octane_rating Subject: 6. What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression
ratio had been constrained by the unwanted "knock" that could
rapidly destroy engines. "Knocking" is a very good description of the
sound heard from an engine using fuel of too low octane.
The engineers had blamed the "knock" on the battery ignition system
that was added to cars 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. They used a Dobbie-McInnes manograph to show that the
knock did not arise from preignition as was commonly supposed, but
arose from a violent pressure rise _after_ ignition. The manograph
had reached the limits of its usefulness, 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 knock. Ricardo had
developed the concept of HUCF ( Highest Useful Compression Ratio )
in using a variable compression engine, however the numbers were not
absolute as there were many variables such as ignition timing,
cleanliness, spark plug postion, engine temperature. etc.
In 1926 Graham Edgar suggested using two hydrocarbons that could be
produced in sufficient purity and quantity. These were "normal heptane"
that was already obtainable in sufficient purity from the distillation
of Jeffrey pine oil, and "an octane, named 2,4,4-trimethyl pentane"
that he first synthesised. Today we call it "iso-octane" or
2,2,4-trimethyl pentane. The octane had a high anti-knock value, and
he suggested using the ratio of the two as a reference fuel.
He demonstrated that all the commercially available gasolines could
be bracketed betweem 60:40 - 40:60 parts by volume heptane:iso-octane.
The reason for using normal heptane and iso-octane is 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.
nC7 = melting point -90.7C, boiling point 98.4C, density 0.684 g/ml
iC8 = melting point -107.45C, boiling point 99.3C, density 0.6919 g/ml
6.2 Why do we need Octane Ratings?
To obtain the maximum energy from the gasoline, the compressed fuel/air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver
an optimum power stroke. However, 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 the reactions form molecules or species
that can autoignite before the flame front arrives, knock will occur.
Simply put, the octane rating of the fuel reflects the ability
of the 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 lead
to piston failure in a few minutes. Knock and preignition are
both favoured by high temperatures, so one may lead to the other,
thus under high speed conditions knock can lead to preignition
which then accelerates engine destruction.
6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives.
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 the 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.
Thus octane rating is determined by the structure of the molecule, with
long, straight hydrocabon chains producing large amounts of easily
autoignitable pre-flame decomposition species, while branched and aromatic
hydrocarbons are more resistant.
In real life, the unburnt "end gases" ahead of the flame front are
subjected to temperatures up to about 700C due to piston motion and
radiant and conductive heating, and begin 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 of O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Anti-knock additives
work by interfering at different points in the pre-flame reactions.
The anti-knock ability of a fuel is related to the "autoignition
temperature" of the hydrocarbons, but it is _not_ substantially related
to:-
1. The energy content of fuel, this should be evident from oxygenates that
have lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
evident from the similarities of the two reference hydrocarbon fuels.
Although flame speed does play a minor part, there are many other
factors that are far more important. ( such as combustion chamber shape,
chemical structure of the fuel, presence of anti-knock additives, number
and position of spark plugs, turbulence etc etc )
Flame speed does not correlate with octane.
6.4 Why are two ratings used to obtain the pump rating?
The (RON+MON)/2 formula is correctly called the "antiknock index".
The initial octane method developed in the 1920s was the Motor Octane
method and, over several decades, a large number of octane test methods
appeared, due to variations to either the engine design or specified
operating conditions. During the 1950-1960s attempts were made to
internationally standardise and reduce the number of Octane Rating test
procedures.
.
During the late 1930s - mid 1960s the Research method became the important
rating because it more closely represented the octane requirements of the
motorist using the fuels/vehicles/roads then available. In the late 1960s
German automakers discovered their engines were destroying themselves on
long Autobahn runs, even though the Reseach Octane was as specified.
They discovered they had to also specify either the MON or the Sensitivity
( the numerical difference between the RON and MON numbers ). Today it's
accepted no one octane rating covers all uses. In fact, during 1994, there
have been increasing concerns in Europe about the Sensitivity of some
commercially available unleaded fuels.
The design of the engine and car significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would run on
the specified research octane fuel, almost regardless of the motor octane,
whereas most 1990s engines have a 'severity" of one, which means the engine
is unlikely to knock if a changes of one RON is matched by an equal and
opposite change of MON.
6.5 What does the Motor Octane rating measure?
The conditions of the motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).
Test Engine conditions - Motor Octane
Test Method ASTM D2700-92
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM,
Intake air temperature 38C
Intake mixture temperature 149C
Coolant temperature 100C
Ignition Advance - variable 14 - 26 degrees BTDC
( varies with compression ratio )
6.6 What does the Research Octane rating measure?
The settings commonly used 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 As specified (eg 51.7C @ )
( varies with barometric pressure )
Intake mixture temperature Not specified
Coolant temperature 100C
Ignition Advance - fixed 13 degrees BTDC
6.7 Why is the difference called "sensitivity"?
RON - MON = Sensitivity.
Because the two test methods use different test conditions, then a
fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels are
expected to have sensitivities around 10. The US 87 (RON+MON/2)
Unleaded gasoline is required to have a 82+ MON.
6.8 What sort of Engine is used to rate fuels?
Octane ratings are determined in a special single cylinder engine with
a variable compression ratio ( CR 3:1 to 30:1 ) known as a CFR engine.
The head and cylinder are one piece, and can be moved up and down to
obtain the desired compression ratio. Only one company manufactures these
engines, the Waukesha Division of Dresser Industries. The engines have a
special four-bowled carburettor to facilitate rapid switching between
reference fuels and samples. A magnetorestrictive detonation sensor measures
the rapid changes in combustion chamber pressure caused by knock and the
amplified signal is measured on a "knockmeter" with a 0-100 scale.
Note that one complete Octane Rating engine costs about $200,000 installed.
6.9 How is the Octane rating determined?
The normal heptane and iso-octane are known as primary reference fuels.
Two blends of these are made, one an octane number above the expected rating,
and another that is one octane number below the expected rating.
To rate a fuel, the engine is set to an appropriate compression ratio that
will produce a knock of about 70 on the knockmeter with the lower octane
blended reference fuel, and the higher octane reference fuel should
produce a number less than 50. The sample is then tested, and if it
doesn't fit between the reference fuels, further reference fuels are
prepared and the engine adjusted to obtain the required knock. The
actual fuel rating is interpolated from the knockmeter readings.
6.10 What is the Octane Distribution of the fuel?
The technique for determining the actual octane requirements of a
vehicle are the Road Octane numbers, and they can depend significantly
on the fuel. If the octane is distributed differently throughout the
boiling range of a fuel is different, 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, as when quickly
accelerating a full vehicle at the start of a hill. 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.
Some 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 tetramethyllead
and tetraethyllead octane profiles were well characteristed, but it can be
a major problem for the new reformulated, low aromatic gasolines,
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's because of the combination of their
driving style, the engine, the octane distribution, fuel volatility and
the octane enhancers used.
6.11 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 engine. That
method has been replaced by the "delta" RON procedure.
The fuel is carefully distilled to obtain a distillate fraction that
boils up to the specified temperature, which is usually 100C.
Both the parent fuel and the distillate fraction are rated using the
Research Octane method. The difference between these is called the
delta RON(100C), usually just called the delta RON.
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant
determinant of octane is the chemical structure of the hydrocarbons
and their response to the addition of octane enhancing additives.
Other factors include:-
Front End Volatility - Paraffins are the major component in gasoline,
and the octane number decreases with increasing chain length or
ring size, but increases with chain branching. Overall the effect is
a significant reduction in octane if front end volatility is lost, as
can happen with inproper or long term storage. Fuel economy on short
trips can be improved by using a more volatile fuel, at the risk
of carburettor icing and increased evaporative emissions.
Final Boiling Point.- Decreases in the final boiling point increase
fuel octane. Aviation gasolines have much lower final boiling points
than automotive gasolines.
Preignition tendency - both knock and preignition can induce each other.
6.13 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. The consequence is that 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 fuels eg 50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher
octane are useless. The lead response of the unleaded fuel doesn't overcome
the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94.
Note that some oxygenated fuels with ordinary gasoline can result in
undesirable increases in volatility due to volatile azeotropes, and
that some oxygenates can have negative lead responses. Also note that the
octane requirement of some engines is determined by the need to avoid run-on,
not to avoid knock.
6.14 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). It's far better to either purchase
a higher octane fuel such as racing fuel, aviation gasoline or methanol
( although the price of chemical grade methanol has almost doubled during
1994 ). If you plan to use alcohol blends ensure you 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 and the service station tank
hasn't just been refilled. They are supposed to wait several hours before
bringing a refilled tank online, but they don't always.
6.15 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. In the 1970s a new
grade 100LL ( low lead = 2ml/US Gal. instead of 4.6 ml/US Gal.) was
introduced to replace the 80/87 and 100/130. However soon after the
introduction there was a spate of plug fouling, and high cylinder
head temperatures resulting in cracked cylinder heads. The old 80/87
grade was reintroduced on a limited scale. The first number is the
Aviation rating ( aka Lean Mixture rating), and the second number
is the Supercharge rating ( aka Rich Mixture rating )
The Aviation rating is determined using the automotive Motor Octane
test procedure, and then corrected to an Aviation number using a table in
the method - it's usually only 1 - 2 Octane units different to the Motor
value up to 100, but varies significant above that eg 110MON=128AN.
The second number on Avgas is the Rich Mixture method Performance Number
( they aren't commonly called octane numbers when they rate above 100 ),
and is determined on a supercharged version of the CFR engine which
has a fixed compression ratio. The method determines the dependance of
the highest permissible power ( in terms of indicated mean effective
pressure ) on mixture strength and boost for a specific light knocking
setting. The Performance Number indicates the maximum knock-free power
obtainable from a fuel compared to iso-octane = 100. Thus a PN=150
indicates that an engine designed to utilise the fuel can obtain 150% of
the knock-limited power of iso-octane at the same mixture ratio. This is an
arbitrary scale based on iso-octane + varying amounts of TEL, derived from
a survey of engines decades ago. Aviation gasoline PNs are rated using
variations of mixture strength to obtain the maximum knock-limited power
in a supercharged engine. This can be extended to provide mixture response
curves which define the maximum boost ( rich about 11:1 stoic ) and
minumum boost ( weak about 16:1 stoic ) before knock.
The 115/145 grade is being phased out, but even the 100LL has more
octane than any automotive gasoline.
|