Question

In an internal combustion engine, during the compression stroke the heat rejected to the cooling water is 50 kJ/kg and the work input is 100 kJ/kg. 

Calculate the change in internal energy of the working fluid stating whether it is a gain or loss.

Answer 1

Heat rejected to the cooling water, Q = – 50 kJ/kg 

(–ve sign since heat is rejected) 

Work input, W = – 100 kJ/kg 

(–ve sign since work is supplied to the system) 

Using the relation, Q = (u₂ – u₁) + W 

– 50 = (u₂ – u₁) – 100 or 

u₂ – u₁ = – 50 + 100 = 50 kJ/kg 

Hence, gain in internal energy = 50 kJ/kg.