The automatic variation of steam supply with the variation of load is called governing and the device used for this purpose is known as a governor. There are two methods commonly used for steam engine governing.
- Throttle governing: In this type of governing, the pressure of a steam admitted to the engine is reduced by throttling before it passes into the engine (Fig. 5.14) and the device used for this purpose is called the throttle valve. The lower pressure of steam during admission process reduces the work developed by the engine. It has been observed that the steam consumption rate is linearly proportional to the indicated power. This line is called Willams line.
- Cut-off governing: In this system of governing, the period of admission of steam entering the engine cylinder is reduced depending on the load on engine (Fig. 5.14b). The volume of steam admitted is proportional to the mass of steam.
The indicated power vs steam consumption per minute for throttle and cut-off governing are shown in Fig. 5.15. It is obvious from the figure that the steam consumption rate at full load is same for both types of governing, but part load steam consumption of throttle governing is higher than that of cut-off governing engine.
The governor used for throttle governing is simple and cheaper. But the thermal efficiency of cut-off governed engine is higher at part load conditions.
Figure 5.14 Methods of governing steam engine: (a) Throttle governing, (b) Cut-off governing
Figure 5.15 Effect of method of governing on indicated power
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