The speed control mentioned above is achieved with the help of a governor and we will study about this device here. I would just like to clarify one confusion here that the main role of the governor is not to increase or decrease the speed which can be done via fuel control system (similar to an accelerator on your car) but once the speed of the engine has been set, the job of the governor is to maintain that speed despite the variations in load. In other words the governor controls the speed variation and keeps the speed within restrained limits despite these variations.
The variations could arise from several factors such as say rough weather. A ship rolling and pitching in heavy weather may temporarily come in such position that its propeller is literally out of water and without the governor the speed of the engine could shoot up to such an extent that it could damage the engine itself.
Governors are also fitted in auxiliary diesel engines on the ship used for power generation, and their function remains the same in this situation as well. The power delivered by the alternator needs to be constant despite load variations and this depends to a great degree on the speed at which the prime mover of the generator diesel engine is rotating since the alternator is getting its movement from that engine only. Hence the role of the governor is equally important in this case as well.
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A Simple Direct Action Governor
The figure below explains the working of an elementary governor known as direct action governor. It is a purely mechanical device working on the principle of centrifugal force acting on rotating/revolving bodies. The working of this governor can be clearly understood if you see the diagram carefully before reading further.
The governor spindle is rotated through a gear mechanism via the engine shaft whose speed needs to be governed. There are flyweights which rotate along with the governor spindle and they are thrown outwards (as shown by arrows in diagram), and the degree of their outward motion is in proportion to the speed at which the shaft and hence the spindle is rotating.
The spring fitted on the spindle acts to return the flyweights to their original position and hence counters the centrifugal force acting on the flyweights.
The net results of all this action is that the spindle moves vertically up or down depending on the position of the flyweights and this motion is transmitted to the appropriate mechanism which results in actual speed change.
We will study about governor characteristics in our next article followed by the study of relay governor with servomechanism.
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Marine Diesel Engines: Principles & Practice by Sanyal, D.K.