ENGINEERING KNOWLEDGE (MOTOR)

GOVERNORS

BASIC GOVERNING TERMS

SPEED DROOP

WHAT IS SPEED DROOP?

Speed droop is the change in governor rotating speed which causes the governors output shaft
(servo) to move from zero to maximum fuel position. It shows how big a speed change is
needed to cause governor output shaft (servo) to travel through its full range.

It is expressed as a percentage change in governing speed corresponding to full travel of servo.

Why speed droop is provided?

Speed droop provides stability in governing system , which in turn results in regulation of the
govern system(diesel engines / steam turbine etc.)This is because the engine fuel rack can take
 only one position for any one speed. Therefore when speed changes the resulting governor action
ceases at particular point that gives the amount of fuel needed for the new load. In this way speed
droop prevent unnecessary governor movement and over-correction (hunting)

Speed droop is necessary when two or more engines are operating in parallel, mechanically
Or electrically to share load proportionately .

What is the effect of small and large speed droop on governing system?
Effect of small speed droop
A small speed droop means a small difference between no load/full load r.p.m, but it also means
Rapid swing from full fuel to no fuel if small speed rise occurs. This makes the governor sensitive.

Due to engine time lag , & sensitive governor,over correction in fuel rack position occurs.This results
In too large change in speed followed by correction in other directions.Equlibrium is reached after
Several oscillations of fuel rack.

Some hydraulic governors can response to speed droop of less than 1% & can shift fuel racks from full l
Load position to no load position & vice versa in less than 1/4 sec.

Effect of large speed droop

A large droop provides slower response to change in speed. Governor will not have strong tendency to
Overshoot & equilibrium is reached with min. oscillation of fuel rack.This is suitable for main propulsion engine
control.

Isochronous operation
Isochronous operation maintains the governed speed the same from no- load up to full load on the engine except
during transient load changes.

It is sometimes desirable to have an isolated prime mover run isochronously.This is achieved by providing transient
Speed droop or temporary speed droop or compensation.

STEADY STATE SPEED REGULATION

Steady state speed regulation is the change in engine speed when the load is changed from full rated at
No load.It is expressed as a percentage of full load speed.

Speed regulation of an engine is directly related to speed droop of the governor.In the practice a governor
Is connected to the engine & the zero & max. positions of fuel racks lie within the zero & max. position
Of fuel rack lie within the zero and maximum travel of the servo.

If the governor was connected to the engine such that 50% of the governor output shaft travel was required
To move the fuel rack from no-load to full load , the regulation would be 2% although the governor speed
Droop is 4%.

TRANSIENT PERFORMANCE
Engine speed deviation and speed recovery time depend on the characteristics of engine and the governor

Following figure shows a typical response curve for an engine

Factors affecting the transient performance are

1. time required to adjust rate of fuel injection to correspond with new load requirement .
this depend on governor.

2. the inertia of flywheel and other rotating parts of the engine and driven apparatus .
3. the time required for engines power output to respond to change in rate of fuel injection
that is acceleration characteristics.
e.g. a 2 cylinder 200 rpm engine takes longer to respond than an eight cylinder 1200 rpm unit.
HUNTING
Hunting is the repeated and sometimes rhythmic variation of speed due to over control by the governor.
Such periodic speed variation may also occur due to fault in engine e.g. misfiring.

If governor is responsible ,the hunting will cease if fuel racks are blocked manually or with a load limit
Control , but it reappears when engine returns to governor control.

STABILITY
The ability of a governor to maintain speed with constant and varying loads without hunting and
Minimum of fuel rack movements.

SENSITIVITY
The smallest speed change that will cause governor to adjust fuel racks. This hesitation results from
Lag in governor action caused by function and lost motion.

COMPENSATION
A mechanical and/or hydraulic action that prevents over-correction . Compensation in a governor produces
Transient speed droop,during speed correction.

STROKE LIMITERS,LOAD LIMITERS

Devices used to limit maximum fuel delivered to engine.

GOVERNOR EFFORT AND WORK OUTPUT

Governor are classified by the work available at the output shaft to which fuel pump linkage is connected
An engine in 2000 to 5000 bhp range will normally require 15 ft.lb output governor.(Stalled force available
Multiplied with stroke of the power piston)The fuel linkage load should be 60% of governor work output
To ensure governor response is not reduced.

MECHANICAL GOVERNOR

CONSTRUCTIONAL FEATURES
The mechanical governor is linked directly to the fuel pump racks. Flyweights in the governor move the fuel racks to
control the fuel injected into the cylinder. The governor is directly connected to the engine by a small drive shaft,
which may be geared up to drive the flyweights at a speed greater than the engine speed.
This increase the sensitivity of the governor to small changes in the engine speed. Speeder spring opposes the
centrifugal force of flyweights, allowing governor to reach equilibrium value.
Such arrangement is common for medium and high speed engines.
Operation
EQUILIBRIUM STATE

Assume the engine is running at half load. The flyweights are in vertical position with equilibrium or static forces
balance existing between the governor spring and the flyweights.

INCREASE IN LOAD
Assume now the engine load increases to full load, and in doing so slows down the engine. The centrifugal force on
the flyweights decreases. Speeder rod moves down and increases the fuel to the engine so that engine can run at higher
load.
When a new equilibrium is achieved the flyweights will not be able to go back to vertical position they had at half load
This is because the more fuel needs to be injected to bring the engine speed at full load . The mechanical governor
has a speed droop , a decrease in speed from no load to full load .

DECREASE IN LOAD
Assume that engine load decreases from half load to no load . Engine speed increases , causing flyweights to move
outwards , pulling the speeder rod up . fuel to the engine is reduced . when the equilibrium is reached engine will be
running at higher speed than the half loaded condition.

LIMITATIONS
The flyweights must be large & spring must be strong enough to move fuel racks.

These governors are adequate for small engines where force required to move fuel racks is small .
For large engines heavier flyweights are necessary .These flyweights have high inertia & not sensitive to engine speed
change .The engine speed would fluctuate in large dead band causing hunting .the mechanical governor has a
speeddroop & cannot be used for isochronous operation.
EFFECT OF TIGHT SLEEVE , WORN FLYWEIGHT PIVOT BEARINGS ON OPERATION OF MECHANICAL
GOVERNOR

If the sleeve becomes tight on the spindle or other causes affect its free up & down movement , the effect on governor
will be that larger changes in the rotational speed of the engine must occur before the governor can act .

When the load on the engine is being increased the spring assisting the downword movement of the sleeve will reduce
the effects of the increased friction but the engine speed will drop more than normal before the engine takes up the
increase of load at the correct engine speed .

With decreasing load the sleeve spring load is additive to the increased friction & a considerable increase in engine
speed must occur before the governor can operate & reduce the fuel supply .The governor droop will be increased &
the speed at which the engine operates will become erratic.

Slackness in flyweight bearings must be taken up when the engine load increases & engine speed falls .this has the
effect of increasing the lag before the governor starts to increase the fuel supply & so increase the engine speed to
normal.

With reduction in load the slack doesnot have to be taken up & the spoeed changes will be approximately normal.

Slackness in the sleeve cod piece will make for erratic speed changes when the load on the engine changes; the effect
of load change on speed will depend on the system of levers & linkage between the governor & the fuel pumps.

HYDRAULIC GOVERNORS

CONSTRUCTIONAL FEATURES
Hydraulic governor consists of three main sections
1 . speed sensitive section , which senses the engine speed & tries to maintain a constant engine speed . it comprises
flyweights , speeder spring & pilot valve .
The gear pump & flyweights are directly driven from engine drive train . A spring drive is provided to isolate cyclic
variation in speed . a sensitive governor would attempt to govern these cyclic irregularities.
2. power section which actuates the fuel racks controlling the flow of fuel to the engine . the gear pump & the
accumulator supply oil to the power piston through the pilot valve . power piston actuates the fuel racks.
compensating section , which compensates the difference in reaction time between the engine & the governor .
This comprises :-
A mechanical linkage which gives a proportional feedback of power piston to the top of speeder spring. The
feedback ratio determining the proportional band of the governor which is called speed droop .
A hydraulic system which feedbacks power piston rate of movement to the pilot valve , thus providing the damping
& stabilizing effect . the needle valve controls the flow of oil in & out of the compensating system.

NEED FOR A COMPENSATING SYSTEM IN A GOVERNOR

The governor tends to react very quickly ,because it is hydraulic , & has few parts with inertia .
on the other hand mass & inertia of the engine is quite high & will not react quickly to speed changes .( see
transient performance ). There must be a circuit or mechanism built in the governor to compensateforthedifference
in reaction time of the engine & the governor . so they will act in accord , rather than at odds with each other .
load limit cam limits the fuel supply to theengineby restricing the travel of pilot valve

GOVERNOR OPERATION

INCREASE IN LOAD
Assume that engine is running at half load at fixedrpm . at this equilibrium position flyweights arevertical & piot valve
is closed .
As the load increases there is drop in rpm . the flyweights move inwards allowing speeder spring to push down the
pilot valve , uncovering the ports.
Oil flows to the bottom of the power piston , while the top is drained . power piston moves up, & increases the fuel
supply to the engine .
Power piston movement I is also transmitted to the top of speeder spring via speed droop lever. This slightly reduces
the set speed to give stabilizing droop characteristics.
Power piston movement is also transmitted through actuating piston to receiving piston . receiving piston moves up
the pilot valve .
As the pilot valve moves upwards it cuts off the oil supply to the power piston & limits its movement .
The receiving piston gradually moves downwards as the oil leaks through needle valve
The forces acting on the pilot valve reach equilibrium & the engine is running stable at its new slightly reduced speed
due to increased load .
The needle valve adjustment is critical to the compensating system always acts to reduce the effect of power
section
Needle valve adjustment is critical to the compensating system . The compensating system always acts to reduce the
effect of power section .
Needle valve adjustment is made after the oil has reached its normal operating temperature.
The leakage through the needle valve must attach the engine acceleration chara