With reference to cylinder liner calibration:
(a) Describe the procedure for calibrating a cylinder liner, stating the purpose of the procedure.
(b) Explain how the accuracy of the readings is ensured.
(c) Describe how EACH of following liner wear patterns are identified, explaining a probable cause in each case:
(i) Clover leafing
(ii) Scuffing.
(a) Procedure for calibrating a cylinder liner
Liner calibration is taken at every unit overhaul to know the wear down of liner.
For slow speed engine, maximum wear down limit is 0.75 to 1 % of original bore diameter.
1. First clean liner thoroughly.
2. Liner calibration is taken by using inside micrometer with extension bar which has been calibrated against a master gauge.
3. Gauging is taken at a number of vertical positions (4 to 6) over the area swept by the piston rings. Measure fore and aft direction and in athwart ships directions.
4. To ensure readings are taken at corresponding points, a template is used.
(b) Reading
(1) To ensure accuracy of the readings, the inside micrometer to be used should have a valid calibration certificate.
(2) Micrometer at the same temperature as the liner.
(3) Temperature of liner recorded to allow for any compensation to be made for expansion.
(4) A gauging point template must be used to make sure the readings are taken at the same place each time.
(c) Liner wear pattern
(i) Clover leafing:
- It is a form of wear on cylinder liners and caused by acid attack usually because of low liner temperatures.
- Sulphuric acid, formed by combustion products due to sulphur in fuel, deposits on liner surfaces when its temperature falls below the dew point.
- It is identified by the excessive wear between the cylinder oil injection points up the length of the liner.
(ii) Scuffing:
- It is a form of localized overheating and welding between particles of the piston rings and liner rubbing surface, resulting in roughening of the surface and a rapid wear.
- Possible causes are reduced cylinder oil film due to excessive temperature, incorrect cylinder lubrication and piston blow-by.
- It is identified by vertical scoring on the rubbing surfaces.
(a) State the circumstantial requirement to renew a cylinder liner of a large diesel engine.
(b) Explain how the old liner is removed and the new liner is fitted.
(c) State the important checks to be made after fitting.
(d) Describe the procedure for replacing a crosshead engine cylinder liner.
(e) State the precautions which must be observed to ensure that the replacement cylinder liner is correctly fitted.
(a) Circumstantial requirement to renew cylinder liner
1. If liner wear limit exceeds maker’s specified value.
- Slow speed engine-0.8 ~ 1% of original diameter
- Medium speed engine-0.4 ~ 0.5mm
- High speed engine-0.25 mm
2. Liner’s ovality exceeds allowable limit.
3. After piston seizure
4. When liner cracks.
5. Liner inner surface, deep spread vertical (longitudinal) scratches
6. Liner inner surface, excessive corrosive wear caused by acidic products of combustion
7. Excessive pitting on cooling-water side of the liner due to erosion caused by cavitations.
(b) To remove liner
1. Make risk assessment and permit-to-work, all staff well understand operation procedure.
2. Drain cooling water from jackets.
3. Remove first cylinder cover with valves, operating gear and all connections
4. Remove piston with rod, cylinder lubricator quills
5. Fit strong back, cross bar and two long bolts on liner carefully.
6. By tightening nuts on top of strong back or by applying oil pressure to jacking nuts,
7. Liner start release and free from its landing surface, guiding sections
8. Attach crane, liner lifted out clear.
To fit new liner
1. Clean and inspect landing spaces of cylinder liner, cooling surface, jacket
2. Before fitting, calibrate new liner
3. Fitted strong back and cross-bar on new liner.
4. Tried new liner in position without sealing rings to ensure clearance.
5. Fitted correct seal rings in appropriate grooves and smeared with a lubricant
6. Applied jointing compound to sealing surfaces of liner flange.
7. Lower new liner carefully into the jacket.
8. Care must be taken to align the liner circumferentially, with markings on jacket and liner.
9. Carried out final landing by nuts on cover studs, until liner landed securely on its joint faces.
(c) Important checks after fitting,
1. After final landing, new liner re-gauged, check any distorting and record.
2. This record should be kept for later reference, to estimate liner wear down.
3. After fitting lubricator quills, test cylinder lubricators, cylinder oil coming out evenly at all holes.
4. After fitting cylinder cover and cooling pipe, fill cylinder jacket water and check water leakage, under pressure by running jacket pump.
(d) Procedure for replacing a cylinder liner
1. Immobilization permit should be obtained.
2. Before starting work, a risk assessment and permit-to-work to be done.
3. All staff must well understand the operation procedure.
4. All the necessary tools assembled and checked.
5. Lifting tools and equipments checked for condition and certification.
6. Jacks checked for leaks.
7. Sufficient spares must be on board
(e) Before fitting a new liner:
1. Check liner for correct part number, class stamps and IAPP certification.
2. Check the landing face in the entablature where the “O” rings seal for corrosion.
3. Check the cooling water space for scale.
4. Gauge the liner.
5. Check lubricator drillings are clear
6. Try the liner in the entablature without the “O” rings. It should slide easily into place. Any resistance must be investigated before final fitting with the “O” rings.
After fitting liner
1. Gauge the cylinder liner and record the readings.
2. Connect lubricators and check for operation.
3. Fill up jacket cooling water up to the top of the liner and check for water leakage
4. After the cylinder head has been fitted and the system filled and pressurized, check for water leakage again.
(a) Sketch the line diagram of the Alpha Lubricator installation on Large SlowSpeed main Engine.
(b) Explain the working principal of Alpha Lubricator.
(c) Describe Cylinder Oil pressure and temperature alarm settings.
(d) What are the advantages of Alpha Lubrication System?
(e) Explain why the injection of cylinder oil in controlled amounts and at specific timing is desirable.
(f) Safety device on alpha lubricator
(a) Alpha Lubricator
(b) Working principle of Alpha Lubricators
- Pump station supplies the Alpha Lubricators with 40 ~ 50 bar oil pressure. A single lubricator unit has five or six plungers, driven by a single hydraulic piston.
- MCU controls the oil injection by activating a solenoid valve of the relevant lubricator. A feedback signal is shown by (LEDs) on intermediate boxes for each cylinder.
Timing
- Timing is based on two signals from the angle encoder, a TDC cylinder “1” marker and a crankshaft position trigger.
- Alpha Lubrication system is accurately timed to inject cylinder oil into piston ring pack during compression stroke.
Quantity
- Cylinder lubrication is based on constant amount of oil being supplied per injection.
- Specific feed rate is controlled by variation of injection frequency.
- Injection frequency is calculated from fuel index and speed.
- Basic cylinder oil feed rate at MCR (100%) calculated as a correlation between number of injections/rpm and stroke of lubricators.
- On the HMI panel, individual cylinder feed rate can be adjusted between 60% and 200%.
- Default value is 100%.
(c)
(d) Advantages of Alpha Lubrication System
(1) Specific oil consumption is less than conventional type. (Conventional type ± 1.2 g/KWh)
(2) Liner and piston ring wear rate decrease due to correct feed rate and timing.
(3) Reduce the possibility of scavenge fire.
(4) Meet the IMO emission requirements.
(e)
1. Oil contains alkaline additives to prevent acid attack of the liner; insufficient oil will not provide the necessary protection.
2. Injecting too much, excessive oil will burn leaving an abrasive calcium ash which builds up on piston crowns and fouls turbocharger nozzles and blades.
3. Oil injected at the correct time, i.e. as the ring pack passes the injection points on the compression stroke.
4. Before injecting means top ring scrape oil upwards
5. After injecting means the lower ring scrape the oil down to the scavenge space.
(f) Safety devices of ALPHA lubricator
1. Suction filter (low pressure side)
2. High pressure side filter
3. Pressure drop indicator
4. Low pressure alarm and starting stand by pump system
5. High and low temperature alarm
6. Feedback sensor
7. Indicating Lamps
8. Pressure gauges
9. Oil heater
With reference to medium speed engine cylinder liners;
(a) Explain the cause and effects of polishing and grazing;
(b) Explain the action of an anti-polishing ring during the operation of the engine
(c) Describe how effective cylinder lubrication is maintained.
(a) Cause and Effect of polishing
- After running in, the liner is left with a microscopic corrugated surface which will hold the lubricating oil.
- Builds up carbon on the top land of the piston touch the liner surface and removing the oil film. As the carbon from combustion is abrasive, it will polish the liner surface.
- Removing the microscopic corrugations gives a highly polished liner surface to which a lube oil film cannot adhere.
- This polished surface leads to increase oil consumption, possible jams the top ring.
Cause and Effect of grazing
- Glazing is the formation of a lacquer on the liner surface due to chemicals from the L.O and F.O.
- This is caused by incorrect running-in procedures or running with long periods on low load.
- The glaze liner causes high oil consumption. High oil consumption may lead to formation of deposits particularly on the piston crown.
(b) Action of an anti-polishing ring
1. Anti-polishing ring has a slightly smaller diameter bore than the liner and is slightly larger than the top of the piston.
2. When the piston is on TDC, the bottom of the anti-polishing ring is just above the top piston ring’s top surface.
3. The anti-polishing ring is a clearance fit in a step at the top of the liner, and can be replaced when it wears.
4. On some engines, it incorporates a cooling space, and anti-polishing ring (or fire band) fitted with O-rings for sealing.
5. As piston reciprocates in cylinder, any carbon build-up on piston top land removed by the anti-polishing ring.
6. This ensures a good lubricant film on the liner surface and reducing wear.
(c)
- On a trunk piston engine, the liner is lubricated by splash lubrication from the revolving crankshaft and reciprocating con rod.
- Piston rings spread oil up length of liner and oil control (scraper) ring scrapes the excess oil back to the crankcase.
- Sufficient oil must be supplied not only to maintain lubrication between ring pack, skirt and liner, but also to keep the ring grooves clean and to reduce formation of sulphuric acid.
Large medium speed engines use two methods for effective lubrication.
(1) Cylinder lubrication: Metered injection pumps supply crankcase oil, via lubricators, and this oil is injected into the top half of the liner to lubricate the ring packs and skirt.
(2) Skirt lubrication: Some of the oil used to lubricate the piston pin and cool the piston is diverted through drillings in the skirt. It lubricates the portion of the skirt transmitting the side thrust to the liner.
(a) Explain why lost motion is provided on some cam shaft.
(b) Draw in detail such arrangement.
(c) Describe reversing operation of this type.
(a) Lost motion
- In 2 stroke engines when necessary to reverse, change fuel injection, air starting & exhaust valve timing
- When engine is operating in the reversed direction, provided camshaft will be lost motion or be retarded through a given angle.
- Rotation of camshaft is totally independent to the rotation of the crankshaft, called lost motion.
- lost motion carried out while engine is at rest.
(b)
(c) Reversing operation
- Camshaft is mounted with reversing servomotor, to get correct positions, for other direction of rotation, for adjusting fuel injection pumps, exhaust valves and air distributor.
- Servomotor consists of a pair of vanes fitted on a camshaft, which move between another pair of vanes fitted within a gear wheel rim.
- Rotation of camshaft is totally independent to the rotation of crankshaft, called lost motion.
- By putting Lubricating oil under pressure about 6 bar between opposite side of vanes, camshaft rotate relative to gearwheel and crankshaft.
- This movement of the camshaft loses motion in given a definite reversing angle.
- This angle is designed that correct fuel timing & other timing relation with pistons, obtained both ahead & astern direction.
- Control oil pressure connect to one side of the vane, other side is connected to drain.
- Vane together with camshaft rotated till end position and held stop.
- lost motion carried out while engine is at rest position.
- Servomotor operated by the engine reversing controls.
With reference to the stuffing box of a large crosshead type marine engine;
(a) Sketch the construction of a stuffing box.
(b) Describe the construction and functions of a stuffing box and material used.
(c) Explain required maintenance and clearance.
(a) Stuffing box
(b) Construction and Functions
- In large two stroke engine, stuffing box mounted on diaphragm, separates under piston space and crankcase.
- Stiffing box prevents contamination of crank case lube oil and scavenges air
- Each of them is equipped with set of sealing rings and scraper rings.
- The rings are kept together round onto the piston rod surface by means of spiral springs.
- The function of seal ring seal against scavenge air, scrape off any residues or dirt from the piston rod during down ward stroke.
- The function of scrapper ring oil control, scrape off excess crank case oil from the piston rod during its upward stroke.
- The oil from lower scraper rings flows through the crankcase.
- A cofferdam machined out between upper rings and lower rings grooves, connected to cock on maneuvering platform through duct.
- By checking outlets from these cocks check the condition of sealing and scraper rings.
- Leakage air shows that sealing ring is not good condition, excessive oil discharge means that scraper rings not good condition
Material – The sealing rings and scraper rings are made of bronze. The stuffing box casing is made of cast iron.
(C) Maintenance
- The stuffing box should be overhauled at regular interval. (normally done at the same time of piston overhaul)
- The correct clearance of ring segments should be checked such as axial, butt and radial direction
- The rings bearing against the piston rod on the entire circumference. Check garter spring tension
- When fitting the scraper rings, it is of utmost importance that the rings are placed correctly.
- Stuffing boxes drain holes clear of deposits of oil sludge and dirt.
- To allow for wear, 6 mm clearance at each joint, 18 mm for 3 ring settlement
- The sealing and scraper ring adjusted in grooves, slide easily without play by maintaining a clearance 0.07 to. 0.12mm.
- Lack of maintenance may lead to
- loss of scavenge air
- contamination of scavenge space and
- overheating of piston rod the formation of hot spot leading to scavenge fire and crankcase explosion.
