Understanding octane

The most interesting and least understood part of a fuel’s properties is the octane quality or number.

The most interesting and least understood part of a fuel’s properties is the octane quality or number.

The octane requirement of an engine is affected by the combustion chamber shape, the bore size, the compression ratio, the load on the engine, the rpm at which the engine is operated, the ignition timing, the cam profile, the air temperature, the humidity, the barometric pressure, the engine operating temperature, the air/fuel mixture, the type of fuel being used, the amount and nature of deposits in the combustion chamber and intake system, and a host of minor factors. But we all would like to have just one number that describes how a fuel will perform in our engines.

When the Wright brothers first took off, they used a fuel with a guesstimated octane number of about 37. Then, as aircraft and automobiles advanced in design and performance, it became apparent that fuels with higher anti-knock characteristics were needed. The early scientists and engineers noted that some fuels knocked more than others, so an inter-industry committee was set up to develop a test that would measure the anti-knocking characteristics of different blends of fuels.

What was developed was the Cooperative Fuels Research (CFR) engine, which has a head and cylinder that can be raised or lowered while the engine is running. Test fuel is run in the engine and the compression ratio is increased until knock is observed. This compression ratio then relates to a specific octane number. The reason it is called octane number is that the pure hydrocarbon fuel, iso-octane, is defined as having an octane number of 100. By contrast, the octane number of normal heptane is defined as zero. If you need a reference fuel with an octane number of 90, you would simply mix 90% iso-octane and 10% normal heptane.

The industry quickly adopted the octane rating system and fuels were marketed with specified octane numbers. But engineers are noted for never leaving things alone. Some of them noted that different types of fuels performed more poorly than others with the same octane number when tested in actual automobiles. They then modified the octane number test to run at higher rpm and intake air temperature. This test was called the motor octane test, while the original test was called the research octane test.

The motor octane numbers for fuels were typically eight to 10 numbers below the research octane numbers. The gasoline marketers were not about to allow the technical people to change to a lower number because everyone knows higher is always better, so for many years gasoline was marketed with just the research octane number posted, even though that number did not correlate well with the real world.

This continued into the 1970s until unleaded fuels came into being. Then the marketing types finally gave in a little and allowed the posting of the average of the research and motor octane ratings. The average did not correlate with the real world nearly as well as the straight motor octane number, but we all know that pure science has no place in the market.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

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