Re: I have never used ethanol - Saab 900 Bulletin Board - Saabnet.com


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Re: I have never used ethanol
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Posted by mag [Email] (#1906) [Profile/Gallery] (more from mag) on Mon, 13 Jul 2009 20:13:08 Share Post by Email
In Reply to: I have never used ethanol,, tibbidoh [Profile/Gallery] , Mon, 13 Jul 2009 15:46:26
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Octane: The research octane of ethanol is 106 vs 90 for gasoline. The (R+M)/2 octane of ethanol is 98. The autoignition temperature is about 200 degrees higher. Ethanol doesn't ping nearly as readily as gasoline, and will tolerate higher compression ratios and stronger and longer boost than will gasoline.

You want to fire ethanol about 5 degrees sooner with ethanol. I've seen no data on what the ideal spark angle should be for C900. Start at 21 and experiment.

Ethanol burns faster than gasoline (faster flame front propagation). Ethanol burns cooler than gasoline, yielding exhaust temps of about 1000 degrees instead of 1400 to 1650 (this is not data from a Saab). It has a higher flash point than gasoline, which makes it harder to start in freezing temperatures.


As for ethanol's energy density, that's not the most interesting property. If you measure energy density by E=MC^2, then ethanol, with its greater mass per gallon, would nuke gasoline. Hmmm. But that's not the kind of energy we want. ;-)

It's popular in the press to compare BTUs, a measure of heating value. Gasoline has about 110,000 BTUs per gallon, ethanol 76,000, and #6 Fuel Oil about 150,000. If you were running a radiant floor heater and had all day, gasoline's 45% higher BTU content would be interesting. But we don't care about BTUs so much because efficiency matters and *time is of the essence*. If BTU was the key property, we would be running Saabs on melted beeswax.

What matters is how efficiently is energy converted to work in the limited time available.


Combusted ethanol produces a greater volume of exhaust gas, at a lower temperaure, than you get with gasoline.

We have some very interesting properties that are nicely suited to an internal combustion engine. It's a stable fuel that doesn't detonate until you tell it to, then it flashes rapidly, it takes compression very well, you can advance the timing giving it a bit more time to combust, leading to more complete combustion, and you get a higher volume of exhaust gas at a cooler temperature. Compared with gasoline, more of the chemical energy is going into expanded gas and into piston motion and less into heat.


I don't have a good handle on how this *really* works. But the possibilities are interesting.

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