6.1 Who invented Octane Ratings?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
6.1 Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression ratio
had been constrained by the unwanted "knock" that could rapidly destroy
engines. "Knocking" is a very good description of the sound heard from an
engine using fuel of too low octane. The engineers had blamed the "knock"
on the battery ignition system that was added to cars along with the
electric self-starter. The engine developers knew that they could improve
power and efficiency if knock could be overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding the exact
cause of knock [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 ).
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