Designing a compressed air car

In summary, an expert summarizer of this conversation believes that it would be possible to take an existing commercial car and remove its IC engine, fuel tank and other unnecessary components to create a compressed air powered car that can travel at a speed of 90 kilometers per hour. The estimated energy used to power this car would be around 120 megajoules, and the estimated cost of materials and construction would be around $10,000. Additionally, the car would need a propulsion system that could work on all pressures from 450 to 1 bar and a CVT would be the most efficient option. The car's ecology would also be a concern, as it would require the burning of gasoline and diesel.
  • #71
dr dodge said:
how would the compressed air be dry, if you are sucking it out of the atmosphere?
Pretty much the same way it gets dry on any other industrial compressor. Traps and mol-sieve

fittings, lines, brackets, and control systems and all things not firmly fastened would become projectiles
After the regulator everything else is at lower pressure. The brake lines in cars at higher pressure but we don't ban brakes in case they become projectiles.

and yes, fire fighters approach anything on fire, its their job
being a bit dramatic, perhaps?
It was a statement by some politician (from an oil producing state?) that electric cars should be banned because firefighters might be electrocuted cutting into them.

It's amazing that a substance I wouldn't be allowed to have in an ugrad lab (explosive, volatile, known carcinogen etc) and in a research lab a small test tube of it would require me to file MSDS sheets, work in a explosion proof fume cabinet with lab coat, goggles and gloves - yet I can fill up 10gallons of the stuff at the supermarket and drive around with it inside my car.
 
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  • #72
there is no regulator if, as the op stated, the 450 bar would be the first stage of the engine, so he would have the torque required.
removing water thru conventional means will remove some of it, not all of it.
as it is removed, your efficiency decreases. any moisture is still going to promote corrosion
I don't think that electrics should be banned, but they should have proper labeling as such, a main disconnect on the outside, like a race car, and the public should be tought the dangers.

and as far as the gasoline statement, I could say the same about alcohol
I need an MSDS, a flammable closet, etc but can buy it drunk at the booze store
thats because corportate liabilities require us to over protect the morons of the works, or pay high insurance payments

dr
 
  • #73
dr dodge said:
I don't think that electrics should be banned, but they should have proper labeling as such, a main disconnect on the outside, like a race car, and the public should be tought the dangers.

dr

I would LOVE to see that when you pull up to a red light.

*CLICK*
No electricity for you.
 
  • #74
As with most things, lots of internal detail not easiley seen at first look, don't lose sight of what I said 300 psi max. and low side 0 psi (maybe lower) everything inside is in motion and full of stored potential energy which gives a flywheel effect to carry each cycle to completion, and then well tuned check valves that control high and low pressures.

The size of compression and expansion is not as important as speed of cycles.

In the interset of safety I tend to think of this as a smaller, constant power battery charger for electric cars.
In a fixed location, a cold air and electric power unit, much the same as heat pumps that already exist, (only the real difference is heat is used for work instead of being dumped as waste.)

RonL

After a little thought, everything in the design in of equal importance. It will be critical for everything to move through transitional states at an appropriate speed to allow heat transfer at the proper rates.
 
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  • #75
chris...you are evil
i like it...lol

dr
 
  • #76
let me present you my ideas again, a bit updated this time.

first, i do not intend to use a pressure reducing valve as this represents a pure loss. my engine would work with pressures from 450 to ~20 bar; if i leave the last 20 bar out (the engine would still work at that pressure, just wouldn't have enough torque), i lose around 2-3% of energy for propulsion, but not for the next compression.
with every pressure between these it should develop a variable torque of 5 to 30 Nm. the amount of torque produced would be simply controlled by a valve and a servo-motor. since 30 Nm is not enough, one pair of gears (say ratio 4.5:1 or 5:1) will have two good effects: increase torque on wheels (135-150 Nm is enough and is there from the very 0 RPM) and reduce leakage of air while accelerating due to higher speed of the engine (less time for leaking). note that i really don't want a gearbox nor a clutch (and so far don't need any of these). besides that, the very core (the turbines) are no larger than 20 cm in diameter and 30 in length, which leaves a lot of space for air tanks. due to this small size the forces exerted by the pressure are kept low and also the problems with rotary vane devices (like friction due to centrifugal forces, bending stress on vanes, etc.). i only have one problem left to solve, see below. if you're interested in details, i can post some more.

yes, the pressure is huge, but valves and pipes should not be the problem. a pressure washer's hose is not a very huge thing, but easily handles 150 bar. moreover, there is enough space for hoses and valves and flow rates are quite low so there's no need for some bizzarre cross-sections.

we already discussed the tanks. so far i trust the standards. amen.

the problem i mentioned is the humidity. i only know stuff about humid air at an atmospheric pressure. see this topic: https://www.physicsforums.com/showthread.php?t=364602"
the first attempt at a solution was to exclude the water from the air during the expansion:
  • a hot and humid air starts to expand
  • it expands to a temperature just above freezing; water condenses
  • since water isn't a problem for a vane turbine (ice is), it's simply removed from the process and ends up on the road
  • dried air heats up and goes to the next stage
in this case only the first one or two stages would be critical - pressure ratio should not exceed, say 2. all i'd have to do is watch not to expand too much at once. oh yes, and there's some heat transferred from the condensing water, which makes the expansion more isothermal, thus doing more work.

but as i see, moisture can be removed during compression. i would like to know more about this, so if you have any links, please, post them here. but i wouldn't want to waste extra energy during compression because of humidity. drying can be done for free. :smile:

please keep in mind I'm not working on a compressor right now. compressors already exist, cars don't. when i finish the car, i'll come back with new ideas for a compressor (during a class i sketched a compressor with no clearance volume)... later.

merry christmas y'all.
 
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  • #77
A couple of points.
A pressure reducing valve doesn't waste anything, and ou are proposing having a valve on the engine to control speed anyway - it's much easier to run at more constant speed and pressure reduce the tank.
The safety factor of a pressure washer hose is low because water doesn't exapan much - the energy stored (an hence the anger) of a substance under pressure depends on how much strain is involved.
Turbines probably aren't the best solution. Most air car designs use piston engines - they are much easier to build and give all the torque you need without a complex gearbox.
The only seals are the piston rings which currently work very well even in the presence of exploding fuel - sealing pistons against 150psi air is pretty trivial.

I don't even want to go into the thermodynamics of how you are somehow heating the air by exapanding it - but basically air cars are going to work in hot dry climates a lot better than cold wet ones.
 
  • #78
mgb_phys said:
A couple of points.
A pressure reducing valve doesn't waste anything, ...

are you sure? consider this equation for isothermal expansion:

W = m R T ln(p1/p2)

say p2 is constant (atmospheric pressure). mass m is defined by car's tanks. higher initial pressure means more work.

or the other way: according to this equation, which represents lost work (losses of exergy)

Wlost = Tsurr Δs,

a change in entropy is directly proportional to lost energy and you can be sure throttling is irreversible and so entropy increases.

Turbines probably aren't the best solution. Most air car designs use piston engines - they are much easier to build and give all the torque you need without a complex gearbox.
The only seals are the piston rings which currently work very well even in the presence of exploding fuel - sealing pistons against 150psi air is pretty trivial.
please note that i wrote turbines in italics, because my engine is not a turbine - it's a set of customised rotary vane pumps in reverse direction - http://journal.fluid-power.net/journal/issue10/fig2.jpg" .

why NOT piston engines?
simply because of losses. if i wanted to have variable torque, i'd have to reduce pressure. see above.
I don't even want to go into the thermodynamics of how you are somehow heating the air by exapanding it - but basically air cars are going to work in hot dry climates a lot better than cold wet ones.
i don't know what do you mean by "heating the air by expanding it". if air expands adiabatically, it cools down. relative humidity increases with lower temperatures and when it reaches the dew point, water starts to saturate.

there are pros and cons in both hot and cold climates:
hot:
+ more heat available from the environment
+ possible collection of heat from the sun
- greater humidity
- smaller density of air in tanks

cold:
+ greater density of air in tanks
+ more heat could be added from the heat reservior in a car - more energy from the same mass of air
+ smaller humidity
- no heat from the environment
- freezing of water

which one is better? it looks like it's quite a complex problem, but i don't feel like it's very important right now - it'll definitely work in both.

should i better close this thread? i think you guys don't follow it anymore. these posts are starting to get random. some are repetitive, the others are wrong. this is the other one.
 
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  • #79
the whole compression/release for work process is very tied to the temp of the air. So, what would be the best efficiencies would be to cool the ambient air as much as possible. this increases density, helps remove water, and then gives you the most "gas per stroke" as possible. If the heat induced in the compression is allowed to abate, then that cool pressurized gas is used to fill the 2nd stage pump, very high density/pressure could be stored. always at Earth temp. because that is the easiest temp to maintain with no energy. heat then could be added at the time that work is being done, but even if it was not, because the density of charge was high at compression efficiencies should still be higher that would normally be

dr
 
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  • #80
  • #81
chayced said:
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.

"...The higher the compression ratio, the more heat that is moved into the propane"




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

"...The energy is not lost, it increases pressure in the propane"


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.

"...Do not lose sight of the fact that all actions are happening on a single shaft, if the propane gas has a set point of 300 psig, this pressure starts driving the main motor which in turn starts the compression of air, flywheel rotation, generator motion, all secondary expansion and recompression."


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

"...This is correct "



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

"...If air is brought in at 100 F and compressed 10:1, the air pressure builds as the pressure of the propane decreases and the liquid gets colder. The compressed air is now able to do work on the same shaft which is already in motion, The air can be expanded to a very cold state." ...(The energy is in the air) "energy can be stored". If each cubic foot of air is dropped in temperature by 150 degrees, what amount of energy is being removed??


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.

"...A number of ways to get started, if you have time the heat is absorbed from the atmosphere or the sun, you can also heat a resistance element with a battery, or you could hand crank the air compressor to some start point.
As for the propane it starts an expansion process as soon as it begins to turn it's motor(s) and put motion into the power shaft..."



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

"...Your right, it won't refill itself magically, it is based on the law that says heat flows spontaneously from hot to cold, now if you do not have time to waite, you can rely on the motion of your system driving the generator or you might draw off of a battery.
No matter how you do it, speed of transfer will depend on the span of high and low temperature..."

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.

"...I think you have missed the whole concept, the propane is the power of the system and the temperature of air in and air exhausted is the sum of energy that must be discharged in some form of work.
The mass energy being cycled inside, might be a 1,000 times greater than what flows in and out with the air flow..."

P.S. I still can't figure out this multi quote thing.:frown:

RonL
 
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  • #82
RonL said:
"...I think you have missed the whole concept, the propane is the power of the system and the temperature of air in and air exhausted is the sum of energy that must be discharged in some form of work.
The mass energy being cycled inside, might be a 1,000 times greater than what flows in and out with the air flow..."

P.S. I still can't figure out this multi quote thing.:frown:

RonL

You still have to compress the propane in the first place. All you are doing by heating the propane tank as you expand the propane is regaining a small amount of the energy initially wasted when compressing the propane.
 
  • #83
chayced said:
You still have to compress the propane in the first place. All you are doing by heating the propane tank as you expand the propane is regaining a small amount of the energy initially wasted when compressing the propane.

chayced, I'm thankful for your comments and questions and will try to get to a point of understanding or clearification, between our thoughts.
I feel that the diccussion needs to move to another place, as I made comments in this thread because of kandelabr making a statement of components working on a single power shaft. I thought our thoughts were more in line with each other.
In the absense of any comment or question from him and the statement that the thread has moved too far from his OP, gives me the impression I have crossed a line.

Thanks
Ron
 

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