FUEL OCTANE RATINGS

FUEL OCTANE


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 engineís design. It rapidly causes high bearing loads and piston temperatures that can lead to major engine damage. Itís 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 fuels.
 Chemical analysis of the fuel was too difficult in those days, and itís still not simple with over 200 different identifiable hydro-carbon compounds in what we call Ďpetrolí, so in the 1920ís 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 occurring.
 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 38įC, the fuel is kept at 150įC 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 itís 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 arenít octane ratings but Ďperformance numbersí 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.
 During the 1960ís another complication arose in fuels with a large difference between their Motor and Research octane numbers (called the fuelís sensitivity). This lead to trouble with some high compression engine on high-speed roads, so in the early 1970ís 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. Todayís 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
Australia.

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