Piston Displacer or Dual Piston Engine

[sham.MY]

Based on this picture,
are this Stirling engine a Piston Displacer Engine or a Dual Piston Engine?
Hope you guys would help me with this.

[Ian S C]

The engine shown is commonly known as an ALPHA type motor, this is what you call a dual piston engine. The format of this one is a Ross Yoke type, developed by Andy Ross in the USA. Hot piston on the left, cold on the right.
Ian S C

[sham.MY]

I have found this formula in a Stirling Engine Design Manual Book

P = Bn x p x f x Vo
P - engine power
Bn - beale number
p - pressure
f - frequency
Vo - ?

i have determine all the value of P, Bn, p and f. I just want to know what is Vo represent,
is Vo a total volume or expansion volume?
If both are not, what is Vo?