A polytropic process is any thermodynamic process that can be expressed by the following equation:

pV^{n} = constant

The polytropic process can describe gas expansion and compression which include heat transfer. The exponent n is known as the polytropic index and it may take on any value from 0 to ∞, depending on the particular process.

There are some special cases of n, which corresponds to particular processes:

the case 1 < n <, in this process heat and work flows go in opposite directions, This process occurs, for example, in vapor compression refrigeration during compression

the case < n < ∞, in this process heat and work flows go in the same direction, This process occurs, for example, in an internal combustion engine (e.g. Otto cycle), in which there are heat loses through the cylinder walls during gas expansion (power stroke).

For a polytropic process between two states:

p_{1}V_{1}^{n} = p_{2}V_{2}^{n}

References:

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).

J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.