The Speculist

Well, it's as cute as those stackable cars. I wonder how much impact the cute factor is going to have on the car market going forward? Everyone seems to be building it into their prototypes. Maybe cute is the new bitchin'. Somebody ask Virginia Postrel.

Compressed air strikes me as more of a companion technology for moving a car (in the context of a hybrid) than it does a good standalone solution.

As for having an on-board air compresser...if the car is a hybrid, why not turn the brakes into an air compression system? You can use compressed air to boost acceleration and then you can use the brakes to compress more air. This is the model currently used with gas/hydraulic hybrids.

This idea strikes me as incredibly inefficient. I believe that within five years cars will be running on all electric via carbon nanotube supercapacitors or a similar technology. The reason is simply that this technology will be a reality very soon and this technology is far more efficient than any kind of mechanical solution (pneumatic or otherwise). Think about it - any air compressor station would have to be powered by an incredible amount of electricity any ways. That electricity would be put to use far more effectively by storing it in a capacitor and using it to power 95% efficient electric motors than converting the electricity into compressed air and then converting the compresses air into mechanical energy via pistons or turbines. I'm not sure what the numbers look like exactly but the compressed air model is probably about 10% efficient and that sucks (i.e. only about 10% of the electricity used to power the air compressor would end up being converted into usable horse power). Whereas electric vehicles have the potential of being about 80% efficient. Remember this - there is simply no kind of motor out there that can compare to electric motors in efficiency even in theory. Our hope lies in the efficient storage and release of electrical energy and we are well on our way to achieving this.

Micah is right about the efficiency of electric motors compared to other prime movers of vehicles.

In situations where compressed air is already a plentiful resource, this type of engine might be practical. Likewise in environments where electric motors would be hazardous, or where any type of internal combustion engine would be hazardous, a pneumatic motor might well be appropriate.

Some comments on this, and on the previous comments:

'Efficiency': these comments seem to be under the impression that efficiency is important here. It's not very, by itself. The problem with plug-in vehicles is not that they use too much electrical power, it's that the vehicles themselves are too expensive, have inadequate range, or take too long to charge. Compressed air vehicles help with all these problems vs. battery-electric cars.

Having said that, compressed air car designs are inherently more efficient than your typical compressed air tools, since they do care about efficiency to the extent that it affects range. The designs I've seen use heat exchangers to expand the air quasi-isothermally, vs. the single stage, unheated expansion you see in simple air motors.

One could get even more range with another heat source, so the expansion occurs at a temperature above ambient. Combustion could do this, or you could use a 'thermal battery' -- an insulated thermal mass that is heated electrically when the vehicle is recharged. The thermal energy/mass in optimized materials can be many times higher than the electrical energy/mass in chemical batteries. This isn't terribly efficient, but, to repeat, efficiency is not the point.

My problem with these compressed air cars is that they aren't good enough. The one shown in the original post is flimsy looking and too expensive. The market has shown that a vehicle has to be a lot like a conventional car or people won't buy it. The Honda Insight hybrid, for example, is too small and compromised: it sold fewer than 700 units in the US last year. Electric cars like GM's EV-1 did even worse when they were being trialed.

If compressed air cars are to succeed at all, they will be as hybrid vehicles in which a conventional IC engine is modified with electronically controlled valves so it can also act as a compressor and air motor. Ford has been looking into this; adapting the engine to be able to store energy in an attached compressed air tank would add about 30 pounds to the weight of the vehicle. This should be significantly cheaper than an electric hybrid, since no expensive batteries or electric motor/generator are required.

See here for an entry on compressed air/IC hybrids from my blog.

Correction: added 30 kilograms, not 30 pounds. Still, that's not very much.

Efficiency is precisely the point. We already have a relatively efficient and convenient method of transportation with the ICE - why would anyone want to downgrade to a laughably inefficient and expensive 'air car'? The reason why the all electric vehicle is better is because it is potentially more efficient which makes it potentially more effective and affordable. Granted that the technology is not quite in place yet but it is very close. With the newest supercapacitors an electric vehicle would have an enormous amount of horsepower and range; it would be highly efficient, and it could be recharged almost instantly. Obviously such a device would have a high market demand and hence it would be produced on a mass scale leading to drastically lower costs than at present. This technology is here now and will be going intp production in the next few years. The air car is a toy. It is a fun idea but to even consider it as some kind of challenger of the status quo is, in my opinion, a misunderstanding of our present circumstances.

The Air Car has its own list of problems, old and new:

1. Range.


2. Lack of infrastructure (gas stations are not presently compressor stations, and the capital investment would be greater)


3. Energy losses in compression.


4. The danger of explosion in an accident (even if the tank does not generate shrapnel, a wall of 3000-psi air can kill).


5. Bulk of air tankage.


6. Thermal difficulties such as freezing of moisture inside engines and valves.

You can beat the crash problem with fusible links and impact disconnects.  The charging time problem is gone.  And the batteries don't make any noise.

The air-car is going to have really good A/C, as long as it's moving (it makes cooling like there's no tomorrow).  However, I don't see it being practical except as a taxicab in warm climates in the near term; the plug-in hybrid offers the best combination of performance and infrastructure compatibility for general-purpose cars in the near term, and batteries outperform air tanks for the long term.