I did a google for "road horsepower" and the first link was for a
Toyota MR2.
http://www.mr2.com/MR2TechData.html
A test by Car & Driver put it at 14.0 hp @ 50 mph. That would be 14*.
746.444 KW, which is what would be needed to cover 50 miles.
10.444/50 = .20888 KW hr/mi. An efficiency of 85% would not be
unrealistic and would give .20888/.85 = .246 KWhr/mi. This thing
looks like it probably has a lower CD than an MR2. BTW that 14.0 hp
was broken down into
Friction and tire losses @ 50 mph ..... 5.5 hp
Aerodynamic drag @ 50 mph ............. 8.5 hp
Bruce
On Sun, 21 Oct 2007 17:09:56 -0700, " snipped-for-privacy@my-deja.com"
I think you missed something here. According to your figures, it will
take just over 200 watts to move the car at 50 mph for one mile.
That's not the figure given of 14 hp, that's more like 1/4 hp expended
to move one mile. You have to expend that power at a constant rate of
14 hp or 10.444 kW. Look at it another way.
W=ExI
Where W=watt
E=voltage
I=current (in amps)
To make the math easer, assume you have a 100 volt battery. Then
solve for current using:
I=W/E I444watts / 100volts I4.44amps
We have a constant current draw of 104.44 amps required to propel the
car at 50 mph. That is the reason why it is really hard to get an
electric car to have a reasonable range (200 miles) at a reasonable
speed (50 mph). You would be looking at a total power expenditure of
over 41.7 kW to drive for 200 miles at 50 mph. That figure assumes no
head wind and level ground. Add in real life conditions and you might
be looking at power expenditures of around 45 to 50 kW. Also, you'd
flat get run over driving around here on the highway at 50 mph. You'd
better be traveling at least 60 mph which would boost your totals even
more.
Even if you had a battery that could deliver that much power, you
still have the problem of getting all that power back into the
battery. In order to replace 41.7 kW using your 120 vac outlet, you
need to draw over 41 amps for 8 hours. That is assuming 100% recharge
efficiency.
You can draw your own conclusions regarding the practicality electric
cars.
Jack
...
No, it takes 10.444 kW to move the car at 50 mph.
It takes just over 200 watt-hours to move the car one mile at 50 mph.
Why not 300V? 600V? 1000V? Wouldn't higher voltage be easier for the
electronics than lower voltage?
It's not the amps, it's the wattage. It's all 10kW no matter how you
slice it. If this was a 1000V system then it would only be 10.4 amps.
10 Amps doesn't sound like a problem.
Those new lithium batteries seem to be able to give 200+ mile range
to electric cars without any problems (except being expensive). I
would imagine you would recharge using a 220V appliance outlet like
they use for electric dryers, electric ovens and air conditioners.
Besides, who drives 200+ miles a day? If you drive only 30 or 40
miles then the battery won't be completely flat and it won't take
as much to recharge.
Anthony
On Sun, 21 Oct 2007 22:16:05 -0700, Anthony Matonak
How can it take 10.444 kW to move a car at 50 mph but only 200 w/hr to
move a car at 50 mph? Remember that a w/hr is a unit of power for a
period of time. A watt is a unit of power without reference to time.
You could use any battery voltage you wish. I chose 100 volts to make
the math easier to follow. The battery voltage would have no effect
on the power requirement.
It IS the amps and the voltage and the current and the wattage. They
are all related. Power in watts is voltage time current. Yes, you
could use a 1000 volt battery and it would only require a current draw
of 10.4 amps. The power would be the same.
You still have to put the power used back into the battery regardless
of what voltage you use to power the charger or what type of battery
you use.
What about the poor guy who has to travel 1000 miles in his electric
car. Does he drive 200 miles then stop for 8 hours while the battery
is recharged?
Jack
Would you prefer if we said that it takes 10.444 kWh to move a car
50 miles in one hour? How about that it takes 1/50th of that to move
the car just 1/50th of that distance in 1/50th of that time?
I'll let you do the math. Hint. 1/50th of 50 miles is 1 mile.
(I'll let someone else describe w/hr and Whr differences.) :)
The wiring in most houses is limited in amperage. Say your car can
only draw 30A from the outlet. If the outlet is 220V then it can get
twice the power than it could if it was 120V. A car that would need
8 hours to recharge on 120V would only need 4 hours on 220V.
Your complaint seems to be that it would take too long to recharge a
car at 120V. I suggest that it wouldn't take too long at 220V.
What about the poor guy who has to travel 4000 miles? What does he
do for speeding tickets and sleep?
Around these parts, it would take over 18 hours to drive 1000 miles
and anyone who has to drive more than a 1000 miles a day has got bigger
problems than where to plug in his car.
Assuming your car gets some 5 miles/kWh and you get to recharge using a
typical 220V 30A outlet. It would only require 6 hours of recharging for
every 200 miles of driving (more or less). This sounds reasonable to me.
Some folks have been suggesting the use of 'quick charge' outlets that
provide higher voltage, current or both. These could recharge the car
while eating lunch (or dinner).
Anthony
That number should be 500miles per day...at least if the driver wants to
stay
legel and safe.Roughly what a solo truck driver does in a day.
But then that also begs to explain as well ,if electric power is cheaper to
operate.And can do the the range across country without delay,with the
potential
of savings for the driver.Why are we starting out with cars instead of
battery powered trains
and 18wheelers?Hell the trains in most of the country already have thier
600v computer controled
traction motor already installed.Now how many of them everlasting batteries
do we for a 6000hp
locomotive.They sometime run more than one locomotive in a consist.....but I
guess we can multiple that one locomotive number on whatever the consist
requires.
At the same ,time I remember some trains were striaght electric.You think
,there might be a reason for most to run diesel on a generator,instead of
electric off an overhead or batteries.
To maintain a speed of 50 mph it takes 10.444 KW (note KW not KWhr).
If you travel at 50 mph for one hour you will have gone 50 miles and
you will have drawn 10.444 KW for one hour, meaning you used 10.444
KWhr. To only travel one mile would only take 1/50=.02 hrs. If you
draw 10.444 KW for .02 hr then you used 10.444*.02=.20888 KWhr or
208.88 watt-hours.
Your complaint seemed to be about the high current draw. Using a
higher voltage reduces the current. And actually it does have an
effect on the power requirement. Part of the power requirement is
providing for power losses due to resistance heating in the batteries,
wiring and motors. Those heat losses are proportional to the square
of the current. Go from 100V to 200V and the resistance losses are
cut to 1/4 of what they were. That also means you can work the motor
harder without over heating it.
Take his other car? Take the bus? Rent another car?
The electric may not be able to do all things, but if it can get the
equivelent of 160 mpg or more for the vast majority of your traveling
then maybe you can afford to make other arangements for those trips
that it isn't suitable for.
Another possibility would be to have an engine just big enough to
provide the power needed for a steady state cruise at the highest
speed you need to go. When going slower the batteries could be
charged on the fly so there would be more power availible for
acceleration or climbing hills. Something like this
http://tinyurl.com/22d3n6
should let you cruise at 70 mph while using .9 gal/hr. That works out
to 77 mpg.
Bruce
Read it again. That is 200 watt hours. Yes, a 1/4 hp motor should be
able to move the car 1 mile in an hour. And a 1 hp motor could move
it 4 miles in an hour. And a 2 hp motor could move it 8 miles in an
hour. And a 14 hp motor could move it 50 miles in an hour. A watt is
energy per unit of time, same as hp. A watt hour is a unit of energy,
as would be a hp hour. Spread that energy out over a long period of
time and the power is low, but the amount of work that can be done is
the same.
The claim made was 120 miles, not 200, and the car should use less
than the 14 hp to go 50 mph. That was the power required to keep a
Toyota MR2 going 50 mph. Even at 14 hp it would only take
10.444*2.4% KW hrs to go 120 miles. It would not be unrealistic to
expect this smaller car with better arodynamics to do 50 mph using 10
hp, which would mean about 18 KW hrs to do 120 miles.
120 volts * 20 amps is 2400 watts. 18 KW hrs / 2400 watts = 7.5 hrs.
They did say to plug it in at night, and that it would be recharged in
a few hours. They were probably stretching things a bit, but not all
that much.
Bruce
18KWhrs at $.12 would cost me about $2.16 in Florida... Thats about
what .75 gal costs this week... 120 miles on .75 gal would be 160 mpg
equivalent.... sounds hi to me... 100mpg equiv I might believe....
Try getting off peak power. Plug it in and let it charge while you
sleep for about half the cost of daytime power. What makes you think
the $/mi should be the same as for gasoline?
On Sun, 21 Oct 2007 22:29:47 -0700, " snipped-for-privacy@my-deja.com"
So 4 hp would move the car 16 miles in an hour (16 mph). A 8 hp motor
would move the car 32 miles. A 16 hp motor would move the car 64
miles. And it would take a 32 hp motor to move the car 128 mph. Holly
smokes, my car should be able to travel at well over 400 mph using its
120 hp motor. Do you see something wrong with this progression?
A watt is a unit of power without regard to time. A wHr is a unit of
work. 10 watts isn't the same as 100 watts but 10 watts applied to a
device for 10 hours will do the same amount of work as 100 watts
applied for 1 hour. The same is true of horsepower, it's a
measurement of power not work. A hpHr is a measurement of work.
So, if it takes 14 hp to move a car at 50 mph without regard to time.
It will take 14 hp to move that car at 50 mph for one minute or one
hour. The only difference is the amount of work done, not the amount
of power needed.
So, his figure of 200 watts to move a car at 50 mph is wrong.
Jack
Sure, I made it linear rather than exponential to keep things simple.
The further you get from the 50 mph starting point the further off you
will be. The point was that 1/4 hp would move the car 1 mile in an
hour. That's traveling 1mph for an hour.
It was 200 watt-hours to move the car one mile in 1/50 hrs. If you
want to go 50 miles in an hour it will take 50 times that.
Sorry but you're the one that seems to have things mixed up.
If you apply a force over a distance you have work. Say you push with
a force of 11 pounds for a distance of 50 feet , you have done 550
foot pounds of work. Time doesn't enter into it. It is 550 foot
pounds of work whether you do it in ten seconds or ten minutes.
Power is how much work is done in a given time. 550 foot pounds per
second is one horsepower. So if we pushed the object above the 50
feet in one second it would have been 1 hp. But it took 10 times that
long so we were producing .1 hp.
Watts are power. 1 hp = 746 watts.
If we specify a power output for a specific period of time the result
is an answer in work. Look at the units. Power is foot pounds
divided time, and you are multiplying by time. The times cancel out
leaving you with foot pounds.
The same thing applies to watts. A watt is current times voltage.
The current is flow per unit of time. So when you specify a time
period for those watts you multiply by that time and get the work done
during that time.
In the original example we knew it took 10,000 watts to maintain a
speed of 50 mph. We know that at that speed it takes .02 hours to
cover 1 mile. Multiply the power (10,000 watts) by the time (.02
hours) and you get an answer in units of work (watt hours). If we had
multiplied the power in horsepower by the time we would have gotten an
answer in horsepower hours which can be converted into foot pounds.
The 200 watt hours seems like a small number compared to 10 KW, but it
is the work done in a short period of time (.02 hours). If you do the
same thing 50 times it will take 200*50,000 watt hours over a
period of 1 hour. Or go at it the other way around. Divide 200 watt
hours by .02 hours. The time units cancel out leaving 10,000 watts.
Starting to make sense?
Bruce
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