Real cicle of a double-acting steam engine.
1) Inlet stroke (A-B).
2) Expansion stroke (B-C).
3) Exhaust stroke (C-D).
4) Exhaust and compression stroke (D-E-F-A).
Opening the inlet-valve (A) the steam enter the cylinder with a pressure a little inferior than that of boiler. Until the inlet-valve is opened the steam pressure in the cylinder keeps constant because the piston goes forward in the right direction.
The inlet stroke is an isobaric stroke (segment A-B parallel to the volume axis).
At the point B the inlet-valve istantly closes. The steam expansion continues and drives the piston to the right dead point.
The steam pressure decreases while the volume increases.
This stroke is made without heat exchange with the outer ambient and is an adiabatic curve.
At the point C, when the exhaust-valve opens, ends the expansion stroke: the steam pressure immediately decreases.
This part of exhaust stroke is an isometric curve represented by the segment C-D parallel to the pressure axis.
This is the fact provided by Carnot: the cold point. If the steam is let out in a condenser, the point D drops, the cicle area increases as well the mechanical work produced.
At the point D the piston begins his return stroke. Up to the point E the pressure keeps constant because the exhaust valve is open.
This part of the cicle is an isobaric.
At the point E the exhaust valve closes. The piston continues his return stroke, the steam, closed inside the cylinder, decreases his volume and increases his pressure.
From the point E to the point F the curve is an adiabatic because none heat exchange is possible.
When the piston reaches the left dead point (point F) the inlet valve opens, the steam pressure immediately increases.
This part of the cicle is an isometric because the volume can't increase.
All begins again.