Heat Engine 

It is a device which convert heat energy into mechanical energy in a cyclic process. 

It has three essential parts : 

i) Source - It is a heat reservoir at high temperature (T1). It has infinite thermal capacity. Any amount of heat can be taken out of it without changing its temperature. 

ii) Sink - It is a heat reservoir at a lower temperature (T2). It also has infinite thermal capacity. Any amount of heat can be added to it without increasing its temperature. 

iii) Working Substance - It is a substance which perform the mechanical work when heat is supplied to it. In steam engine, steam acts as a working substance while in a diesel engine, a mixture of fuel vapours and air acts as the working substance. 

Working of Heat Engine : 

In each cycle, the working substance absorbs a definite amount of heat (Q1) from the source at temperature T1, convert a part of this energy into mechanical work (W) and reject the remaining heat (Q2) to the sink at lower temperature (T2). The work done by the working substance is transferred to the environment by some arrangement.

Efficiency of a heat engine (η)

It is defined as the ratio of net work done by the engine in one cycle to the amount of heat absorbed by the working substance from the source. 

This is a cyclic process i.e. system returns to its initial state. Hence, change in temperature, ΔT=0 so ΔU=0
According to first law of thermodynamics
ΔQ=ΔU+ΔW

ΔQ=0+ΔW

ΔQ=ΔW

Q1-Q2=ΔW

Efficiency (η) = ΔW/Q1

η=(Q1-Q2)/Q1
η=1-(Q2/Q1)

Note : 

⭐ Efficiency of a heat engine is always less than unity. When Q2=0 then η=1 or 100% which is not possible. 

⭐ The efficiency of steam engine varies from 12% to 16%.

⭐ The efficiency of petrol engine is 26%.

⭐ The efficiency of diesel engine is 40%.