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 [24]. They used a Dobbie-McInnes manograph to demonstrate that the knock
did not arise from pre-ignition, as was commonly supposed, butarose 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 [9]. 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 [11]. These were "normal heptanes", 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 volume heptane:iso-octane.
The reason for using normal heptane and iso-octane was because they both have
similar volatility properties, specifically boiling point, thus the varying
ratios 0:100 to 100:0 should not exhibit large differences in volatility that
could affect the rating test.
Heat of
Melting Point Boiling Point Density
Vaporization
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 ).
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 favored by high
temperatures, so one may lead to the other. Under high-speed conditions knock
can lead to preignition, which then accelerates engine destruction
[27,28].
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:-
The energy content of fuel, this should be obvious, as oxygenates have lower
energy contents, but high octanes.
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.
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 [39].
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
summarized by Campbell and Boyd [103]. 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 [103]. During the
1950-1960s attempts were made to internationally standardize 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 [32]. I should note that the Research method was only
formally approved in 1947, but used unofficially from 1942.
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 [104]
Engine Cooperative Fuels Research (
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
(e.g. 14 - 26 degrees BTDC)
Carburetor Venturi 14.3 mm
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 [105]
Engine Cooperative Fuels Research (CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
(e.g. 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
Carburetor Venturi Set according to engine altitude
(e.g. 0-500m=14.3mm, 500-1000m=15.1mm )
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 [39]. 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 [106].
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 carburetor that can adjust individual bowl
air-fuel ratios. This facilitates rapid switching between reference fuels and
samples. A magneto restrictive 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 "knock meter" 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.
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 knock meter for the sample when the air-fuel
ratio is adjusted on the carburetor 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 knock meter readings
[104,105].
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, and 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 characterized, 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.
What is a "delta Research Octane number"?
To obtain an indication of behavior 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 [27]. 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 [107]. 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 the US.
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 carburetor 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.
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 cannot 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 drivability 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 drivability.
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
realizing that you have, could be expensive engine damage.
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.
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 e.g. 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.
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 drivability. 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 cannot utilize. 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 drivability and performance, using
anything more is waste of money, and anything less could result in an
unscheduled, expensive visit to your mechanic.
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 chip.
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) [108]. 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.
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 (a.k.a. Lean Mixture rating), and the second number is the
Supercharge rating (a.k.a. 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
[110]. 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 vary significantly above that e.g. 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 that 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 utilize 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 that define the maximum boost (rich - about 11:1
stoichiometry) and minimum boost
(weak about 16:1 stoichiometry) before knock [110].
The 115/145 grade is being phased out, but even the 100LL has more octane than
any automotive gasoline.
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 [11], and during the 1930s and 40s, the
ratings increased by approximately 20 units as alkyl leads and improved refining
processes became widespread [12].
Naphthalene has a blending motor octane number of 90 [52], 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 problems.
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
recognized that the general public seldom drive as severely as the testers, and
so may be satisfied by a lower octane fuel [28].
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
[23].
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 cannot deliver as much of the ideal
combustion power to the flywheel. For a typical carbureted 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.
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 drivability [38].
An average increase of 2 (R+M)/2 ON is required for each 1.0 increase (leaning)
of the air-fuel ratio [111]. 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 in backfiring.
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 Center) 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 [27]. 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 [38].
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.
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 airflow, 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 [112].
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 [76].
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.
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 [111].
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.
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
compression ratio.
They also reduce thermal conductivity, thus increasing the combustion chamber
temperatures.
They catalyze undesirable pre-flame reactions that produce end gases with low
autoignition temperatures.
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:
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 [28].
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
vs. 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 [28].
The Road Octane Number lies between the MON and RON, and the difference between
the RON and the Road Octane number is called 'depreciation" [111]. Because
nominally identical new vehicle models display octane requirements that can
range over seven numbers, a large number of vehicles have to be tested [28,111].
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 temperature.
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 [27]. Other studies report the octane number
requirement decreased by 1.0 - 1.9 RON/300m without specifying altitude [38].
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.
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].
What does water injection achieve?.
Water injection, as a separate liquid or emulsion with gasoline, or as a vapor,
has been thoroughly researched. If engines can be 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 [113].
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 vaporization 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% [114]. 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 vaporization,
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 [110].
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
normally 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.
.
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 glowing deposits ignite the air-fuel mixture
prematurely, or hot surfaces such as exhaust valves and spark plugs. If it
continues, it can increase in severity and become Run-away Surface Ignition (RSI),
which prevents the combustion heat being converted into mechanical energy, thus
rapidly melting pistons. The Ricardo method uses an electrically heated wire in
the engine to measure preignition tendency. The scale uses iso-octane as 100 and
cyclohexane as 0.
Some common fuel components:-
paraffins 50-100
benzene 26
toluene 93
xylene >100
cyclopentane 70
di-isobutylene 64
hexene-2 -26
There is no direct correlation between antiknock ability and preignition
tendency, however high combustion chamber temperatures favor 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 carburetors have an idle stop solenoid
that partially closes the throttle blades when the ignition key was off, and
thus (if set correctly) prevent run-on.
