How Can We Design a Compressed Air Car for 300 km Travel?

[russ_watters]

For those taking the negative reactions to the idea personally, keep in mind a couple of important things:

1. You are, for the most part, dealing with experienced professionals one side of the discussion and amateurs on the other. There is a considerable knowledge gap between them. This isn't elitism, it is simply a reality.

2. Any engineering project must go through a feasibility study phase before it is undertaken. Real engineers don't just decide to do something, then set about doing it because there is a very high risk of wasting time and money on a dead end. The real engineers here are spending most of their time examining the overall feasibility rather than helping with the design because they know that that step has been skipped. They are trying to save the OP from wasted time and money spending years/decades on a project that may be doomed to fail.

 

[TVP45]

now you're getting rude. the purpose of the first video was to illustrate a tank's behaviour when emptied suddenly. if you noticed the case where a tank stays in its place, there's actually nothing dangerous.
by wishing me good luck, i hope you're abandoning this topic. you're doing nothing but grunting anyway.

Chris:
A safer bet would be lots of smaller cylinders, but how would you stop them from moving.

that's just what i have said. i'd have them installed in some sort of foam or in a box. i'd also leave a lot of holes for ventilation in case of inconviency.

I apologize. I didn't mean to be rude to you. I meant to be scornful of youtube and I wrote too quickly, without reviewing. I really do wish you well.

 

[xxChrisxx]

now you're getting rude. the purpose of the first video was to illustrate a tank's behaviour when emptied suddenly. if you noticed the case where a tank stays in its place, there's actually nothing dangerous.
by wishing me good luck, i hope you're abandoning this topic. you're doing nothing but grunting anyway.

Chris:
A safer bet would be lots of smaller cylinders, but how would you stop them from moving.

that's just what i have said. i'd have them installed in some sort of foam or in a box. i'd also leave a lot of holes for ventilation in case of inconviency.

You are misunderstanding the problem, those tests only involve valve shear. Which is fairly predictable.

The problem you face is that in a crash any flaw in the material (which everything has) can propogate into a crack. Meaning that it could very well blow a huge chunk of the cylinder away from the side which no box or foam would contain.

 

[mgb_phys]

The aircar's CF tanks meet European crash standards, the are designed to unravel releasing the energy slowly rather than burst. In general CF pressure vessels don't burst like ductile metal ones.

 

[redargon]

A thought just occurred to me... If the pressure vessel did vent (so as to say, not explode with initial harm) into the car, which would burst first (if all the windows and doors were closed) your head or the windows? What I'm trying to say, is exploding tanks are not necessarily the only danger caused by a faulty tank. They might unravel nicely, but that hig pressure air has to go somewhere.

 

[kandelabr]

redargon:
tanks should be completely separated from car's passenger space, and i don't think that should be a big challenge.

if you look back how this topic developed, you'll notice i usually gave you ideas and you tried to find fragile spots in my design. that's actually just what i want, but you've found nothing major that i haven't already thought of. yes, probably i haven't put enough thinking effort into air tanks, but since these things are, as mgb said, standarized already, i didn't even intend to.

you should know that I'm second year of faculty of mechanical engineering in ljubljana. my plan was first to develop a rough design (like the thing i presented here), without major holes/faults (like you found here), and then try to put it into reality as a project at my university.
there has already been a similar project (check out http://www.studentroadster.com/ ). but this would indeed be a much bigger and more complicated one.
keep in mind that we also have some very good professors in machine dynamics (did some research for renault, audi, nuclear power plant, etc...), thermodynamics, composite materials (that professor is in a stage of developing new standards for climbing ropes), machine elements, etc, etc. whatever i would need to know, if this project was good enough to be accepted by faculty, i'd surely get support from real professionals. i'd also get some money support from faculty, university, slovenian fund and various european funds.

so, do you think this idea of mine is feasible or am i doomed to fail?

 

[xxChrisxx]

I'd say its feasible from what has been discussed so far, now that the pressure vessel issue has been sorted as that was my only major concern about the project.

However it does require extensive study into all areas, for anything that is impossible to really pick up on a forum.

 

[kandelabr]

[...] However it does require extensive study into all areas, for anything that is impossible to really pick up on a forum.

indeed, I'm completely aware of that fact.

however, I'm not trying to make a commercially available, tested and optimised vehicle, more like a prototype, a presentation to show we're not necessarily doomed to use oil until it runs out. well, something in this manner.

 

[redargon]

ok, the compressed air tanks aside, let's get to the motors.

would you use available air powered motors, or design your own and maybe you could explain them in a bit more detail? Do you have any specs for available air motors? Then we can discuss each element at a time (like we did for the air cylinders)

 

[kandelabr]

you can find air motors on sites like globalspec, but they aren't quite useful; only work at low pressures (~ 6 bar), have low torque (~ 10, 15 Nm), etc.

a part of my engine would look like this:

this is very similar to a vane pump. i'd make it be able to vary displacement from center of housing (named "d" in sketch). with that, a variable relationship between smallest place (beginning of the expansion, left arrow pointing in) and greatest (right arrow). the part on the lower side wouldn't be sealed so air would be free to go out.

i have written a MATLAB function that accepts as input parameters R (outer radius), r (inner rotor radius), d (displacement), h (height), fi (angle of rotation) and returns V1/V2 (volumes) and torque at given angle.
here's a sample result:

R: 150, r: 90, d: 30, h: 80 [mm]
Exponent of polytropic expansion: 1.30
Max pressure: 3.164
Lowest/highest torque: 63.689 / 70.853 [Nm]
Average torque 1: 68.474
Average torque 2: 71.272
RPM: 1000
Power 1: 3.732 kW

Average torque 1 and 2 are values, calculated in two different ways. The first way was to calculate volume according to fi and out of that pressure, from pressure and vane lenghts i got torque.
the second was simply W = M×fi, which, i believe is more accurate, but doesn't tell the M(fi) dependence.
max pressure is simply: p_max = p_atm * (V_max/V_min)^n,
where p_atm is atmospheric pressure (say this is the last stage of expansion), V_max is volume at fi = 30° (we have 6 vanes) and n is 1.30.

it would be possible to make this thing run very smooth (like putting vanes on rollers), the only thing that i can't tell are seals. that can be a problem.
see this topic: https://www.physicsforums.com/showthread.php?t=310106
and this: https://www.physicsforums.com/showthread.php?t=310412

air that goes out would go to a heat exchanger and then back to the next, bigger turbine.

it would also be possible to surround this thing with a pressurized gas from tanks - that would ease the pressure on seals and also, if any air leaked out, it would go to next stage instead of going directly into the atmosphere.

 

[gumpfer]

The fact that scuba tanks get too hot and might burst if filled too quickly leads to the most interesting question. If you had a new system for compressing air based on mixing inside the main tank, the atmosphere could be gotten into the tank with a supercharger. Air could be compressed so cheaply that the high pressure tanks could be dispensed with or at least not charged externally.

An intake pipe has a series of check valves inside the tank and the small volume between the check valves is pulsed with tank air using a large quick acting 2 way valve tee'd into it. The compressor just feeds atmosphere to this equalizer and has little work to do; most of the compressing is done by tank air. Thus most of the compression heat ends up concentrated in a small space which raises the intake atmosphere's pressure way above tank pressure and the result is a supercharged pulsejet pump inside an air tank driven by tank air.

No it isn't perpetual motion. The energy comes from the heat of ordinary atmosphere so it has a high COP like a heat pump. An analogous process has been used with steam boilers since 1850. Engineers argued that the boiler injector was impossible decades after it was available at Boilers R Us.

Thanks for the forum.

 

[xxChrisxx]

Several things seem oh so very wrong with this. There mere fact that you have to immediately jump to saying this 'isn't perperual motion' sets off the ******** detector alarm in my head.

First of all the above needs to be explained much much better, as its difficult to visualise.

What is driving this supercharger in the first place?

What are the flow rates of the two streams of air (I don't mean numerical just an indication of the relative flows)?

Why do you think that the atmospheric pressure would go 'way above' tank pressure?

How do you hope to get more energy out of the air than you initially put in as there appear to be no other sources of energy.
(And don't say that COP allows more energy, as it doesnt. Heat is not the same as work)

 

[kandelabr]

the compressor has nothing to do with this topic at all. i assume there is a compressor that stores cool pressurized air and also stores heat that was produced during compression.
that's all that's needed to know about compressors in this very case.

 

[RonL]

Several things seem oh so very wrong with this. There mere fact that you have to immediately jump to saying this 'isn't perperual motion' sets off the ******** detector alarm in my head.

First of all the above needs to be explained much much better, as its difficult to visualise.

What is driving this supercharger in the first place?

What are the flow rates of the two streams of air (I don't mean numerical just an indication of the relative flows)?

Why do you think that the atmospheric pressure would go 'way above' tank pressure?

How do you hope to get more energy out of the air than you initially put in as there appear to be no other sources of energy.
(And don't say that COP allows more energy, as it doesnt. Heat is not the same as work)

What did Rudolf Clausius mean when he made this statement??

" Mechanical work may be transformed into heat,and conversely heat into work, the magnitude of the one being always proportional to the other"

This question is not meant to be rude, it has been the foundation of most of the thoughts that I present, that most people misinterpret as out of context.

Ron

 

[xxChrisxx]

Work -> Heat -> Work

is totally fine.

The way it reads in gumpfers post is that say, 1KW work gives 5KW heat (COP of 5) so therefore you have more energy to play with. But when you convert that back to work the most you can get out of the cycle is 1KW. And Mr Clausius' wonderful concept of entropy states you won't even get that.

I've possibly misunderstood what he was on about, so its why I wanted some clarificaiton on a few issues, but in my experience anyone who uses the phrase 'its not perperutal motion honest guv' is usually talking about just that.

 

[gumpfer]

Let's compare two ways of compressing air. Typically it is squeezed mechanically into a smaller volume. Or there is pressure equalization, filling a volume from a higher pressure volume. (A third way to compress air is by directly heating it.) Any way you do it, for a given weight of air at a given set of conditions, to get a certain pressure there is the same amount of heat involved.

But heat and temperature aren't the same thing. Scuba tanks are filled by pressure equalization. Filling them too fast is not safe because a lot of heat develops quickly and there's no time for it to dissipate. Over-pressuring the tank is the result.

My design goal is to turn this phenomena to my advantage. The potential for a normal amount of air to develop more pressure than expected because of very fast delivery is easily explained.

In mechanical compression the compression and resulting heating takes place in the compressor, not the plumbing or tank. Air is delivered fairly slowly to the tank after substantial cooling has already taken place. In pressure equalization, the compression and same resulting heating all takes place in the tank.

Textbooks warn about keeping oil out of the plumbing. A few drops of oil in a closed pipe that is suddenly filled with high pressure air might explode and burst the pipe. This class of phenomena can be advantageous but oil is not needed.

Imagine a normal piston compressor feeding a tank that is pre-filled to 200 psig. This tank (not the compressor) is the source for compression by equalization of a smaller volume. For the sake of fast equalization the smaller volume to be filled from this source is placed inside the tank. This is the equalizer.

The intake pipe extends into the tank and is closed by a series of two check valves with a space between them. The intake pipe has to be large enough to accomadate the amount of atmosphere that will enter the equalizer at slightly above atmospheric pressure.

In the intake pipe between the two check valves is a large port that is open and shut by a fast acting valve that can be controlled by some convenient means.

When the equalizer is full of atmosphere the big valve on its side opens and if not for compression heat the resulting pressure would be about 199 psig in both tank and equalizer. But thermal equilibrium is not reached as quickly as pressure equalization. If the equalizer is well insulated there won't be time for the "no work" final result of free expansion. The rush of tank air into the equalizer compresses the air in the equalizer and the compression heat is trapped in the equalizer by the closure of the valve. If the valve were left open for a long time, thermal equilibrium would be reached and the resulting pressure in both tank and equalizer would be slightly less than 200 psi. But this is a way of overshooting equilibrium and the whole tank has contributed a little of its heat to the contents of the equalizer which blast into the tank en masse.

The sudden departure of the equalizer contents creates a momentary depression or relatively low pressure zone at the equalizer's intake check valve and the next charge of atmosphere enters under its own impetus with the help of the compressor which is only acting as a supercharger. Not resisting tank pressure.

The air compressor as we know it is an outdated machine. Simplistic thinking about compressed air has people assuming that pressure is just pressure, never mind the details. But the pressure in any container can change according to the motion of the air inside. Still air gives the maximum reading on the pressure gauge, that is static pressure. If the air moves in the tank, the pressure goes down till the air stops moving. There are any number of ways to get air into a tank. As the skeptics inform us, trying to design a more efficient air engine is a waste of time. A cheap way of compressing air is what is needed.

The equations for pressure equalization are simple but hard to find. They are implied in a few air brake manuals but spelled out in only a very few.

The condition of a given batch of air is constant as defined by the combined gas laws pv/t = Constant.

Full equalization of two conditions (waiting for thermal equalization too) results in the sum of the two original constants. pv/t of the compressing source + pv/t of the compressed destination = a new pv/t.

For the process I'm talking about we don't wait for thermal equalization so the sum of the two original pv/t conditions = the sum of the two new pv/t conditions.

Thanks for the forum.

Gumpfer

 

[redargon]

The equations for pressure equalization are simple but hard to find. They are implied in a few air brake manuals but spelled out in only a very few.

The condition of a given batch of air is constant as defined by the combined gas laws pv/t = Constant.

Sorry, I got a little bored and didn't read through your entire essay, but I thought you might like to have some more information about the combined gas law: It's combined, it's about a gas, but it only applies to ideal gases. Real gases can be taken into account by using a compressibility factor, otherwise your equations break down at high pressures and (low)temperatures

also, if you're getting your theory from air brake manuals, I'm a bit worried. Try wikipedia or your local library for a good text on thermodynamics, they're easy to find.

 

[gumpfer]

The equations for pressure equalization are not spelled out in any thermodynamics textbook I have seen, nor in any compressed air textbook I have seen. I was hoping to hear from people who are interested enough to read my post, if you're too bored to read my post before responding to it then perhaps you are trying to keep yourself awake by insulting an idea you have not thought about? No thanks, I am bored with skeptics who know all without investigating first.

 

[RonL]

The equations for pressure equalization are not spelled out in any thermodynamics textbook I have seen, nor in any compressed air textbook I have seen. I was hoping to hear from people who are interested enough to read my post, if you're too bored to read my post before responding to it then perhaps you are trying to keep yourself awake by insulting an idea you have not thought about? No thanks, I am bored with skeptics who know all without investigating first.

Well I did try to read your post, and like redargon, could not follow it all the way through, but I do think you are trying to describe this concept (or basically a jet ejector system) which I think has merit.



http://www.aircaraccess.com/images/magkvalv.jpg

Ron

 

[redargon]

The equations for pressure equalization are not spelled out in any thermodynamics textbook I have seen, nor in any compressed air textbook I have seen. I was hoping to hear from people who are interested enough to read my post, if you're too bored to read my post before responding to it then perhaps you are trying to keep yourself awake by insulting an idea you have not thought about? No thanks, I am bored with skeptics who know all without investigating first.

yes, but the fundamentals of pressure equalization has to do with thermodynamics: Boyles Law, Ideal gas law etc. I was just hoping you were open enough to accept some constructive criticism about how the combined law may (or may not) be applied and how this can be adjusted for by using compressibility factors. I'm not being skeptical, because I did not read through your whole post, but your approach to the combined gas law jumped out at me and I thought I'd offer some insight.

P.S. I tried reading through your post again and started getting confused (I'm apparently no the only one). Maybe you could add some sketches and also the equations of pressure equalization that you have so that we can discuss those as well and so that the idea is a bit more accessible to everyone.

 

[gumpfer]

Here's the sketch Ron linked to.

This is old stuff, based on patent no. 2030759 but complicates it. My idea is to simplify that idea by Bob Neal (1936, now in public domain) not complicate it.

The patent shows a tank with an intake pipe entering and dumping air into the tank through two check valves. That's all. No explanation of why it should work, but the inventor did show a working model in order to get the patent. This sketch adds hardware to make it maybe seem feasible. I don't know about the jet pump idea though, the compressor in the tank that supplies drive air might have to move a lot of air and I'm saying it can be done very simply.

The general idea of the "Neal tank" is to make a new kind of compressor that doesn't resist the pressure that's in the tank already but puts atmosphere into the tank "somehow" and let's the compression take place by equalization with what is already in the tank. The "somehow" is the question. The energy source is the internal energy or heat of the incoming atmosphere adding to the pool of energy already in the tank.

Info on air brakes is not to be sneezed at, it stops trains so it had better be right. But the reduction to math is not easy to find. I did find it though about 1.5 times. And it's easy math.

PV/T = constant. Roughly. I know air is not an ideal gas but all the books use this for all practical purposes. So if a fixed quantity of air (a certain weight, a fixed number of molecules) has a condition pv/t, then if you change its pressure, volume, and or temperature, the value of pv/t will remain constant. The subnotations used below refer to different conditions with each separate batch of air being a different condition. For example 1 is atmosphere of a certain pressure, volume and temperature. The subnotation 2 is compressed air of a certain pv/t, a different constant. The subnotation 3 is a new condition referring to the new condition of a new batch of air, the mixture of the first two. This assumes complete pressure equalization and thermal equalization.

But if you don't wait for equilibrium then the result is two new conditions. For example if you fill a scuba tank (condition 1) from a higher pressure tank (condition 2), then the result is a hot scuba tank and a slightly cooled source tank. Conditions 3 and 4. Complete equalization is if you have perfect insulation on both tanks, leave the valve open, and wait for thermal effects to spread out through the combined volume of both tanks till all is equal. That doesn't happen (except in Joule's experiment which proves the energy is conserved, there is no external work done), you take your warm tank with you, the scuba filling station doesn't have time to reclaim their lost heat.

So I'm talking about a simple math I learned from air brake manuals because no one else cares about pressure equalization except odd stuff like player pianos and pipe organs and ear doctors. Google it--I'm not exaggerating. Pressure equalization is an ignored aspect of science except for stopping jillions of tons of train. Thermal effects aren't important in that application because large events are being handled by equalizing relatively large volumes with other relatively large volumes through a small pressure differential. My application involves equalizing a relatively large high pressure volume with a relatively small low pressure volume. It is the difference in size and pressure between the two conditions that creates a drastic surge of heat between the two check valves in the tank. The equalizer is suddenly filled with tank air and if it doesn't get super hot than why do we have to fill scuba tanks slowly? Overshooting equilibrium is dangerous, so it has to be done inside the tank.

Here is the math.

Complete equalization (aka free expansion, unbalanced expansion, unresisted expansion, partially unresisted expansion; discovered by Joule and once called "Joule's Law"):

P₁V₁/T₁ + P₂V₂/T₂ = P₃V₃/T₃

Incomplete equalization (call it a surge-driven equalizer):

P₁V₁/T₁ + P₂V₂/T₂ = P₃V₃/T₃ + P₄V₄/T₄

To be perfectly honest, even the air brake manuals didn't have the last equation, since they ignore thermal effects until a pipe bursts, then I guess they have to clean oil out of their pipes so it won't keep happening. I think if the first equation is true then the second one is also correct. When you mix two pv/t conditions, you add them together to get the new condition.

This is derived mathematically from Boyle's Law, Charles Law, and the combined law, nothing mysterious. If you can ignore thermal effects then you just delete all the T's from the equations. My idea is to make the thermal effects do something with a forward surge past equilibrium and then not allow the disturbance to return. There is no time for full equalization; the heat causes all the air between the check valves to blast into the tank and a depression is left behind in the equalizer so that the compressor is only working against very little pressure instead of tank pressure.

Thanks again for any constructive criticism.

Gumpfer

 

[Peter T.]

Hi friends,

maybe following story has something to do with the issue:...A friend of mine had nearly a flat and hot tire at his car. He drove to a gasoline station and tried to fill it up. At 1,7 bar that tire exploded. The service man said, that this would happen sometimes. ..Ok... the tire was hot (100 C.?) The surrounding temperature 15 C. According to the rule of Gay-Lussac the tire should have a temp. of 500 C, to have a pressure of 5 bar...where normally tires can blow up.
Where is the trick? Is it the time you need to fill up tanks (tires)?

Peter T.

 

[mgb_phys]

A friend of mine had nearly a flat and hot tire at his car. He drove to a gasoline station and tried to fill it up. At 1,7 bar that tire exploded. The service man said, that this would happen sometimes. ..

Most likely he had damaged the tire by driving on it while it was flat and started a crack somewhere.

 

[boab]

This is a bit outdated reply to the thread, but I thought that kandelabr had a good idea with using air to drive his ideal car.
What I think was lost in the discussion was applying compressed air drive beyond its practical design limits, such as cost, range, size, etc. I've more than once thought of making "a small" vehicle using compressed air myself. It'd be unique, relatively simple to build, and fun to make something different. However...efficient it would not be. Nor would it have a very long range. A compressed air driven engine is nothing more than a steam engine in principle, without the heat in the boiler. Consequently it has to have the boiler filled by a compressor.
I was somewhat surprised at the negative comments though on some aspects of his idea...out of hand. There are air powered forklifts, and they were used extensively in coal mines at one time. Why, because they work in the environments where there was a chance of a spark causing an explosion, or in contaminating the working environment atmosphere. Those requirements made them practical to use.
If IC engines are so much more practical than compressed air, why are so many things like jack hammers compressed air powered. There's no practical reason a jack hammer couldn't be IC, and there are some, but on the whole, compressed air still rule in practicality for use in jack hammers and various hydraulic applications.
On the other hand, trying to replace an IC engine in a car, as wasteful and inefficient as it is, with a compressed air powered engine is not practical. Same applies to electric cars that use batteries, or fuel cells. "I LIKE" electric motors in cars. In certain aspects they have many advantages over IC engines. One of which is the efficiency of the motor which runs about 90%, verses say 23% for IC's. BUT, the batteries to power the motors are heavy, costly, have a low power density, and need replacement every 4 or 5 years. They also have to be charged from an outside source. Usually from just about as inefficient a charging source as the IC itself.
Fuel cells...they are lighter, extremely expensive, and require specially prepared fuels that must be absolutely clean of containment's. They also do not have rapid response, or high load capabilities.
And again, in the case of the hydrogen fuel cell, it requires the making of the hydrogen which does not occur in nature in large quantities in pure form. And again, the hydrogen making process is about as bad as running an IC engine in the first place. This is ignoring the fact that such a hydrogen economy will require a whole new development in infrastructure to supply it.
So in short...what have you "REALLY GAINED" when you drive your non polluting electric or fuel cell car? Actually, very little. The best electric car batteries have only about the energy storage of a gallon and a half of gasoline.
In many cases, it pays to look to the railroad and marine industries for efficient power systems. Someone in the thread mentioned diesel locomotives driving electric traction motors. More efficient than having it done by a mechanical drive train, and has been around since the 50's. Notice that hybrid cars are now powered by the same basic system. They use the best of both.
If you think about it, the horse was a pretty good way to travel. A bit slow, labor intensive, and a methane polluter, but it lasted for thousands of years before being outdone by the IC engine. There was a reason for that mode of transport lasted so long. It fit the need, and was practical at the time.
If you really want to get a clean mode of transportation, nothing beats the old fashion sail. It all depends on what you consider practical, and staying in the limits of that mode of power generations efficiency curve.

Boab

 

[kandelabr]

excuse me, boab, but i don't get what did you actually wanted to say.

anyway, I'm still developing my engine: there's only one problem left to be solved and then i can launch my autocad.

 

[boab]

If you don't get what I was trying to say, that is okay. I wish you luck on the project.

Boab

 

[kandelabr]

If you don't get what I was trying to say, that is okay. I wish you luck on the project.

Boab

thanks, i appreciate that.

 

[RonL]

This is a bit outdated reply to the thread, but I thought that kandelabr had a good idea with using air to drive his ideal car.
What I think was lost in the discussion was applying compressed air drive beyond its practical design limits, such as cost, range, size, etc. I've more than once thought of making "a small" vehicle using compressed air myself. It'd be unique, relatively simple to build, and fun to make something different. However...efficient it would not be. Nor would it have a very long range. A compressed air driven engine is nothing more than a steam engine in principle, without the heat in the boiler. Consequently it has to have the boiler filled by a compressor.
I was somewhat surprised at the negative comments though on some aspects of his idea...out of hand. There are air powered forklifts, and they were used extensively in coal mines at one time. Why, because they work in the environments where there was a chance of a spark causing an explosion, or in contaminating the working environment atmosphere. Those requirements made them practical to use.
If IC engines are so much more practical than compressed air, why are so many things like jack hammers compressed air powered. There's no practical reason a jack hammer couldn't be IC, and there are some, but on the whole, compressed air still rule in practicality for use in jack hammers and various hydraulic applications.
On the other hand, trying to replace an IC engine in a car, as wasteful and inefficient as it is, with a compressed air powered engine is not practical. Same applies to electric cars that use batteries, or fuel cells. "I LIKE" electric motors in cars. In certain aspects they have many advantages over IC engines. One of which is the efficiency of the motor which runs about 90%, verses say 23% for IC's. BUT, the batteries to power the motors are heavy, costly, have a low power density, and need replacement every 4 or 5 years. They also have to be charged from an outside source. Usually from just about as inefficient a charging source as the IC itself.
Fuel cells...they are lighter, extremely expensive, and require specially prepared fuels that must be absolutely clean of containment's. They also do not have rapid response, or high load capabilities.
And again, in the case of the hydrogen fuel cell, it requires the making of the hydrogen which does not occur in nature in large quantities in pure form. And again, the hydrogen making process is about as bad as running an IC engine in the first place. This is ignoring the fact that such a hydrogen economy will require a whole new development in infrastructure to supply it.
So in short...what have you "REALLY GAINED" when you drive your non polluting electric or fuel cell car? Actually, very little. The best electric car batteries have only about the energy storage of a gallon and a half of gasoline.
In many cases, it pays to look to the railroad and marine industries for efficient power systems. Someone in the thread mentioned diesel locomotives driving electric traction motors. More efficient than having it done by a mechanical drive train, and has been around since the 50's. Notice that hybrid cars are now powered by the same basic system. They use the best of both.
If you think about it, the horse was a pretty good way to travel. A bit slow, labor intensive, and a methane polluter, but it lasted for thousands of years before being outdone by the IC engine. There was a reason for that mode of transport lasted so long. It fit the need, and was practical at the time.
If you really want to get a clean mode of transportation, nothing beats the old fashion sail. It all depends on what you consider practical, and staying in the limits of that mode of power generations efficiency curve.

Boab

I think I have a clear understanding of what you are saying, however I think very high pressures in larger containment is not the best solution.

Kandelabr has made many comments that fall completely inline with most of my thoughts, and I want to commend him for withstanding the many negative and criticizing comments of some.

Your comment about a steam engine "A compressed air driven engine is nothing more than a steam engine in principle, without the heat in the boiler. Consequently it has to have the boiler filled by a compressor." leads me to ask a question.

Considering a propane system of proper design, will build to a pressure of close to 300 psi if it is in direct sunlight on a 100 degree day, with no work being performed, now will the gas vapor not drive a compressor for some duration, starting an air compression cycle ?? If several compressing and expanding units are working on the same power shaft and they are inside the propane tank, will they not deposit waste heat in the now cold liquid, thus continuing the cycle??

I believe kandelabr is alluding to this type of heat/work relationship and one key is that everything is working on one shaft, any movement at all results in compression and expansion taking place at the same time. Air brings heat in, it is transferred into motion and work transferred out keeps the system from overheating.

Ron

 

[chayced]

Ron,
Have you ever put your hand on an air compressor discharge line? It's hot because of heat of compression. This is not free energy, it's work done by the compressor that ends up as wasted heat. The same applies in reverse, as you expand a gas it absorbs heat from the environment, but this cannot be more heat that it gave up when it was compressed. If you are dealing with a liquid like propane, then you are adding a phase change in either direction, but the principle is the same.

You don't gain anything, from heating your tank that you didn't already lose when you compressed your air.

Sorry, but that's just how it works.

 

[russ_watters]

boab, as far as I can tell, all of those air powered devices you mentioned lack a tank and that's part of the reason that their usefulness is completely unrelated to whether a car could be air powered.

 

[RonL]

Ron,
Have you ever put your hand on an air compressor discharge line? It's hot because of heat of compression. This is not free energy, it's work done by the compressor that ends up as wasted heat. The same applies in reverse, as you expand a gas it absorbs heat from the environment, but this cannot be more heat that it gave up when it was compressed. If you are dealing with a liquid like propane, then you are adding a phase change in either direction, but the principle is the same.

You don't gain anything, from heating your tank that you didn't already lose when you compressed your air.

Sorry, but that's just how it works.

I really have to wonder what you are thinking when you read my comment ??
The compressor is inside the tank, immersed in the liquid and all heat of compression is absorbed, what happens when propane is heated ? it builds pressure.
You now have both high pressure air and high pressure propane. Can they both perform work? I hope you answer yes.

The rest of the design is all about, how much compression and how much expansion, and how the heat is exchanged within the two systems as they counter flow through the heat exchangers.
On the compression side heat flows into the propane, and on the expansion side heat moves out of the propane.

Is there any difference between heat liberated from compressed air and heat liberated from burned fuel?

The air and propane are isolated from each other, the air is an open system and the propane is a closed system. Work has to be performed in order to keep the propane from overheating. The work out is exactly equal to the difference of air temperature drawn in and the temperature of the air discharged.

Ron

 

[chayced]

The compressor is inside the tank, immersed in the liquid and all heat of compression is absorbed, what happens when propane is heated ? it builds pressure.

The higher the discharge temperature of the compressor the harder it has to work to pressurize the air. The cooler the discharge temperature the less it has to work, but this also reduces the amount of heat transferred to the propane. All you really have is a compressor which is also acting as a propane heater.

Can they both perform work? I hope you answer yes.

Sure, but you lose energy initially from the compressor. Nothing gained.

The rest of the design is all about, how much compression and how much expansion, and how the heat is exchanged within the two systems as they counter flow through the heat exchangers.

Even with counter-flow heat exchangers that are infinitely long with no resistance, the best you can ever achieve is for each fluid to reach the others initial temperature. (With the same heat capacity and flow-rate of each fluid.) Nothing to win here.

On the compression side heat flows into the propane, and on the expansion side heat moves out of the propane.
Ron

So what you are really saying is that the propane is only a heat transfer medium.

Is there any difference between heat liberated from compressed air and heat liberated from burned fuel?

Yes, you have to put the energy into the air with a compressor, you don't with the fuel.

The air and propane are isolated from each other, the air is an open system and the propane is a closed system. Work has to be performed in order to keep the propane from overheating.

What is performing this work? As you have described it, the air compressor has nothing driving it, it just exists. Now if the propane is driving the air compressor, then it is the energy stored in the tank of propane that drives the whole thing. So you recoup part of your losses by heating the propane tank. You don't win anything because the tank had to be cooled in the first place when it was compressed. Also when start taking propane out of the tank it cools off. So all you are doing is regaining some of the losses that occurred when the propane tank was compressed.

The work out is exactly equal to the difference of air temperature drawn in and the temperature of the air discharged.

No it's not. The propane tank does not magically refill itself. You are trading potential energy in the tank for a reverse brayton cycle on the air.
http://en.wikipedia.org/wiki/Brayton_cycle

You lose energy in the process one way or the other. All you have is a machine that uses compressed propane to cool off air. Not a very efficient AC.

 

[dr dodge]

I have to note a couple things here.
If you are using outside air to compress, it will have very high, and to a large extent unpredictable varying levels of water. it could be good, or bad, but it will be there.
so now we have all stainless steel components or monel for corrosion resistance. (big $$$)
500 bar (~7500 psi) is significant. The tank discussion aside, lines, control systems, valving all would need to be big, thick and heavy walled. lots of hard steel, very little flexable stuff.
Please remember the standard is usually safety factor of 3-5, that's 22000-36000 psi (1500-2500 bar)
given an ideal gas (and with the water content it's not) if the tank was 10 cu ft, there is basicly 5000 cu ft of gas crammed in there. If a tank ruptured, that gas volumn would expand out rapidly. anything less than a perforated cage covering every pressure member would be foolish, from a safety point.
this is pneumatics in a hydraulic pressure relm, and the variable displacement motors the OP described are already in use with hydraulic systems in heavy equipment.
I have sketched numerous variations of this kind of process on my own time, but, due to the size, weight, and safety concerns this system is better suited as a stationary energy storage unit. A simple car would weight as much as a dump truck.
think, slow compressor (one stroke/day) lowers the heating, multistage compressing (still at 1 stroke/day) and buried/walled in high pressure containers.
I can tell you one thing for sure, I work on a daily basis with pressures in that range (up to 15kpsi gas, 72kpsi hydraulic) and if you take it for granted, someone wil get hurt or die.
(once a device failed at 15k (due to stupidity) and the 2" stainless cap took a big chunk out of the blockhouse wall, and then went thru the wall sending 3/8 bolts flying like 30/30 bullets, NOT FUN!)

dr

 

[mgb_phys]

levels of water...so now we have all stainless steel components or monel for corrosion resistance.

Only for the compressor - the compressed air would be dry

The tank discussion aside, lines, control systems, valving all would need to be big, thick and heavy walled. lots of hard steel, very little flexable stuff.

The tanks in cars are carbon fibre, the lines to the motor and the motor operate at lower pressures.

If a tank ruptured, that gas volumn would expand out rapidly. anything less than a perforated cage covering every pressure member would be foolish, from a safety point.

The carbon tanks are designed to unravel safely rather than puncture.
The actual energy stored is relatively low and is easily distributed.

The same argument has been applied to electric cars, that they are unsafe because firefighters in an accident might be electrocuted cutting through a wire - presumably the ff love approaching a burning gas tank.
If you tried to introduce a gasoline powered car today with the same safety standards that you need to handle gasoline in the chemistry lab - it would also be impractical.

 

[dr dodge]

- propulsion
to use as much energy as possible, i'd need an air motor that would work on all pressures from 450 to 1 bar, .

that is not low pressure.

how would the compressed air be dry, if you are sucking it out of the atmosphere? upon cooling it would condense in the tank. to much and it would hydro-lock your motor. if you are drying the air, then add more efficiency roll-off.

it doesn't matter if the construction of the tank unravels or not (which I would have to see to believe), fittings, lines, brackets, and control systems and all things not firmly fastened would become projectiles

good ideas, wrong application (automobile)
and yes, fire fighters approach anything on fire, its their job
being a bit dramatic, perhaps?

dr