The Air Car - A Breath Of Fresh Air Or A Waste Of Breath ?

The primary impact of peak oil will be felt on our transportation systems. As road transportation is the primary consumer of oil, this is where we will need to make the most changes in order to adapt to less available oil. There are a number of ways of adapting - most, if not all, of which have been discussed at length in the peak oil blogosphere. These include expanding mass transit systems, redesigning cities and towns to make them easier to walk or cycle around (or combining both of these approaches in "transit oriented development"), making greater use of electric cycles (or mopeds), using lightweight materials in vehicle construction, and - most commonly - switching to electric vehicles (particularly, in the medium term, plug-in hybrids).

One variation on the oil free car alternative is the "air car", which is powered by compressed air. The Age recently reported that IT MDI-Energy will be setting up a manufacturing plant in Melbourne, with cars expected to go on sale next year for less than $8000 and with running costs "80% lower than current comparable vehicles" (2 L per 100 km).

All of this sounds very promising (and the company promotes it as the solution for both peak oil and global warming). The question is - is it for real ?

The company is claiming that the vehicles will be able to attain speeds of up to 110 kilometres an hour, and travel 150 kilometres on compressed air alone. The Age article also mentions another (hybrid) mode of operation where the air is heated using a fuel source, such as ethanol or diesel, which would make it "possible to travel from Perth to Brisbane without refuelling".

The car is refuelled by plugging it into the compressed air supply found at most service stations, and founder Louis Arnoux is claiming that the "engine technology could also be used to power homes". In other words, it is another way of implementing the vehicle to grid (or V2G) concept - which would be an interesting development as one of the main obstacles for this idea (once plug-in hybrids appear in significant numbers) is the impact of constantly discharging and recharging on battery lifespans (though recent developments in this area are promising too).

Compressed air is similar to hydrogen - it is an energy storage medium, not an energy source. Critics point out that using compressed air simply shifts energy production from oil based engines to power stations - usually coal fired ones, particularly in Australia. On the other side of the ledger, compressed air is a safe, well-known storage mechanism (already in large scale use to store power produced by wind farms, for example), and the energy generation infrastructure can (and hopefully will) be converted from fossil fuel based sources to cleaner alternatives over time.

The Air Car was created by MDI (Moteur Developpement International) which is headquartered in Luxemburg, while the prototype factory is in the south of France. Originally conceived by former Formula 1 engineer Guy Negre back in 1991, the official names for the “Air Cars” are the OneCAT, CityCAT and MiniCAT. The OneCAT is expected to sit three or five people, with the MiniCAT and CityCAT models expected to follow.

MDI recently signed a deal with India’s Tata Motors, to build the air-powered vehicles in India. Zero Pollution Motors is looking to market the car in the US, and the Thai government has also invited Tata to manufacture the car in Thailand. A Colombian company (MDI Andina S.A) is also looking to produce the cars and sell them in Latin America.

The company has been talking about producing cars since at least 2000, so it is worthwhile remaining skeptical until cars start rolling off a production line somewhere.

WebHubbleTelescope had a brief look at the Air Car back in 2004.

The Air Car has gotten the press excited on and off over the years. The French design, which has received the most publicity, uses compressed air as an energy delivery mechanism. It has the potential for providing a clean-burning solution, but as usual it takes net energy to compress the air. No free lunch, unless wind or solar energy are involved to run the air compressors. And even there, we require energy to make the windmills and solar conversion devices.

As a sanity check here are two ways to calculate the energy value of 1 liter of compressed air. Remember that the gold standard is 1 GJ/30 liters for gasoline (or 33,000,000 joules/liter). First, if you compress air completely you actually get liquid. So we take the energy value of liquid nitrogen (air consists of 70% nitrogen by volume).

1. Energy Density/Specific Energy of liquid nitrogen = 320 KJ/l or 320,000 joules/liter
2. Heat of Vaporization of liquid nitrogen = 161 KJ/l or 161,000 joules/liter (to double-check the above value)

Looking at specific energy, this is at best 100 times less energy content than gasoline. On the plus side, the transfer to mechanical power is better than for gasoline (burning gas generates much wasted heat). Granted that advantage, we still have to generate the compressed air by using energy, and to top it off, we also have much worse energy density (i.e. energy per volume) than gasoline. You understand why consumers and corporations like gasoline (little energy overhead to extract a free lunch).

James Fraser at The Energy Blog had a look at the air car earlier this year when the Indian deal was announced, coming to the following conclusion:

This technology competes with the electric car. The claimed advantage of compressed air over electric storage is that it is less expensive, has a faster recharge time and pressure vessels have a longer lifetime compared to batteries. Both technologies have hurdles to overcome, demonstrating that the air engine/compressed air system is as light, efficient and cheap as available electric motors/batteries. The main issues to me are that the air engine has not been proven to be dependable and advanced batteries are still too expensive. ...

A discussion of the energy efficiencies of an air engine vehicle vs an electric vehicle would breakdown into the efficiency of the air compressor and air engine vs the efficiency of batteries and motors in the electric car, which I am sure the electric car would win. However because of the potentially low initial cost, low maintence cost and low operating cost compared to a fossil fueled vehicle the "air car" could find a niche market if it could be marketed before low cost batteries are available.

The Australian operation, IT-MDI Energy Pty Ltd, is a merger betweeen MDI and IT Mondial, Louis Arnoux’s IT business. The IT MDI-Energy venture has other ambitions besides transport, with its (in my mind, very confusing) website detailing plans to provide home power generation (shades of the key to Richard Smalley's "distributed energy grid" idea) and even broadband internet services in a “green” manner, using a combination of solar power and some sort of cogeneration technology. While the air car idea seems to have quite a lot of history behind it, much of the rest smells a lot like vapourware based on the information on the website.

When the article in The Age came out, Kyle Schuant posted a few back-of-the-envelope calculations to The Bullroarer comparing the air car to a small petrol fuelled car in terms of fuel costs and carbon emissions, in which the air car fared pretty well.

If I remember my high school physics and chemistry right, the energy E required to compress air at 25C is,

E = 110,000 x ln (P1/P2) /m3/mol

There are about 45mol air in 1m3, so,

E = 110,000 x ln (P1/P2) /m3

This howstuffworks article tells us that an air car tank might have 300lt at 4,561psi, which is 29,999,087.707 - call it 30,000 kPa. Atmospheric pressure is 101.3kPa. 300lt at 30,000kPa will be 90,000lt at atmospheric pressure, or 90m3. And so we get,

E = 110,000 x ln (30,000 / 101.3) x 90
= 110,000 x 5.69 x 90
= 56,331,000J
which is 15.6kWhr

However, a company which supplies air compressors tells us that "Most systems typically waste 25 to 50 percent of the energy required to generate compressed air that actually provides useful work."

Let's be optimistic and assume that with lots of air cars zooming around, service stations will buy the most efficient (expensive) compressors. So we get just a 25% loss. This brings us to 20.9kWhr.

Let's round it up to 21kWhr to refill the tank. Again, this isn't the air car referred to in the article, but it gives us an idea of the order of magnitude.

21kWhr to travel 200km.

A regular small city car gets about 10km/lt. Petrol costs about $1.30/lt, and causes 2.32kg CO2e/lt. So to go 200km in a regular car would cost $26 and cause 46.4kg CO2e in emissions.

Electricity from coal cost $0.1355/kWh and 1.21kg CO2e/kWh, so the 200km journey would cost $2.85 and cause 34.9kg CO2e in emissions.

Electricity from wind costs $0.19/kWh and causes 0.04kg CO2e/kWh. So the 200km journey would cost $3.99 and cause 0.84kg CO2e in emissions.

The average Australian car is driven 15,000km annually. That'd be 75 refills, or 1,575kWh energy in all. That's not bad when the average household uses 6,000kWhr annually.

Presumably service stations could do things better than we could at home, since they can buy the big heavy and efficient equipment; if service stations supply so much compressed air, they'll start charging more for it, more than the power costs. Still, it seems that running it on compressed air will be significantly cheaper in money terms.

However, if the air is compressed by electricity got from coal, the greenhouse gas emissions will be comparable to simply burning petrol in the car.

Again, not perfect calculations, but the best we can do with the data we've got, and they give us an order of magnitude idea of the numbers involved.

There is another Australian company pursuing air powered vehicles - the Di Pietro Rotary Air Engine, which doesn't seem to have made much progress commercialising their technology, though it still appears in the press from time to time. From a recent ABC interview:

BLANCH : As the world wakes up to global warming, petrol prices rise and greenhouse gases pollute the atmosphere, what better than a car that creates zero pollution by running on nothing but compressed air? The dream started seven years ago for a Melbourne engineer, Angelo di Pietro, to advance his innovative air-driven 'Engineair' vehicle that he conceived, designed and developed and which could have an enormous impact on future motor-driven applications. I asked Angelo to list after zero-pollution, what he considered to be important improvements that his engine delivered over other motors.

ANGELO DI PIETRO : Our motor delivers high torque and low rpm, very high efficiency, low noise and it's a fraction of the weight of a traditional piston motor. It is cheaper to produce and is better for the environment, as less material and energy is used in its production.

BLANCH : So your motor is based on a rotary piston. How does your engine design differ from existing rotary engines?

ANGELO DI PIETRO : Uses a single rotary piston and pivoting dividers which runs almost frictionless.

BLANCH : Your motor's seven times smaller than the piston air motor currently in use, so what power does the engine develop with what about of compressed air?

ANGELO DI PIETRO: Although our motor is seven times smaller than the piston air motor, we develop much more power with considerable less energy, even by using our early motor's testing results of 2002, conducted by Monash University, we only use 770 litres per minute per horse power compared with the piston motor's 896 litres. We have advanced our technology today enormously and our scientific model predictions suggest that the new motor could be made at least four times more efficient for the same power output, compared to its commercial competitors.

BLANCH : So how do you get your motor to operate at a higher torque or with greater efficiency?

ANGELO DI PIETRO : By regulating air pressure and timing or manipulating the compressed air to perform the reverse function from when it was compressed.

BLANCH : You've designed the engine to be suited to a variety of applications and these range from commercial vehicles and motor scooters, buses, boats, trains and cars. Well that's a whole spectrum of transport, isn't it? So how does your engine adapt to such a range of vehicles?

ANGELO DI PIETRO : The engine can be scaled up or down in its size and will be built from different materials specific to each use, for example, carbon fibre or other plastics or even stainless steel for marine use. Our engine is best suited to a new generation of vehicles that can be built lighter as the need to build current heavier structures to support large heavy motors and all that goes with them is no longer required. This reduction in the weight of the engine and the elimination of many other components translates into fuel efficiency and economic benefits. ...

Cross posted from Peak Energy

Uh-oh, now someone will tell me that I didn't remember my high school physics right! Sorry, it was a long time ago and the only physics experiments I've done since then involved firing rifles.

It's probably worth mentioning that a 300lt tank is pretty significant. That's about the size of your average sedan's boot. Given the pictures we've seen of these prototypes, either the tank is cunningly hidden underneath, or else it's accidentally oops omitted from the image. Or perhaps the engine is magically as small as a walnut and the tank's in there.

I think it's an awesome idea if coupled with renewable energy, though an atrocious idea if coupled with fossil fuel generated electricity (better just to burn the stuff directly in the car). I'm just sceptical about the engineering and economics of it. They've had the better part of a decade to secure the patents, so there's no reason they can't put the technical details out there - except of course if it's just geek dreamware...

The idea of cars having accidents with a 3500psi container in each of the cars is a major concern. Have you ever seem the results of a hot water storage tank exploding due to the failure of the temp/pressure valve and faulty thermostat. It can destroy a house and the pressures here are substantially less than is proposed for the air car. Not a chance!

LPG is at a similar pressure and has trundled around in the back of cars for decades. Of course, they don't have 300lt tanks, but rather 30-75lt, but still.

I've yet to hear of a hot water system's explosion "destroying a house". The wiring or gas can go wrong and start a fire, but that's why - here in Victoria at least - they legally have to be outside the house.

Absolutely having all that air under pressure would be a safety concern. But so is having 40-75lt of petrol or LPG, travelling at 100km/h, and so on. We try to make those safe.

Me, I'd rather we were rid of cars entirely, even if they were powered by sunshine, strawberries, and pretty girl's smiles. But I'm not optimistic that'll happen any time soon, so instead I'm interested to hear of ideas like this - it's just a pity they always turn out to be some geek's wet dream.

I can assure you that 300l of hot water/steam can do a lot of damage if they explode. I used to work in the elect industry and have had quite a lot to do with how water systems. In NSW at least, it is quite legal to have HWS inside a house and they do make a good drying cupboard and also improve eff through lower losses.
I agree LPG is under high pressure but there have a few accidents but they are few.
However, it is only postponing the inevitable, we have to get off the car dominated society.

So they "do a lot of damage", they don't "destroy the house". Those are two very different things.

NSW is a dangerous and savage land. HW systems should be outside the house, not in it.

If we can use a high pressure flammable gas safely, then we can use a high pressure non-flammable gas safely.

I agree that cars suck. Here I rant about how and why I hate them ;)

NSW is a dangerous and savage land. HW systems should be outside the house, not in it.

If we can use a high pressure flammable gas safely, then we can use a high pressure non-flammable gas safely.

Agreed on all counts :-)

Welding cylinders run 2000 psi+ and are trucked around regularly with safety.

Actually, they are exceedingly dangerous in a vehicle fire, especially acetylene which is not stored at very high pressure because it will spontaneouly explode.

In the UK nobody will go anywhere near a fire with welding cyliders involved - that includes the fire brigade.

Several times this year alone major motorway routes have been closed for twenty four hours after a vehicle has been left to burn itself out before anybody will go within several hundred metres of it.

Oh, they certainly have the potential to "destroy a house." But the trick is that with 40 gallons or so at 300psi and umpteen degrees...when the pressure is released - it flash boils. So you get that first crack in the water heater and it all turns into water vapor KAPOW! So the power there comes from 40 gallons of liquid turning into a gas - there's some serious expansion there. The MDI car's tank is designed to rupture in a certain way to direct the explosion and diffuse it as much as possible. At 3000 psi though, it's still a fricken bomb.

"The MDI car's tank is designed to rupture in a certain way to direct the explosion and diffuse it as much as possible."

One of the miracles of composite materials. CNG is also stored at this pressure on vehicles with few safety concerns, so given that air is a significantly less flammable gas I'm not so concerned.

Water? Outdoors? But it will all freeze solid in the winter! And in parts of fall and spring, too At least where I live it would. :) In the non-tropical parts of the world, the hot water heater is indoors and a matter of course.

LPG is at a similar pressure and has trundled around in the back of cars for decades. Of course, they don't have 300lt tanks, but rather 30-75lt, but still.

From Wikipedia:

The pressure at which LPG becomes liquid, called its vapor pressure, likewise varies depending on composition and temperature; for example, it is approximately 220 kilopascals (2.2 bar) for pure butane at 20 °C (68 °F), and approximately 2.2 megapascals (22 bar) for pure propane at 55 °C (131 °F).

Even taking the higher number for propane, 2.2 MPa is 13.5 times lower than the 30MPa used in the above calculation. 30MPa is 300atm. or the pressure exercised 3000 meters below sea level. An explosion of such a tank would be a dangerous thing to happen.

Let's assume that they can get 80% efficiency out of their expander, and they need a range of 150 miles, at about 100 watt-hours per mile. These are all optimistic assumptions.

So we're talking about most of 69 megajoules, the energy equivalent of 1/2 gallon of gasoline, released in a fraction of a second.
A better comparison may be - that's the energy equivalent of 36 pounds of TNT.

edit: corrected for higher efficiency

I am sure that there will be many on this thread who will do the simple math and also point out that for reasons of safety, short range, massive difficulties in repressurising and sheer inefficiency this is yet another example of why there are, as yet, still no adequate alternatives to gasoline or diesel for road transport.

It seems we have no option but to make gasoline somehow or other. This is why the only currently suitable alternatives to fuels from crude oil are the limited sources of ethanol, bio-diesel or gasoline from tar.

To give you some idea what all the potential alternatives are up against remember gasoline is a highly concentrated energy source, it is also relatively cheap.

Drive into a garage and the petrol pump will fill up the tank at a rate of about 1 litre per second.

One litre of petrol yields 34,000,000 joules of energy when it is burnt.

A petrol pump could therfore supply energy at a rate of 34 million watts. This is an huge amount of energy,

A typical coal-fired power station might only supply at a rate of 2000MW - this is the equivalent of roughly 60 petrol pumps!

"there are, as yet, still no adequate alternatives to gasoline or diesel for road transport."

"... for road transport of individuals in a one-tonne vehicle", I think you meant to say.

Because otherwise, you know, we have these things called "legs", and later inventions like a "bicycle" or a "bus" (they can make those electric, too - with batteries, and/or overhead lines).

for road transport of individuals in a one-tonne vehicle", I think you meant to say.

Oh yes, I was just keeping it to the point of compressed air car, or any other potential alternative energy private car. :-)

Can you even imagine a compressed air powered 42 tonne delivery truck or a 'road train'?

Actually, there are around 6,000,000,000 people in the world who already don't travel much by car - if you have a car you are one of the all time lucky few, make the most of it.

I am surprised no one seems to have brought up flywheels as a more efficient way of storing rotary energy. Take a look at this article about
Main points :

  • Compressed air stores around 83 W-h/kg (320 kJ/kg) while modern flywheels around 130 W-h/kg (500 kJ/kg) - once you put in the weight of the container for the compressed air, you will find that the flywheel has at least twice the energy density.
  • The flywheel occupies perhaps one quarter of the volume of a compressed air system storing a similar amount of energy - less space needed in vehicle
  • The flywheel can be charged at home in a few hours from a regular electric socket or in around 15 minutes at a charging station
  • The flywheel is contained in a shield and when it explodes does so without any damage to the surroundings.
  • The flywheel will slow down over a period of several days whereas compressed air remains in the tank.

For both approaches, a tram or a bus is a more appropriate vehicle.

I think the real advantage of compressed air is low cost. It can very well be some limited kind of solution for third world countries.

Flywheels would be much more expensive and have certain safety issues too (albeit more manageable).

"Me, I'd rather we were rid of cars entirely, even if they were powered by sunshine, strawberries, and pretty girl's smiles."

It would be a horrible waste to use that for driving cars!

After the corn runs out.....

"Have you ever seem the results of a hot water storage tank exploding due to the failure of the temp/pressure valve and faulty thermostat." No, but I have seen fully charged scuba tanks survive intact in the trunk of a car that had been rear ended on a highway. I probably wouldn't want to repeat the experiment too often though.

When I was training to be a fireman, I had to bring my SCBA (Self-contained Breathing Apparatus) with me to the classes in either my car or the fire companies truck. When I packed the tank, I always made sure that the valve was pointing towards the front so that if it got knocked off, the tank would go out the back of the vehicle and not through me.

On one the many sites this company (MDI) publish there is a photograph showing three tanks installed under the floor. (Sorry, I haven't got the link)

I guess they have used 3 tanks for better weight distribution and to minimise the the force/noise of a rupture of any one tank.

It may also allow a tank-at-a time refill from the implied 'onboard compressor' when it is running in 'dual mode'.

Its OK Kiashu, Your physics haven't failed you and you're right the tank will be quite significant.

The true problem is the thermodynamics of compressing and expanding the air. Joules are lost in both phases. There are notes that these guys have or will consider reheating the expanding air. The big gain is in economic investment and operating off the motor fuel tax hijacking.

But the biggest potential is in Di Pietro's design. While knowing that energy input is required both to compress and reheat it would be a huge stroke of design innovation to pair Di Pietro's motors in a Stirling setup. One driven by heated gas producing the torque and the other re compressing the gas cooled by doing work. Even more efficiency can be gained because a Stirling uses constant combustion instead of intermittent combustion as in an IC engine. See the idea in more detail at

The car is refuelled by plugging it into the compressed air supply found at most service stations...

What service station now has a compressed air supply sized to recharge an air car? The only use they currently have for compressed air is running pneumatic tools and the hose used to fill up tires. For those purposes a 150 psi 5 h.p. compressor with maybe a 50 gallon tank would be all I'd expect to see. No way would that be up to the job in terms of pressure or volume.

Sure in theory they could be installed, but as with hydrogen car proposals there is a "catch 22": The stations are not going to spend many dollars on high volume high pressure compressors if they don't have lots of customers for the air, and will enough cars be sold if there is no place other than at an "at home" recharging station to get high pressure air?

From EcoGeek's review:

Refueling is simple and will only take a few minutes. That is, if you live nearby a gas station with custom air compressor units. The cost of a fill up is approximately $2.00. If a driver doesn't have access to a compressor station, they will be able to plug into the electrical grid and use the car’s built-in compressor to refill the tank in about 4 hours.

Somewhere on one of MDIs many websites they clearly state they require the rollout of "high volume high pressure compressors". Sure, this is a problem but not insurmountable.

What is the cost of an 4500psi air compressor? How much will it be to install it in sufficient number of locations? There needs to be infrastructure in place otherwise people won't buy the car.

A much better approach IMO would be to install the compressor with the car, so that it can recharge from any electric outlet like an electric car.

As noted above, the car includes an onboard compressor.

If you want a quick refill, go to a service station with heavy duty equipment. If you don't mind waiting 4 hours, plug the car into the wall and do it at home.

Yes, when I saw this I smelled scam. There is no way existing compressors in gas stations can do the job. Just the power requirements of one would be respectable - 20kWh for a reasonable 10 minutes makes it to 120kW. I don't think they have the power lines to handle such loads.

I’m with you it seems to be a scam.
To provide perspective on a 300 liter air tank, it could be a 32.7 inch dia sphere or three cylinders each 12 inches in dia and 53 inches long with spherical ends.
The three cylinders would no doubt be much safer and practical.

20 HP for 2 hours at 60 MPH is 30Kwh of energy.
How much energy is required to compress 30,000 liters of air into 300 liters of air at 3000 PSI. Of course the refill supply could be stored in much larger containers at higher pressure.

Yes, it has "SCAM" written all over it.

  • Claims of 80% thermal efficiency from the engines, with no details or caveats.
  • The aforementioned claim that standard gas-station compressors would do the job of refilling, when their air pressure falls short by a factor of 30.
  • The talk about a non-fossil infrastructure without any mention of how much energy would be required or where it would come from.

I don't agree that the power limits are a problem, though.  A filling station could have a compressor that runs all night, putting air into a series of high-pressure tanks.  During the day, the tanks would be drained; the only immediate power required would be to top up the difference between the highest-pressure tank and the maximum filling pressure.  This is pretty much how SCUBA tanks are filled from sets of high-pressure storage tanks.

Given that the methods are old hat, why weren't they mentioned on the site?  This also yells "SCAM", because they couldn't even be bothered to refer to established practice to bolster their credibility.

There is no reason why compressed air cannot be piped to places like service stations from large central compressing stations.

Compressing air creates a lot of heat - this can be used to heat water that can be used by houses, factories and offices especially in the winter.

Clearly, the more effective one is at cooling the air while it is being compressed, the more efficient the whole operation is - the closer it is to an isothermic process. I suspect multi-stage compression is the way to go.

I don't seem to have noticed anyone pointing out that in Australia there is a lot of sunshine and heat in the summer. If the compressed air is warmed up (perhaps by passing it over a heat-exchanger on the roof), the same tankful will take you quite a bit further. Also, the exhaust from the engine can be used to cool the interior of the car most effectively!

There is no reason why compressed air cannot be piped to places like service stations from large central compressing stations.

The reason is called "frictional losses".

Good point  

To say that the wind power to compressed air storage is a commercial entity is not correct. The Iowa site referenced isn't even under construction yet. The project isn't scheduled for operation until 2011.

Yes - sorry - that was a bit garbled - I meant compressed air is already in use for large scale energy storage (as per the examples in Germany and Alabama) and will be used in conjunction with wind power shortly (as per the planned developments in Iowa and Texas).

PV = NRT When you pump in normal atmospheric air, it contains water vapor. The pressure in the tank increases and the tank gets warm to hot depending upon rate of pressure increase versus the cooling of the tank due to normal conductive and radiative cooling. Eventually your full tank cools to ambient temperature.

When you are releasing the compressed air, the air inside the tank cools and the water in the water vapor condenses out. Eventually, you have to empty the tank of water. If I have to do this with a compressed air tank that goes up to 200psi, then is it extremely likely I will have to do this to a tank that goes up to 10,000psi. How do you empty a carbon wound tank of this collected water?

Also, what prevents ice build up in the relief valve as the tank decompresses? Does anyone know or have any information on this?

The fill-up time and cost have to be clarified. A normal tire fill-up at the gas station takes several minutes. Even if the gas station has a gigantic air compressor, a few minutes sounds optimistic. $2.00 for a fill-up sounds optimistic as well.

This energy storage mechanism has some advantages over hydrogen vehicles, which are present major obstacles in energy losses, transporting hydrogen, complex energy cells, and the high potential for major explosions.

How do you heat the interior, defrost the windows, and power the lights and radio?

You could build a safe compressed air tank. It wouldn't be light or cheap to make.
You could use a tank of liquid nitrogen to power a car. Again, not light or cheap to make.
Both compressed air and liquid nitrogen are cheaper than gasoline in terms of distance traveled. They are both much less compact.
Compressed air and liquid nitrogen also make excellent air conditioning systems.
My idea of using hot sloth grease to power a car would make a really good combined system. Instant on air conditioning in the summer, instant on heating in the winter...
An air engine with power from expanding air and a boost from hot sloth grease is practical. You don't get much range but you don't need to worry about obtaining either gasoline or lead acid batteries.
But we'll go with synfuels instead because we have the technology and infrastructure in place.

A Car Powered by Liquid Nitrogen!

University of North Texas CooLN2Car

If the setup as similar to a industrial gas filling station, you'd have a gas receiver downstream of the compressors. A quick car fill would be very doable. The trick would be making the filling connection foolproof.

There is some discussion about these issues at the Popular Mechanics article ( :

"How will they power brake, head lights?" How do you tink they are powered on a conventional internal combustion engine? The air engine will still generate a vacuum to run a brake servo and I'm sure they will fit an alternator to charge a battery for lights and other electrical items.

I remember the Aircar being presented while I was writing my thesis in the field of thermodynamics in 1995. The car was then due out "within months". This has been the claim ever since!!! It sounds like a good idea - using compressed air as batteries, but if it is such a good idea why does it never come any closer to appearing on the road? I was originally very optimistic, and I understand the theory behind the concept but I have become less and less optimistic each year as there is never any realism in the predictions of when the vehicle can be commercialised. I repeat - 1995!!!! That is 12 years ago!

The one absolute with an air car is that it will never be as fast or have the acceleration of an ICE. In an SUV culture of course they never took off. Their time may be coming, though.

Compressed air (or any compressed gas) is a very poor, inefficient technique for energy storage. When the gas is compressed it is an adiabatic process and the gas temperature rises according to a standard adiabatic compression formula. It is then sent to a storage tank where it sits until the energy is needed. While in storage it cools to ambient temperature. In the process it loses energy.

When energy is needed the compressed gas is fed to an expansion engine. During expansion it cools (again according to the adiabatic compression formula). The gas exiting the expansion engine is substantially cooler than ambient. The temperature difference between cool gas and ambient is a form of waste energy.

I have heard of compressed gas being used for large scale energy storage, but only where underground caverns are used to hold the gas, and the peak pressure is very little above atmospheric pressure, and the cavern volume is very large.

As a boy sixty years ago I used to get excited by articles in Popular Mechanics about ideas like this. I think there were attempts to make compressed air powered automobiles in the late 19th and early 20th century. So this idea has been known for over a century. Still there is no successful implementation. Maybe it really doesn't work.

Exactly! This is why I can't get too enthused about any sort of compressed air storage scheme .... the inherent energy losses are just too great.

Compressed air is basically a form of a 'spring', but unlike a mechanical spring which has some minor hysteresis losses, compressed air has, as you correctly pointed out, some serious losses as the result of fundamental thermodynamics.

This is why WW II compressed-air torpedoes usually has some sort of auxilliary burner (usually alcohol) that heated the expanding air to prevent it from become too cold and thus less energetic.

Then, of course, we have the inherent safety issue of having a large pressure vessel filled with a highly compressed gas and the tremendous amount of energy that would be suddenly released if it ruptured. Not an insurmountable problem, but a fairly serious one nonetheless.

Compressed air has its place, but I don't think that place is in a passenger vehicle.

I'm skeptical about losses as energy gets transferred between more forms, i.e. from electricity>compressed air>motor, rather than electricity>motor.

But I wonder how a car powered by a coiled spring as you suggest would fair - its inexpensive and seems to work pretty well for toy cars!

You are right about the energy lost during compression.

The faster a gas is compressed, the greater the temperature rise. So when the gas cools it loses energy, having lowered pressure. Likewise, the higher the pressure the higher the temp, thus the greater the loss. Only way to circumvent this is to keep the gas hot until use, or compress the gas very slowly, both of which are not feasible in this application.

However, during the expansion process the reverse is true. The lower temperature of the gas means some of the pressure energy was converted to work along with some of the temperature energy (enthalpy). The energy gained from this stage of gas being at lower temp is less than that of heat lost during compression. If total process efficiency (compression & expansion) is less than 80% then we better stick with straight electric cars.

You've never heard of insulated tanks?

Insulated or not.. You refill, and automatically have a huge heat source inside of your tiny-winy air car.

It's not 'cool', at least in Australia..

But it could be good for the inner-city public transport:

geek7 you're right, compressed air is just a highly wound spring that's more manipulable. Without the reheat a lot of energy from the spring escapes. Or add lots more heat and really pump the spring, but these all consume more joules, which betrays the whole point. For more see:

While I'm not all that excited about a compressed air powered car, I am curious about how this could be used to store energy in a household. Either from household solar, wind, hydro, or from the grid as a form of household to grid balancing.

I think it would be easier to make use of the heat and cold you get from compressing and decompressing the air. That might improve the overall efficiency a bit. I'm not sure how much heat we're talking about, but you could use it year round to preheat the domestic hot water. And the cold would certainly be useful in hot climates during the summer, or for food storage year round.

I am curious about how this could be used to store energy in a household.

It has large conversion losses, presents explosion hazards, and it's noisy.

If you tried to put one in your house, you'd probably be shut down under laws banning operation of dangerous industrial equipment in residential areas.  Stick with batteries, they're superior on almost every figure of merit (save cycle life).


A 300 Liter tank is equal to about 11 cubic ft. or 0.3 meter cubed (1m x 1m x 0.3m). This is not a really large volume, but at 4560 psi, this is a lot of energy that could be released in a very short time. If the tank ruptured with a hole of 2 square inches (12.9 sqr cm), the force of the escaping air would be 9120 lbs. This would be enough to propell the car at a 3g acceleration - very fast! Safety of the air tank is very important here.

To get a source for 4560psi air would require special compressors, perhaps a cascade system where one high efficiency scroll type compressor would feed into another, with two or three stages required. Single stage compression is too inefficient for this psi. So the cost of the equipment to provide large volumes at this pressure would be $30,000 to $50,000, which is quite a bit to invest in for limited application. Then you have the maintenance of said compressors. Thus the source of the air would be a problem. The new type high efficiency compressor noted in the article is for small volume applications, we don't know if it can be scaled to large sizes required for station supply.

The concept sounds good on paper, but seems to have too many complicating factors. Just like hydrogen, the largest capital cost is for the primary energy production and storage.

I think this is the sort of thing such a filling station might need if things got at all busy :

This is a Compair model H5417 compressor

It can supply 18.7 cu ft of air at 5100 psi per minute, or 1.7 full air car fills per minute with the numbers we have been given

Its a 4 stage water cooled unit, weighs 750 kg, about 2 cubic meters in volume for the machinery, has a 15Kw electric motor which needs 400 volt 3 phase power, needs a flow rate of about 1,100 Liters per hour of cooling water at "city mains" temp.

Call them for quote :)

Interesting, the numbers do not add up - if you could fill up 1.7 cars per minute with a 15kW motor, each car would store just 0.14kWth times the efficiency of the process, so probably less than 0.1kWth - and this won't bring you farther than a kilometer.

I think you misinterpret the 18.7 cu ft. number. It should be the amount of the uncompressed air at the input, otherwise it simply does not add up.

Judging from the motor power this piece of equipment would fill up an 20kWth air car for about 2 hours... for more powerful ones probably the price quote will look even better.

Your right, my numbers don't add up! apologies to all here

This site:

gives the math from first principals which seems corect for calculating the potentional energy stored in a tank of air, and some scary stories of exploding scuba tanks :(

comes out to Joules stored aprox. = pressure (Bar) X Volume of tank (liters) X 450

for the air car with 300 liters at 300 bar when full we would have

40,500,000 joules /1000/ 3600 = 11.25 KwH which obviously can't be done with a 15 Kw motor running for less than a minute regardless of how good the compressor is.

It can supply 18.7 cu ft of air at 5100 psi per minute, or 1.7 full air car fills per minute with the numbers we have been given

I think if you examine the figures more carefully you'll see that it's SCFM, Standard Cubic Feet per Minute.  In other words, the volume is before compression, not after; it's a great deal smaller at 5100 PSIA.

Well this (9000 lbs of rocket thrust, 3G acceleration) obviously leads to a great opportunity. They're all aiming at the wrong market. See "The Fast and the Furious" and its low-budget badly-acted follow-ons to find the real market: kids who like fast cars.

Dick Lawrence

I've seen an air car on video and it is EXTREMELY noisy. It's like a cross between an air-cooled VW and a jackhammer. I don't think anyone would want to subject themselves to such noise pollution.

The following details the history of the Air Car including "Industrial Production in Various Countries" in 2003!!. Only one minor detail has been omitted - the fact that the production never actually took place although a number of licenses were sold and a number of international investors were possibly cheated out of their money:

The name of the company has changed many times (due to legal proceedings?), and the names Zero Emission Vehicle with Compressed Air Technology "ZevCat", the "AirCar" and "Motordeaire" etc have all been used at different times. Interestingly MDI "Moteur Development International" has the same logo as an import company which imports all kinds of things. I smell a scam. Anyone who can confirm my suspicions?

Hype cycle:

Or, I like some of the variations on the definition of "hype" here:

"Hype" is what sells almost everything in our culture.

Try this:

"1. Intensive, exaggerated or artificially induced excitement about, or enthusiasm for, something or someone.

Thesaurus: hoop-la (US), ballyhoo, build-up, excitement, publicity.

2. Exaggerated and usually misleading publicity or advertising; a sales gimmick.
3. A publicity stunt, or the person or thing promoted by such a stunt.
4. A deception."

Or this:

"1. A hypodermic needle.
2. A drug addict.
3. Something which artificially stimulates, especially a drug."

Or this:

To inject oneself with a drug. See also hypo1.

Form: hype up (usually)

This is what sells political campaigns and movies and wars and other consumer products. Hype.

And yet once in a great while some kind of dream comes true.

Who dreams that dream? When does it come true? When does it lead to a lower circle of Hell than one already inhabits?

I am skeptical.

Of course, if compressed air would work, liquid air should work even better:

Unfortunately, even in liquid form, the air takes up a huge volume. As can be seen by the "car" above, most of the vehicle become fuel tank, and even with the weight of the vehicle pared down to a minimum, the performance is very poor.

I once did a study of liquid air/compressed air going all the way back to the birth of the idea, and fould that attempts at using air as a storage medium go back to the birth of the century. One of the cars in the first "motor race" in Chicago was a compressed air car, and it actually completed the race (albeit with a very light chassis car and frequent recharging)

The only advantages that liquid air offer over a well designed battery storage system are (a)no rare minerals or complex chemistry is needed, it's good old fashioned pneumatics, and (b) the storage technology does not degrade with use, i.e., virtually unlimited cycle life.

These are not to be dismissed, but batteries (lithium ion and lithium polymer in particular) are increasing in cycle life, and the materials can be recycled, and are being reduced as much as possible by efficiencies of manufacturing. This reduces the "advantage" of compressed air/liquid air as a storage medium, in particular when you take into account the volume requirements of the compressed or liquid air tank.

Of much greater interest is the application of liquid air/compressed air in stationary storage systems. This to me still shows real possibility. The volume of the tank is not nearly so great a liability in a stationary installation.
The advantage of no decline in performance over many charge/discharge cycles is a big factor in a stationary installation.

Liquid air is not as good as lithium ion batteries on a Kwh per pound basis, but is better than lead acid batteries by a considerable margin. But in a stationary installation, weight, like volume, are not priority factors as they would be in a transportation application.

Even given this, compressed air and liquid air have not been competitive as a energy storage medium in most cases, even in stationary situations (there have been some exceptions, the most noted in a CAES (Compressed Air Energy Storage) system in Alabama. This system uses underground cavarns of very large volume.

But we can pretty much assume that if compressed air/liquid air has not as of yet been accepted as a stationary storage medium where it's weaknesses of weight/volume inefficiency can be overlooked, it will find the hurdles of space efficiency/weight efficiency much more daunting to overcome in a moving vehicle.

The pioneers of many of these alternatives would be advised to first demonstrate the application in stationary installations first, and work to transportation alternatives later. That is the way it was done with steam and electric motors. Only gasoline engines moved to transportation almost immediately, demonstrating the astounding powr to weight ratio/power to volume efficiency of the gasoline engines.


"Even given this, compressed air and liquid air have not been competitive as a energy storage medium in most cases, even in stationary situations"

I should note that one summer I worked at a sheet metal factory, and all f the machines there, except for two big programmable cutters, were driven by compressed air. I worked on this big-arsed old machine made about 1950 (it had brass fittings!), a guillotine that could cut through 8mm steel plates.

As I understood it, they used pneumatic machines because the power requirements on a purely electric one would be enormous, so that they'd have to pay the power company to put in high tension wires, and then there'd be all the safety concerns with that, the place would be like a neighbourhood power substation with huge transformers and so on.

The pneumatics were good for providing a big push over a short period. We used to go through a 100lt "air liquide" bottle each day for our eight or so machines (all used less than mine, since they were cutting through 2mm aluminium and the like).

The pressure of the bottles I don't know. None exploded while I was there, despite the generally unsafe workplace and indifference of workers and foreman. I do know the things were too heavy to lift even for me and the big Samoan together, and we were the strongest ones there (the rest of the crew were little Vietnamese guys), having previously lifted up to a quarter-tonne together.


You hit on exactly the best application for compressed air...
"The pneumatics were good for providing a big push over a short period."

The weight issue you mentioned is a real problem in transportation applications....

I have been looking at applications of compressed air for small scale vertical axis wind turbines for several years. The compressed air could provide a short term storage medium to smooth out the variability, and provide a "pneumatic transmission", converting the slow speed torgue of a vertical axis drag type turbine to high speed power to drive the generator. It could work technically, the cost issue per kilowatt would be the deciding issue.

Given that low speed vertical axis drag type windmills do not turn fast, but make mountains of torque, it would essentially be a way to convert torque to kilowatts. Now if there was any real market for wind power here in coal country...:-)


I agree on the stationary storage approach liquid/compressed air storage seems to be a natural addition to solar/wind energy production. I think its more a case of not enough intermittent energy sources.

Compressed air is similar to hydrogen - it is an energy storage medium, not an energy source.

I am always bemused by such statements. It implies that oil, coal or natural gas are more than just a storage medium and are in fact an energy source. Rubbish! It seems to me that an awful lot of effort over an awful lot of years went into storing that ancient sunlight in what ultimately became that fossil fuel.

We're in love with our petrol because it's a wonderful energy storage medium, not because it's a source of energy! That's why we're having a debate about air-cars. We are only really debating about how good/bad the energy storage medium is, not the source of that energy.


EXACTLY correct. You must understand that oil is worshipped here as a god.

I once made exactly the point you made about oil here on TOD and closed with this line...
Oil is good. But it's not that damn good." I still feel the same way.
Some day, we will realize that the "source" is solar. Cut out the middleman.


Yikes - I hope you don't think I worship oil as a god - I thought I was just making a simple statement of fact.

For the record, I think oil is a poisonous liquid and I can never understand why people fetishise it. I think we should replace it entirely, and would think so even if the peak was 30 years off...

I think the point is that the work required to store the energy in oil, gas etc was done for us, whereas with hydrogen, compressed air etc we need to do the work to store the energy ourselves.

All energy stored (in whatever form) comes from one of 2 places - the sun or the centre of the earth...

If we have an insulated pressure tank (or at least the amount of heat lost before reuse can be neglected, what is
the thermodynamic limit to storage efficienct (work-out/work-in)? I susupect it is 100%, entropy is increased by conductance of heat (deltaQ/T). So the key is adiabatic (w/o heat transfer) compression/expansion and have enough insulation that you don't lose heat. That seems incompatible with low volume or weight per unit of energy. If you let the air-tank cool, you will get much less mechanical energy out, and might have a problem with ice formation in your engine.

It still might be useful for cheap portable energy, where absolute energy efficiency isn't crucial (think lawn mowers here).

A large centrifugal compressor would have an adiabatic efficiency in the 85-90% range. A piston compressor (reciprocating) would have an adiabatic efficiency of 90%+. The piston compressor is the only thing capable of 3000+ psi. A turbine expander would have an efficiency in the mid 80's. I know nothing about the rotary engine used in the car; but high pressure piston expanders used in early air plants (circa 1950) were around 70%. It would not be unreasonable to assume the engine will be in the high 80's.

Given those efficiencies and allowing for transfer losses the power recovery would be around 70%. This less than for battery systems (about 80% I think) but it's also a lot cheaper.

Some other points:

There will be no liquid water in the tank. There will be no liquid or vapor phases at those high pressures. If the tank is blown down some water may collect; this is usually taken care of with a quick purge during filling. Most of the atmospheric water is removed in the initial compression.

Nobody is going to use a on demand compressor at a filling station. It will be much cheaper to use a gas receiver and a smaller compressor that runs all the time.

Given those efficiencies and allowing for transfer losses the power recovery would be around 70%.

I think you're seriously overestimating the efficiency.  Filling the tanks to 300 bar with a 3-stage compressor means each stage has a pressure ratio of about 6.7:1.  By the adiabatic gas law Pv=C (and of course Pv=nRT), the temperature will increase by a factor of (P/P0)(γ-1)/γ, where γ is the ratio of constant-pressure specific heat to constant-volume specific heat (about 1.4 for air at room temperature and pressure).  This temperature ratio is about 1.72.  You'll have to cool the air between stages, so all this heat energy gets thrown away.  The lost energy is equal to .72/1.72=42%, so the compression process is only 58% efficient at best (it will be lower if the air isn't cooled all the way to ambient between stages).

The air motor can make up for this by pre-heating or re-heating air between expansions (the 2-cylinder engine would appear to be capable of 2-stage expansion, at best).  This may be the source of the 80% efficiency claim; it may refer only to fuel used aboard the vehicle, not to energy used to compress the air.  However, absent a source of heat the temperature of the air during expansion is going to be much less than the temperature during compression, so the recovered energy will be much smaller than the input (energy is equal to the integral of pressure over the change in volume).  You can't escape the laws of physics even if Mother Nature is a bitch.

Having actually done compressor efficiency measurements in the field I can say you're wrong. Vendors get 76% isothermal efficiencies and high 80's polytropic with multistage compressors. I would suggest you actually do the calculations.

They get this with how many stages to what pressure?  The more stages and the lower the pressure, the closer you get to isothermal operation.  I doubt that a garage-scale system going to 300 bar will be able to justify the capital expense for that, though.

several years ago I stumbled across the aircar and signed up for email notification of the release of their product. Still waiting and sure it is a hoax. The only report I have read of its motorability said it ran out of air after only a few miles.
It will get more attention as oil depletes and people get desperate, just look at the number of hits on google for this search.
434,000 for "car runs on water."

I don't think the car can't be built. For instance there is also a shop in France already producing small numbers. The competitiveness may be grossly depending on tax issues and things like that. The big question is: does it make sense with regard to efficiency of energy use. In contrast to conventional vehicle drives, the energy balance is not yet suffciently transparent.
Some posters have mentioned the energy loss occurring due to heat loss during compression. An important additional source of loss has not been mentioned yet, as far as I understood. Certainly the pneumatic motor has some limited range of working pressure that must be considerably lower than the tank fillup pressure (or else the tank size must be greatly increased to provide ample dead volume). During operation, the compressed air has to be expanded to that working pressure - almost certainly by mere throttling. The pressure drop is a significant loss which is entirely due to the use of a compressed gas.
I think it doesn't make a lot of sense to discuss air-driven vehicle technique as long as the working cycle isn't clear in a quantitative manner.

They could have made the car a bit less ugly. that could hamper sales. Who would be seen in it?

Sorry to post this here as it is off topic but does anyone have more information about this?

"You're looking at an increase equivalent to the same amount of energy as conventional oil reserves in the world today," says petroleum geologist Steve Larter of the University of Calgary in Canada, a member of the team investigating the microbial process. "It's potentially a game changer if it can be demonstrated."

Article with this subject was posted on Saturday's DrumBeat and discussed Sunday DrumBeat (16-12-07).

Problem as I see it is getting enough concentration of microbes and having proper temp for them to work, then very slow process of converting to waste (CH4 gas).

Compressed air vehicles were experimented with in the late 19th and early 20th century. For the most part these were converted steam locomotives. One streetcar experiment had compressors and hot water tanks at each end of the line. The streetcar was charged with both the compressed air and hot water. Its implied the water was heated by the air compression and this stored thermal energy was transfered to the street car tanks.
There is the claim that the heat of compression is lost to the atmosphere as the tank cools. If the aircar had an heat exchanger like those in common air conditioners then heat would be absorbed from the air. Of course the heat exchanger for an air car would be much larger roughly the size of the boiler in Stanley Steamers.
When I first saw the MDI air motor I jumped to the conclusion that it is unneccesarily complicated. It uses an offset crank so the piston has more time at the TDC where it can absorb heat from the cylinder head. Considering the small surface area involved and the short time period not much energy could be absorbed per stroke. It would have been better to have used a scaled up version of the CO2 motors used in some model airplanes. CO2 is injected at the top of the stroke and exhausted via a port at the bottom of the stroke. Since not all the expanded gas leaves the cylinder it is recompressed and therefore heated on the up stroke. This residual heated gas mixes with the next charge giving up some of its heat and making the motor more efficient. Nothing new here so move along.

In general:
I think the idea of mass transit vehicles which store energy that they collect on only part of their route is very promising. A light rail multiple unit system could be written up which charges only in lowspeed areas near & at the stations, and need only carry only enough battery power to make two segments of its route.

This reduces the need for expensive high-tension wires, for dangerous third-rails (dangerous for grade crossings), for complicated trolleybus overhead interleaves. It restricts the high voltage to an area which can be controlled with fencing & surveillance, and which you can run on a single transformer substation, reducing RoW requirements.

Energy density of the fuel isn't nearly the issue on a huge train that it is on a car - and if you don't need to make it extremely long range, you could electrify some of these things with moderately expensive rolling stock upgrades like a battery/supercapacitor car, rather than giant civil engineering project involving hundreds of workers stringing and maintaining specially tensioned wire. It's especially attractive when the difficulties of using that wire at high speed come into view - and adding to the length of the train adds trivial aerodynamic drag.

I wish EngineerPoet could comment on this technology. IMO, it is one of the worst Thermodynamic cycle efficiency that you can design.

Why would you say that?

It doesn't matter what the efficiency is because we are wasting cheap (compared to gasoline) electricity. The issue is how much energy can we store in that air tank, not how much electricity does it take.

All we have to do is cool the tank while we fill it. Some, well maybe half, of the energy in the compressor will go to heating coolant but so what?

Thanks to insomnia, I've been sprinkling bits down the thread even before I saw your wish.

Consider it granted.

Great comments. RE: the potential 3 G's of acceleration, the way I picture it is 2 G's straight up, probably for several hundred feet, though that's a guess. Time enough for your life to flash before your eyes several times along with a damn good view out the windshield.

You really do quickly reach a point of diminishing returns in compressed air. Once example of high efficiency would be a low-friction pneumatic shock absorber type system - efficiency reasonably approaching 100%. As you stuff more air into a small space, so much is lost in heating, and phase change of the water, etc that the efficiency plummets... as others have pointed out with actual numbers.

For that reason, it would seem that the best use of compressed air would be in a regenerative braking system... say in a passenger bus that stops a lot. In that case, a relatively low-pressure system could probably go to zero and back up to 40 with high efficiency and get by on a much smaller liquid-fuel motor. Although I tend to think a direct mechanical flywheel might beat it in efficiency.

Compressed air is good for some stuff, but highly compressed air is an inefficient way to store energy. I do vote yea for the low pressure start-stop bus idea though, that might work well and drastically lower the amount of liquid fuel needed for non-electrified public transport like busses. And there will be a lot of areas in which electrified public transit won't happen all that quickly. A tiny engine with efficient start-stop regeneration using compressed air might be ideal.

Buying stock in air car companies is probably a bad idea...

From :

Mexico City, one of the most polluted cities in the world, could see air-powered taxis on its streets by 2002.

Factories producing a car billed as non-polluting are due to open in Mexico next June, and the first taxis are expected to roll off the production line eight months later...

That would have been June 2001.

For deliverable energy, an air cylinder as tall as a car is wide is challenged by a butane barbecue-igniter. A worked example.

--- G.R.L. Cowan
How shall the car gain nuclear cachet?

I can picture grandma trying to fuel her air car at a station. Then we have the millions who can't even keep their tires properly inflated. Then there's the question of insurance cost, which sounds like it would be expensive. The thermodynamic objections sound quite serious. Like fuel-cell cars, I think the air-car will remain a concept car; too many factors don't add-up.

As a number of people have pointed out above, compressed air cars will most likely be poor performers from an energy efficiency point of view even if they can be manufactured with a reasonable driving range. One possible economic advantage relative to electric cars is low cost. Air tanks are likely to have low up front costs and very long lifetimes compared to electrochemical batteries. Also, if the compressed air at filling stations came out of large storage tanks then it would be possible to run the air compressors using variable output from renewable resources. That is if wind or solar power in excess of the current electricity demand is available then the power could be used to compress air. Stationary compressors would be logistically easier to manage than a large fleet of mobile automobile batteries in a V2G scheme.

Have you ever seen a room-full of industrial air compressors?  They shake the building; they are that loud.  Then you've got the hazards of high-pressure gas in large volume.  No way are you going to site these things near residential areas!

Hi, it's my first post on TOD. I know Jerome from Dailykos, and maybe a few others here to.

There's a problem with your assumption of 10 km/lt fuel consumption for a small city car. That converts to 23.5 MPG which is more typical for a midsize sedan (saloon to you Brits). I'm talking about a regular gasoline, non-hybrid, non-diesel car that typically has a 150-160hp 4 cylinder engine. The MDI air cars are much smaller and slower. They're in a completely different universe of comfort and performance. A small city car would be something like a Smart ForTwo or a 660cc Kei car from Japan. Those usually get 40-55 MPG, and they still have better acceleration and top speed than these air cars. It's hard to do an apples to apples matchup, but I'm pretty familiar with the auto market, so I'll have more on that soon. There are a few EVs that come close to their gasoline twins, but most are NEVs and don't compete.

You're confusing theoretical performance from the test track with actual performance which depends on local conditions.

For example, if your car achieves X mpg at 60 miles/hr on the track, then because it's designed to achieve peak fuel efficiency at that top speed, it might achieve 0.8X mpg at 20 miles/hr travelling in the city - that's a pretty typical average speed in cities. Add in congestion and you find that about 20% of a car's fuel is burned while sitting still. So now you're getting 0.6X mpg.

Here Down Under cars in rural areas get on average 14km/100lt, while those in the city get 6km/100lt. The national average is 14,700km travelled annually using 1,820lt of petrol, or 8km/lt. That's the national average over about 7 million private cars. We've another 7.5 million motorbikes, trucks, buses and so on, each with their own efficiencies and particular conditions.

In practice, when you add together road-building and maintenance, fuel supply, foreign wars to secure fuel supply, congestion on roads, lethal accidents, the actual achieved fuel efficiencies and so on, private cars are an extraordinarily expensive inefficient way of getting people from A to B. Most of those considerations would apply whatever the cars were powered by. In the end, using 1.5 tonnes of metal to move 70-140kg of people (average of 1.59 people per trip here Down Under) is just never going to be very resource or energy-efficient.

There's a lot of issues in that, but I'll try to address each in turn.

About the difference between city and highway efficiency- When I gave a single mileage number like 24 MPG for a midsize sedan, I meant an average of a mix of city and highway driving, but we can look at city and highway in more detail. Let's look at the Toyota Camry, the best selling car in the US. It gets 21 MPG city, 31 MPG highway, and 25 MPG combined. It has a 158 hp 4 cylinder engine, a ton of luxury/convenience features, and more interior room than a full size sedan from the 80's. Looking at smaller cars, a Honda Civic gets 25 MPG city, and the Smart Fortwo gets a surprisingly low 31 MPG city. Kei cars aren't sold in the US, but the most efficient ones like a Daihatsu Move CVT can get over 50 MPG without a diesel or hybrid powertrain. However, it's hard to compare directly because Japan has a different test cycle than the US for measuring fuel consumption. Which brings us to...

Testing standards- You gave the example of measuring fuel consumption at a constant 60mph or 20mph, but most testing works differently. The US EPA has a standard city cycle and highway cycle. All cars are tested according to the same standard, and the resulting figures are the only ones that manufacturers are allowed to advertise. The test was revised for 2008 to reflect real world driving more accurately, so all cars took a hit in EPA mileage from 2007 to 2008. It's easy to check with cars unchanged from 2007 to 2008 model year. The pre-2008 test was unchanged since the 70's and gave very optimistic numbers. Read more about the new and old testing procedures here.

Now when you see a claimed range for an EV or this aircar, you need to take it with a huge grain of salt unless it's measured with a standard driving cycle. There actually is one for EVs, SAE J1634. The Toyota RAV4 EV NiMH had a range of 94 miles in that driving cycle. Range at a constant 60 mph was lower at 86.9 miles. You'll see that a lot with EVs. An accurate range is important to calculating efficiency because you're dividing range by the energy consumed in a battery charge. If real world range is, say, half of the claimed maximum range ("up to"), effiency gets cut in half too.

The main thing is, I disagreed with the diarist's assumption about 10 km/lit fuel consumption for a small city car.

A regular small city car gets about 10km/lt.

Actually, Big Gav was quoting Kyle Schaunt in The Bullroarer. So it was the latter's mistake. 10 km/lit converts to 23.5 MPG which is horrible mileage for a city car like a Smart or a Daihatsu Move. It's closer to the combined city/highway mileage of a midsize sedan or, if you're more pessimistic, the city mileage of a compact sedan. It's very important to do an apples to apples comparison between the gasoline and air car/EV.

In a future with expensive energy, we'll most likely be driving slow, less powerful cars (if we drive at all), but for now people need cars with enough power and acceleration to safely merge with traffic, or else they'll be relegated to toy status, e.g. neighborhood electric vehicles (NEVs).

One final note, I think the bit about lethal accidents is a red herring. I'm working on a blog post about it, but the short version is... there's risk in all transportation. Any vehicle more efficient than a car will probably be smaller and lighter than a car, and they'll be less safe than a car as long as you're sharing the road with cars, delivery trucks and buses. The US has such a culture of fear and safety. Asians and Europeans ride bicycles and motor scooters, and it's no big deal to them.

IT MDI – Energy – What’s it about?
It’s great to feel the excitement, interest and intrigue about the MDI “air car” as posted on The Oil Drum – Australia and New Zealand. However, a large number of those comments are in my view mistaken and made without the required knowledge and expertise. In this posting I wish to clarify a few matters.
To call the MDI vehicles an “air car” is a misnomer. It is no more an “air car” than a “gasoline car”, or a “diesel car” or an “ethanol car” or a “rapeseed oil car” or a “what-have-you car.” The point being that the MDI engines (there is a whole series of them) are being designed to accept a very wide range of primary energy inputs including fossil fuels, bio fuels, waste heat recycling from other processes and thermal solar energy.
Transport is only on aspect of the applications of the MDI technology. Power generation at the point of use, on customer premises is another. Power generation applications are at least as important as the transport applications.
“Is it for real?” many people ask. I did understand that question when it was asked ten years ago. We are now well beyond that stage. I speak as one of the few people in the world to have driven an MDI vehicle. After due diligence, my company acquired the rights to the MDI technology for Australasia in 1999. MDI has sold similar rights for over 40 MDI licensed manufacturing plants globally. This year, as noted on TOD, Tata Motors, the 5th largest automotive manufacturer in the world, acquired a licence for India. So yes it is for real and we intend to eventually cover a range of applications including cars, buses, small to heavy trucks, industrial tractors, farm machinery, marine and light aircraft applications.
It is important to realise that we are not dealing here with just a new engine or car. The MDI development extends to a radically new manufacturing process and new business models. IT MDI – Energy Ltd, the result of a joint venture between MDI and my own company, IndraNet Technologies (IT), aims to deploy an entirely new class of integrated energy, transport and communication infrastructures, creating entirely new industries and new markets in Australasia and beyond.
To understand the importance of our initiative one must stand back and consider the global situation as it is currently emerging. Most readers and contributors to TOD are aware of “Peak Oil”. Yet most of the comments I have seen on the so-called “air car” seem to me to misunderstand the situation.
The world is currently passing through the peak of yearly oil deliveries. Very soon it will also pass the peak of natural gas supplies and immediately thereafter that for coal and uranium. Within 20 years, 30 years at most, all supplies will be in relentless decline. The most recent data makes this very clear (e.g. IEA, ASPO, EWG). However, talking in those terms does not go to the heart of the matter. What matters much more is available supply per head of global population. The peak for oil and gas combined (hydrocarbons) per head was passed in 1979. We are already 28 years into the Post-Oil Era. Another way of looking at this is to say that the world has wasted 28 years in sheer denial and not developing and deploying viable alternatives.
However, looking at supplies per head still does not go to the heart of the matter. Few people would confuse the cost of raw material inputs to a process with annual sales of the end products and net profits after depreciation. And yet this is what most people do with energy. It is rather pointless to focus on yearly crude oil inputs to the global economy. What matters is net energy available for all forms of economic activity, that is to say, the net energy available when one has deducted all the costs and depreciated all of the energy capital involved in getting energy supplies to market. As for any other investment, the key parameter is EROI, the Energy Return on Energy Investment. The point is that EROIs for all fossil fuels are in sharp decline. This means that those of us working in this field now consider that net energy from oil and natural gas will be about nil probably within 10 years and net energy from all fossil fuels including coal is on track to be about nil well before 2040. Unless alternatives are deployed in time at nil net energy everything grinds to a halt. The precautionary principle tells us not to try such a route.
I am the sort of person that prefers to call a spade a spade and to consider realities straight. The above considerations do not yet go fully to the heart of the matter. Combine the consequences of the peaking of fossil fuels and uranium, with the consequences of climate change and that of the numerous other forms of ecological damage caused by close to 7 billion humans and it is clear that humankind as a whole has placed itself on a fast track to extinction (e.g. humankind already substantially exceed the Earth’s carrying capacity).
For well over a century most of humankind has been living in a fake world, a kind of fantasyland; fake because it is entirely predicated on a series of myths - the myth of “cheap fossil fuels for ever”; the myth of neo-classical self-interested economic utility; the myths related to a dualistic approach to the world and fellow humans (in jargon terms dual thinking refers to the use of Aristotelian logic; in colloquial terms, roughly, it means thinking in terms of either/ors - as in “is the guy across going to do me in or shall I?”). This fake world has begun crashing around people’s ears in a kind of eerie slow motion.
Most experts consider that it is now far too late to avoid major disruptions to the world economy and possibly much worse. More imbecile a prolonged set of attitudes and business-as-usual (BAU) decision-making over the last 100 years is hard to imagine. In short BAU leads to highly non-BAU and highly unpalatable outcomes. It is high time to stop beating about the proverbial bush and begin to act intelligently for once, in order to take a radical turn towards sustainable ways of life.
There is no wiggle room in this matter. There are at best 30 years to invent and institute a global civilisation change and the immediate first 10 years of those 30 years are key. This entails the entire re-engineering of all global energy, transport and communication infrastructures – none of the present ones are sustainable. Historically it has taken 40 to 50 years to deploy new major infrastructures globally. To say that humankind is in serious trouble would be an understatement. The situation is seemingly intractable. In fact, there are solutions that can be deployed within the required time frame, just. However, they all require intelligence. The necessary changes are not just confined to the “what to do”. They also concern the “how to do it”, and more specifically the “how to think” to rapidly get out of trouble.
It is not that energy is scarce. Yearly the earth receives many orders of magnitude more energy from the sun than humankind could ever require. Humans create scarcity not nature and they do so because of how they have been thinking so far.
What is required is a change of paradigm. Currently most infrastructures are heavily centralised and structured hierarchically (think oil wells, coal mines, large refineries, large power stations, large car manufacturing facilities, etc.). A consequence of this arrangement is that most of the fossil energy we use goes to waste heat. For example, moving a person on a road is generally less than 10% energy efficient. I estimate that on average and roughly at least 80% of all energy we use is wasted. That means that humans spend over US$3.5 trillion per year on waste heat – not a very bright thing to do. This is a large “pot of gold”, big enough to pay for the complete re-engineering required for both industrialized and developing countries.
However, this pot of gold is not accessible to current centralised and hierarchical infrastructures. The people holding the gold are the end users who pay the bills at the petrol stations, and at the end of the month for electricity, gas and telecoms, and whose money in the main goes to paying for waste heat. This is where the change of paradigm comes in. The beauty of the situation is that solar energy is inherently distributed and energy, transportation and communications users are also distributed.
The paradigm shift can be summarised as a shift from thinking in terms of million barrels of oil per day and very low net energy technologies to megawatts per square kilometre of solar energy and high net energy technologies. The latter means point-of-use energy harvesting, matching the grades of energy sources and energy use (e.g. use low grade energy sources for low grade heating requirements), storing small amounts of energy in highly distributed fashion and non-hierarchical, networks of networks, mesh communication and energy networking. In brief, the solution is in emulating what nature does with solar energy. Nature does not harvest solar energy in one place and transport it thousands of kilometres away to use it wastefully. It does things locally and makes an abundant use of non-hierarchical meshed networks of networks for both energy transport and communications (e.g. within individual plants and within forests and ecosystems).
Within the available very short time frame very few technologies meet the requirements for sustainability. Most cost too much, take too long to develop and deploy, do not return high enough net energy (or have a negative net energy balances), harm ecosystems, impinge unduly on food supplies or fibre supplies (for textiles and/or building and furniture materials) or a combination of all of the above. To the best of my knowledge, the combined IT MDI integrated Information Communication and Energy Technology package (ICET) is one of the very few technology sets that, by design, meets sustainability and timeframe requirements.
As I have stressed in introduction we aim to deploy an entirely new class of integrated energy, transport and communication infrastructures, creating entirely new industries and new markets in Australasia. Once we have established it there we plan to share our know-how globally. I have elaborated on the above in a short book that is being presently edited and that we plan to make available on our website towards the end of January 2008.
Matthew Simmons, Chairman of Simmons & Company International, one of the world’s largest oil and energy investment bank, is famous for having stressed at ASPO-3, in 2003, that in his opinion “the world does not have a Plan B” to substitute for oil and other fossil resources fast enough. We do have a “Plan B” that includes four main components:
 Reducing transport drastically and replacing it with advanced broadband communications (i.e. replacing the transport of people with that of data, voice and visuals) in energy efficient ways;
 Deploying high efficiency, distributed energy networks;
 Deploying novel forms of high efficiency transport for people and goods; with
 All of the above being based on solar energy (e.g. wind, thermal solar, photovoltaics, biomass, etc.).
To find out more please visit IT MDI – Energy’s website at

Hi Louis,

Thanks for your comments.

Given the article is a little old now I doubt many people will read them though.

I was wondering if you'd like to do an interview style follow-up post, where I run through some of the various questions and objections people have come up with and let you address them point by point.

Let me know if you are interested (biggav at gmail dot com) and we can do this in the New year - once site traffic returns to normal levels...