Considered by many merely as a means of speed regulation, the governor is, in fact, a very refined component, which in its most developed form is able to load limit, load share, load sense, regulate rates of acceleration. Not all these features, however, are available or even required in any one unit. A governor is usually made to suit the service demands on the engine to which it is to be fitted. It may provide single speed running conditions irrespective of load changes (isochronous), or be able to respond to increases in load so that acceleration is regulated to a level compatible with the effective and safe running of the engine.
The early forms of ‘inertia’ type governors were, in essence, overspeed trips, and were not able to increase fuel to suit increases in load. Although these governors are now largely obsolete, the principle behind them is still used in ‘overspeed trips’. These trips shut down an engine in the event of an excessive and rapid increase in speed, such as may occur if the propeller shaft were to fracture. These units commonly have a ‘fly’ or ‘bob’ weight restrained by a spring. When the engine exceeds a predetermined speed, the weight moves out to strike some form of fuel cut off. Te important thing about this action is that as soon as the weight begins to move, its centre of gravity moves radially outward from the centre of the shaft, increasing the centrifugal force so that the weight moves outward with increasing force. This process is therefore very positive in action and no hunting or hesitation occurs. Once the pre-set speed is reached, the overspeed cut out operates, very rapidly.
Centrifugal (CF) governors, unlike the above units, are able to both increase and decrease the fuel setting as loads either rise or fall. However, it is not possible to make a centrifugal governor truly, isochronous (constant speed). When an increase in load is experienced by an engine it will tend to slow down. This causes the fly weights of the CF governor to move inwards under spring force. This, in turn, causes the fuel racks to be pulled out to increase fuel, but if the racks are in a new position then the slide of the governor will also be in a new position, and so the balance between the flyweights and spring will be achieved at some new running speed. This change in final, steady running speed is known as the ‘permanent variation’, whereas the fall or rise in speed which occurs as loads change is called ‘temporary variation’. The temporary variation fluctuates above and below the desired speed value as the governor attempts to settle down to a steady running speed again. This period of hunting that occurs across the desired value is a function of the size of weights; the smaller the weights, and therefore the more sensitive, the longer the period of fluctuations in speed. Large weights settle to a final steady speed more quickly than small weights, but the magnitude of their temporary variation is greater. Thus it can be seen that CF governors are not suitable for regulating engines which drive alternators, where frequency stability is important, but are adequate for dc generation and the control of any other prime mover where strict adherence to a set running speed is not important. One of the major drawbacks of a CF governor is its limited ‘governor effort’, i.e. the power it develops to operate the fuel racks of the engine. Although large powers can be developed, fairly large fly weights are required; the governor then becomes less sensitive and large changes in running speeds will occur before a steady condition is reached. Although the governor effort can be increased by gearing up the rotational speed of the flyweights, usually to a maximum and optimum of 1500 rev/min, there is still a limit to the power output.
Where routine maintenance is concerned, the governor should be checked regularly for adequate lubrication. The operating range is usually quite small, giving a tendency for them to wear over limited areas. Then a sudden and larger change in speed than normal may carry the governor onto a ridge of debris or gummy oil deposit so deposit so that it may stick at that point. To restrict this problem as far as possible, the governor pivots, slide, etc. should be cleaned and well lubricated whenever possible. It should be borne in mind that the governor structure is such that, were would be able to move outwards and shut the engine down. The connections from the governor to the fuel racks should be designed in such a way that the governor can both increase and decrease the fuel within the bounds of some hand setting; i.e. the governor can adjust the fuel settings but cannot release more fuel to the engine than is dictated by some predetermined value set by the engine operator.
To overcome the above limitations of the CF governor, ‘servo governors’ have been developed. These use the power of hydraulics to pump the heavier fuel rack systems associated with the larger engines into the desired position. This ‘powered’ operation is rapidly and accurately achieved, and the hydraulic flow can be either electrically or mechanically controlled. The speed sensing can be done fey a tachometer arrangement that can be set to the desired speed, and on sensing any variation a solenoid operated flow valve allows pressurised hydraulic flow into a servo system of pistons/plungers which resets the rack positions. Although this system is quite effective, more mechanically controlled governors are fitted to larger engines. These use the principle of the CF governor, i.e. fly weights acting against spring pressure, but instead of the slide working directly onto the fuel racks it simply regulates the flow of hydraulic fluid to the servo pistons controlling the rack positions. This system therefore has the proven reliability and sensitivity of the small CF governor and yet develops quit a large governor effort through the hydraulic fluid.