Polytropic processes on a PV plot

[eurekameh]

1 -> 2 : Isochoric heat addition
2 -> 3 : Polytropic expansion
3 -> 1 : Isobaric compression back to the initial state

Sketch this cycle on a pressure-volume plot in relation to the saturation curve.
Now, my question is, why is the polytropic process on the PV plot concave up and not down, as shown on http://imageshack.us/photo/my-images/685/unledmhi.png/ ?

 

[Andrew Mason]

The plot of the polytropic process depends on the value of n in $PV^n = K$. If n = Cp/Cv it is an adiabatic process. If n = 1 then it is an isothermal process (PV = nRT = constant). If n=0 it is an isobaric process. None of those will provide a convex PV graph.

AM