Gas mileage and headers

[Carl Ijames]

> I'm not talking mpg, I'm talking WOT.  Power output per CID.  Alcohol does
> allow higher compression, but not much higher than racing fuel.  I believe
> the big gain in alky is the presence of oxygen,  and oxygen is the limiting
> factor of any IC engine.  Even if BTU's per pound are less than non oxygen
> enriched fuel, the density of liquid fuel does not begin to compare with the
> density of air.  (even if alcohol is half the density of gas it more than
> makes up for the BTU difference because it brings in O2).  

[Warning - this is a pet peeve of mine :-).]

The short answer is that I highly recommend reading the gasoline FAQ.  The
longer answer is that the oxygen in oxidizers like oxygen gas and nitrous
oxide (and nitromethane) is available for combustion, while the oxygen in
oxygenated fuels is not.  A simplistic way to view combustion is to take
all the input molecules (fuel and oxidizer) and break every chemical bond
so that you are left with individual atoms.  This costs so much energy per
bond, as different bonds have different strengths.  Then, convert every
carbon into a molecule of carbon dioxide (CO2), and every hydrogen into a
molecule of water (H2O).  Forming these C-O and H-O bonds releases energy,
and if the energy released exceeds the energy input to break the bonds the
overall process is exothermic (releases energy).  Good fuels release a lot
of energy per molecule of fuel.  It doesn't take much energy to break the
O-O bond in oxygen gas or the C-C and C-H bonds in a hydrocarbon so they
are good fuels.  However (finally, you say :-)), an alcohol like methanol
carries it's oxygen in the form of H3C-O-H, or to say it another way it
has one C-O bond and one O-H bond.  Now, it takes as much energy to break
a bond as you get back by making one, and these are the same kinds of
bonds you make when you form CO2 (okay, I fudged here as this really has
double bonds so you get a little more energy back) and H2O, so the oxygen
in methanol contributes very little to the total released energy.  In
fact, the oxygen in an oxygenated fuel such as methanol or MTBE is
basically inert.  If you need X BTU/hour to maintain some speed and you
switch to oxygenated fuel that is partially inert (so it has fewer
BTU/gallon) you must burn more gallons of it to go the same speed.  In an
engine with an O2 sensor and feedback control to maintain stoichiometric
combustion conditions on average, more fuel will be injected and your
mileage will go down.  On a carbureted engine that meters fuel flow on air
flow, the same amount of oxygenated fuel will be consumed as
non-oxygenated fuel but since part of the fuel is inert less "real" fuel
is consumed and so the motor runs leaner.  This usually reduces CO
emissions, and so oxygenated fuels are a solution invented in the 70's,
based on 60's technology, implemented in the 90's when most cars
incorporated O2 sensors.

Compounds such as nitrous oxide (N2O) and nitromethane (CH3NO2) have their
oxygen bonded so that it can be released with much less energy than is
released by forming CO2 and H2O and so they are oxidizers.  Because they
are injected as liquids their density is much higher than that of air and
so much more oxygen can be drawn into a cylinder than a cylinder's volume
of air contains, so they are also called "chemical superchargers". 
Nitrous also gives a tremendous cooling effect which raises the apparent
octane of gasoline, which lets it work well with 93 octane pump gas. 
Nitromethane is even more special chemically because it is both a fuel and
an oxidizer (as are most explosives like TNT).  It contains more fuel than
oxygen so by itself it burns rich, which is why you still need air :-). 

Enough, sorry for the wandering rant.

-- 
Regards,
Carl Ijames     ijames at netaxs.com


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