It was invented in 1788 by James Watt to control his steam engine where it regulates the admission of steam into the cylinder(s). Its widest use was on steam engines during the Steam Age in the 19th century. It was also found on internal combustion engines and variously fueled turbines, and in some modern striking clocks.
Feedback mechanism to change speed as needed and to maintain a speed once reached
thus that mechanism was provided to the governor of engine. A governor was essentially a speed-sensitive device, designed to maintain a constant engine speed regardless of load variation. Since all governors used on diesel engines control engine speed through the regulation of the quantity of fuel delivered to the cylinders, these governors may be classified as speed-regulating governors. As with the engines themselves there are many types and variations of governors. In this module, only the common mechanical-hydraulic type governor would be reviewed.
The major function of the governor was determined by the application of the engine. In an engine that was required to come up and run at only a single speed regardless of load, the governor was called a constant-speed type governor. If the engine was manually controlled, or controlled by an outside device with engine speed being controlled over a range, the governor was called a variable speed type governor. If the engine governor was designed to keep the engine speed above a minimum and below a maximum, then the governor was a speed-limiting type.
The last category of governor was the load limiting type. This type of governor limits fuel to ensure that the engine was not loaded above a specified limit. Note that many governors act to perform several of these functions simultaneously.
Operation of a Governor
The following was an explanation of the operation of a constant speed, hydraulically compensated governor using the Woodward brand governor as an example. The principles involved are common in any mechanical and hydraulic governor.
The Woodward speed governor operates the diesel engine fuel racks to ensure a constant engine speed was maintained at any load. The governor was a mechanical-hydraulic type governor and receives its supply of oil from the engine lubricating system. This means that a loss of lube oil pressure would cut off the supply of oil to the governor and cause the governor to shut down the engine. This provides the engine with a built-in shutdown device to protect the engine in the event of loss of lubricating oil pressure.
Simplified Operation of the Governor
The governor controls the fuel rack position through a combined action of the hydraulic piston and a set of mechanical flyweights, which are driven by the engine blower shaft.
As the engine speeds up or down, the weights move in or out. The movement of the flyweights, due to a change in engine speed, moves a small piston (pilot valve) in the governor's hydraulic system. This motion adjusts flow of hydraulic fluid to a large
hydraulic piston (servo-motor piston). The large hydraulic piston was linked to the fuel
rack and its motion resets the fuel rack for increased/decreased fuel.
Fig 28 simplified Mechanical-Hydraulic Governor
Detailed Operation of the Governor
With the engine operating, oil from the engine lubrication system was supplied to the
governor pump gears, as illustrated in Figure 29. The pump gears raise the oil pressure to a value determined by the spring relief valve. The oil pressure was maintained in the annular space between the undercut portion of the pilot valve plunger and the bore in the pilot valve bushing. For any given speed setting, the spring speeder exerts a force that was opposed by the centrifugal force of the revolving flyweights. When the two forces are equal, the control land on the pilot valve plunger covers the lower ports in the pilot valve bushing.
Under these conditions, equal oil pressures are maintained on both sides of the buffer piston and tension on the two buffer springs was equal. Also, the oil pressure was equal on both sides of the receiving compensating land of the pilot valve plunger due to oil passing through the compensating needle valve. Thus, the hydraulic system was in balance, and the engine speed remains constant.
When the engine load increases, the engine starts to slow down in speed. The reduction in engine speed would be sensed by the governor flyweights. The flyweights are forced inward (by the spring), thus lowering the pilot valve plunger (again, due to the downward spring force). Oil under pressure would be admitted under the servo-motor piston (topside of the buffer piston) causing it to rise. This upward motion of the servo-motor piston would be transmitted through the terminal lever to the fuel racks, thus increasing the amount o f fuel injected into the engine. The oil that forces the servo-motor piston upward also forces the buffer piston upward because the oil pressure on each side of the piston was unequal.
This upward motion of the piston compresses the upper buffer spring and relieves the pressure on the lower buffer spring.
The oil cavities above and below the buffer piston are common to the receiving compensating land on the pilot valve plunger. Because the higher pressure was below the compensating land, the pilot valve plunger was forced upward, recentering the flyweights and causing the control land of the pilot valve to close off the regulating port. Thus, the upward movement of the servo-motor piston stops when it had moved far enough to make the necessary fuel correction.
Oil passing through the compensating needle valve slowly equalizes the pressures above and below the buffer piston, thus allowing the buffer piston to return to the center position, which in turn equalizes the pressure above and below the receiving
compensating land. The pilot valve plunger then moves to its central position and the
engine speed returns to its original setting because there was no longer any excessive
outward force on the flyweights.
The action of the flyweights and the hydraulic feedback mechanism produces stable
engine operation by permitting the governor to move instantaneously in response to the load change and to make the necessary fuel adjustment to maintain the initial engine speed.
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