The primary criteria used by most consumers to gauge the quality of gasoline is the octane rating. High octane ratings do not mean high quality. The octane rating of a fuel is a method of comparing the antiknock qualities of that fuel to standard test fuel. The octane rating scale was developed by American society for Testing Materials (ASTM). The scale begins at zero and runs to numbers above 100. Fuels on the lower end of scale (0 to 85) tend to “knock” during the combustion process inside an engine. Engine “knock” is a pinging or knocking sound produces by the engine while running, usually under loaded conditions. Knock is a serious problem because valves, pistons and bearings are subjected to abnormal forces and temperatures.

Octane rating recommendations for a given engine are usually stated by the engine mfg. the compression ratio of an engine usually determines the recommended octane rating of the fuel to be used in the engine. Engines with high compression ratios, such as 8.5 to 1, can use higher octane fuels more efficiently. The higher he compression of an engine, the higher the power output of the engine. This is because more of the fuel energy is converted to useful power.

Engines with lower compression ratios (7 to 1) may not need high octane fuel. If the engine performs satisfactorily without knocking on lower octane fuels, use of higher octane fuel is not only more expensive, but wasteful. More crude oil is needed to make one gallon of high octane fuel than is needed to make one gallon of low octane fuel.

Octane ratings found on gasoline pumps at gas stations will usually be an average of the motor octane rating and the research octane rating. Both motor and research ratings are test methods used to determine octane ratings of fuel. Premium, regular and super grades of fuel are rough measures of octane ratings. Regular unleaded fuel will usually be an octane rating of 87. Premium grades will have an octane rating of 89 and super grades will be 91 octane or more.

The measurement of the Octane number of gasoline using NIR spectroscopy is both fast and reliable utilizing Guided Wave NIR analyzers as described here. This method minimizes the need for laboratory sample collection. Results are available in real-time (seconds) for multiple parameters in complex streams. The systems are capable of measuring Octane number and many other parameters such as benzene, density, distillation points, etc for both blended gasoline (all grades) and individual blending components.