ACTUAL INDICATOR DIAGRAM

Indicator diagram is simply a graph between pressure of steam in a cylinder against the steam volume.

The following assumptions are made in drawing the theoretical indicator diagram:

1. Clearance is neglected.
2. Steam supplied to cylinder is at constant pressure.
3. Ports are opened and closed instantaneously.
4. There is no drop in pressure due to condensation.
5. Expansion of steam in cylinder follows the hyperbolic law.

The theoretical and actual indicator diagrams are shown in Fig. 5.8. The actual indicator diagram is found to be different from the theoretical diagram due to following reason:

1. A definite clearance is necessary to prevent the piston striking the cylinder head. Provide cushioning effect to piston and allow small quantity of water to be collected in the cylinder due to condensation of steam. Therefore the actual supply of steam starts from point ‘a’ instead of point A. The admission pressure is also lower due to condensation of Steam on the cooler cylinder walls and throttling effect during flow of steam through the inlet port. Therefore, the supply steam pressure is represented by ab instead AB.
2. The instantaneous cut-off is not possible as it takes place during few degrees of crank rotation. Therefore, the diagram gets rounded up near point b.
3. The expansion of steam in the cylinder is not perfectly hyperbolic and is represented by some other curve bc instead of BC. This is due to condensation of steam during expansion.
4. The exhaust opens before the piston reaches ODC position (say point C) as instantaneous drop in pressure is not possible. Therefore, fall in pressure starts from C and actual pressure drop is represented by ‘cd’.
5. The exhaust pressure is slightly above the condenser or back pressure as the steam has to be forced out from cylinder. The actual exhaust pressure is represented by ‘de’.
6. The supply of exhaust steam to exhaust port is stopped at point ‘c’ by closing the port before the piston reaches IDC positions. The entrapped steam is compressed during the remaining stroke and provides a cushioning effect. This is represented by ‘ef’. The inlet port opens just before the piston reaches the IDC position and the pressure inside the cylinder increases.

Figure 5.8 Theoretical and actual indicator diagrams

The area between the theoretical and actual indicator diagrams represents the lost work. The ratio of the area of the actual indicator diagram to the area of theoretical indicator diagram is called the diagram factor.