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Author Topic: The Compressed air engine: Is it a scam?  (Read 30413 times)

Offline syhprum

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The Compressed air engine: Is it a scam?
« Reply #25 on: 12/09/2007 09:25:35 »
5.48*10^7 Joules

Please tell why my calculation is so wrong!

I assume the air is enclosed in a cylinder of 1 meter cross sectional area and 96 meters long!
The compressed air at 300 bar is in the top 32 cm giving a force of 3*10^7 Newtons on the piston.
It is now allowed to expand pushing the piston 96 meters with an average force over the stroke of 1.5*10^7 Newtons which I calculate (wrongly!) as 1.44*10^9 Newton meters (Joules).

Failed "O" level maths 1941.
« Last Edit: 12/09/2007 09:27:35 by syhprum »
 

Offline McQueen

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The Compressed air engine: Is it a scam?
« Reply #26 on: 12/09/2007 10:18:36 »
I think your original estimate of 1.38 KW Hr was nearer to the mark syhprum. McQueen
« Last Edit: 12/09/2007 10:20:25 by McQueen »
 

Offline McQueen

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The Compressed air engine: Is it a scam?
« Reply #27 on: 12/09/2007 15:03:20 »
The compressed air car is  a much desired development in the search for an alternative cleaner, non-polluting form of motivating power that can be used in transportation. For the past 150 years the IC Piston engine, working on the Otto cycle and first invented in 1866 by Nicholaus Otto, has been the mainstay of personal and commercial road transport.  Yet with all the billions of dollars spent on trying to improve the performance of the IC Piston engine, every working sequence of the engine being examined and tested with the latest computer technology and simulations, by millions of people around the world, it is amply clear that if a solution is to be found for a more efficient engine, it wont be based on IC piston technology. Even with all the 21st. Century innovations and improvements, such as MPFI, double overhead cams, multiple exhaust and inlet valves etc., the IC piston engine efficiency remains at 20%!
Think of it, an amazing and unacceptable 80% of the energy in the fuel used is wasted, by comparison, rockets and jets have an efficiency of 70% and better, or to put it another way, are more than two and a half times as efficient as piston engine cars, they are also, relatively, pollution free. Unfortunately, both turbines and rockets require or have required until this date, continuous combustion to deliver this order of efficiency. When used in road transport they are therefore very fuel inefficient using about 8 10 times the fuel used by a piston engine of similar size.
The compressed air car comes as a welcome and audacious challenge to IC Piston technology, it is an almost viable technology. The Compressed air car engine does work, for those of you who have doubts, you can check out this link on model airplanes that use compressed CO2 to run a piston engine. http://blacksheepsquadron.com/index.html
Of course one has to take into account that CO2 is far denser than air and would therefore work better as a compression agent, however the point is that the compressed air engine technology is viable, or as I had previously stated, almost  viable. The reason I say this is that a tank of compressed air with a capacity of 340 litres, (taken from the MDI Car specifications web-site) the compressed air would last for a maximum of 2 minutes. You can perform the calculations for yourself. A tank holding 340 litres corresponds to a cube with 2.25 ft sides. This tank is filled with compressed air at 4500 psi or 300 bars approx. The optimum rpm of the engine is 4500rpm, compressed air engines tend to perform best at a given rpm. Now if the cylinder capacity is about 8 cu ins. then at 4500 rpm, the engine would use 4500/60 = 75 x 8 = 600 cu ins in one second and 36,000 cu ins in one minute. The capacity of the tank is 27 x 27 x 27 cu ins or 19,683 cu ins. But the piston needs only about 500 psi  to work, so to get an accurate figure we have to divide 36,000/9 = 4000.  So the energy in the tank will last for 19,683/4000 = 5 minutes approx.  The point is that it often takes five minutes just to get the car out of the garage also once you use all that compressed air there is no way to re-charge the compressed air, so after about 4 5 minutes you have to go to a service station and recharge. To recharge air at 4500 psi takes about a compressor working at 500 KW! 500KW, is enough to supply electricity to a fairly large town.  The longer range of 200km 300 km is only achievable when an IC piston engine is onboard. Again to recompress the air with an onboard motor would take at least 3 4 kours. So while it is a wonderful technology, it is far from viable at the moment. TheRotary Pulse Jet Engine, has none of these shortcomings and if allowed I will discuss it in my next post. McQueen
« Last Edit: 12/09/2007 15:17:54 by McQueen »
 

Offline lightarrow

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The Compressed air engine: Is it a scam?
« Reply #28 on: 12/09/2007 18:02:54 »
5.48*10^7 Joules

Please tell why my calculation is so wrong!

I assume the air is enclosed in a cylinder of 1 meter cross sectional area and 96 meters long!
The compressed air at 300 bar is in the top 32 cm giving a force of 3*10^7 Newtons on the piston.
It is now allowed to expand pushing the piston 96 meters with an average force over the stroke of 1.5*10^7 Newtons which I calculate (wrongly!) as 1.44*10^9 Newton meters (Joules).

Failed "O" level maths 1941.
You are missing Boyle's law: PV = k = constant at T constant, so P = k/V; pressure decreases with volume and the average pressure is not given by an arithmetic average (if that's the way you computed the average force).
You have to do an integral to compute the work done by the piston:
∫P(V)dV from V1 = 0.32 m3 to V2 = 96 m3
where P(V) = nRT/V, so: ∫nRT/V dV = nRT ln(V2/V1); you get n from P1V1 = nRT. I chose T = 300K (27C) to simplify my computations.
 

Offline syhprum

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« Reply #29 on: 12/09/2007 19:22:38 »
I realised that my simple assumption that the average pressure on the piston would be half the maximum would lead to an error but as my figure was 26.3 times yours I did not think this was the only cause.
My next guess was that the force would drop of at an exponential rate 1/2.718 but the error is still 18 times!.
your figure equates to 16.2 KWh which would not take the vehicle very far!.
The penny has now dropped I failed to take account how quickly the pressure would drop as the piston moved.
 

Offline McQueen

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The Compressed air engine: Is it a scam?
« Reply #30 on: 15/09/2007 00:30:25 »
Quote
Can anyone well versed in physics and good at arithmetic tell me how many Joules of energy are stored in 320 liters of air compressed to 300 Bar.
I don't think it amounts to a lot

I make it about 1.38 KW hours, a small slow vehicle might mamage on about 5 KW and run for 15 minutes or so ! 
Your calculations are actually surprisingly close to the manufacturers own claims, they state that 320 litres of air at 300 bar is equivalent to about 1.5 litres of petrol (or ).3 US gallons) and has an energy equivalent to 1.5KW hours. So yes it should run for 15 minutes, so how come the claims are for a range of 200 - 300 Kms? I think that is only when an IC engine is present onboard to assist. McQueen
 

Offline lightarrow

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« Reply #31 on: 15/09/2007 01:45:15 »
5.48*10^7 Joules

Please tell why my calculation is so wrong!

I assume the air is enclosed in a cylinder of 1 meter cross sectional area and 96 meters long!
The compressed air at 300 bar is in the top 32 cm giving a force of 3*10^7 Newtons on the piston.
It is now allowed to expand pushing the piston 96 meters with an average force over the stroke of 1.5*10^7 Newtons which I calculate (wrongly!) as 1.44*10^9 Newton meters (Joules).

Failed "O" level maths 1941.
I don't think you have failed, if you can still make these computations !
However 1.44*10^9 Joule = 400 kWh not 1.38 kWh.
 

Offline syhprum

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« Reply #32 on: 15/09/2007 06:50:18 »
Not really understanding the subtleties of calculus I did a numerical integration moving the the piston sufficiently each time to actually half the pressure which gives very nearly the correct answer.
at school they drummed some calculus into us parrot fashion but never gave us any idea how to applie it to real life situations.
I don't know how I got 1.38 KWh my second attempt gave 400 KWh
 

lyner

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The Compressed air engine: Is it a scam?
« Reply #33 on: 15/09/2007 23:07:21 »
Isothermal processes are not good value in terms of efficiency.
Boyle's Law refers to isothermal changes.  A piston engine using compressed air would gradually cool down  because the  changes in pressure and volume would lead to cooling  cooling.
Compressing a gas produces a lot of heat which has to be taken away - or will leak away in time. The energy involved is a significant proportion of the input energy so efficiency is compromised.
However, there is no reason to discount compressed gas as a source of energy. (Or, more correctly, a way of storing energy.) A very simple engine can convert the energy into a useful form - how about a siren using CO2 canisters, for instance? Very loud and the power source lasts for years and years without running down. Unlike batteries.
Utility rather than efficiency is sometimes the prime concern.
« Last Edit: 15/09/2007 23:13:17 by sophiecentaur »
 

Offline McQueen

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The Compressed air engine: Is it a scam?
« Reply #34 on: 16/09/2007 04:21:28 »
Quote
A very simple engine can convert the energy into a useful form - how about a siren using CO2 canisters, for instance? Very loud and the power source lasts for years and years without running down. Unlike batteries.
In fact the uses of compressed air in Industry, apart from those that you have quoted, that readily come to mind such as a dentists drill and the Jack hammer, are too numerous to ennumerate.  There is no doubt that the aircar does work, but as you point out it just does not work efficiently. McQueen
« Last Edit: 16/09/2007 04:23:04 by McQueen »
 

Offline lightarrow

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The Compressed air engine: Is it a scam?
« Reply #35 on: 16/09/2007 14:50:02 »
Isothermal processes are not good value in terms of efficiency.
Boyle's Law refers to isothermal changes.  A piston engine using compressed air would gradually cool down  because the  changes in pressure and volume would lead to cooling  cooling.
Compressing a gas produces a lot of heat which has to be taken away - or will leak away in time. The energy involved is a significant proportion of the input energy so efficiency is compromised.
However, there is no reason to discount compressed gas as a source of energy. (Or, more correctly, a way of storing energy.) A very simple engine can convert the energy into a useful form - how about a siren using CO2 canisters, for instance? Very loud and the power source lasts for years and years without running down. Unlike batteries.
Utility rather than efficiency is sometimes the prime concern.
Yes, and what is worse is that a compressed air engine couldn't even be totally isothermal, but something between isothermal and adiabatic, so the work produced is even less.
 

Offline syhprum

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« Reply #36 on: 27/09/2007 20:09:39 »
There is an intersting article in the 'New Scientist' about the use of windmills in Iowa to compress air and store it in natural underground reservoirs so that the power generated by the windmills overnight is available during the day.
The compressed air is not used directly to drive turbines but to replace the the compressor stage in conventional gas fired turbines leading to a large increase in efficiency.
« Last Edit: 27/09/2007 20:12:44 by syhprum »
 

Offline Bartvl

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The Compressed air engine: Is it a scam?
« Reply #37 on: 12/11/2008 12:43:11 »
I don't exactly know if this massage is here at the good place. But we have to investigate a motorcycle (maximum speed 30 kmh) what is going forward with compressed air. Do you know what parts we need therefor? I think we need a air engine and a heater. But I don't know what other parts we need. Can you help us?

Thank you very much.

Bart
 

lyner

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The Compressed air engine: Is it a scam?
« Reply #38 on: 12/11/2008 18:55:18 »
Something to store the compressed gas in?
Buy/ hire CO2 canisters from a brewery if this is a brief investigation. Diving cylinders (air) would be expensive to buy but you could go 'down the road' to get them charged and the regulators would not be too costly. HEALTH and SAFETY could be an issue for you - or, at least, it should be for your supervisors.
I would think that a turbine would be best bet for getting something working quickly.
This is not a trivial exercise unless you have access to some serious machine tools.
Scrap Heap Challenge comes to mind. Could be fun.
 

Offline yor_on

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The Compressed air engine: Is it a scam?
« Reply #39 on: 13/11/2008 15:22:52 »
So if you allow for the air to cool of you will have it under pressure at room temperature.
But then, when filling it to another tank, won't it want to expand inside that new area, and won't that motion transfer to energy/heat??

And as more air is pressed in from the already pressured tank.
Won't that air, also wanting to expand, collide with the molecules already there, producing even more heat/energy?

So the faster you fill it the less heat, would that be correct?
But then the friction in the connections between the two tanks will be a problem, or?
I need to stop reading this:)
 

lyner

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The Compressed air engine: Is it a scam?
« Reply #40 on: 13/11/2008 16:51:11 »
Re the motorcycle and Yor-on: Wouldn't the obvious thing be to vent the gas to the outside after it's done its expanding / work? Or are you not talking of the motorcycle?

What two tanks?
 

Offline yor_on

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The Compressed air engine: Is it a scam?
« Reply #41 on: 14/11/2008 00:09:29 »
sorry Sophie, got to eager there.
Was reading the first page and the discussion of heat.

Totally missed page two I'm afraid, as I started to wonder how the compressed air would behave if 'let out' to an empty tank.
Reading your answer to the Question posed I agree.
It sounds like an advanced challenge if they are going to modify a bike engine to air?
Perhaps its a theoretical exercise?

And yes, my thoughts do seem to fit in somehow?
Strange:)
 

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The Compressed air engine: Is it a scam?
« Reply #41 on: 14/11/2008 00:09:29 »

 

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