MODIFIED RANKINE CYCLE

In modified Rankine cycle to reduce friction the stroke length is reduced.

Figure 5.6(a) and 5.6(b) show the modified Rankine cycle on p-v and T-s diagrams respectively, by neglecting pump work.

It may be observed that p-v diagram is very narrow at the toe. i.e., point 3′. The work obtainable near to 3′ is very small which is too inadequate to overcome friction due to reciprocating parts of engine. Therefore the adiabatic process 2-3′ is terminated at point 3. The pressure drop decreases suddenly whilst the volume remains constant. This operation is represented by the line 3-4. By doing so the stroke length is reduced. The loss of work is represented by the hatched area 3-3′-4-3, which is negligibly small.

The work done during the modified Rankine cycle can be calculated as follows:

Let p₁, v₂, u₂, h₂ represents the initial conditions at point 2.

p₂, v₃, u₃, h₃ corresponds to condition of steam at point 3.

Figure 5.6 Modified Rankine cycle: (a) p-v diagram, (b) T-s diagram

p₃, h₄ correspond to condition of steam at point 4.

Work done during the cycle per kg of steam = area 1 – 2 – 3 – 4 – 5 – 1

= area 1 – 2 – b – 0 – 1 + area 2 – 3 – a – b – 2 area – 0 – b – a – 4 – 5 – 0

= p₁ v₂ + (u₂ – u₃) – p₃ v₃

Heat supplied, q_s = h₂ – h_f4