giving up some of its stored energy, which moves the
connecting fuel linkage to an increased delivery
position. This additional fuel delivered to the
combustion chambers would result in an increase in
horsepower, but not necessarily an increase in engine
When the truck moves into a downhill situation, the
operator is forced to back off the throttle to reduce the
speed of the vehicle; otherwise, the brakes or
engine/transmission retarder has to be applied. The
operator can also downshift the transmission to obtain
additional braking power. However, when the operator
does not reduce the throttle position or brake the vehicle
mass in some way, an increase in road speed results.
This is due to the reduction in engine load because of the
additional reduction in vehicle resistance achieved
through the mass weight of the vehicle and its load
pushing the truck downhill. This action causes the
governor weights to increase in speed, and they attempt
to compress the high-speed spring, thereby reducing the
fuel delivery to the engine. Engine overspeed can result
if the road wheels of the vehicle are allowed to rotate
fast enough that they, in effect, become the driving
The governor assembly would continue to reduce
fuel supply to the engine due to increased speed of the
engine. If overspeed does occur, the valves can end up
floating (valve springs are unable to pull and keep the
valves closed) and striking the piston crown. Therefore,
it is necessary in a downhill run for the operator to
ensure that the engine speed does not exceed maximum
governed rpm by application of the vehicle, engine, or
Favorable, as well as unfavorable, characteristics
are to be found in mechanical governors. Advantages
are as follows:
They are inexpensive.
They are satisfactory when it is not necessary to
maintain exactly the same speed, regardless of
They are extremely simple with few parts.
Disadvantages are as follows:
They have large deadbands, since the speed-
measuring device must also furnish the force to
move the engine fuel control.
Their power is relatively small unless they are
They have an unavoidable speed droop, and
therefore cannot truly provide constant speed
when this is needed.
Although hydraulic governors have more moving
parts and are generally more expensive than mechanical
governors, they are used in many applications because
they are more sensitive, have greater power to move the
fuel control mechanism of the engine, and can be timed
for identical speed for all loads.
In hydraulic governors (fig. 5-7), the power which
moves the engine throttle does NOT come from the
speed-measuring device, but instead comes from a
hydraulic power piston, or servomotor. This is a piston
that is acted upon by fluid pressure, generally oil under
the pressure of a pump. By using appropriate piston size
and oil pressure, the power of the governor at its output
shaft (work capacity) can be made sufficient to operate
the fuel-changing mechanism of the largest engines.
The speed-measuring device, through its speeder
rod, is attached to a small cylindrical valve, called a
pilot valve. The pilot valve slides up and down in a
bushing, which contains ports that control the oil, flow
to and from the servomotor. The force needed to slide
the pilot valve is very little; a small ball head is able to
control a large amount of power at the servomotor.
The basic principle of a hydraulic governor (fig.
5-7) is very simple. When the governor is operating at
control speed or state of balance, the pilot valve closes
the port and there is no oil flow.
When the governor speed falls due to an increase in
engine load, the flyweights move in and the pilot valve
Figure 5-7.Hydraulic governor.