Ideal working gas for a Stirling engine

[cmb]

I was reading around Stirling engines and there's something I am not sure about, can't see a logical explanation.

So the ideal gases discussed are hydrogen and helium on account of their higher thermal conductivities.

But I am unclear on what motivation there would be to use hydrogen which has a significantly higher thermal capacity than other gases.

Is it better to have a gas with a high or a low thermal capacity in a heat engine?

I'd have assumed you'd want a gas with the lowest possible thermal heat capacity, because that would require the least amount of energy transfer to heat it up and cool it down (i.e., would be quicker) yet still deliver the same mean effective pressure as the working volumes change. (Ideal gas laws law still applies, irrespective of heat capacity.)

I mean, if hydrogen and helium have the same thermal conductivity but hydrogen has 3 times (ish) the heat capacity, then it'd take 3 times as long to heat up and cool down the hydrogen than helium.

So in fact, the best gas for a Stirling engine is one where the ratio of thermal conductivity to heat capacity is the highest, not one that has a high value for both characteristics.

Is that right?

 

[jrmichler]

There are a large number of variables, of which you identified two. I suggest getting one or two books on the subject, and enjoy some good reading. Two good resources, both of which are currently available from Amazon, are:

Stirling Engine Design and Feasibility for Automotive Use, edited by M.J. Collie, 1979.
Air Engines The History, Science, and Reality of the Perfect Engine, by T. Finkelstein and A.J. Organ, 2009.

From the Collie book: "Hydrogen is the best because it has the highest thermal conductivity, the lowest viscosity, and a low heat capacity ... However, hydrogen permeates through metals ... On the other hand, helium is inert and can be permanently contained in metal. It has an even lower volumetric heat capacity than hydrogen and almost as good a thermal conductivity, but the viscosity is twice that of hydrogen. Michels [76e] showed that a Stirling engine can be designed to use either hydrogen, helium, or nitrogen all with the same efficiency for the same temperature. However, the heater, cooler, and regenerator of each engine would be designed quite differently."

Would it surprise you to know that once upon a time, I also was interested in Stirling engines? Enough to buy books on the subject, but not enough to actually try to build one.