MHT-CET Physics Crash Course “Heat Engine” Class 12

“Heat Engine” MHT-CET Class 12 Physics Crash Course | Chapter Thermodynamics

In this session G.S. Khairnar discussed the theory and MCQ’s of “Heat Engine”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.

Heat Engine:-

          Hello dear students, we have learnt about the laws of thermodynamics, thermodynamic process and their expressions. According to second law of thermodynamics, the mechanical work can be completely converted in to heat but heat cannot be completely converted in to mechanical work. Thus we can say the energy given to the system always used by it in two parts i.e. part of energy to do the work and the part rejected or lost in surroundings.

In this article we are going to discuss about the most important application of thermodynamic system and law of thermodynamics.

Let’s learn the concept heat engine….!

The device which uses heat energy and converts it in to mechanical work is known as heat engine.

 Working of heat engine is cyclic process and involves the thermodynamic changes of variables like pressure, volume and temperature. The block diagram of common heat engine is given below.

Parts of heat engine:-

Heat engine mainly consists of three parts. 1) Hot reservoir (Working substance), 2) Cylinder and 3) Cold reservoir.

  • Working substance is considered as the source of heat energy. It uses mixture of fuel vapour (petrol/ diesel) and air to produce high amount of heat which is further used to do the work. Due to its high temperature this part is termed as hot reservoir.
  • Cylinder is consists of massless and frictionless piston connected to the mechanical body of engine which performs the work.
  • Cold reservoir takes the heat rejected from the cylinder and sends it in to surroundings by means of exhaust. It’s also known as sink.

Working of heat engine:-

⸎ Let Q1 be the heat produced at the hot reservoir. This heat is then transferred to the cylinder.

⸎ Due to high temperature, the gas in the cylinder gets more pressure and momentum due which helps the gas for expansion.

⸎ The gas falls on the piston and exerts momentum on the walls of piston so that it moves and the volume of cylinder gets expanded.

⸎ Here the piston does work on the mechanical part. Let ‘W’ be the work done by the piston due to heat Q1.

⸎ As we know according to 2nd law of thermodynamics, the heat cannot be completely used to produce the mechanical work as it gets lost in some extent in to surrounding.

⸎ Let Q2 be the heat rejected or the lost during this process is then transferred to the cold reservoir and then finally sent to surrounding by the means of exhaust valve.

⸎ Once the heat is converted in to work, the piston again comes back to its initial position by the means of compression. This decreases the volume again and the process is repeated again and again.

⸎ Hence the working of heat engine is cyclic process.

Efficiency of heat engine:-

Then the efficiency heat engine is the ratio of amount of work done by the engine to the amount of heat supplied to the engine,

From the above equation if there is no heat lost in surrounding i.e. Q2=0, then the efficiency of heat engine would be η=1

But in practice there is always loss of heat is takes place and hence no heat engine can be 100 % efficient.