Agree with this.
When the engine is cold (or strictly speaking when the 'hot end' is at the same temperature as the 'cold end' there is no change in the volume of the air as the displacer goes up and down and moves the air about inside the engine If you then turn the engine in this state you actually get small amounts of resisting 'push' and 'pull' against the power piston as it compresses or expands the air around its neutral volume at that temperature.
However, as the 'hot end' is warmed relative to the 'cold end' you get very small 'pushes' and 'pulls' on the power piston just as the air is heated and cooled when the displacer moves the air around the engine. The 90 degree phase lag between the displacer piston and the power piston means that these small 'pushes' and 'pulls' on the power piston are timed just right to keep building momentum in a smoothly running system.
It is. a bit like giving a child's swing a small push at just the right time in each cycle. After a while, the small inputs can build to deliver quite a large output.
Again, I disagree with some of this. On a gamma engine:
There is no 'pull', ever. There is only pressure, more pressure and less pressure, on either side of the power piston. On a cold unpressurized power cylinder, there is an appearance of 'pull' when one mechanically moves the power piston in such a way as to increase the volume in the sealed area (lowering its' pressure) and then the atmospheric pressure on the outside acts to -push- the power piston 'inward'.
The sealed area has a volume which changes as per the location of the power piston (not the displacer). The working gas (air or otherwise) fills
that volume at a pressure determined by the temperature and the 'amount' (not volume) of air, which can be increased by pressurization (adding more molecules).
The pressure on the piston is not necessarily 'very small'. I read of a marine engine with a bore and stroke measured in -feet-, it is unlikely the torques involved in that case would be considered small.
I feel the difficulties encountered on some very small engines, even if relatively well sealed could arise because of the relative relationship of the low
available motive power to friction. There is just not very much of a delta there. As the engine size goes up, the available power goes up faster than the additional friction.
Short connecting rods on very small engines might also lead to problematic side load friction. Most of the smooth and slowly running examples I have seen have long connecting rods (and sizeable flywheels).
I admit to being new to machining, fairly new to Stirling theory, but not so much a newbie with mechanics and/or physics.
And maybe to being a little sensitive to being called out as 'wrong'.