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5kw metro engine?
Topic Started: Nov 20 2010, 11:06 PM (1,880 Views)
dayle1960
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Fastest Hampster EVER

A buddy of mine asked me a question today at lunch. I said I would throw it to the smart guys at geometroforum and see if they could answer it.

If a railroad locomative has a diesel engine which runs electric motors which in turn moves the wheels on a VERY heavy train, could a 5kw gas powered electric generator run electric motors in a metro and make the metro move at 55mph?

My answer was NO. The energy from the 5kw gas powered electric generator would not equal the output needed to move the weight of the metro. He thought that my explaination was BS. Oh well.

Please chime in and let tell me if my answer is correct, or if he could really run a metro on a 5kw gas powered electric generator.

Thanks

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Stately
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How much (if any) battery capacity are you allowed?
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Coche Blanco
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Troll Certified

what is the phase of the moon during said run?
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duff_remle
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[ *  *  *  * ]
1hp = 745.7 watts

metro = 50 hp = 37285 watts

so 5000 watts doesnt sound like nearly enough to me
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dayle1960
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Fastest Hampster EVER

Stately, he said no batteries were needed. Just use the power supplied from the generator which goes directly to the motors on the wheels.

His thinking is that if his "5kw generator can supply power to his house during an electrical blackout caused by an ice storm, surely that same generator can supply enough power to run a car".
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Woodie
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It's roughly 7 horsepower, no way. Takes about 10 just to cruise at 55 mph, three times that much to accelerate up to speed.
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dayle1960
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Fastest Hampster EVER

That is exactly what I told him, woodie. The guy just doesn't get the concept of potential and kinetic energy and forces used to get an object to move. I told him there would not be enough energy to get the car moving, but once moving it would have enough energy to keep the car in motion, except when going up hill.

He doesn't realize the size of a compressor in a refrigerator is smaller than the size of motor needed to turn one wheel on a car. Now quadruple the number of motors, then the 5kw generator does not have enough potential energy to spin four motors and move 1600 pounds of car. He just does not get it.

Thanks
Edited by dayle1960, Nov 22 2010, 10:08 PM.
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Woodie
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The horsepower number should clarify things for him. If he's an average guy, he's probably got a 200 hp car and thinks you're a madman for driving a 55 hp car. Explain to him that his car has 147 kilowatts, and yours has 40 kilowatts, now he wants to get by on 5?

We wouldn't even be having this discussion in England, because they rate car engines in kilowatts anyway, he would already know that is riding lawn mower territory.
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Stately
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dayle1960
Nov 21 2010, 05:26 AM
The guy just doesn't get the concept of potential and kinetic energy and forces used to get an object to move.
Exactly--without batteries, *massive* flywheel, or other "potential" storage it wouldn't work.

But, hey, he could always power his A/C with the generator:

Posted Image
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JellyBeanDriver


Quote:
 
The guy just doesn't get the concept of potential and kinetic energy and forces used to get an object to move.


Actually he's ignoring rolling and air resistance, and how long he's willing to wait to get to speed. You could get a metro to 100MPH with a 1HP engine under the right hypothetical conditions (total vacuum of space and rock hard tires).
He's doing the conversion of mechanical energy to electrical so he could maximize torque at any speed. If he's willing to take an infinite amount of time to reach his target speed, even 1HP or less would get the metro up to speed.

So he's right, but it only works on paper and if you ignore all the losses in the real world.
Edited by JellyBeanDriver, Nov 21 2010, 01:02 PM.
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JellyBeanDriver


Here's an extreme case of what I'm talking about - propulsion of space crafts using 'solar wind'
http://science.nasa.gov/science-news/science-at-nasa/2000/ast28jun_1m/

Quote:
 
A common misconception is that solar sails are pushed by the solar wind just as sailboats are propelled by the wind on Earth. This is not so. The solar wind is an extremely tenuous flow of particles streaming away from the Sun. It exerts very little force on anything it hits. The propulsive force for a solar sail arises from the pressure of photons (light) from the Sun or from lasers. Sunlight at 1 Astronomical Unit (1 AU is Earth's distance from the sun = 150 million km or 93 million miles) exerts a force of 9 Newtons per square kilometer (0.78 pounds per square mile) on a solar sail.


Very little energy input (photons), very low losses, spacecraft is NOT light, and eventually it reaches a speed of 90KM/second!
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nerys
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Grr

Yes that could work and no it can't work.

First electric is orders of magnitude more efficient than gasoline so you don't need ANYWHERE NEAR the watts that a gas engine "equates to"

but your problem is not going 55mph. thats easy actually you only need about 75-150 amps (likely less for a metro).

your problem is ACCELERATING where you can use400-500 + amps

so you might be able to use an electric motor and genny to KEEP the metro going 55mph but you would have a VERY VERY hard time using it to "get to" 55mph without blowing the breaker in very short order.

on the other hand a 5kw genny is not that much more efficient than a metro engine if at all so you might actually end up using MORE FUEL if you could make it work.

50mpg is 1 gallon of gas per hour. at 50 mph

my 5.5kw genny burns through just under a gallon an hours but at only 50% load. at full load it burns through MORE than a gallon an hour IE worse than my metro.
Edited by nerys, Nov 21 2010, 09:02 PM.
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JellyBeanDriver


nerys
Nov 21 2010, 09:00 PM
Yes that could work and no it can't work.

First electric is orders of magnitude more efficient than gasoline so you don't need ANYWHERE NEAR the watts that a gas engine "equates to"

but your problem is not going 55mph. thats easy actually you only need about 75-150 amps (likely less for a metro).

your problem is ACCELERATING where you can use400-500 + amps
amps mean nothing if you don't include what voltage you're using.
Edited by JellyBeanDriver, Nov 21 2010, 09:04 PM.
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Woodie
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No one asked about efficiency or whether you would save fuel, he simply asked "would it work?"

5KW = 7 HP
55mph = 10 HP

Nope, won't work.
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nerys
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Grr

I don't require you to "ask me" something to "provide it" woodie. and off course you flat out ignored the entire first 80% of my post which had nothing to do with efficiency and everything to do with if it would work or not. I then added my own thoughts on the issue. if you don't like that, well, tough.

I think 7 hp from an electric motor would be enough to cruise at 55mph your ignoring the efficiency of the electric motor it is NOT equal to the gas motor.

I don't think it would be enough for acceleration unassisted.

typically 72 volts so that would be what? 5400 watts and I think the Metro would take less. but again this would be CRUISE CONTROL. if you tried to accelerate your going to have issues :-)
Edited by nerys, Nov 22 2010, 08:02 AM.
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