Examination of the optimization table reveals that the optimum phase angle ?* tends to be lower for poorer mechanisms and for higher temperature ratios. Although 90 is usually considered near enough to the optimum for all practical purposes in gamma engines, the table shows values lower by more than 10 in some cases. Of course, in most split-cylinder engines, the phase angle can be easily changed to best suit operating conditions.

On the other hand, engine performance is not extremely sensitive to phase angle. Figure 10.5 shows how the engine of Figure 10.4 responds to variations in its phase angle. The swept volume ratio for this example was taken to be ? = 0 .75, for which chosen value the best phase angle is near 80 . The table in the figure gives the calculated specific work values in a neighborhood of this point; it shows that shaft output is virtually the same for 10 on either side of the optimum 80 . Note that the maximum indicated work in this example occurs at 90 , for the fixed swept volume ratio.

Figure 10.5: Variation of specific indicated and shaft work with phase angle for the gamma engine of Figure 10.4 with a fixed swept volume ratio of 0.75.