WHAT IS HEAT ENGINE IN THERMODYNAMICS?

We were discussing the concept of control volume system and concept of specific enthalpy in the field of thermal engineering in our previous posts, where we have seen the fundamentals of a control volume system as well as the specific heat and types of specific heat also.

Let us go ahead to discuss the concept of a heat engine and a heat pump in thermodynamics.
We will see here the fundamentals of heat engine and after that we will see the principle of heat pump in our next post. We will finally find out here the thermal efficiency of a heat engine.

So let us first see here the concept of a heat engine

Heat engine is basically defined as a device which will be operated in a thermodynamic cyclic process and will convert the heat energy or thermal energy in to mechanical energy. Following figure, displayed here, indicates the fundamentals of a heat engine.

Heat engine

So as we have studied above, during definition of a heat engine, that heat engine will provide mechanical energy i.e. work energy or we may say that output of a heat engine will be mechanical energy i.e. work energy.

As we know that heat engine converts thermal energy or heat energy in to mechanical energy, therefore we can say that thermal energy or heat energy will be termed as the input energy to a heat engine.

As we can see above in figure, heat engine will have three important terms and these are as mentioned below

Thermal reservoir at higher temperature or source of heat energy
Working media or substance
Thermal reservoir at lower temperature or sink of heat energy

According to the second law of thermodynamics, heat energy will be given by higher temperature thermal reservoir i.e. from the source of heat to heat engine and heat engine will convert a part of input heat energy in to useful work energy and rest part of input heat energy will be directed towards the sink of heat i.e. lower thermal reservoir.

Efficiency of a heat engine

Let us determine the efficiency of a heat engine, before going for calculation we need to know the each term used here and these are as mentioned below.

Temperature of hot thermal reservoir is T_H

Temperature of cold thermal reservoir is T_C

Heat energy provided by source of heat or hot thermal reservoir is Q_H

Useful work energy produced by heat energy is W

Rest quantity of heat energy rejected to sink of heat or cold thermal reservoir is Q_C

Efficiency of a heat engine will be determined by the ratio of output term i.e. useful work energy to the input term i.e. input heat energy. Efficiency of a heat engine could be easily written as displayed in following figure.