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AV Fuel..... the real deal

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This was posted a long long time ago on scream and fly. WELL worht the read.

Aviation gasoline is still readily available in 3 formulations:

80 which is now completely lead free ( red in color )

100LL which has < 1 gram of Tetraethylead/gal. (blue in color)

100 which has > 1 gram of Tetraethylead/gal. ( green in color )

80 which was originally called 80/87 has a research octane number of 87 and a motor octane number of 80. This means using the gas station pump octane system of the average ( R+M)/2 yields a pump number of about 83.5. 100 green or what was originally called 100/130 has a pump number of 115. 100 low lead is the newest and does not follow the same convention its 100 octane rating is a good approximation of its pump number. Its research octane is usually about 106.5 with a motor number of about 94.

I am going to assume you are wondering about 100LL Blue which was developed for aircooled, primarily forced induction, 2700 rpm, fourstrokes as a fuel for watercooled, naturally aspirated, 9000 rpm, two stroke. The important thing to remember when searching for the optimum gasoline for a spark ignition internal combustion engines is this: PRODUCTION OF MAXIMUM POWER REQUIRES THE PREVENTION OF DETONATION BUT THE ABSENSE OF DETINATION DOES NOT MEAN MAXIMUM POWER IS BEING PRODUCED. I know this sounds like a contradiction but it is similar to all squares are rectangles but not all rectangles are squares . As an example you could purchase gallons of Toulene ( research octane of about 118 ) at your local paint store and add them to pump premium at a ratio of 2 to 5 to produce a 100 octane fuel. This fuel would surely eliminate detonation in any outboard ever conceived partially because of the octane but also because so little power would be produced ( so less heat would be produced ). The deficiency of 100LL for a modern outboard fuel starts with its specific gravity which is at .65-.68g/ml when pump premium is usually .72-74g/ml means the fuel delivery volume must be increased by 6-9% to maintain the same air fuel ratio. Remember as a teach my Thermodynamics students the proper stochiometric air/fuel ratio of 14.7 is by weight not volume. So when the weight per volume (g/ml) drops you must introduce more volume to maintain the same weight. Im sure some of you have experienced this to a much larger extent when experimenting with Ethanol, Methanol, and Nitropropanes. So now you are saying so what if I have to enrich the mixture. Well in terms of producing the maximum horsepower from a wet induction motor this extra fuel vapor in the ports displaces air. Think about it like this, the ports only flow a fixed VOLUME of vapor when a large percentage of that is fuel then less air can reach the combustion chamber. So now you are probably asking how can an alcohol motor make so much power when such a large portion of the available volume must be fuel? Well the answer is that alcohols are oxygen bearing while gasolines are not. If you just consider the chemical nature of the intake charge an alcohol fueled motor has both more oxygen and more BTUs of energy pre volume than a gasoline charge. But I digress the second drawback of 100LL for a lakeracer is its latent heat of vaporization pump premium is about 560*10^3 J/kg, 100LL is about 690*10^3 J/kg, ethanol for comparison is about 850*10^3 J/kg. I realize that the joule as a unit of energy has very little meaning to Americans, so let me summarize buy saying that this is directly related to the charge cooling of the motor. This drop in charge temperature will increase power maybe 1.25% in a 250CID two stroke cylinder say 7.5% for a 150CID V6. This will offset and mask most of the power loss from the specific gravity issue, but the negative is this the cooler charge temperatures lead to greatly increased condensation inside the motor when you shut it off. When you tear the head off a motor running 100LL on a typical Midwestern or southern summer day 80-90 degree intake charge temperature ( high humidity, Dew point above 65 Degrees ) after it has cooled down you will find more water in the cylinder than you would with a lower heat of vaporization fuel. This problem is pronounced in alcohol motors and smart racers have learned to run there motors on pump gasoline before storing them. This is not a problem for aviation engines this fuel designed for because there intake charge temperatures are over 250 degrees and the fuel acts like an intercooler removing some of the heat. You can solve this problem by doing like the alcohol racers do or by fogging your motor heavily after every use in the above conditions. The biggest drawback of 100LL is that it contains volatile compounds released under temperature and pressure ( during combustion ) designed to dissolve lubricating oils and flash them during combustion. The reason for this is that the bane of the forced induction carbureted aviation motor is oil fouled plugs. This single characteristic is why people say 100LL runs dry. When they dissemble the motor they see very little lubricating oil in the combustion chamber where normally due the designed in porosity of Nicasil and to a lesser extent Nicom the bore always feels oily. This problem can NOT be rectified by increasing the fuel/oil ratio. I have seen testing where 100LL reduced the ring life by half versus standard pump fuels. This is not a problem in an aviation motor where peak piston speeds are more like 1000 foot/minute than a 2.5s 4000 F/M.

100LL is essentially immune to tampering or cheating, but it is quite a poor choice for a high performance outboard fuel.

Dr. Pat Farrell

Engine research Center

U-Wisc Madison

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