Method of assigning an octane number to a fuel

A method of obtaining an octane number of a first fuel, the method comprising:

• operating an engine at a constant established compression ratio;
• obtaining a first knock intensity while operating the engine on a first reference fuel having a first assigned octane number,
• obtaining a second knock intensity while operating the engine on a second reference fuel having a second assigned octane number,
• obtaining a third knock intensity while operating the engine on a third reference fuel having a third assigned octane number, and
• obtaining a fourth knock intensity while operating the engine on a fourth reference fuel having a fourth assigned octane number,
• wherein each of the the first assigned octane number, the second assigned octane number, the third assigned octane number, and the fourth assigned octane number differ from each other by a value of no less than 0.5 octane number and no greater than 2.5 octane number,
• wherein the second assigned octane number has a lowest assigned octane number, the fourth assigned octane number has a highest assigned octane number, and the first assigned octane number that is greater than the assigned octane number of the third reference fuel, and
• wherein the constant established compression ratio is based on the first reference fuel or the third reference fuel and the knock intensities are in the range of 0 KI to 250 KI;
• running the first, second, third, and fourth reference fuels through the engine in an order in which fuels with the next highest or next lowest octane number are not run consecutive with one another;
• plotting the first, second, third, and fourth knock intensities relative to their respective first, second, third, and fourth assigned octane numbers;
• fitting a line to the plotted first, second, third, and fourth knock intensities with a linear method to determine a spread of the engine based on calculating knock intensity units per octane number of the engine for the at least three reference fuels;
• obtaining an R squared value for the fitted line, comparing the R squared value with a minimum acceptable R squared value, and validating the fitted line if the R squared value is at least the minimum acceptable R squared value;
• obtaining a prototype fuel knock intensity while operating the engine on a prototype fuel having an assigned octane number;
• obtaining a sample fuel knock intensity while operating the engine on the sample fuel having an unassigned octane number, and wherein the prototype fuel and the sample fuel differ from one another by up to +/−4 octane number;
• and
• if the fitted line is validated, assigning an octane number of the sample fuel based on the prototype fuel knock intensity, the sample fuel knock intensity, and the fitted line,
• wherein the assigning is based on dividing a difference between the prototype fuel knock intensity and the sample knock intensity by the spread to determine a delta between the sample fuel and the prototype fuel, and adding the delta to the obtained octane number of the prototype fuel.