Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the
injectors. Because a diesel engine is not self-speed-limiting, it requires not only a means of
changing engine speed (throttle control) but also a means of maintaining the desired speed. The
governor provides the engine with the feedback mechanism to change speed as needed and to
maintain a speed once reached.
A governor is 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 will be reviewed.
The major function of the governor is determined by the application of the engine. In an engine
that is required to come up and run at only a single speed regardless of load, the governor is
called a constant-speed type governor. If the engine is manually controlled, or controlled by an
outside device with engine speed being controlled over a range, the governor is called a variablespeed
type governor. If the engine governor is designed to keep the engine speed above a
minimum and below a maximum, then the governor is a speed-limiting type. The last category
of governor is the load limiting type. This type of governor limits fuel to ensure that the engine
is not loaded above a specified limit. Note that many governors act to perform several of these
Operation of a Governor
The following is 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 is maintained at any load. The governor is 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 will 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.
Figure 28 provides an illustration of a functional diagram of a mechanical-hydraulic
governor. The position of the flyweights is determined by the speed of the engine. 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 is linked to the fuel
rack and its motion resets the fuel rack for increased/decreased fuel.
With the engine operating, oil from the engine lubrication system is 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 is 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
is 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
piston and tension on the two buffer springs is equal. Also, the oil pressure is 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 is 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 will 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 will be admitted under the servo-motor piston (topside
of the buffer piston) causing it to rise. This upward motion of the servo-motor piston will
be transmitted through the terminal lever to the fuel racks, thus increasing the amount of
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 is
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 is below the
compensating land, the pilot valve plunger is 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 has 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 is 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