At the beginning of the 20th century, engineers were perplexed by aircraft engines that were self-destructing for no apparent reason: One day, they would run fine, the next, the engine would hole a piston. Soon, they figured out that fuel detonation was the culprit, tracked the problem down to varying fuel qualities, and it became apparent that a standard fuel rating system was needed.
At the time, batches of fuel that weighed and seemed identical were not--there was a great difference in quality, even amongst ones that came from the same factory.
So the fuel manufacturers tried to rate fuels by through a series of chemical tests, which proved unreliable in determining which blend would "knock," or detonate, once used in the real world.
Consequently, special single-cylinder engines with variable compression ratios were developed as a standard test platform--all you had to do, theoretically, was crank the compression up until the fuel being tested began to knock, and then record it's High est Useable Compression Ration (HUCR). These engines were distributed to different fuel labs, and a standard was born.
Or so they thought.
After testing at different sites, it was found that the exact same fuel would produce different HUCR numbers, depending on the atmospheric conditions. It was then decided to pick two pure, readily available substances to calibrate all the test machines/si tes. Presumably, pure substances would give predictable, constant performance by which a standard "low" and "high" rating could be set.
The two primary reference fuels, Isooctane (2,2,4 trimethyl pentane) and n-heptane were arbitrarily chose and, again arbitrarily, assigned "octane" numbers of 100 and zero, respectively. Then, all test engines could be "zeroed" in with n-heptane, while an upper range could also be quantified with Isooctane.
Octane, by the way, turned out to be a poor name since the "octane rating" of the molecule n-octane (C8H18) is actually -17!
A fuel's "octane" rating is found by comparing it's knock characteristics with various mixtures of n-heptane and Isooctane. For example, a "octane" rating of 92 means that the fuel being tested, under standard conditions and running in a sta ndard engine, performs the same as a mixture that is 92 parts Isooctane, and eight parts n-heptane. Numbers higher than 100 simply specify the potential to perform better that pure Isooctane.
But since those early days of fuel standardization, the testing procedure has fragmented into several testing procedures, the most relevant being the Research Octane Number (RON), Motor Octane Number (MON) and the Pump Octane Number (PON).
The RON and MON tests use the same single-cylinder, variable-compression engine, but differ in that the MON test specifies a higher test RPM and inlet
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