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Alcohol and gas mix
I suspect it would depend on the engine, but your math is not correct. You are obtaining about 17% less milage with the 10% alcohol.
Alcohol is going to give you less BTU's per gallon just as the winter blend of diesel gives you less BTU's and worse economy. With a 10% mix it should only give you a little less.
Remember that you have 95/5 ethanol to water mix when they distill ethanol as it aziotropes at this ratio. To get higher you must back distill from another solvent like benzene. The alcohol in the fuel will absorb any water in the tanks. Therefore the local fuel retailer gets his tanks cleaned of condensation and you get more than the .5% water you originally had in the gas and considerably less performance. Naturally this applies not only to the local retailer but also to the bulk dealer, pipeline and the tank truck. Do we need a spot test to check for water in the gasoline?
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Alcohol and gas mix
Well I am a scientist and engineer, but too lazy to look up all the numbers in Perrys etc. I suspect that the higher the compression the worse off you will be. Ethanol will have slightly less BTU value per gallon (76K vs. 116 K BTUs), say 35 percent. At ten percent add ratio that should give maybe a 3.5 % decrease in fuel value per gallon. Naturally the water will decrease the fuel value by another 0.5 %
The ethanol adds oxygen content to the fuel which decreases the formation of hydrocarbons in combustion and other pollutants. It also decreases the temperature of combustion decreasing the level of NOX produced. I am not sure if it is the water or the ethanol that provides this function.
Octane value in a gasoline engine is not just the BTU’s but the suppression of the ignition point, there is very little difference in the BTU value of premium and regular gasoline. The addition of ethanol will decrease the octane rating as it will increase the flash point. Pre-ignition on a high compression engine will decrease the power as on a modern engine the spark timing is automatically decreased to accommodate the lower value fuel on older engines we heard the knock. The higher the compression the more the spark must be retarded and the less value you will obtain from the fuel.
Naturally if the engine is designed to burn the alcohol fuel by itself like a GP car, a top fuel dragster or a Brazilian auto the problems are diminished.
Personally I think to get as large of a decrease as he experienced with his motorcycle engine he was ingesting more than 0.5% water. I have done no technical studies to support my hypothesis, but you do.
This is no longer fun and becoming too much like work, but possibly more informative.
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Alcohol and gas mix
Brendon, the octane rating is a comparison of the performance in burn rate and knock (flash point) tests when compared to heptane value 0 and iso-octane value 100. The tests try and define the delay and speed of the burn.
The fuel value or BTUs per gallon is not represented in the octane rating. Methane, propane and butane all have less fuel value than gasoline in terms of BTU gallon. Pure ethanol has a greater vapor pressure and lower boiling point than iso-octane. Therefore in pure form we expect it to have a lower octane rating. We expect it to burn more like hexane. In formulation? Gasoline has thousands of different compounds in it and how it interacts will the ethanol? MTBE is added to the gasoline primarily for the oxygen content, but it also increases the Octane rating. The other factor is the water content in the ethanol. This will delay the burn and possibly increase the power available, (remember that you are using all the energy available in the gasoline only a fraction) if at the right concentrations. People are still trying to sell water injection to increase your power.
Can you have a good octane rating, lower fuel value and poorer milage? Yes.
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