# Would it be possible to create a sterling engine out of liquid nitrogen and air?

Is the heat difference enough to cause a noticeable power output in some type of sterling engine? Liquid nitrogen is quite cheap (alternative fuel source?)

russ_watters
Mentor
Setting aside for now the practical difficulties in sterling engines (they are efficient, but produce a low power density) and in using liquid nitrogen:

The heat of vaporization of nitrogen is 202 kJ/kg
The heat of combustion of gasoline is 47,000 kJ/kg

You'd need more than 200 times as much vaporized liquid nitrogen as burned gasoline to ge the same amount of energy output.

Hmmm, the vaporation heat will just be the waste heat you have to dump. The amount of useful work will be about a factor 3 larger.

When you price fuels, you have to do it on a \$/(delivered kJ) basis. Nitrogen looks very expensive on that basis.

Perhaps you could get radioactive waste free of charge and use that to power an engine. I think that caesium 137 will yield 1 Watt per gram. The half life in 30 years, so you can imagine a car being powered by a block of ceasium that would only have to be replaced every ten years or so.

Perhaps you could get radioactive waste free of charge and use that to power an engine. I think that caesium 137 will yield 1 Watt per gram. The half life in 30 years, so you can imagine a car being powered by a block of ceasium that would only have to be replaced every ten years or so.
Where would I get a 1 kilowatt (about 1 HP) Cesium source? Assuming about 1 MeV per decay, this corresponds to about 6.26 x 1015 decays per second, or 169,000 Curies. I once measured the shielding elffect of lead around a 1 milliCurie cesium source, and decided that 2 inches of lead would attenuate the radiation by about a factor of 10. So how many tons of lead are required to shield this source?
Note: spelling: Stirling engine.