Stirling cycle air engines (especially of the unpressurised variety) are not renowned for power output. Most struggle to overcome their own friction.
They've had very little influence on the world, except for a brief burst of popularity in the 19th century. This was before the advent of IC engines, and when not requiring a boiler certificate was enough advantage to offset the fundamental handicap of low specific output (by size and cost).
But, Stirling engines have been fascinating people for the nearly 200 years since Robert Stirling demonstrated his first version in 1816.
And, many engineers and enthusiasts continue to invest lives and fortunes in their design, development, and promotion.

As the standard power measurement systems by which engines are compared do not showing Stirling engines in an advantageous way, I believe they should have their own special power scale (with apologies to Watt and a nod to Beaufort):

To be called:

The Jude*Scale

Jude 0: No discernable preference for rotational direction.
Jude 1: Clear rotational direction preference but not self sustaining.
Jude 3: Almost self sustaining; tantalisingly close to.
Jude 4: Occasionally self sustaining, but sometimes stops spontaneously.
Jude 5: Mostly self sustaining- almost never stops spontaneously.
Jude 6: Easily self sustaining- with excess enough to run its cooling fan.
Jude 7:Some power; Could spin the "Lotto" wheel (for small numbers).
Jude 8: Useful power; like from half an unfit person on an exercise machine.
Jude 9:Really useful output; enough to power a mobility scooter (but not uphill).
Jude 10: Lots ? (never yet attained by any unpressurised air engine).

*After Saint Jude the apostle- and patron saint of lost causes

Peter Lynn, Ashburton New Zealand , March 2012

PS: Having made 14 or so original Stirling engines since 1969, I'm barely averaging Jude 5 so far- and can claim two zero's- one of them just last year.

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