How is the Gasoline "Pump Octane Number" Calculated?

Every driver routinely does it-maybe he does it once a week or maybe even more often. What is that? They fill their motor vehicle with gasoline from the filling station. Most of us seek to shave a little off the fuel cost by buying "regular" grade gasoline. That grade is probably rated 87 by the pump octane number (PON) system. What does that number stand for, i.e. how is it calculated?

Read Your Pump

Look all over your pump, and you will likely see the octane number you've chosen was calculated by the (R+M)/2 method. What is that? The R stands for Research Octane Number (RON). The M stands for Motor Octane Number (MON). Adding the two numbers (RON + MON) together and then dividing by two gives the Pump Octane Number. So where do those two numbers come from?

Research Octane Number

The research octane number of a sample of gasoline is burned inside a test engine and compared with standards of known value burned in the same test engine. The standards consist of mixtures of iso-octane and n-heptane. Since iso-octane has a rating of 100 and n-heptane has a rating of 0, mixtures of intermediates can be created to meet a specific number in between those limits.

Motor Octane Number

Motor octane number (MON) is normally less in value than RON. As an example, a fuel evaluated as 91 RON could give a MON of only 83. Adding these two together and dividing by two would give (91 + 83)/2 = 87 PON. How is the motor octane number determined? It is done in somewhat similar fashion to the method of determining the research octane number. One of the differences is the test engine is run at 900 rpm instead of 600 rpm. This provides an octane value determined under load conditions, and tempers the value for operation under more realistic functioning conditions.

Gasoline - A Composite Mixture

Brands of gasoline differ in chemistry, depending on the source of the oil, the method of cracking, blending, additives, and other factors. Each component has a theoretical octane value, even as iso-octane has a value of 100 and n-heptane a value of 0. Thus it is not practical to determine the exact amount of each component, figure a percentage, etc. The simplest and only practical way to evaluate a batch of gasoline is to use this test engine method.

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References and Resources:

U. of St. Francis - "Analysis of Octane Values Based on Gasoline Components of Various Brands" (Slide Show)

NIST - Uniform Engine Fuels, Petroleum Products, and Automotive Lubricants Regulation

Federal Trade Commission - The Low-Down on High Octane Gasoline

Personal Experience