Efficiency : The fraction of the total amount of heat absorbed by an engine which it can convert into work is known as efficiency of the engine. thus,

Efficiency of a heat engine (η) = Work done by  the engine / total heat absorbed at a upper temperature T₂

η = w / q₂ [ from equation (12)]

= T₂ – T₁/ T₂  Equation………..(14)

Since T₂-T₁/T₂ is invariably less than 1, the efficiency of a heat engine is always less then unity. In other words, no heat engine has yet been constructed whose efficiency is equal to unity. Such a possible is denied by second law of thermodynamics.

It also follows from equation (14) that the efficiency of the heat engine depends upon the magnitude of difference between temperature T₂ and T₁, i.e, greater the difference between the temperature of the source and sink, greater is the efficiency of the heat engine. This also justifies the use of superheated steam in steam engines as compared to simple steam.

According to the first law of thermodynamics,

“Net work done by the system = net amount of heat absorbed by the system.“

Since net work done by the system is w and net vamount of heat absorbed by the system q is equal to q2-q1, therefore,

w = q₂- q₁

on combining this equation with (14), we get,

= q₂-q₁/q₂ = T₂-T₁/T₂

Efiiciency (η) = q₂-q₂ / q₂ = T₂-T₁/T₂  Equation………..(15)

The above equation has been derived by assuming that various steps in the above cycle were brought about in a thermodynamically reversible manner so as to get maximum work. Since this cannot be achieved in actual practice, therefore, efficiency of the heat engine is even less that given by the above equation.