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Far more than anybody ever needed to know about Petrol, Gasoline,
/ Petroleum / Gas / Petrol FAQ
More than you ever
needed to know about fuels, detonation, pre-ignition
You DO NOT NEED TO
READ THIS! At least not to build Nitrous System.
Taken from a newsgroup a few years ago by a guy that seems to
understand far too much about Fuel... He should get a life - I found it most
interesting but I have far too much time. With permission.
Since 1912 the spark ignition internal combustion engine's
had been constrained by the unwanted "knock" that could
engines. "Knocking" is a very good description of the sound
heard from an
engine using fuel of too low octane.
The engineers had blamed
on the battery ignition system that was added to cars along
electric self-starter. The engine developers knew that they
power and efficiency if knock could be overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding
cause of knock . They used a Dobbie-McInnes manograph to demonstrate
that the knock did not arise from preignition, as was commonly
arose from a violent pressure rise after ignition.
The manograph was not
suitable for further research, so Midgley and Boyd developed a
camera to see what was happening. They also developed a
"bouncing pin" indicator that measured the amount of knock .
alternative concept of HUCF ( Highest Useful Compression Ratio
) using a
variable-compression engine. His numbers were not absolute, as
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
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
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 volume heptane:iso-octane.
The reason for using normal heptane and iso-octane was because
have similar volatility properties, specifically boiling
point, thus the
varying ratios 0:100 to 100:0 should not exhibit large
volatility that could affect the rating test.
Melting Point Boiling Point Density
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
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
mixture inside the combustion chamber needs to burn evenly,
from the spark plug until all the fuel is consumed. This would
optimum power stroke. In real life, a series of pre-flame reactions
occur in the unburnt "end gases" in the combustion chamber before
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
unburnt end gases to resist spontaneous autoignition under the
conditions used. If autoignition occurs, it results in an extremely
pressure rise, as both the desired spark-initiated flame front, and
undesired autoignited end gas flames are expanding. The combined
peak arrives slightly ahead of the normal operating pressure peak,
to a loss of power and eventual overheating. The end gas pressure
superimposed on the main pressure wave, leading to a sawtooth
pressure oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce
failure in a few minutes. Knock and preignition are both favoured by
temperatures, so one may lead to the other. Under high-speed
knock can lead to preignition, which then accelerates engine
6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the
end gases to spontaneously ignite under the specified test
Within the chemical structure of the fuel is the ability to
pre-flame conditions without decomposing into species that will
before the flame-front arrives. Different reaction mechanisms,
various stages of the pre-flame compression stroke, are responsible
undesirable, easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are
one at a time from the molecule by reactions with small radical
(such as OH and HO2), and O and H atoms. The strength of
bonds depends on what the carbon is connected to. Straight chain HCs
normal heptane have secondary C-H bonds that are significantly
the primary C-H bonds present in branched chain HCs like iso-octane
The octane rating of hydrocarbons is determined by the structure of
molecule, with long, straight hydrocarbon chains producing large
easily-autoignitable pre-flame decomposition species, while branched
aromatic hydrocarbons are more resistant. This also explains why the
ratings of paraffins consistently decrease with carbon number. In
the unburnt "end gases" ahead of the flame front encounter
to about 700C due to piston motion and radiant and conductive
commence a series of pre-flame reactions. These reactions occur at
thermal stages, with the initial stage ( below 400C ) commencing
addition of molecular oxygen to alkyl radicals, followed by the
transfer of hydrogen atoms within the new radical to form an
oxygen-containing species. These new species are susceptible to
branching involving the HO2 radical during the intermediate
stage (400-600C), mainly through the production of OH radicals.
the most important reaction that produces chain branching is the
one hydrogen atom radical with molecular oxygen to form O and OH
The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Antiknock
work by interfering at different points in the pre-flame reactions,
the oxygenates retarding undesirable low temperature reactions, and
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 "auto ignition temperature"
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates
lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this
evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other
that are far more important. ( such as compression ratio,
combustion chamber shape, chemical structure of the fuel,
antiknock additives, number and position of spark plugs,
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
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
summarised by Campbell and Boyd . In 1928 the Co-operative Fuel
Committee formed a sub-committee to develop a uniform knock-testing
apparatus and procedure. They settled on a single-cylinder,
water-cooled, variable compression engine of 3.5"bore and 4.5"
knock indicator was the bouncing-pin type. They selected operating
for evaluation that most closely match the current Research Method,
correlation trials with road octanes in the early 1930s exhibited
discrepancies that conditions were changed ( higher engine speed,
temperature, and defined spark advance profiles ), and a new
Octane rating method was produced. This method is similar to the
conditions of the current Motor Octane procedure [12,103]. Over
decades, a large number of alternative octane test methods appeared.
were variations to either the engine design, or the specified
conditions . During the 1950-1960s attempts were made to
standardise and reduce the number of Octane Rating test procedures.
During the late 1940s - mid 1960s, the Research method became the
rating because it more closely represented the octane requirements
motorist using the fuels/vehicles/roads then available. In the late
German automakers discovered their engines were destroying
long Autobahn runs, even though the Research Octane was within
They discovered that either the MON or the Sensitivity ( the
difference between the RON and MON numbers ) also had to be
it is accepted that no one octane rating covers all use. In fact,
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
requirement for both RON and MON. In the 1930s, most vehicles would
been sensitive to the Research Octane of the fuel, almost regardless
Motor Octane, whereas most 1990s engines have a 'severity" of one,
means the engine is unlikely to knock if a changes of one RON is
an equal and opposite change of MON . I should note that the
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
high load driving. For most hydrocarbon fuels, including those with
lead or oxygenates, the motor octane number (MON) will be lower than
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
( 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
conditions, especially the intake mixture temperatures and engine
then a fuel that is sensitive to changes in operating conditions
a larger difference between the two rating methods. Modern fuels
have sensitivities around 10. The US 87 (RON+MON)/2 unleaded
recommended to have a 82+ MON, thus preventing very high sensitivity
. Recent changes in European gasolines has caused concern, as
sensitivity unleaded fuels have been found that fail to meet the 85
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
with a variable compression ratio ( CR 4:1 to 18:1 ) known as a
Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the
114.3mm, giving a displacement of 612 cm3. The piston has four
rings, and one oil control ring. The intake valve is shrouded. The
cylinder are one piece, and can be moved up and down to obtain the
compression ratio. The engines have a special four-bowl carburettor
can adjust individual bowl air-fuel ratios. This facilitates rapid
between reference fuels and samples. A magnetorestrictive detonation
in the combustion chamber measures the rapid changes in combustion
pressure caused by knock, and the amplified signal is measured on a
"knockmeter" with a 0-100 scale [104,105]. A complete Octane Rating
system costs about $200,000 with all the services installed. Only
company manufactures these engines, the Waukesha Engine Division of
Industries, Waukesha. WI 53186.
6.9 How is the Octane rating determined?
To rate a fuel, the engine is set to an appropriate compression
will produce a knock of about 50 on the knockmeter for the sample
air-fuel ratio is adjusted on the carburettor bowl to obtain maximum
Normal heptane and iso-octane are known as primary reference fuels.
blends of these are made, one that is one octane number above the
rating, and another that is one octane number below the expected
These are placed in different bowls, and are also rated with each
ratio being adjusted for maximum knock. The higher octane reference
should produce a reading around 30-40, and the lower reference fuel
produce a reading of 60-70. The sample is again tested, and if it
fit between the reference fuels, further reference fuels are
the engine readjusted to obtain the required knock. The actual fuel
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
for octane that depend on the fuel. If the octane is distributed
throughout the boiling range of a fuel, then engines can knock on
of 87 (RON+MON)/2, but not on another brand. This "octane
especially important when sudden changes in load occur, such as high
full throttle, acceleration. The fuel can segregate in the manifold,
the very volatile fraction reaching the combustion chamber first
that fraction is deficient in octane, then knock will occur until
volatile, higher octane fractions arrive [27,28].
Some fuel specifications include delta RONs, to ensure octane
throughout the fuel boiling range was consistent. Octane
seldom a problem with the alkyl lead compounds, as the tetra methyl
and tetra ethyl lead octane volatility profiles were well
it can be a major problem for the new, reformulated, low aromatic
as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have
that an 87 (RON+MON)/2 from one brand has to be substituted with an
(RON+MON)/2 of another, and that is because of the combination of
driving style, engine design, vehicle mass, fuel octane
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
segregation, an octane rating procedure called the Distribution
Number was used. The rating engine had a special manifold that
the heavier fractions to be separated before they reached the
chamber . That method has been replaced by the "delta" RON
The fuel is carefully distilled to obtain a distillate fraction that
to the specified temperature, which is usually 100C. Both the parent
and the distillate fraction are rated on the octane engine using the
Research Octane method . The difference between these is the
RON(100C), usually just called the delta RON. The delta RON ratings
not particularly relevant to engines with injectors, and are not
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant
octane is the chemical structure of the hydrocarbons and their
the addition of octane enhancing additives. Other factors include:-
Front End Volatility - Paraffins are the major component in
the octane number decreases with increasing chain length or ring
increases with chain branching. Overall, the effect is a
reduction in octane if front end volatility is lost, as can happen
improper or long term storage. Fuel economy on short trips can be
by using a more volatile fuel, at the risk of carburettor icing
increased evaporative emissions.
Final Boiling Point.- Decreases in the final boiling point increase
octane. Aviation gasolines have much lower final boiling points
automotive gasolines. Note that final boiling points are being
because the higher boiling fractions are responsible for
quantities of pollutants and toxins.
Preignition tendency - both knock and preignition can induce each
6.13 Can higher octane fuels give me more power?
On modern engines with sophisticated engine management systems, the
can operate efficiently on fuels of a wider range of octane rating,
remains an optimum octane for the engine under specific driving
Older cars without such systems are more restricted in their choice
as the engine can not automatically adjust to accommodate lower
Because knock is so destructive, owners of older cars must use fuel
not knock under the most demanding conditions they encounter, and
continue to use that fuel, even if they only occasionally require
If you are already using the proper octane fuel, you will not obtain
power from higher octane fuels. The engine will be already operating
optimum settings, and a higher octane should have no effect on the
system. Your driveability and fuel economy will remain the same. The
octane fuel costs more, so you are just throwing money away. If you
already using a fuel with an octane rating slightly below the
using a higher octane fuel will cause the engine management system
to move to
the optimum settings, possibly resulting in both increased power and
fuel economy. You may be able to change octanes between seasons (
octane in winter ) to obtain the most cost-effective fuel without
Once you have identified the fuel that keeps the engine at optimum
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
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
meeting the octane requirement, the engine will rapidly suffer major
due to knock. You must not use fuels that produce sustained audible
as engine damage will occur. If the octane is just sufficient, the
management system will move settings to a less optimal position, and
only major penalty will be increased costs due to poor fuel economy.
Whenever possible, engines should be operated at the optimum
long-term reliability. Engine wear is mainly related to design,
manufacturing, maintenance and lubrication factors. Once the octane
run-on requirements of the engine are satisfied, increased octane
no beneficial effect on the engine. Run-on is the tendency of an
continue running after the ignition has been switched off, and is
in more detail in Section 8.2. The quality of gasoline, and the
package used, would be more likely to affect the rate of engine
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
lower octane, followed by 50% of higher octane. The fuels may not
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
tank is half full. In general the octane response will be linear for
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
octane are useless for most commercial gasolines. The lead response
unleaded fuel does not overcome the dilution effect, thus 50:50 of
and 91 unleaded will give 94. Some blends of oxygenated fuels with
gasoline can result in undesirable increases in volatility due to
azeotropes, and some oxygenates can have negative lead responses.
requirement of some engines is determined by the need to avoid
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
the management system may move the engine settings into an area of
efficient combustion, resulting in reduced power and reduced fuel
You will be losing both money and driveability. If you use a fuel
octane rating higher than what the engine can use, you are just
money by paying for octane that you can not utilise. The additive
are matched to the engines using the fuel, for example intake valve
control additive concentrations may be increased in the premium
If your vehicle does not have a knock sensor, then using a fuel with
octane rating significantly below the octane requirement of the
that the little men with hammers will gleefully pummel your engine
You should initially be guided by the vehicle manufacturer's
however you can experiment, as the variations in vehicle tolerances
mean that Octane Number Requirement for a given vehicle model can
over 6 Octane Numbers. Caution should be used, and remember to
if the conditions change, such as carrying more people or driving in
different ambient conditions. You can often reduce the octane of the
you use in winter because the temperature decrease and possible
changes may significantly reduce the octane requirement of the
Use the octane that provides cost-effective driveability and
using anything more is waste of money, and anything less could
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
and once you have satisfied the optimum octane requirement, you are
optimum overall performance area of the engine map. Tuning changes
more power will probably adversely affect both fuel economy and
Unless you have access to good diagnostic equipment that can ensure
regulatory limits are complied with, it is likely that adjustments
regarded as illegal tampering by your local regulation enforcers. If
skilled, you will be able to legally wring slightly more performance
your engine by using a dynamometer in conjunction with engine and
analyzers and a well-designed, retrofitted, performance engine
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not
and a survey in 1989 showed the cost of increasing the octane rating
US gallon by one unit ranged from 10 cents ( methanol ), 50 cents
$1.00 ( TEL ), to $3.25 ( xylenes ) . Refer to section 6.20 for
discussion on naphthalene ( mothballs ). It is preferable to
higher octane fuel such as racing fuel, aviation gasolines, or
Sadly, the price of chemical grade methanol has almost doubled
If you plan to use alcohol blends, ensure your fuel handling system
compatible, and that you only use dry gasoline by filling up early
morning when the storage tanks are cool. Also ensure that the
storage tank has not been refilled recently. Retailers are supposed
several hours before bringing a refilled tank online, to allow
undissolved water to settle out, but they do not always wait the
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two
common grades being 80/87, 100/130, and 115/145 [109,110]. The first
the Aviation rating ( aka Lean Mixture rating ), and the second
number is the
Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new
100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was
replace the 80/87 and 100/130. Soon after the introduction, there
spate of plug fouling, and high cylinder head temperatures resulting
cracked cylinder heads . The old 80/87 grade was reintroduced
limited scale. The Aviation Rating is determined using the
Octane test procedure, and then converted to an Aviation Number
table in the method. Aviation Numbers below 100 are Octane numbers,
numbers above 100 are Performance numbers. There is usually only 1 -
Octane units different to the Motor value up to 100, but Performance
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
compression ratio. The method determines the dependence of the
permissible power ( in terms of indicated mean effective pressure )
mixture strength and boost for a specific light knocking setting.
Performance Number indicates the maximum knock-free power obtainable
fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that
designed to utilise the fuel can obtain 150% of the knock-limited
iso-octane at the same mixture ratio. This is an arbitrary scale
iso-octane + varying amounts of TEL, derived from a survey of
performed decades ago. Aviation gasoline PNs are rated using
mixture strength to obtain the maximum knock-limited power in a
engine. This can be extended to provide mixture response curves
the maximum boost ( rich - about 11:1 stoichiometry ) and minimum
( weak about 16:1 stoichiometry ) before knock .
The 115/145 grade is being phased out, but even the 100LL has more
than any automotive gasoline.
6.20 Can mothballs increase octane?
The legend of mothballs as an octane enhancer arose well before WWII
naphthalene was used as the active ingredient. Today, the majority
mothballs use para-dichlorobenzene in place of naphthalene, so
carefully if you wish to experiment :-). There have been some
the toxicity of para-dichlorobenzene, and naphthalene mothballs have
become popular. In the 1920s, typical gasoline octane ratings were
, and during the 1930s and 40s, the ratings increased by
units as alkyl leads and improved refining processes became
Naphthalene has a blending motor octane number of 90 , so the
a significant amount of mothballs could increase the octane, and
soluble in gasoline. The amount usually required to appreciably
octane also had some adverse effects. The most obvious was due to
melting point ( 80C ), when the fuel evaporated the naphthalene
precipitate out, blocking jets and filters. With modern gasolines,
naphthalene is more likely to reduce the octane rating, and the
required for low octane fuels will also create operational and
determine octane requirement?
What is the Octane Number Requirement of a Vehicle?
The actual octane requirement of a vehicle is called the Octane
Requirement (ONR), and is determined by using series of standard
that can be blends of iso-octane and normal heptane ( primary
or commercial gasolines ( full-boiling reference ). In Europe, delta
(100C) fuels are also used, but seldom in the USA. The vehicle is
under a wide range of conditions and loads, using decreasing octane
from each series until trace knock is detected. The conditions that
maximum octane are not consistent, but often are full-throttle
from low starting speeds using the highest gear available. They can
at constant speed conditions, which are usually performed on chassis
dynamometers [27,28,111]. Engine management systems that adjust the
requirement may also reduce the power output on low octane fuel,
in increased fuel consumption, and adaptive learning systems have to
preconditioned prior to testing. The maximum ONR is of most
interest, as that
usually defines the recommended fuel, however it is recognised that
general public seldom drive as severely as the testers, and so may
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
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency
engine according to the standard equation
Efficiency = 1 - (1/compression ratio)^gamma-1
where gamma = ratio of specific heats at constant pressure and
volume of the working fluid ( for most purposes air is the working
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1
The efficiency gains are best when the engine is at incipient knock,
why knock sensors ( actually vibration sensors ) are used. Low
ratio engines are less efficient because they can not deliver as
much of the
ideal combustion power to the flywheel. For a typical carburetted
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
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and
7.3 What is the effect of changing the air-fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1
ratio, but was very engine specific. Modern engines, with engine
systems, now have their maximum octane requirement near to 14.5:1.
given engine using gasoline, the relationship between thermal
air-fuel ratio, and power is complex. Stoichiometric combustion (
ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither
power - which occurs around air-fuel 12-13:1 (Rich), nor maximum
efficiency - which occurs around air-fuel 16-18:1 (Lean). The
is controlled at part throttle by a closed loop system using the
in the exhaust. Conventionally, enrichment for maximum power
is used during full throttle operation to reduce knocking while
better driveability . An average increase of 2 (R+M)/2 ON is
for each 1.0 increase (leaning) of the air-fuel ratio . If the
is weakened, the flame speed is reduced, consequently less heat is
to mechanical energy, leaving heat in the cylinder walls and head,
potentially inducing knock. It is possible to weaken the mixture
that the flame is still present when the inlet valve opens again,
7.4 What is the effect of changing the ignition timing
The tendency to knock increases as spark advance is increased. For
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing
octane fuel, retarding the spark 4 degrees lowers the octane
91, whereas advancing it 8 degrees requires 96 octane fuel . It
be noted this requirement depends on engine design. If you advance
the flame front starts earlier, and the end gases start forming
the cycle, providing more time for the autoigniting species to form
the piston reaches the optimum position for power delivery, as
the normal flame front propagation. It becomes a race between the
and decomposition of the increasingly-squashed end gases. High
produce end gases that take longer to autoignite, so the good flame
reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine
intended to use. The timing of the spark is advanced sufficiently to
that the fuel-air mixture burns in such a way that maximum pressure
burning charge is about 15-20 degree after TDC. Knock will occur
this point, usually in the late compression - early power stroke
The engine management system uses ignition timing as one of the
variables that is adjusted if knock is detected. If very low octane
are used ( several octane numbers below the vehicle's requirement at
settings ), both performance and fuel economy will decrease.
The actual Octane Number Requirement depends on the engine design,
some 1978 vehicles using standard fuels, the following (R+M)/2
Requirements were measured. "Standard" is the recommended ignition
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
The actual ignition timing to achieve the maximum pressure from
combustion of gasoline will depend mainly on the speed of the engine
flame propagation rates in the engine. Knock increases the rate of
pressure rise, thus superimposing additional pressure on the normal
combustion pressure rise. The knock actually rapidly resonates
chamber, creating a series of abnormal sharp spikes on the pressure
The normal flame speed is fairly consistent for most gasoline HCs,
of octane rating, but the flame speed is affected by stoichiometry.
the flame speeds in this FAQ are not the actual engine flame speeds.
CR gasoline engine at 1500 rpm would have a flame speed of about
and a similar hydrogen engine yields 48.3 m/s, but such engine flame
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
reduce automotive pollution. The good news for the consumer is their
to maintain the efficiency of gasoline combustion, thus improving
economy. The bad news is their tendency to hinder tuning for power.
basic modern engine system could monitor and control:- mass air
flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ),
( vibration sensor ), EGR, exhaust gas temperature, coolant
intake air temperature. The knock sensor can be either a nonresonant
installed in the engine block and capable of measuring a wide range
vibrations ( 5-15 kHz ) with minimal change in frequency, or a
that has excellent signal-to-noise ratio between 1000 and 5000 rpm
A modern engine management system can compensate for altitude,
temperature, and fuel octane. The management system will also
start settings, and other operational parameters. There is a new
that the engine management system also contain an on-board
function that warns of malfunctions such as engine misfire, exhaust
failure, and evaporative emissions failure. The use of fuels with
such as methanol can confuse the engine management system as they
more hydrogen which can fool the oxygen sensor  .
The use of fuel of too low octane can actually result in both a loss
economy and power, as the management system may have to move the
settings to a less efficient part of the performance map. The system
the ignition timing until only trace knock is detected, as engine
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
indicate how it is affected by charge temperature variations.
load increases both the engine temperature, and the end-gas
the likelihood of knock increases as load increases. Increasing the
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
in the end gases to occur, thus reducing the tendency to knock. On
with management systems, the ignition timing may be advanced with
speed and load, to obtain optimum efficiency at incipient knock. In
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
same engine after 25,000 km. This Octane Requirement Increase (ORI)
is due to
the formation of a mixture of organic and inorganic deposits
both the fuel and the lubricant. They reach an equilibrium amount
of flaking, however dramatic changes in driving styles can also
dramatic changes of the equilibrium position. When the engine starts
more oil, the octane requirement can increase again. ORIs up to 12
uncommon, depending on driving style [27,28,32,111]. The deposits
the ORI by several mechanisms:-
- they reduce the combustion chamber volume, effectively increasing
- they also reduce thermal conductivity, thus increasing the
- they catalyse undesirable pre-flame reactions that produce end
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
consequently there are standard procedures for evaluating the
of the gasoline in an engine. The most common are:-
1. The Modified Uniontown Procedure. Full throttle accelerations are
from low speed using primary reference fuels. The ignition timing
adjusted until trace knock is detected at some stage. Several
fuels are used, and a Road Octane Number v Basic Ignition timing
obtained. The fuel sample is tested, and the trace knock ignition
setting is read from the graph to provide the Road Octane Number.
a rapid procedure but provides minimal information, and cars with
management systems require sophisticated electronic equipment to
adjust the timimg .
2. The Modified Borderline Knock Procedure. The automatic spark
disabled, and a manual adjustment facility added. Accelerations
performed as in the Modified Uniontown Procedure, however trace
maintained throughout the run by adjustment of the spark advance.
of ignition advance v engine speed is made for several reference
and the sample fuels. This procedure can show the variation of
with engine speed, however the technique is almost impossible to
on vehicles with modern management systems .
The Road Octane Number lies between the MON and RON, and the
between the RON and the Road Octane number is called 'depreciation"
Because nominally-identical new vehicle models display octane
that can range over seven numbers, a large number of vehicles have
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
of air temperature and humidity are considered, it is often possible
one octane grade in summer, and use a lower octane rating in winter.
Motor octane rating has a higher charge temperature, and increasing
temperature increases the tendency to knock, so fuels with low
( the difference between RON and MON numbers ) are less affected by
7.11 What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study
a decrease of the octane requirement of 1.4 RON/300m from sea level
and 2.5 RON/300m from 1800m to 3600m . Other studies report the
number requirement decreased by 1.0 - 1.9 RON/300m without
altitude . Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number
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
- fuel is metered according to air volume, consequently as density
the stoichiometry moves to rich, with a lower octane number
- manifold vacuum controlled spark advance, and reduced manifold
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
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
vapour, has been thoroughly researched. If engines can calibrated to
with small amounts of water, knock can be suppressed, hydrocarbon
will slightly increase, NOx emissions will decrease, CO does not
significantly, and fuel and energy consumption are increased .
Water injection was used in WWII aviation engine to provide a large
in available power for very short periods. The injection of water
decrease the dew point of the exhaust gases. This has potential
problems. The very high specific heat and heat of vaporisation of
means that the combustion temperature will decrease. It has been
a 10% water addition to methanol reduces the power and efficiency by
3%, and doubles the unburnt fuel emissions, but does reduce NOx by
A decrease in combustion temperature will reduce the theoretical
possible efficiency of an otto cycle engine that is operating
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
are used to provide increased power for short periods, up to 40%
assuming adequate mechanical strength of the engine. The 40/60 or
water-methanol mixtures are used as boost fluids for aviation
water would freeze. Methanol is just "preburnt" methane,
consequently it only
has about half the energy content of gasoline, but it does have a
of vaporisation, which has a significant cooling effect on the
Water-methanol blends are more cost-effective than gasoline for
cooling. The high Sensitivity of alcohol fuels has to be considered
engine design and settings.
Boost fluids are used because they are far more economical than
fuel. When a supercharged engine has to be operated at high boost,
mixture has to be enriched to keep the engine operating without
extra fuel cools the cylinder walls and the charge, thus delaying
of knock which would otherwise occur at the associated higher
The overall effect of boost fluid injection is to permit a
increase in knock-free engine power for the same combustion chamber
temperature. The power increase is obtained from the higher
In practice, the fuel mixture is usually weakened when using boost
injection, and the ratio of the two fuel fluids is approximately 100
of avgas to 25 parts of boost fluid. With that ratio, the resulting
performance corresponds to an effective uprating of the fuel of
irrespective of its original value. Trying to increase power
40% is difficult, as the engine can drown because of excessive
Note that for water injection to provide useful power gains, the
management and fuel systems must be able to monitor the knock and
both stoichiometry and ignition to obtain significant benefits.
engines are designed to accommodate water injection, most automobile
are not. Returns on investment are usually harder to achieve on
do not normal extend their performance envelope into those regions.
injection has been used by some engine manufacturers - usually as an
expedient way to maintain acceptable power after regulatory
baggage was added to the engine, but usually the manufacturer
produces a modified engine that does not require water injection.
How can I identify
and cure other fuel-related problems?
What causes an empty fuel tank?
* You forgot to refill it.
* Your friendly neighbourhood thief "borrowed" the gasoline - the
one took the vehicle.
* The fuel tank leaked.
* Your darling child/wife/husband/partner/mother/father used the
* The most likely reason is that your local garage switched to an
gasoline, and the engine management system compensated for the
content, causing the fuel consumption to increase ( although the
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
glowing deposits or hot surfaces - such as exhaust valves and spark
If it continues, it can increase in severity and become Run-away
Ignition (RSI) which prevents the combustion heat being converted
mechanical energy, thus rapidly melting pistons. The Ricardo method
electrically-heated wire in the engine to measure preignition
scale uses iso-octane as 100 and cyclohexane as 0.
Some common fuel components:-
There is no direct correlation between antiknock ability and
tendency, however high combustion chamber temperatures favour both,
one may lead to the other. An engine knocking during high-speed
will increase in temperature and that can induce preignition, and
any preignition will result in higher temperatures than may induce
Misfire is commonly caused by either a failure in the ignition
fouling of the spark plug by deposits. The most common cause of
was the alkyl lead additives in gasoline, and the yellow glaze of
lead salts was used by mechanics to assess engine tune. From the
recess to the tip, the composition changed, but typical compounds (
from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4;
Run-on is the tendency of an engine to continue running after the
has been switched off. It is usually caused by the spontaneous
the fuel-air mixture, rather than by surface ignition from hotspots
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
off. The engine may refire, thus taking the conditions out of the
range for a couple of cycles, and then refire again, until overall
of the engine drops it out of the critical region. The octane rating
fuel is the appropriate parameter, and it is not rare for an engine
require a higher Octane fuel to prevent run-on than to avoid knock
Obviously, engines with fuel injection systems do not have the
idle speed is an important factor. Later model carburettors have an
stop solenoid which partially closes the throttle blades when the
key was off, and ( if set correctly ) thus prevents run-on.
8.3 Can I prevent carburetter icing?
Yes, carburettor icing is caused by the combination of highly
high humidity and low ambient temperature. The extent of cooling,
the latent heat of the vaporised gasoline in the carburettor, can be
as 20C, perhaps dropping below the dew point of the charge. If this
water will condense on the cooler carburettor surfaces, and will
the temperature is low enough. The fuel volatility can not always be
to eliminate icing, so anti-icing additives are used. In the US,
additives are seldom required because of the widespread use heated
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
inhibits ice crystal formation. These are usually added at
of 30-150 ppm.
- cryoscopic additives that depress the freezing point of the
so that it does not turn to ice. Alcohols ( methanol, iso-propyl
etc. ) and glycols ( hexylene glycol, dipropylene glycol ) are
concentrations of 0.03% - 1%.
If you have icing problems, the addition of 100-200mls of alcohols
to a full
tank of dry gasoline will prevent icing under moderately-cold
If you believe there is a small amount of water in the fuel tank,
of isopropyl alcohol as the first treatment, and isopropyl alcohol
preferred for more severe conditions. Oxygenated gasolines using
can also be used.
8.4 Should I store fuel to avoid the oxygenate season?
No. The fuel will be from a different season, and will have
different volatility properties that may induce driveability
can tune your engine to perform on oxygenated gasoline as well as it
traditional gasoline, however you will have increased fuel
to the useless oxygen in the oxygenates. Some engines may not
perform well on some oxygenated fuels, usually because of the
different volatility and combustion characteristics. A good mechanic
be able to recover any lost performance or driveability, providing
is in reasonable condition.
8.5 Can I improve fuel economy by using quality gasolines?
Yes, several manufacturers have demonstrated that their new gasoline
packages are more effective than traditional gasoline formulations.
claimed their new vapour-phase fuel additive can reduce existing
up to 30%, improve fuel economy, and reduce NOx tailpipe emissions
when compared to other advanced liquid phase additives . The
claims have been successfully disputed in court by Chevron - who
that their existing fuel additive already offered similar benefits.
reputable gasoline manufacturers will have similar additive packages
premium quality gasolines . Quality gasolines, of whatever
ratings, will include a full range of gasoline additives designed to
consistent fuel quality.
Note that oxygenated gasolines must decrease fuel economy for the
If your engine is initially well-tuned on hydrocarbon gasolines, the
stoichiometry will move to lean, and maximum power is slightly rich,
either the management system ( if you have one ) or your mechanic
increase the fuel flow. The minor improvements in combustion
oxygenates may provide, can not compensate for 2+% of oxygen in the
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.
gasoline has been allowed to get warm, thus catalysing olefin
reactions, and perhaps also losing volatile material in unsealed
Such fuel will tend to rapidly form gums, and will usually have a
reduction in octane rating. The fuel can be used by blending with
volume of new gasoline, but the blended fuel should be used
otherwise teh old fuel will catalyse rapid decomposition of the new,
resulting in even larger quantities of stale fuel. Some stale fuels
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
dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb
and will not significantly affect combustion. Once you have mopped
water with IPA, small, regular doses of any anhydrous alcohol will
keep the tank dry. This technique will not work if you have very
amounts of water, and the addition of greater amounts of IPA may
Water in fuel tanks can be minimised by keeping the fuel tank near
filling in the morning from a service station that allows storage
stand for several hours after refilling before using the fuel. Note
oxygenated gasolines have greater water solubility, and should cope
small quantities of water.
8.8 Can I used unleaded on older vehicles?
Yes, providing the octane is appropriate. There are some older
cut the valve seats directly into the cylinder head (eg BMC minis).
absence of lead, which lubricated the valve seat, causes the very
oxidation products of the exhaust valve to wear down the seat. This
seat recession is usually corrected by installing seat inserts,
the seats, or use of specific valve seat recession protection
(such as Valvemaster). Most other problems arise because the fuels
different volatility, or the reduction of combustion chamber
These can usually be cured by reference to the vehicle manufacturer,
will probably have a publication with the changes. Some vehicles
perform as well on unleaded with a slightly lower octane than
leaded fuel, due to the significant reduction in deposits from