For most of the second half of the 20th century, from WWII onward, gasoline has been the
least expensive fluid aside from water that the consumer could purchase. After the latest
round of petroleum price increases around $60 barrel this price comparison may not hold
true. It is true that gasoline has been one of the most abundant and low cost fluids that
most of us deal with. Gasoline is unquestionably the most important product from
petroleum. The market demand for gasoline in the United States is such that oil refiners
need to produce 45-50 barrels of gasoline from every 100 barrels of crude oil refined.
Historically this has not always been the case. Originally the most important product from
petroleum was kerosene. In the mid-19th century oil lamps illuminated many homes. The
most popular fuel for these lamps was whale oil. As whalers slaughtered more and more
whales and whale oil became less available, kerosene supplemented whale oil as a fuel
As the 20th century dawned, Henry Ford began to realize his dream of building an
inexpensive, reliable automobile that the average citizen could afford. Not only did the
coming of the automobile revolutionize American society, but it was fueled by gasoline, the
waste product of the oil industry. Kerosene remained the most important petroleum
product until about 1920. Then two separate factors converged. First, the increasing
availability of low cost automobiles made it easy for more people to own cars. This led to
more driving, which in turn increased the demand for gasoline. Second, the steady
expansion of electric power networks made electric lighting increasing available, and this
in turn reduced the demand for kerosene.
Because of these factors the roles reversed. Gasoline became the dominant petroleum
product with kerosene the ‗throw-away.‘ Before tears are shed about this crude out cast
understand that since the 1960‘s kerosene has made a comeback due to the wide
acceptance and proliferation of jet aircraft for civilian air transport (jet fuel is basically a
refined form of kerosene), but gasoline is still the main petroleum product by far.
Two issues dominate the production and use of gasoline: its performance in the engine,
and the yield (the amount produced) per barrel of crude oil. This paper will consider
engine combustion performance first, since the issue has a bearing on strategies used for
The area of interest is the sequence of events during the power stroke of engine operation.
Ignition of the gasoline/air mixture by the spark plug causes a flame to progress through
the mixture as combustion proceeds. The ignition (reaction) causes an immediate rise in
the temperature (and also pressure thanks to the work of Robert Boyle) of the mixture in
the cylinder. As the hot mixture of gases expands, it pushes down on the piston, doing the
WORK necessary to propel the vehicle. It is enlightening to note that as the mixture of
reacting gasoline and air and combustion products expands when the piston moves, its
temperature and pressure will necessarily drop. This is a classic example of a heat
WEC 04/06 IET 210
engine. In normal operation the hot reactants expand smoothly and deliver its chemical
energy to the pistons as WORK.
Under certain conditions of engine operation, the pressure and temperature of the fuel can
get so high that the remaining unburned fuel/air mixture explodes, rather than continuing to
burn smoothly. When the two flame fronts (one spark ignited, one auto ignited) collide, the
resulting shock wave is so violent that it is audible above normal engine noise. Consider
this fact; on an equal weight basis, a gasoline/air mixture is a more devastating explosive
than dynamite. This phenomenon is known in the vernacular as engine knock (detonation
to gear heads).
Engine knock is undesirable for a number of reasons. It reduces gas mileage; that is it
wastes gasoline. It reduces power and acceleration of the vehicle. It leads to considerable
wear and tear on engine parts (daylights piston crowns) and increased maintenance.
Engine knock became more and more of a problem beginning in the 1930‘s, as engine
designs evolved to higher compression ratios in the search for more power. Henry Fords‘
Model T from the 1920‘s had a compression ratio of about 4:1. Compression ratio has an
important effect on the combustion of the fuel and the resulting work done on the pistons.
A typical mid-sized car in today‘s market will have a compression ratio of about 8:1. And
high-performance cars may have ratios in the 10:1 range. Knock is affected by two
parameters: engine design and fuel quality.
The key feature of engine design is compression ratio. The higher the compression ratio
the higher will be the pressure inside the cylinder at the moment of ignition. But the higher
the initial pressure inside the cylinder at the instant the fuel ignites, the easier and more
likely it will be for the pressure to get high enough to cause the engine to knock. In other
words the higher the compression ratio, the more likely the engine is to knock with a given
type of fuel.
An important property of hydrocarbon liquids is their auto-ignition temperature, the
temperature at which liquid will ignite and burn with out a source of ignition. The auto-
ignition temperature of hydrocarbons is related to its molecular composition and structure.
Large straight chain hydrocarbon molecules have much lower auto-ignition temperatures
than do branched-chain and smaller molecules. Because it consists of mainly of smaller
molecules, gasoline has a fairly high auto-ignition temperature. This is the reason for
spark ignition systems in gasoline engines. It is entirely possible in modern gasoline
engines for temperatures and pressures to get high enough for auto-ignition to occur.
When this takes place the engine ‗pings‘ and under severe conditions ‗knock‘ occurs. Both
of these acoustical announcements are usually evidenced by looking at the spark plug.
Engines under going detonation will shortly deposit small round shiny balls of aluminum on
the center electrode of the spark plug. If not attended to immediately this condition will
enrich the local auto technician at your expense.
The octane number of gasoline is a measure of its ability to burn smoothly without
knocking. Straight-chain alkanes are less thermally stable and burn less smoothly than
WEC 04/06 IET 210
branched-chain alkanes. The ‗straight run‘ gasoline fraction obtained directly from
petroleum stock is a poor motor fuel and needs additional refinement because it contains
primarily straight-chain hydrocarbons that preignite too easily to be suitable for use as a
fuel in an internal combustion engine.
To investigate the relationship between gasoline composition and engine knock,
automotive engineers studied a variety of pure chemical compounds and their behavior in
engines under standardized test conditions.
To select a way of rating the propensity of a gasoline to cause knocking, a Cooperative
Fuel Research Committee was set up in 1927 comprising representatives of the American
Petroleum Institute, the American Manufacturers Assn., the National Bureau of Standards,
and the Society of Automotive Engineers. A single-cylinder engine with a variable
compression ratio had been built by John Campbell at General Motors. Graham Edgar at
the Ethyl Corporation prepared samples of various pure hydrocarbons. including normal
heptane distilled from the sap of the Jeffrey Pine. The engine enabled researchers to burn
mixtures of Edgar's pure hydrocarbons while varying the compression, to see at what point
In 1929, T. A. Boyd proposed to the committee that a variable-compression engine be the
basis for rating gasolines. Some committee members felt that such an engine would be
too complicated for routine use, but the Waukesha Engine Company volunteered to build a
prototype. By 1931 Waukesha was able to display its engine at a meeting of the American
Petroleum Institute; skeptics were persuaded and thousands of the engines were
subsequently built. (In fact, in 1980 the American Society of Mechanical Engineers
designated the engine an ―engineering landmark.‖)
It is possible if you wish to call up Waukesha Motors and order yourself an ASTM-CFR test
engine. Make sure you have about $250,000 available on your VISA before you order it.
This single-cylinder wonder has a four bowl carburetor and a movable cylinder head that
can vary the compression ratio between 4:1 to 18:1 while the engine is running (do not try
this at home).
In the committee‘s opinion, no one test was able to give a rating useful over the whole
range of operating conditions, and so two methods were defined: the Motor Method (ASTM
d 357) MON and the Research Method (ASTM d 908) RON. Both methods are based on
comparing the performance of the gasoline being tested with the performance of a mixture
of 2,2,4, trimethyl pentane (also erronously called iso-octane) and normal heptane. The
octane number is the percentage of iso-octane in that mixture whose performance (in
regard to knocking) is the same as that of the gasoline under test. For example, if the
performance of the gasoline under test is the same as that of a mixture of 80%
2,2,4,trimethyl pentane and 20% normal heptane, the gasoline is 80 octane. Octane
numbers above 100 are found by mathematical extrapolation.
The higher the octane number of a gasoline the lower its tendancy to knock. As a result,
increasing the octane number of the gasoline conteracts the effects of increasing the
WEC 04/06 IET 210
compression ratio of the engine. Increased compression ratios will deliver more power
from a given fuel load in an engine, as long as detonation ‗knock‘ is controled.
Presently there are three grades of gasoline sold at most service stations; 86-87 octane is
intended for most small and mid-sized cars (econo boxes); 89 octane is used in some mid-
size or larger cars; 92-94 octane is used in cars with large engines or in high performance
cars (Roush Mustangs, Saleen Corvettes).
Fuel is just one of many factors affecting whether an engine will knock. Consequently in
any particular engine gasolines with the same octane number but from different blenders
may perform differently: one may cause knock and the other may not. Similarly a gasoline
that causes knock in one engine model may not in another. This is not proof that the
octane rating was inaccurate.
Based on market demand, and on the need to meet engine performance requirements,
there are two key points regarding gasoline; it is desirable to have gasoline availble in the
86-94 octane range, and to produce 45-50 barrels of gasoline for every 100 barrels of
crude oil refined. The octane number of gasoline can be increased either by increasing
the percentage of branched chain and aromatic hydrocarbons or by adding so called
octane enhancers (or a combination of both).
Straight-Run Gasoline from Distillation
One way to think about, and classify, refinery processes is to consider all the kinds of
things that can be done with molecules. They can be separated without any chemical
change at all (that is, these could be callled physical processes, because they take
advantage of physical properties of molecules, and not their chemical properties). It is
possible to take small molecules and put them together to make bigger ones, so callled
buid-up processes. Also the inverse, taking large molecules and break them apart to
make smaller ones in breakdown processes. And, finally, it is possible to change the
molecular structure without changing its size (sometimes called change process).
Distillation is an example of a physical process. The gasoline that is produced from the
distillation of crude oil is called, straight-run gasoline. There are two problems with straight
run gasoline; first, even with the best Pennsylvania-crude quality oils it is probable to get
only 20 barrels of gasoline per 100 barrels of crude; with poor-quality crudes, the yield may
be less than 10 barrels of straight-run gasoline. Second the octane number of straight-run
gasoline is about 35 (not good enough for the Briggs lawnmower). As a result there is a
need to enhance both the yield of gasoline and its octane number to meet present-day
market demand and performance requirements.
The key to enhancing yield comes from the aproximate relationship between boiling point
and size of molecules. As long as substances compared are fairly similar chemically, it is
found that the higher the boiling point, the larger the size of the molecules, and vice versa.
So to increase the yield of gasoline two strategies can be used. Molecules of five or fewer
carbon atoms can be combined to build up molecules in the gasoline size range. Or, large
WEC 04/06 IET 210
molecules that have more than nine carbon atoms can be broken apart to make smaller
molecules that are in the gasoline-size range. Normally, many refineries do both.
Alkylation is the building up of small molecules of three or four atoms to products of six to
eight carbon atoms. Build up processes are used to form larger molecules of high octane
number and boiling in the gasoline range by combining smaller molecules. Small
molecules created as a by product of the thermal cracking process can be used in the
alkylation process to form compounds in the gasoline range with very high octane
numbers. The produce is called motor fuel alkylate.
In 1938, it was discovered that gasoline of a much higher octane than typical straight-run
gasolines could be produced by treating small hydrocarbon molecules with sulfuric acid.
This process calle sulfuric acid alkylation, provided most of the high octane aviation fuel
used during the Second World War and became, for a time, one of the most important
processes in the refinery.
Cracking is the process of breaking down molecules of more than nine carbon atoms to
the range of five to nine carbon atoms. Thermal cracking is now used in the United States
only on distillation residua (resids) to make solid coke and to increase yields of light
products. Unfortunately the products of thermal cracking tend to be in the 55-75 octane
number range, not good enough for today‘s engines.
As the 1930‘s drew to a close, it was apparent that the automobile engines were becoming
more powerful, of higher compression ratio, and demanded gasoline of higher octane
rating. By 1935 the octane rating had been boosted to 71 with corresponding increases in
The most widely used breakdown process today is catalytic cracking, which efficiently
converts gas oils into high-octane gasoline and other lighter products. Gas oils are
petroleum fractions that boil in the range fo about 450—800°F, such as diesel fuel, heating
oils,and fuel oils. Around the time of the Second World War Eugene Houdry discovered
that if cracking was done in the presence of clay minerals, not only did the large molecules
break apart, but also the products were converted to branched paraffins, naphthenes, or
aromatics. That is, both the yield and the octane number go up. The clay does not enter
into the cracking reaction; it serves only to speed up the reaction and to enhance the
octane number of the product.
The first commercial catalytic cracking plant went into operation in 1936 using bentonite
clay as the catalyst. By the time the Second World War broke out there were 12 plants in
the United States, providing 132,000 barrels per day of high-octane gasoline. By the end
of the war, some 34 plants were in operation with a capacity of 500,000 barrels per day.
WEC 04/06 IET 210
Catalytic cracking enables the production of 40-50 barrels of gasoline per 100 barrels of
crude with octane numbers over 90. Some historians of technology consider the
development of catalytic cracking to be the greatest triumph of chemical engineering.
The octane number of gasoline can also be increased by adding special chemicals called
antiknock agents or (gasp) octane enhancers. In 1921 Thomas Midgely discovered that
tetraethyllead was an outstanding antiknock agent for low grade fuels. It became the most
widely used antiknock agent in the United States, up to 1975. The additio of three grams
of tetraethyllead per gallon of gasoline increase the octane number by 10-15. From 1925
to 1975, both regular and premium grades of gasoline contained an average of three
grams of tetraethyllead per gallon. This product was called ‗leaded gasoline,‘ although it
most certainly did not contain metallic lead, as the name might imply. In some cases this
fuel was called ‗ethyl‘ gasoline. Eventually ‗lead‘ was removed from gasoline due to the
concern that ‗lead‘ could poison or foul the catalysts used in catalytic converters installed
on car exhaust systems to reduce air pollution. Without lead, less expensive gasoline
blends with low octane ratings did not burn and perform efficiently. This problem was
partially alleviated by engine redesign (read low compression, smog motors) and partially
blending higer octane components (such as benzene and toluene) into the fuel, but both
approaches increased gasoline prices significantly.
The exhaust emissions of automobile engines contains, CO, oxides of nitrogen and
unburned (non reacted) hydrocarbons, all of which contribute to air pollution. As city
(urban) air pollution worsend in the 1950s and 1960s, Congress eventually passed the
Clean Act of 1970, which, among many things, required that 1975-model-year cars emit no
more than 10% of the carbon monoxide and hydrocarbons emitted by 1970 models. The
solution to lowering these emissions was a platimum-based catalytic converter. The only
major problem other than cost was that it required a lead-free gasolines, since lead
deactivates the platinum catalyst by coating its surface. As a result, automobiles
manufactured since 1975 have been required to use lead-free gasolines to protect the
catalytic converter. It is now virtually impossible to purchase leaded gasoline at filling
stations in the United States.
Because tetraethyllead can no longer be used in the U.S. and a few other countries, other
octane enhancers are now added to gasoline. These include toluene, tertiary butyl
alcohol, methyl tertiary butyl ether (MTBE), methanol, and ethanol. The most popular
octane enhancer has been MTBE. Unfortunately, MTBE is not only a good octane
enhancer, it is also fairly soluble in water. In recent years, there has been rapidly growing
concer about gasoline containing MTBE leaking out of underground storage tanks (at
service stations, for example) and eventually mixing with ground water. Although gasoline
does not disolve in water, the MTBE will. This leads to contamination of ground water,
and, potentially, of drinking water. A move is afoot in many locations to ban the use of
MTBE in gasolines.
WEC 04/06 IET 210
To build a modern 200,000 barrel-per-day refinery today from the ground up would cost
well over a billion dollars. This 200,000 barrel-per-day plant would produce about 100,000
barrels of gasoline, 5,200 barrels of liquefied petroleum gas, 12,000 barrels of jet fuel,
7,000 barrels of kerosene, nearly 42,000 barrels of heating oil and diesel fuel, and coke,
asphalt, lubricating oil stocks, and even 170 tons of sulfur.
The 1990 Clean Air Act Amendments require cities with excessive carbon monoxide
pollution to use oxygenated gasolines during winter. Oxygenated gasolines are blends of
gasoline with organic compounds that contain oxygen, such as MTBE, methanol and
ethanol. Oxygenated gasolines burn more completely than nonoxygenated gasoline and
can reduce carbon monoxide emissions in urban areas by up to 17%. The 1990
regulations also require oxygenated gasolines to contain 2.7% oxygen by weight. The use
of oxygenated gasoline is currently mandated in about 40 cities in the United States.
Although these oxygenated compounds have high octane numbers, a high octane number
relates only to a tendancy for engine knocking. An octane number does not relate directly
to the energy content–in Btu per pound or Btu per gallon—of a fuel. In general, all
oxygenated additives contain less energy than gasoline because they are already partially
oxydized. In a practical sense, this means that there is less knocking in the engine but
less gas ‗mileage‘—the number of miles traveled per gallon of fuel used. Despite this
drawback, because ethanol and methanol can be made from biomass or other renewable
sources, they are considered to be good liquid fuel options by advocates of recucing our
dependence on fossil fuels.
Although ethanol and methanol add oxygen to gasoline, they are not totallynon-polluting
fuels. Both can produce toxic aldehydes on incomplete oxidation and, like all carbon
containing fuels they produce CO2. Consumers sometimes express one other complaint;
additives, such as ethanol, methanol, and MTBE increase the cost of gasoline without
increasing the energy it provides, (oxygenates actually decrease the energy provided per
WEC 04/06 IET 210
Gasoline Gallon Equivalent (GGE) Table
Fuel Type Unit of BTUs Gallon
Measure Per Unit Equivalent
Gasoline, regular unleaded, gallon 114,100 1.00 gallon
Gasoline, RFG, (10% MBTE) gallon 112,000 1.02
Diesel, (typical) gallon 129,800 0.88
Liquid natural gas (LNG), gallon 75,000 1.52
Compressed natural gas (CNG), cubic foot 900 126.67 cu.
Liquefied petroleum gas (LPG or gallon 84,300 1.35
Methanol (M-100) gallon 56,800 2.01
Methanol (M-85) gallon 65,400 1.74
Ethanol (M-100) gallon 76,100 1.50
Ethanol (E-85) gallon 81,800 1.40
Bio Diesel (B-20) gallon 129,500 0.88
Electricity kilowatt 3,400 33.53
WEC 04/06 IET 210