Octane Invention

Since 1912 the spark ignition internal combustion engine's compression ratio had been constrained by the unwanted "knock" that could rapidly destroy engines.

"Knocking" is a very good description of the sound heard from an engine using fuel of too low octane. The engineers had blamed the "knock" on the battery ignition system that was added to cars along with the electric self-starter.

The engine developers knew that they could improve power and efficiency if knock could be overcome. Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, butarose from a violent pressure rise *after* ignition. The manograph was notsuitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock.

Ricardo had developed analternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc.

In 1927 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity.

These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane.

The octane had a high antiknock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.

The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test.

Name ------------Melting Point(C)---- Boiling Point (C) --Density (g/ml) --Heat of Vaporisation
normal heptane --: -90.7 -------------- +98.4------------ 0.684 -----------0.365 @ 25C
iso octane --------:-107.45 ------------+ 99.3 ------------0.6919 ----------0.308 @ 25C
Having decided on standard reference fuels, a whole range of engines andtest conditions appeared, but today the most common are the Research OctaneNumber ( RON ), and the Motor Octane Number ( MON ).