Pressure Envelope Effects

The "Net" work produced by a machine may be reduced by the pressure envelope. For example, a piston may have 1 atmosphere of pressure on one side, 4 on the other. The net force is 3 atmospheres. As the volume expands, the inside pressure falls to 1 atmosphere, the net force falls to zero. So, although the piston can be driven further, and that would result in greater conversion of heat to work, it takes more work to continue expansion than it produces. (In fact it produces negative work.)

Some machines, such as Automobile Engines, have characteristics that part of each cycle works against the Pressure Envelope - the Power Stroke, and part works with the pressure Envelope - the Compression stroke. The Pressure Envelope has effectively no net change to the exportable work. However, it still creates limits on the machine, a power stroke must maintain pressure greater than the Envelope, or do negative net work on part of its travel.

Steam turbines' "volumes" are constantly increasing in size - pushing the Pressure Envelope away. The steam eventually collapses back into water, so the cycle is closed, but closed outside the turbine engine. The net work of these engines is directly reduced by the envelope. However, even a tiny pressure increase, less than the pressure of the Envelope, can continue the Turbine's movement.

Exportable work, is the wok deliverable by a Heat Engine to the outside world. The work done by a vapor to expand its volume is independent of the outside of the Heat Engine. The work delivered by the machine, piston, turbine, etc. is the net Force time Distance the engine can deliver. One part of making a 100% efficient engine, is that the net work done inside the expanding volume equals the net work the engine can supply.