1. Introduction and Intent
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?
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/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?
7. What parameters determine octane requirement?
7.1 What is the effect of Compression ratio?
7.2 What is the effect of changing the air/fuel ratio?
7.3 What is the effect of changing the ignition timing
7.4 What is the effect of engine management systems?
7.5 What is the effect of temperature and Load?
7.6 What is the effect of engine speed?
7.7 What is the effect of engine deposits?
7.8 What is the Road octane requirement of an vehicle?
7.9 What is the effect of air temperature?.
7.10 What is the effect of altitude?.
7.11 What is the effect of humidity?.
7.12 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?
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
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.
( billion = 1 x 10^9. trillion = 1 x 10^12 ). Crude Oil Proved Reserves R/P Ratio Middle East 89.6 billion tonnes 95.1 year USA 4.0 9.9 years Total World 136.7 43.1 years Coal Proved Reserves R/P Ratio USA 240.56 billion tonnes 267 years Total World 1,039.182 236 years Natural Gas Proved Reserves R/P Ratio USA 4.7 trillion cubic metres 8.8 years Total World 142.0 64.9 years.
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. Lead levels were increased, and 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. In 1990 the Clean Air Act started forcing major compositional changes on gasoline, and these changes will continue into the 21st Century because gasoline is a major pollution source.
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 [10].
normal = continuous chain of carbons ( Cn H2n+2 )
normal heptane C-C-C-C-C-C-C C7H16
iso = branched chain of carbons ( Cn H2n+2 )
iso octane = C C
( aka 2,2,4-trimethylpentane ) | |
C-C-C-C-C C8H18
|
C
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
cyclohexane = C
/ \
C C
| | C6H12
C C
\ /
C
C
| C5H10
2-methyl-2-butene C-C=C-C
Alkynes ( aka acetylenes, have carbon-carbon triple bonds )
_
Acetylene C=C C2H2
Arenes ( aka aromatics )
C C
// \ // \
C C C-C C
Benzene | || Toluene | ||
C C C C
\\ / \\ /
C C
C6H6 C7H8
Polynuclear Aromatics ( aka PNAs or PAHs )
C C
// \ / \\
C C C
Naphthalene | || | C10H8
C C C
\\ / \ //
C C
Ethanol C-C-O-H C2H5OH
C
|
Methyl tertiary butyl ether C-C-O-C C4H90CH3
(aka tertiary butyl methyl ether ) |
C
They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary
butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg
ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). Most oxygenates used in
gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbons. 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. Production has increased at the rate of 10 to 20%
per year, and the spot market price in June 1993 was around $270/tonne [11].
The "ether" starting fluids for vehicles are usually diethyl ether
( liquid ) or dimethyl ether ( aerosol ). Note that " petroleum ether " is
actually a volatile hydrocarbon fraction, it is 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. The major reduction in the reactivity of exhaust and evaporative emissions will occur with reformulated gasolines, due to be introduced in January 1995, which have oxygenates and major composition changes to the hydrocarbon component. Oxygenates do not necessarily reduce all individual 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 EPA in the US, with waivers being granted for some combinations. The change to reformulated gasoline requires oxygenates, but also that the hydrocarbon composition must be significantly more modified than the existing oxygenated gasolines to reduce unsaturates, volatility, benzene, and the reactivity of emissions.
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%). 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 efficiency that oxygenates provide on some engines usually do not completely compensate for the oxygen [12].
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 [13,14].
Originally, iodine was the best anti-knock 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 anti-knock, 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 [7,8,16]. 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 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 [17] 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 90The alkyl lead anti-knocks 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 [13,14].
A refinery will distill crude oil into various fractions and, depending on the desired final products, will further process and blend those fractions. Typical final products could be:- gases for chemical synthesis and fuel (CNG), liquified gases (LPG), butane, aviation and automotive gasolines, aviation and lighting kerosines, diesels, distillate and residual fuel oils, lubricating oil base grades, paraffin oils and waxes. Many of the common processes are intended to increase the yield of blending feedstocks for gasolines.
Typical modern refinery processes for gasoline components include
Two important reactions are:-
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 + 8H2Othus 1.000 kg of C7H16 required 3.513 kg of O2 = 15.179 kg air.
The chemical stoichiometric combustion of hydrocarbons with oxygen can be written as:-
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2OOften, 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 obtained 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
Typical Heats of Combustion are [22]:-
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* 112*
g 40.967
2-methylbutene-2 44.720 176* 141*
Because all the data is available, the calorific value of fuels can be
estimated quite accurately from hydrocarbon fuel properties such as the
density, sulfur content, and aniline point ( which indicates the aromatics
content ).It should be noted that because oxygenates contain oxygen that can not provide energy, they will have significantly lower energy contents. They are added to provide octane, not energy. For an engine that can be optimised for oxygenates, more fuel is required to obtain the same power, but they can burn slightly more efficiently, thus the power ratio is not identical to the energy content ratio. They also require more energy to vaporise.
Energy Content Heat of Vaporisation Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0
Typical values for commercial fuels in megajoules/kilogram are [23]:-
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. The
calorific value is the maximum energy that can be obtained from the fuel,
but the reality of modern SI engines is that efficiencies of 20-40% may be
obtained, this limit being due to engineering and material constraints
that prevent optimum combustion conditions being used. The CI engine can
achieve higher efficiencies, usually over a wider operating range as well.
D4814-93a 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:-
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/ethanol blends to meet the required specifications.
Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum.
Typical US gasoline levels are 0.03 %mass.
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. 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.
Because phosphorus adversely affects exhaust catalysts, the EPA limits phosphorus in all gasolines to 0.0013 gP/L.
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 1/May/1997. The second stage, the "Complex Model" would be developed, with the following parameters likely to be controlled - reid vapour pressure, benzene, oxygen, sulfur, olefins distillation ( 90% Evaporated ), and aromatics. Each refiner must have their RFG recertified using the Complex model by 1/May/1997 [27].
1990 Clean Air Act CARB
benzene 2 % 1 % maximum 1.0 vol% maximum
oxygen 0.2 % 2 % minimum 1.8-2.0 mass%
sulfur 150 ppm no increase 40 ppm
aromatics 32.0 % 25 % maximum 25 vol% maximum
olefins 9.9 % 5 % maximum 6 vol% maximum
reid vapour pressure 60 kPa 56 kPa (north) 48 kPa
50 kPa (south)
90% evaporated 170 C - 149 C
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% 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 3.5 wt% in
1979/1982, and tert-butyl alcohol to 3.5 wt% in 1981.
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% [28].
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 [29]). Note that the actual temperature in the combustion chamber is also determined by other factors, such as load and engine design. The addition of oxygenates changes the pre-flame reaction pathways, and also reduces the energy content of the fuel. The levels of oxygen in the fuel is regulated according to regional air quality standards.
Stability
Motor gasolines may be stored up to six months, consequently they must not form gums which may precipitate. Gums are usually the result of copper-catalysed reactions of the unsaturated HCs, 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 minutes test period has been found to be sufficient for most storage and distribution systems.
Corrosiveness
Sulfur in the fuel creates corrosion, and when combusted will form corrosive gases that attack the engine, exhaust and environment. Sulfur also adversely affects the alkyl lead octane response and may poison exhaust catalysts. The copper strip corrosion test and the sulfur specification are used to ensure fuel quality. The copper strip test measures active sulfur, whereas the sulfur content reports the total sulfur present.
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:-) )[30]. Most suppliers of quality gasolines will formulate similar additives into their products, and cheaper lines are less like to have such additives added. As different brands use different additives and oxygenates, it is probable that important parameters, such as octane distribution, are 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.
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/May/1997 Complex model, will probably introduce major reductions in the distillation range, as well as 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 base, and the extrapolation of this 20% to 100%. This 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 blending octanes for the aromatics and olefins [31].
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.
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 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 and generate smoke, and 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/residues, and does not dissolve or poison the car catalyst or owner, is based on knowledge of the gasoline composition.
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.
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 [33]. Alkyl lead additives can only substitute for some of the aromatics in gasoline, consequently they do not eliminate aromatics, which will produce benzene emissions [34]. Alkyl lead additives also require toxic organohalogen scavengers, which also react in the engine to form and emit other organohalogens, including highly toxic dioxin [35]. 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.
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 light (wavelengths of less than 430nm) to form ground-level (tropospheric) ozone, which is one of the major components of photochemical smog [36]. 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 [37]. The issue is whether it is natural, or induced by human activities. 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 [38].
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 [39];-
Cars Region 1990 1991 1992 1993 Africa 222 213 194 201 Asia-Pacific 12,064 12,112 11,869 11,467 Central & South America 800 888 1,158 1,524 Eastern Europe 2,466 984 1,726 1,783 Middle East 35 24 300 377 North America 7,762 7,230 7,470 8,172 Western Europe 13,688 13,286 13,097 11,124 Total World 37,039 34,739 35,815 34,649 Trucks ( including heavy trucks and buses ) Region 1990 1991 1992 1993 Africa 133 123 108 109 Asia-Pacific 5,101 5,074 5,117 5,054 Central & South America 312 327 351 417 Eastern Europe 980 776 710 708 Middle East 36 28 100 110 North America 4,851 4,554 5,371 6,037 Western Europe 1,924 1,818 1,869 1,345 Total World 13,336 12,701 13,627 13,779To fuel all operating vehicles, considerable quantities of gasoline and diesel have to be consumed. Major consumption in 1993 of gasoline and middle distillates ( which may include some heating fuels, but not fuel oils ) in million tonnes.
Gasoline Middle Distillates
USA 335.6 233.9
Canada 25.0 24.4
Western Europe 166.0 264.0
Japan 56.4 89.6
Total World 802.0 989.0
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
335.6 million tonnes in 1993. In 1993 the total world production of crude oil
was 3164.8 million tonnes, of which the USA consumed 787.5 million tonnes
[40]. 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.
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."[41].
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 [42].
The health risks to service station workers, who are continuously exposed to refuelling emissions remain a concern [44]. 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.
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 unburned hydrocarbons are related to the fuel composition (volatility, olefins, aromatics, final boiling point), as well as state of tune, engine condition, and age/condition of the engine lubricating oil [46]. Particulate emissions, especially the size fraction smaller than ten micrometres, are a serious health concern. The current major source is from compression ignition ( CI = 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 1997 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. The rules 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 [33].
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 [33].
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 the five compounds listed as toxics under the Clean Air Act,
then the beneficial effects of catalysts are obvious [33].
FTP-weighted emission rates. (mg/mi)
Benzene Formaldehyde Acrolein
Gas Reform Gas Reform Gas Reform
Noncatalyst 156.18 138.48 73.25 85.24 11.62 13.20
Oxidation Cat. 27.57 25.01 28.50 35.83 3.74 3.75
3-way Catalyst 19.39 15.69 7.27 7.61 1.11 0.74
Adaptive Learn. 19.77 20.39 4.87 8.43 0.81 1.16
Acetaldehyde 1,3 Butadiene
Gas Reform Gas Reform
Noncatalyst 19.74 21.72 2.96 1.81
Oxidation Cat. 11.15 11.76 0.02 0.33
3-way Catalyst 4.43 3.64 0.07 0.05
Adaptive Learn. 3.07 4.71 0.00 0.14
The author reports analytical problems with the 1,3 Butadiene, and only
Noncatalyst values are considered reliable.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 and particulates, are omitted. The 1994 tests signal the transition from 50,000 to 75,000 mile compliance testing, and I have not listed the subsequent 50,000 mile limits [47,48].
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 4.1 34.0 4.0 6
1972 3.0 28.0 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 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 ? TLEV 0.13 3.4 0.4
1994 75,000 0.31 4.2 0.6 ?
1997 LEV 0.08 3.4 0.2
1997 ULEV 0.04 1.7 0.2
1998 ZEV 0.0 0.0 0.0
2004 0.13 1.8 0.16 ?
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 [49] 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 [50]. 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 2-40ng/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.
6.4 methanol 9.0 ethanol 11.7 MTBE 12.1 ETBE, TAME 14.6 gasoline without oxygenatesThe 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 [47].
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%, may increase formaldehyde by 45% [19]. 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 known catalyst "poisons" 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. Mazda claim that fuel efficiency is improved by 5-8% while meeting all emission regulations [52] .
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 and increase pollution.
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 [53].
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.
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 [56]. 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, 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 toxic.
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. A judgement on the use of "renewable" oxygenates is expected in early 1995.
Be prepared to try several different brands of 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.
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
Typical carbonyls are:-
The most famous of these remote sensing systems is the FEAT ( Fuel Efficiency Automobile Test ) team from the University of Denver [62]. 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 [63]. 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. They offered to purchase two vehicles at blue book price. They were declined, and so attempted to modify those vehicles, even though their condition did not justify the expense.
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 [64]. 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.
Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock [16]. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, but arose from a violent pressure rise _after_ ignition. The manograph was not suitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock [7]. Ricardo had developed an alternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc.
In 1926 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity [9]. These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane. The octane had a high anti-knock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.
The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test.
Heat of
Melting Point Boiling Point Density Vaporisation
C C g/ml MJ/kg
normal heptane -90.7 98.4 0.684 0.365 @ 25C
iso octane -107.45 99.3 0.6919 0.308 @ 25C
Having decided on standard reference fuels, a whole range of engines and
test conditions appeared, but today the most common are the Research Octane
Number ( RON ), and the Motor Octane Number ( MON ).
Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used. If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction [17].
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species (such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen bonds depends on what the carbon is connected to. Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker than the primary C-H bonds present in branched chain HCs like iso-octane [13,14].
The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number. In real life, the unburnt "end gases" ahead of the flame front encounter temperatures up to about 700C due to piston motion and radiant and conductive heating, and commence a series of pre-flame reactions. These reactions occur at different thermal stages, with the initial stage ( below 400C ) commencing with the addition of molecular oxygen to alkyl radicals, followed by the internal transfer of hydrogen atoms within the new radical to form an unsaturated, oxygen-containing species. These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600C), mainly through the production of OH radicals. Above 600C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can significantly affect the pre-flame reaction pathways. Anti-knock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and the alkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence [13,14].
The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:-
The initial octane method developed in the 1920s was the Motor Octane method and, over several decades, a large number of octane test methods appeared. These were variations to either the engine design, or the specified operating conditions [65]. During the 1950-1960s attempts were made to internationally standardise and reduce the number of Octane Rating test procedures.
During the late 1930s - mid 1960s, the Research method became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available. In the late 1960s German automakers discovered their engines were destroying themselves on long Autobahn runs, even though the Research Octane was within specification. They discovered that either the MON or the Sensitivity ( the numerical difference between the RON and MON numbers ) also had to be specified. Today it is accepted that no one octane rating covers all use. In fact, during 1994, there have been increasing concerns in Europe about the high Sensitivity of some commercially-available unleaded fuels.
The design of the engine and car significantly affect the fuel octane requirement for both RON and MON. In the 1930s, most vehicles would run on the specified Research Octane fuel, almost regardless of the Motor Octane, whereas most 1990s engines have a 'severity" of one, which means the engine is unlikely to knock if a changes of one RON is matched by an equal and opposite change of MON [19].
Test Engine conditions Motor Octane
Test Method ASTM D2700-92 [66]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm
Test Engine conditions Research Octane
Test Method ASTM D2699-92 [67]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )
Some fuel specifications include delta RONs, to ensure octane distribution throughout the fuel boiling range was consistent. Octane distribution was seldom a problem with the alkyl lead compounds, as the tetra methyl lead and tetra ethyl lead octane volatility profiles were well characterised, but it can be a major problem for the new, reformulated, low aromatic gasolines, as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have discovered that an 87 (RON+MON/2) from one brand has to be substituted with an 89 (RON+MON/2) of another, and that is because of the combination of their driving style, engine design, vehicle mass, fuel octane distribution, fuel volatility, and the octane-enhancers used.
The fuel is carefully distilled to obtain a distillate fraction that boils to the specified temperature, which is usually 100C. Both the parent fuel and the distillate fraction are rated on the octane engine using the Research Octane method [68]. The difference between these is the delta RON(100C), usually just called the delta RON.
Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel. The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.
Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and that some oxygenates can have negative lead responses. Also note that the octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.
You should initially be guided by the vehicle manufacturer's recommendations, however you can experiment, as the variations in vehicle tolerances can mean that Octane Number Requirement for a given vehicle model can range over 6 Octane Numbers. Caution should be used, and remember to compensate if the conditions change, such as carrying more people or driving in different ambient conditions. You can often reduce the octane of the fuel you use in winter because the temperature decrease and possible humidity changes may significantly reduce the octane requirement of the engine.
Use the octane that provides cost-effective driveability and performance, using anything more is waste of money, and anything less could result in an unscheduled, expensive visit to your mechanic.
The second Avgas number is the Rich Mixture method Performance Number ( PN - they are not commonly called octane numbers when they are above 100 ), and is determined on a supercharged version of the CFR engine which has a fixed compression ratio. The method determines the dependence of the highest permissible power ( in terms of indicated mean effective pressure ) on mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock-free power obtainable from a fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine designed to utilise the fuel can obtain 150% of the knock-limited power of iso-octane at the same mixture ratio. This is an arbitrary scale based on iso-octane + varying amounts of TEL, derived from a survey of engines performed decades ago. Aviation gasoline PNs are rated using variations of mixture strength to obtain the maximum knock-limited power in a supercharged engine. This can be extended to provide mixture response curves which define the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry ) before knock [71].
The 115/145 grade is being phased out, but even the 100LL has more octane than any automotive gasoline.
Efficiency = 1 - (1/compression ratio)^gamma-1where gamma = ratio of specific heats at constant pressure and constant volume of the working fluid ( for most purposes air is the working fluid, and is treated as an ideal gas ). There are indications that thermal efficiency reaches a maximum at a compression ratio of about 17:1 [15].
The efficiency gains are best when the engine is at incipient knock, that's why knock sensors ( actually vibration sensors ) are used. Low compression ratio engines are less efficient because they can not deliver as much of the ideal combustion power to the flywheel. For a typical carburetted engine, without engine management [17,24]:-
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.
The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel/air mixture burns in such a way that maximum pressure of the burning charge is about 15-20 degree after TDC. Knock will occur before this point, usually in the late compression/early power stroke period. The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used ( several octane numbers below the vehicle's requirement at optimal settings ), both performance and fuel economy will decrease.
The actual Octane Number Requirement depends on the engine design, but for some 1978 vehicles using standard fuels, the following (R+M)/2 Octane Requirements were measured. "Standard" is the recommended ignition timing for the engine, probably a few degrees before Top Dead Centre [24].
Basic Ignition Timing
Vehicle Retarded 5 degrees Standard Advanced 5 degrees
A 88 91 93
B 86 90.5 94.5
C 85.5 88 90
D 84 87.5 91
E 82.5 87 90
The actual ignition timing to achieve the maximum pressure from normal
combustion of gasoline will depend mainly on the speed of the engine and the
flame propagation rates in the engine. Knock increases the rate of the
pressure rise, thus superimposing additional pressure on the normal
combustion pressure rise. The knock actually rapidly resonates around the
chamber, creating a series of abnormal sharp spikes on the pressure diagram.
The normal flame speed is fairly consistent for most gasoline HCs, regardless
of octane rating, but the flame speed is affected by stoichiometry. Note that
the flame speeds in this FAQ are not the actual engine flame speeds. A 12:1
CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s,
and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds
are also very dependent on stoichiometry.
A modern engine management system can compensate for altitude, ambient air temperature, and fuel octane. The management system will also control cold start settings, and other operational parameters. There is a new requirement that the engine management system also contain an on-board diagnostic function that warns of malfunctions such as engine misfire, exhaust catalyst failure, and evaporative emissions failure. The use of fuels with alcohols such as methanol can confuse the engine management system as they generate more hydrogen which can fool the oxygen sensor [47] .
The use of fuel of too low octane can actually result in both a loss of fuel economy and power, as the management system may have to move the engine settings to a less efficient part of the performance map. The system retards the ignition timing until only trace knock is detected, as engine damage from knock is of more consequence than power and fuel economy.
- they reduce the combustion chamber volume, effectively increasing the compression ratio. - they also reduce thermal conductivity, thus increasing the combustion chamber temperatures. - they catalyse undesirable pre-flame reactions that produce end gases with low autoignition temperatures.
The octane rating engines do not reflect actual conditions in a vehicle, consequently there are standard procedures for evaluating the performance of the gasoline in an engine. The most common are:- 1. The Modified Uniontown Procedure. Full throttle accelerations are made from low speed using primary reference fuels. The ignition timing is adjusted until trace knock is detected at some stage. Several reference fuels are used, and a Road Octane Number v Basic Ignition timing graph is obtained. The fuel sample is tested, and the ignition timing setting is read from the graph to provide the Road Octane Number. This is a rapid procedure but provides minimal information. 2. The Modified Borderline Knock Procedure. The automatic spark advance is disabled, and a manual adjustment facility added. Accelerations are performed as in the Modified Uniontown Procedure, however trace knock is maintained throughout the run. A map of ignition advance v engine speed is made for several reference fuels and the sample fuels. This procedure can show the variation of road octane with engine speed.
- reduced air density provides lower combustion temperature and pressure. - fuel is metered according to air volume, consequently as density decreases the stoichiometry moves to rich, with a lower octane number requirement. - manifold vacuum controlled spark advance, and reduced manifold vacuum results in less spark advance.
Some aviation SI engines still use boost fluids. The water/methanol mixtures are used to provide increased power for short periods, up to 40% more - assuming adequate mechanical strength of the engine. The 40/60 or 45/55 water/methanol mixtures are used as boost fluids for aviation engines because water would freeze. Methanol is just "preburnt" methane, consequently it only has about half the energy content of gasoline, but it does have a higher heat of vaporisation, which has a significant cooling effect on the charge. Water/methanol blends are more cost-effective than gasoline for combustion cooling. The high Sensitivity of alcohol fuels has to be considered in the engine design and settings.
Boost fluids are used because they are far more economical than using the fuel. When a supercharged engine has to be operated at high boost, the mixture has to be enriched to keep the engine operating without knock. The extra fuel cools the cylinder walls and the charge, thus delaying the onset of knock which would otherwise occur at the associated higher temperatures.
The overall effect of boost fluid injection is to permit a considerable increase in knock-free engine power for the same combustion chamber temperature. The power increase is obtained from the higher allowable boost. In practice, the fuel mixture is usually weakened when using boost fluid injection, and the ratio of the two fuel fluids is approximately 100 parts of avgas to 25 parts of boost fluid. With that ratio, the resulting performance corresponds to an effective uprating of the fuel of about 25%, irrespective of its original value. Trying to increase power boosting above 40% is difficult, as the engine can drown because of excessive liquid [71].
Note that for water injection to provide useful power gains, the engine management and fuel systems must be able to monitor the knock and adjust both stoichiometry and ignition to obtain significant benefits. Aviation engines are designed to accommodate water injection, most automobile engines are not. Returns on investment are usually harder to achieve on engines that do not normal extend their performance envelope into those regions. Water injection has been used by some engine manufacturers - usually as an expedient way to maintain acceptable power after regulatory emissions baggage was added to the engine, but usually the manufacturer quickly produces a modified engine that does not require water injection.
Preignition occurs when the air/fuel mixture is ignited prematurely by glowing deposits or hot surfaces - such as exhaust valves and spark plugs. If it continues, it can increase in severity and become Run-away Surface Ignition (RSI) which prevents the combustion heat being converted into mechanical energy, thus rapidly melting pistons. The Ricardo method uses an electrically-heated wire in the engine to measure preignition tendency. The scale uses iso-octane as 100 and cyclohexane as 0.
Some common fuel components:-
paraffins 50-100
benzene 26
toluene 93
xylene >100
cyclopentane 70
di-isobutylene 64
hexene-2 -26
There is no direct correlation between anti-knock ability and preignition
tendency, however high combustion chamber temperatures favour both, and so
one may lead to the other. An engine knocking during high-speed operation
will increase in temperature and that can induce preignition, and conversely
any preignition will result in higher temperatures than may induce knock.Misfire is commonly caused by either a failure in the ignition system, or fouling of the spark plug by deposits. The most common cause of deposits was the alkyl lead additives in gasoline, and the yellow glaze of various lead salts was used by mechanics to assess engine tune. From the upper recess to the tip, the composition changed, but typical compounds ( going from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4; 3Pb3(PO4)2.PbClBr.
Run-on is the tendency of an engine to continue running after the ignition has been switched off. It is usually caused by the spontaneous ignition of the fuel/air mixture, rather than by surface ignition from hotspots or deposits, as commonly believed. The narrow range of conditions for spontaneous ignition of the fuel/air mixture ( engine speed, charge temperature, cylinder pressure ) may be created when the engine is switched off. The engine may refire, thus taking the conditions out of the critical range for a couple of cycles, and then refire again, until overall cooling of the engine drops it out of the critical region. The octane rating of the fuel is the appropriate parameter, and it is not rare for an engine to require a higher Octane fuel to prevent run-on than to avoid knock [17].
Two types of additive are added to gasoline to inhibit icing:-
- surfactants that form a monomolecular layer over the metal parts that inhibits ice crystal formation. These are usually added at concentrations of 30-150 ppm. - cryoscopic additives that depress the freezing point of the condensed water so that it does not turn to ice. Alcohols ( methanol, ethanol, iso-propanol, etc. ) and glycols ( hexylene glycol, dipropylene glycol ) are used at concentrations of 0.03% - 1%. If you have icing problems, the addition of 100-200mls of methanol to a full tank of dry gasoline will prevent icing under most conditions. If you believe there is a small amount of water in the fuel tank, add 500mls of isopropanol as the first treatment. Oxygenated gasolines using alcohols can also be used.
Note that oxygenated gasolines must decrease fuel economy for the same power. If your engine is initially well-tuned on hydrocarbon gasolines, the stoichiometry will move to lean, and maximum power is slightly rich, so either the management system ( if you have one ) or your mechanic has to increase the fuel flow. The minor improvements in combustion efficiency that oxygenates may provide, can not compensate for 2+% of oxygen in the fuel that will not provide energy.
Water in fuel tanks can be minimised by keeping the fuel tank near full, and filling in the morning from a service station that allows storage tanks to stand for several hours after refilling before using the fuel. Note that oxygenated gasolines have greater water solubility, and should cope with small quantities of water.
There is one aftermarket additive that may be cost-effective, the lubricity additive used with unleaded gasolines to combat valve seat recession on engines that do not have seat inserts. The long-term solution is to install inserts at the next top overhaul.
Some other fuel additives work, especially those that are carefully formulated into the gasoline by the manufacturer at the refinery.
A typical gasoline may contain [17,19,24]:-
* Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm to prevent its misuse as an industrial solvent * Antioxidants, typically phenylene diamines or hindered phenols, are added to prevent oxidation of unsaturated hydrocarbons. * Metal Deactivators, typically about 10ppm of chelating agent such as N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper, which can rapidly catalyze oxidation of unsaturated hydrocarbons. * Corrosion Inhibitors, about 5ppm of oil-soluble surfactants are added to prevent corrosion caused either by water condensing from cooling, water-saturated gasoline, or from condensation from air onto the walls of almost-empty gasoline tanks that drop below the dew point. If your gasoline travels along a pipeline, it's possible the pipeline owner will add additional corrosion inhibitor to the fuel. * Anti-icing Additives, used mainly with carburetted cars, and usually either a surfactant, alcohol or glycol. * Anti-wear Additives, these are used to control wear in the upper cylinder and piston ring area that the gasoline contacts, and are usually very light hydrocarbon oils. Phosphorus additives can also be used on engines without exhaust catalyst systems. * Deposit-modifying Additives, usually surfactants. 1. Carburettor Deposits, additives to prevent these were required when crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controls were introduced. Some fuel components reacted with these gas streams to form deposits on the throat and throttle plate of carburettors. 2. Fuel Injector tips operate about 100C, and deposits form in the annulus during hot soak, mainly from the oxidation and polymerisation of the larger unsaturated hydrocarbons. The additives that prevent and unclog these tips are usually polybutene succinimides or polyether amines. 3. Intake Valve Deposits caused major problems in the mid-1980s when some engines had reduced driveability when fully warmed, even though the amount of deposit was below previously acceptable limits. It is believed that the new fuels and engine designs were producing a more absorbent deposit that grabbed some passing fuel vapour, causing lean hesitation. Intake valves operate about 300C, and if the valve is is kept wet deposits tend not to form, thus intermittent injectors tend to promote deposits. Oil leaking through the valve guides can be either harmful or beneficial, depending on the type and quantity. Gasoline factors implicated in these deposits include unsaturates and alcohols. Additives to prevent these deposits contain a detergent and/or dispersant in a higher molecular weight solvent or light oil whose low volatility keeps the valve surface wetted. 4. Combustion Chamber Deposits have been targeted in the 1990s, as they are responsible for significant increases in emissions. Recent detergent-dispersant additives have the ability to function in both the liquid and vapour phases to remove existing carbon and prevent deposit formation. * Octane Enhancers, these are usually formulated blends of alkyl lead or MMT compounds in a solvent such as toluene, and added at the 100-1000 ppm levels. They have been replaced by hydrocarbons with higher octanes such as aromatics and olefins. These hydrocarbons are now being replaced by a mixture of saturated hydrocarbons and and oxygenates. If you wish to play with different fuels and additives, be aware that some parts of your engine management systems, such as the oxygen sensor, can be confused by different exhaust gas compositions. An example is increased quantities of hydrogen from methanol combustion.
Methanol Ethanol Unleaded Gasoline
RON 106 107 92 - 98
MON 92 89 80 - 90
Heat of Vaporisation (MJ/kg) 1.154 0.913 0.3044
Nett Heating Value (MJ/kg) 19.95 26.68 42 - 44
Vapour Pressure @ 38C (kPa) 31.9 16.0 48 - 108
Flame Temperature ( C ) 1870 1920 2030
Stoich. Flame Speed. ( m/s ) 0.43 - 0.34
Minimum Ignition Energy ( mJ ) 0.14 - 0.29
Lower Flammable Limit ( vol% ) 6.7 3.3 1.3
Upper Flammable Limit ( vol% ) 36.0 19.0 7.1
Autoignition Temperature ( C ) 460 360 260 - 460
Flash Point ( C ) 11 13 -43 - -39
The major advantages are gained when pure fuels ( M100, and E100 ) are used,
as the addition of hydrocarbons to overcome the cold start problems also
significantly reduces, if not totally eliminates, any emission benefits.
Methanol will produce significant amounts of formaldehyde, a suspected
human carcinogen, until the exhaust catalyst reaches operating temperature.
Ethanol produces acetaldehyde. The cold-start problems have been addressed,
and alcohol fuels are technically viable, however with crude oil at
<$30/bbl they are not economically viable, especially as the demand for then
as precursors for gasoline oxygenates has elevated the world prices.
Methanol almost doubled in price during 1994. There have also been trials
of pure MTBE as a fuel, however there are no unique or significant advantages
that would outweigh the poor economic viability [11].
CNG has been extensively used in Italy and New Zealand ( NZ had 130,000 dual-fuelled vehicles with 380 refuelling stations in 1987 ). The conversion costs are usually around US$1000, so the economics are very dependent on the natural gas price. The typical 15% power loss means that driveability of retrofitted CNG-fuelled vehicles is easily impaired, consequently it is not recommended for vehicles of less than 1.5l engine capacity, or retrofitted onto engine/vehicle combinations that have marginal driveability on gasoline. The low price of crude oil, along with installation and ongoing CNG tank-testing costs, have reduced the number of CNG vehicles in NZ. The US CNG fleet continues to increase in size ( 60,000 in 1994 ).
LPG ( Liquified Petroleum Gas ) is predominantly propane with iso-butane and n-butane. It has one major advantage over CNG, the tanks do not have to be high pressure, and the fuel is stored as a liquid. The fuel offers most of the environmental benefits of CNG, including high octane. Approximately 20-25% more fuel is required, unless the engine is optimised ( CR 12:1 ) for LPG, in which case there is no decrease in power or increase in fuel consumption [17,76].
methane propane iso-octane
RON 120 112 100
MON 120 97 100
Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712
Net Heating Value (MJ/kg) 50.0 46.2 44.2
Vapour Pressure @ 38C ( kPa ) - - 11.8
Flame Temperature ( C ) 1950 1925 1980
Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31
Minimum Ignition Energy ( mJ ) 0.30 0.26 -
Lower Flammable Limit ( vol% ) 5.0 2.1 0.95
Upper Flammable Limit ( vol% ) 15.0 9.5 6.0
Autoignition Temperature ( C ) 540 - 630 450 415
Unfortunately the flame temperature is sufficiently high to dissociate atmospheric nitrogen and form undesirable NOx emissions. The high flame speeds mean that ignition timing is at TDC, except when running lean, when the ignition timing is advanced 10 degrees. The high flame speed, coupled with a very small quenching distance mean that the flame can sneak past inlet narrow inlet valve openings and cause backfiring. The advantage of a wide range of mixture strengths and high thermal efficiencies are matched by the disadvantages of pre-ignition and knock unless weak mixtures, clean engines, and cool operation are used.
Interested readers are referred to the group sci.energy.hydrogen for details about this fuel.
Temperature Flame Speed
Fuel Oxidant ( C ) ( m/s )
Acetylene Air 2400 1.60 - 2.70
" Nitrous Oxide 2800 2.60
" Oxygen 3140 8.00 - 24.80
Hydrogen Air 2050 3.24 - 4.40
" Nitrous Oxide 2690 3.90
" Oxygen 2660 9.00 - 36.80
Propane Air 1925 0.45
Natural Gas Air 1950 0.39
Nitrous oxide is not yet routinely used on standard vehicles, but the
technology is well understood.
This post is an edited version of some posted after JdA posted some claims from a hot-rod enthusiast reporting that triptane + 4cc TEL had a rich power octane rating of 270. This was followed by another that claimed the unleaded octane was 150.In WWII there was a major effort to increase the power of the aviation engines continuously, rather than just for short periods using boost fluids. Increasing the octane of the fuel had dramatic effects on engines that could be adjusted to utilise the fuel ( by changing boost pressure ). There was a 12% increase in cruising speed, 40% increase in rate of climb, 20% increase in ceiling, and 40% increase in payload for a DC-3, if the fuel went from 87 to 100 Octane, and further increases if the engine could handle 100+ PN fuel [85]. A 12 cylinder allison aircraft engine was operated on a 60% blend of triptane ( 2,2,3-trimethylbutane ) in 100 octane leaded gasoline to produce 2500hp when the rated take-off horsepower with 100 octane leaded was 1500hp [10].
Triptane was first shown to have high octane in 1926 as part of the General Motors Research Laboratories investigations [86]. As further interest developed, gallon quantities were made in 1938, and a full size production plant was completed in late 1943. The fuel was tested, and the high lead sensitivity resulted in power outputs up to 4 times that of iso-octane, and as much as 25% improvement in fuel economy over iso-octane [10].
All of this sounds incredibly good, but then, as now, the cost of octane enhancement has to be considered, and the plant producing triptane was not really viable. the fuel was fully evaluated in the aviation test engines, and it was under the aviation test conditions - where mixture strength is varied, that the high power levels were observed over a narrow range of engine adjustment. If turbine engines had not appeared, then maybe triptane would have been used as an octane agent in leaded aviation gasolines. Significant design changes would have been required for engines to utilise the high anti-knock rating.
As an unleaded additive, it was not that much different to other isoalkanes, consequently the modern manufacturing processes for aviation gasolines are alkylation of unsaturated C4 HCs with isobutane, to produce a highly iso-paraffinic product, and/or aromatization of naphthenic fractions to produce aromatic hydrocarbons possessing excellent rich-mixture antiknock properties.
So, the myth that triptane was the wonder anti-knock agent that would provide heaps of power arose. In reality, it was one of the best of the iso-alkanes ( remember we are comparing it to iso-octane which just happened to be worse than most other iso-alkanes), but it was not _that_ different from other members. It was targeted, and produced, for supercharged aviation engines that could adjust their mixture strength, used highly leaded fuel, and wanted short period of high power for takeoff, regardless of economy.
The blending octane number, which is what we are discussing, of triptane is designated by the American Petroleum Institute Research Project 45 survey as 112 Motor and 112 Research [31]. Triptane does not have a significantly different blending number for MON or RON, when compared to iso-octane. When TEL is added, the lead response of a large number of paraffins is well above that of iso-octane ( about +45 for 3ml TEL/US Gal ), and this can lead to Performance Numbers that can not be used in conventional automotive engines [10].
The following is edited from a post in a debate over the advantages of water injection. I tried to demonstrate what modifications would be required to convert my own 1500cc Honda Civic into something worthwhile :-).There are many variables that will determine the power output of an engine. High on the list will be the ability of the fuel to burn evenly without knock. No matter how clever the engine, the engine power output limit is determined by the fuel it is designed to use, not the amount of oxygen stuffed into the cylinder and compressed. Modern engines designs and gasolines are intended to reduce the emission of undesirable exhaust pollutants, consequently engine performance is mainly constrained by the fuel available.
My Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5 litre engine was only permitted to operate on 102 Research Octane fuel, and had limits placed on the amount of fuel it could use during a race, the maximum boost of the turbochargers was specified, as was an additional 40kg penalty weight. Standard 102 RON gasoline would be about 96 R+M/2 if sold as a pump gasoline. The normally-aspirated 3.0 litre engines could use unlimited amounts of 102RON fuel. The F1 race duration is 305 km or 2 hours, and it's perhaps worth remembering that Indy cars run at 7.3 psi boost.
Engine Standard Formula One Year 1986 1987 1989 Size 1.5 litre 1.5 litre 1.5 litre Cylinders 4 12 12 Aspiration normal turbo turbo Maximum Boost - 58 psi 36.3 psi Maximum Fuel - 200 litres 150 litres Fuel 91 RON 102 RON 102 RON Horsepower @ rpm 92 @ 6000 994 @ 12000 610 @ 12500 Torque (lb-ft @ rpm) 89 @ 4500 490 @ 9750 280 @ 10000Lets consider the transition from Standard to Formula 1, without considering materials etc.
For subsequent years the restrictions were even more severe, 150 litres and 36.3 maximum boost, in a still vain attempt to give the 3 litre, normally-aspirated engines a chance. Obviously Honda took advantage of the reduced boost by increasing CR to 9.4:1, and only going to 15% rich air/fuel ratio. They then developed an economy mode that involved heating the liquid fuel to 180F to improve vaporisation, and increased the air temp to 158F, and leaned out the air-fuel ratio to just 2% rich. The engine output dropped to 610hp @ 12,500 ( from 685hp @ 12,500 and about 312 lbs-ft of torque @ 10,000 rpm ), but 32% of the energy in the fuel was converted to mechanical work. The engine still had crisp throttle response, and still beat the normally aspirated engines that did not have the fuel limitation. So turbos were banned. No other F1 racing engine has ever come close to converting 32% of the fuel energy into work [87].
11.1 Books and Research Papers
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Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (1984).
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2. Hydrocarbons from Fossil Fuels and their Relationship with Living
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5. BP Statistical Review of World Energy - June 1994.
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6. Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
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8. ?
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22. Chemical Engineers' Handbook - 5th edition
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24. Automotive Gasolines - Recommended Practice.
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25. Standard Specification for Automotive Spark-Ignition Engine Fuel.
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26. Criteria for Quality of Petroleum Products.
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27. Meeting the challenge of reformulated gasoline.
R.J. Schmidt, P.L.Bogdan, and N.L.Gilsdorf.
Chemtech, February 1993. p.41-42.
28. The Relationship between Gasoline Composition and Vehicle Hydrocarbon
Emissions: A Review of Current Studies and Future Research Needs.
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W.Kreucher, and E.Kulik.
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31. Knocking Characteristics of Pure Hydrocarbons.
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35. Determination of PCDDs and PCDFs in Car Exhaust.
A.G.Bingham, C.J.Edmunds, B.W.L.Graham, and M.T.Jones.
Chemosphere, v.19 p.669-673 (1989).
36. Volatile Organic Compounds: Ozone Formation, Alternative Fuels and
Toxics.
B.J.Finlayson-Pitts and J.N.Pitts Jr..
Chemistry and Industry (UK), 18 October 1993. p.796-800.
37. The rise and rise of global warming.
R.Matthews.
New Scientist, 26 November 1994. p.6.
38. Energy-related Carbon Dixode Emissions per Capita for OECD Countries
during 1990.
International Energy Agency. (1993)
39. Market Data Book - 1991, 1992, 1993 and 1994 editions.
Automobile News
- various tables
40. BP Statistical Review of World Energy - June 1994.
- Crude oil consumption p.7.
41. Automotive Gasolines - Recommended Practice
SAE J312 Jan93.
- Section 4
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
42. The Rise and Fall of Lead in Petrol.
IDG Berwick
Phys. Technol., v.18 p.158-164 (1987)
43. E.C. seeks gasoline emission control.
Hydrocarbon Processing, September 1990. p.43.
44. Health Effects of Gasoline Exposure. I. Exposure assessment for U.S.
Distribution Workers.
T.J.Smith, S.K.Hammond, and O.Wong.
Environmental Health Perspectives Supplements. v.101 s.6 p.13 (1993)
45. Atmospheric Chemistry of Tropospheric Ozone Formation: Scientific and
Regulatory Implications.
B.J.Finlayson-Pitts and J.N.Pitts, Jr.
Air & Waste, v.43 p.1091-1100 (1993).
46. Trends in Auto Emissions and Gasoline Composition.
R.F.Sawyer
Environmental Health Perspectives Supplements. v.101 s.6 p.5 (1993)
47. Reference 6.
- Volume 9. Exhaust Control, Automotive.
48. Achieving Acceptable Air Quality: Some Reflections on Controlling
Vehicle Emissions.
J.G.Calvert, J.B.Heywood, R.F.Sawyer, J.H.Seinfeld
Science v261 p37-45 (1993).
49. Radiometric Determination of Platinum and Palladium attrition from
Automotive Catalysts.
R.F.Hill and W.J.Mayer.
IEEE Trans. Nucl. Sci., NS-24, p.2549-2554 (1977).
50. Determination of Platinum Emissions from a three-way
catalyst-equipped Gasoline Engine.
H.P.Konig, R.F.Hertel, W.Koch and G.Rosner.
Atmospheric Environment, v.26A p.741-745 (1992).
51. Alternative Automotive Fuels - SAE Information Report.
SAE J1297 Mar93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
52. Lean-burn Catalyst offers market boom.
New Scientist, 17 July 1993. p.20.
53. Catalysts in cars.
K.T.Taylor.
Chemtech, September 1990. p.551-555.
54. Advanced Batteries for electric vehicles.
G.L.Henriksen, W.H.DeLuca, D.R.Vissers.
Chemtech, November 1994. p.32-38.
55. The great battery barrier.
IEEE Spectrum, November 1992. p.97-101.
56. Exposure of the general Population to Gasoline.
G.G.Akland
Environmental Health Perspectives Supplements. v.101 s.6 p.27-32 (1993)
57. Court Ruling Spurs Continued Debate Over Gasoline Oxygenates.
G.Peaff.
Chemical & Engineering News, 26 September 1994. p.8-13.
58. The Application of Formaldehyde Emission Measurement to the
Calibration of Engines using Methanol as a Fuel.
P.Waring, D.C.Kappatos, M.Galvin, B.Hamilton, and A.Joe.
Sixth International Symposium on Alcohol Fuels.
- Proceedings, v.2 p.53-60 (1984).
59. Emissions from 200,000 vehicles: a remote sensing study.
P.L.Guenther, G.A.Bishop, J.E.Peterson, D.H.Stedman.
Sci. Total Environ., v.146/147 p.297-302 (1994)
60. Remote Sensing of Vehicle Exhaust Emissions.
S.H.Cadle and R.D.Stephens.
Environ. Sci. Technol., v.28 p.258A-264A. (1994)
61. Real-World Vehicle Emissions: A Summary of the Third Annual CRC-APRAC
On-Road Vehicle Emissions Workshop.
S.H.Cadle, R.A.Gorse, D.R.Lawson.
Air & Waste, v.43 p.1084-1090 (1993)
62. IR Long-Path Photometry: A Remote Sensing Tool for Automobile
Emissions.
G.A.Bishop, J.R.Starkey, A.Ihlenfeldt, W.J.Williams, and D.H.Stedman.
Analytical Chemistry, v.61 p.671A-677A (1989)
63. A Cost-Effectiveness Study of Carbon Monoxide Emissions Reduction
Utilising Remote Sensing.
G.A.Bishop, D.H.Stedman, J.E.Peterson, T.J.Hosick, and P.L.Guenther
Air & Waste, v.42 p.978-985 (1993)
64. A presentation to the California I/M Review Committee of results of
a 1991 pilot programme.
D.R.Lawson, J.A.Gunderson
29 January 1992.
65. Methods of Knock Rating. 15. Measurement of the Knocking
Characteristics of Automotive Fuels.
J.M.Campbell, T.A.Boyd.
The Science of Petroleum. Oxford Uni. Press. v.4 p.3057-3065 (1938).
66. Standard Test Method for Knock Characteristics of Motor and Aviation
Fuels by the Motor Method.
ASTM D 2700 - 92. IP236/83
Annual Book of ASTM Standards v.05.04 (1994).
67. Standard Test Method for Knock Characteristics of Motor Fuels by the
Research Method.
ASTM D 2699 - 92. IP237/69
Annual Book of ASTM Standards v.05.04 (1994).
68. Preparation of distillates for front end octane number ( RON 100C )
of motor gasoline
IP 325/82
Standard Methods for Analysis and Testing of Petroleum and Related
Products. Wiley. ISBN 0 471 94879 9 (1994).
69. Octane Enhancers.
D.Simanaitis and D.Kott.
Road & Track, April 1989. p.82,83,86-88.
70. Specification for Aviation Gasolines
ASTM D 910 - 93
Annual Book of ASTM Standards v.05.01 (1994).
71. Reference 1.
- Chapter 19. R.A.Vere
72. Automotive Sensors Improve Driving Performance.
L.M.Sheppard.
Ceramic Bulletin, v.71 p.905-913 (1992).
73. Reference 23.
- Chapter 7.
74. Investigation of Fire and Explosion Accidents in the Chemical, Mining
and Fuel-Related Industries - A Manual.
Joseph M. Kuchta.
US Dept. of the Interior. Bureau of Mines Bulletin 680 (1985).
75. Natural Gas as an Automobile Fuel, An Experimental study.
R.D.Fleming and J.R.Allsup.
US Dept. of the Interior. Bureau of Mines Report 7806 (1973).
76. Comparative Studies of Methane and Propane as Fuels for Spark Ignition
and Compression Ignition Engines.
G.A.Karim and I.Wierzba.
SAE Paper 831196. (198?).
77. The Outlook for Hydrogen.
N.S.Mayersohn.
Popular Science, October 1993. p.66-71,111.
78. Reference 6.
- Volume 11. Fuel Cells.
79. The Clean Machine.
R.H.Williams.
Technology Review, April 1994. p.21-30.
80. Hybrid car promises high performance and low emissions.
M. Valenti.
Mechanical Engineering, July 1994. p.46-49.
81. ?
Automotive Industries Magazine, December 1994.
82. Instrumental Methods of Analysis - 6th edition.
H.H.Willard, L.L.Merritt, J.A.Dean, F.A.Settle.
D.Van Nostrand. ISBN 0-442-24502-5 (1981).
83. Research into Asymmetric Membrane Hollow Filter Device for Oxygen-
Enriched Air Production.
A.Z.Gollan. M.H.Kleper.
Dept.of Energy Report DOE/ID/12429-1 (1985).
84. New Look at Oxygen Enrichment. I. The diesel engine.
H.C.Watson, E.E.Milkins, G.R.Rigby.
SAE Technical Paper 900344 (1990)
85. Thorpe's Dictionary of Applied Chemistry - 4th edition.
Longmans. 1949.
- Petroleum
86. Detonation Characteristics of Some Paraffin Hydrocarbons.
W.G.Lovell, J.M.Campbell, and T.A.Boyd.
Ind. Eng. Chem., v.23 p.26-29. (1931)
87. Secrets of Honda's horsepower heroics.
C. Csere.
Road & Track/Car & Driver?, May 1991. p.29.
1. Modern Petroleum Technology - any edition.
Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (5th=1984).
2. Hydrocarbon Fuels.
E.M.Goodger.
MacMillan. (1975)
3. Alternative Fuels
E.M.Goodger.
MacMillan. ISBN 0-333-25813-4 (1980)
4. Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
Editor, M.Howe-Grant.
Wiley. ISBN 0-471-52681-9 (1993)
- especially Alcohol Fuels, Gasoline and Other Motor Fuels, and
Fuel Cells chapters.
5. The Automotive Handbook. - any edition.
Bosch.
6. SAE Handbook, volume 1. - issued annually.
SAE. ISBN 1-56091-461-0 (1994).
- especially J312, and J1297.
7. Proceedings of the xxth International Symposium on Alcohol Fuels.
- Held every two years and most of the 10 conferences have lots of
good technical information, especially the earlier ones.
- various publishers.
8. Alternative Transportation Fuels - An Environmental and Energy
solution.
Editor, D.Sperling.
Quorum Books. ISBN 0-89930-407-9 (1989).
9. The Gasohol Handbook.
V. Daniel Hunt.
Industrial Press. ISBN 0-8311-1137-2 (1981).