Here's my attempt at describing what I think a practical Stirling engine cycle is. This one's for you Matt, to help explain what I meant in your hot space v. cold space thread. Note that this is also why I am keen on either modifying the cycle, or moving to a hybrid open/otto cycle that is much easier to maintain in continuous operation.
To do this I'll use a purely hypothetical situation to help visualize the cycle as I see it. As always, this is just my opinion.
We'll start with the basic 300k-600k cycle.
Now for the hypothetical. Bob is our engine builder, and he lives on a planet that has an atmospheric condition of 450k at 14.7psi. This planet happens to have 300k ground water, and abundant geothermal hot springs of 600k.
Bob says, "Wow, I have free heating AND cooling available to influence my atmospheric conditions".
Bob builds a Gamma type engine, and lets its internal pressure equalize with his atmosphere while the power piston is in the middle of its stroke. In this way the engine at base temperature will have internal pressure>atm at TDC, and internal pressure<atm at BDC. In this way Bob has created a perfectly balanced compression and expansion ratio.
Now we apply the temperature difference and our Stirling cycle is complete.
Now back to Earth. It's all about perspective. We have the same conditions available to us, we just don't see it as such because of the atmosphere we live in.
As stated, in practice it's hard to keep the internal mass of air in the sweet spot, and we end up with a bias of less mass(overall more expansion work) or more mass(overall more compression work) in the system.