Post by Rich
combustion is not used (BUT in MHO it could be). The mixture is always on
the rich side to guarantee predictable and repeatable ignition without any
dead cycles. The rich mixture is another inefficiency because it results in
less than complete combustion. There are some other kinetic factors like
Most modern engines run super-lean and recirculate exhaust to mitigate the
detonation (this reduce oxygen % and thus flame speed).
Post by Rich
Increasing the compression would create a richer oxygen environment to
produce a hotter burn (more heat) which is the whole objective of the IC
engine. But too much oxygen would move the process from burn to explode
which would damage the engine. There is the real limit on mileage and
A suitable ECU can cope with 100% oxygen (see above). The cylinder
metallurgy cannot cope with 100% oxygen.
Post by Rich
efficiency: How much heat you can get from the burn of the fuel without
exploding it. In diesel engines the compression ratio is much higher and it
is so oxygen rich that a spark is not required to initiate the burn. But
here, again, the efficiency limit is imposed by the rate of oxidation; too
fast and BOOM, you wrecked your engine. The reason diesel can tolerate
higher oxygen ratios is because there is less BTU per unit volume than for
gasoline, of whatever octane.
Actually Diesel fuel has higher BTU/cu. ft than Otto, precisely because of
higher compression. And Diesel can cope with 100% oxygen with
recirculation as above. The reason Diesel is more efficient is the higher
compression and the different burn characteristics of heavy fuel (it won't
really detonate). The same increase in efficiency reduces the heat load on
the cylinder and allows more boost. The metallurgy is the actual limiting
factor for Diesel boost, as anyone running tuned Diesels will tell you,
there is no such thing as too much boost for a Diesel, but melting the
engine or making the exhaust glow red hot is a definite possibility.
High efficiency compound power stations use a blown Diesel followed by
reheat and a steam turbine to reach very high efficiencies. Alternatively
a gas turbine is used instead of the Diesel.
Post by Rich
So to get more mileage you need to figure out how to get more heat per
unit fuel, a more complete burn and keep on the safe side of explosion.
Now here is an interesting point. Some people talk about alternative
energy on one side of the page and better efficiency on the other side.
So they came up with ethanol. Ethanol has less BTU per unit volume than
Gasoline. To save energy they advocate mixing ethanol with the gasoline and
at some pumps it is already premixed. Question! How is mixing something
with less energy with something with more energy going to result in
increased efficiency, or better mileage, and so on. The heat produced by
the burn will actually decrease, resulting on lower power. The problem of
repeatable ignition cycles requiring a richer mixture remains. Is it an
oxymoron? Or is it simply a moron idea? I am baffled by it.
It is an oxy (pun!) moron, and the joke is on the buyer. Ethanol is
'pre-oxydized'. Fuel is sold by the gallon but it should be sold by the
pound (weight) or more exactly, by the BTU, converted to weight. All
professional fuel measurements are in lbs/hr (or HP or mile or kW such) at
given BTU (or equivalent kg/hr). Also ALL bulk fuel sales are by lbs or
tons. There is no such thing as selling by the gallon excepting for the
suckered end customer. Even when bulk barrels are sold temperature is
accounted for to arrange for the amount in bulk barrels at STP. The reason
is that fuel varies in volume, with temperature, besides having various
BTU grades. When one buys gas in winter one buys more gas by weight per
gallon than in summer. The difference is large enough that it can wipe out
smaller effects (like economizers and engine tuning).
The current way to sell fuel by the gallon regardless of temperature while
leaving the makers to 'blend' it as they see fit is a license to steal
imho. Meanwhile the state authorities that tax fuel tax it happily by the
gallon regardless of density and BTU content. Switching to E85 increases
tax income (at the same tax rate) by up to 30% for the same mileage. So if
E85 would get a '10% tax break' then the tax authority would still gain by
a large margin.
Ethanol mixed into gas (specifically E85 and the like) is effectively a
hole in the consumer's pocket and liable to bring a bad name to Ethanol
fuel. Not only does the gas prevent the engine to be compressed up to 15:1
as pure ethanol and ethanol/water (incidentally W85 defined as 85% ethanol
and 15% water would be a workable fuel) would, but it adds all the
problems of limited storage time (if any water gets in E85 the fuel will
separate) and reduces the BTU thus increasing consumption by the gallon
gauge by up to 30%, as recently compared by an auto magazine's independent
testing. A boon for the taxman and for fuel Co's, and a bad name for
Ethanol. With this the taxman and the fuel co's hit two rabbits with
somebody else's bullet imho.
A fair way to sell fuel is by BTU, specified by mass (lbs or kgs). This
deletes the temperature effect and even makes E85 comparisons fair since
the price should reflect $/BTU and not the treacherous $/gallon (or
And to run an engine on ethanol, one runs it on 100% ethanol or 85-95%
ethanol + water + nitrous or such, at 15:1 and higher (possible with
exhaust recirculation) compression, where the added efficiency by
compression increase more than compensates for the lower BTU/lb (not
gallon!) of ethanol. In fact, to be fair, car mileage should be given in
miles/BTU (or miles/kWh or equivalent). That would allow electric and
other cars to be compared too, and would be a direct measure of
And, incidentally, CO2 exhaust, 'green footprint' etc is also related to
fuel mass and type, not volume. Look up any chemical reaction and you'll
see that *everyone* balances reactions by mass, not by volume, from 9th
grade on at least. That makes suckers who buy fuel by volume in summer
less intelligent than 9th graders imho.
And if you think I'm just babbling, read about a lawsuit in Utah that
tries to force authorities to make gas retailers add temperature
compensation to gas pumps:
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