Originally Posted by gotchiguy
Couple of questions for you who seem to be so clued up on the petrol front because I am intrigued! Is RON the percentage octane level of the fuel? Or is it some other measure? Because octane itself is aliphatic of course and the companies refine these to produce branched and cyclic isomers which promote efficient combustion but would not be called octane despite having 8 carbons. Also, how can ethanol increase the RON if it is a measure of octane, or is RON nothing to do with the level of octane rather a measure of the energy density of the fuel?
The biggest problem with ethanol on boats is storage of the petrol in the tanks. If left for long period (eg winter) condensation within the tank is a much more serious problem. Cars are much less likely to be stood for extended periods of time as boats are so the problem does not exist.
1/. Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.
Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.
Motor Octane Number (MON)
There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.
2/. Correct about storage, any water will lower the boost from ethanol in both octane, rvp and vli. (typically 4-5 rvp at 5% ethanol. this is further complicated by the denaturant used in the ethanol prior to blending)