Re: Energy cost calcs: all gas vs. all electric
- From: "The Pre-Meltdown Kid" <entropic3.14decay@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 12 Feb 2008 23:59:38 -0500
Oh, might should have mentioned the overall strategy of the previous calcs:
Notice that there is no mention of engines, cylinders, or any other
specifics.
The calculation of *theoretical mpg's* hinges on just a few general
variables, some of which we can't do much about:
gas engine eff, energy content of gas, the CdA, and internal losses of the
car.
Oh, and the the speed at which one drives.
Only the CdA (wind resistance factor) can be manipulated to realize greater
theoretical efficiences.
And driving slower in top gear. Altho a previous thread on this topic
suggested that this was not always the case, ie, slow speed in top gear =
best mpg's, but likely "second order" diffs.
In *actual* efficiencies, the specifics of engine design of course come into
play.
In my Honda Fit, the only way to raise the "theoretical max mpg's" of 54 is
to lower the driving speed, or further reduce the CdA.
It seems, tho, that for bitty cars like the Honda Fit genre, 50-60 mpg is
about the max mpg's that can be realized in a gas powered car--not the 100+
that some ballyhoo about.
However, for all electric cars, the equivalent of 200-500 mpg's seem
realistic--barring calculational errors.
--
------
Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
Stop Corruption in Congress & Send the Ultimate Message:
Absolutely Vote, but NOT for a Democrat or a Republican.
Ending Corruption in Congress is the *Single Best Way*
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AND,
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entropic3.14decay at optonline2.718 dot net; remove pi and e to reply--ie,
all d'numbuhs
"The Pre-Meltdown Kid" <entropic3.14decay@xxxxxxxxxxxxxxxxxx> wrote in
message news:47b2737d$0$25054$607ed4bc@xxxxxxxxx
Awl--
Even tho "gasoline mpg's" may be obsolete with the advent of electric
cars, I thought it would be interesting to see at least what the upper
limit of gasoline mpg's are.
The calcs reveal an additionally compelling feature about electric cars,
namely that in addition to a myriad of other electric car advantages, they
operate under a *fundamentally higher thermodynamic efficiency*, because
the efficiency of a charging cycle of a battery is double-triple that of a
Carnot-limited heat engine.
My calcs and values follow--would be interesting to see what others might
get.
The drag equation ( http://en.wikipedia.org/wiki/Drag_equation) is
Force = .5 p v^2 Cd A
p = density of air, v = velocity, Cd = drag coeff, A = net frontal area of
object.
F x 1 mile is the energy needed to overcome aerodynamic drag for one mile.
A gallon of gas has about 131 megajoules.
Net engine efficiency is about 32% (about 60% thermodynamic/Carnot eff,
and we realize about half of that).
Aerodynamic drag supposedly accounts for 60% of the energy needed to
propel a car, at highway speeds.
Using a CdA of about 5.5 for my Honda Fit (auto), at 65 mph, and the 60/40
energy apportionment, I get a "theoretical" mpg of about 54.
On an upstate drive, I was thrilled to achieve 46.4 of those 54 mpg's,
much better than Consumer Reports results.
City driving, however, esp. with Yonkers hills, is not much better than
25-30.
Hummers have a CdA of about 25, which means they consume 5 times the
aerodynamic energy of my Fit.
As stated above, perhaps the biggest energy hit occurs from middling gas
engine efficiency, where 67% of the energy in a gallon of gas goes right
out the radiator/tailpipe--up the proverbial chimney. wow....
And, another 40% of the paltry energy extracted from said gallon goes to
sundry mechanical inefficiencies, likely consistent with the large
difference between crankshaft hp and rear-wheel hp: Transmission
losses, driveline losses, sundry power-assists, etc.
So from a pure energy pov, the comparison between gas and electric looks
like this:
Gas: 131 Megajoules x 32% (thermo) x 60% available for aero drag = 25
megajoules applied to aero drag.
Electric: 131 x 80% (batt/elec motor eff) x 85% (assuming only 15% loss
for car internals) = 89 megajoules applied to aero drag.
Of course, the electric is not using a gallon of gas, this is just
supplying a 131 megajoule "initial equivalent".
Thus, an electric car can be 350% the energy efficiency of an IC engine
driven car. Mebbe more.
One can fudge the numbers quite a bit either way, but it is unlikely that
an electric will fall below 200% the total energy efficiency of gas.
In addition, the energy/resources consumed in the mfr of an electric car
are likely a small fraction of that of a gas car, esp. if we dispense with
the 300 hp bullshit.
VWs did fine with 42 hp.
If we factor in the overall lower-power of an electric, say, 40 vs 200 hp,
we have *another* factor of 500%.
Overall, the energy savings of automotive transportation could drop to 1/5
to 1/10 of current usage, which is pretty astounding.
Also, the automotive service industry will be revolutionized--or
destroyed, depending on your perspective. "Check the brushes on your
permanently lubed DC motors, sir?"
Might not even have brushes!
Yeah, batteries are still a problem, but proly not for long.
With a small good hub motor on each wheel, even lead-acid batts are
viable, at least in-town.
If all this indeed happens, I guess guys will have to go back to wearing
decorated/bejweled codpieces, instead of their Vettes, Vipers, and
Carreras. :)
Info for the above calcs:
p = rho = 1.293 (metric units); v = 65 mph; CdA = 5.5 sq ft
Conversions:
.447 mph to m/s
10.76 sq ft to sq m
1610 meters/ mi
.6 = fraction of energy applied to aero drag
131,000,000 joules/gal
.32 gas engine eff.
All data from the below peer-reviewed sources: :)
http://en.wikipedia.org/wiki/Drag_equation
http://en.wikipedia.org/wiki/Drag_coefficient 60%
http://en.wikipedia.org/wiki/Automobile_drag_coefficients
http://en.wikipedia.org/wiki/Automotive_aerodynamics CdA's
http://ecen.com/content/eee7/motoref.htm enginen eff
http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf 95% batt eff.
--
------
Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
Stop Corruption in Congress & Send the Ultimate Message:
Absolutely Vote, but NOT for a Democrat or a Republican.
Ending Corruption in Congress is the *Single Best Way*
to Materially Improve Your Family's Life.
The Solution is so simple--and inexpensive!
AND,
Make sure whomever you do vote for believes in
ABSOLUTE separation of Church & State--ferchrissakes
entropic3.14decay at optonline2.718 dot net; remove pi and e to reply--ie,
all d'numbuhs
.
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