Prelecture 31: Slide 8

On the last slide, we calculated the efficiency of a Stirling engine and found it to be less than the Carnot efficiency. In the next prelecture, we will develop a general expression for the efficiency of a heat engine in terms of the increase in the entropy of the engine and its environment which will explain exactly why the Stirling efficiency is less than the Carnot efficiency.

We close with a final note of interest to those of you who may be wondering how to actually build a Stirling engine. Since it is not practical to repeatedly heat and then cool the same walls of a cylinder, the usual design uses some method to move the gas back and forth between the hot part and the cold part of the cylinder so that the temperatures of these parts never have to change. In the design shown the gas is moved between the hot bottom and cold top of the cylinder by a displacer. Notice that the displacer is smaller than the cylinder so it does not compress the gas, it just moves the gas around. We will discuss real-world designs more in the next prelecture.