What is the Difference Between Isentropic and Polytropic Process

The key difference between isentropic and polytropic process is that isentropic process always shows a lower efficiency, whereas polytropic process always shows a higher efficiency.

Isentropic process is a thermodynamic process in which both adiabatic and reversible natures can be observed. Polytropic process, on the other hand, is any reversible process on any open or closed system of gas or vapor involving both heat and work transfer in such a way that a specified combination of properties is maintained throughout the process.

1. Overview and Key Difference
2. What is an Isentropic Process
3. What is a Polytropic Process
4. Isentropic vs Polytropic Process in Tabular Form
5. Summary – Isentropic vs Polytropic Process

What is an Isentropic Process?

An isentropic process is a thermodynamic process in which both adiabatic and reversible natures can be observed. In this process, the work transfers of the system tend to be frictionless and occur without the transfer of heat or matter. This is an idealized process that is useful in engineering as a model for comparison with real processes. We can idealize it as a reversible process that does not occur in reality. This is because a process that is both adiabatic and reversible has to have equal initial and final entropies, which leads to naming it as an isentropic process. However, we can interpret this term in another way, i.e., a system that has its entropy unchanged.

Isentropic vs Polytropic Process in Tabular Form

Figure 01: In a T–s (entropy vs. temperature) diagram of an isentropic process. Here, the entropy remains constant

Spontaneous processes increase the entropy of the universe. When this happens, either the system entropy or the surrounding entropy may increase. An isentropic process happens when the system entropy remains constant.

A reversible adiabatic process is an example of an isentropic process. Moreover, the constant parameters in an isentropic process are entropy, equilibrium, and heat energy.

What is a Polytropic Process?

A polytropic process can be described as any reversible process on any open or closed system of gas or vapor involving both heat and work transfer in such a way that a specified combination of properties is maintained throughout the process. It occurs with a heat transfer. However, the heat transfer happens reversibly in this process. When a gas undergoes this type of heat transfer, the following equation is true for a polytropic process.

PVn = constant

Here, P is the pressure, V is the volume, and n is a constant. Hence, to hold PV constant in the polytropic gas expansion/compression process, both heat and work interchange takes place between the system and its surrounding. Therefore, polytropic is a non-adiabatic process.

What is the Difference Between Isentropic and Polytropic Process?

An isentropic process is a thermodynamic process in which both adiabatic and reversible natures can be observed. A polytropic process is a reversible process on any open or closed system of gas or vapor involving both heat and work transfer. The key difference between isentropic and polytropic process is that an isentropic process always shows a lower efficiency, whereas a polytropic process always shows a higher efficiency.

The below infographic presents the differences between isentropic and polytropic process in tabular form for side by side comparison.

Summary – Isentropic vs Polytropic Process

Isentropic processes and polytropic processes are important in physical chemistry. An isentropic process is a thermodynamic process in which both adiabatic and reversible natures can be observed. A polytropic process is a reversible process on any open or closed system of gas or vapor involving both heat and work transfer. The key difference between isentropic and polytropic process is that an isentropic process always shows a lower efficiency than a polytropic process.