Friday, August 15, 2008

Mechanical Governer

Diesel Fuel Systems

Mechanical Governors
This Meeting Guide is the third in a series dealing with the basic
diesel engine fuel system and components. It is about the diesel

Fig. 01

Each Caterpillar diesel engine is equipped with a governor. Why?
Diesel engines can accelerate-increase speed-at the rate of more
than 2000 revolutions per second. Yes, PER SECOND. Without a
governor a diesel engine can quickly destroy itself.

Fig. 02


Never operate a diesel engine without a governor controlling it. If
you were to move the fuel rack of a diesel engine to the full “ON”
position without a load and with the governor not connected, the
engine speed might climb and exceed safe operating limits before
you could shut it down. One second...two seconds...before you
knew what was happening, the engine may have been seriously
damaged by overspeeding.
This warning - never operate a diesel engine without a governor
controlling it - is concerned with one of the purposes of governors:
to prevent engine overspeeding. Governors also keep the engine at
the desired speed and increase or decrease engine power output to
meet load changes.

Fig. 03

This presentation introduces and explains the mechanical governor.
The mechanical governor is the simplest of the various types of
governors and is basic to their operation.
Besides the mechanical governor, Caterpillar engines use: servomechanical
governors, hydraulic governors and electronic
governors. These governors will be discussed in future

Fig. 04

This tractor is equipped with a mechanical governor. We can see the
governor control lever, the control linkage, the governor and the fuel
injection pump housing.

Fig. 05.
This is a closeup of the governor, mounted on the rear of the fuel
injection pump housing.

Let’s look at the construction and operation of the mechanical
governor using schematic illustrations.

Fig. 06

Diesel engine mechanical governors consist of two basic
mechanisms: the speed measuring mechanism and the fuel changing

Fig. 07
The speed measuring mechanism senses engine speed changes, and
the . . . .
Fig. 08

. . . fuel changing mechanism increases or decreases the amount of
fuel supplied the engine to correct these changes.
Let’s look at each basic mechanism separately and learn how it

Fig. 09
The speed measuring mechanism is simple, has few moving parts
and measures engine speed accurately. The main parts are:
1) gear drive from the engine,
2) flyweights, and
3) spring.

Fig. 10

The flyweights and “L” shaped ballarms which pivot are mounted
on the governor drive.

Fig. 11

The flyweights are rotated by the engine.

Fig. 12

As the flyweights rotate, they exert a centrifugal force outward. The
flyweights move outward pivoting the ballarms upward. The amount
of outward force depends on the speed of rotation.
Centrifugal force is the basic operating principle of the speed
measuring mechanism. Now, what is centrifugal force?

Fig. 13
If we tie a ball on a string . . . .

Fig. 14

. . . . . and swing it around and around . . .

Fig. 15

faster and faster, an outward force-centrifugal force- is exerted on
the ball. This centrifugal force swings the ball outward and upward
until the ball is nearly straight out.
And, we can see that the faster we swing it, the greater the pull on
the string and the farther outward it swings.

Fig. 16

This force - centrifugal force - is the basic principle used in the
speed measuring operation of the diesel engine governor. Keep
centrifugal force in mind as we discuss the other parts of the speed
measuring mechanism. Remember, the greater the engine speed, the
greater the centrifugal force and, therefore, the greater the
movement of the flyweights and ballarms.

Fig. 17

We need to control this centrifugal force, so we have the governor
spring. The spring acts against the force of the rotating flyweights
and tends to oppose them. The force exerted by the spring depends
on the governor control setting.

Fig. 18

A lever connected to the governor control pushes on or compresses
the spring. The spring force opposes the flyweights to regulate the
desired engine speed setting.
The governor control, shown here as a simple push-pull knob, may
be a hand operated control lever or a foot operated accelerator

Fig. 19
As long as the spring force equals the flyweight centrifugal force,
engine speed remains constant.

Fig. 20

The speed measuring mechanism, then, senses and measures engine
speed changes. The fuel changing mechanism links the speed
measuring mechanism with the fuel injection pumps to control

Fig. 21

The fuel changing mechanism consists of the:
1) connecting linkage,
2) rack and
3) the fuel injection pump.

Fig. 22

Flyweight movement - outward in this example - due to engine
speed changes, are transferred through the simple linkage to the
rack and, therefore, to the fuel injection pump plunger.

Fig. 23

When the engine load increases - as when a dozer digs in - the
speed decreases. The flyweight force decreases, and the spring
moves the linkage and rack to increase the fuel to the engine. The
increase fuel position is held until the engine speed returns to the
desired setting, and the flyweight force again balances the spring

Fig. 24

When the engine load decreases, the speed increases. The flyweight
force increases, overcoming the spring force, moving the rack to
decrease fuel to the engine. The decrease fuel position is held until
engine speed returns to the governor control setting, and the spring
force again balances the flyweight force.

Fig. 25

In summary, the basic governor consists of the:
drive gears, flyweights, spring, and control lever of the speed
measuring mechanism, and the connecting linkage, rack and fuel
injection pump of the fuel changing mechanism.

Fig. 26

The rack which meshes with the injection pump plunger gear
segments extends from the injection pump housing into the
governor. The rack and fuel injection pumps are parts of the fuel
injection pump housing assembly.

Fig. 27

As you recall, Meeting Guide 43, Fuel Systems: Part 2, explained
fuel injection pump operation and how the fuel injected into each
cylinder is increased or decreased.

Fig. 28

In this cutaway governor and fuel injection pump housing, we see
that the rack extends into the governor. Rack movement controls the
amount of fuel injected in each cylinder.
Let’s look at a closer view of our cutaway governor.

Fig. 29

In this cutaway section of our housing, see the flyweights, spring,
spring seat and thrust bearing. The thrust bearing (not previously
mentioned) is an anti-friction bearing between the flyweight
ballarms which rotate and the spring seat which, of course, does not

Fig. 30

The governor is driven by the lower gear bolted to the fuel injection
pump camshaft.
The control lever has been removed from its shaft in the governor
housing and set in place to show how it is positioned.

Fig. 31

Looking closer, we can see (from right to left) the drive gear ,
flyweights , spring, spring seats, control lever and the collar and bolt
which connects to the rack. The purpose of the collar is explained

Fig. 32

This governor cross section illustrates: (1) lever, (2) spring seat, (3)
spring, (4) spring seat and thrust bearing and (5) flyweight
The arrows indicate drive gear rotation and rack movement.

Fig. 33

Two adjusting screws limit the travel of the governor control lever
between LOW IDLE position and the HIGH IDLE position.
The low idle stop and high idle stop are simply minimum and
maximum engine rpm settings with no load on the engine.

Fig. 34

The high and low idle adjusting screws are located under the cover
on the governor.

Fig. 35

Notice that the holes in the cover are shaped to lock the screws and
prevent them from turning after they are adjusted.

Fig. 36

The operators control is positioned at the desired governor setting:
low idle, high idle or fuel off.

Fig. 37

When the lever in the governor is in the LOW IDLE position, a
spring loaded plunger in the lever assembly contacts the low idle
stop of the adjusting screw.

Fig. 38

When the lever in the governor is in the HIGH IDLE position, the
lever contacts the high idle adjusting screw.

Fig. 39

To shut the engine down, the governor control is moved full forward
- past . . . .

Fig. 40

. . . the low idle stop. It is necessary to force the plunger over the
shoulder on the low idle screw . . .

Fig. 41

. . .to move the rack to the FUEL OFF position.

Fig. 42

Looking, again, at the governor cross section see
(1) the high idle adjusting screw and
(2) the low idle adjusting screw. The lever is against the HIGH IDLE screw.
The low idle and high idle screws, then limit minimum and
maximum engine rpm with no load on the engine. What limits
engine power output when the engine is fully loaded?

Fig. 43

A collar and stop bar limit rack travel and, therefore, the power
output. The collar is secured by a bolt connecting the rack linkage.
The stop bar is mounted in the governor housing. With the rack
moved to the FULL LOAD position, the collar just contacts the stop

Fig. 44

When our engine is operating with the governor at high idle (1) and
picks up a load, the speed decreases, flyweight centrifugal force
lessens, and the spring moves the rack to give the engine more fuel
increasing power. The collar (2) and stop bar (3) limit the distance
the spring can move the rack. As the collar contacts the stop bar,
full load position is reached. This limits the fuel delivered to the
engine so as not to exceed design limitations.

Fig. 45

Returning to the governor cross section, note the location of the:
(1) collar,
(2) stop bar,
(3) bolt and
(4) rack.
Like other diesel engine components, the governor must be
lubricated for long life. Let’s look at a governor lubrication system

Fig. 46

The governor is lubricated by the engine lubricating system. Oil
from the diesel engine oil manifold is directed to the governor drive
bearing. All other governor parts are lubricated by splash.
The oil drains from the governor, through the fuel injection pump
housing, back to the engine crankcase.

Fig. 47
In summary, we have discussed the mechanical governor’s primary
components and principle of operation. Remember a governor has
two basic mechanisms: the speed measuring mechanism and the
fuel changing mechanism.

Fig. 48

In our cross section we located the lever, spring, spring seats,
flyweights, thrust bearing, drive gears and rack. We also discussed
the high and low idle settings and the full load stop.
At the beginning of this lesson we warned: NEVER OPERATE A
IT. Why are governors so important to a diesel engine?

Fig. 49
Note: The instructor should make clear we are not saying
gasoline engines never have a governor. Some
gasoline engines use a governor for the same reasons as
a diesel: to control engine speed and to regulate engine power output.
First, gasoline engines are self-limiting. Engine speed is controlled
by a butterfly valve in the intake manifold which limits the air
supply Limiting the amount of air taken in for combustion, limits
engine speed.

Fig. 50

Diesel engines, however, are not self-limiting. Engine air intake is
not limited, and the cylinders always have more air than is needed
to support combustion. The amount of fuel injected into the
cylinders controls engine speed.

Fig. 51

And, as the fuel is injected directly into the cylinders rather than
into the air intake manifold, engine response is immediate. This,
resulting greater power stroke, adds up to very rapid acceleration.
As we said earlier, diesel engines can accelerate at a rate of more
than 2000 revolutions per second. Because of this rapid
acceleration, manual control is difficult, if not impossible.

Fig. 52


Fig. 53

At this point, we have built up the basic diesel mechanical governor.
This governor works fine on engines whose engine speed is held
fairly constant and the governor is controlled by hand. However, on
other engines, the force needed to compress the governor spring or
to move the rack -just operating the governor - could be very tiring
to the operator.

Fig. 54

With the servo-mechanical governor, the work operation of
compressing the governor spring is done with engine oil pressure.

Fig. 55

With the hydraulic governor, the work operation of moving the fuel
injection pump rack is done with engine oil pressure.
These governors are discussed in . . . .

Fig. 56

. . . . Meeting Guide 60, “Servo Mechanical Governors.”

Fig. 57
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