alt.energy.renewable

On Sun, 21 Oct 2007 22:29:47 -0700, "bsr3997@ "
wrote:

>On Oct 21, 10:09 pm, Retired VIP wrote:
>> On Sun, 21 Oct 2007 17:09:56 -0700, "bsr3...@ "
>>
>>
>>
>>
>>
>> wrote:
>> >On Oct 10, 1:55 pm, Eeyore
>> >wrote:
>> >> BobG wrote:
>> >> > Actually looks pretty slippery. .25KWhr per mile perhaps?
>>
>> >> In a flat country like Holland maybe ?
>>
>> >> Graham
>>
>> >I did a google for "road horsepower" and the first link was for a
>> >Toyota MR2.
>>
>> > /
>>
>> >A test by Car & Driver put it at hp @ 50 mph. That would be 14*.
>> >746= KW, which is what would be needed to cover 50 miles.
>> > /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 hp
>> >was broken down into
>>
>> >Friction and tire losses @ 50 mph ..... hp
>> >Aerodynamic drag @ 50 mph ............. hp
>>
>> >Bruce
>>
>> 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 kW. Look at it another way.
>
>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.

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


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