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Originally Posted by Old Tele man
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That's not per equivalence oxygen though. Per equivalence oxygen I think the alcohol fuels have about the same heat of combustion, maybe ever slightly more, maybe slightly less I can't remember. I am pretty sure the usual quoted heats of combustion are going from liquid state fuel, so they include latent heat of vaporization. I believe gaseous ethanol combusting produces about the same amount of heat.
With insufficient timing the slower burn of ethanol could possibly give rise to higher exhaust temperatures, but I imagine this would be very hard to actually accomplish. Moreover, the most heat sensitive thing in the exhaust stream is not the valves but rather the catalyst, and manufacturers tune their engines to protect the cats.
The thing with ethanol is that because its latent heat of vaporization is significant, even compared to its heat of combustion, it's very sensitive to how the fuel is delivered and vaporized. Ideally for efficiency you'd want all the fuel to vaporize in the chamber after the intake valve seals, to reduce the work on the compression stroke. Direct injection systems inject on the intake stroke and maybe a second time on compression because they need time for the mixture to form, so no engines actually take full advantage of the cooling effect for efficiency. What you can do is cool down the surfaces in the engine a bit to quench potential hot spots that could cause knock, and cooling the intake charge a little helps too.
However some engines end up with unvaporized ethanol at the end of the compression stroke, and these will see near the full 34% loss of "fuel economy". I think this happens to some extent on a lot of current crop flex fuel engines because they're not doing so well in fuel efficiency. In this case, the exhaust temperature is definitely going to be lower.