# air car



## spacemule (Sep 12, 2008)

Plausible? http://www.cnn.com/2008/TECH/08/08/air.car/index.html



> CNN) -- You've heard of hybrids, electric cars and vehicles that can run on vegetable oil. But of all the contenders in the quest to produce the ultimate fuel-efficient car, this could be the first one to let you say, "fill it up with air."
> 
> That's the idea behind the compressed air car, which backers say could achieve a fuel economy of 106 miles per gallon.
> 
> ...


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## oldsaw (Sep 12, 2008)

Mechanical methods of compressing air are very inefficient, becoming even more so at high pressures.

From a physics standpoint, this doesn't make any sense to me. You will lose a bunch of energy in the compression phase, far more than just running the car on the gas used to compress the air. You would double the regular mechanical inefficiencies, then add the pressure inefficiencies on top of that. Just using the gasoline would be significantly more efficient.

Nice thing to talk about, but I haven't had anyone explain how this will work. Most stories are like the one you posted, lots of glowing praise, but no facts. Journalists are such suckers.

Mark


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## spacemule (Sep 12, 2008)

oldsaw said:


> Mechanical methods of compressing air are very inefficient, becoming even more so at high pressures.
> 
> From a physics standpoint, this doesn't make any sense to me. You will lose a bunch of energy in the compression phase, far more than just running the car on the gas used to compress the air. You would double the regular mechanical inefficiencies, then add the pressure inefficiencies on top of that. Just using the gasoline would be significantly more efficient.
> 
> ...



I doubt anyone buys 100 mpg--I'm just trying to figure out how an "external combustion" engine works.


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## raycarr (Sep 13, 2008)

picture a campfire


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## JohnL (Sep 17, 2008)

The only way to get any kind of mileage would be to compress the air on an external compressor, and not include that energy used in the ballance. Then extend the mileage by heating up the tanks with the gas burners to raise the pressure after it falls to a certain level. 


If they included the outside compressor's energy draw, I think it would be un-impressive.


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## Dubai Vol (Sep 17, 2008)

spacemule said:


> I'm just trying to figure out how an "external combustion" engine works.



Steam locomotives are considered external combustion engines-for one.

Think of it this way: the "engine" is the device that changes some form of energy into mechanical motion. So a steam engine uses steam to move the piston (or turbine blades, these days) but the source of the steam could be miles away from the engine, just connected by a pipe (a well-insulated pipe!) You could have one central steam boiler and dozens of steam engines scattered around a factory, for example. 

This guy is just doing the same thing with air, heating it outside the actual engine to make more pressure. If you look at the pic from the article, the "heater" could be anywhere in the car, because it's not part of the engine, it's just connected by a pipe. In practice it will be close, to stop the air cooling off.

At the end of the day this is just another energy-storage device. The efficiencies come from using cheap electricity to compress the air at home just as a plug-in hybrid stores cheap power from the grid in batteries, but as Oldsaw points out, there are inefficiencies aplenty to deal with. Plus I don't care for sitting on tanks of compressed air.


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