By Maklub Al Mostofa
Q. What is scavenging? Name the types of scavenge used for large two stroke engine. Describe the advantage of Uniflow scavenging.
SCAVENGING:
It is the removal of residual exhaust gas and its replenishment with fresh air in an internal combustion of the engine. The fresh air intake and exhaust gas expel operation are not simultaneous fully but some degrees of overlap period are provided for better efficiency.
There are 3 types of scavenging process.
1. Loop scavenging
2. Cross scavenging
3. Uniflow scavenging
Loop scavenging:
In this type of scavenging air passes over the piston crown and rises to form a loop. Ports are cut in the cylinder liner wall for this operation. The scavenge ports and exhaust ports are in the same side of the liner.
Image credit: Introduction To Marine Engineering by D A TAYLOR
Cross scavenge:
In this type of scavenging the scavenge air is directed upwards and expelled from the opposite side of the liner. Scavenge and exhaust Ports are cut in the cylinder liner wall for this operation. The scavenge ports and exhaust ports are in opposite side of each other.
Image credit: Introduction To Marine Engineering by DA TAYLOR
Uniflow scavenging:
In this type of scavenging scavenge air enters through ports near the bottom of the liner then travels straight up the liner and forcing the exhaust gas to expel from the top through valve arrangements. This gives maximum efficiency of scavenging.
Image credit: Introduction To Marine Engineering by D A TAYLOR
Advantages of Uniflow scavenging:
• Highest efficiency of scavenging and Very little amount of exhaust and scavenge intermixing.
• Provides higher degrees of overlapping period hence increasing efficiency of the engine.
• Allow simplicity of liner as fewer ports are cut.
• More liner cooling is achieved/Thermal stresses are less compared to other method of scavenging.
• Avoid long piston skirt.
• Make satisfactory & economical of cylinder lubrication/ Cylinder L.O. consumption is reduced.
• Efficient design for long stroke engine.
“Pulse” system:
Image credit: MARINEDIESELS.UK.COM
The Pulse system takes advantage of the higher pressures and temperatures of the exhaust gases during Blow—down period and with rapidly opening of exhaust valves or ports.
The gases leave the cylinder at high velocity as pressure energy is effectively converted into kinetic energy to create a pressure wave or pulse in the exhaust pipe.
The pipe so constructed in small diameter is quickly pressurised and boosted up to form a pressure pulse.
The pulsating pressure waves reach up to the turbine nozzles and further expansion takes place.
The pressure in the exhaust pipe before the turbine shows a cyclic variation as is evident from the pressure crank angle diagram.
Advantages of pulse system:
1) High available energy at turbine.
2) Good performance at low speed & part load.
3) Good turbocharger acceleration.
4) Highly responsive to any changing load due to small volume of exhaust ducting
5) Required no scavenge assistance at any changing load
Disadvantages:
1. The exhaust grouping is complicated.
2. Different sizes of exhaust pipes are needed for spare.
3. Exhaust tuning and grouping requires special attention to avoid back flow and incorrect tuning could seriously affect the engine performance.
Constant Pressure System:
Image credit: MARINEDIESELS.UK.COM
In the constant-pressure system, exhaust gases from all cylinders pass into a common large diameter exhaust manifold.
This is of sufficient capacity so that the intermittent exhaust from the different cylinders does not cause pressure fluctuations.
Because of the fairly constant gas pressure and temperature at the turbine inlet; it operates at about its optimum efficiency.
This system is most suitable for high output engines, and there is no need to group the cylinder exhausts into multiples of three.
A major disadvantage of the constant-pressure system is that when running at reduced speed and especially when starting up, the pressure energy available at the turbine inlet is insufficient to drive it fast enough to supply the quantity of air at the scavenging pressure necessary for efficient scavenging and combustion.
Therefore, it is necessary to be made an additional source of air compression, such as reciprocating pump or turbo blower.
Turbocharger Arrangement in Constant Pressure System:
• No exhaust grouping required and pipe distribution is simple.
• Exhaust gases enter into large common manifold and then to turbine
• Firing order not considered
Advantages:
1. Good performance in high load.
2. More suitable for high output engine.
3. There is no need to group the cylinders exhaust into multiple of three.
4. High turbine efficiency due to steady flow of exhaust.
5. Reduction in SFOC (Specific Fuel Oil Consumption)
Disadvantages:
1. When running at reduced speed and starting up low available energy at turbine. Thus it supplies inadequately air quantity of the scavenge pressure necessary for efficient scavenging and combustion.
2. It requires Auxiliary Blowers.
3. Poor response in changing load.
Turbocharger Surging:
• It is the aerodynamic phenomena which produces a back flow of air from discharge to suction side (impeller and diffuser) when the discharge volute pressure exceeds the pressure build up in the suction side and it causes heavy noise and vibration of turbocharger.
Causes of Turbocharger Surging:
1. Suddenly load change by heavy sea
2. Scavenge space fire
3. Exhaust trunk fire
4. Poor power balance
5. Dirty nozzle and blades
6. Individual cylinder misfire
7. Chocked scavenge /exhaust ports
8. Incorrect matching of turbocharger to engine.
9. Poor scavenging or leaky exhaust valve
Advantages of Inter Cooling the Charged Air:
• Reduce scavenge air temperature
• increase the density of air delivered to the cylinders
• Increasing the power output delivered by the engine.
• Cooled scavenge air reduces cylinder and exhaust gas temperature at a given power level.
• Maintain temperatures within the acceptable limits.
Types of Turbocharger Lube Oil System
There are two methods of lubrication:
1. Own sump
2. External lube oil supply system
Types of Turbocharger Bearings
Q. Describe the type of T/C bearings. Discuss their strength & weakness.
Ans: There are two types of T/C bearings:
1. Sleeve Type.
2. Ball or roller type.
Ball type bearing (rolling type):
The ball and roller bearings are mounted in resilient mountings incorporating spring damping to prevent damage due to vibration. These bearings have their own integral oil pumps and oil supply, and have a limited life.
Advantages of ball or roller type bearing:
• Less friction losses.
• Minimal lubrication
• Greater alignment accuracy.
Disadvantages of ball or roller type bearing:
• More expensive.
• Limited life time
• Need higher grade lubrication.
• More susceptible to vibration and fatigue
Sleeve type bearings:
• Sleeve type bearings are lubricated by external L.O supply system. This is achieved either by
– Gravity from independent header tank situated about six meters above the bearings to ensure that LO pressure never drops below 1.6 bar. Or by
– A main L.O pump leads to the bearings with a separate L.O pipe line. It has also gravity tank in case of failure of L.O supply. The system incorporates fine filters to avoid impurities and foreign particles.
Advantages of sleeve type bearing:
• Load carrying capacity is more.
• It can run at higher temperature.
• Better inlet flow.
• Shorter rotor length
• Higher efficiency under full load condition.
• Time between overhauling is more than the other types of bearings.
Disadvantages of sleeve type bearing:
• At low speed/load create high friction.
Function of Labyrinth Seals:
• Labyrinth seals are of specially designed metallic fin type seals that separate the bearings from the blower and turbine.
• These seals are sealed by air supplied from the compressor volute casing is led into a space to prevent oil entering the blower and to prevent contamination of the oil by the exhaust gas.
Q. What is exhaust tuning?
• Ans: Exhaust tuning is the arrangement of exhaust pipe lay out with suitable length & proper v/v timing to exhaust into the same pipe without affecting the engine performance.
Q. How the exhaust of one cylinder is prevented from interfering the scavenging of another in the pulse system?
Answer: In pulse system, the exhaust of one cylinder is prevented from interfering the scavenging of another as follows.
– Exhaust system is tuned so that the manifold pressure pulsation is not reflected back to engine as backpressure.
– As the exhaust valve of a diesel engine opens, the gases in the cylinder rapidly expand and gain velocity and kinetic energy as they pass into the exhaust pipe.
– As the exhaust gas travels through the manifold it causes partial vacuum behind it.
– The cylinders are so connected to a common exhaust pipe so that the exhaust valve of the second cylinder is opened when the exhaust from the first cylinder has created a vacuum in the pipe
– Exhaust from the second cylinder will be discharged more easily.
– Thus the tuned exhaust system with the exhaust pipes a suitable length and arranging for two or three cylinder with suitable exhaust valve timing prevented from interfering the others.
Q. State the indication of scavenges fire.
Ans: Indication of scavenge fire:
• High exhaust temperature of corresponding unit.
• High local temperature in scavenge trunk.
• Black smoke in exhaust gas.
• Burning smell
• Surging of T/C.
• Flame, spark or smoke emitted from the scavenge drains.
• Loss in power, irregular and fluctuating rpm of engine.
Q. Describe the causes & prevention of scavenge trunk fire?
Ans: Causes of scavenge fire:
• Accumulation of unburned fuel or cylinder L.O. in scavenge trunk.
• Blow past of the engine due to over load or bad piston rings..
• Bad combustion due to bad fuel management.
• Faulty injector and fuel pump timing.
• Excessive cylinder lubrication.
Prevention of scavenge fire:
• Regular draining, cleaning and monitoring of the scavenge space.
• Correct rate of cylinder lubrication.
• Proper maintenance & correct adjustments of piston rings, cylinder liner and fuel injection equipments.
• Ensure balanced engine.
• Safety devices such as electrical temperature sensing, pressure relief valves should be fitted in the scavenge trunk.
Q. How scavenge fire can be avoided? What are the safety devices incorporated in an engine for scavenge fire.
Ans: Step to avoid scavenge fire:
• Proper maintenance of the fuel injection equipments, piston rings as per maker instruction.
• Not to over load the engine.
• Periodically cleaning and inspection of scavenge trunk.
• Regular draining of scavenge trunk.
• Monitor the under piston scavenge space temperature
Safety devices for scavenge trunk:
• Scavenge trunk relief valve.
• High temperature sensing device with alarm.
• Fixed fire extinguishing system.
• Sight glass on scavenge drain line.
• Non-return valve.
Q. Describe the action to be taken in case of scavenge fire.
Action to be taken:
• Activate fire alarm.
• Inform bride & chief engineer.
• Stop the engine but L.O. pump keep on running.
• Engage turning gear & rotate the engine.
• Apply fire extinguishing medium to scavenge trunk.
• Arrange boundary cooling.
For more details, you can visit the the link: http://marinestudy.net/diesel-engine-scavenge-fire/
Q. With reference to large T/C state-
a) How air & gas tightness between rotor & casing achieved?
b) Explain how the gas can raise the pressure of air higher than its own?
c) What is the purpose of nozzle ring?
Ans: a) Air & gas tightness between rotor & casing achieved by allowing some of the air from compressor outlet through a labyrinth arrangement which act as a seal.
b) The gas can raise the pressure of air higher than its own because of T/C.
• The inducer guides the air flow smoothly into the eye of impeller which throws the air outwardly increasing velocity due to centrifugal force at high rotational speed.
• The diffuser reduces velocity by converting kinetic energy into pressure energy and leads the air to the volute casing.
• The volute casing further decreases the velocity and increases the air pressure.
c) Purpose of nozzle ring:
• Expand the exhaust gas and direct exhaust gas to turbine blade.
• Convert the pressure energy of gas to kinetic energy giving a high velocity.
Q. Purpose of inducer.
Ans: .
• The purpose of inducer is to guide the air smoothly into the eye of the impeller.
• It also takes shock of the incoming air therefore supplying smooth air into the impeller.
Function of Diffuser in turbochargers?
• To direct the air smoothly into volute casing
• Convert kinetic energy to pressure energy.
What is K value in turbochargers?
• It is the distance between the rotor shaft end and the flange of bearing cover measured by blower side.
• It is critical clearance necessary to avoid contact rotating impeller and stationary blower casing cover in case of thrust bearing worn out.
Q. With reference to T/C give reasons for corrosion on the: a) gas side b) air side c) water side.
Ans: Reasons for corrosion on gas side:
• Poor combustion and carryover of oil causes carbon build up in nozzles and blades. It prevents proper heat exchange.
• Oxide of sulphur from combustion may cause acidic corrosion under light load with low exhaust temperature.
• Vanadium & sodium ash from fuel combustion may deposit & causes local burning.
• Deposit of calcium sulphate cylinder L.O. may causes corrosion.
Reasons for corrosion on air side:
• Oil vapour & Dust from engine room may sucked by the compressor & causes corrosion on air side.
• Salty moisture which is present in the air at sea sucked by the compressor & causes corrosion.
Reasons for corrosion on water side:
• Water side may be corroded by electro- chemical if the pH value of water is low i.e. water is acidic.
• If there is inadequate protection against sea water corrosion.
• Improper cleaning schedule or PMS.
Q. With reference to the T/C state-
a) Why turbine blades are loosely attach to the rotor?
b) How axial movement of the turbine blade is prevented?
c) How blade vibration is dampened over a wide range of speed?
d) What is the purpose of sealing air?
Ans:
a) The root is usually a slack fit to allow for differential expansion of the rotor and blade and to assist damping vibration. Blade roots are of fir tree shape which gives positive fixing and minimum stress concentration at the conjunction of root and blade.
b) Axial movement of the turbine is prevented by fixing the locking strip.
– In case of ball & roller bearing turbocharger axial thrust is balanced by lamina damping spring assemblies in bearing housing.
– In case of sleeve type bearing turbocharger axial thrust is balanced by sealing air which is fed to the chamber behind the turbine disc from compressor outlet at sufficient r.p.m. of rotor.
– But at start up, shut down & very low speed axial thrust is taken by thrust bearing.
c)
d) Blade vibration is dampened over a wide range of speed by binding the blade with a lacing wire. The wire passes through holes in the blades and damps the vibration due to friction between the wire and blade. It is not fixed to each individual blade.
IMAGE CREDIT: MARINEDIESELS.UK.COM
e) Purpose of sealing air:
• Seal the labyrinth gland to protect L.O. contamination by exhaust gas at turbine side.
• Seal the labyrinth gland to protect L.O. & air mixture at blower side.
• Cool the rotor shaft & turbine disc.
• Oppose the axial thrust towards compressor.
Q. With reference to the T/C state- Cooling water casing got holed, how to run T.C?
Internal water leakage:
– Stop the engine.
– Stop water supply
– Arrange compressed air supply as coolant.
– Attention must be paid to the operating temperatures especially for the turbine end bearing.
Q. Advantage of charge air cooler.
Ans:
• Increased density of air ( allow a greater mass of air to be compressed)
• Increases scavenge efficiency.
• Increases output power.
• Increases fuel burning capacity.
• Engine is maintained at safe working temperature.
• Reduced thermal stress on piston rings, piston & liner.
Q. How will you know turbocharger air filter chocked?
Ans: Filter condition can be assessed by the following parameters and symptoms:
• Greater differential pressure at turbo charger filter.
• Reduced engine power.
• Black smoke from the funnel
• Less Scavenge pressure
How will you run engine in case of turbocharger failure?
• Engine to be stopped.
• Rotor to be blocked
• Exhaust gas to be by-passed the turbocharger
• Run engine with reduced speed with remaining turbocharger
• Use Auxiliary Blower
• Maintain all temperature and pressure of fuel, cooling water and lubrication within limit
Q: During normal engine operation a turbocharger rapidly loses speed and the speed reduction is accompanied by appreciable noise.
(a)State with reasons the possible causes.
The two factors of loud noise and rapid speed reduction indicates that rotational
friction has dramatically increased, or the rotor is in contact with the stator, which
could be due to
◊Bearing failure: Bearing failure will affect the rotor clearances, causing the rotor to contact the stator. Bearing may fail due to
– Lack of lubrication
– Dirty lubricant
– Lack of cooling
– Excess vibration
– Failure due to fouling
– Failure of the resilient mounting
– Over running hour.
– Faulty sealing system.
◊Mechanical damage to the rotor: If any component from the combustion chamber is admitted to the exhaust side of the rotor, the rotor will be imbalanced and may leading to possible bearing failure.
◊Failure of the water coolant casing: This will admit water to the gas inlet and cause a reduction in the rotor speed
b. State with reasons the factors which may limit engine operating speed with the turbocharger out of operation
Ans:
The exact power limit with a defective turbocharger will depend upon the actual configuration of the system and how many turbochargers remain in operation
The following factors need to be considered when operating the engine:
◊Exhaust temperatures: The actual operating conditions should be taken into consideration and engine manual would be consulted as to the maximum limit
◊Exhaust smoke levels: Exhaust color should be monitored. Allowance should be made for the change of rate acceleration to prevent excessive thermal load.
◊Maximum engine power: This will be limited due to the factors stated above, and this will probably be approximately 50% power with the loss of one of the two turbochargers.
Hence the engine power would be reduced in consideration with all the above factors.
With reference to turbo-chargers:
a) state how the in-service performance checks are undertaken for EACH of the
Following:
i) the gas side
ii) the compressor
iii) the suction filter
iv) The after cooler
b) State with reasons the action require to maintain satisfactory performance of
i) The turbine
ii) The compressor
b) Should in-service vibration be experienced state with reasons the possible causes?
GAS SIDE: The main monitoring data is the temperature differential across the T/C for a given turbocharger speed. It gives an indication of energy converted by the gas side, i.e: nozzle ring and blade. The fall off performance of these components would be mainly due to fouling.
COMPRESOR SIDE: The compressor performance would be reduced by fouling of the compressor wheel and diffuser. This fouling would also cause the scavenge air pressure to fall, whilst the
T/C was maintaining a stable speed and all other parameters were normal. Hence the delivery pressure of the compressor would be monitored.
SUCTION FILTER: The suction filter performance can be checked by the followings:
– Scavenge air pressure
– Pressure drop across the filter. A manometer fitted across the filter will show an increase in differential pressure.
AFTER COOLER:
The after cooler performance would reduce when fouling is present.
– If the fouling is at the air side this would be monitored by measuring the air pressure drop across the cooler.
– If the fouling is at the water side this would be monitored by the reduction in temperature difference between the cooling water inlet and outlet.
The performance of the turbine is maintained by:
– The fuel combustion should be kept to an optimum condition.
– The cylinder oil doses should be avoided.
– Regular in-service cleaning, The in-service cleaning can be carried out using either dry cleaning involving dry particle cleaning, and/or water washing of the turbocharger using warm water and air blast .
– Manual cleaning of the turbocharger on a yearly operational running hour routine.
The performance of the compressor is maintained by:
– Regular in-service cleaning, supplemented by manual cleaning of the compressor and diffuser on a yearly operational running hour routine.
– The in-service cleaning can be carried out using a small quantity of warm water injected into the turbocharger suction housing at full T/C speed. This cleaning can be carried out twice weekly.
– The air drawn into the compressor should be as clean as possible. The air filter should be maintained in good condition.
The T/C can vibrate due to the following causes:
– Rotor imbalance following inadequate in-service cleaning.
– Vibration can be caused by the blade failure due to corrosion, fatigue, failure of a portion of lacing wire.
– Rotor imbalance due to failure of the rotor support bearings.
– Rotor imbalance due to rotor damage from an object displaced from the cylinder, such as a broken piston ring or exhaust valve.
– Surging of the turbocharger, when the unit is operated close to the surge line possibly due to a fouled hull.
– Vibration transmitted to the turbocharger unit from an adjacent machinery unit.
Following a recent turbocharger overhaul it is observed that the scavenge air pressure is lower than it was previously and the engine power output is also reduced.
a. State, with reasons, the possible causes of the problem.
b. State, with reasons, what engine operational information should be gathered in order to enable the possible cause of the problem to be detected.
c. Explain how the defect may be rectified.
d. State the instructions which should be issued with respect to future turbocharger overhauls in order to prevent similar incidents.
Answer:
a.
A turbocharger overhaul consists of removing the rotor and nozzle ring for inspection and cleaning, renewing the bearings where necessary, inspection of the labyrinth glands, cooling water spaces and gas inlet protection grid.
Causes of low scavenge pressure are:
– Dirty air filter. Overlooked cleaning the air filter before refitting.
– Scavenge dump valves left open.
– Damage to turbine blades. This could have occurred when removing refitting rotor. The efficiency of the turbine will be reduced if the blade profile is damaged. May also cause vibration.
– Damaged impeller; again while removing or refitting. See above.
– Defective or incorrectly fitted bearings allowing rotor to rub against casing labyrinth glands may be damaged as a result.
– Foreign bodies in protection grid, nozzle ring, turbine blades, compressor or diffuser. Compressor axial clearances incorrect due to badly fitted bearing; allowing air to leak back from discharge to suction. May also cause back of compressor wheel to rub against casing.
– Waste gate or a charge air by pass valve, may be jammed open or not operating correctly (if fitted)
b. Operational information to be gathered:
– Turbocharger speed: The faster the turbocharger revolutions, the greater mass flow of air produced, and the higher the scavenge pressure. Slow turbocharger rpm could be an indication of all of the above faults.
– Vibration (and noise): Vibration can indicate damaged unbalanced rotor; noise indicates rotor in contact with casing.
– Exhaust gas inlet temperature to turbo-charger: A higher than normal temperature (along with higher exhaust valve temperatures)are an indication of a high exhaust gas back pressure could be caused by choked nozzle ring.
– Exhaust gas temperature difference across turbine: A smaller than normal differential will indicate that the exchange in energy is not taking place across the turbine.
– Pressure drop across air inlet filter.
-Temperature differential across air cooler water inlet and outlet.
– Surging of the turbocharger
c.
If the T/C is vibrating or unusually noisy:
– Engine must be stopped and the cause investigated. This will entail removal of the rotor. Check compressor wheel, shaft, and labyrinth, turbine wheel for signs of damage or rubbing. Check casing and nozzle ring for damage. If damage has occurred then it must be rectified before the rotor is rebalanced and refitted with the correct bearings.
If there is no vibration or noise:
– Check dump valves, bypass and waste gate if fitted.
– Check that the air filter is clean.
– clean the air cooler on the air side.
If none of these are successful, then it may be necessary to open up the turbo-charger and check that it has been assembled correctly with correct axial clearances and that there are no blockages in the nozzle ring or protection grid.
d.
The overhaul of the turbocharger should be overseen senior member of the engineering staff as appointed by the Chief Engineer.
– Only chemicals recommended by the manufacturer are to be used at the recommended strengths.
– After cleaning the turbine rotor and compressor wheels are to be examined closely for any sign of damage before re-assembly.
– New bearings to be fitted if the running hours are within 20% of the recommended maximum hours for the bearing.
– Only manufacturers spares, sealed in tins are to be used.
– Axial clearances must be checked and recorded after comparison with the original clearances
– Ensure that no gloves, rags, tools etc are left in the casing or exhaust pipes before boxing up, and that the protection grids are clean and in position.
– The air filters and air cooler is to be cleaned.
– Bypass valve and waste gates are to be checked for correct operation on completion of overhaul.
Turbocharger vibration:
Sudden vibration:
– Sudden imbalance due to blade damage
– Sudden imbalance due to partial dirt removal.
– Breakage of damping wires
– Breakage of resilient mounting.
– Bearing failure
– Poor cylinder combustion.
– Damaged support
– External excitation
Remedial action:
– Regular and effective in service cleaning.
– Effective LO management.
– Regular inspection of bolts and casing.
– Maintain optimum cylinder combustion condition.
– Protection grid condition
Gradual vibration:
– Ineffective in service cleaning.
– Gradual fouling
– Slackening of bolts
– Deterioration of resilient mounts for bearings.
– Ineffective PMS.
Remedial action:
– Maintain PMS
– Effective Water/solid wash.
– Regular bearing exchange
– Regular inspection and maintenance of the supports.
Q: State the Methods of operating with defective turbocharger.
Methods of running the engine with defective turbocharger depend upon design consideration and the repair facility.
Answer:
Bypass arrangement:
Some design provides a different path for the exhaust gas to pass to the uptake.
Engines with exhaust by-pass
1. Stop the engine.
2. Lock the turbocharger rotor.
3. Remove the blanking plate from the exhaust by-pass pipe.
4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
5. Run engine with reduced load.
Engines without exhaust by-pass:
This system allows the damaged rotor to be taken out of the engine. This may also allow repair work of the rotor is possible.
1. Stop the engine.
2. Remove the rotor and nozzle ring of the turbocharger.
3. Insert blanking plates.
4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
5. Cooling should be maintained for the turbine casing if possible.
6. Run engine with reduced load.
Engine with locking the rotor:
1. Stop the engine.
2. Lock the rotor of the defective turbocharger.
3. Rather than completely blanking off the compressor an orifice plate is fitted to allow controlled amount of cooling air to pass to prevent overheating of the rotor due to conduction
4. Cooling should be maintained for the turbine casing if possible.
5. LO supply to stop.
6. Run engine with reduced load.
Q. Describe how the following conditions are caused and dealt with
• Gradually decreasing in scavenges air pressure.
• Suddenly increasing in scavenges pressure.
• Scavenge air temperature at consistently above normal.
(a)Scavenging air pressure drops in spite of the normal speed of the turbocharger means that resistance has increased in the passage between air inlet and the scavenge air valves.
• Problem with the turbocharger
• Negative pressure set up inside scavenge trunk.
• Air suction filters fouling.
• Faulty scavenge air non-return valve.
• Charge air cooler fouling.
• Turbocharger deceleration.
• Scavenge fire.
• One of the cylinder units misfiring.
• Fouling exhaust system.
• Scavenge manifold leakage and scavenge drain valve open.
• Engine room pressure is negative due to bad weather.
(b) The scavenge air pressure may be increased suddenly because of the following facts:
• Adverse weather and strong wind.
• Overloading of engine.
• Faulty exhaust booster system.
• Auxiliary blower cut in.
• Just after water washing.
(c) The scavenging air temperature may be rising due to the following conditions:
• Fouling of charge air cooler.
• High engine room ambient temperature.
• Faulty cooling water control bypass system.
• Engine overload.
• Scavenge fire.
• Piston blow-past.
Remedies
So as to avoid these conditions already mentioned above, to carry out the following measures:-
• Charged air cooler is cleaned when the pressure differential exceeds the max limit, by reading the differential pressure at manometer fitted at charged air cooler for airside cleaning.
• Turbocharger blower side air filter is cleaned by checking the pressure different shown at “U” tube manometer attached at Blower side air filter housing.
• Charge air cooler’s seawater is cleaned by checking the cooling S.W inlet and outlet temperature difference.
REFERENCES:
DIESEL ENGINES BY A.J WHARTON
MARINEDIESELS.UK.COM
REEDS MOTOR ENGINEERING KNOWLEDGE
INRODUCTION TO MARINE ENGINEERING BY D A TAYLOR
