A LITTLE HISTORY
Detonation is the name given to the very rapid and uncontrolled combustion
that occurs in petrol engines when the fuel octane being used is too low for the engine’s design.
It rapidly causes high bearing loads and piston temperatures that can lead to major engine damage.
It’s been with us since the dawn of the internal combustion engine, but in those early days of very
low output engines, it was at first just an annoying rattling noise coming from the engine.
During the First World War there was a sudden increase in the need for higher output engines
to power planes and tanks. The available fuels were very prone to detonation and caused many early
engine failures, so some of the best scientific brains of the day concentrated on finding a cure.
They soon discovered that petrol made from the crude oil from different regions of the world had
very different detonation characteristics. Harry Ricardo in England and Thomas Midgely in the USA
were two people who became famous for their work on fuels. Their discoveries lead to the use of
tetra-ethyl lead as an octane booster and the use of laboratory fuels like iso-octane to rate
Chemical analysis of the fuel was too difficult in those days, and it’s still not simple
with over 200 different identifiable hydro-carbon compounds in what we call ‘petrol’, so in the
1920’s attention turned to testing fuels in special engines. The Cooperative Fuels Research (CFR)
committee was jointly formed by automotive and oil companies to design the test engine. The
resulting CFR engine was built by the Waukesha Engine Company in 1928 and is still used today,
albeit with some modern extras like computerised control.
The CFR engine is a single cylinder, 600-cc, 4-stroke engine with a compression ratio
variable from about 4 to 1, up to as high as 18 to 1, while running. It uses a carburettor
with several float chambers so that the fuel being tested can be quickly switched with the
the turn of a tap. The engine also has heaters to control the temperature
of both intake air and fuel, and gauges to both monitor the engines and show when detonation is
The first test developed is today called the Motor Octane Number (MON) test. The CFR
engine runs at 900 rpm, inlet air is kept at 38°C, the fuel is kept at 150°C and ignition timing
is varied to match compression ratio. This test represents engines under high load, high throttle
opening conditions. The fuel under test is run and it’s knock or detonation characteristics at
different compression ratios were noted. Then the test was repeated this time using a laboratory
fuel, which is a mixture of just two compounds, N-heptane and Iso-octane.
N-heptane has an octane of 0 (zero) and iso-octane has an octane of 100. By varying the
mixture of an octane number anywhere from 1 to 100 is possible. When the laboratory mixture had
the same knock characteristics as the test fuel, the amount of iso-octane used became the octane
number of the test fuel. Thus a fuel, which knocks with 60% iso-octane and a 40% n-heptane brew
has an octane rating of 60. The highest octane theoretically possible is 100 (100% iso-octane),
but there are fuels with octane ratings over 100. This is done by using charts that allow the
test results to be disregarded past 100 according to how much compression it can over a 100%
iso-octane brew. Strictly speaking numbers over 100 aren’t octane ratings but ‘performance
numbers’ however, for convenience we just use octane.
Later it was discovered that some lightly load engines, like cars on relatively flat
roads, could still get into detonation trouble even with fuels that performed well in the
MON test, so another test was developed that used a similar CFR engine with different criteria.
This test is called the Research Octane Number (RON). The CFR engine runs at 600 rpm, the inlet
air temp is varied to match barometric pressure, but the ignition timing is fixed. This test
better represents the engines under part throttle conditions. The Research Octane number is
usually higher than the Motor number for most automotive fuels.Previous page
During the 1960’s another complication arose in fuels with a large difference between
their Motor and Research octane numbers (called the fuel’s sensitivity). This
lead to trouble with some high compression engine on high-speed roads, so in the early 1970’s the
AKI method was adopted in the USA. AKI is the average of RON and MON and is a better indicator of
any fuel with a high sensitivity, when only one octane number is advertised.
To reduce consumer confusion over different octane numbers, the USA passed laws so that AKI is
the octane rating shown on pumps at petrol stations in that part of the world. Because of this
it is also sometimes called the Pump Posted Octane Number.
Other versions of the CFR engines are used for aviation fuels (with supercharging) and for
diesel fuels to get Cetane ratings. Today’s CFR engine costs about US$120,000 and comes with
computerized data acquisition to speed up testing and improve reliability. Modern computerized
chemical analysis is also used for refinery quality control so that tests in the CFR engine are
not required so often.
By Paul Dawson M.I.A.M.E
Bombardier Recreational Products