An improved, free-
piston, Stirling
machine having at least three pistons series connected in an alpha Stirling configuration. Each cylinder is stepped so that it has a relatively larger
diameter interior wall and a coaxial, relatively smaller
diameter interior wall. Each
piston is also stepped so that it has a first component
piston having an end face facing in one axial direction and matingly reciprocatable in the smaller
diameter cylinder wall and a second component piston having an end face facing in the same axial direction and matingly reciprocatable in the larger diameter, cylinder wall. One of the piston end faces bounds the compression space and the other end face bounds the expansion space. Preferably, each stepped piston has
peripheral, cylinder walls that are axially adjacent and joined at a shoulder forming the end face of the larger diameter component piston. Stirling machines with these stepped features are also arranged in various opposed and duplex configurations, including arrangements with only one load or
prime mover for each opposed pair of pistons. Improved balancing or vibration reduction is obtained by connecting expansion and compression spaces of a four cylinder in-line arrangement in a 1, 3, 2, 4 series sequence. Three cylinder embodiments provide a highly favorable volume
phase angle of 120° and are advantageously physically arranged with three, parallel, longitudinal axes of reciprocation at the apexes of an equilateral triangle.