Posted tagged ‘engine speed’

MECHANICAL GOVERNORS

September 13, 2011

 

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
governor.
Fig. 01Each 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
GOVERNORSNever 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. WARNING

Fig. 03This 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
presentations.
MECHANICAL
Fig. 04This 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. 06Diesel engine mechanical governors consist of two basic
mechanisms: the speed measuring mechanism and the fuel changing
mechanism.
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
operates.

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. 10The flyweights and “L” shaped ballarms which pivot are mounted
on the governor drive.

Fig. 11
The flyweights are rotated by the engine.
Fig. 12As 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. 16This 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
pedal.
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
engine.
Fig. 21The 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. 23When 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
force.

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. 27As 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. 29In 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
rotate.

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. 31Looking 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
later.
Fig. 32
This governor cross section illustrates: (1) lever, (2) spring seat, (3)
spring, (4) spring seat and thrust bearing and (5) flyweight
assembly.
The arrows indicate drive gear rotation and rack movement.
Fig. 33Two 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. 36The 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. 38When 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. 42Looking, 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
bar.
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
schematic.
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. 48In 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
DIESEL ENGINE WITHOUT A GOVERNOR CONTROLLING
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. 52NEVER OPERATE A DIESEL ENGINE WITHOUT A
GOVERNOR CONTROLLING IT.

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. 54With 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

 

Fuel Injectors

September 8, 2011
Each cylinder has a fuel injector designed to meter and inject fuel into the cylinder at the proper instant. To accomplish this function, the injectors are actuated by the engine’s camshaft. The camshaft provides the timing and pumping action used by the injector to inject the fuel. The injectors meter the amount of fuel injected into the cylinder on each stroke. The amount of fuel to be injected by each injector is set by a mechanical linkage called the fuel rack. The fuel rack position is controlled by the engine’s governor. The governor determines the amount of fuel required to maintain the desired engine speed and adjusts the amount to be injected by adjusting
the position of the fuel rack.

Each injector operates in the following manner. As illustrated in Figure 26, fuel under pressure enters the injector through the injector’s filter cap and filter element. From the filter element the fuel travels down into the supply chamber (that area between the plunger bushing and the spill deflector). The plunger operates up and down in the bushing, the bore of which is open to the fuel supply in the supply chamber by two funnel-shaped ports in the plunger bushing.


Figure 26 Fuel Injector Cutway

The motion of the injector rocker arm (not shown) is transmitted to the plunger by the injector follower which bears against the follower spring. As the plunger moves downward under pressure of the injector rocker arm, a portion of the fuel trapped under the plunger is displaced into the supply chamber through the lower port until the port is closed off by the lower end of the plunger. The fuel trapped below the plunger is then forced up through the central bore of the plunger and back out the upper port until the upper port is closed off by the downward motion of the plunger.

With the upper and lower ports both closed off, the remaining fuel under the plunger is subjected to an increase in pressure by the downward motion of the plunger.
When sufficient pressure has built up, the injector valve is lifted off its seat and the fuel is forced through small orifices in the spray tip and atomized into the combustion chamber. A check valve, mounted in the spray tip, prevents air in the combustion chamber from flowing back into the fuel injector. The plunger is then returned back to its original position by the injector follower spring.

On the return upward movement of the plunger, the high pressure cylinder within the bushing is again filled with fresh fuel oil through the ports. The constant circulation of fresh, cool fuel through the injector renews the fuel supply in the chamber and helps cool the injector. The fuel flow also effectively removes all traces of air that might otherwise accumulate in the system.

The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifold and then back to the fuel tank, is adjacent to the inlet opening and contains a filter element exactly the same as the one on the fuel inlet side. In addition to the reciprocating motion of the plunger, the plunger can be rotated during operation around its axis by the gear which meshes with the fuel rack. For metering the fuel, an upper helix and a lower helix are machined in the lower part of the plunger. The relation of the helices to the two ports in the injector bushing changes with the rotation of the plunger.

Changing the position of the helices, by rotating the plunger, retards or advances the closing of the ports and the beginning and ending of the injection period. At the same time, it increases or decreases the amount of fuel injected into the cylinder. Figure 27 illustrates the various plunger positions from NO LOAD to FULL LOAD. With the control rack pulled all the way (no injection), the upper port is not closed by the helix until after the lower port is uncovered.

Consequently, with the rack in this position, all of the fuel is forced back into the supply chamber and no injection of fuel takes place. With the control rack pushed all the way in (full injection), the upper port is closed shortly after the lower port has been covered, thus producing a maximum effective stroke and maximum fuel injection. From this no-injection position to the full-injection position (full rack movement), the contour of the upper helix advances the closing of the ports and the beginning of injection.


Fig 27 Fuel Injector Plunger

 

Governor

September 8, 2011
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 variable speed 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 functions simultaneously.

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.


Fig 28 simplified Mechanical-Hydraulic Governor

Detailed Operation of the Governor
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 bushing.


Fig 29 Cutway of Woodward Governor

Under these conditions, equal oil pressures are maintained on both sides of the buffer 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 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 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 speed.