car that runs on compressed air

[knowledge..w/o..power]

A French company has developed a pollution-free car which runs on compressed air. India's Tata Motors has the car under production and it may be on sale in Europe and India by the end of the year.

The air car, also known as the Mini-CAT or City Cat, can be refueled in minutes from an air compressor at specially equipped gas stations and can go 200 km on a 1.5 euro fill-up -- roughly 125 miles for $3. The top speed will be almost 70 mph (112 km/h) and the cost of the vehicle as low as $7000.

The car features a fiberglass body and a revolutionary electrical system and is completely computer-controlled. It is powered by the expansion of compressed air, using no combustion at all, and the exhaust is entirely clean and cool enough for use in the internal air conditioning system.

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[+DieChecker]

Very interesting. Nothing wrong with the concept. I would ask how they get so much power from expanding air.

If a normal engine runs at 2000 rpm, then that is 2000 revolutions per minute, where each revolution means the pistons need to move, right? Then assume that you need something like a half liter of air to turn the pistons each revolution. Then you would need 1000 liters of air to go for 1 minute. Assuming that liquid air is similar in density to water, then that would fit into a liter bottle, as air is a thousand times less dense then water. One minute for one liter of liquid air. So an hour would need a 60 liter tank. Seems do-able.

There are problems with using such high compression tanks and of course the electricity used to make the compressed air might very well be from oil or coal power plants.

One thing to consider is that such quick changes in the air's pressure and volume will create a chilling effect. It would not take long till the engine was freezing cold. I'm sure there is ways to get around such a problem however.

http://auto.howstuffworks.com/question133.htm

All very interesting. (And based in real world physics, I might add.)

[RabidCat]

Very interesting. Nothing wrong with the concept. I would ask how they get so much power from expanding air.

If a normal engine runs at 2000 rpm, then that is 2000 revolutions per minute, where each revolution means the pistons need to move, right? Then assume that you need something like a half liter of air to turn the pistons each revolution. Then you would need 1000 liters of air to go for 1 minute. Assuming that liquid air is similar in density to water, then that would fit into a liter bottle, as air is a thousand times less dense then water. One minute for one liter of liquid air. So an hour would need a 60 liter tank. Seems do-able.

There are problems with using such high compression tanks and of course the electricity used to make the compressed air might very well be from oil or coal power plants.

One thing to consider is that such quick changes in the air's pressure and volume will create a chilling effect. It would not take long till the engine was freezing cold. I'm sure there is ways to get around such a problem however.

http://auto.howstuffworks.com/question133.htm

All very interesting. (And based in real world physics, I might add.)

Early shaft mining underground rails used compressed air locomotives. These were quite efficient.

Must remember that expansion engines such as air or steam engines don't need rpm to develop their torque. Steam engines have greatest torque at very low rpm. Also, multiple expansion, such as double or triple expansion allows for greater usage of the pressure drop. Usually this consists of larger pistons in the lower pressure cylinders.

The cars mentioned are multiple expansion units. As commuter vehicles or short range utility vehicles they should be quite good.

Use of coal etc to generate power in a centralized location offers greater control over pollution, at least. At best, substantially greater efficiency than gasoline. Major loss would be heat generated in compression, not usually recoverable. Takes heat to make the engine run, no matter what.