​It is a relative index representing resistance of gasoline to premature detonation (capability of the gasoline to burn down gradually without detonation). This index is reflected as a gasoline grade displayed in the filling stations. The larger the number is the better resistance of gasoline to premature detonation is. This index is determined using special equipment through the comparison of resistance to premature detonation of reference blend of two components and the gasoline under test. The octane number of a sample of fuel is determined by burning the gasoline in an engine under controlled conditions, e.g., of spark timing, compression, engine speed, and load, until a standard level of knock occurs.