Human Powered Air-Kart (Help!)

The_Solution?

First time posting, and I appreciate any replies. I've had a couple thoughts about building a small go-kart powered by a pneumatic motor. Please join me on a vision quest as I explain a few thoughts, and ask for advice:

Assumption: 1 person through pedal power can exert between 100-300 watts of energy continuously. This roughly equates to 0.13 - 0.4 horsepower.

Would it be possible to equate this horsepower, through the use of a bicycle type set up, to turn an air compressor pump enough to fill an accumulation tank to force enough psi at such a rate to a pneumatic engine to provide forward motion continuously?

I know there's a lot of variables, and I'm talking with an engineer buddy of mine about it too. Sadly I don't know enough about physics and the laws of thermodynamics to do this kind of thing on my own.

Ultimately, the Kart I would like to build would be a two-seater with the bicycle/pump on the passenger's side to allow one to compress while the other drives. Perhaps even incorporating a hand crank on the driver's side to assist with the needed torque to overcome the gearing necessary to get up adequate RPMs for the compressor.

I just really don't know. I'm sorry, I know this is a wild hair-brained idea, and I'm really swinging for the fences here. But I figured if anyone could give me adequate advice, it would come out of this forum.

Thanks!

zoobyshoe

Off the top of my head: there's a limit to the efficiency of any air compressor, and a limit to the efficiency of any pneumatic motor. I can't think of a reason to expect they represent high efficiency devices, therefore, I suspect you'd just be doubling the inefficiency of a more direct pedal to wheel drive (as on a bicycle).

The_Solution?

Interesting, so just the fact of adding another device, you're diminishing the efficiency?

Would this be true even if you were stepping up the Rpms through gearing?

zoobyshoe

Yep. A bicycle has a certain efficiency in turning human power into motion. A pedal operated air compressor has a certain efficiency turning human power into the potential energy represented by the compressed air. There's no particular reason for me to think the latter is any more efficient than the former, so in the latter case you've expended the same amount of power as the former, but still don't have your motion yet. The next step, turning the compressed air into motion by means of the pneumatic motor, is guaranteed to entail more losses to friction, etc. There's no 100% efficient motor. So the more devices required before you get from power input to motion, the more losses you should expect.

If, in practice, you could find or make, a compressor and pneumatic motor that were twice as efficient as you'd normally expect (which seems unlkely in my mind) then you'd be OK.
A transmission would do the same thing a transmission always does. The losses in your cart would happen before the power got to the transmission, so there would be less power input into the transmission.

Think of a bicycle. Normally you just work the pedals with your feet. Your cart is like using your feet to first operate an air compressor that stores compressed air, which is then used to turn the pedals your feet would normally have been directly turning to begin with. The extra step, in principle, means extra friction to overcome, before you ever get to the transmission.

uby

The disadvantage of decreased efficiency might be offset in some circumstances by the added advantage of energy storage - allowing for the pedaller to take breaks.

HauntedMines

A lay-man's way of thinking about it: Every air compressor I've ever used, from a bicycle pump to a large shop-compressor, heats up when it runs. Considerably. I've also been around pneumatic motors, and they too get hot when in use. That tells me they're loosing energy thru heat. If I compare that to a simple chain and sprocket mechanical setup (a bicycle), I seldom notice heat from that. So as a common-sense guess I think a compressed-air unit would be less efficient.

However, if you consider using the compressor and storage tank for slowing/braking, perhaps you could capture the braking energy for use later to assist the pedaling.

fwiw.
- Steve

The_Solution?

Well thank you all for your advice. I'll take it all into consideration as I build.

I hope you'll check out my blog and comment on the progress:

http://air-kart.blogspot.com/

Currently I'm working on the electrical system. More to come.

Thanks again!

Emreth

Is there a specific reason you want to use compressed air? It would be more inefficient than an electrical based system like a hybrid car. You can instead have somebody pedal an electrical generator at an optimum rate continuously, and divert some/all the output to the main motor or the battery depending on the throttle. It would then be straightforward to incorporate regenerative braking, which would help a lot in a track.

Victorymike18

I agree with everyone who has who replied. (And ZoobyShoe really nailed a few points well).

Air compressors get very hot when in operation due to the friction of the various moving components. I'm sure you could revamp a high quality ($$$) compressor and make it more efficient than a homeowner-grade one, but I still think there will be too much loss of energy for it to be sensible.

*Edit* I just re-read your first post. As people have mentioned, the air compressor is going to be a big energy-sapping component in this process. What you need to consider is that you are trying to convert your input energy (pedaling motion which is rotating mechanical energy) right back into rotating mechanical energy at the wheels... A bicycle does this very efficiently, and weighs about 30-35 Lbs.

At the end of the day you could do the following with the same results: Take a bicycle and stick the dead weight of an air compressor and added weight of gears and chains on the bike. And when you go to ride it, squeeze the brakes a bit so they are scrubbing your speed... This may come off a little harsh, but it's actually a pretty fair example of the results, based on your description.

Regarding a transmission: A transmission will multiply or divide torque, but will oppositely effect horsepower (essentially like a lever). A transmission does not get you any energy for free, it just allows you to use the energy from the powerplant most effectively for the job. For example: I could build a human pedal powered device that can lift 2,000 Lbs. of weight. But I would have to make the gearing so extreme that it would take a significant quantity of pedal revolutions (and time) to lift that weight one inch off the ground...

I took a look at your website and saw your framing for the solar panels, which you say are solely to operate some onboard electronics. I have to ask: When you factor in the weight of the framework (looks like you're using electrical conduit) and the panels themselves, you are adding a lot of weight to the vehicle. Would it not make sense to use rechargeable batteries to power the on-board electronics and eliminate that weight?

What electronics do you need to operate?

Steve (HauntedMines) mentioned regenerative braking to spin the compressor. That's a great idea in itself, but you're still putting energy through an inefficient method of conversion. Which leads me to my next question: Have you considered using a flywheel for energy storage?

I recently read an article where a university student used a small CVT (continuously variable transmission) to spin up a flywheel when braking his bicycle. He could then reapply that energy back through the CVT to assist with accelerating. The system as a whole was pretty simple, and seemed effective enough to be worth exploring further.

Flywheels are significantly more efficient in terms of input energy vs. output energy. They are a different challenge, so I will digress for now unless you want more information.

I'm not too knowledgeable when it comes to physics, but I hope some of this is helpful. I'd be happy to share my limited knowledge and point you to various articles and resources if that would help.

- Mike

Q_Goest

I like VictoryMike's flywheel idea.

Not only are air compressors inefficient (maybe 80% so loss of 20% of energy input) but air motors are horribly inefficient, loosing another 50% of the energy input.

pantaz

Mechanical friction is part of it, but compressing a gas increases its temperature. Searching these forums should turn up some previous discussions. Also, look up "Ideal Gas Law", and "Charles' Law"

The_Solution?

@Mike, I appreciate the in depth feedback! Let me answer in turn. I disagree with the adding weight on your back and riding a bicycle analogy. The engine is not dead weight, and while the accumulation take may be, its not much (~20lbs). The torque from a pneumatic engine is higher at lower RPMs than a human can generate. While we can beat it out for endurance and an exchange of energy over the long haul, released compressed air wins out for low speeds. At least I think it does from what I've read haha. In regard to the power system, yes I am incorporating a small system. Please refer to my blog for some explanation, I made a post specifically talking about it. (http://air-kart.blogspot.com/) And on the note of a Flywheel, I will actually be incorporating 2 of them. More to come on that one later. Thank you for your ideas and I hope you'll keep em coming!

@Steve, Great point on the Heat issue. I have been considering some fixes that could mitigate that. But, I think in the scale that I'll be working, it won't be so bad that it will damage the system. As far as energy loss, yup, I will be losing a lot of energy to heat. But even gas engines get a fairly low exchange of gas energy to energy used for movement vs energy made into heat. Why not have the same inefficiencies but with something that doesn't pollute? Maybe we should put our minds on developing a way to compress air without generating so much heat, or utilizing that heat somehow to create an electric power we can use. Huh? What do you think?? Thanks for the response!

@uby, You bet! This vehicle will not only incorporate an accumulation tank, but an electrical system that will power a 12v compressor when the psi dips below a certain level. This will help the person pedaling, and will at least in a small way, compensate for some of those inefficiencies. Check out my blog for some info on that power system. (http://air-kart.blogspot.com/) Any other thoughts? Thanks!

@Emreth, Hit the nail on the head. This vehicle will in fact be somewhat hybrid in that it will use solar energy and Human energy to compress air. Now, you have a great question there, why not use an electric engine? It really comes down to weight and electric power needed. Electric is heavy, and it needs a lot of power. Human/Solar couldn't keep up for that long. BUT, replace that heavy electric engine with a light-weight pneumatic motor that creates a significant amount of torque at lower rpms, and you can slap on some gears, up your rpm at where the rubber meets the road (literally, not a figure of speech here) and you're in business. Thank you for your suggestion! Got any ideas on how to utilize some of that regenerative breaking for air compression?

Again, thank you all, and please visit my blog (http://air-kart.blogspot.com/) for updates as I'll answer questions on here, and on there. Ultimately, I want you all to be part of this build! I want your ideas, and I want to make them work. I think a lot of people would say that it's impossible, and while good intentions aren't going to change physics, we can definitely use them to motivate some amazing ideas!

NascentOxygen

Modern electric motors are efficient, light weight, can be constructed as part of the wheel hub, and don't need gearing.

I really like the concept of the pneumatic engine. They would perform most reliably in the tropics, but in icy climates I think they would ice up and be quite useless. So, ideally, you would reside in a warm country close to a servo where you can get free compressed air.