The performance of a spark-ignited gasoline-fueled engine (i.e., an Otto cycle engine) is improved when the gasoline burns and advances quietly with a smooth flame front as it drives the engine's cylinders' pistons rather than knocking or pinging, which indicates that the fuel is detonating instead of burning smoothly. The octane number is a measure of the anti-knock quality of gasoline ... in other words, the ability of the gasoline to resist knocking when it is burned in a spark-ignited Otto cycle engine. Pure iso-octane, which has an excellent anti-knock quality, has been designated as having an octane number of 100. Samples of gasoline are tested by running them in a single-cylinder engine mounted in a laboratory. For example, if a gasoline's anti-knock performance during the test is 85 percent as good as the performance of pure iso-octane in the same test, then that gasoline is said to have an octane number of 85. As another example, if a gasoline's anti-knock performance during the test is 10 percent better than that of pure iso-octane, then that gasoline is said to have and octane number of 110. There are two different sets of engine operating conditions used during the tests to measure the octane number of a gasoline. One set of conditions is used to determine the so-called Research octane number (RON), and another set of conditions is used to determine the so-called Motor octane number (MON). The average of the RON and MON is abbreviated to (R + M)/2 and that is the octane rating which is usually posted at service stations in the US.
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