Heat Engine | Efficiency, Definition, Advantages, FAQs

Any "cyclic" device by which heat is converted into mechanical work is called a heat engine. Efficiency, Definition, Advantages, FAQs

Heat Engine

Any "cyclic" device by which heat is converted into mechanical work is called a heat engine. For a heat engine there are three essential requirements :

  • Source:- A hot body, at a fixed high temperature T1 from which the heat can be drawn heat, is called source or hot reservoir.
  • Sink:- A cold body at a fixed lower temperature T2 to which any amount of heat can be rejected, is called sink or cold reservoir.
  • Working Substance:- The material, which on being supplied with heat, performs mechanical work is called the working substance.
Heat Engine
Heat Engine

In a heat engine, the working substance takes in heat from the source, converts a part of it into external work, gives out the rest to the sink and returns to its initial state. This series of operations constitute a cycle. The work can be continuously obtained by performing the same cycle over and over again.

Suppose the working substance takes in an amount of heat Q1 from the source, and gives out an amount Q2 to the sink. Let W be the amount of work obtained. The net amount of heat absorbed by the substance is Q1 - Q2, which has been actually converted into work. Applying the first law of thermodynamics to one complete cycle. We get

Q1 - Q2 = W

Thermal Efficiency

The thermal efficiency (e) of an engine is defined as the ratio of the work obtained to the heat taken in from the source, that is,

`\e=\frac W{Q_1}=\frac{Q_1-Q_2}{Q_1}`


This equation indicates that the efficiency of the heat engine will be unity (efficiency 100%) when Q2 = 0. This is, however, not possible in practice, This means that the engine cannot convert all the heat taken in from the source into work.

Advantages of Heat Engines

  • It helps mankind to discover various machines.
  • It helps to regulate temperatures.
  • The cycling procedures are more advantageous than normal procedures.
  • As the fluids are allowed to high temperatures, we can yield high efficiency.
  • It consumes very less time and less cost.
  • The procedure and apparatus are also easy to use and simple to fix.


Hence, the heat engine is a system of converting heat energy into mechanical work. The efficiency of the heat engine is the difference between a hot reservoir and a cold reservoir. We have delivered the formula to find out the efficiency of a heat engine. Also, we can't get 100% efficiency for any heat engine.

Frequently Asked Questions (FAQs)

Que:- Can room be cooled by leaving the door of an electric refrigerator open?

Ans:- A room cannot be cooled by having opened the door of the refrigerator placed in it. On the contrary, the room will warm up. The reason is as follows :

The refrigerator removes heat from its interior and expels it into the surrounding air, thus warming the air. For doing this, additional energy is supplied to the refrigerator by an electric motor. The heat expelled into the air is the sum of the energy from the motor and that removed from the interior of the refrigerator. In other words the refrigerator adds more heat into the room than it removes from its anterior. On opening its door or it will run continuously and hence add even more heat to the room than when its door is closed.

Que:- Is it Possible to Build a 100% Efficient Heat Engine?

Ans:- Of course not. The thermal efficiency can be defined as a fraction of the amount of heat given to the amount of heat acquired from the heat engine. If we provide 100% heat energy, it gets reduced to its lowest level after the process has been done. So at every point in time, we will observe a certain amount of difference between those two amounts and that difference is nothing but the thermal efficiency which could be neither 1% nor 100%.

Read also

  1. Laws of Thermodynamics
  2. Concepts of Thermodynamics
  3. Van der waal's Equation

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