[Diy_efi] Is E85 worth it?

Klaus Allmendinger klaus
Sat Aug 26 02:37:27 UTC 2006

Yes, you are right, you compare BTU (work) output vs. input. What I meant was that the engine efficiency is higher with ethanol than with gasoline. This means you can't assume the same efficiency number. Or expressed differently, with 40% lower BTU content you don't neccessarily have 40% lower mileage, as that assumes constant efficiency. As to the more moles of combustion product, that is very simple if you use the O2 in the charge as initial metric. One O2 molecule can produce one CO2 molecule from the carbon combustion (assuming stoichiometric combustion). But it can produce 2 H2O molecules from H2 combustion. A higher hydrogen/carbon ratio means the available moles of O2 are converted into more resultant gas volume (or pressure as in a fixed volume like a comb. chamber). This is one of the causes of higher engine efficiency with ethanol (and even higher with methanol). Hence, even though ethanol has lower BTU content, the cylinder pressure (and therefore torque) produced is not proportionally lower. Again, another way of expressing raised efficiency.
What all these discussions don't take into account is that we have gone about as far as we can push engines on gasoline regarding efficiency. Lean burn stratified charge has been tried with a little success, HCCI is still in the experimental stage. 
With ethanol these boundaries are by far not yet reached. Ethanol can tolerate a wider mixture range, both on the lean as on the rich side than gasoline. It can also tolerate much higher EGR dilution than gasoline. Engine efficiency in part throttle operation on gasoline is 10-15% or lower because of the lower dynamic compression ratio with intake vacuum. Throttling by EGR, thereby essentially lowering the O2 content of the end gas, allows to raise that part throttle efficiency quite a lot. But that is only possible with highly dilution tolerant fuel.
Flame front speed is higher (although initial kernel delay is longer) which means even about the same ign. advance can be used, it produces less negative torque from pressure rise BTDC and so on.


From: diy_efi-bounces at diy-efi.org on behalf of Daniel Nicoson
Sent: Fri 8/25/2006 6:26 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] Is E85 worth it?


I am curious of your source of the #4 passage below.  I think your science
is somewhat flawed.  I do agree that any normal engine will maximize it's
efficiency by configuring the engine to take advantage of the higher octane
available.  This usually means a higher compression ratio that will allow
better fuel efficiency.

I question the issue of not comparing the BTU in versus work out.  That is
how you determine efficiency, how much energy out versus the amount input.
You have to compare apples to apples.  I think what you really mean to say
is that the layman's measure of efficiency miles per gallon is really an
economic measure of work out (miles driven) per dollar input.

Actual thermodynamic efficiency would entail comparing the BTU input of the
fuel compared to the work output (converted to BTU for direct comparison).

For long term success in the marketplace, the "layman's efficiency" is what
will really count (it of course is determined by thermodynamic efficiency
and the price of various fuels).  The long term success of ethanol or any
other alternative fuel will boil down to its cost relative to good old

I curious what you mean by saying the ethanol will produce more gas
molecules with lower BTU input resulting in a higher pressure?  I think you
are trying to say that the ethanol is more powerful in an ICE with lower BTU
input?  That's just not true.

I do appreciate the clarification on the pipeline compatibility issue.  It
sounds like BP could have used a bunch of ethanol going through its
pipelines on the north slope of Alaska the last few years...

Take care,

Dan Nicoson

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
Behalf Of Klaus Allmendinger
Sent: Friday, August 25, 2006 5:58 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] Is E85 worth it?

4. Greenhouse gas production
The higher hydrogen/carbon ratio of ethanol or methanol means that a lot of
the energy is produced from the combustion of hydrogen vs carbon. This
lowers the tailpipe emissions of CO2 and CO dramatically for the same energy
unit. Also a comparison based purely on BTU (thermal) content as a basis for
milage prediction is unfair to these lower alcohols. As mentioned before,
they produce more gas molecules as combustion product and therefore higher
cylinder pressure with lower BTU input. After all, we are running internal
combustion engines, not steam engines, where a BTU comparison would be fair.
The lower flame-front temperatures of burning alcohol means also that less
nitrous oxides are produced.

Just my $0.02 worth.


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