(a) What are the bearing design and requirement?
(b) Explain function of oil grooves and wedges.
(c) Explain following.
(1) Bearing crush
(2) Surface finish
(3) Spark erosion
(a) Bearing design and requirement
1. Bearing sliding surface geometry.
2. Roughness surface of journal or pin, which determines the permissible bearing pressure and required oil film thickness. for effective and safe functioning of the bearing.
3. The correct flow of the cooling oil to prevent heat accumulation, provided either through the clearance between the journal & the bearing bore or through axial grooves in the bearing sliding surface.
(b) Function of oil grooves and wedges
1. To improve the oil distribution over the load carrying surfaces.
2. In crosshead bearings, to assist hydrodynamic oil film between the load-carrying surfaces.
3. To provide oil cooling (oil grooves). to perform these functions, oil flow freely from the oil grooves, past the oil wedges, and into supporting areas, where oil film carries the load.
(c)
(1) Bearing crush
- Bearing shells diameter is slightly greater than the housing.
- Difference between these two diameters is bearing crush.
- The crush provides the friction to prevent bearing shell from rotating in housing.
(2) Surface finish
- Machined surfaces are never perfectly smooth.
- Finely finished surfaces damaged less than rough surfaces by the metal-to-metal contact that occurs under boundary lubrication conditions.
(3) Spark erosion
- Spark erosion is caused by a voltage discharge between the main bearing and journal surface.
- Spark erosion is only observed in main bearings and journals.
- To protection spark erosion, an earthing device is installed; check regularly effectiveness
- Keep potential level at maximum 80 mV.
(a) Regarding the two stroke engine, how to calibrate following bearings
(i) Main Bearing
(ii) Crankpin Bearing
(iii) Crosshead guide shoes bearing
(b) What are the causes of bearing damage?
(a) Calibrate bearing
(i) Calibration of main bearing
Too large difference in crankshaft deflection readings, check individual bearing
Method (1): After removing the bearing top cover and shell, a special ‘Bridge’ gauge is placed.
- Clearance is taken by inserting a feeler gauge between bridge gauge and the journal.
Method (2): A special retractable feeler gauge is inserted between upper shell and journal to take reading.
Method (3): Remove screws of lub.oil pipe from main bearing cap, set measuring tool (dial gauge) to zero and insert into bore of screw to take reading.
- The clearance must not exceed 0.1 mm than original.
(ii) Calibration of Crankpin bearing
- Turn crank to BDC.
- Insert a feeler gauge at bottom of the bearing shell in both sides and note down.
- When adjusting bottom end bearing, use of thin lead wire is better result than feeler gauge.
- Clearance must not exceed 0.1 mm than original.
- If limit exceeds, crankpin bearing disassembled for inspection.
(iii) Calibration of Crosshead guide shoes bearing
1. The crank pin stand in a horizontal position 90˚ towards fuel pump side.
- Crosshead is automatically pressed by con-rod against rails surfaces on exhaust side and clearance is taken on fuel pump side with feeler gauge.
2. Crosshead bears on one side fully.
- Take clearances take on both exhaust and fuel pump sides.
- One side give a ‘zero’ value.
3. The clearance must not exceed 0.1mm than original.
- Crosshead bearing disassembled for inspection.
(b) Causes of Bearings damage:
1. Insufficient supply of Lube oil.
2. Wrong grade of Lube oil.
3. Dirty and impurity content in Lube oil.
4. Bearing corrosion by combination of fuel and Lube oil, resulting in weak acid formation.
5. Vibration by incorrect power balance, running in critical speed and misalignment.
6. Mishandling during inspection and maintenance.
With reference to crossheads
(a) State why the lubricating oil supply pressure is higher than that required for the main bearings;
(b) State, with reasons THREE possible causes of top end bearing failure.
(a) lubricating oil supply pressure is higher than that required for the main bearings
- Main bearings are hydrodynamically lubricated.
- Crankshaft rotation allows supply oil from top(pressure of 3 ~ 4 bar), to be swept into the loaded lower-half
- Oil wedge built up preventing metal to metal contact between main bearing and shaft.
- Higher pressure better forming of wedge action.
- Crosshead bearing is swinging about the pin, changing direction every stroke so that oil cannot be swept into the loaded lower half bearing. Oil must be supplied under pressure.
- On the Sulzer RTA engine, oil is injected between pin and bearing at about 12 bars supplied by booster pumps.
(b) THREE possible causes of top end bearing failure
(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) 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.
(a) Why top end bearings of large slow speed engine are more prone to failure than other bearings?
(b) Why crosshead pin diameter is greater in proportion to pin length?
(a)
Top end bearings are more prone to failure than other bearings due to the following facts:
(1) High Sudden Load: The combustion gas pressure, load on crosshead pin is about 600 tons when cylinder pressure is at maximum. That load directly effects to the bearings.
(2) No Load Reversal: In two stroke engine, load is always vertically downward and no load reversal takes place. So the bottom halves of the bearings carry all loads.
(3) Distortion: Combustion pressure is applied on the centre of the pin and reaction on either end which causes deflection of crosshead pin and bearings, resulting poor alignment and uneven load distribution.
(4) High Bearing Pressure: Space for crosshead assembly is limited and it reciprocates full length of stroke. The bearing area is limited and result in high pressure on bearing. (about 140 bar or more).
(5) Lubrication problem: There are two difficulties of crosshead bearing lubrications.
- Reciprocating movement: oil supply is disturbed by vertical movement of pin and bearing, and difficult to get smooth flow of lubrication.
- Oscillating movement: connecting rod swings about the pin just 25°~30˚ travel, it is difficult to build up full fluid film, boundary lubrication exists due to oscillation movement.
(b) The loads on the crosshead pin and bearing are very high.
Load (F) = Pressure (P) x Area (A)
- If area of pin increase, the effective pressure on pin and bearing will reduce.
- The area of pin depends on two factors, (1) diameter of pin and (2) pin length.
- If pin diameter is smaller in proportion to pin length, less stiffened to withstand high firing load
- Longer pin acts as simple support beam with central load and two reactions, resulting in more deflection of the pin and bearing which may lead to the bearing failure.
- Space for crosshead is limited.
In some modern engine design, the pin diameter is increased beyond the theoretically required.
- A larger surface area reduces the pressure on bearing.
- Large diameter pin has more rigidity and greater relative velocity (speed) than small diameter pin.
- Creates to carry more oil amount and increases the kinetic energy of oil film, reducing the boundary lubrication effect to the crosshead bearing.
- Pin deflection is also less than small diameter pin and reduces possibility of bearing failure.
- In some designs, diameter of crosshead pin is nearly bore of piston to withstand the high pressure load and reduces the effective pressure on the crosshead bearings.
Define each of the following terms and state its relevance to combustion with aids of sketches.
(a) Ignition delay period
(b) Atomization
(c) Penetration
(d) Turbulence
(a) Ignition delay period
- Time duration from point of injection (fuel valve opening) to point of ignition (combustion pressure rise).
- If time delay too long, large amount of fuel injected before burning begins.
- When ignition start, very rise of pressure causes diesel knock.
- Ignition Delay 0.5 to 10 millisecond.
- Heavy oil has longer ignition delay time than lighter oil.
- Long delay time is unsuitable for starting of cold engine.
- Reduced by increasing compression pressure and temperature.
(b) Atomization
- Atomization is splitting up of fuel into very small droplets when it is injected into cylinder.
- Fuel injector forcing fuel, at high pressure through small atomizer holes.
- The size of droplet will depend upon
- size of holes and
- Pressure difference between fuel pump discharge and compressed air in the combustion chamber.
- Atomization is necessary to get complete combustion.
(c) Penetration
Penetration is distance oil droplets travel into combustion space before mixing with air and igniting.
Penetration depends upon
- Size of the fuel droplets
- Velocity of fuel injected and
- Condition of combustion chamber
Good Penetration is
- to penetrate the fuel into the whole combustion space for good mixing with air,
- but not to impinge internal surface before burning.
(d) Turbulence
- Turbulence is movement of compressed air and fuel, within combustion space before combustion occurs.
- Turbulence improves mixing of fuel and air, for effective and rapid combustion.
Explain below topics, causes and effect with aids of indicator diagrams
(a) Early ignition
(b) Late ignition
(c) After burning
(d) Exhaust valve timing
(a) Early ignition
It can be seen from Fig. that this causes abnormal high peak pressure at about TDC.
Causes
1. Fuel Cetane no: is higher than normal.
2. Fuel pump incorrect timing
3. Broken or wrongly-set injector spring
4. Overheating of parts within the cylinder
Effect
1. Abnormal high Pmax
2. High thermal efficiency
3. Low exhaust temperature
4. Heavy shock load to the running gear and bearings with knocking sound
5. Shock load and vibration cause damage to components
(b) Late ignition
It can be seen from Fig. that this causes low peak pressure well after TDC.
Cause:
1. Fuel cetane no: of is lower than normal
2. Fuel pump leaking or incorrect fuel timing
3. Excessive injector spring setting
4. Under cooling of parts within the cylinder.
5. Poor quality fuel and atomization
6. Low compression
7. Insufficient supply of combustion air
Effect:
1. Low Pmax
2. High exhaust temperature and smoke.
3. Loss of power due to incorrect fuel burning
4. Combustion continue during the expansion stroke (Afterburning)
(c) After burning
During the expansion stroke, slow or late combustion of fuel, during lower part of stroke expansion line is rise
Cause:
1. Bad fuel oil
2. Incorrect fuel grade
3. Incorrect fuel temperature
4. Incorrect injection timing
5. Poor atomization
6. Poor or excess penetration
7. Insufficient air supply
Effect:
- High exhaust temperature and pressure
- Bum exhaust valves and foul the exhaust system, with risk of turbocharger surging or uptake fires.
- High temperatures within the cylinder cause deterioration in lubrication and possible damage to liner surface and piston rings.
- Burning of the piston crown.
(d) Exhaust valve timing
On a slow running engine exhaust valve timing checked by light spring indicator diagram. This will not give an accurate timing check but compare with a normal diagram, if valve opening is early or late.
Causes
1. Defective exhaust valve actuator
2. Incorrect tappet clearance
3. Incorrect exhaust valve driving oil pressure
4. Incorrect air spring pressure
5. Excessive wear of air piston or oil piston
Effects
1. Exhaust valve burning
2. Incomplete combustion
3. Power loss
4. Fouling of exhaust system
