In applying the optimization Table 10.1, it is important to keep in mind the interchange between mechanical efficiency and maximum shaft output. As can be seen from the table, at the maximum shaft output point, the mechanical efficiency is most often significantly below the mechanism effectiveness. A better mechanical efficiency can be obtained by taking a swept volume ratio smaller than ?*. In most cases, just a slightly smaller ? will considerably improve mechancal efficiency while resulting in only a relatively small reduction in shaft output. This is illustrated in Figure 10.4, which gives a graphical picture of how the swept volume ratio affects specific indicated work, specific shaft work, and mechanical efficiency for the case of an engine with parameter values ? = 0 .5, ? = 0 .5, E = 0 .75, and ? = ?/2.

Figure 10.4: An example of the variation of specific indicated work, specific shaft work, and cyclic mechanical efficiency with respect to swept volume ratio for a gamma-type Schmidt Stirling engine.

The specific works for the example of Figure 10.4 are calculated just as in the optimization, that is, relative to total swept volume and mean cycle pressure. Although the peak specific shaft output occurs at a swept volume ratio just below 0.8, there is not much reduction in shaft output with ? = 0 .6 or even 0.5, and mechanical...