Mechanical Question about project Urgent!

[MrForbes]

Originally Posted by brennerator

Yeah I know that, but wouldnt this be useful in a city environment?

I can't see how it could be any use at all for saving fuel...but it could help teach you about different forms of energy, so it's a very useful project in that regard.

[brennerator]

When youre braking wouldnt the fans help conserve some energy. So in a city environment where one is constantly braking...

[Alan Anderson]

Originally Posted by brennerator

When youre braking wouldnt the fans help conserve some energy.

Not really, no. Unless you're traveling extremely fast (wind turbines generate power approximately proportional to the fourth power of wind speed), or unless your fan blades are huge (think helicopter), you will lose more energy by having to accelerate the weight of the fans than you can possibly recover by letting them spin as you slow down.

You can get a much greater overall benefit by carefully streamlining the vehicle to avoid wind resistance. That helps reduce energy needs the entire time you are moving, where the fans can only provide a theoretical benefit for the first few seconds of braking.

[Al Skierkiewicz]

Bren
Could you be a little more specific about your idea? We are trying to understand your thoughts on braking as opposed to driving. Brush type, DC permanent magnet motor act as generators when they are driven from an external source. If you have ever pushed your robot on the floor when it is turned off you will see this effect as the fans on the Victors and the LEDs will light due to the voltage and current generated by the drive motors backfeeding the Victors.

[MrForbes]

Originally Posted by brennerator

When youre braking wouldnt the fans help conserve some energy. So in a city environment where one is constantly braking...

First a bit of information about energy use in cars, at different speeds. There are three basic ways that typical cars use energy, all of the energy comes from the gasoline powered engine, but goes to different things.

The first is overcoming rolling resistance....it takes energy to overcome the tire's resisance to roll. This load is mostly proportional to the speed of the car, so the faster you drive, the more power it takes to keep the car moving. To get a feel for this, try (safely!) pushing a car, it will be easy to push it slowly, but it becomes harder to push the faster it goes.

The second is overcoming air resistance, this is aerodynamic drag.....it takes energy to move the air out of the way of the body of the car. This load is mostly proportional to the square of the speed of the car. That means that it is very small at low speeds, but very high at high speeds. To get a feel for this, see if a light breeze will push a car, and also see how it is different driving into a headwind at highway speed, vs. driving with a tailwind.

The third is accelerating the mass of the car. This energy is not wasted, unless you use the brakes to slow down (then your kinetic energy turns into heat energy, which is dissipated by the brakes). Hybrid cars get about the same mileage in town as they do on the highway, because they recover the kinetic energy, and store it in a battery for use in accelerating the car again.

I think that you are trying to recover a very small part of the energy that is being used to overcome aerodynamic drag. In city driving, there is not much energy lost to drag, most is lost to the brakes and overcoming rolling resistance. And still, the best way to reduce losses to air resistance, is to make the car "smoother" to reduce the coefficient of drag, and to reduce the frontal area (height and width).

If you don't understand any part of this, please ask more questions!

[Bob22341]

If you are looking to recover energy lost by a car, you would be better off trying to recover heat from the engine and brakes.

[brennerator]

Thanks a lot guys; this is really informative.

I understand your arguments, and I too believe that I will not recoup enough. But I think it is a useful experiment. At the ver yleast I can learn more on this subject.

So I just basically stacked the PAPST large CPU fans on the hood of the car, perpendicular to the ground. I have 3 of these large fans. They are wired in series and connected to a resistor. I will use hte multimeter to read voltages when driving hte car at a variety of speeds. After this I will calculate the power and then I can see how much I generate. Unfortuantely I will not be able to see the increase of my drag coefficient with this assembly.

Any thoughts?

Once again; I cant thank you guys enough for your input.

[Al Skierkiewicz]

Brenn,
What is the part number of your fans. Most of the Pabst line is brushless.

[brennerator]

I was able to use my fan as a generator.

I used a 1 k-ohm resistor and read the voltage across the two leads.

My data as follows:

15 mph = 1.20 V
20 mph = 2.06 V
25 mph = 2.68 V
30 mph = 2.83 V
35 mph = 2.77 V
40 mph = 2.82 V
45 mph = 2.70 V
50 mph = 2.52 V
55 mph = 2.26 V
65 mph = 2.21 V

The fan probably was not designed to handle such high rpm's past 40 mph.

Can I make any conclusions from this data taht will allow me to test the efficacy of using fans. Please note that I was not able to calculate how much the overall drag coefficient increased on my car.

Thanks so much for all your help.

[MrForbes]

You are making about one hundredth of one watt of power...that's not much!

P = E^2 / R

[brennerator]

hmm...is htere any practical usage for one hundredth of one watt of power? Remember that I have 5 of these fans. So its more like one twentieth of one watt of power.

[TubaMorg]

Originally Posted by brennerator

hmm...is htere any practical usage for one hundredth of one watt of power? Remember that I have 5 of these fans. So its more like one twentieth of one watt of power.

Hey Bren, try wiring the fans in parallel and see what happens.