222 213 194 201 209
Asia-Pacific 12,064 12,112 11,869 11,463 11,020
Central & South America 800 888 1,158 1,523 1,727
Eastern Europe 2,466 984 1,726 1,837 1,547
Middle East 35 24 300 390 274
North America 7,762 7,230 7,470 8,172 8,661
Western Europe 13,688 13,286 13,097 11,141 12,851
Total World 37,039 34,739 35,815 34,721 36,289
Trucks ( including heavy trucks and buses )
Region 1990 1991 1992 1993 1994
Africa 133 123 108 101 116
Asia-Pacific 5,101 5,074 5,117 5,057 5,407
Central & South America 312 327 351 431 457
Eastern Europe 980 776 710 600 244
Middle East 36 28 100 128 76
North America 4,851 4,554 5,371 6,037 7,040
Western Europe 1,924 1,818 1,869 1,718 2,116
Total World 13,336 12,701 13,627 14,073 15,457
To fuel all operating vehicles, considerable quantities of gasoline
and diesel have to be consumed. Major consumption in 1994 of gasoline
and middle distillates ( which may include some heating fuels, but
not fuel oils ) in million tonnes.
Gasoline Middle Distillates
USA 338.6 246.3
Canada 26.8 26.1
Western Europe 163.2 266.8
Japan 60.2 92.2
Total World 820.4 1029.0
The USA consumption of gasoline increased from 294.4 (1982) to 335.6 (1989)
then dipped to 324.2 (1991), and has continued to rise since then to reach
338.6 million tonnes in 1994. In 1994 the total world production of crude oil
was 3209.1 million tonnes, of which the USA consumed 807.9 million tonnes
[68]. Transport is a very significant user of crude oil products, thus
improving the efficiency of utilisation, and minimising pollution from
vehicles, can produce immediate reductions in emissions of CO2, HCs, VOCs,
CO, NOx, carbonyls, and other chemicals.
5.2 Why are there seasonal changes?
Only gaseous hydrocarbons burn, consequently if the air is cold, then the
fuel has to be very volatile. But when summer comes, a volatile fuel can
boil and cause vapour lock, as well as producing high levels of evaporative
emissions. The solution was to adjust the volatility of the fuel according
to altitude and ambient temperature. This volatility change has been
automatically performed for decades by the oil companies without informing
the public of the changes. It is one reason why storage of gasoline through
seasons is not a good idea. Gasoline volatility is being reduced as modern
engines, with their fuel injection and management systems, can automatically
compensate for some of the changes in ambient conditions - such as altitude
and air temperature, resulting in acceptable driveability using less volatile
fuel.
5.3 Why were alkyl lead compounds removed?
" With the exception of one premium gasoline marketed on the east coast
and southern areas of the US, all automotive gasolines from the mid-1920s
until 1970 contained lead antiknock compounds to increase antiknock quality.
Because lead antiknock compounds were found to be detrimental to the
performance of catalytic emission control system then under development,
U.S. passenger car manufacturers in 1971 began to build engines designed to
operate satisfactorily on gasolines of nominal 91 Research Octane Number.
Some of these engines were designed to operate on unleaded fuel while others
required leaded fuel or the occasional use of leaded fuel. The 91 RON was
chosen in the belief that unleaded gasoline at this level could be made
available in quantities required using then current refinery processing
equipment. Accordingly, unleaded and low-lead gasolines were introduced
during 1970 to supplement the conventional gasolines already available.
Beginning with the 1975 model year, most new car models were equipped
with catalytic exhaust treatment devices as one means of compliance with
the 1975 legal restrictions in the U.S. on automobile emissions. The need
for gasolines that would not adversely affect such catalytic devices has
led to the large scale availability and growing use of unleaded gasolines,
with all late-model cars requiring unleaded gasoline."[69].
There was a further reason why alkyl lead compounds were subsequently
reduced, and that was the growing recognition of the highly toxic nature of
the emissions from a leaded-gasoline fuelled engine. Not only were toxic
lead emissions produced, but the added toxic lead scavengers ( ethylene
dibromide and ethylene dichloride ) could react with hydrocarbons to produce
highly toxic organohalogen emissions such as dioxin. Even if catalysts were
removed, or lead-tolerant catalysts discovered, alkyl lead compounds would
remain banned because of their toxicity and toxic emissions [70,71].
5.4 Why are evaporative emissions a problem?
As tailpipe emissions are reduced due to improved exhaust emission control
systems, the hydrocarbons produced by evaporation of the gasoline during
distribution, vehicle refuelling, and from the vehicle, become more and
more significant. A recent European study found that 40% of man-made
volatile organic compounds came from vehicles [72]. Many of the problem
hydrocarbons are the aromatics and olefins that have relatively high octane
values. Any sensible strategy to reduce smog and toxic emissions will also
attack evaporative and tailpipe emissions.
The health risks to service station workers, who are continuously exposed
to refuelling emissions remain a concern [73]. Vehicles will soon be
required to trap the refuelling emissions in larger carbon canisters, as
well as the normal evaporative emissions that they already capture. This
recent decision went in favour of the oil companies, who were opposed by the
auto companies. The automobile manufacturers felt the service station
should trap the emissions. The activated carbon canisters adsorb organic
vapours, and these are subsequently desorbed from the canister and burnt in
the engine during normal operation, once certain vehicle speeds and coolant
temperatures are reached. A few activated carbons used in older vehicles
do not function efficiently with oxygenates, but carbon cannister systems
can reduce evaporative emissions by 95% from uncontrolled levels.
5.5 Why control tailpipe emissions?
Tailpipe emissions were responsible for the majority of pollutants in the
late 1960s after the crankcase emissions had been controlled. Ozone levels
in the Los Angeles basin reached 450-500ppb in the early 1970s, well above
the typical background of 30-50ppb [74].
Tuning a carburetted engine can only have a marginal effect on pollutant
levels, and there still had to be some frequent, but long-term, assessment
of the state of tuning. Exhaust catalysts offered a post-engine solution
that could ensure pollutants were converted to more benign compounds. As
engine management systems and fuel injection systems have developed, the
volatility properties of the gasoline have been tuned to minimise
evaporative emissions, and yet maintain low exhaust emissions.
The design of the engine can have very significant effects on the type and
quantity of pollutants, eg unburned hydrocarbons in the exhaust originate
mainly from combustion chamber crevices, such as the gap between the piston
and cylinder wall, where the combustion flame can not completely use the HCs.
The type and amount of unburnt hydrocarbon emissions are related to the fuel
composition (volatility, olefins, aromatics, final boiling point), as well
as state of tune, engine condition, and condition of the engine
lubricating oil [75]. Particulate emissions, especially the size fraction
smaller than ten micrometres, are a serious health concern. The current
major source is from compression ignition ( diesel ) engines, and the
modern SI engine system has no problem meeting regulatory requirements.
The ability of reformulated gasolines to actually reduce smog has not yet
been confirmed. The composition changes will reduce some compounds, and
increase others, making predictions of environmental consequences extremely
difficult. Planned future changes, such as the CAA 1/1/1998 Complex model
specifications, that are based on several major ongoing government/industry
gasoline and emission research programmes, are more likely to provide
unambiguous environmental improvements. One of the major problems is the
nature of the ozone-forming reactions, which require several components
( VOC, NOx, UV ) to be present. Vehicles can produce the first two, but the
their ratio is important, and can be affected by production from other
natural ( VOC = terpenes from conifers ) or manmade ( NOx from power
stations ) sources [62,63]. The regulations for tailpipe emissions
will continue to become more stringent as countries try to minimise local
problems ( smog, toxins etc.) and global problems ( CO2 ). Reformulation
does not always lower all emissions, as evidenced by the following aldehydes
from an engine with an adaptive learning management system [55].
FTP-weighted emission rates (mg/mi)
Gasoline Reformulated
Formaldehyde 4.87 8.43
Acetaldehyde 3.07 4.71
The type of exhaust catalyst and management system can have significant
effects on the emissions [55].
FTP-weighted emission rates. (mg/mi)
Total Aromatics Total Carbonyls
Gasoline Reformulated Gasoline Reformulated
Noncatalyst 1292.45 1141.82 174.50 198.73
Oxidation Catalyst 168.60 150.79 67.08 76.94
3-way Catalyst 132.70 93.37 23.93 23.07
Adaptive Learning 111.69 105.96 17.31 22.35
If we take some compounds listed as toxics under the Clean Air Act, then the
beneficial effects of catalysts are obvious. Note that hexane and iso-octane
are the only alkanes listed as toxics, but benzene, toluene, ethyl benzene,
o-xylene, m-xylene, and p-xylene are aromatics that are listed. The latter
four are combined as C8 Aromatics below [55].
Aromatics FTP-weighted emission rates. (mg/mi)
Benzene Toluene C8 Aromatics
Gas Reform Gas Reform Gas Reform
Noncatalyst 156.18 138.48 338.36 314.14 425.84 380.44
Oxidation Cat. 27.57 25.01 51.00 44.13 52.27 47.07
3-way Catalyst 19.39 15.69 36.62 26.14 42.38 29.03
Adaptive Learn. 19.77 20.39 29.98 29.67 35.01 32.40
Aldehydes FTP-weighted emission rates. (mg/mi)
Formaldehyde Acrolein Acetaldehyde
Gas Reform Gas Reform Gas Reform
Noncatalyst 73.25 85.24 11.62 13.20 19.74 21.72
Oxidation Cat. 28.50 35.83 3.74 3.75 11.15 11.76
3-way Catalyst 7.27 7.61 1.11 0.74 4.43 3.64
Adaptive Learn. 4.87 8.43 0.81 1.16 3.07 4.71
Others 1,3 Butadiene MTBE
Gas Reform Gas Reform
Noncatalyst 2.96 1.81 10.50 130.30
Oxidation Cat. 0.02 0.33 2.43 11.83
3-way Catalyst 0.07 0.05 1.42 4.59
Adaptive Learn. 0.00 0.14 0.84 3.16
The author reports analytical problems with the 1,3 Butadiene, and only
Noncatalyst values are considered reliable. Other studies from the
Auto/Oil research program indicate that lowering aromatics and olefins
reduce benzene but increase formaldehyde and acetaldehyde [20]
Emission Standards
There are several bodies responsible for establishing standards, and they
promulgate test cycles, analysis procedures, and the % of new vehicles that
must comply each year. The test cycles and procedures do change ( usually
indicated by an anomalous increase in the numbers in the table ), and I
have not listed the percentages of the vehicle fleet that are required to
comply. This table is only intended to convey where we have been, and where
we are going. It does not cover any regulation in detail - readers are
advised to refer to the relevant regulations. Additional limits for other
pollutants, such as formaldehyde (0.015g/mi.) and particulates (0.08g/mi),
are omitted. The 1994 tests signal the federal transition from 50,000 to
100,000 mile compliance testing, and I have not listed the subsequent
50,000 mile limits [28,76,77].
Year Federal California
HCs CO NOx Evap HCs CO NOx Evap
g/mi g/mi g/mi g/test g/mi g/mi g/mi g/test
Before regs 10.6 84.0 4.1 47 10.6 84.0 4.1 47
add crankcase +4.1 +4.1
1966 6.3 51.0 6.0
1968 6.3 51.0 6.0
1970 4.1 34.0 4.1 34.0 6
1971 4.1 34.0 6(CC) 4.1 34.0 4.0 6
1972 3.0 28.0 2 2.9 34.0 3.0 2
1973 3.0 28.0 3.0 2.9 34.0 3.0 2
1974 3.0 28.0 3.0 2.9 34.0 2.0 2
1975 1.5 15.0 3.1 2 0.90 9.0 2.0 2
1977 1.5 15.0 2.0 2 0.41 9.0 1.5 2
1980 0.41 7.0 2.0 6(SHED) 0.41 9.0 1.0 2
1981 0.41 3.4 1.0 2 0.39 7.0 0.7 2
1993 0.41 3.4 1.0 2 0.25 3.4 0.4 2
1994 50,000 0.26 3.4 0.3 2 TLEV 0.13 3.4 0.4 2
1994 100,000 0.31 4.2 0.6 2
1997 LEV 0.08 3.4 0.2
1997 ULEV 0.04 1.7 0.2
1998 ZEV 0.0 0.0 0.0 0
2004 0.125 1.8 0.16 2
It's also worth noting that exhaust catalysts also emit platinum, and the
soluble platinum salts are some of the most potent sensitizers known.
Early research [78] reported the presence of 10% water-soluble platinum in
the emissions, however later work on monolithic catalysts has determined the
quantities of water soluble platinum emissions are negligible [79]. The
particle size of the emissions has also been determined, and the emissions
have been correlated with increasing vehicle speed. Increasing speed also
increases the exhaust gas temperature and velocity, indicating the emissions
are probably a consequence of physical attrition.
Estimated Fuel Median Aerodynamic
Speed Consumption Emissions Particle Diameter
km/h l/100km ng/m-3 um
60 7 3.3 5.1
100 8 11.9 4.2
140 10 39.0 5.6
US Cycle-75 6.4 8.5
Using the estimated fuel consumption, and about 10m3 of exhaust gas per
litre of gasoline, the emissions are 2-40 ng/km. These are 2-3 orders
of magnitude lower than earlier reported work on pelletised catalysts.
These emissions may be controlled directly in the future. They are currently
indirectly controlled by the cost of platinum, and the new requirement for
the catalyst to have an operational life of at least 100,000 miles.
5.6 Why do exhaust catalysts influence fuel composition?
Modern adaptive learning engine management systems control the combustion
stoichiometry by monitoring various ambient and engine parameters, including
exhaust gas recirculation rates, the air flow sensor, and exhaust oxygen
sensor outputs. This closed loop system using the oxygen sensor can
compensate for changes in fuel content and air density. The oxygen sensor
is also known as the lambda sensor because the actual air-fuel mass ratio
divided by the stoichiometric air-fuel mass ratio is known as lambda or the
air-fuel equivalence ratio.
The preferred technique for describing mixture strength is the fuel-air
equivalence ratio ( phi ), which is the actual fuel-air mass ratio divided
by the stoichiometric fuel-air mass ratio, however most enthusiasts use
air-fuel ratio and lambda. Lambda is the inverse of the fuel-air equivalence
ratio. The oxygen sensor effectively measures lambda around the
stoichiometric mixture point. Typical stoichiometric air-fuel ratios are
[80]:-
6.4 methanol
9.0 ethanol
11.7 MTBE
12.1 ETBE, TAME
14.6 gasoline without oxygenates
The engine management system rapidly switches the stoichiometry between
slightly rich and slightly lean, except under wide open throttle conditions
- when the system runs open loop. The response of the oxygen sensor to
composition changes is about 3 ms, and closed loop switching is typically
1-3 times a second, going between 50mV ( lambda = 1.05 (Lean)) to 900mV
(lambda = 0.99 ( Rich)). The catalyst oxidises about 80% of the H2, CO,
and HCs, and reduces the NOx [76].
Typical reactions that occur in a modern 3-way catalyst are:-
2H2 + O2 -> 2H2O
2CO + O2 -> 2CO2
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
2CO + 2NO -> N2 + 2CO2
CxHy + 2(x + (y/4))NO -> (x + (y/4))N2 + (y/2)H2O + xCO2
2H2 + 2NO -> N2 + 2H2O
CO + H20 -> CO2 + H2
CxHy + xH2O -> xCO + (x + (y/2))H2
The use of exhaust catalysts have resulted in reaction pathways that can
accidentally be responsible for increased pollution. An example is the
CARB-mandated reduction of fuel sulfur. A change from 450ppm to 50ppm, which
will reduce HC & CO emissions by 20%, was shown to increase formaldehyde by
45%, but testing in later model cars did not exhibit the same effect [32,58,
59]. This demonstrates that continuing changes to engine management systems
can also change the response to fuel composition changes.
The requirement that the exhaust catalysts must now endure for 10 years or
100,000 miles will also encourage automakers to push for lower levels of
elements that affect exhaust catalyst performance, such as sulfur and
phosphorus, in both the gasoline and lubricant. Modern catalysts are unable
to reduce the relatively high levels of NOx that are produced during lean
operation down to approved levels, thus preventing the application of
lean-burn engine technology. Recently Mazda has announced they have
developed a "lean burn" catalyst, which may enable automakers to move the
fuel combustion towards the lean side, and different gasoline properties may
be required to optimise the combustion and reduce pollution [81]. Mazda
claim that fuel efficiency is improved by 5-8%, while meeting all emission
regulations, and some Japanese manufacturers have evaluated lean-burn
catalysts in limited numbers of 1995 production models.
Catalysts also inhibit the selection of gasoline octane-improving and
cleanliness additives ( such as MMT and phosphorus-containing additives )
that may result in refractory compounds known to physically coat the
catalyst, reducing available catalyst and thus increasing pollution.
5.7 Why are "cold start" emissions so important?
The catalyst requires heat to reach the temperature ( >300-350C ) where it
functions most efficiently, and the delay until it reaches operating
temperature can produce more hydrocarbons than would be produced during
the remainder of many typical urban short trips. It has been estimated that
70-80% of the non-methane HCs that escape conversion by the catalysts
are emitted during the first two minutes after a cold start. As exhaust
emissions have been reduced, the significance of the evaporative emissions
increases. Several engineering techniques are being developed, including the
Ford Exhaust Gas Igniter ( uses a flame to heat the catalyst - lots of
potential problems ), zeolite hydrocarbon traps, and relocation of the
catalyst closer to the engine [76].
Reduced gasoline volatility and composition changes, along with cleanliness
additives and engine management systems, can help minimise cold start
emissions, but currently the most effective technique appears to be rapid,
deliberate heating of the catalyst, and the new generation of low thermal
inertia "fast light-up" catalysts reduce the problem, but further research
is necessary [76,82].
As the evaporative emissions are also starting to be reduced, the emphasis
has shifted to the refuelling emissions. These will be mainly controlled
on the vehicle, and larger canisters may be used to trap the vapours emitted
during refuelling.
5.8 When will the emissions be "clean enough"?
The California ZEV regulations effectively preclude IC vehicles, because
they stipulate zero emissions. However, the concept of regulatory forcing
of alternative vehicle propulsion technology may have to be modified to
include hybrid or fuel-cell vehicles, as the major failing of EVs remains
the lack of a cheap, light, safe, and easily-rechargeable electrical
storage device [83,84]. There are several major projects intending to
further reduce emissions from automobiles, mainly focusing on vehicle mass
and engine fuel efficiency, but gasoline specifications and alternative
fuels are also being investigated. It may be that changes to IC engines and
gasolines will enable the IC engine to continue well into the 21st century
as the prime motive force for personal transportation [77,85]. There have
also been calls to use market forces to reduce pollution from automobiles
[86], however most such suggestions ( increased gasoline taxes, congestion
tolls, and emission-based registration fees ) are currently considered
politically unacceptable. The issue of how to target the specific "gross
polluters" is being considered, and is described in Section 5.14.
5.9 Why are only some gasoline compounds restricted?
The less volatile hydrocarbons in gasoline are not released in significant
quantities during normal use, and the more volatile alkanes are considerably
less toxic than many other chemicals encountered daily. The newer gasoline
additives also have potentially undesirable properties before they are even
combusted. Most hydrocarbons are very insoluble in water, with the lower
aromatics being the most soluble, however the addition of oxygen to
hydrocarbons significantly increases the mutual solubility with water.
Compound in Water Water in Compound
% mass/mass @ C % mass/mass @ C
normal decane 0.0000052 25 0.0072 25
iso-octane 0.00024 25 0.0055 20
normal hexane 0.00125 25 0.0111 20
cyclohexane 0.0055 25 0.010 20
1-hexene 0.00697 25 0.0477 30
toluene 0.0515 25 0.0334 25
benzene 0.1791 25 0.0635 25
methanol complete 25 complete 25
ethanol complete 25 complete 25
MTBE 4.8 20 1.4 20
TAME - 0.6 20
The concentrations and ratios of benzene, toluene, ethyl benzene, and xylenes
( BTEX ) in water are often used to monitor groundwater contamination from
gasoline storage tanks or pipelines. The oxygenates and other new additives
may increase the extent of water and soil pollution by acting as co-solvents
for HCs.
Various government bodies ( EPA, OSHA, NIOSH ) are charged with ensuring
people are not exposed to unacceptable chemical hazards, and maintain
ongoing research into the toxicity of liquid gasoline contact, water and soil
pollution, evaporative emissions, and tailpipe emissions [87]. As toxicity
is found, the quantities in gasoline of the specific chemical ( benzene ),
or family of chemicals ( alkyl leads, aromatics, olefins ) are regulated.
The recent dramatic changes caused by the need to reduce alkyl leads,
halogens, olefins, and aromatics has resulted in whole new families of
compounds ( ethers, alcohols ) being introduced into fuels without prior
detailed toxicity studies being completed. If adverse results appear, these
compounds are also likely to be regulated to protect people and the
environment.
Also, as the chemistry of emissions is unravelled, the chemical precursors
to toxic tailpipe emissions ( such as higher aromatics that produce benzene
emissions ) are also controlled, even if they are not themselves toxic.
5.10 What does "renewable" fuel or oxygenate mean?
The general definition of "renewable" is that the carbon originates from
recent biomass, and thus does not contribute to the increased CO2 emissions.
A truly "long-term" view could claim that fossil fuels are "renewable" on a
100 million year timescale :-). There was a major battle between the
ethanol/ETBE lobby ( agricultural, corn growing ), and the methanol/MTBE
lobby ( oil company, petrochemical ) over an EPA mandate demanding that a
specific percentage of the oxygenates in gasoline are produced from
"renewable" sources [88]. On 28 April 1995 a Federal appeals court
permanently voided the EPA ruling requiring "renewable" oxygenates, thus
fossil-fuel derived oxygenates such as MTBE are acceptable oxygenates [89].
Unfortunately, "renewable" ethanol is not cost competitive when crude oil
is $18/bbl, so a federal subsidy ( $0.54/US Gallon ) and additional state
subsidies ( 11 states - from $0.08(Michigan) to $0.66(Tenn.)/US Gal.) are
provided. Ethanol, and ETBE derived from ethanol, are still likely to be
used in states where subsidies make them competitive with other oxygenates.
5.11 Will oxygenated gasoline damage my vehicle?
The following comments assume that your vehicle was designed to operate on
unleaded, if not, then damage such as exhaust valve seat recession may occur.
Damage should not occur if the gasoline is correctly formulated, and you
select the appropriate octane, but oxygenated gasoline will hurt your pocket.
In the first year of mandated oxygenates, it appears some refiners did not
carefully formulate their oxygenated gasoline, and driveability and emissions
problems occurred. Most reputable brands are now carefully formulated.
Some older activated carbon canisters may not function efficiently with
oxygenated gasolines, but this is a function of the type of carbon used.
How your vehicle responds to oxygenated gasoline depends on the engine
management system and state of tune. A modern system will automatically
compensate for all of the currently-permitted oxygenate levels, thus your
fuel consumption will increase. Older, poorly-maintained, engines may
require a tune up to maintain acceptable driveability.
Be prepared to try several different brands of oxygenated or reformulated
gasolines to identify the most suitable brand for your vehicle, and be
prepared to change again with the seasons. This is because the refiners can
choose the oxygenate they use to meet the regulations, and may choose to set
some fuel properties, such as volatility, differently to their competitors.
Most stories of corrosion etc, are derived from anhydrous methanol corrosion
of light metals (aluminum, magnesium), however the addition of either 0.5%
water to pure methanol, or corrosion inhibitors to methanol-gasoline blends
will prevent this. If you observe corrosion, talk to your gasoline supplier.
Oxygenated fuels may either swell or shrink some elastomers on older cars,
depending on the aromatic and olefin content of the fuels. Cars later than
1990 should not experience compatibility problems, and cars later than 1994
should not experience driveability problems, but they will experience
increased fuel consumption, depending on the state of tune and engine
management system.
5.12 What does "reactivity" of emissions mean?
The traditional method of exhaust regulations was to specify the actual HC,
CO, NOx, and particulate contents. With the introduction of oxygenates and
reformulated gasolines, the volatile organic carbon (VOC) species in the
exhaust also changed. The "reactivity" refers to the ozone-forming potential
of the VOC emissions when they react with NOx, and is being introduced as a
regulatory means of ensuring that automobile emissions do actually reduce
smog formation. The ozone-forming potential of chemicals is defined as the
number of molecules of ozone formed per VOC carbon atom, and this is called
the Incremental Reactivity. Typical values ( big is bad :-) ) are [74]:
Maximum Incremental Reactivities as mg Ozone / mg VOC
carbon monoxide 0.054
alkanes methane 0.0148
ethane 0.25
propane 0.48
n-butane 1.02
olefins ethylene 7.29
propylene 9.40
1,3 butadiene 10.89
aromatics benzene 0.42
toluene 2.73
meta-xylene 8.15
1,3,5-trimethyl benzene 10.12
oxygenates methanol 0.56
ethanol 1.34
MTBE 0.62
ETBE 1.98
5.13 What are "carbonyl" compounds?
Carbonyls are produced in large amounts under lean operating conditions,
especially when oxygenated fuels are used. Most carbonyls are toxic, and the
carboxylic acids can corrode metals. The emission of carbonyls can be
controlled by combustion stoichiometry and exhaust catalysts, refer to
section 5.5 for typical reductions for aldehydes.
Typical carbonyls are:-
* aldehydes ( containing -CHO ),
- formaldehyde (HCHO) - which is formed in large amounts during lean
combustion of methanol [90].
- acetaldehyde (CH2CHO) - which is formed during ethanol combustion.
- acrolein (CH2=CHCHO) - a very potent irritant and toxin.
* ketones ( containing C=0 ),
- acetone (CH3COCH3)
* carboxylic acids ( containing -COOH ),
- formic acid (HCOOH) - formed during lean methanol combustion.
- acetic acid (CH3COOH).
5.14 What are "gross polluters"?
It has always been known that the EPA emissions tests do not reflect real
world conditions. There have been several attempts to identify vehicles on
the road that do not comply with emissions standards. Recent remote sensing
surveys have demonstrated that the highest 10% of CO emitters produce over
50% of the pollution, and the same ratio applies for the HC emitters
- which may not be the same vehicles [91-102]. 20% of the CO emitters are
responsible for 80% of the CO emissions, consequently modifying gasoline
composition is only one aspect of pollution reduction. The new additives can
help maintain engine condition, but they can not compensate for out-of-tune,
worn, or tampered-with engines. There has recently been some unpublished
studies that demonstrate that the current generation of remote sensing
systems can not provide sufficient discrimination of gross polluters without
also producing false positives for some acception vehicles - more work
is required, and in some states I&M emissions testing using dynamometers
is being introduced to identify gross polluters.
The most famous of the remote sensing systems is the FEAT ( Fuel Efficiency
Automobile Test ) team from the University of Denver [99]. This team is
probably the world leader in remote sensing of auto emissions to identify
grossly polluting vehicles. The system measures CO/CO2 ratio, and the
HC/CO2 ratio in the exhaust of vehicles passing through an infra-red light
beam crossing the road 25cm above the surface. The system also includes a
video system that records the licence plate, date, time, calculated exhaust
CO, CO2, and HC. The system is effective for traffic lanes up to 18 metres
wide, however rain, snow, and water spray can cause scattering of the beam.
Reference signals monitor such effects and, if possible, compensate. The
system has been comprehensively validated, including using vehicles with
on-board emissions monitoring instruments.
They can monitor up to 1000 vehicles an hour and, as an example,they were
invited to Provo, Utah to monitor vehicles, and gross polluters would be
offered free repairs [100]. They monitored over 10,000 vehicles and mailed
114 letters to owners of vehicles newer than 1965 that had demonstrated high
CO levels. They received 52 responses and repairs started in Dec. 1991, and
continued to Mar 1992.
The entire monitored fleet at Provo (Utah) during Winter 1991:1992
Model year Grams CO/gallon Number of
(Median value) (mean value) Vehicles
92 40 80 247
91 55 1222
90 75 1467
89 80 1512
88 85 1651
87 90 1439
86 100 300 1563
85 120 1575
84 125 1206
83 145 719
82 170 639
81 230 612
80 220 500 551
79 350 667
78 420 584
77 430 430
76 770 317
75 760 950 163
Pre 75 920 1060 878
As observed elsewhere, over half the CO was emitted by about 10% of the
vehicles. If the 47 worst polluting vehicles were removed, that achieves
more than removing the 2,500 lowest emitting vehicles from the total tested
fleet.
Surveys of vehicle populations have demonstrated that emissions systems had
been tampered with on over 40% of the gross polluters, and an additional 20%
had defective emission control equipment [101]. No matter what changes are
made to gasoline, if owners "tune" their engines for power, then the majority
of such "tuned" vehicle will become gross polluters. Professional repairs to
gross polluters usually improves fuel consumption, resulting in a low cost to
owners ( $32/pa/Ton CO year ). The removal of CO in the Provo example above
was costed at $200/Ton CO, compared to Inspection and Maintenance programs
($780/Ton CO ), and oxygenates ( $1034-$1264/Ton CO in Colorado 1991-2 ), and
UNOCALs vehicle scrapping programme ( $1025/Ton of all pollutants ).
Thus, identifying and repairing or removing gross polluters can be far more
cost-effective than playing around with reformulated gasolines and
oxygenates. A recent study has confirmed that gross polluters are not always
older vehicles, and that vehicles have been scrapped that passed the 1993 new
vehicle emission standards [102]. The study also confirmed that if estimated
costs and benefits of various emission reduction strategies were applied to
the tested fleet, the identification and repair techniques are the most
cost-effective means of reducing HC and CO. It should be noted that some
strategies ( such as the use of oxygenates to replace aromatics and alkyl
lead compounds ) have other environmental benefits.
Action Vehicles Estimated % reduction % reduction
Affected Cost per $billion
(millions) ($billion) HC CO HC CO
Reformulated Fuels 20 1.5 17 11 11 7.3
Scrap pre-1980 vehicles 3.2 2.2 33 42 15 19
Scrap pre-1988 vehicles 14.6 17 44 67 2.6 3.9
Repair worst 20% of vehicles 4 0.88 50 61 57 69
Repair worst 40% of vehicles 8 1.76 68 83 39 47
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...part III
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