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Designing a compressed air car

by kandelabr
Tags: compressed air car
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mgb_phys
#19
Apr29-09, 11:04 AM
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There is a french company (aircar?) with a prototype Smart car style compressed air powered car.

Advantages:
Low weight (with CF tanks) compared to battery
Quick refill at service station
Simple engine - basically a 2 stroke ICE without the 'C'
Long life of tanks compared to battery
Cooling effect of gas drives air conditioning.

Disadvantages
No way to do regenerative braking (onboard compressor would be too large) not a problem for a small in town car.
Cooling effect of gas - in winter
Filling infrastructure more complex , you need a compressor rather than an extention cable.
kandelabr
#20
Apr29-09, 11:12 AM
P: 102
well, as i said before, these cars are nothing but a bathtub with a motor that has been accidentally found there. these things are small and dangerous (in case of crash it's better to be in a heavy vehicle than in that nutshell) and useless - i'd like to take my girlfriend with me, wouldn't you too? and after all, they aren't efficient at all, we just get an impression they are, because they don't need a lot of energy. if you look closely, there's no regenerative braking, isentropic expansion, no use of heat generated during compression, etc.
and there are pretty cold winters, need heating, not A/C.

edit: i found that: http://www.mdi.lu/english/
omg just look at those artifacts. i wouldn't drive it if they paid me. their maximum range is 180 to 220 km, not to mention at what speed. besides that, they all have some sort of gasoline engine built in.
FredGarvin
#21
Apr29-09, 11:27 AM
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As I see it there are some huge, real problems with compressed air:

1) Safety. A compressed air car made from carbon fiber will never pass FMVSS safety standards for crashes. Also, compressed air at those pressures make very nice bombs. Not that gas cars can't explode, but if you rupture a gas tank, it won't immediately explode. A compressed air tank at 400-500 bar will.

2) Filling rate required: If you try to fill a compressed air tank quickly at those high of pressures, you better have supplemental cooling for the tank. If you don't, I don't know of many people who would be happy with a day long refill of their tank when the best you're looking at range wise is a couple of kilometers. Also, the notion of pulling up to a refilling station is ludicrous. I wouldn't want to be anywhere near a tank when it is being filled to high pressure. The tanks should be submerged in water when they get filled. Look for any accident report of a scuba tank exploding and you'll see why this should be mandatory for filling.

3) Heat Exchangers: Because you need a heat exchanger in the system, you are limited in the delta t between the inlet and outlet temps. This drives the size and the weight up.

4) There are way too many other uses for oil and fossil fuels other than automobiles: There are bigger fish to fry out there.
giggoman
#22
Apr29-09, 12:44 PM
P: 4
My gawd, this is how new ideas get shut down..... an automatic transmission is a term used to describe displacement. Not in the traditional form IE, A turbo 400... A design that be lighter and more suitable for an electric vehicle should be the focus.

Why don't we discuss the problems and how to overcome them, instead of mindless rhetoric, that is both condescending and the non-productive.
kandelabr
#23
Apr29-09, 12:46 PM
P: 102
i'm totally aware of all things you said:
1) Safety.
i have a few ideas on how not to be killed if an air tank explodes. when i asked a professor at our faculty what happens if a tank explodes, he said "why would it explode at all?"
well, i know it would explode, but i don't really imagine how this happens.
i'd try to make some kind of (kevlar fiber?) reinforced bag around the tank. it wouldn't be sealed and it wouldn't prevent the tank from rupturing, but would at least ease the explosion by more evenly distributing pressure.
the second idea was to built a tank from a cylinder with separate caps. these caps would be held together separately, not by the cylinder wall, but by another cylinder around the smaller one. if the tank was about to explode, only caps on either side would be blown off (a few centimeters) and the tank would empty without spitting dangerous stuff around.
the third option is to use foams that are already used to absorb impact energy.
of course there wouldn't be only one tank in a car, i'd use more and distribute them around different places in a car.
i know 100 MJ is quite a lot of energy and these ideas are only some thinking i've done. a lot of research would need to be made in this field though.

2) Filling rate required
there is not a chance air would be filled directly into car's tank. a special compressor would be needed with heat recovery system (warming up that heat reservoir). that compressor would prepare compressed air and the hot stuff and only push compressed air into car's tank.
there also wouldn't be any compressors onboard the car. that would be silly.

3) Heat Exchangers:
you probably meant temperature difference, right? i don't know why is the problem here. air, expanded to three times the starting volume, can quickly reach temperatures well below -50C, and 50 or 70 K is a difference, large enough to have a reasonably small heat exchanger.

4) There are way too many other uses for oil and fossil fuels other than automobiles: There are bigger fish to fry out there
yup, i know that. but cars are one of the biggest polluters since their cleaning abilities are very little, efficiencies low, but quantity of cars is huge. there's at least not that many trucks and larger machines, and even those can be i.e. transferred to railroad (trucks) or have installed a sort of cleaning device or whatsoever.
TVP45
#24
Apr29-09, 12:58 PM
P: 1,127
Quote Quote by kandelabr View Post
i'm totally aware of all things you said:
1) Safety.
i have a few ideas on how not to be killed if an air tank explodes. when i asked a professor at our faculty what happens if a tank explodes, he said "why would it explode at all?"
well, i know it would explode, but i don't really imagine how this happens.
i'd try to make some kind of (kevlar fiber?) reinforced bag around the tank. it wouldn't be sealed and it wouldn't prevent the tank from rupturing, but would at least ease the explosion by more evenly distributing pressure.
the second idea was to built a tank from a cylinder with separate caps. these caps would be held together separately, not by the cylinder wall, but by another cylinder around the smaller one. if the tank was about to explode, only caps on either side would be blown off (a few centimeters) and the tank would empty without spitting dangerous stuff around.
the third option is to use foams that are already used to absorb impact energy.
of course there wouldn't be only one tank in a car, i'd use more and distribute them around different places in a car.
i know 100 MJ is quite a lot of energy and these ideas are only some thinking i've done. a lot of research would need to be made in this field though.

2) Filling rate required
there is not a chance air would be filled directly into car's tank. a special compressor would be needed with heat recovery system (warming up that heat reservoir). that compressor would prepare compressed air and the hot stuff and only push compressed air into car's tank.
there also wouldn't be any compressors onboard the car. that would be silly.

3) Heat Exchangers:
you probably meant temperature difference, right? i don't know why is the problem here. air, expanded to three times the starting volume, can quickly reach temperatures well below -50C, and 50 or 70 K is a difference, large enough to have a reasonably small heat exchanger.

4) There are way too many other uses for oil and fossil fuels other than automobiles: There are bigger fish to fry out there
yup, i know that. but cars are one of the biggest polluters since their cleaning abilities are very little, efficiencies low, but quantity of cars is huge. there's at least not that many trucks and larger machines, and even those can be i.e. transferred to railroad (trucks) or have installed a sort of cleaning device or whatsoever.
Please listen to FredGarvin about safety. For a SCUBA sized tank at those pressures, you're looking at a stick of dynamite. Bigger tanks, well.... And, it takes little more than a scratch or small dent to propagate. When explosion proof housings are tested, it is standard to use compressed water rather than air due to the danger. Your expendable endcap would be high velocity shrapnel.
xxChrisxx
#25
Apr29-09, 01:01 PM
P: 2,048
Quote Quote by giggoman View Post
My gawd, this is how new ideas get shut down..... an automatic transmission is a term used to describe displacement. Not in the traditional form IE, A turbo 400... A design that be lighter and more suitable for an electric vehicle should be the focus.

Why don't we discuss the problems and how to overcome them, instead of mindless rhetoric, that is both condescending and the non-productive.
We'll start discussing the problem when you stop talking utter ********. Read up what the word transmission means and stop posting rubbish until you do.

It has never been and never will be used to describe displacement of any kind as we already have a word for that. And that word is displacement.
kandelabr
#26
Apr29-09, 01:10 PM
P: 102
carbon fuel tanks don't tear apart in shrapnels, these tanks end up in two, maybe three pieces. they have already been tested and used in such applications.
my endcaps would stay in one piece and only move as much as a centimeter or two away from the inner cylinder and then be stopped by something that would be designed to stop them. nothing would really be damaged except the outer cylinder (it wouldn't be a cylinder at all, rather some carbon or kevlar ropes or something like that, haven't really gone deep into this stuff).
xxChrisxx
#27
Apr29-09, 01:12 PM
P: 2,048
Quote Quote by TVP45 View Post
Please listen to FredGarvin about safety. For a SCUBA sized tank at those pressures, you're looking at a stick of dynamite. Bigger tanks, well.... And, it takes little more than a scratch or small dent to propagate. When explosion proof housings are tested, it is standard to use compressed water rather than air due to the danger. Your expendable endcap would be high velocity shrapnel.
This is so true, i've seen photos of industrial pressure tests that used gas that lead to failure (this was back in the day before health and saftey)

A block of concrete weight the best part of 2 tons blown 200 yards through a brick wall and into the car park.

Quote Quote by kandelabr
1) Safety.
i have a few ideas on how not to be killed if an air tank explodes. when i asked a professor at our faculty what happens if a tank explodes, he said "why would it explode at all?"
If a high pressure vessel fracures it'll do one of two things depending on the container. It'll either

a: not fail completely, in which case it'll take off like a rocket and cause carnage.
b: the crack will propogate and the vessel will be blown apart: which basically means it becomes a bomb.

Both rockets and bombs are considered to be mildly dangerous.
giggoman
#28
Apr29-09, 01:18 PM
P: 4
A multi-stage tank system could also be used, with an atmosphere sensitive valve. Air compression is very powerful, a ruptured tank during an accident, could be dangerous, but easily averted. Many industries have implemented fail safe measures that address this potential problem. Once again A traditional Compressor is not what I am referring to. It is the function of the device. A piston, a valve, a compressor. Oh sure you could buy one from Home Depot, But what fun would that be?

I agree, originality, Learn from the past, Build something new.

L
xxChrisxx
#29
Apr29-09, 01:21 PM
P: 2,048
Quote Quote by kandelabr View Post
carbon fuel tanks don't tear apart in shrapnels, these tanks end up in two, maybe three pieces. they have already been tested and used in such applications.
my endcaps would stay in one piece and only move as much as a centimeter or two away from the inner cylinder and then be stopped by something that would be designed to stop them. nothing would really be damaged except the outer cylinder (it wouldn't be a cylinder at all, rather some carbon or kevlar ropes or something like that, haven't really gone deep into this stuff).

I dont think you grasp the sheer amount of pressure you are talking about. Pressure vessels with that much air in them do not just go pop. I'm am not joking when I say they go through brick walls with ease.
giggoman
#30
Apr29-09, 01:28 PM
P: 4
My apologies, I didn't realize this was an Elitist Forum... I didn't know this was thread was about semantics. I guess I should brush up a little on my Glossary and Communication skills.

BTW, I have already built several Transmission systems, although Rudimentary, in design, I double the speed of a golf cart. Without changing the Motor. I'm just a lowly mechanic, What the hell do I know?

L
xxChrisxx
#31
Apr29-09, 01:35 PM
P: 2,048
I personally hate arguing over semantics, but there is a difference between being correct and being incorrect. That is not semanics.

What you have said so far makes little to no sense. Transmissions have nothing to do with what we are talking about and certainly dont describe displacements.

In fact just what are you talking about when you said 'automatic transmissions'?
kandelabr
#32
Apr29-09, 01:52 PM
P: 102
i have a feeling you're starting to lean towards "not possible" without trying to think "how to make it possible".
ok, you're probably right about the danger. how many pressure vessels have you seen or heard exploding?
take a look at this video:
http://www.youtube.com/watch?v=tyINN...eature=related
and this:
http://www.youtube.com/watch?v=ejEJGNLTo84&NR=1
i actually wouldn't give the tank 6 ft to accelerate.

the fact is that i would definitely have to split a giant tank or two to many smaller tanks. an explosion like seen in first or second movie i gave links doesn't seem too lethal if handled correctly.

here's an idea: i could design a tank so that there would be one know spot where it would definitely break in case of crash. from that spot it would be possible to at least prolong explosion duration.

and finally, so much discussion goes on about a subject that is actually most thoroughly researched. i know which standard describes composite pressure vessels (got an answer on this forum), and there's also quite a lot of cars and other machines running on compressed air.
xxChrisxx
#33
Apr29-09, 02:16 PM
P: 2,048
I dont deal with pressure vessels so i've seen none. However I have heard of many exploding. Most pressure tanks like that scuba one take the rocket route. It tends to be the large industrial pressure vessels that bomb. But at the pressures you are taling about , unless you have a very thick walled pressure vessel then it could go either way.

'Doesnt seem too lethal?' for a start that tank is tiny compared to the application you are looking at. And in the second case it went through a solid wall, if you put a person in the way it would go through them too. Thats faily lethal.

The other compressed air cars certainly arent looking at pressures or volumes of air of this scale.

I'm not leaning towards 'not possible' merely 'not safe' in its current form.

A safer bet would be lots of smaller cylinders, but how would you stop them from moving.
TVP45
#34
Apr29-09, 03:01 PM
P: 1,127
Quote Quote by kandelabr View Post
i have a feeling you're starting to lean towards "not possible" without trying to think "how to make it possible".
ok, you're probably right about the danger. how many pressure vessels have you seen or heard exploding?
take a look at this video:
http://www.youtube.com/watch?v=tyINN...eature=related
and this:
http://www.youtube.com/watch?v=ejEJGNLTo84&NR=1
i actually wouldn't give the tank 6 ft to accelerate.

the fact is that i would definitely have to split a giant tank or two to many smaller tanks. an explosion like seen in first or second movie i gave links doesn't seem too lethal if handled correctly.

here's an idea: i could design a tank so that there would be one know spot where it would definitely break in case of crash. from that spot it would be possible to at least prolong explosion duration.

and finally, so much discussion goes on about a subject that is actually most thoroughly researched. i know which standard describes composite pressure vessels (got an answer on this forum), and there's also quite a lot of cars and other machines running on compressed air.
I'm sorry. Had I known you had youtube data, I would not have posted, using simple engineering standards.
Good luck to you.
kandelabr
#35
Apr29-09, 05:33 PM
P: 102
Quote Quote by TVP45 View Post
I'm sorry. Had I known you had youtube data, I would not have posted, using simple engineering standards.
Good luck to you.
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.
russ_watters
#36
Apr29-09, 05:35 PM
Mentor
P: 22,303
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.


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