When we were discussing fundamentals of thermal engineering, we have discussed various basic concepts of thermodynamics such as “Thermodynamic cycle” and also “Work and heat in thermodynamics”.

Today we will see here the classification of thermodynamic cycles in the subject of thermal engineering with the help of this post.

### Brief introduction and importance of a thermodynamic cycle

Before going to classify the thermodynamic cycles, we must have to remind the concept of a thermodynamic cycle and also we need to understand here the importance or requirement of a thermodynamic cycle. Why we need to read the thermodynamic cycle? We will secure this answer here in this post.

Thermodynamic process or series of processes will be termed as thermodynamic cycle, when thermodynamic system undergoes through such changes of state that initial state is similar or identical with initial state.

Now we will try to understand here the importance of a thermodynamic cycle. We have seen during studying second law of thermodynamics that heat energy could be converted in to work energy and work energy could be converted in to heat energy.

But we must note it here that in order to secure continuous conversion of heat energy in to work energy or work energy in to heat energy, a thermodynamic system will have to undergo through a cyclic process. Therefore we can say that a thermodynamic cycle is required for securing the continuous conversion of heat energy in to work energy or work energy in to heat energy.

Now as we have seen in our previous post “thermodynamic reversible cycle” that a thermodynamic cycle will be termed as reversible cycle if each process of cycle is reversible otherwise cycle will be termed as irreversible cycle. For a reversible cycle, each process must be reversible process. If anyone process of a cycle is not reversible process then that cycle will be termed as irreversible cycle.

### So let us see the classification of thermodynamic cycles

Thermodynamic cycles are basically classified in two categories, first one is heat engine cycles and second one is heat pump or refrigeration cycles.

If a thermodynamic cycle is receiving heat energy as input energy and producing work energy as output energy or we can say that if a thermodynamic cycle is converting heat energy in to work energy then that thermodynamic cycle will be termed as heat engine.

As we are securing here mechanical energy in terms of work energy from heat engine, therefore heat engine is also termed as power cycle. For more detailed explanation about thermodynamic heat engine, we must find the post "Heat engine in thermodynamics".

If a thermodynamic cycle is converting work energy in to heat energy then that thermodynamic cycle will be termed as heat pump or also refrigeration cycle. For more detailed explanation about thermodynamic heat pump, we must find the post "Heat pump in thermodynamics",

On the basis of phase or state of working fluid, heat engine cycles or power cycles are classified in two categories. First one is vapour power cycle and second one is gas power cycle.

### Vapour power cycles

If the working fluid of power cycle changes its state during completing the cyclic process i.e. some process of cycle will have working fluid in the state of liquid and some process of cycle will have working fluid in terms of vapour then such a cyclic process or thermodynamic cycle will be termed as power cycle.

#### Vapour power cycles are classified in following types

1. Carnot Cycle
2. Rankine cycle
3. Regenerative cycle
4. Reheat cycle

### Gas power cycles

If the working fluid of power cycle is in the state of gas and does not change its state during completing the cyclic process i.e. such power cycle will have working fluid in the state of gas only then such a cyclic process or thermodynamic cycle will be termed as gas power cycle.

#### Gas power cycles are classified in following types

1. Otto cycle
2. Diesel cycle
3. Dual cycle
4. Gas turbine cycle

We will discuss each and every thermodynamic cycle as listed above in our upcoming posts.

Further heat engines are also classified, in following two types, on the basis of methods of supplying of heat to the working fluid or system.
1. Internal combustion
2. External combustion

### Internal combustion

Heat energy will be added here to the working fluid of cycle by burning the fuel within the system boundaries such as automobile engines

### External combustion

Heat energy will be added here to the working fluid of cycle by one source external to the system such as steam power plants.

We will see another topic i.e. “A brief introduction to steam power plant” in our next post in the category of thermal engineering.

Do you have suggestions? Please write in comment box

### Reference:

Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by Prof S. K. Som