How Are Octane Ratings Tested?

How Are Octane Numbers Measured And Who Measures Them?

How Are Octane Ratings Tested?The most reliable way of testing the octane value of a fuel is to use a CFR (Cooperative Fuel Research) engine (AKA knock engine). The CFR testing method was developed in the 1920s as a reliable way to measure octane levels, and the ASTM (American Society for Testing and Materials) still uses is to this day. Not the exact came engine though.

The CFR texting engine measures octane values by combusting the fuel and measuring the pre-ignition or detonation that occurs.  The Cooperative Fuel Research engine has highly repeatable measurements within 0.2 RON within the same lab, and results can be reproduced within 0.7 RON across different engines, in different testing labs. Each test result is compared to the ASTM Method D2699, which dictates what RON to give the fuel.

1928 Cooperative Fuel Research Engine
1928 Cooperative Fuel Research Engine

This certified testing method allows refineries and development laboratories to test their product accurately. But more importantly, this allows other entities, like the EPA, to confirm their claimed numbers.

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Cooperative Fuel Research Engine Details

The Cooperative Fuel Research unit is a carburetted, single-cylinder engine, but the variable-compression ratio is what sets it apart from the engine in your dirt bike.

The cylinder head can raise and lower as needed to adjust the compression ratio, and therefore increase or decrease the possibilities of pre-ignition or detonation.  By registering the “knock” energy at a given compression ratio, the engineer can conclude the octane grade of the tested fuel.

Modern Cooperative Fuel Research Engine
Modern Cooperative Fuel Research Engine

The engine has to be heated and sustain a 100 to 130 degree (Fahrenheit) oil temperature and a 2-3Hg (inches of mercury) manifold vacuum. The Air/fuel ratio is also adjusted for elevation and must be maintained.

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Before every test, toluene a hydrocarbon-based solvent mixture is run through the engine to remove contaminants. A reference fuel is then run through the engine for calibration checks.

The Research Octane Number test is conducted at 600 RPM. The intake and air/fuel charge temperatures are maintained at 126 degrees Fahrenheit.  Ignition timing is set to 13 degrees before piston top dead center.

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