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Subject: Gasoline FAQ - Part 3 of 4
This article was archived around: Thu, 15 Jan 2004 22:16:58 +1300
Last-modified: 17 November 1996
Subject: 6. What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression ratio
had been constrained by the unwanted "knock" that could rapidly destroy
engines. "Knocking" is a very good description of the sound heard from an
engine using fuel of too low octane. The engineers had blamed the "knock"
on the battery ignition system that was added to cars along with the
electric self-starter. The engine developers knew that they could improve
power and efficiency if knock could be overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding the exact
cause of knock . 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 . 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 1927 Graham Edgar suggested using two hydrocarbons that could be produced
in sufficient purity and quantity . 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 antiknock 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
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.
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 ).
6.2 Why do we need Octane Ratings?
To obtain the maximum energy from the gasoline, the compressed fuel-air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver an
optimum power stroke. In real life, a series of pre-flame reactions will
occur in the unburnt "end gases" in the combustion chamber before the flame
front arrives. If these reactions form molecules or species that can
autoignite before the flame front arrives, knock will occur [21,22].
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
6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives. Different reaction mechanisms, occurring at
various stages of the pre-flame compression stroke, are responsible for the
undesirable, easily-autoignitable, end gases.
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 [21,22].
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 compression 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. Antiknock 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 [21,22].
The antiknock ability is related to the "autoignition temperature" of the
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates have
lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other factors
that are far more important. ( such as compression ratio, stoichiometry,
combustion chamber shape, chemical structure of the fuel, presence of
antiknock additives, number and position of spark plugs, turbulence etc.)
Flame speed does not correlate with octane.
6.4 Why are two ratings used to obtain the pump rating?
The correct name for the (RON+MON)/2 formula is the "antiknock index",
and it remains the most important quality criteria for motorists .
The initial knock measurement methods developed in the 1920s resulted in a
diverse range of engine test methods and conditions, many of which have been
summarised by Campbell and Boyd . In 1928 the Co-operative Fuel Research
Committee formed a sub-committee to develop a uniform knock-testing
apparatus and procedure. They settled on a single-cylinder, valve-in-head,
water-cooled, variable compression engine of 3.5"bore and 4.5" stroke. The
knock indicator was the bouncing-pin type. They selected operating conditions
for evaluation that most closely match the current Research Method, however
correlation trials with road octanes in the early 1930s exhibited such large
discrepancies that conditions were changed ( higher engine speed, hot mixture
temperature, and defined spark advance profiles ), and a new tentative ASTM
Octane rating method was produced. This method is similar to the operating
conditions of the current Motor Octane procedure [12,103]. Over several
decades, a large number of alternative octane test methods appeared. These
were variations to either the engine design, or the specified operating
conditions . During the 1950-1960s attempts were made to internationally
standardise and reduce the number of Octane Rating test procedures.
During the late 1940s - 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 vehicle significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would have
been sensitive to the Research Octane of the 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 . I should note that the Research
method was only formally approved in 1947, but used unofficially from 1942.
6.5 What does the Motor Octane rating measure?
The conditions of the Motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).
Test Engine conditions Motor Octane
Test Method ASTM D2700-92 
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
6.6 What does the Research Octane rating measure?
The Research method settings represent typical mild driving, without
consistent heavy loads on the engine.
Test Engine conditions Research Octane
Test Method ASTM D2699-92 
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 )
6.7 Why is the difference called "sensitivity"?
RON - MON = Sensitivity. Because the two test methods use different test
conditions, especially the intake mixture temperatures and engine speeds,
then a fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels typically
have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is
recommended to have a 82+ MON, thus preventing very high sensitivity fuels
. Recent changes in European gasolines has caused concern, as high
sensitivity unleaded fuels have been found that fail to meet the 85 MON
requirement of the EN228 European gasoline specification .
6.8 What sort of engine is used to rate fuels?
Automotive octane ratings are determined in a special single-cylinder engine
with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative
Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the stroke is
114.3mm, giving a displacement of 612 cm3. The piston has four compression
rings, and one oil control ring. The intake valve is shrouded. The head and
cylinder are one piece, and can be moved up and down to obtain the desired
compression ratio. The engines have a special four-bowl carburettor that
can adjust individual bowl air-fuel ratios. This facilitates rapid switching
between reference fuels and samples. A magnetorestrictive detonation sensor
in the combustion chamber measures the rapid changes in combustion chamber
pressure caused by knock, and the amplified signal is measured on a
"knockmeter" with a 0-100 scale [104,105]. A complete Octane Rating engine
system costs about $200,000 with all the services installed. Only one
company manufactures these engines, the Waukesha Engine Division of Dresser
Industries, Waukesha. WI 53186.
6.9 How is the Octane rating determined?
To rate a fuel, the engine is set to an appropriate compression ratio that
will produce a knock of about 50 on the knockmeter for the sample when the
air-fuel ratio is adjusted on the carburettor bowl to obtain maximum knock.
Normal heptane and iso-octane are known as primary reference fuels. Two
blends of these are made, one that is one octane number above the expected
rating, and another that is one octane number below the expected rating.
These are placed in different bowls, and are also rated with each air-fuel
ratio being adjusted for maximum knock. The higher octane reference fuel
should produce a reading around 30-40, and the lower reference fuel should
produce a reading of 60-70. The sample is again tested, and if it does not
fit between the reference fuels, further reference fuels are prepared, and
the engine readjusted to obtain the required knock. The actual fuel rating
is interpolated from the knockmeter readings [104,105].
6.10 What is the Octane Distribution of the fuel?
The combination of vehicle and engine can result in specific requirements
for octane that depend on the fuel. If the octane is distributed differently
throughout the boiling range of a fuel, then engines can knock on one brand
of 87 (RON+MON)/2, but not on another brand. This "octane distribution" is
especially important when sudden changes in load occur, such as high load,
full throttle, acceleration. The fuel can segregate in the manifold, with
the very volatile fraction reaching the combustion chamber first and, if
that fraction is deficient in octane, then knock will occur until the less
volatile, higher octane fractions arrive [27,28].
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.
6.11 What is a "delta Research Octane number"?
To obtain an indication of behaviour of a gasoline during any manifold
segregation, an octane rating procedure called the Distribution Octane
Number was used. The rating engine had a special manifold that allowed
the heavier fractions to be separated before they reached the combustion
chamber . That method has been replaced by the "delta" RON procedure.
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 . The difference between these is the delta
RON(100C), usually just called the delta RON. The delta RON ratings are
not particularly relevant to engines with injectors, and are not used in
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of
octane is the chemical structure of the hydrocarbons and their response to
the addition of octane enhancing additives. Other factors include:-
Front End Volatility - Paraffins are the major component in gasoline, and
the octane number decreases with increasing chain length or ring size, but
increases with chain branching. Overall, the effect is a significant
reduction in octane if front end volatility is lost, as can happen with
improper or long term storage. Fuel economy on short trips can be improved
by using a more volatile fuel, at the risk of carburettor icing and
increased evaporative emissions.
Final Boiling Point.- Decreases in the final boiling point increase fuel
octane. Aviation gasolines have much lower final boiling points than
automotive gasolines. Note that final boiling points are being reduced
because the higher boiling fractions are responsible for disproportionate
quantities of pollutants and toxins.
Preignition tendency - both knock and preignition can induce each other.
6.13 Can higher octane fuels give me more power?
On modern engines with sophisticated engine management systems, the engine
can operate efficiently on fuels of a wider range of octane rating, but there
remains an optimum octane for the engine under specific driving conditions.
Older cars without such systems are more restricted in their choice of fuel,
as the engine can not automatically adjust to accommodate lower octane fuel.
Because knock is so destructive, owners of older cars must use fuel that will
not knock under the most demanding conditions they encounter, and must
continue to use that fuel, even if they only occasionally require the octane.
If you are already using the proper octane fuel, you will not obtain more
power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the management
system. Your driveability and fuel economy will remain the same. The higher
octane fuel costs more, so you are just throwing money away. If you are
already using a fuel with an octane rating slightly below the optimum, then
using a higher octane fuel will cause the engine management system to move to
the optimum settings, possibly resulting in both increased power and improved
fuel economy. You may be able to change octanes between seasons ( reduce
octane in winter ) to obtain the most cost-effective fuel without loss of
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.
6.14 Does low octane fuel increase engine wear?
Not if you are meeting the octane requirement of the engine. If you are not
meeting the octane requirement, the engine will rapidly suffer major damage
due to knock. You must not use fuels that produce sustained audible knock,
as engine damage will occur. If the octane is just sufficient, the engine
management system will move settings to a less optimal position, and the
only major penalty will be increased costs due to poor fuel economy.
Whenever possible, engines should be operated at the optimum position for
long-term reliability. Engine wear is mainly related to design,
manufacturing, maintenance and lubrication factors. Once the octane and
run-on requirements of the engine are satisfied, increased octane will have
no beneficial effect on the engine. Run-on is the tendency of an engine to
continue running after the ignition has been switched off, and is discussed
in more detail in Section 8.2. The quality of gasoline, and the additive
package used, would be more likely to affect the rate of engine wear, rather
than the octane rating.
6.15 Can I mix different octane fuel grades?
Yes, however attempts to blend in your fuel tank should be carefully
planned. You should not allow the tank to become empty, and then add 50% of
lower octane, followed by 50% of higher octane. The fuels may not completely
mix immediately, especially if there is a density difference. You may get a
slug of low octane that causes severe knock. You should refill when your
tank is half full. In general the octane response will be linear for most
hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher
octane are useless for most commercial gasolines. 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 some oxygenates can have negative lead responses. The octane
requirement of some engines is determined by the need to avoid run-on, not
to avoid knock.
6.16 What happens if I use the wrong octane fuel?
If you use a fuel with an octane rating below the requirement of the engine,
the management system may move the engine settings into an area of less
efficient combustion, resulting in reduced power and reduced fuel economy.
You will be losing both money and driveability. If you use a fuel with an
octane rating higher than what the engine can use, you are just wasting
money by paying for octane that you can not utilise. The additive packages
are matched to the engines using the fuel, for example intake valve deposit
control additive concentrations may be increased in the premium octane grade.
If your vehicle does not have a knock sensor, then using a fuel with an
octane rating significantly below the octane requirement of the engine means
that the little men with hammers will gleefully pummel your engine to pieces.
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.
6.17 Can I tune the engine to use another octane fuel?
In general, modern engine management systems will compensate for fuel octane,
and once you have satisfied the optimum octane requirement, you are at the
optimum overall performance area of the engine map. Tuning changes to obtain
more power will probably adversely affect both fuel economy and emissions.
Unless you have access to good diagnostic equipment that can ensure
regulatory limits are complied with, it is likely that adjustments may be
regarded as illegal tampering by your local regulation enforcers. If you are
skilled, you will be able to legally wring slightly more performance from
your engine by using a dynamometer in conjunction with engine and exhaust gas
analyzers and a well-designed, retrofitted, performance engine management
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not cost-effective
and a survey in 1989 showed the cost of increasing the octane rating of one
US gallon by one unit ranged from 10 cents ( methanol ), 50 cents (MMT),
$1.00 ( TEL ), to $3.25 ( xylenes ) . Refer to section 6.20 for a
discussion on naphthalene ( mothballs ). It is preferable to purchase a
higher octane fuel such as racing fuel, aviation gasolines, or methanol.
Sadly, the price of chemical grade methanol has almost doubled during 1994.
If you plan to use alcohol blends, ensure your fuel handling system is
compatible, and that you only use dry gasoline by filling up early in the
morning when the storage tanks are cool. Also ensure that the service station
storage tank has not been refilled recently. Retailers are supposed to wait
several hours before bringing a refilled tank online, to allow suspended
undissolved water to settle out, but they do not always wait the full period.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two numbers, with
common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is
the Aviation rating ( aka Lean Mixture rating ), and the second number is the
Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade,
100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to
replace the 80/87 and 100/130. Soon after the introduction, there was a
spate of plug fouling, and high cylinder head temperatures resulting in
cracked cylinder heads . The old 80/87 grade was reintroduced on a
limited scale. The Aviation Rating is determined using the automotive Motor
Octane test procedure, and then converted to an Aviation Number using a
table in the method. Aviation Numbers below 100 are Octane numbers, while
numbers above 100 are Performance numbers. There is usually only 1 - 2
Octane units different to the Motor value up to 100, but Performance numbers
varies significantly above that eg 110 MON = 128 Performance number.
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 .
The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.
6.20 Can mothballs increase octane?
The legend of mothballs as an octane enhancer arose well before WWII when
naphthalene was used as the active ingredient. Today, the majority of
mothballs use para-dichlorobenzene in place of naphthalene, so choose
carefully if you wish to experiment :-). There have been some concerns about
the toxicity of para-dichlorobenzene, and naphthalene mothballs have again
become popular. In the 1920s, typical gasoline octane ratings were 40-60
, and during the 1930s and 40s, the ratings increased by approximately 20
units as alkyl leads and improved refining processes became widespread .
Naphthalene has a blending motor octane number of 90 , so the addition of
a significant amount of mothballs could increase the octane, and they were
soluble in gasoline. The amount usually required to appreciably increase the
octane also had some adverse effects. The most obvious was due to the high
melting point ( 80C ), when the fuel evaporated the naphthalene would
precipitate out, blocking jets and filters. With modern gasolines,
naphthalene is more likely to reduce the octane rating, and the amount
required for low octane fuels will also create operational and emissions
Subject: 7. What parameters determine octane requirement?
7.1 What is the Octane Number Requirement of a Vehicle?
The actual octane requirement of a vehicle is called the Octane Number
Requirement (ONR), and is determined by using series of standard octane fuels
that can be blends of iso-octane and normal heptane ( primary reference ),
or commercial gasolines ( full-boiling reference ). In Europe, delta RON
(100C) fuels are also used, but seldom in the USA. The vehicle is tested
under a wide range of conditions and loads, using decreasing octane fuels
from each series until trace knock is detected. The conditions that require
maximum octane are not consistent, but often are full-throttle acceleration
from low starting speeds using the highest gear available. They can even be
at constant speed conditions, which are usually performed on chassis
dynamometers [27,28,111]. Engine management systems that adjust the octane
requirement may also reduce the power output on low octane fuel, resulting
in increased fuel consumption, and adaptive learning systems have to be
preconditioned prior to testing. The maximum ONR is of most interest, as that
usually defines the recommended fuel, however it is recognised that the
general public seldom drive as severely as the testers, and so may be
satisfied by a lower octane fuel .
7.2 What is the effect of Compression ratio?
Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation
Efficiency = 1 - (1/compression ratio)^gamma-1
where 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 for
gasoline fuels in an SI engine .
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 [27,38]:-
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.
7.3 What is the effect of changing the air-fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel
ratio, but was very engine specific. Modern engines, with engine management
systems, now have their maximum octane requirement near to 14.5:1. For a
given engine using gasoline, the relationship between thermal efficiency,
air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel
ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum
power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal
efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio
is controlled at part throttle by a closed loop system using the oxygen sensor
in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio
is used during full throttle operation to reduce knocking while providing
better driveability . An average increase of 2 (R+M)/2 ON is required
for each 1.0 increase (leaning) of the air-fuel ratio . If the mixture
is weakened, the flame speed is reduced, consequently less heat is converted
to mechanical energy, leaving heat in the cylinder walls and head,
potentially inducing knock. It is possible to weaken the mixture sufficiently
that the flame is still present when the inlet valve opens again, resulting
7.4 What is the effect of changing the ignition timing?
The tendency to knock increases as spark advance is increased. For an engine
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93
octane fuel, retarding the spark 4 degrees lowers the octane requirement to
91, whereas advancing it 8 degrees requires 96 octane fuel . It should
be noted this requirement depends on engine design. If you advance the spark,
the flame front starts earlier, and the end gases start forming earlier in
the cycle, providing more time for the autoigniting species to form before
the piston reaches the optimum position for power delivery, as determined by
the normal flame front propagation. It becomes a race between the flame front
and decomposition of the increasingly-squashed end gases. High octane fuels
produce end gases that take longer to autoignite, so the good flame front
reaches and consumes them properly.
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 BTDC .
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.
7.5 What is the effect of engine management systems?
Engine management systems are now an important part of the strategy to
reduce automotive pollution. The good news for the consumer is their ability
to maintain the efficiency of gasoline combustion, thus improving fuel
economy. The bad news is their tendency to hinder tuning for power. A very
basic modern engine system could monitor and control:- mass air flow, fuel
flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ), knock
( vibration sensor ), EGR, exhaust gas temperature, coolant temperature, and
intake air temperature. The knock sensor can be either a nonresonant type
installed in the engine block and capable of measuring a wide range of knock
vibrations ( 5-15 kHz ) with minimal change in frequency, or a resonant type
that has excellent signal-to-noise ratio between 1000 and 5000 rpm .
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  .
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.
7.6 What is the effect of temperature and load?
Increasing the engine temperature, particularly the air-fuel charge
temperature, increases the tendency to knock. The Sensitivity of a fuel can
indicate how it is affected by charge temperature variations. Increasing
load increases both the engine temperature, and the end-gas pressure, thus
the likelihood of knock increases as load increases. Increasing the water
jacket temperature from 71C to 82C, increases the (R+M)/2 ONR by two .
7.7 What is the effect of engine speed?.
Faster engine speed means there is less time for the pre-flame reactions
in the end gases to occur, thus reducing the tendency to knock. On engines
with management systems, the ignition timing may be advanced with engine
speed and load, to obtain optimum efficiency at incipient knock. In such
cases, both high and low engines speeds may be critical.
7.8 What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than the
same engine after 25,000 km. This Octane Requirement Increase (ORI) is due to
the formation of a mixture of organic and inorganic deposits resulting from
both the fuel and the lubricant. They reach an equilibrium amount because
of flaking, however dramatic changes in driving styles can also result in
dramatic changes of the equilibrium position. When the engine starts to burn
more oil, the octane requirement can increase again. ORIs up to 12 are not
uncommon, depending on driving style [27,28,32,111]. The deposits produce
the ORI by several mechanisms:-
- they reduce the combustion chamber volume, effectively increasing the
- they also reduce thermal conductivity, thus increasing the combustion
- they catalyse undesirable pre-flame reactions that produce end gases with
low autoignition temperatures.
7.9 What is the Road Octane Number of a Fuel?
The CFR 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 trace knock ignition timing
setting is read from the graph to provide the Road Octane Number. This is
a rapid procedure but provides minimal information, and cars with engine
management systems require sophisticated electronic equipment to adjust
the ignition timing .
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 by adjustment of the spark advance. 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, however the technique is almost impossible to perform
on vehicles with modern management systems .
The Road Octane Number lies between the MON and RON, and the difference
between the RON and the Road Octane number is called 'depreciation" .
Because nominally-identical new vehicle models display octane requirements
that can range over seven numbers, a large number of vehicles have to be
7.10 What is the effect of air temperature?
An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]. When the combined effects
of air temperature and humidity are considered, it is often possible to use
one octane grade in summer, and use a lower octane rating in winter. The
Motor octane rating has a higher charge temperature, and increasing charge
temperature increases the tendency to knock, so fuels with low Sensitivity
( the difference between RON and MON numbers ) are less affected by air
7.11 What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m . Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude . Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:-
- 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.
7.12 What is the effect of humidity?.
An increase of absolute humidity of 1.0 g water/kg of dry air lowers the
octane requirement of an engine by 0.25 - 0.32 MON [27,28,38].
7.13 What does water injection achieve?.
Water injection, as a separate liquid or emulsion with gasoline, or as a
vapour, has been thoroughly researched. If engines can calibrated to operate
with small amounts of water, knock can be suppressed, hydrocarbon emissions
will slightly increase, NOx emissions will decrease, CO does not change
significantly, and fuel and energy consumption are increased .
Water injection was used in WWII aviation engine to provide a large increase
in available power for very short periods. The injection of water does
decrease the dew point of the exhaust gases. This has potential corrosion
problems. The very high specific heat and heat of vaporisation of water
means that the combustion temperature will decrease. It has been shown that
a 10% water addition to methanol reduces the power and efficiency by about
3%, and doubles the unburnt fuel emissions, but does reduce NOx by 25% .
A decrease in combustion temperature will reduce the theoretical maximum
possible efficiency of an otto cycle engine that is operating correctly,
but may improve efficiency in engines that are experiencing abnormal
combustion on existing fuels.
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 .
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.
Subject: 8. How can I identify and cure other fuel-related problems?
8.1 What causes an empty fuel tank?
* You forgot to refill it.
* Your friendly neighbourhood thief "borrowed" the gasoline - the unfriendly
one took the vehicle.
* The fuel tank leaked.
* Your darling child/wife/husband/partner/mother/father used the car.
* The most likely reason is that your local garage switched to an oxygenated
gasoline, and the engine management system compensated for the oxygen
content, causing the fuel consumption to increase ( although the effect on
well tuned engines is only 2-4% ).
8.2 Is knock the only abnormal combustion problem?
No. Many of the abnormal combustion problems are induced by the same
conditions, and so one can lead to another.
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:-
There is no direct correlation between antiknock 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 [27,28].
Obviously, engines with fuel injection systems do not have the problem, and
idle speed is an important factor. Later model carburettors have an idle
stop solenoid which partially closes the throttle blades when the ignition
key was off, and thus ( if set correctly ) prevents run-on.
8.3 Can I prevent carburetter icing?
Yes, carburettor icing is caused by the combination of highly volatile fuel,
high humidity and low ambient temperature. The extent of cooling, caused by
the latent heat of the vaporised gasoline in the carburettor, can be as much
as 20C, perhaps dropping below the dew point of the charge. If this happens,
water will condense on the cooler carburettor surfaces, and will freeze if
the temperature is low enough. The fuel volatility can not always be reduced
to eliminate icing, so anti-icing additives are used. In the US, anti-icing
additives are seldom required because of the widespread use heated intake
air and fuel injection .
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, iso-propyl alcohol,
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 alcohol to a full
tank of dry gasoline will prevent icing under moderately-cold conditions.
If you believe there is a small amount of water in the fuel tank, add 500mls
of anhydrous isopropyl alcohol as the first treatment, and isopropyl
alcohol is also preferred for more severe conditions. Oxygenated gasolines
using alcohols can also be used. It's important to ensure the alcohol is
anhydrous, as some grades contain up to 30% water.
8.4 Should I store fuel to avoid the oxygenate season?
No. The fuel will be from a different season, and will have significantly
different volatility properties that may induce driveability problems. You
can tune your engine to perform on oxygenated gasoline as well as it did on
traditional gasoline, however you will have increased fuel consumption due
to the useless oxygen in the oxygenates. Some engines may not initially
perform well on some oxygenated fuels, usually because of the slightly
different volatility and combustion characteristics. A good mechanic should
be able to recover any lost performance or driveability, providing the engine
is in reasonable condition.
8.5 Can I improve fuel economy by using quality gasolines?
Yes, several manufacturers have demonstrated that their new gasoline additive
packages are more effective than traditional gasoline formulations. Texaco
claimed their new vapour-phase fuel additive can reduce existing deposits by
up to 30%, improve fuel economy, and reduce NOx tailpipe emissions by 15%,
when compared to other advanced liquid phase additives . The advertising
claims have been successfully disputed in court by Chevron - who demonstrated
that their existing fuel additive already offered similar benefits. Other
reputable gasoline manufacturers will have similar additive packages in their
premium quality gasolines . Quality gasolines, of whatever octane
ratings, will include a full range of gasoline additives designed to provide
consistent fuel quality.
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.
8.6 What is "stale" fuel, and should I use it?
"Stale" fuel is caused by improper storage, and usually smells sour. The
gasoline has been allowed to get warm, thus catalysing olefin decomposition
reactions, and perhaps also losing volatile material in unsealed containers.
Such fuel will tend to rapidly form gums, and will usually have a significant
reduction in octane rating. The fuel can be used by blending with twice the
volume of new gasoline, but the blended fuel should be used immediately,
otherwise teh old fuel will catalyse rapid decomposition of the new,
resulting in even larger quantities of stale fuel. Some stale fuels can drop
several octane numbers, so be generous with the dilution.
8.7 How can I remove water in the fuel tank?
If you only have a small quantity of water, then the addition of 500mls of
dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb the water,
and will not significantly affect combustion. Once you have mopped up the
water with IPA, small, regular doses of any anhydrous alcohol will help
keep the tank dry. This technique will not work if you have very large
amounts of water, and the addition of greater amounts of IPA may result in
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.
8.8 Can I used unleaded on older vehicles?
Yes, providing the octane is appropriate. There are some older engines that
cut the valve seats directly into the cylinder head ( eg BMC minis ). The
absence of lead, which lubricated the valve seat, causes the very hard
oxidation products of the exhaust valve to wear down the seat. This valve
seat recession is usually corrected by installing seat inserts, hardening
the seats, or use of specific valve seat recession protection additives
( such as Valvemaster ). Most other problems arise because the fuels have
different volatility, or the reduction of combustion chamber deposits.
These can usually be cured by reference to the vehicle manufacturer, who
will probably have a publication with the changes. Some vehicles will
perform as well on unleaded with a slightly lower octane than recommended
leaded fuel, due to the significant reduction in deposits from modern
unleaded gasolines. If premium unleaded petrol containing relatively
high levels of aromatics is used, some carburetted engines from the 1960s
may experience spark plug fouling, however most vehicle manufacturers
have guides to ensure careful engine tuning will eliminate most of the