(a) What are the function of governors?
(b) Sketch and describe hydraulic governors
(c) Explain the following terms of governor? (i) Droop (ii) Hunting
(a) Purpose or Function of Governor
Governor is a device which controls speed of engine automatically in prescribed limit. The governor does in two steps.
(1) Measuring the speed
(2) Control the amount of fuel supply to the engine.
The governor which keeps the steady speed regardless of the load is called a speed governor.
(b) Hydraulic governor
- Hydraulic governor consists of an oil pump, an accumulator, a pilot valve, power piston, speeder spring and ball head assembly
- Pump supplies pressure oil to governor.
- Excess oil to governor sump and reduce pressure of oil
- If governor speed is normal, pilot valve closes port, no flow of oil.
- Due to engine load increase, governor speed “falls” , flyweights move in and pilot valve moves down, pressure oil goes to power piston and piston move upward to increase fuel.
- Due to engine load decrease, governor speed “raises”, flyweights move out and pilot valve moves up, pressure oil releases to sump and power spring forces piston down, decreasing fuel.
- For stability, necessary to feedback from power piston.
- Feedback link connecting control piston to the power piston as shown in figure.
(c) Terms of governor
- When engine and governor combination, difference between no load speed and full load speed is governor droop.
- A small governor droop means a small difference between full load and no load steady speed.
- A large governor droop gives a slower response to the change in speed.
- If engine load changes, governor tends to over-control and undercontrol.
- This over-and-under control causes a fluctuation in engine speed call hunting.
(a) Describe, with the aid of sketches, an electronic governor suitable for a main propulsion engine.
(b) Explain how the governor described (a) functions to increase fuel to the engine in the event of a load increase.
(c) State TWO defects which can impair operation of the governor described in Q (a).
(a) Electronic governor
- A magnetic pickup transmits a signal representing engine speed.
- This signal is compared to the signal set value.
- The difference between signals (error) is amplified and sent to electro hydraulic servo actuator.
- This signal converts to a mechanical which operates the fuel racks.
- The governor programmed to avoid running in a barred speed range and to increase the load slowly during a build up to full revolutions (ramp generator).
- Advantage (1) fast response with no friction (2) control not affect by heat (3) less vibration
- If cables are unshielded, magnetic interference from other electrical sources cause the governor to malfunction.
- Wear or stiffness in the mechanical actuator linkages cause the governor to over compensate in load change leading to hunting.
(a) Describe main engine over speed governor.
(b) Explain in detail with aids of sketch the operation of a mechanism, which shut out the fuel supply.
(c) Indicate clearly how this is achieved without varying the engine control.
(a) Main engine Over speed governor
- If shaft speed exceeds by about 10%, governor comes into operation prevents engine from excessive racing at heavy weather.
- Governor is driven by gear train through spring loaded flexible coupling.
- A hydraulic power piston to operate fuel oil control linkages against forces of a spring
- Sudden speed increase fly ball sense set point and it transforms movement of servo slide valve (F) via collar (A) and bell crank levers (B).
- Slide valve (F) controls oil inlet on under face of servo piston (E) and rotates shaft which changes fuel pump setting for lower speed.
(b) The operation of fuel shut off mechanism
As speeds increase, up, fly-balls move outwards.
1. Spring carrier sleeve and bell crank levers (B) push servo slide valve (F)
2. Control edge of slide valve (F) uncovers the port, oil flows into under face of servo piston (E).
3. Servo piston (E) moves and comes to a stop as soon as the port is covered again.
4. Fuel control shaft is turned towards no admission.
As speed decreases, fly-balls return to their original position.
1. Servo slide valve (F) moves back to its position.
2. Servo cylinder is now communicated to oil outlet and the servo piston (E) is forced back to its original position by spring force.
(c) Indicate clearly how this is achieved without varying-the engine control
- Main engine governor is designed for an over speed control with servo piston as amplifier of force.
- It is inoperative at minor speed fluctuations and at lower speed range.
With reference to the main engine bottom end bearing:
(a) Describe FIVE faults which might be found on the bearing and/or the crankpin during a survey inspection, indicating a possible cause for each fault.
(b) Describe the procedure for checking and adjusting a bottom end bearing clearance.
(1) Ovality: caused by downward load on crankpin as crankshaft rotates.
Not exceed 25% of bearing clearance or danger of losing hydrodynamic lubrication.
(2) Circumferential scoring of pin and bearing: caused by hard impurities in oil, enter into soft bearing metal.
(3) Corrosion of pin and bearing material:
- caused by S.W or F.W contamination and acidic LO due to oxidation or bacterial contamination.
- Both are identified by pitting. Corrosion of tin alloy in bearing material produces hard scabs of stannic oxide,
- Cause scoring and serious damage to bearing and pin.
(4) Wiping of the bearing:
- metal to metal contact between sliding surfaces causes increased frictional heat, resulting in plastic deformation of white metal.
- Caused by overloading, poor surface finish of pin (due to scoring or pitting), or water contamination or incorrect oil viscosity.
- All of these will affect the formation of hydrodynamic oil film.
- If wiping is not severe or only affects the over layer, inspection of the bearing will show a smearing of material.
(5) Cracking of the bearing material:
- broken away from steel backing shell.
- Caused by faulty manufacture, overloading or bearing steel shell move in bearing housing or fretting between housing and shell.
(b) Procedure for checking and adjusting a bottom end bearing clearance
(1) On large two stroke engines, to measure clearance on a bottom end bearing turn the unit concerned to BDC by inserting a feeler gauge at the bottom of the bearing shell in both sides.
(2) For a four-stroke engine, procedure is similar but measure the clearance from outside the engine
(3) For thin shell bearing types, clearance is not adjustable and when clearance reaches a maximum, replace bearing
(4) In case of thick shell bearings, the clearance is adjusted by fitting or removal of shims between the bearing halves.
(5) As bearing wears, clearance will increase, shims removed or reduced in thickness after bearing is opened.
(6) Shims on both sides have equal thickness.
(7) On large two stroke, typical bearing clearance is about 0.5 mm.
(8) An average wear rate of 0.01 mm per 10,000 hours is regarded as normal.
(a) What is thin shell bearing and explain the white metal bearing corrosion.
(b) Explain factors which appear to contribute towards the formation of tin oxides and associate problems.
(a) Thin shell bearing
- In main engine, thin shell bearings are mostly used.
- Consist of a steel backing strip coated with a layer of white metal.
WHITE METAL BEARING CORROSION
- White metals are tin based; contain a higher proportion of tin than other compounds. (tin, Antimony, lead)
- Due to electrolyte corrosion, tin forming extremely hard, brittle in presence of moisture.
- These oxides are a grey to grey-black coloured, surface layer on the white metal, either in local patches or completely covering bearing.
- The hardness of this brittle oxide layer twice that of steel
- if it became detached, serious damage to bearing and journal surfaces occur.
- The formation of the oxide layer is reduced clearances, overheating and seizure etc.
(b) Factors which appear to contribute towards the formation of tin oxides are:-
- The tin in the white metal can oxidize.
- Factors which appear to contribute towards the formation of tin oxides are:-
- Boundary lubrication at starting conditions
- Surface discontinuities
- Concentration of electrolyte
- Oil temperature
- Stresses in the bearing metal
- Additives in the lub oil can add some degree of protection as efficient centrifuging.
- Stannic Oxide is much harder than the white metal and can cause two problems:
- Prevents absorption of dirt particles abrasive particles are stuck to the surface of the white metal.
- Local overheating and melting occurs
- The oxide is brittle and crack with piece edge projecting out
- Both these results in scoring of the journal;
- In addition, the presence of water in L.O cause the oxidation of metals in the bearing causing metal to grow
- This can reduce bearing clearance and can lead to bearing failure.
Write short notes:
(a) Main causes of bottom end bearing bolt failure.
(b) Causes of piston ring failure.
(c) What do you understand by VIT mechanism?
(d) State problems which may occur running an engine with one unit hang up and possible remedies.
(a) Main causes of bottom end bearing bolt failure
- Stress concentration e.g. Sharp corners in way of change of section, shank and bolt head, damaged fillet and surface finish.
- Over stretching or over tightening of bolts resulting in permanent damage due to plastic deformation.
- Uneven tightening, resulting in overloading of some bolts.
- Inadequate pretension, resulting in high ‘fluctuation of stress’ in bolts and fatigue failures.
- Improper seating of bolt head or nut resulting in bending stresses in the bolts.
- Corrosive attack due to contaminated lube oil.
(b) Causes of piston ring failure
1. Insufficient ring and groove axial clearance
2. Insufficient ring gap
3. Insufficient lubrication, cooling system
4. Excessive wear of piston rings, cylinder liner
5. Excessive ring chamfer
6. Excessive combustion ratio
7. Excessive clearance between piston and liner
8. Prolong running with engine overload
9. Turbocharger fouling in exhaust side
10. Partially choked turbocharger filter
(c) VIT mechanisms
VIT mechanism is linked to governor load setting shaft and built in cam system, which controls, through linkages, simultaneously timing of suction valve closure (beginning of delivery) and spill valve opening (end of delivery).
- Injection timing altered by changing position on eccentrics controlling suction valve and spill valve during running.
- If position tappet controlling, suction valve is lowered, injection commences earlier and fuel quantity increased.
- If tappet controlling, spill valve is raised, end of injection is made earlier, increased quantity of fuel delivered is reduced and brought back to its normal level.
- Timing of fuel injection is now advanced without any change in the amount of fuel delivered.
- When using poorer ignition quality fuel at the same valve timing, Pmax will drop.
- To obtain normal Pmax, fuel quality setting lever is manually adjusted VIT mechanism.
(d) Problem which may occur running an engine with one unit hang up & remedies
While running an engine with one unit hung up, the following problems may occur
(1) Torsional vibrations may occur at certain engine speeds.
(2) Critical Speed range may change.
(3) Turbocharger surging
(4) a dead spot for starting when manoeuvring.
The engine run at reduced speed as recommended by manufacturer.
With reference to the bottom end bearing bolts for medium speed four stroke engine;
(a) explain why bottom end bolts will ultimately fail under normal operation conditions;
(b) Describe the features incorporated into bottom bolts design to inhibit failure
(c) Describe a strategy for preventing failure.
1. On a 4 stroke engine, bolts subjected to tensile stress when tightened and additional varying tensile stress during operation.
2. Connecting rod is in compression during compression and power stroke. in tension during inlet and exhaust strokes.
3. This increases the tension in the bottom end bolts, leading to cyclic stressing. This repeated cyclic stressing will cause the bolt to fail after a number of reversals.
Shank of bolt smaller diameter
- Over tensioning of bolts should be avoided as this increases the stress level.
- If bolts are damaged during maintenance, stress raisers will result, thereby increasing the risk of fatigue.
- When inspecting the bolt, check for:
- Corrosion – acid attack from LO.
- Mechanical damage – dents, burrs, etc.
- Fretting on landing faces
- Thread condition
- Cracks – NDT, dye penetrants.
- Only tighten to manufacturers’ recommendations – hydraulic jacks, stretch measurement, or using jig and feeler gauges.
- Do not overload the engine either as a whole or individual units.
- In order to minimize the risk of fatigue failure, bottom end bolts should be replaced if the bolt elongation has reached a maximum (go–no go gauge) or after about 15,000 running hours.