Hybrid Car – More Fun with Less Gas

prius worth getting? - Page 6

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Posted by Gene Nygaard on November 17, 2004, 1:30 am
nobody@home.net wrote in message

That's a wasted step, just changing to a different unit of mass, and
one which is of no utility whatsoever in getting to the units of
energy you use below, British thermal units, which are not in the
subsystem of units which includes slugs.   Of course, miles are also
not part of the system which includes slugs, nor are hours, so you've
also had to do a conversion of your speed numbers to use slugs with
any meaningful results.  Another wasted step.  Then you've had to
convert the resultant ftlbf to Btu.  Better just to use the pounds in
the KE formula, and mi/h for speed, then use a conversion factor from
lb mi^2/h^2 to Btu--and you can factor in the constant 1/2 factor as
well.  You can figure that out once and reuse it, rather than having
to waste time in so many intermediate calculations.

But what is this, a drone, a driverless car?  Don't you need to add in
the mass of the driver?  What is that listed mass for, a vehicle with
an empty tank?  Or one 1/4 full only?  Let's just throw in a little
more for driver and fuel (say you drive it from full down to a quarter
tank, so use an average value of 5/8 of a tank of fuel would seem
reasonable), and call the total mass 3100 pounds.

Better yet, just use the mass in kilograms and energy in joules, and
energy density of gasoline in joules per liter.  The calculations are
simpler yet.

You also have 44 more accelerations from 0 to cruising speed, each
requiring additional expenditure of energy over what would be used
just in normal maintining of speed in the face of friction losses and
those due to terrain, going up and down.

Gene Nygaard

Posted by nobody on November 17, 2004, 2:03 pm
Thanks for the response, Gene.  Interesting post.  My comments below.

On 16 Nov 2004 17:30:24 -0800, gnygaard@nccray.com (Gene Nygaard)

Weight and the slug are not different units of mass.  The units of
weight are force units, not mass. The slug is a mass unit as are
kilograms. As to the units of energy, I can use any I prefer.  If you
like Joules or kwh or ft-lbs(f), use them.

No big deal. Did the numbers on a spreadsheet.  Retired my slide rule
decades ago.

If you plug weight (pounds force) into the KE equation I gave you're
going to get the wrong answer.

Of course it's a drone.  I was just making a rough estimate to satisfy
my curiosity, ball park as some would say. Make any assumptions you
prefer. A weight of 3100 lbs would increase the kinetic energy by 7%
over my figure.

That's true indeed. Matter of fact, I did do it that way just for
kicks as well as the way I posted. I was raised in the English system
or something like it, so I still use it. Old habits are hard to break.
I even measure lengths in feet and inches.  Going to get a metric tape
measure some day. Would make life simpler.

Not relevant. All I needed was the kinetic energy stored in the moving
vehicle. I did tacitly assume level terrain. Should have stated that.

By the way, since this is the Prius newsgroup, do you have any
comments or opinions about the car?


Posted by Gene Nygaard on November 18, 2004, 2:31 am
 nobody@home.net wrote in message

No, "weight" is not a unit of anything.  Pounds are units of mass.
Slugs are units of mass.

You are one confused fool, failing to understand the simple fact that
"weight" is an ambiguous word, one with several different meanings.

The word "weight" entered Old English over 1000 years ago, meaning the
quantity measured with a balance.  That quantity is mass, not force.
It was used as a measure of how much stuff people had, for the purpose
of trade.  We still use the very same word today, with the very same
meaning, for the very same purposes.

Weight is never a force when anybody talks about "net weight" of
anything, nor about "tare weight" of its container.  Naturally, when
products in American supermarkets and auto parts stores and whatever
include pounds and ounces for this weight, they are every bit as much
units of mass as the grams and kilograms which appear right alongside
them on the same label.  In fact, we no longer have independent
standards for those pounds; since a 1959 international agreement, the
common, worldwide definition of these pounds is as units of mass
exactly equal to 0.45359237 kg.

Weight is never a force when anybody talks about "troy weight" of
anything.  That's one way the troy units differ not only from their
avoirdupois cousins, but from grams and kilograms as well--they have
never spawned units of force of the same name.  There is no troy ounce
force, never has been.

Weight is never a force when anybody talks about "carat weight" of
anything (5 carats = 1 gram in the modern definition).


Fuel tank capacity (l) 45
. . .
Load Capacity
Kerb weight (kg) 1300
Gross vehicle weight (kg) 1725

Toyota Prius

Curb weight is 2,733 pounds--about 250 pounds heavier than a Corolla,
and 330 pounds more then a Ford Focus sedan.


Curb weight: 2,890 lbs. (1,311 kg)

[Note that this is the number you used, Rod--GN]

http://autos.msn.com/research/vip/spec_Exterior.aspx?modelid 047&trimid=-1&src=VIP

Curb Weight - Automatic (lb.) 2890

<end of quotes>

Now, Rod, it is your turn.  Like you said, those kilograms are units
of mass.  Now, why in the hell don't you just explain to us all
exactly what is done differently when this "curb weight" is measured
in pounds, from what is done when it is measured in kilograms.

While you are at it, tell us why in the world the manufacturers would
measure two different quantities for this purpose in the first place?

Or just wake up and smell the coffee.  Let's add another to my list
above: weight is never a force when anybody talks about "curb weight"
or "kerb weight."  It doesn't make any difference how they spell it.
It doesn't make any difference whatsoever what units they use to
express it.  Nobody in the whole wide world ever measures curb weight
in newtons, and nobody in the whole wide world ever measures curb
weight in pounds force.

You can.  But the only units of energy in the only subsystem which
includes slugs are foot-pounds force.

You don't have pounds force.  You have pounds mass.

If you use pounds mass and feet per second in your kinetic energy
formula, the result is in foot poundals.  It is just as easy to
convert foot poundals to Btu, as your calculations converting foot
pounds force to Btu were.

If the curb weight is indeed 1311 kg, as listed in one of the sites
above, at exactly what areas on the surface of the earth would that
much mass exert a force of 2890 pounds force, measured to the nearest
pound force?  Note that even if you limit yourself to sea level on the
surface of the earth, a mass of 1311 +/0.5 kg will exert a force of
from 2881.4 lbf to 2898.9 lbf (throw in elevation, including Mt.
Chimborazo, the highest mountain on earth in both ways relevant to
this discussion, and the variation on the surface of the earth is even
greater).  But 1311 kg is 2890 lb anywhere on earth, or anywhere else.

If curb weight were in pounds force, as you erroneously believed,
you'd never have more than 2 significant digits, if all you know about
location is that it is somewhere on earth.

It is indeed relevant to figuring any change in the fuel efficiency

I'm intrigued by the idea, but I've never even seen a Prius.

Gene Nygaard

Posted by Bill on November 18, 2004, 3:06 am

and nothing else deserved consideration.

Posted by nobody on November 18, 2004, 12:38 pm

 Thanks Bill.  You are right. He has no interest in the Prius, just
wants to show off his knowledge of an arcane subject. How do you
suppose he landed on the Prius newsgroup?  Do you suppose he Googles
newsgroups looking for people who abuse units? Chief of the Units
Police maybe? Anyway, I just put him in the kill-filter box and closed
the lid.

Rod, the happily confused fool.

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