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Old 02-28-2011, 01:32 PM   #1 (permalink)
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Power and Weight

Build light as possible. The idea that adding more batteries will make the car go farther can backfire. The problem is that if weight is doulbled the energy to accelerate that mass is 4 times as much. The same is true of the distance to stop, it is also 4 times. Choose the lightest motor that will do the job and then the lightest battery pack that will supply the motor to its peak rpm and torque.
Everything must pull its own weight. Add up the weight of everything and divide that total weight by the horsepower or torque. If you are pulling 20 lbs per horse verses 80 lbs per horse, the first model will take 1/16 the energy to accelerate to highway speed as compared with the second.
Weight also increases drag which is a separate issue from accelerating a mass up to a constant speed.

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Old 02-28-2011, 02:41 PM   #2 (permalink)
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I for one would like to test this one day.

If the various sources of used 6v golf cart batteries we've been recycling through the ForkenSwift's 48v 8-battery pack ever dry up, I believe I would look seriously at four 12v batteries. Partly for weight, partly for up front cost.

Or perhaps by then some used lithium cells will be kicking around...
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Old 02-28-2011, 08:44 PM   #3 (permalink)
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Here's a counterpoint:

Weight is important, but not as important as drivetrain efficiency, or as aerodynamic drag. The reason is, that as you note weight adds to the kinetic energy, so you can reclaim it by coasting, and/or by using regenerative braking if it is a EV. So, yes it takes more to accelerate, but you can get back some of the energy you put in; preferably by coasting.

Aerodynamic drag though, is a total loss. And rolling friction (like wheel alignment, bearing friction, dragging brakes, and tire rolling resistance, etc.) are also total loss; but they are also fairly easy to minimize.
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Old 03-01-2011, 02:21 AM   #4 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Here's a counterpoint:

Weight is important, but not as important as drivetrain efficiency, or as aerodynamic drag. The reason is, that as you note weight adds to the kinetic energy, so you can reclaim it by coasting, and/or by using regenerative braking if it is a EV. So, yes it takes more to accelerate, but you can get back some of the energy you put in; preferably by coasting.

Aerodynamic drag though, is a total loss. And rolling friction (like wheel alignment, bearing friction, dragging brakes, and tire rolling resistance, etc.) are also total loss; but they are also fairly easy to minimize.
Ok good point with reguard to coasting but, please consider that weight makes bearing friction worse and also it presses the tires down to make rolling resistance worse. If you try to fix it with lighter wheels they will be crushed at the next pot-hole. Regenerative systems will return a fraction of the energy spent.
I got a ride in a car that was 1400 lbs over weight and despite what people might think, the Leaf which is built lighter will go twice as far and the Tesla roadster 4 times as far because they built it very light. The same can be said for the British mini that reduced weight by using wheel motors. 280 miles on a charge is very good! 50 miles on a charge is a lead weight on wheels. So you see, I am just makeing some real world observations as to what has produced results and what has failed.

If you try this out on a bicycle you will get a better feel for what is going on.
First ride 8 miles with out any weight in your back pack. The next day, carry 100 pounds of lead in your back pack and make the same trip up and down hills. Reclaim the energy that your legs put in when going down the other side of the hills. The ride will be very difficult on day two and you will feel it in your legs.
I have had my hands into the construction of two aircraft and believe it or not, all of these factors are important for any vehical.
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Old 03-01-2011, 02:50 AM   #5 (permalink)
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Quote:
The problem is that if weight is doulbled the energy to accelerate that mass is 4 times as much.
Kinetic energy is proportional to weight. W=mv²/2
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Old 03-01-2011, 05:44 AM   #6 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
The reason is, that as you note weight adds to the kinetic energy, so you can reclaim it by coasting, and/or by using regenerative braking if it is a EV.
It's still a net loss, even if you can reclaim some of it.
You can't always coast down to a stop.
Regen braking slows you down in the process, so you lose even more as you're not getting as far, nor is the conversion to stored electrons 100% .
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Old 03-01-2011, 08:22 AM   #7 (permalink)
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Right, lower weight is better; all else being equal. And yes, higher weight adds to rolling resistance. Of course, coasting energy = acceleration - aero and rolling losses - necessary braking. I was a guest member of the Edison2 X-Prize team remember, and I paid close attention! And regenerative braking is only available on EV's and hybrids, and is lossy.

But aerodynamic losses completely swamp all other friction losses, often by orders of magnitude; at all but school or local residential zone speeds. Aerodynamic losses are total.

The two vehicles that are case in point for the aero drag being more important than weight are Allert Jacobs Honda streamliner, and Dave Cloud's Dolphin. Also, the X-Prize Knockout Round results confirm this.

The most important factor for high efficiency is the drivetrain.
The second most important factor is aerodynamic drag.
The third most important factor is weight.
Rolling efficiency is critical and must be good, but is the easiest to achieve.

Obviously, all four of the critical factors are interrelated. ICE drivetrains may be lighter than EV's, but they require more cooling, which adds aero drag. EV drivetrains are 3-4X more efficient, and make regenerative braking possible, but they tend to weight more. Packaging an EV is a bit easier, because the motor(s) are more compact, and they have no exhaust system, and they have minimal cooling requirements.
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Old 03-01-2011, 10:30 AM   #8 (permalink)
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didn't we just have the power to weight conversation? sort of

http://ecomodder.com/forum/showthrea...e-15028-4.html

I still think that a home built EV project doesn't look into the balance of the cars weight which could add or detract from its overall handling and performance maybe even adding to its overall range as well. If there is no other option for weight reduction make sure the weight is in the right places. Balancing the car to the occupants if you only have 1 person in the car add the drivers weight to the passenger side in roughly same location.

That may be something for you to look at MetroMpg when you swap out the batteries try and see if you can distribute the cars weight by relocating the batteries to reach the 60% front and 40% rear with equal lateral balance depending on your driving needs. (aka passenger or not) would like to see if it did extend your range.
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Old 03-01-2011, 11:04 AM   #9 (permalink)
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Quote:
Originally Posted by byte312 View Post
The problem is that if weight is doulbled the energy to accelerate that mass is 4 times as much. The same is true of the distance to stop, it is also 4 times.
Just to repeat: this is false. Double the weight is only 2x the energy to accelerate to the same speed. And the braking distance might not increase at all, as it's usually traction limited.

But the idea that you can reclaim the energy is also mostly false. A regeneration cycle is very inefficient -- 25% to 30% is typical.

The regeneration efficiency is higher for specialty vehicles, but omitting the transmission, differential and driveshafts (e.g. four CV joints) isn't practical for an on-road car. And we still aren't quite there with regeneration into a high recovery ratio capacitor bank (vs. using the motor controller to push energy back into the batteries). Every attempt has been high publicized, giving the false impression that it's common and successful.
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Old 03-01-2011, 01:20 PM   #10 (permalink)
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Ok but add the effects of drag

Quote:
Originally Posted by DJBecker View Post
Just to repeat: this is false. Double the weight is only 2x the energy to accelerate to the same speed. And the braking distance might not increase at all, as it's usually traction limited.

But the idea that you can reclaim the energy is also mostly false. A regeneration cycle is very inefficient -- 25% to 30% is typical.

.
Ok I think that you are correct. But is it fair to say that drag will increase with added weight? The kenetic energy is x2 for twice the weight but now add the increased drag from the tires being pressed into the pavement and the bearing friction then figure in the reduced effeciency of the motor as it passes more current almost like a straight piece of wire under heavy load. This is what will rapidly discharge your batteries.

I will take the advice of anyone who gets thier car to go 200 miles on a charge and worship them as a god. Results are on the bottom line.

regeneration of 25% to 30% is fantastic. If you can get that much energy back, your range will be 25 to 30% better. When your car stops on the road 10 miles from home you will wish you had an extra 10% You would not throw 25% of your money in the street and call it practical.
If you do I will pick it up at the risk of being charge with a violation of the laws of physics. 85%, 95%, 100% is not overunity.

The most practical cars do not have a full transmission, CV joints, and differential.

Some good examples are: (and yes I am saying it again) the Tesla cars, or the British Mini with a range of 280 miles. The Leaf is not bad with a range of 100 miles. That would make me happy. There are different and better ways to build a car. Pick the best example and work from there.

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