(a) Why correct functioning of the charge air cooler is extremely important in relation to efficient engine operation?
(b) Sketch the charge air system for large two stroke main propulsion diesel engine?
(c) What are the causes of scavenge air temperature high?
(d) Explain the effect of fouling charge air cooler on engine.
(a)
- Air passed through cooler to engine in order to reduce its temperature and restore density of charge air to optimum conditions.
- Correct functioning of cooler is extremely important in relation to efficient engine operation.
- Any fouling which occurs reduce heat transfer from air to cooling medium
- Estimated 1˚C rise in temperature of air delivered to engine increase exhaust temperature by 2˚C.
- Reduction in air pressure at cooler outlet due to increased resistance is also a direct result of fouling.
(b)
(C) The causes of Scavenge air temperature high
(a) Overloading of the engine
(b) Deficiency of charge air cooler due to fouling on S.W side and air side
(c) Scavenge fire
(d) Ambient air temperature is too high
(e) Due to hot gas blow pass
(f) Choke of exhaust port
(g) Intermixing of scavenge air and exhaust gas at loop and cross scavenging system.
(d) Effect of fouling charge air cooler on engine
1. Decreasing engine efficiency
2. Increasing charge air temperature caused by fouling charge air cooler, results in a rise of combustion air temperature and an increased specific volume of the air.
3. Reduce mass of air to cylinders. causes a rise of the exhaust gas temperature and an increased thermal load on the engine.
What are the causes and remedies of following operation defects?
(a) Crack cylinder liner
(b) Failure of white metal on bottom end bearing
(c) Crank pin corrosion
(d) Heavy pitting on gear teeth
(a) Crack cylinder liner
Causes
1.Design failure or improper fitting
2. Improper fuel quality
3. Lack of cylinder lubrication
4. Thermal stress due to high difference Jacket water temperature
5. Insufficient cooling
6. Rapid contraction of metal
7. hoop stress on liner
8. Over tightening of cylinder head bolt
9. Slack tie bolts
10. Foundation chock loosen
11. After immerse scavenge fire
Prevention:
1. Provide sufficient cooling with correct temperature control
2. Supply correct cylinder lubrication quantity and quality
3. Never use excess-worn liner, if possible,
4. Maintain good engine performance,
5. Correct treatment to cooling water,
6. Follow correct procedure of fitting piston assembly liner and cylinder head
7. Avoid operational overloading of the engine
Remedy – Renew the cracked cylinder with new one.
(b) Failure of white metal on bottom end bearing
Causes
1. Design failure or improper fitting
2. Incorrect bearing nip
3. Insufficient lube oil supply
4. Excessive water content in lube oil
5. Tin oxide corrosion (promote electro chemical reaction)
6. Crank pin ovality and roughness
7. Wiping due to insufficient clearance or misalignment or very slow start-up
8. Fatigue cracking (Crazed cracking)
9. Squeezing ( beyond compressive strength)
Remedies
1. Renew bearing according to maker recommended R/H
2. Avoid over tightening
3. Apply sufficient L.O supply to bearing
4. Keep good L.O filtration, purification, regular lab test
5. Care to correct fitting of bearing halves and maintain correct clearances
6. Always check bearing end play and L.O distribution whenever crankcase inspected,
7. Always apply L.O by hand pump, before starting the engine. (For G/E) or keep L.O priming pump always ON during stand still.
(c) Crank pin corrosion
Causes
1. Using high sulphur fuel; SOx may contaminate with crankcase oil (by condensation) and form acidic crankcase oil. When oil is circulated, acid will attack to metal. The attack appears as very fine pitting holes and gradually extends to large area pitting. The load carrying capacity of the pin is decreased; scores to pin may wipe out bearing.
2. Cooling F.W leaking from jacket and piston cooling system promote oxidation and microbial degradation, and cause corrosion.
3. Due to water contamination, electrochemical corrosion occurs between two different crank pin and bearing metals.
Remedy
1. Use low sulphur content fuel
2. Maintain good engine performance
3. Maintain earthing arrangement of the shafting in good condition
4. Overhaul stuffing box regularly.
5. Periodically do L.O onboard test
6. Stop any leakage of water
7. carry out continuous purification of sump oil
8. Batch purification whenever opportunity has attained.
(d) Heavy pitting on gear teeth
Causes
- It is a form of fatigue failure which mostly occurs with new gear during run-in period; small alignment error, surface irregularities which subject to repeatedly overstress.
- They are always found around the pitch line or in the mid height of the tooth.
- Initial pitting indicates that the gear is heavily loaded; remedial action must be taken to avoid heavy pitting on gear teeth.
Heavy pitting on gear teeth could be caused by one or a combination of the following;
1. Poor material or Design fault
2. Shaft misalignment
3. Overload running
4. Overheat running
5. Water and other impurities in gear oil
6. Using unsuitable gear oil
Remedies
1. Use correct grade of gear oil
2. Use magnetic filter
3. Avoid overload running
4. Avoid water ingress in gear oil or sump oil
5. Periodical L.O test
6. Check shaft alignment and re-align if necessary
7. Use a higher viscosity oil
(a) State with reasons, the engine operating conditions which would indicate that a cylinder liner was cracked.
(b) Describe the action to be taken in the event of discovering that a main engine cylinder liner is cracked whilst the engine is operating at sea.
(a)
1. Loss of water from the expansion tank with no sign of external leakage.
2. High cylinder jacket water outlet temperature: The crack on liner allowing the combustion gas to enter the cooling water space and heat the cooling water.
3. Cooling F.W pressure gauge pointer will be fluctuating.
4. In extreme cases, the cooling water will boil off in the area of the crack.
5. Low exhaust temperature, loss of power, poor combustion leading to black smoke: water may enter the cylinder
when the piston is at the bottom of the stroke because the C.F.W pressure is higher than cylinder pressure at this position.
This water will vaporize during compression stroke.
Due to high specific heat capacity of water the compression temperature will be lower than normal, resulting in longer ignition delay, and loss of power.
(b)
On a 2 stroke engine, the liner is cracked while the engine is operating at sea and the engine cannot be stopped for long enough to change the liner due to operational or navigation reasons, the following actions must be taken to allow the engine to be operated with the unit out of operation: (Man B&W-MC engine)
1. Isolate cooling water from cylinder and drain.
2. Lift fuel pump follower clear of cam by using special lifting tool.
3. Put the exhaust valve out of operation:
(a) high pressure hydraulic pipe removed
(b) oil supply to the exhaust valve actuator blanked off
(c) actuator piston is lifted using a special tool so that the follower is clear of the cam.
(d) exhaust valve air spring supply is closed so that the valve held open.
4. Starting air supply to the defective cylinder to be blanked off.
5. Cylinder lubrication must be maintained whilst engine is running.
- The engine can now be operated at restricted power.
- Care must be taken not to exceed manufacturers recommended settings to avoid damage to the crosshead and bottom end bearings because there will now be no gas load on the piston.
- The engine will have a dead spot when starting. If this occurs, then give the engine a kick on air in the opposite direction.
- Surging of the turbocharger may be experienced and care must be taken to ensure that there are no excessive crankshaft vibrations, reducing load if this is experienced.
- Renew cylinder liner as soon as possible.
(a) What are the purposes of fitting cross-head in a diesel engine and what are difficulties usually found on its’ bearing concerning lubrication?
(b) How many different methods to overcome crosshead bearing problems?
(a) Purposes of fitting cross head in a diesel engine
1. To minimize the force upon cylinder liner by piston
2. To absorb side thrust between piston and liner due to angularity of connecting rod.
3. To provide long stroke of piston
4. Get more engine output
Difficulties found concerning cross head bearing lubrication
Load on bearing – Fluctuated load and high sudden load act continuous downward on lower bearing, half engine cycle.
Bearing lubrication is difficult due to the following factors.
1. Reciprocating movement – Oil supply is disturbed by vertical movement of pin and bearing.
2. Almost downward forces and no load reversal – does not provide a squeeze film into loaded part of bearing
3. Slow oscillating movement- Connecting rod swings through 25˚~30˚ (small angle), it is difficult to build up full fluid film. Boundary lubrication exits due to oscillation movement (no relative movement).
4. Full effect of combustion pressure -Cross-head bearings get maximum load and lube oil film in it is extremely thin.
(b) Different methods adopted to overcome crosshead bearing problems
(1) By using pumps mounted on crosshead- inject high-pressure oil into bearing surface when boundary lubrication exists.
(2) By increasing lubricating oil pressure- to crosshead to a level, where separation of surfaces, during light load period, such as scavenging, providing a squeeze film during high load period.
(3) By using conjugate bearings- deflection of pin and bearing remains in-line, results in lower and uniform load.
(4) By using large diameter, stiff, short pin – increases bearing speed, resulting higher bearing lube oil pressure while crosshead bearings moving on pin and retains sufficient oil pressure when at dead stops.
(5) Continuous full length bearing face under pin – Low specific load on bearing and load is transmitted directly downwards.
(a) Describe, with the aid of a sketch, the crosshead assembly showing the connections for the piston rod, connecting rod and slippers.
(b) Describe how the crosshead slippers, top and bottom end bearings are supplied with lubricating oil.
(a) Crosshead
- Piston rod is bolted to crosshead pin which reciprocates in crankcase.
- Top end of connecting rod is forged to form housing, for continuous lower bearing (made=Tin-Aluminum).
- White metal lined bearing cap incorporates a slot, to allow piston rod foot to be bolted to top of crosshead pin.
- Large diameter pin and continuous lower bearing shell gives maximum surface area, reducing load on bearing to a minimum.
- Connecting rod swings about crosshead pin and transfers downward thrust to crankshaft converting it to rotary motion.
- Large diameter, finely finished hardened crosshead pin reduce at each end, to form journals on which guide shoes or slippers locate
- Retaining covers hold shoes in place on journal.
- Guide shoes which run in crosshead guides are free to rotate, allow limited amount for misalignment.
- To allow this rotation, locating bores of guide shoe line with white metal.
- Side thrust is transferred into engine frame via guide shoes or slippers which reciprocate in crosshead guides machined in engine frames.
(b) Crosshead slippers, top and bottom end bearings lubrication
- Oil is supplied to crosshead slippers, top and bottom end bearing via a telescopic pipe (MAN B&W) or swinging arm link (Sulzer).
- On MAN B&W engine, oil is supplied to crosshead slippers, top and bottom end bearing from main L.O system
- Lower crosshead bearing shells and slippers have oil grooves machined to assist oil distribution
- On the Sulzer engine, oil is supplied drilling in pin to crosshead slippers, top and bottom end bearing from oil is boosted in pressure to about 12 bars.
(a) Sketch a cross section of a main engine structure comprising bed plate frames and entablature showing the bolt in position.
(b) Explain, why tie bolt need to be used in some large slow speed engine. The purpose of tie bolt on large engine.
(c) Explain in detail how the bolts are tensioned. Procedure of checking and tightening the bolt.
(d) Which precaution you have to done before tie bolt tightening. Precaution to be taken before and after tightening.
(e) Effect of tie bolt slack.
(f) What are important during tie bolts maintenance.
(g) Explain why tightening of tie bolts to within prescribed limit important.
(a) Cross section of main engine bed plates, frame, entablature
(b) Purpose of tie bolt
- To secure bed plate, column and the cylinder block firmly, to assembly engine structure.
- To transfer firing load down to main bearing girders
- To minimize bending stress in bed plate
- To achieve mechanical stability of the whole engine assembly under running condition.
- Fit two tie bolts, on each transverse girders as close as possible to the shaft centerline.
- To prevent vibration, tie bolt fitted with anti-vibration bush and pinching screws.
(c) Procedure for checking and tightening
- Before tightening, take and record crankshaft deflection.
- Slack pinching screws on the bolts and loosen main bearings thrust bolts.
- Remove protecting cap and clean contact surface and thread of bolts.
- Use a dial gauge to detect relative movement between tie bolt and entablature.
- Record the movement throughout the operation, before and after checking and tightening of each tie bolts.
- Fit the hydraulic tensioning device correctly and apply pressure as per maker’s instructions.
- Tightening done in pairs at a time starting from the middle of the engine and working outwards alternately.
- Raise Hydraulic pressure in two stages. 350bar in first stage, 520bar in second stage.
- Insert feeler gauge through inspection slot to check clearance between tie bolt nut and landing surface, turn nut again for possible movement.
- If no clearance exist or no movement, tie bolts are in properly tightened condition.
(d) Precaution
Before tightening
- Shut off starting air supply
- Engage turning gear
- Take and record crankshaft deflection
- Slack pinching screws
- Slack main bearing Jack bolts as per maker instruction (if fitted)
After tightening
- Apply anti-corrosive coating, fit back protective caps.
- Retighten pinching screw.
- Retighten the main bearing thrust bolt (jack bolt) correctly.
- Take crankshaft deflection again and compare with recorded value
(e) Effect of tie bolt slack
- Misalignment of cylinder liner and piston.
- Bending movement in bolts.
- Uneven stress in the bolts cause nearly fatigue failure.
- Tie bolts cannot take the proper load.
- Crosshead guide faces and bars will be slackened.
- Fretting the landing surfaces of cylinder block and tie bolt nut.
- Whole structure will be destroyed.
(f) Important points during tie bolts maintenance
Before doing any job on tie bolts
- Take crankshaft deflection
- Slack main bearing jack bolts
- Slack pinching Screws
- Always tighten correct sequence as per maker instruction
- Bolts must not tightened and slacked in one stage
- Tightened back main bearing jack bolts and pinching screws.
- Take crankshaft deflection again and compare with record value.
(g) Tie bolts tightening within limits
Overtightening the bolt
- Increase tensile stress
- Increase bending moment between centre line of transverse girder and tie bolts
- Increase cracking of transverse girders
- Slack the bolt
