jdaniel

Here you go... everything you need to know about gas.

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 preignition, as was commonly supposed, but
arose from a violent pressure rise *after* ignition. The manograph was not
suitable for further research, so Midgley and Boyd developed a high-speed
camera to see what was happening. They also developed a "bouncing pin"
indicator that measured the amount of knock [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 heptane", that
was already obtainable in sufficient purity from the distillation of Jeffrey
pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first
synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane. The
octane had a high antiknock value, and he suggested using the ratio of the
two as a reference fuel number. He demonstrated that all the commercially-
available gasolines could be bracketed between 60:40 and 40:60 parts by
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 Vaporisation
C C g/ml MJ/kg
normal heptane -90.7 98.4 0.684 0.365 @ 25C
iso octane -107.45 99.3 0.6919 0.308 @ 25C

Having decided on standard reference fuels, a whole range of engines and
test conditions appeared, but today the most common are the Research Octane
Number ( RON ), and the Motor Octane Number ( MON ).

6.2 Why do we need Octane Ratings?

To obtain the maximum energy from the gasoline, the compressed fuel-air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver an
optimum power stroke. In real life, a series of pre-flame reactions will
occur in the unburnt "end gases" in the combustion chamber before the flame
front arrives. If these reactions form molecules or species that can
autoignite before the flame front arrives, knock will occur [21,22].

Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure
peak arrives slightly ahead of the normal operating pressure peak, leading
to a loss of power and eventual overheating. The end gas pressure waves are
superimposed on the main pressure wave, leading to a sawtooth pattern of
pressure oscillations that create the "knocking" sound.

The combination of intense pressure waves and overheating can induce piston
failure in a few minutes. Knock and preignition are both favoured by high
temperatures, so one may lead to the other. Under high-speed conditions
knock can lead to preignition, which then accelerates engine destruction
[27,28].

6.3 What fuel property does the Octane Rating measure?

The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives. Different reaction mechanisms, occurring at
various stages of the pre-flame compression stroke, are responsible for the
undesirable, easily-autoignitable, end gases.

During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed
one at a time from the molecule by reactions with small radical species
(such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen
bonds depends on what the carbon is connected to. Straight chain HCs such as
normal heptane have secondary C-H bonds that are significantly weaker than
the primary C-H bonds present in branched chain HCs like iso-octane [21,22].

The octane rating of hydrocarbons is determined by the structure of the
molecule, with long, straight hydrocarbon chains producing large amounts of
easily-autoignitable pre-flame decomposition species, while branched and
aromatic hydrocarbons are more resistant. This also explains why the octane
ratings of paraffins consistently decrease with carbon number. In real life,
the unburnt "end gases" ahead of the flame front encounter temperatures up
to about 700C due to compression and radiant and conductive heating, and
commence a series of pre-flame reactions. These reactions occur at different
thermal stages, with the initial stage ( below 400C ) commencing with the
addition of molecular oxygen to alkyl radicals, followed by the internal
transfer of hydrogen atoms within the new radical to form an unsaturated,
oxygen-containing species. These new species are susceptible to chain
branching involving the HO2 radical during the intermediate temperature
stage (400-600C), mainly through the production of OH radicals. Above 600C,
the most important reaction that produces chain branching is the reaction of
one hydrogen atom radical with molecular oxygen to form O and OH radicals.

The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Antiknock additives
work by interfering at different points in the pre-flame reactions, with
the oxygenates retarding undesirable low temperature reactions, and the
alkyl lead compounds react in the intermediate temperature region to
deactivate the major undesirable chain branching sequence [21,22].

The antiknock ability is related to the "autoignition temperature" of the
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates have
lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other factors
that are far more important. ( such as compression r

mdvaldosta

Damn I started to read that till I realized it was... hold on using a counter... 56,254 characters and 8,986 words...

jdaniel

LOL... Thats why I put his answer in bold print. I read it all before I copied and pasted and was very interesting. A pretty good read to me.

artisan00

gonna save this and read it later - really interesting but reeeally long - good crapper reading maybe?

oh and now that i notice the subject of the thread again, and after seeing what i just wrote....

just wanted to say , no pun intended!