The indicator diagram is a representation of the variations of pressure and volume of steam inside the cylinder on p-V diagram for one complete cycle of operation. The theoretical p-v diagram for a single acting steam engine is shown in Fig. 5.7 without and with clearance.
The sequence of operations is as follows:
Process 1-2: The steam is supplied to cylinder at constant pressure p1. The steam is cut-off at point 2.
Process 2-3: The steam expands in the cylinder till the piston reaches the ODC position. The expansion process is hyperbolic isothermal, i.e. pv = const.
Process 3-4: The pressure falls from point 3 to point 4 at constant volume instantaneously due to the opening of the exhaust. The point 3 is called the point of release as the pressure is allowed to fall suddenly to the back pressure pb.
Process 4-5: It represents the exhaust of used steam at constant pressure
Process 5-1: It represents steam admission to cylinder at constant volume. The pressure suddenly rises form pb to p1.
The working cycle is completed within two strokes of piston or one revolution of crank.
Figure 5.7 Theoretical indicator diagram for a steam engine: (a) Without clearance, (b) With clearance
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