Pneumatic Transmission?

[Indubitably]

I've been farting around with the idea of using a high speed rc motor for my next build, simply because the little buggers are cheap as frak, crazy efficient, and weigh a fraction of what a comprably rated hub would weigh. The only problem is that no matter how I slice it, I keep coming back to the fact that I need to run them through a heavy, expensive, and inefficient transmission. In the best of situations it seems I can hope to run a simple jackshaft setup that manages to get my motor somewhere near its happy place without breaking the bank, but the moment I drop even a nexus 3 into the mix my efficiency / dollar suddenly goes to shit. Everything else seemed to invariably lead me down an endless rabbit hole of prohibitively time consuming and expensive customization, that would yield at best a marginal improvement in efficiency.

So I started looking around for pretty much any means of power transmission whatsoever, that came in a modular format versital enough that I could put something custom together withoutout having to reinvent the wheel, yet was common enough that I could actually get my hands on it at a reasonable price, and I just kept coming back to pnuematics. Now, I know that compressed air isn't really a viable energy storage medium for a bike because of its poor power density, but why not use it as a means of power transmission? I'm thinking the way to go would be a closed loop with a series of check valves, a pressure reservoir, and a vacuum reservoir. The system would be thermally isolated, so that the heat created by compression in the pressure reservoir (and lack there of created by decompression in the vacuum reservoir) would only serve to further drive the pressure differential, and what ever thermal bleed over might occur, would at least be kept within the system so that thermal power loss would be minimized.

Presumably, with large enough volume, a small amount of energy could be stored by simply allowing the wheel to drive air into the vacuum reservoir, which would act like a kind of regenerative breaking capacitor, or you could just let the system turn the electric motor to charge the batteries, or some combination of both.

The basic idea is to get the natural torque conversion of a pneumatic system, but with the power density of batteries, while allowing the electric motor to do its thing at a happy rpm where it will never be in danger of over heating, and to do it with generic parts widely available at a fairly reasonable price so it can be easily assembled as a single module that can be conveniently fitted to the bike and or removed for maintainance (with standard replacement parts).

What do you guys think? Is this doable? Would I be better off with hydraulics? Am I going to wind up spending just as much as I would on a gear head and a nexus 3 anyway, or perhaps just missing some fundamental drawback that makes it more of a pain in the ass than its worth? Comments and corrections are greatly appreciated, but I haven't even drawn this thing out on paper yet, so please bare with any obvious mistakes.

[Punx0r]

I imagine it would be *horribly* inefficient!

Just look at air tools: compact, lightweight and cool-running, but take a 3HP compressor running flat out to power them. Whereas the equivalent mains-powered tool uses a 500W motor.

I can't think of any vehicle that has used pneumatic drive?

IIRC hydraulic drives are only around 50% efficient. Years ago a 2WD motorcycle drove the front wheel by hydraulics. Might have been a Paris-Dakar entrant.

Gears are cheap, simple, light and ~99% efficient

[Tench]

Ohlins developed the hydraulic front wheel drive system for Yamaha who did indeed fit it on a limited production run of WR450F enduro bikes called 2 Trac, they also had prototypes running on R1,s and TT600,s.
I cant see pneumatics being anywhere near as efficient as hydraulics as a meens of power transmission due to the fact air will compress and for arguments sake oil doesnt but both would have poor efficiency compared to a mechanical system and more complexity, i dont see a benefit?

Some examples of pneumatic and hydraulic efficiency in this artical;

http://www.machinerylubrication.com/Rea ... department

[StudEbiker]

Yes, your "solution" sounds like the proverbial guy that finds out he has a heart condition and cancels his health insurance to save money for all the medical bills he anticipates.

[Indubitably]

I get that conventional wisdom says that compressed gas is inefficient, which is all well and good because most people don't stop to think about the fact that the majority of the kinetic energy being pumped into your system is going into thermal excitation of the gas molecules, but it is not the gas itself that is "inefficient", its all that kinetic energy you loose via using your cylendar as a space heater during compression. Theoretically it all comes down to simple conservation of energy, if I really can keep it isolated, then energy that goes into the system, stays in the system, until I give it a way out via the wheel. Obviously I'm going to loose some heat no matter how good my insulation is, especially if it gets much hotter or colder than the surrounding environment, and there will naturally be accoustic losses and the like, but if I can more or less keep the hot side hot, and the cold side cold, the system should almost work like a thermal flywheel. If I was going to compress a large volume of gas for a long time, thermal isolation would be much more difficult, but the hot compressed air is going to be flowing to the cold vacuum almost as fast as I can make it, and I will be pulling energy out of the system through the wheel almost as fast I can pump it in, so the heat won't really have time to get out of the system before I can transfer it's energy to the wheel.

I'm also not terribly convinced by this "99% efficient" mechanical transmission. I've just been around too many mechanics that like to throw around the "expect to lose 25% of your power in the transmission" rule of thumb. I don't know if that is something that should be taken literally or not, but somewhere between the engine and the wheels they seem to be seeing a significant loss of power, and I am inclined to suspect that they are not completely mistaken in that respect.

[Jeremy Harris]

First off, lets define what we mean by efficiency when it comes to a transmission system. The simple definition I'd used is power out divided by power in, expressed as a percentage, so a bike transmission that has a power input of 100 watts and a power output of 97 watts is 97% efficient.

Next, let's compare a few drive systems:

- Chain drive - can be around 96% to 98% efficient if clean and lubricated.
- Toothed belt drive - typically around 94 to 96% efficient if a modern tooth profile (like HTD or GT2 is used).
- Spur gears - similar to toothed belts, at around 94% to 96% if run in oil baths, slightly lower losses if run open, but only when kept clean and well lubricated.
- Hydraulic drive - around 60% to 80% if the pump and motor are well-matched, but with higher losses at low power levels.
- Air pump and air motor - around 30% to 60%, depending on pump and motor design. Air tolls are typically around 30%, compressors are around 50% typically.

The reason air is an exceptionally poor fluid for energy transmission has to do with it's compressibility and its very low specific heat. When you compress air it heats up, from PV/T = PV/T, increase the pressure, keep the volume the same (it's a closed system) and the temperature has to increase, basic physics. You will always compress air if you try and push it into a pipe at greater than atmospheric pressure, again from simple physics. If you don't increase the pressure then you don't do useful work, so cannot transmit power. Because air has a low specific heat it heats up very easily and the heat generated from compressing it passes to the compressor and pipework, so a lot of energy is lost as heat. The air passed to the air motor has only around half the the energy that went in to the compressor. Some further energy is lost due to flow in the pipes (similar to the losses in a hydraulic system) and yet more is lost in the air motor, as it requires a pressure differential to operate, which means the exhaust air will be higher than atmospheric pressure, and will therefore have a fair bit of wasted energy.

Hydraulic systems are more efficient than air systems, but still have substantial losses, because the viscosity of oil means that there will be frictional loss in the pipes and pump/motor that will be dissipated as heat. The same goes for an electrical transmission, where a generator is used to drive a motor, although in that case it's usually electrical resistance in the generator, wiring and motor that causes most of the loss.

If you happy with a system that wastes around 60 to 80% of the battery energy you have available, then by all means look at using a pneumatic transmission. Most people would like to see something that was less wasteful though, I believe.

[Indubitably]

I appreciate that you're taking the time to explain where I'm going wrong, because that's how my ideas improve, but there is obviously some sort of disconnect, because all I'm seeing here is a reiteration of the basic reasoning behind my argument, only with a contradicting conclusion.

What I'm saying is that I actually want compression, because it should give me a a kind of infinitely variable transmission effect, and what your argument appears to imply is that compression, in and of itself, has some sort of immutable energy negating effect that would annihilate some large portion of the energy in the system even if I actually could isolate it perfectly from the environment. Perhaps there is something I'm missing in your comments about heat transfer within the system? As I understand it, this isn't really and issue, because while the pump must work harder (or rather, longer, since my plan is to move a coninuously smaller volume of air as the differential increases) and consume more power as the differential increases, it similarly requires less power as more heat migrates from the the hot side to expand gas on the cold side. I suppose I could see how for any one momentary snapshot of the system that power might seem to just sort of disappear, but hysteresis should correct for this in the long run.

The idea is rather like using a giant rubber band as a belt in a belt drive, only the belt has some sort of pinching system that forces it to contract only in one direction, so that any energy stored in stretching it must eventually contribute to forward motion of the system somewhere. At the moment when the belt is stretched it is longer but has not contributed to forward motion, so energy appears to be lost, but in fact merely hides itself in tension. What you seem to be saying is that the belt will contract without ever releasing the tension, yet the tension will be gone none the less, as would be the case with the closed check valve pressure system that never lost heat but still lost energy without doing work. If I think of the belt as just getting longer and longer without ever contracting if left in a stretched state for too long, but periodically snipping off the excess as I go, I can get something like the compression system loosing heat to the environment, but presumably the belt is reinforced with insulation that increases the amount of time it can be stretched so that I will see only negligible permanent stretching.

What specifically am I missing here? I mean, I'm really not trying to bust your balls, but if there is some fundamental flaw in my reasoning, I need to know where it is, so I can correct my understanding. You don't seem to be saying that it is simply impossible to effectively insulate the system, yet your argument does seem to assume that heat has left the system through something other than kinetic energy transferred to the wheel, and I'm just not seeing where it went.

[Punx0r]

I'm afraid I'm not smart enough to explain it, but I think there is a flaw somewhere in your reasoning.

I can only try to speculate that in order to extract work from the compressed gas, it must be allowed to expand (fairly) freely, and this inherently means thermal energy escaping your system.

I was wrong earlier, there have been vehicles that used pneumatic drives: http://en.wikipedia.org/wiki/Pneumatic_motor

I was looking for, but failed to find, the efficiency of a pneumatic motor itself - not as part of a compressor system. I suspect it will be substantially lower than an electric motor, controller and mechanical drive combined.

There's nothing wrong with considering new ideas

[Jeremy Harris]

I appreciate that you're taking the time to explain where I'm going wrong, because that's how my ideas improve, but there is obviously some sort of disconnect, because all I'm seeing here is a reiteration of the basic reasoning behind my argument, only with a contradicting conclusion.

What I'm saying is that I actually want compression, because it should give me a a kind of infinitely variable transmission effect, and what your argument appears to imply is that compression, in and of itself, has some sort of immutable energy negating effect that would annihilate some large portion of the energy in the system even if I actually could isolate it perfectly from the environment. Perhaps there is something I'm missing in your comments about heat transfer within the system? As I understand it, this isn't really and issue, because while the pump must work harder (or rather, longer, since my plan is to move a coninuously smaller volume of air as the differential increases) and consume more power as the differential increases, it similarly requires less power as more heat migrates from the the hot side to expand gas on the cold side. I suppose I could see how for any one momentary snapshot of the system that power might seem to just sort of disappear, but hysteresis should correct for this in the long run.

The idea is rather like using a giant rubber band as a belt in a belt drive, only the belt has some sort of pinching system that forces it to contract only in one direction, so that any energy stored in stretching it must eventually contribute to forward motion of the system somewhere. At the moment when the belt is stretched it is longer but has not contributed to forward motion, so energy appears to be lost, but in fact merely hides itself in tension. What you seem to be saying is that the belt will contract without ever releasing the tension, yet the tension will be gone none the less, as would be the case with the closed check valve pressure system that never lost heat but still lost energy without doing work. If I think of the belt as just getting longer and longer without ever contracting if left in a stretched state for too long, but periodically snipping off the excess as I go, I can get something like the compression system loosing heat to the environment, but presumably the belt is reinforced with insulation that increases the amount of time it can be stretched so that I will see only negligible permanent stretching.

What specifically am I missing here? I mean, I'm really not trying to bust your balls, but if there is some fundamental flaw in my reasoning, I need to know where it is, so I can correct my understanding. You don't seem to be saying that it is simply impossible to effectively insulate the system, yet your argument does seem to assume that heat has left the system through something other than kinetic energy transferred to the wheel, and I'm just not seeing where it went.

OK, some basic physics.

When you compress a gas (or a mixture of gasses, like air) then it gets hot. Gasses have a limited capacity to store heat, set by a constant called their specific heat, which means that the temperature rise can be very high - in simple terms if you compress it a lot it gets very hot. Heat is a form of energy, so when you compress the gas some of the energy you put in is stored as potential energy in the compressed gas and some is lost as heat that escapes, both through the casing, pipe work etc and also through the exhaust air. This means you've already lost some energy from that used to drive the compressor as heat.

Next you pass the hot, compressed, air to the air motor. This extracts some of the potential energy in the compressed gas, but not all of it, because there needs to be enough "left over" pressure to move the air out of the exhaust port, just like a car engine.

If you make the system closed, like a hydraulic system, then all you do is reduce the amount of useful energy that the air motor can deliver, because you will increase the exhaust pressure by piping it back to the compressor. The amount of energy extracted is directly proportional to the pressure difference across the air motor.

If you were to build a closed system like this, and manage to perfectly insulate it, then the temperature of the air inside would rapidly increase to the point where something would fail I expect. Just like a closed cycle steam engine, you would need some form of external cooling to make the thing work. This cooling would extract energy from the system as heat. The rate of heat loss is proportional to the difference in temperature and follows a non-linear characteristic. The hotter something is the more energy it loses per unit time (see Newtons Law of Cooling for an explanation). This means that the energy loss from the heat in a closed system like this would quickly reach pretty high levels.

If you want more background on adiabatic compression, then take a look at this Wiki page and some of the links to similar areas of thermodynamics: http://en.wikipedia.org/wiki/Adiabatic_process

What you have been describing is a perfect adiabatic process, something that doesn't exist.

BTW, the rubber band analogy is a good one, as rubber bands get hot as they are stretched, this heat gets lost and therefore they always return less energy when allowed to relax than it took to stretch them.

[TylerDurden]

BTW, the rubber band analogy is a good one, as rubber bands get hot as they are stretched, this heat gets lost and therefore they always return less energy when allowed to relax than it took to stretch them.

As a kid I thought of making an air-cooling system using rubber bands - I felt them get cooler as I gently released their tension against my cheek.

[Hillhater]

...heat migrates from the the hot side to expand gas on the cold side.

This is the crux of the issue.
You are assuming you can fully contain the heat from the compression of the gas,...and fully recover it again during the expansion phase in the motor.
..That is impossible to achieve. !
Frictional losses alone in the compressor and motor would be considerable ( 20-30% )

A more efficient ( power and $$$ wise ) use of the RC motor would be one of the friction drive systems which have been developed on here.

[fechter]

I've looked at the idea of using magnetic 'gears'. There are some existing systems out there but they don't perform very well. It might be possible to use magnetic gearing in a harmonic drive or epicyclic configuration to give a high reduction ratio and still have very high efficiency. The limitation is in how much torque you can get from a given size of magnet arrangement.

[cal3thousand]

Frictional losses that you cannot recoup will be too large.

And if you could recoup them, the cost to build this system is way to much for a bike. One principle always applies here: KISS

[Punx0r]

Lockable torque converter?

[Holocene]

Here's one such approach:
http://www.instructables.com/id/Air-powered-bicycle./

[Holocene]

Another approach is an air-driven tesla turbine (possibly embedded in the rear wheel)
http://www.youtube.com/watch?v=3bFTYaETsdE

[Punx0r]

Quoting from the youtube video:

With shop air, turbine drove the loaded system at a maximum 929 RPM and produced 21 W. The theoretical power of shop air delivered was 350 W, so the efficiency of the system is 6%.

Further up it mentioned the air supply was 20CFM @ 80psi.

A 3HP compressor will deliver about 9CFM FAD. Being generous, let's say 6HP to give 20CFM. So that's ~4.5KW power in, 21W power out


The bike with the air drill is interesting, but the range given was 1 minute at 4mph. I make that 117 yards on a full charge

[Indubitably]

Yeah, looks like my main problem was that I was simply failing to scale my mental model correctly. I was thinking we were talking coffee cup hot, when its looking a lot more like we're talking espresso machine hot, or hotter. If you visualize it in this range the whole thing goes to shit in no time flat unless the pressure containment and insulation properties drastically increase, at which point the weight of the the system alone makes it useless. I knew there was going to be some loss just because things that get all mixed up like that (the way that the kinetic energy gets all mixed up when it becomes heat) just never do seem to want to go back to behaving nicely, but I hadn't really accounted for it getting that bad that fast. I'm not even sure myself whats happening when I let it go much further than this, at some point everything just plain goes all screwy and my brain keeps wanting to tell me that it will suddenly start to magically make a whole shitload of heat all by itself, so I'm definitely not getting the whole picture just yet.


If we're going to go mechanical...

Why not use that same slippage effect that you get between the magnetic field and the rotor, but just do it with gears instead. Like maybe use one of those peerless lawn tractor differentials, only run it backwards. On one side you attach a standard brushless motor with adjustable speed, on the other side a run of the mill brushed DC motor with an rpm happy place at pack voltage that is somewhere near the far end of that asymptotic section of the bldc's rpm-effiency curve (just that past the peak, but before it really falls off). Then simply adjust the bldc's speed, when the bldc's speed is at the low end of the asymptotic part of the effiency curve far from the brushed motor speed, you get your "maximum" slippage, as the spider gears attached to the housing "slip back" (like the rotor does relative to the rotation of the magnetic field in a motor), so that the housing of the differential will turn at your maximum output speed (arbitrarily chosen for ideal output characterisitcs and motor stress) with minimum torque. By increasing the bldc's speed you will get less and less output speed, but more and more output torque until both pinion gears are matched in speed and the housing stops moving. Past this point the slippage should reverse and the housing will move backwards. This should put both your maximum efficiency and power output for the system togethwrvat the torque end of spectrum where it is needed most.

I'le have to actually get my hands on a differential to be absolutely certain that this spider gear mechanical slippage effect isn't just a visual artifact of the mental-mechanisms behind my imagining of the machine's construction and operation, but if its not there, then I'm going to have to completely rebuild my understanding of a differential, because it employs exactly the same concept that I use to see the "forward" application of a differential. I haven't figured any sort of load reaction in the motors into the system either, so something whacky might go down when things go dynamic, but the basic idea should be sound.

[Miles]

viewtopic.php?f=28&t=10998

viewtopic.php?f=28&t=7358

[Indubitably]

http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=10998

http://www.endless-sphere.com/forums/vi ... =28&t=7358

Hah! Excellent, so I'm not entirely crazy after all. I was afraid I might have to couple a pair of them to get true torque reduction though, and this does seem to confirm that such is the case. Perhaps it could be done in such a way that the differential housing would serve as the sun gear for an planetary system? I'll have to think about this one for a while.


PS: Just out of curiosity, why are we not building motors with concentric ironless rotating coils sets and running multiple controllers instead of the whole permanent magnet thing? Is it the difficulty of rotating contacts, or windings not being well enough balanced? Just seems to me that this whole process would be made a lot less complicated if we let magnetic fields do the high speed part of the rotation for us (does that make the timing too complicated?)

If I was going do this from the ground up I'd imagine that I'd do a hub motor, but with some combination of multiple rotating coils sets, and I'd maybe wind them with a shitload of really thick aluminum transformer wire or something, and put them way out at the rim instead of the hub. More because I just don't want to pay for a bunch of copper and Neo dymnium magnets, but I'd figure it would solve our torque issues to boot. I'm also kind of curious if there wouldn't be some way to have a stator induce a current in an intermediate rotating coil set that would interact with a third coil.