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Alpha vs Beta

Posted: Tue Aug 28, 2018 10:11 pm
by larfalitl
Have read most that can be found on the 'net, & keep finding what I'll call "unsupported statements". They may very well be true, but I can't see the basis of them. I'm hoping someone can shed some light on them for me.

The first being:-
"The Alpha configuration apparently has a better power-to-weight ratio than the Beta, because the Alpha has two pistons" - from here viewtopic.php?f=1&t=2288
I've read this plenty of other places also.

Having two pistons requires two sets of seals. Having one larger piston would give a smaller sealing area per area of piston face.
If we have a 100cc engine with one 10 square cm piston or a 100cc with two 5 square cm pistons. Same volume, same piston face, same compression ratio, same heater, cooler, regen surface. Why is the Alpha a more powerful engine?

It may be due to this from page 7 of Andy Ross' "Making Stirling Engines"

"a drive rod from the displacer extend through the piston, which is a subtle but important mechanical disadvantage."

The displacer rod must seal in a gland somewhere. In a cylinder head in a gamma or in the piston in a beta.

Andy also states that a gamma is not as efficient as an alpha on page 21. Maybe it's an issue with the displacer, as free piston engines have not taken over the world either.

Hoping you can enlighten me.


Re: Alpha vs Beta

Posted: Wed Aug 29, 2018 3:08 am
by Ian S C
I can assure you that an ALPHA motor has a higher power to weight over either a BETA, or GAMMA motor. The swept volume of the cylinders of an ALPHA is equal, ie 1:1, I'v only built one unpressurised Ross Yoke ALPHA motor, 35 mm bore x 22 mm stroke, about 5 Watt at the crankshaft.
Ian S C

Re: Alpha vs Beta

Posted: Fri Aug 31, 2018 2:41 am
by MikeB
So if the power-to-weight advantage is due to the 1:1 ratio, why isn't that used for the other configurations?

Re: Alpha vs Beta

Posted: Fri Aug 31, 2018 4:50 am
by Ian S C
No the ALPHA motor is a different motor, the hot and cold cylinders both generate power, where as the other Stirling type motors have the one power cylinder, and use some of that power to drive the displacer. My second build is based on Dyna, a BETA motor designed by James G. Rizzo, it has a bore of 1 3/4", a power stroke of 3/4", and a displacer stroke of 1 3/16", and has a similar power output to the above ALPHA motor, it's still going well, built in 1994. It did over 1000hrs in its first year, and wore out the original crankshaft, and conrods.
Ian S C