octane number

A rating of a gasoline in terms of its propensity to cause knocking.

Gasoline is not a chemical compound; it is a mixture of many different compounds.

By 1882, experimenters noted that spark ignition internal combustion engines knocked more on some gasolines than on others. Ideally, when the spark ignites the fuel-air mixture in the cylinder of a gasoline engine, the flame front spreads out smoothly from the spark into the unburned part of the mixture, giving a gradually increasing push to the piston. However, as the flame front spreads, the hot products of combustion behind the flame front compress the unburned part of the fuel-air mixture. Compressing a gas raises its temperature. Radiation from the burning fuel can also raise the temperature of the unburned fuel. The unburned fuel-air mixture can be heated so much that some of the hydrocarbons in it reach their ignition temperature and ignite all at once–explosively, causing knock. Knock can destroy engines.

To select a way of rating the propensity of a gasoline to cause knocking, a Cooperative Fuel Research Committee was set up in 1927 comprising representatives of the American Petroleum Institute, the American Manufacturers Assn., the National Bureau of Standards, and the Society of Automotive Engineers. A single-cylinder engine with a variable compression ratio had been built by John Campbell at General Motors. Graham Edgar at the Ethyl Corporation prepared samples of various pure hydrocarbons. including normal heptane distilled from the sap of the Jeffrey Pine. The engine enabled researchers to burn mixtures of Edgar's pure hydrocarbons while varying the compression, to see at what point knock occurred.

In 1929, T. A. Boyd proposed to the committee that a variable-compression engine be the basis for rating gasolines. Some committee members felt that such an engine would be too complicated for routine use, but the Waukesha Engine Company volunteered to build a prototype. By 1931 Waukesha was able to display its engine at a meeting of the American Petroleum Institute; skeptics were persuaded and thousands of the engines were subsequently built. (In fact, in 1980 the American Society of Mechanical Engineers designated the engine an “engineering landmark.”)

In the committee’s opinion, no one test was able to give a rating useful over the whole range of operating conditions, and so two methods were defined: the Motor Method (ASTM d 357) and the Research Method (ASTM d 908). Both methods are based on comparing the performance of the gasoline being tested with the performance of a mixture of 2,2,4, trimethyl pentane (also called iso-octane) and normal heptane. The octane number is the percentage of iso-octane in that mixture whose performance (in regard to knocking) is the same as that of the gasoline under test. For example, if the performance of the gasoline under test is the same as that of a mixture of 80% 2,2,4,trimethyl pentane and 20% normal heptane, the gasoline is 80 octane. Octane numbers above 100 are found by extrapolation.

The two test methods give different results, and the difference in the results differs from gasoline to gasoline. As a broad generalization, the motor method captures the gasoline's performance at high engine speeds and loads, and the research method at low speeds. The octane rating on American gasoline pumps is usually the average of the research and motor octane numbers, which is sometimes called the anti-knock index. In Europe, pumps have traditionally displayed the research octane number.

Fuel is just one of many factors affecting whether an engine will knock. Consequently in any particular engine gasolines with the same octane number but from different blenders may perform differently: one may cause knock and the other may not. Similarly a gasoline that causes knock in one engine model may not in another. This is not proof that the octane rating was inaccurate.

C. A. Amann.
The Automotive Spark-Ignition Engine – An Historical Perspective.
in History of the Internal Combustion Engine.
E. F. C. Somerscales and A. A. Zagotta, editors.
ICE--volume 8.
New York: American Society of Mechanical Engineers, 1989.
Page 36.

1926 Am Chem Soc paper


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