It has taken me far too long to realize the true nature of PV=nRT. I struggled with how to relate it to a free expansion, where the piston is allowed to travel faster than the expanding gas. Instead, and it may be obvious to many, PV=nRT as related to an expansion, describes the maximum affect possible for a given volume of heated gas in real world conditions. 100cc of 300k gas at 1 bar, becomes 200cc of 600k gas at 1 bar. Along the way it did work on the atmosphere, and in fact, all the work it could ever possibly do in a real engine.
The buffer pressure represents the starting base load that also sets the conditions for inside the power cylinder. If the starting conditions remain the same, yet more load is added externally(10lbs of weight), the gas will not be able to expand to twice its volume. While the gas will still expand to 1 bar at a smaller than 200cc volume, and lift the extra weight, this is an inefficient use of heat.
The extra load, presented in weight, took away a large percentage of the overall energy available in the expanding system.
We can remove the buffer pressure entirely and replace it with weights(mass). If the piston is one square inch we can use a 15lb weight to replace the buffer pressure. It is now much easier to see the work that was done in the initial expansion.
Let's now make the initial starting condition 100cc of 300k gas at 10 bar with a 150lb weight(in place of a 10 bar buffer pressure). If the external load is still only 10lbs, it is now only a fraction of the overall energy available in the expanding system. That means only a fraction of the temperature is required, and the added gas density makes heat transfer more effective.
Added compression ratio, by going beyond the zero point, will certainly allow an atmospheric or low pressure engine to make more power and appear more efficient. But the added efficiency of compression in an external combustion engine is only an illusion. The extra expansion and compression work is wasted in the pursuit of higher power output, when higher buffer pressure and much lower temperatures can be used instead. That is not to say it's a wasted effort, as high output atmospheric engines are likely to be adopted first.
There was no need to substitute the buffer pressure with a 150lb weight, other than to show that along with satisfying the initial 10lb load, we also now have a 150lb oscillating mass. If that sounds inconsequential, just put your finger under it.