# Designing a compressed air car

1. ### kandelabr

102
hello,
lately i've been thinking how to roughly design a compressed air powered car. i'd take an existing commercial car, take out its IC engine, fuel tank and all the other stuff and mount my own machine and tanks in.

i want it to travel 300 km (185 miles) at speed of 90 km/h (55 mph).

here are some very rough estimates:

- energy
aerodynamically superior modern cars (audi a2, toyota prius, honda insight, ...) have drag coefficients around 0.26 and frontal area cca 2.5 m2, they weight around 1200 kg. good tires should have coefficient of rolling friction around 0.005 - 0.01.
energy used in a 300 km constant-speed travel would be:

E = (Ffriction + Fdrag) × s = (approx.) 120 MJ.

this number seems fine to me, since an ordinary small car burns 6 liters of fuel per 100 km, every liter has 35 MJ of energy, assume 20% efficiency: 3 (300 km) × 6 (liters per 100 km) × 35 (MJ per liter) × 0.2 (eff) = 126 MJ.

storing 120 MJ in an air tank would require quite a pressure (let's say 450 bar, i've heard 500 bar compressed air applications exist already), and under that pressure quite a huge tank (about 500 liters). i assumed expansion would be isothermal or at least close to that (multi-staged and inter-warmed ).

- 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, torque should not be a problem. a CVT would definitely come in handy and fancy (i know what you think, see "efficiency" below).
a lot of existing air-powered vehicles use a decompression valve so their motors work on constant pressure (10 bar or so). that's like throwing out 90% of available energy, only increasing entropy. that's probably because of air engines, but that's just not good enough.

- efficiency
i know compressing air is very close to brutally wasting energy, but i thought the heat generated during compression could be captured in a chemical way like this:
http://en.wikipedia.org/wiki/Sodium_acetate
the air would then expand a bit, cool down, go through some heat exchanger, first get heat from the surrounding atmosphere and then from this heat reservoir.

i mentioned a CVT, these transmissions can get quite hot, this heat could also be used for warming air.
and regenerative braking - i'd somehow cool the brakes with a liquid that would then warm the air.

- ecology
i know you'll start screaming the standard "where do the electricity come from? - fossil fuel..." yada yada. i'm not trying to create energy from nothing, i'm just thinking it would be fine to get rid of gasoline and diesel for running cars. imagine a city with clear air (no brown clouds, huh?) and to me, more important thing - no noise. i just hate it when i can't hear my own mp3 player despite there's not a single car moving in front of a red traffic light.
exhausts from large power plants are quite clean nowadays, and in 20 years we'll have fusion power plants anyway, the problem is only in energy storage - chemical is the most dense by far, but can't be used in other way than burning. and we all know what are the efficiencies of heat engines. batteries could be the second, but (in my opinion) they are even less clean than fossil fuels - buying a new 200kg lithium pack every 4 years isn't very clean. in fact, composite pressure vessels can outreach batteries in sense of mass energy density.

now my questions:
- is it possible to put 500 liters of air tanks into a medium sized car? note that steel tanks would weight 2 or 3 tons, carbon tanks only 100 or 200 kilos. i can sacrifice the trunk, although without excess joy.
- speaking of aerodynamics, small and big cars have very similar frontal area surface and drag coefficient. does that mean that it would be easier to put all this compressed air into a bigger car (more space for tanks), since a bit heavier car wouldn't consume that much more energy?
- could i reduce drag by installing some weird panes on the back of the car (something like horizontally turned pyramid or a cone)?
- since a lot of fresh air would be needed to warm cold, expanded air between expansion stages, i could make a long and huge pipe to the rear part of the car, ending as huge exhaust pipes somewhere in the middle of the back of the car (vertically). would that contribute to lower drag coefficient?
- what's the efficiency of existing air motors? can they be connected in series, let's say each stage would expand air volume 3 times, so 6 stages would do the job. would the first motor (the one working on 450 bar) actually work with such pressure?
- i could use a vane pump-like air motor. many motors with different radiuses could be connected to a single shaft, and a computer would decide which motors would the air pass through. i actually already have all the calculations for that motors made (dimensions and dependent torque), my main concern momentarily is sealing. a itzy-bitzy vane pump (a 10 cm diameter (4")) and to put in 450 bar...

i'd be very glad to hear your comments on my thinking. i'm quite a n00b in mechanical engineering, but i'm totally into it, just need a few dozens of tons of experiences.

2. ### giggoman

4
I have given this subject much thought in the last 12 months.

kandelabr said:

- efficiency
i know compressing air is very close to brutally wasting energy, but i thought the heat generated during compression could be captured in a chemical way like this:
http://en.wikipedia.org/wiki/Sodium_acetate
the air would then expand a bit, cool down, go through some heat exchanger, first get heat from the surrounding atmosphere and then from this heat reservoir.

Consider changing the core of you mechanical set up. To include recapturing the movement of the vehicle. I don't know, maybe a 5th wheel to pavement, that would be the size of a 26inch bike tire. Or for that matter, one (tire and or Flywheel) that is turned by one of the four tires already used by the vehicle in a horizontal position. By using and automatic shifting mechanism on a 12 speed set, so as to minimize start up loads and maximize cycles. Motorcycles have the alternators located with the crank. Imagine as that bike tire turns it itself generates power (reclaims). Then it would thru gear and an independent drive train, turn a Compressor. Replacing the Gas tank with an air tank.

You could design an internal turbine system that would, with proper jetting, spin a generator, even supplement a flywheel using timed controlled burst of air to maintain revolutions, even while stopped at a light, stopped at a store, etc.

Of course a efficient design would include 2 or more tanks for air to ensure pressurization
and redundancy. Automatically switching from tank to tank as target range has been reached. It would also have brake generation, LED lighting, Solar paneling, and a DC motor with AL cast case. In all there could be 6-8 generators on a vehicle, feeding a regulator, battery bank/capacitor. Using this systems heat exchanger to supply cabin environment stability. '

I toyed with idea of using a smaller engine to turn the main generator for an 2000 watt system. Honda makes one that weighs 10 lbs. Even a 8000 watt system can run under load all day (8+ hours) on 5 gallons of gas. (if 208v is needed) The charging stations, for electric cars are never more than 30 amps. And able to recharge most electric cars presently on the market right now. Theoretically you could just install a low profile 8000 watt gas engine generator a bank of batteries, and a DC motor in the place of a engine in a car and you would get 500 miles a tank. This has been used by the railroad for many years. Yes, DC Motors drive Locomotives. Diesel engines turn Generators, that supply them with power. Not much different. But, Air WILL be one of the power generators of the future.

Also, Think of Rims that have cooper windings embedded, with the rotors they attach to with Magnets. Four tires....well you do the math.

Finally, never design a vehicle without a good Automatic Transmission system. Direct drive motors limit speed, and irresponsible. Putting undue strain on Motors the longer they are in FLA mode. Eliminate waste, everywhere. Motor-Generator combo's are another direction you could go in. I believe, the most crucial piece of the above mentioned power train, would be the fly wheel. A properly weighed/balanced Flywheel will spin forever. It could easily be built under the chassis above the frame. But it must weigh enough to compensate, the applied load. Ample enough to spin Long after you've settle in for the day/night.

Like I said, Just some ideas. Forget about those unable to see your vision. People laughed at the Wright Bros. After they were Bicycle mechanics. And all of the material and technology has already been made, at this point in history, you just have arrange them differently, and abandon the tried and true.

kandelabr said:

- is it possible to put 500 liters of air tanks into a medium sized car? note that steel tanks would weight 2 or 3 tons, carbon tanks only 100 or 200 kilos. i can sacrifice the trunk, although without excess joy.
- speaking of aerodynamics, small and big cars have very similar frontal area surface and drag coefficient. does that mean that it would be easier to put all this compressed air into a bigger car (more space for tanks), since a bit heavier car wouldn't consume that much more energy?

There are a couple companies utilizing the frame of the vehicle as a tank. Steel tanks would be overkill. I think 14 GAUGE AL with Carbon/Graphite lining, should lighten the load considerably, or a similar combo. Using battery power for Start up. Taking the combustion system from Engine to Fuel will give you 700 lbs or so, and a few Cubic Feet to play with. Capturing as much Kinetic energy, will at the very least expand your range per charge, leaning toward perpetual. Engineering a system of generators that minimizes drag. Capacitors will be key in boosting available Amperage, for an overdrive feel if used with 2 speed/ 3 speed motor (s).

I believe creating an independent Electrical Power plant, while integrating the drive train, is how an engineer should begin. I would love to assist in problem solving, and consulting you in accomplishing this goal. I am an Mechanic/Master Electrician with extensive Aerospace manufacturing assembly experience. There isn't any reason this planet should continue relying on Fossil Fuels. This practice must end.

L

Last edited: Apr 29, 2009
3. ### redargon

348
1-I just did a rough ideal gas calc (which is probably very inaccurate at such high pressures), but i get a mass of 300kg of air for 500l at 500bar. Plus the tank weights, it seems to be getting pretty heavy. The volume of your container would then also be at least500l, which would be roughly a cylinder 0.5m in diameter and 2.6m long. So far, looking a bit difficult to fit ito a standard car. I think you may be better off custom designing a chassis rather than trying to hijack a current model. Not sure...
2- The frontal surface area and drag coefficient of you car are important. A small increase in length probably won't damage your total drag too much. Just remember that a difference in drag coefficient from 0.25 and 0.3 (seemingly small) is a 20% increase in drag. And more and more important as your velocity increases
3- You could perhaps reduce the drag due to vortices that are generated on the trailing edge of your car, but that would require some serious research into the flow profile of the car you choose or design. Usually adding anything will increase the drag.
4- I'm not sure I understand this idea of yours, maybe you can rephrase it a little or add a sketch.
5- Not sure (would be interested to find out too actually)
6- Sealing a high pressure system can be tricky (especially with gases as leaks due to material permeability also becomes a factor). The regulator that you would use to reduce the air pressures to those required by the motor would aslo need to be pretty well designed.

Giggoman: your post sounds very close to a discussion about perpetual motion machines, which; as I understand it, is not what the OP is after. The OP is trying to be realistic. Energy recapturing is a tricky subject and should be thoughtfully considered. Recapturing energy from vehicle motion (using a flyweel attached to a drive wheel for example) inhibits vehicle motion and requires more power (can't be over unity efficiency) to overcome the inhibition the recapturing system uses. KERS is different for example, where you use otherwise wasted energy (thermal energy from heavy braking) to provide useful energy using recapturing methods.

4. ### TVP45

I don't mean to be pedantic (well, a little perhaps), but the Wright brothers were not bicycle mechanics. They began as mechanics, moved into manufacture, and then trained themselves to become engineers. They used wind tunnels. They understood (albeit in a more qualitative form) control surfaces. They were not amateurs, but professionals.

5. ### xxChrisxx

Adding length and tapering the back down to a point is acutally much more beneficial than reducing the frontal area on Cd.

The main drag on a car is pressure drag from seperated flow By making it longer you can reduce the angle of the taper and therefore keep the flow attached to the surface. This is the reason why you see sports cars with no rear window wipers and why many roof box/racks with sloped fronts are more efficient when put on backwards.

The ideal aero shape you want for a car is a long thin teardrop. But that looks both silly and is impractical for road use.

6. ### xxChrisxx

You are joking arent you. If you want to eleiminate inefficienies the first thing you chuck out is an automatic transmission. Fluid couplings are garbage and heavy.

No it wont.

7. ### Ranger Mike

1,557
xxChrisxx you got it...most in effective..
my question is why does he want to go with compressed air?
takes a lot of power to drive air compressor and a whole lot of room to store it..

nothing wrong with IC motors..humans give off more co2 jogging than the IC?
plenty of oil out there
i just had earth day bon fire to celebrate earth day..i used pleanty of 5- 30 race car oil and old tires to stoke it..was great..

Last edited: Apr 29, 2009
8. ### RonL

907
I agree that compressed air is the way to go for future designs, however lower pressure will reduce a lot of expense in the final design.
Recovery of waste heat in the compression stage is most important, and can be done in a quite simple way.
Flywheels can play an important part of a design, but need to react to the air movement that is already flowing past the car, due to it's motion.(speed of the flywheel might not be the most important part of it's function)
The discharge of air from any system, should be directed into the draft area, this will be a most rewarding area of efficiency designing.

As was mentioned before, a complete new design might be best, it might be that some vehicles can be retrofitted, but not many.

Ron

9. ### kandelabr

102
you must be american, am i right?
you know, you don't eat oil or coal and then go jogging. your CO2 footprint is neutral, that means you breathe out only the stuff that was taken from the atmosphere.
yes, a lot is wrong with IC motors. they are 200 years old, have very limited efficiency, are noisy, they stink, cause vibrations, they are complicated, and no significant improvements have been made in past 20 years. a bit better power-to-weight ratio and lower fuel consumption.
your attitude is exactly the attitude that many have and it's taking you nowhere. there's nothing wrong with cooking on a fireplace, so why did you switch to gas/electric cooker?

10. ### Ranger Mike

1,557
i would cook on a fire but i clear cut all the wood and don't want to spend  to take my diesel pick up and pick up the wood from the forest perserve...

far as attitude..this be the land of the free and home of the brave..so if it is so bad a place, why does everyone in the world want to come here?
attitude..??
it is called capitalism and works..
now for the emperical stuff..dude..nothing now or in the near 50 years will replace the IC..it is way to economical to make and use..refined to the max..so oil is king and wil be for a long time to come...
people been trying to develop alternatives since 1970..it ain't happening and will not..
no one wants a pregnant roller skate to drive....

11. ### kandelabr

102
ok, i don't know anyone here who wants to go to america, but we'd better leave this theme for some other topic.
ICEs are well developed, that's true, but they have their theroetical limit and they are already pretty close to it. someone will have to think out something new. nowadays, nothing new is being made, only old things optimised. that's not bad, but can't go this way forewer.

12. ### Ranger Mike

1,557
in the late 1880s the US Patent office was going to be closed because " everything had been invented" and no reason to keep the bureau open..mush the same attitude in the late 1970s ..then the personal computer hit the free market..then the Internet revolution hit..humans have a sick way of inventing things to energize the market place and make money...many have looked at energy alternatives and it just ain't gonna happen until oil is zeroed out ( not happening for 500 years) or cheap alternative happens..if it was gonna happen ..it would have already...
I gotta go.. my whaling partner is here and we have to sharpen our harpoons..the blue whales are spawning..easy targets

13. ### redargon

348
kandelabr: instead of trying to argue with a cynic, why not rather use your time more constructively to post some more of your ideas and reactions to the comments that people have made in response to your actual questions as posted in your original post.

xxChrisxx and RonL have made some appropriate remarks. What do you think about those?

TVP is only replying to giggoman (who in my opinion is trying to introduce crackpot over unity efficiency ideas)

Ranger Mike is being rather cynical so early in this thread, seeing the relatively honest approach that you are taking for this idea. You have shown evidence of actually wanting to design something better (based on normal physical properties), not produce a magic perpetual motion machine. So I think you deserve some reasonable answers.

14. ### kandelabr

102
giggoman:
i didn't really understand what do you mean with "energy recapturing". things are simple - kinetic energy spend with acceleration can be recaptured when braking, everything else goes to heat.
i'm going to develop a completely new type of engine and i'm almost half done, but still choking with a few questions i can't get information for.
i don't think flywheel is a good idea. it has lower energy density than compressed air, needs very very good bearings (magnetic levitation that is), and needs special treatment when rotation axis turns and here comes a bunch of problems.
as for brakes, i'd thermally isolate the disc and only cool brake pads with a liquid medium. that would be the simplest and lightest solution.
i was thinking of everything you said, that is batteries, LED lights, solar panels, etc., except without gasoline.
there are air cars under development that use gasoline burners to heat air to increase range, but that's really stupid. if i have to burn gasoline to go somewhere, i'll surely burn it in an ICE.

redargon:
yes, it's about 300 kilos according to ideal gas equation, and guess what, i calculated work done by the gas according to van der waals equation and it's the same (a few joules different).
anyway, it's not the weight that's problematic, it's the volume. it goes well beyond 500 liters for 120 MJ and 500 bar (calculated using real gas equations), so i'd need to get my car to use at most 100 MJ.

check out the attachments. if i managed to make the added "teardrop rear" retractable, then it would activate at speeds higher than (some speed i guess ) and retract during city driving.
i alse attached a skecth of supposed multi-staged expansion motor. note that turbines should either be of different sizes on the same shaft or i'd use sort of a trick, e.g. this one:
http://auto.howstuffworks.com/hybrid-car7.htm

as far as the exhausts i mentioned are concerned, imagine rear lights on this car were exhausts of fresh air:

and as far as a pressure regulator is concerned, i won't touch it. my first idea was to make a kind of pressure reduction valve so i'd have constant pressure for the engine. just calculate how much work can be extracted from 300 kilos of air at 10 bar and how much on 450 bar. or am i wrong here somewhere?

there's another thing that's confusing me.
say we have a convergent nozzle and an airflow through it. air velocity increases on the smaller end, thus reducing pressure. in a divergent nozzle, exactly the opposite happens, pressure increases.
why do sport cars have diffusors on the rear then? shouldn't the airflow force them to the ground?
well, in my case, i'd want the car to half fly to reduce tire friction... just kidding

i've also heard the air under car is moving with the car, so frontal area includes a hole between the road and car bottom. is that true? how could i manage to make the air under the car to stay where it was, not to move along?

ranger mike:
yes, it would take a lot of power to drive air compressor, but exactly the same amount of power is required to drive an ICE driven car, actually even more.
and i'm not going to make a bathtub on wheels that will embarass me in front of anybody, like current air cars are like. the demands i stated are quite high and i'm not going any lower (ok, maybe 250 km and 80 km/h, but that's the limit. ).

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15. ### xxChrisxx

I'll will say one thing, I do find americans to be slighty set in their ways with regards to the car engine.

Mike is right though, with second generation biofuels that are essentially carbon neutral the combustion engine will be the way to go until fuel cell technology comes down in price. And you are wrong about IC development, SI engines hace stalled slightly, but diesels have come on massively in the last 10 -15 years. Mainly due to advances in turbo technology such as variable guide vanes etc.

Compressed air cars are a good idea in theory but a bit of a dead end practically. I was going to ask this Q in the other thread.

How do you see compressed air technology comparing to hydrogen fuel cell tech in the future? Why would compressed be better?

16. ### kandelabr

102
well, honestly, if i had money, i'd already have honda fcx. i'm quite a fan of fuel cell technology, although there are still some big problems with it (like leaking hydrogen). but i like the compressed air idea because we're severely limited by size of tank we can put into a car. because of that, we'll try to make things more efficient and economical instead of building bigger and bigger engines and only using brute force to achieve what we want. building a car that will run 300 miles on 100 MJ is a bigger challenge than putting 16 cylinders together to achieve 1000 HP.
at least that's my opinion.
oh, and another, i can't make a fuel cell car at home...

17. ### xxChrisxx

Ah the attachments are working for me now. So you are running it from a set of turbines not using the compressed air to acutate a piston? Interesting.

Are you goiing to be running the LP turbines down to 1 bar? And how is this going to work, are you thinking directly driven or is it going to power an electric motor?

18. ### kandelabr

102
i thought a turbine would actually be a vane pump, just inverted - i'd attach all of them to the same shaft and make their sizes vary, so that the smallest space in the smallest turbine would be 450-times smaller than the biggest one in the biggest turbine. an engine like this would be ridiculously small and would have huge torque (in the beginning, at 450 bar). gor lower pressures, i'd add some bigger turbines to maintain approximately constant torque. a computer would control valves that would turn air flow into desired turbines.

19. ### mgb_phys

8,952
There is a french company (aircar?) with a prototype Smart car style compressed air powered car.

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.

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.

20. ### kandelabr

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.

Last edited: Apr 29, 2009