As the designer of the engine1 featured in Dr. Mahkamov’s paper,2 I welcome his pioneering application of CFD modeling to the Stirling cycle. The technique would have been helpful in 1996 when this engine was on the drawing board and then during testing in 2003, when absence of dynamic pressure measuring equipment hindered diagnosis of the problem he subsequently identified by modeling—flow restriction caused by a dimensional error in the crown of the power piston.

Unfortunately, however, his models do not appear to have achieved the close agreement between predicted and actual power outputs required to justify fundamental design decisions or, as in this case, a change of operating mode from the engine’s original gamma layout to the less efficient alpha regime. Some explanations for the poor correlation between prediction and reality, set out below, may be found in...

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