Main Engine PREP/OPERATION Flashcards
What will you do as 2/E, if main engine lubricating oil temperature abnormally high?
Inform bridge & reduce engine speed
Check engine overload or not (Exhaust temp:, fuel rack,..)
Check L.O sump & L.O cooler & L.O purifier temperature (set value)
Check L.O sump tank heating valve.
Shut L.O cooler by-pass totally after stopping (or) too high temperature not fall
Clean L.O cooler
Check sump tank heating coil leakage
Make L.O onboard test (esp Viscosity)
Check lubricating oil piping system leakage or blockage
Make inspection & check bearing clearance & loosing attachment
Check ampere (or) load when turn the turning gear
What will you do as 2/E, when increase sump lubricating oil level by marine engineer?
Check piston cooling system (water)
Check L.O purifier (gravity disc is correct or not) [L.O purifier water outlet sight glass]
Check filling valve from storage tank
Check L.O cooler/although oil pressure is greater than sea water pressure.
What will you do as 2/E, when decrease in sump lubricating oil level by marine engineer?
Check rate of decreasing if slowly decrease, fill up L.O and find the leakage without stopping engine.
If rapidly decrease, inform to bridge and stop the engine. Find the leakage and repair Possible leakage points, these being the following
- Bed plate crack (check engine room bilge)
- Piston cooling L.O system (check scavenge space & under piston space {entablature})
- L.O cooler & L.O purifier
- All pipes and connection
- Check L.O return valve from crankcase to sump tank close or not
- Check oil scraper rings & stuffing box
What will you do as 2/E, when decrease in lubricating oil pressure observed?
Start stand by pump
Change & clean L.O filter
After engine stopping, check bearing clearance and L.O pipe connection
Check L.O pump discharge & suction pressure
Check L.O temperature
What will you do as 2/E, if lube oil is contaminated with sea water?
When sump oil is contaminated with SW, find sources of leakage (may be from LO cooler during ME stopped) stoppage and rectified.
In port or while ME is stopped, transfer contaminated oil through purifier or transfer pump into settling tank, settled for at least 24 hours at about 60 C°, and water and sludge drained out periodically.
Oil passed through purifier at 78° C with optimum efficiency, and pump back to settling tank.
When sump tank is empty, interior cleaned and examined.
Purified oil sent to laboratory and tested
During this time, new oil should be used
Oil should be reused, if lab results recommended that it is fit for further use. (Straight mineral oil 3% water washed. Additive oil 1% water washed).
what are the causes, effect and remedies of Contamination of Lubricating Oil by Water
Causes:
Leakages from cylinder cooling water system.
Leakages from piston cooling water system (for water cooled pistons).
LO cooler water leakages (can be seawater or freshwater).
Leakages from sump tank heating coils.
Condensation of water vapour inside crankcase.
Effects:
Acid formation in lube oil for trunk type piston engines.
Reduction in cooling efficiency.
Reduction in load carrying capacity of lube oil.
Reduction in lube oil properties such as TBN.
Formation of sludge.
Corrosion in various parts of the machinery.
Microbial degradation of lube oil.
Remedies:
Proper purification of lube oil with minimum throughput.
Renewal of lube oil.
What is the purpose of charge air cooler on ships ?
To reduce air temperature & increase density of charge air.
More fuel can be burnt and more power can be obtained.
Reduce exhaust temperature and engine thermal load.
Increase scavenging efficiency, safe working temperature.
what maintenance is carried on cooler to maintain optimum efficiency?
Cooler should be checked any deposit of lime, scale or oil sludge may be present in cooler; it should be cleaned.
Cooler of water side can be done with soft tube brush and oil side with carbon tetra chloride solution in reverse direction to normal flow with hand pump for about 4 hours.
After cleaning the cooler are hydraulically pressure tested normally 1.5 times the working pressure.
If 10% of the tubes have been leaked retubing in necessary. Normal leaking tubes may be stopped by plugging.
Corrosion can be protected by means of preservative coating (Anti corrosive paints) inside the shell and water boxes and by means of anodes such as zinc fitting inside water boxes.
Essential cooler for optimum efficiency can be maintained by controlling of temperature of fluid or sea water.
What is the purpose of division plate in cooler ?
Provided to increase numbers of pass.
This increase the cooling efficiency.
how would you deal with L.O cooler leakage ?
When engine running,
Oil comes out at the cooling water overboard.
Sump tank oil level will fall down.
L.O pressure will drop
If L.O cooler leaks the engine should be stopped with permission from bridge.
The leakage can be detected by carrying out a hydraulic pressure test to the oil side.
After cooling down the engine stop main circulating L.O pump and main S.W cooling pump.
Close necessary valves
Open water box covers cooler both side.
Blank off discharge pipe of cooler oil side.
Connect hydraulic pump to inlet of cooler.
Apply oil pressure normally 1.5 times the working pressure. Then check the leakage at cooler both sides
Normally leaking tubes may be stopped by plugging
If 10% of the tubes have been leaked retubing is necessary.
Then the engine is put back normal running.
how would you deal with main engine air cooler water leakage ?
Check water level insight glass fitted at cooler drain pipe.
Drain the cooler / taste the water
If the water continuous comes out, the cooler is leakage. Also in the funnel white & dense smoke.
Small amount of water leakage is detected by the shore lab analysis carried out on scrape down samples.
Then the engine should be stopped with permission from bridge.
Normal leaking tubes can be stopped by plugging.
Then the engine is put back normal running.
How to check cooler efficiency ?
Check sea water in/out temperature difference. Less difference means poor efficiency of cooler (must be high)
Check coolant medium in / out pressure (Pressure drop across the cooler: should be low)
Feel over cooler shell, upper hot, middle warm, down cool is normal.
Check pump and by pass valve.
What is the purpose of baffle plate in coolers ?
To support the tube stack.
To guide the flow of fluid
To increase cooling surface area
To minimize the tube vibration
what is fouling of LO cooler
Fouling is the formation of biological coatings on a surface, which makes the transfer of heat more difficult.
Fouling of main engine lube oil cooler may result in high lube oil temperature and low lube oil pressure. Lube oil coolers can be plate type or shell and tube type.
However the reasons for poor cooler performance can be due to fouling of oil side, or Fouling of cooling water side
for plate type coolers are concerned, cleaning is much easier. Shell side of lube oil coolers are normally designed with baffles and fins for enhanced heat exchange. Chemical cleaning is carried out regularly for the shell side. This removes sludge adhered inside and increases heat transfer efficiency.
what are the Reasons for Poor LO Cooler Performance
Fouling of either lube oil side or cooling water side or both
Insufficient circulation of cooling water
Malfunctioning or improper adjustment of temperature controller / three way valve
Air lock inside the cooler
Choked cooling water inlet filter (for plate type coolers)
Broken or misplaced baffle plates (for shell and tube coolers)
how can you maintain LO cooler performance
Keep the vents for cooler open, ensure vent is not choked and both lube oil and cooling water sides are free from air lock
Carry out chemical cleaning of cooling water side
Back flush the cooler
Check the cooling water pressure and quantity flowing through the cooler
Check adjustment of temperature controller / position of three way valve
Clean inlet filter for cooling water
Ensure baffle plates are in position
If necessary, take out tube nest out of the cooler for cleaning sludge (for shell and tube coolers)
Clean cooler plates manually (for plate type coolers)
a) In the case of a main engine and Controllable Pitch Propeller not responding to bridge control describe the routine for changing to Engine Room Control (8)
b) Explain how manual control of fuel pump delivery is achieved when
emergency manoeuvring on a large 2 stroke crosshead engine. (8)
a.) contact bridge and C/E and inform them that you want to switch to engine room control. then go to local control panel. afterwards you can switch from bridge to engine room control. ensure to maintain communication with bridge at all times and carry out any orders immediately. use the manoeuvring table for pitch and speed references and adjust speed when required from the telegraph in ECR. ensure to stay in ECR at all times to adjust speed of engine and maintain control.
b.) Inform bridge and C/E then set up manual control for fuel pump delivery by changing fuel pump control shaft from local to manual, then put blocking arm or mechanical lever into position for engine side control. then move the handwheel to disconnect fuel pump from governor control and connect to local manual handwheel to operate fuel rack. ensure control air valve is on local. match the speed in ECR to engine side and switch control from ECR to local control at engine side. by operating the manual handwheel which operates the fuel rack your controlling the speed of the engine by controlling the fuel supply for pump.
Describe the sequence of actions when preparing the main engine, from cold, for sea service. Assume that the engine has been shut down for a long period of time. (16)
first before doing anything contact the bridge and chief engineer to let them know engine is going to be prepared for sea service. then test the steering gear. when preparing main engine from cold the jacket water cooling system must be prepared. this is because the cooling water temperature at this point would have decreased to well below operating temperature. (60-65deg) to prepare the system first check the expansion tank and ensure its at the correct level if not then top. this tank as well as allowing for thermal expansion is where the cooling water for the engine is taken from so sufficient water must stored inside for cooling purposes. check the temperature of the jacket water. in order to ensure good engine operation is being maintained jacket water must be about 60-65 degrees when starting main engine from cold. if jacket water is below this temperature then use the jacket water pre-heater to gradually raise the operating temperature over a period of time to avoid thermal shocking. jacket water must be preheated to operating temperature to adequately cool the engine components and not raise the internal temperature of the components which will lead to thermal stress and component failure. once operating temperature is reached start the jacket water transfer pump and open bypass valve.
the next systems to prepare is the LO system followed by steam tracing system and then fuel oil circulating system. prepare lube oil system first because it can be heated to operating temperature by jacket cooling water system circulating through main engine. fuel oil has to be heated by steam tracing system which takes longer to prepare.
To prepare the LO system start by checking the main engine lube oil sump tank level and replenish it if required. this tank is replenished by the LO purifier treatment system so this must also be started. This sump tank keeps all the treated excess lube oil from main engine and ensure that lube oil supply pump maintains correct submerged oil level. then start the LO pump to begin circulating oil around the main engine. the LO will be heated up to operating temperature by the jacket water circulating through the main engine. ensure to monitor the pressures and temperatures of jacket water system and LO system wait for them to stabilize.
Then prepare the steam tracing system
by starting boiler from auto mode ensure boiler runs ok and reaches adequate steam pressure. then line up the steam system so that fuel system receives external heating to raise fuel to service temperature which is about 90degrees. start up fuel oil purifier treatment system and drain any water from fuel service and settling tank using drain valves located at the bottom
once systems are prepared contact the bridge to inform them engine is being turned over. then open indicator cocks and engage the turning gear if 2 stroke engine activate cylinder lubrication (4 stroke use splash lubrication). ensure engine is turned for at least a minimum of 15mins to allow for 2revs of crankshaft so its sufficiently lubricated. then disengage turning gear.
then prepare the air starting system. To prepare system drain any moisture from starting air system, including air compressor and air bottles. start air compressor on auto and ensure pressure in air bottles is around 28bar. line up system to direct starting air to air distributor. then open air distributor valve to supply starting air to main engine starting air valves. Then “kick” engine on air and observe the indicator cocks to ensure cylinders are purged of debris and oil or water moisture. then close the indicator cocks and begin test engine on fuel by opening the necessary valves and starting fuel pump. if engine is preforming normally and parameters are okay make contact with the bridge to say engine is ready. finally fill up the engine room log book.
With reference to the operation of main propulsion engine, outline the
importance of each of following:
a) Maintaining the temperature of the scavenge air above the dewpoint; (4)
b) Maintaining the fuel at the correct viscosity for injection; (4)
c) Regular on board testing of the lubricating oil; (4)
d) Ensuring rotation of the exhaust valves. (4)
a.) Dew point is the temperature at which air condenses into saturated water vapour. scavenge air dew point being (40-45deg normally). Any time scavenge air temperature drops below it becomes saturated water vapour. this would then cause condensation on cylinder liners which then washes off any lube oil and degrades it causing rust and components corroding. also water will accumulate in scavenge space and so it will flow through scavenge space drains to bilges. if water vapour mixes with exhaust gas it forms sulphuric acid causing acid corrosion. If you have scavenge temperature highly above dew point then water vapour in exhaust gas vaporizes during combustion and if enough condensation accumulates in scavenge space it takes the sulphur from the fuel forming sulphuric acid. Dew point temperature will also depend on where you are so dew point of air has to be measured as well as seawater temperature and kept above this dew point temperature.
b.) Maintaining correct fuel viscosity is essential for correct atomization of fuel. This is important for maintaining correct fuel to air ratio and good combustion. for instance if you have high viscosity of fuel this leads to improper atomization at fuel injectors because there is under pressurization of fuel. This is because viscosity is the resistance flow rate so high viscosity fuel has more resistance and requires more energy to flow so pressure decreases. on the other hand too low viscosity will mean fuel has less resistance so flow rate is increased. Problem with this would be that flow rate is too high for fuel injectors to handle so there is over pressurization of fuel. To control viscosity you control temperature and thereby controlling flow rate.
c.) lube oil is tested regularly to check for any contaminants such as water or metal that can cause damage or wear and tear on engine components. lube oil test will also check the degradation of oil. by this i mean lube oil after a period of time degrade due to oxidisation so it has to tested for quality an purity. oxidisation referring to a series of chemical reactions that occur and have a negative effect on lube oil due to the presence of oxygen. water contains oxygen so if lube oil is contaminated by water the oxygen will then attack the lube oil and make it less effective. process speeds up with the presence of heat. oxidisation cant be stopped but only sowed down with the presence of anti-oxidants and then replace lube oil when anti-oxidants are depleted. essentially lube oil is tested for all of these contaminants and once life time expectancy is reached its changed.
d.) Exhaust valves must rotate as it helps to maintain the exhaust valves lifespan due to preventing carbon deposits from building up around the seat, stem and guide and therefore keeping exhaust valves clean. Carbon deposits are a by-product of combustion and any build up can cause engine to overheat as well as reduced fuel efficiency, increased emissions and a loss in engine power. a typical exhaust valve rotates about 360 degrees per minute to remove these carbon deposits. The rotation also ensures that heat is distributed evenly across the face of the valve, thereby preventing any hotspots from occurring. hotspots and uneven heat distribution will cause damage to the valve. to ensure valve rotation 4 strokes have roto caps fitted where as 2 strokes have winged parts on the valve stem and high pressure exhaust gas from combustion pushes on these winged parts and causes valve to rotate.
a) State the reason for turning the engine with the turning gear prior to
starting.(4)
b) State the reason for leaving the indicator cocks on main engine cylinders open when the engine is turned initially with the turning gear. (4)
c) State the reason for leaving the lubricating oil circulating after “Finish with Engines”. (4)
d) State why diesel alternator cooling water may be circulated through the main engine after shutdown. (4)
a.) Using turning gear for engine prior to starting ensures engine is free to turn it also ensures cylinders are cleared from debris or fluid (oil/water moisture). Ensures correct rotation of engine and builds up sufficient lubrication for engine parts, if cylinders aren’t cleared from the debris and oil or water moisture prior to start up you can cause cylinders to seize up, as well as wear and tear damage and potentially a hydraulic lock.
b.) Leaving indicator cocks open allows cylinders to be cleared of and fluids (oil/water moisture) or debris that’s been trapped inside the cylinders. It also decompresses the cylinders so that the turning gear can operate.
c.) The lubricating oil is circulated after engine has been stopped because engine components can contain residual heat, which if not gradually cooled can cause thermal stress damage to engine components. For example gradual cooling to the pistons has to maintained to avoid a piston head cracking due to thermal stress and or residual heat another example is to provide cooling to bearings to avoid seizure due to thermal stress. additionally it can be done to maintain operating temperature. if long term lube oil circulating is done use a low pressure priming pump as oppose to supply pump (higher pressure) to avoid erosion. This is fatigue attack on bearings due rapid pressure changes.
d.) Cooling water is circulated through the main engine after shutdown because again engine parts contain residual heat. Therefore, by utilising cooling water its possible to remove the residual heat and prevent thermal stress damage to cylinder liner and other components. Additionally, it ensures that correct main engine starting/ operating temperature is maintained after engine shutdown by using the jacket water preheater. If engine was to be started from cold, it can be subjected to low temperature corrosion so preheating the cooling water ensures that main engine is maintained in good working condition and therefore reducing the time needed to prepare main engine.
Describe, with the aid of sketches, the operation of 4-stroke cycle diesel
engines.
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Induction/intake stroke
Air intake valve opens during induction stroke to allow fresh air into engine cylinder. Motion/ timing of intake valve is controlled by the camshaft. When fresh air enters piston moves from top dead centre to bottom dead centre. Exhaust valve is closed during the intake stroke when piston moves downwards motion of piston create a partial vacuum inside the cylinder the pressure difference generated from motion of piston moving down pulls air molecules into cylinder through the air manifold and then the intake valve closes at the end of the intake stroke.
Compression stroke
The piston moves from the bottom dead centre to top dead centre compressing the air inside the cylinder both the intake and exhaust valve remain closed, thereby isolating the combustion chamber from ambient air. By compressing air when piston moves up to top dead centre the pressure and temperature of this air is increased.
Power stroke
During power stroke fuel is injected 10 degrees before top dead centre at the end of the compression stroke. Fuel is injected by fuel injector in a finally atomised form into the cylinder when the hot air mixes with the fuel this results in combustion. The energy from the combustion of fuel pushes the piston down from top dead centre to bottom dead centre. Both exhaust valve and inlet valve is closed during this stroke. The downward motion of the piston provides energy for the crankshaft to rotate.
Exhaust stroke
combustion of fuel produces exhaust gas. This gas contains unburnt fuel particles and harmful pollutants. Use last valve controlled by camshaft opens during this stroke and therefore allowing the exhaust gas to flow through the exhaust manifold. The air intake valve remains closed. During this stroke the pistons upward motion from BDC to TDC allows the exhaust gas to be driven out from the cylinder.
The four-stroke cycle is completed in four strokes of the piston, or two revolutions of the crankshaft. In order to operate this cycle the engine requires a mechanism to open and close the inlet and exhaust valves. Consider the piston at the top of its stroke, a position known as top dead centre (TDC). The inlet valve opens and fresh air is drawn in as the piston moves down (Figure 2.1 (a)). At the bottom of the stroke, i.e. bottom dead centre (BDC), the inlet valve closes and the air in the cylinder is compressed (and consequently raised in temperature) as the piston rises (Figure 2.1(b)). Fuel is injected as the piston reaches top dead centre and combustion takes place, producing very high pressure in the gases (Figure 2. l(c)). The piston is now forced down by these gases and at bottom dead centre the exhaust valve opens. The final stroke is the exhausting of the burnt gases as the piston rises to top dead centre to complete the cycle (Figure 2.1(d)). The four distinct strokes are known as ‘inlet’ (or suction), ‘compression’, ‘power’ (or working stroke) and ‘exhaust’. The angle of the crank at which each operation takes place is shown as well as the period of the operation in degrees.. For different engine designs the different angles will vary, but the diagram is typical.
a) Explain why the correct tappet clearance is essential on the inlet and exhaust valves of a 4-stroke diesel engine. (8)
b) State the results of the tappet clearance being:
(i) Too large; (4)
(ii) Too small. (4)
a.) Correct tappet clearance ensures valves are closed when required. This is needed because the valve spindle will expand in length under operation due to high temperature. If there isn’t a clearance the valve stem would expand against the rocket arm and the cam would expand against the cylinder causing a loss of compression and burning of valve and seat. Correct size tappet clearance is therefore essential to ensure good engine operation and max power output by allowing for a 0.5-millimetre clearance for thermal expansion.
b.) i.)
If the tappet clearance is too large valves will open late and close early. This leads to poor scavenging of exhaust gas and a reduced engine power output.
ii.) If the tappet clearance is too small this will cause valves to open early and close late leading to reduced engine power output due to a loss in compression. If clearance is very small due to thermal expansion exhaust valve remain open allowing for combustion gases to blow past exhaust valve seat leading to a possible scavenge fire.
Describe, with the aid of sketches, the operating principle of 2-stroke diesel engines.
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at the start of the air intake the piston is moving towards BDC at the end of a power stroke. piston uncovers the inlet ports and an air blower delivers fresh air into the cylinder. the exhaust valve opens piston reaches BDC and starts to move upwards incoming air scavenges exhaust gases out of the cylinder. piston continues upwards covers the inlet ports and cuts of the air supply from the air blower the exhaust valve then closes the piston approaches TDC compressing the air and raising its temperature. just before piston reaches TDC the fuel injector sprays in fuel, heat of the compressed air ignites the fuel. pressure from the expanding gases forces piston to move down the cylinder in a power stroke, before piston reaches BDC exhaust valve opens and burnt gases escape. air inlet ports are uncovered and fresh air enters form the air blower, piston at this point has reached BDC and cycle repeats again. All events occur in 2 strokes of the crankshaft so 1 revolution.
The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements. First, the fresh air must be forced in under pressure. The incoming air is used to clean out or scavenge the exhaust gases and then to fill or charge the space with fresh air. Instead of valves holes, known as ‘ports’, are used which are opened and closed by the sides of the piston as it moves. Consider the piston at the top of its stroke where fuel injection and combustion have just taken place (Figure 2.3(a)). The piston is forced down on its working stroke until it uncovers the exhaust port (Figure 2.3(b)). The burnt gases then begin to exhaust and the piston continues down until it opens the inlet or scavenge port (Figure 2.3(c)). Pressurised air then enters and drives out the remaining exhaust gas. The piston, on its return stroke, closes the inlet and exhaust ports. The air is then compressed as the piston moves to the top of its stroke to complete the cycle (Figure 2.3(d)). A timing diagram for a two-stroke engine is shown in Figure 2.4.
Explain the constructional differences between 2-stroke slow speed and 4-stroke medium speed diesel engines.
- Two stroke engines use a crosshead to connect the piston to
the connecting rod. Four stroke engines connect the piston to
connecting rod directly using a gudgeon pin. - As a result the cylinder liners on four strokes are considerably
shorter to allow transverse movement of connecting rod. - Piston skirts on 4 stroke engines are comparably longer to
absorb the side loading against the cylinder liner created by the
rotating crankshaft. - The cylinder liners on two stroke engines have scavenge ports to
allow entry of fresh combustion air, whereas four stroke cylinders do
not. - Typically two stroke engines have one large exhaust valve on the
cylinder head, whereas four stroke engines have multiple inlet and
exhaust valves. - Two stroke engines will utilise a Aux Blower during start up and low
loads, a four stroke will only have a turbo charger - Two Stroke engines will have two separate lubrication systems
( cylinder and system) Four stroke engines will have one common
lubrication system - 2 stroke engines are formed of 3 main sections. Bedplate, A-Frame and entablature. 4 Stroke engines are usually cast from one single piece of material.
with two stroke low speed engine the crankcase oil is separated from the scavenge space and the underside of the piston by a division within the engine structure. the piston rod is fixed to the piston and passes through a gland called a stuffing box and on to the cross head bearing. as the piston is segregated from the crankcase there is no lubrication to the piston from the crankcase oil as there is in the 4 stroke engine. therefore, the cylinder and piston rings on the two stroke, slow speed engine are lubricated by the total loss cylinder oil system.
With reference to medium Speed Diesel Engine, Sketch and label a typical
arrangement for the Piston and connecting rod.
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Explain the constructional difference between a slow speed 2 stroke and a medium speed 4 stroke diesel engine with respect to the connection between the piston and the crankshaft.
b) Describe the function of the diaphragm and stuffing box.
a.) On 2 stroke engines, the connecting rod is attached to the piston rod via the crosshead bearings, which transfer much of the engine thrust to the crosshead guides in the engine
frames. Four stroke engines connect the piston to connecting rod directly using a gudgeon pin. As a result the cylinder liners on four strokes are considerably
shorter to allow transverse movement of connecting rod.
Piston skirts on 4 stroke engines are comparably longer to
absorb the side loading against the cylinder liner created by the
rotating crankshaft
The cylinder liners on two stroke engines have scavenge ports to
allow entry of fresh combustion air, whereas four stroke cylinders do
not.
b.) Stuffing box separates the
scavenge space from the
crankcase, preventing cross
contamination of oil and
keeping the scavenge air out
of crankcase, via scraper and
sealing rings
the diaphragm is positioned between the crankcase and cylinder of a 2 stroke engine, to segregate the 2 areas. Its purpose is to prevent crankcase oil being carried into the scavenge space, and to prevent scavenge air, used cylinder oil and the products of combustion all from contaminating the crankcase oil.
a) Sketch a typical power indicator card for a slow speed marine diesel
engine.(6)
b) Explain how the card may be used to assess the power developed in the
cylinder. (10)
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b.) the total are of the diagram is divided by the length of the diagram to obtain the mean height. mean height can then be multiplied by the spring scale of the indicator mechanism giving indicated mean effective pressure for the cylinder.
W^.i= Pi x l x A x n
W^.i= indicated power
(for one cylinder only)
Pi = indicated mean effective pressure
l = stroke or length of the cylinder
A = area of the cylinder A = TTD^2/4
n = number of power strokes per second (rev/sec)
D = diameter of cylinder (m)
power calculated from the diagram is called indicated power of the engine. its the power developed inside the cylinder of the engine. mean effective pressure can be found by dividing the area of the diagram by the length. the result is multiplied by the spring rate of the indicator spring, giving an average cylinder pressure. its used in the expression for indicated power and used as a value for comparison between engines.
4 stroke = n is divided by 2 because for one power there is revs for crankshaft.
The area within the diagram represents the work done within the measured cylinder in one cycle. The area can be measured by an instrument known as a ‘planimeter* or by the use of the mid-ordinate rule. The area is then divided by the length of the diagram in order to obtain a mean height. This mean height, when multiplied by the spring scale of the indicator mechanism, gives the indicated mean effective pressure for the cylinder. The mean effective or ‘average’ pressure can now be used to determine the work done in the cylinder. To obtain a measure of power it is necessary to determine the rate of doing work, i.e. multiply by the number of power strokes in one second. For a four-stroke-cycle engine this will be rev/sec -r 2 and for a two-stroke-cycle engine simply rev/sec.
Describe the function of each of the following components of a diesel engine: (4) marks each, a selection of 4 come in different questions.
a) Chocks;
b) Bedplate;
c) Tie rods;
d) Entablature;
e) Holding down bolts;
f) Crankshaft.
a.) chocks, are used to
restrict sideways and forward
motion of the engine, both
against the static weight of
engine but also the dynamic
loads created by running
machinery.
b.) The bedplate forms the base
of the engine. It supports the
crankshaft during rotation and
is strong to avoid deflections
of the shaft.
c.) The Tie Rods are hydraulically
tightened bolts that compress
the crankcase sections together.
They are used to resist the forces
of combustion that are pulling the
engine apart.
d.) The entablature is mounted on
top of the A frame. The cylinder
liners are supported within the
entablature.
e.) Holding down bolts are used to
connect the bedplate of the
engine to the ship. They will
keep the engine in position
avoiding crankshaft deflection
and vertical movement.
f.) Crankshaft is used to convert the
reciprocating motion of the pistons to
rotational motion used to drive the
ship via a propeller. It can also be used to transmit LO to different components via drillings.
Draw a line diagram of a main lubricating oil system for a large 2-stroke crosshead type diesel engine. Label all the main components of the system. (16)
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With reference to a small diesel engine that requires the use of a starting handle:
a) State the procedure for starting (8)
b) State EIGHT reasons that would cause difficulty in starting. (8)
a.) * Check LO, FO, cooling water level and activate the starting handle.
- Compressor must be unloaded and drain valve opened.
- Check gear lever is in neutral.
- Set the decompression lever to the decompression position in order keep the air inlet valve open to gain momentum for the flywheel, swing round the starting handle building up speed until maximum speed, then push over decompression lever whilst continuing to maintain speed with the starting handle. then when there is enough momentum deactivate the lever handle. When cranking ensure to control the fuel supply.
b.) when engine is in cold climate engine cranking will be more difficult because the LO viscosity would have increased.
partially blocked fuel injector nozzle
no fuel in tank
insufficient cranking because of air lock due to engaged load from the compressor ( need to drain compressor)
insufficient compression due to cold climate temperature
malfunction of decompression lever meaning air inlet valve is kept open
leaky inlet and exhaust valve or cylinder cover gaskets
too high viscosity of fuel due to cold climate
a) Sketch a camshaft timing chain arrangement indicating how the chain tension is adjusted. (12)
b) State two items of ancillary equipment that can be driven via the timing chain. (4)
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b.) Air starting distributor
Cylinder lubricators
With reference to marine diesel engines:
a) state the purpose of a camshaft; (2)
b) state two methods of driving a camshaft on a large marine diesel engine; (4)
c) state the speed of the camshaft relative to the crankshaft on:
i. a four-stroke engine; (2)
ii. a two-stroke engine (2)
d) sketch a fuel cam for a unidirectional diesel engine indicating EACH of the
following:
i. Point of Injection; (2)
ii. Peak; (2)
iii. Slow return (2)
a) Each engine cylinder will have a set of
Cams, which govern and drive the fuel pumps and control exhaust valve timing.
b) cam gears - allow the rotation of the camshaft to be in time with, the rotation of the crank shaft and thereby allows the valve opening, valve closing, and injection of fuel to occur at precise moments in the piston’s travel.
cam chain drive - The Cam shaft can be driven with a chain connected from the crank shaft. A chain drive is used to transmit motion from the crankshaft of the engine to the camshaft. This drive is known as timing chain and it is responsible for rotation of the camshaft.
c.) The speed of the camshaft on a four stroke engine is half the speed of the crankshaft
The speed of the camshaft on a 2 stroke engine is the same speed as the crankshaft
d.) SEE EOOW ORAL/IAMI Sketch Pack for Drawing
With reference to the reversing of an engine:
a) describe THREE methods by which it may be achieved; (12)
b) explain what is meant by lost motion. (4)
a.) one method of reversing which is done by MAN BMW is to change the position of the cam follower. The Cam is symmetrical which means that the rise, peak and dwell will be the same regardless of the direction of rotation. when the order for astern is given the compressed air from air starting system is used to actuate the pneumatic valve which displaces each cam follower unit. if position isn’t properly locked fuel delivery is stopped. this is carried out by a sensor fitted on each pump once the cam follower has shifted fuel timing changes and the firing order changes. air supplied to each cylinder will now be carried out according to the new reverse firing order.
The simplest method for reversing is through utilising two sets of cams per cylinder and moving the camshaft axially to align the cams in the ahead or astern position. These are used in Mitsubishi engines the camshaft contains one for ahead and one for astern direction for each unit. Entire camshaft is axially moved to change the cam and thereby changing the firing order.
one method of reversing cam is Sulzer reversible engines utilise a servo motor to rotate only the fuel cam.
When the astern signal is given the hydraulic oil is supplied into the servo motor and acting against the rotating vane will rotate the cam to the desired position .
b.) lost motion is the angular period between the top dead centre points for ahead and astern running. The lost-motion clutch uses a rotating vane which is attached to the camshaft and moves in relation to the camshaft drive from the crankshaft. The vane is shown held in the ahead operating position by oil pressure. When oil is supplied under pressure through the drain, the vane will rotate through the lost-motion angular distance to change the fuel timing for astern operation. The starting air system is retimed, either by camshaft movement or by a directional air supply being admitted to the starting air distributor, to reposition the cams.
what is TDC and BDC
TDC = Top dead centre - The point at which pistons direction of travel, changes from an upward to a downward stroke.
BDC = Bottom dead centre - The point at which pistons direction of travel, changes from a downward to an upward stroke.
what is meant by a total loss cylinder oil system?
A total-loss oiling system is an engine lubrication system whereby oil is introduced into the engine, and then burned. the cylinder oi sued for this system is specially formulated to combat the acids that are found in an engine burning high sulphur fuel oil. when ship has to change over to low sulphur fuel a different set of circumstances will exist and there is different requirement for the oil. engineers must change the cylinder oil at the same time as fuel is changed from high to low sulphur fuel.
what is a diesel engine?
The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points.
draw a two stroke timing diagram?
draw a four stroke timing diagram?
describe the cross section of a four stroke engine
The engine is made up of a piston which moves up and down in a cylinder which is covered at the top by a cylinder head. The fuel injector, through which fuel enters the cylinder, is located in the cylinder head. The inlet and exhaust valves are also housed in the cylinder head and held shut by springs. The piston is joined to the connecting rod by a gudgeon pin. The bottom end or big end of the connecting rod is joined to the crankpin which forms part of the crankshaft. With this assembly the linear up-and-down movement of the piston is converted into rotary movement of the crankshaft. The crankshaft is arranged to drive through gears the camshaft, which either directly or through pushrods operates rocker arms which open the inlet and exhaust valves. The camshaft is ‘timed’ to open the valves at the correct point in the cycle. The crankshaft is surrounded by the crankcase and the engine framework which supports the cylinders and houses the crankshaft bearings. The cylinder and cylinder head are arranged with water-cooling passages around them.
describe the cross section of a two stroke engine
The piston is solidly connected to a piston rod which is attached to a crosshead bearing at the other end. The top end of the connecting rod is also joined to the crosshead bearing. Ports are arranged in the cylinder liner for air inlet and a valve in the cylinder head enables the release of exhaust gases. The incoming air is pressurised by a turbo-blower which is driven by the outgoing exhaust gases. The crankshaft is supported within the engine bedplate by the main bearings. A-frames are mounted on the bedplate and house guides in which the crosshead travels up and down. The entablature is mounted above the frames and is made up of the cylinders, cylinder heads and the scavenge trunking.
what’s the difference between 2 and 4 stroke cycle
The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1.8. For a particular engine power the two-stroke engine will be considerably lighter—an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke. The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil. Each type of engine has its applications which on board ship have resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel operating on the two-stroke cycle. At this low speed the engine requires no reduction gearbox between it and the propeller. The four-stroke engine (usually rotating at medium speed, between 250 and 750 rev/ min) is used for auxiliaries such as alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 80 and 100 rev/min.
what are two possible measurements of engine power?
There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator. The shaft power is the power available at the output shaft of the engine and can be measured using a torsion meter or with a brake.
with the aid of a sketch explain what an engine indicator is?
It is made up of a small piston of known size which operates in a cylinder against a specially calibrated spring. A magnifying linkage transfers the piston movement to a drum on which is mounted a piece of paper or card. The drum oscillates (moves backwards and forwards) under the pull of the cord. The cord is moved by a reciprocating (up and down) mechanism which is proportional to the engine piston movement in the cylinder. The stylus draws out an indicator diagram which represents the gas pressure on the engine piston at different points of the stroke, and the area of the indicator diagram produced represents the power developed in the particular cylinder. The cylinder power can be measured if the scaling factors, spring calibration and some basic engine details are known. The procedure is described in the Appendix. The cylinder power values are compared, and for balanced loading should all be the same. Adjustments may then be made to the fuel supply in order to balance the cylinder loads.
what is the function of a torsion meter?
A marine shaft power (torsion) metermeasures the real time torque (torsion), speed,and power output on a propeller shaft, this way power can be adjusted and engine efficiency.
If the torque transmitted by a shaft is known, together with the angular velocity, then the power can be measured,
i.e. shaft power = torque x angular velocity
The torque on a shaft can be found by measuring the shear stress or angle of twist with a torsion meter.
explain what is meant by the gas exchange process?
A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air. Charging is the filling of the engine cylinder with a supply or charge of fresh air ready for compression. With supercharging a large mass of air is supplied to the cylinder by blowing it in under pressure. Older engines were ‘naturally aspirated’—taking fresh air only at atmospheric pressure. Modern engines make use of exhaust gas driven turbo chargers to supply pressurised fresh air for scavenging and supercharging. Both four-stroke and two-stroke cycle engines may be pressure charged. On two-stroke diesels an electrically driven auxiliary blower is usually provided because the exhaust gas driven turbo blower (turbocharger) cannot provide enough air at low engine speeds, and the pressurised air is usually cooled to increase the charge air density. At high enough engine speed the turbocharger is used, it consist of a single shaft. The single shaft has an exhaust gas turbine on one end and the air compressor on the other. Suitable casing design and shaft seals ensure that the two gases do not mix. Air is drawn from the machinery space through a filter and then compressed before passing to the scavenge space. The exhaust gas may enter the turbine directly from the engine or from a constant-pressure chamber. Each of the shaft bearings has its own independent lubrication system, and the exhaust gas end of the casing is usually water-cooled.
draw a exhaust gas driven turbocharging arrangement for a slow-speed two-stroke cycle diesel engine
explain the importance and difference in scavenging on 4 and 2 stroke?
Efficient scavenging is essential to ensure a sufficient supply of fresh air for combustion. In the four-stroke cycle engine there is an adequate overlap between the air inlet valve opening and the exhaust valve closing. With two-stroke cycle engines this overlap is limited and some slight mixing of exhaust gases and incoming air does occur. A number of different scavenging methods are in use in slow-speed two-stroke engines. In each the fresh air enters as the inlet port is opened by the downward movement of the piston and continues until the port is closed by the upward moving piston. The flow path of the scavenge air is decided by the engine port shape and design and the exhaust arrangements. Three basic systems are in use: the cross flow, the loop and the uniflow. All modern slow-speed diesel engines now use the uniflow scavenging system with a cylinder-head exhaust valve. In a Four Stroke Cycle IC Engine Separate exhaust stroke pushes burnt gases through Exhaust Valve so in this case scavenging is achieved by valve overlapping.
Explain with the aid of sketches the different methods of scavenging for 2 strokes
In cross scavenging the incoming air is directed upwards, pushing the exhaust gases before it. The exhaust gases then travel down and out of the exhaust ports. Figure 2.10(a) illustrates the process. In loop scavenging the incoming air passes over the piston crown then rises towards the cylinder head. The exhaust gases are forced before the air passing down and out of exhaust ports located just above the inlet ports. The process is shown in Figure 2.10(b).
Loop scavenge arrangements have low temperature air and high temperature exhaust gas passing through adjacent ports, causing temperature differential problems for the liner material. Uniflow is the most efficient scavenging system but requires either an opposed piston arrangement or an exhaust valve in the cylinder head. All three systems have the ports angled to swirl the incoming air and direct it in the appropriate path.
With uniflow scavenging the incoming air enters at the lower end of the cylinder and leaves at the top. The outlet at the top of the cylinder may be ports or a large valve. The process is shown in Figure 2.10(c). Each of the systems has various advantages and disadvantages. Cross scavenging requires the fitting of a piston skirt to prevent air or exhaust gas escape when the piston is at the top of the stroke. With uniflow scavenging the two-stroke engine is designed to have the exhaust at one end of the cylinder (top) and scavenge air entry at the other end of the cylinder (bottom) so that there is a clear flow traversing the full length of the cylinder. This design means that the scavenge air does not have to travel up the cylinder and down again, as with the other designs, to purge the exhaust gas from the previous cycle, hence the name UNI-flow.
what are scavenge fires
Cylinder oil can collect in the scavenge space of an engine. Unburned fuel and carbon may also be blown into the scavenge space as a result of defective piston rings, faulty timing, a defective injector, etc. A build-up of this flammable mixture presents a danger as a blow past of hot gases from the cylinder may ignite the mixture, and cause a scavenge fire. A loss of engine power will result, with high exhaust temperatures at the affected cylinders. The affected turbo-chargers may surge and sparks will be seen at the scavenge drains. Once a fire is detected the engine should be slowed down, fuel shut off from the affected cylinders and cylinder lubrication increased. All the scavenge drains should be closed. A small fire will quickly burn out, but where the fire persists the engine must be stopped. A fire extinguishing medium should then be injected through the fittings provided in the scavenge trunking. On no account should the trunking be opened up. To avoid scavenge fires occurring the engine timing and equipment maintenance should be correctly carried out. The scavenge trunking should be regularly inspected and cleaned if necessary. Where carbon or oil build up is found in the scavenge, its source should be detected and the fault remedied. Scavenge drains should be regularly blown and any oil discharges investigated at the first opportunity.
with the aid of a sketch describe the operation of a mechanical governor?
A flyweight assembly is used to detect engine speed. Two flyweights are fitted to a plate which rotates about a vertical axis driven by a gear wheel (Figure 2.22). The action of centrifugal force throws the weights outwards; this lifts the vertical spindle and compresses the spring until an equilibrium situation is reached. The equilibrium position or set speed may be changed by the speed selector which alters the spring compression. As the engine speed increases the weights move outwards and raise the spindle; a speed decrease will lower the spindle. The hydraulic unit is connected to this vertical spindle and acts as a power source to move the engine fuel controls. A piston valve connected to the vertical spindle supplies or drains oil from the power piston which moves the fuel controls depending upon the flyweight movement. If the engine speed increases the vertical spindle rises, the piston valve rises and oil is drained from the power piston which results in a fuel control movement. This reduces fuel supply to the engine and slows it down. It is, in effect, a proportional controller. The actual arrangement of mechanical engine governors will vary considerably but most will operate as described above.
what is the purpose of an engine governor
The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed. Governors for diesel engines are usually made up of two systems: a speed sensing arrangement and a hydraulic unit which operates on the fuel pumps to change the engine power output.
what is an electric governor?
The electric governor uses a combination of electrical and mechanical components in its operation. The speed sensing device is a small magnetic pick-up coil. The rectified, or d.c., voltage signal is used in conjunction with a desired or set speed signal to operate a hydraulic unit. This unit will then move the fuel controls in the appropriate direction to control the engine speed
what is the purpose of a cylinder relief valve?
The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal. A spring holds the valve closed and its lifting pressure is set by an appropriate thickness of packing piece. Only a small amount of lift is permitted and the escaping gases are directed to a safe outlet. The valve and spindle are separate to enable the valve to correctly seat itself after opening. The operation of this device indicates a fault in the engine which should be discovered and corrected. The valve itself should then be examined at the earliest opportunity.
what is a crankcase OMD?
The presence of an oil mist in the crankcase is the result of oil vaporisation caused by a hot spot. Explosive conditions can result if a build up of oil mist is allowed. The oil mist detector uses photoelectric cells to measure small increases in oil mist density. A motor driven fan continuously draws samples of crankcase oil mist through a measuring tube. An increased meter reading and alarm will result if any crankcase sample contains excessive mist when compared to either clean air or the other crankcase compartments. The rotary valve which draws the sample then stops to indicate the suspect crankcase. The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air. The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for crosshead type engines and the level model for trunk piston engines
purpose of a crankcase explosion relief valve?
As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase. Various designs and arrangements of these valves exist where, on large slow-speed diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used. A light spring holds the valve closed against its seat and a seal ring completes the joint. A deflector is fitted on the outside of the engine to safeguard personnel from the out flowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase. After operation the valve will close automatically under the action of the spring.
purpose of the engine turning gear?
The turning gear or turning engine is a reversible electric motor which drives a worm gear which can be connected with the toothed flywheel to turn a large diesel. A slow-speed drive is thus provided to enable positioning of the engine parts for overhaul purposes. The turning gear is also used to turn the engine one or two revolutions prior to starting. This is a safety check to ensure that the engine is free to turn and that no water has collected in the cylinders. The indicator cocks must always be open when the turning gear is operated
what is the procedure for reversing an engine?
Engine reversing
When running at manoeuvring speeds:
1.Where manually operated auxiliary blowers are Fitted they should be started.
2.The fuel supply is shut off and the engine will quickly slow down,
3.The direction handle is positioned astern.
4.Compressed air is admitted to the engine to turn it in the astern direction.
5. When turning astern under the action of compressed air, fuel will be admitted. The combustion process will take over and air admission cease.
When running at full speed:
1.The auxiliary blowers, where manually operated, should be started.
2.Fuel is shut off from the engine.
3.Blasts of compressed air may be used to slow the engine down.
4.When the engine is stopped the direction handle is positioned astern.
5.Compressed air is admitted to turn the engine astern and fuel is admitted to accelerate the engine. The compressed air supply will then cease
explain the reason for starting air overlap?
Starting Air Overlap
There must be some overlap between the operation of starting air valves to the different cylinders of an engine, so that as one cylinder valve is closing another one is opening just at the correct moment to ensure a continued rotation of the engine before the fuel is introduced. This is essential to ensure a positive angular motion of the engine crankshaft with sufficient momentum to give a positive start. The usual minimum amount of overlap provided in practice is 15°.
Starting air is admitted on the working stroke and the period of opening is governed by practical considerations with three main factors to consider:
1.The firing interval of the engine.for example, with a four-cylinder two-stroke engine the firing interval is 90°, that is, 360/4 and if each cylinder valve covered 90° of the cycle then the engine would not start if it had come to rest in the critical position with one valve fractionally off closure and another valve just about to start opening.
2.The valve must close before the exhaust commences. It is rather pointless blowing high pressure air straight to exhaust and it could be dangerous.
3.The cylinder starting air valve should allow the air to enter the cylinder after its associated piston has passed TDC to give a positive turning moment in the correct direction.
In fact some valves are arranged to start to open as much as 10° before TDC because the engine is past this position before the valve is effectively open and the compressed air is having an effect. Any reverse turning effect is negligible as the turning moment exerted on a crank very near dead centre is small indeed.Consider figure 5.1a for a four-stroke engine. With the timings as shown the air starting valve opens 15° after dead centre and closes 10° before exhaust begins. The air start period is then 125°. The firing interval for a six-cylinder four-stroke engine is 720/6 = 120°. The period of overlap is 5° which is insufficient. Although this example could easily be modified so as to give sufficient (say 15°) overlap by reducing the 15° after dead centre and the 10° before exhaust opening, it can become very difficult to arrange with very early exhaust opening on turbo-charged engines. A seven-cylinder four-stroke engine is much easier to arrange. Consider figure 5.1b which represents a two-stroke engine. This has an air start period of 115°. Firing interval for a three-cylinder two-stroke engine = 360/3 = 120°. This means no overlap. Modification can arrange to give satisfactory starting with this example but for modern turbo-charged two-stroke engines having exhaust opening as early as 75° before BDC (outer) it becomes virtually impossible. A four-cylinder two-stroke engine is much easier to arrange and would be adopted. Consider figure 5.1c which is a cam diagram for a two-stroke engine with four cylinders. The air open period is 15° after dead centre to 130° after dead centre, that is, a period of 115°. This gives 25° of overlap (115 – 360/4) which is most satisfactory. Take care to note the direction of rotation and this is a cam diagram so that for example, No. 1 crank is 15° after dead centre when the cam would arrange to directly or indirectly open the air start valve. The firing sequence for this engine is 1 4 3 2. This is very much related to engine balancing and no hard and fast rules can be laid down about crank firing sequences as each case must be treated on its merits. It may be useful to note that for six-cylinder, two-stroke engines a very common firing sequence is 1 5 3 6 2 4 and similarly for seven and eight cylinders 1 7 2 5 4 3 6 and 1 6 4 2 8 3 5 7 respectively are often used. The cam on No. 1 cylinder is shown for illustration as it would probably be for operating say cam operated valves, obviously the other profiles could be shown for the remaining three cylinders in a similar way. The air period for cylinder numbers 1, 4, 3 and 2 are shown respectively in full, chain dotted, short dotted and long dotted lines and the overlap is shown shaded
explain the purpose of starting air valves?
Each cylinder is fitted with a starting air valve which is operated pneumatically by one of the starting air distributor control valves. These are arranged radially around the starting air distributor cam. At the correct engine position an air signal from the control valve is directed to the cylinder starting air valve upper chamber and acts on the piston to open the valve. As this is happening the air from the lower chamber is vented to atmosphere through the control valve. At the end of the starting air admission period control air is redirected to the lower chamber to close the valve while the upper chamber is vented to atmosphere through the control valve. The valve opens and closes quickly with air cushioning at the end of the closing motion to reduce shock on the valve seat. If the pressure in the cylinder is substantially higher than the starting air pressure, the valve will not open. This prevents hot gases entering the starting air manifold. During engine operation the air inlet to the starting valve should be regularly checked. A hot inlet would indicate a leaking starting air valve allowing hot combustion gases to enter the air manifold which may lead to an explosion if starting air is admitted
explain the purpose of starting air distributor?
There are many designs of air distributor all with the same basic principles, that is, to admit air to the pistons of cylinder relay valves in the correct sequence for engine starting. Valves not being supplied with air would be vented to the atmosphere via the distributor. Some overlap of timing would obviously be required. The starting control valves are arranged radially around the starting air distributor cam, which is driven via a vertical shaft from the camshaft. When the engine starting lever is operated air is admitted to the distributor forcing all control valves, against the return spring, onto the cam. The control valve of a cylinder which is in the correct position for starting, will be pushed into the depression in the cam and assume the position shown in figure 5.3b. In this position air from the starting system will be directed to the upper part of the cylinder starting air valve causing the valve to open. At the same time air from the lower chamber of the cylinder starting air valve will be vented to atmosphere. At the end of the cylinder starting air period the distributor cam moves the control valve to the position. In this position air from the starting air system is directed to the lower chamber of starting air valve causing the valve to close. Air from the upper chamber is vented through the control valve to atmosphere. The starting control valves are held off the distributor cam by springs when starting air is shut off the engine
why don’t 4 strokes require a reversing mechanism?
The general trend in four-stroke practice is to utilise a unidirectional engine, coupled, via a reduction gearbox, to a controllable pitch propeller. The need for reversing mechanisms on the four-stroke engines is, therefore, no longer required.
with the aid of a sketch explain a lost motion clutch?
Refer now to figure 5.5. The design in this figure which is based on older Wartsila Sulzer engine practice has a lost motion on the fuel pump camshaft of about 30°. When reversal is required oil pressure and drain connections are reversed. Oil flowing laterally along the housing moves the centre section to the new position, that is, anti-clockwise as shown in figure 5.5. The oil pressure is maintained on the clutch during running so that the mating clutch faces are kept firmly in contact with no chatter. Figure 5.5 Lost motion clutch There are a number of variations on this design but the principle of operation is similar although not all types rotate the clutch to its new position before starting and merely allow the camshaft to ‘catch on’ with the crankshaft rotation when lost motion is completed. It is worth pausing for a moment and reflecting upon the limitations of the mechanical designs. For example, think how difficult it would be to arrange a fuel cam with a profile that gave a pre-ignition and post-ignition phase to the injection process
what is the easiest way of reversing a 2 stroke engine?
One of the easiest ways of changing the settings on a two-stroke engine is to reposition the fuel, exhaust valve and starting air cams on the camshaft, with their associated equipment, so that the engine operating in reverse can utilise one cam. This avoids the complication of moving the camshaft axially but it also means that it is necessary to provide a ‘lost motion clutch’ on the camshaft.
why is exhaust gas harmful to environment?
Exhaust gases from engines and boilers contain atmospheric pollutants which are principally nitrogen oxides (NOX), sulphur oxides (SOX), carbon oxides and unburnt hydrocarbon particulates. These various pollutants contribute to smog and acid rain, and carbon oxides contribute to the greenhouse effect, which is increasing global temperatures. The IMO Marine Environment Protection Committee is considering ways to reduce the pollutants in exhaust emissions. IMO is to add a new Annex to MARPOL 73/78 to deal with atmospheric pollution. The SOX content of emission may be reduced by either a reduction of the sulphur content in fuels or an exhaust gas treatment system. New engine technology may reduce NOX formation and thus emissions, while carbon oxides can be reduced by good plant maintenance. Selective Catalytic Reduction Systems are in use on some vessels, which are said to reduce NOX emissions by 90 per cent and carbon oxides by 80 per cent.
define term medium speed diesel?
The term medium speed refers to diesels that operate within the approximate speed range of 300–800 rev/min. High speed is usually 1,000 rev/min and above.
what are the advantages of the medium-speed diesel?
1.Compact and space saving. The vessel can have reduced height and broader beam – useful in some ports where shallow draught is of importance. The considerable reduction in engine height compared to direct drive engines and the reduced weight of components means that lifting tackle, such as the engine room crane, is reduced in size as it will have lighter loads to lift through smaller distances. More cargo space is made available and because of the higher power to weight ratio of the engine a greater weight of cargo can be carried.
2.Through using a reduction gear a useful relationship between ideal engine speed and ideal propeller speed can be achieved. For optimum propeller speed hull form and rudder have to be considered, the result is usually a slow turning propeller (for large vessels this can be as low as 50–60 rev/min). Gearing enables the naval architect to design the best possible propeller for the vessels without having to consider any dictates of the engine. Engine designers can ignore completely propeller speed and concentrate solely upon producing an engine that will give the best possible power weight ratio.
3.Modern tendency is to utilise unidirectional medium-speed geared diesels coupled to either a reverse reduction gear, controllable pitch propeller (CPP) or electric generator.
The second two of these methods are the ones primarily used and the advantages to be gained are considerable.
They include:
a.Less starting torque required, clutch disengaged or CPP in neutral.
b.Reduced number of engine starts, hence starting air capacity can be greatly reduced and compressor running time minimised. Classification society requirements are six consecutive starts without air replenishment for non-reversible engines and twelve for reversible engines. Cylinder liner wear rate increases during starting.
c. Engines can be tested at full speed with the vessel alongside a quay without having to take any special precautions.
d. With the mechanical drive arrangement and the engine or engines running continuously, power can be taken off via a clutch or clutch/gear drive for the operation of electric generators or cargo pumps, etc. Hence the main engine has become a multi-purpose ‘power pack’.
e. Improved manoeuvrability, vessel can be brought to rest within a shorter distance by intelligent use of the engines and CPP.
f. Staff work load during ‘stand-by’ periods is reduced and the system lends itself ideally to simple bridge control.
4.With two engines coupled via gearing one may be disengaged, while the other supplies the motive power, and overhauled. This reduces off hire time as the voyage is continued at slightly reduced speed with a fuel saving.
5.Spare parts are easier to store and manhandle, therefore unit overhaul time will be greatly reduced.
Reverse reduction gear
These gear systems are mainly restricted, at present, to powers of up to about 4800 kW for twin-engined single-screw installations. Their obvious advantages are as follows:
1.Uni-directional engine.
2.No c.p. propeller required.
3.Ability to engage or disengage either engine of a twin-engine installation from the bridge by a relatively simple remote control.
4.Improved manoeuvrability, etc.
When dealing with higher powers the friction clutches used in the system can become excessively large, great heat generation during engagement may require a cooling system, the overall arrangement becomes more expensive and it may be cheaper to use direct reversing engines – however it would also, for reasons previously outlined, be prudent to use a c.p. propeller. Two systems of reverse reduction gear are shown in figures 8.3 and 8.4. In figure 8.3, the engine drives a steel drum which has two inflatable synthetic rubber tubes bonded to its inner surface. These tubes have friction material, like brake lining, on their inner surface. Air is supplied through the centrally arranged tube, or the annulus formed by the tube and shaft hole to one or the other of the inflatable tubes. Two flanged wheels are connected via hollow shafts and gears to the main gear wheel and shaft. For operation ahead, air would be supplied to inflatable tube A. which would then by friction on flanged wheel B bring gears 1 and 2 up to speed; gears 3, 4 and 5 together with flanged wheel D would be idling. For astern operation, air would be supplied to inflatable tube C (A evacuated) and by friction on flanged wheel D gears 3, 4, 5 and 2 would be brought up to speed, gear 1 and drum B would be idling. For single reduction, gears 3 and 4 would be the same size and so would be gears 1 and 5.An alternative system, either single or double reduction but probably the latter, is shown in figure 8.4. Friction clutches A and B are pneumatically controlled from some remote position. Gears 1, 2, 3 and 4 would have to be the same size if the gear were to be single reduction – but this is most unlikely.
with the aid of a sketch explain fluid coupling for 4 stroke?
These are completely self-contained, apart from a cooling water supply, they require no external auxiliary pump or oil feed tank. A scoop tube when lowered picks up oil from the rotating casing reservoir and supplies it to the vanes for coupling and power transmission; withdrawal of the scoop tube from the oil stops the flow of oil to the vane which then drains to the reservoir. During power transmission a flow of oil takes place continuously through the cooler and clutch. fluid clutches operate smoothly and effectively. They use a fine mineral lubricating oil and have no contact and hence no wear between driving and driven members. Torsional vibrations are dampened out to some extent by the clutch and transmitted speeds can be considerably less than engine speed if required by suitable adjustment of the scoop tube. It is possible to have a dual entry scoop tube for reversible engines, this obviates the use of c.p. propellers or reversible reduction gears but the control problem is considerably more complex with reversible engines, they have to be stopped and started and if four-stroke engines are used camshafts have to be moved, etc. (figure 8.2).
what are flexible couplings
These are used between engine and gearbox to dampen down torque fluctuations, reduce the effects of shock loading on the gears and engine, cater for slight misalignments. They are also used in conjunction with clutches for power take-off when required. In construction they may be similar to the well-known multi-tooth type to be found in turbine installations or employ diaphragms or rubber blocks. Those types that use rubber or synthetic rubber, such as Nitrile, give electrical insulation between driving and driven members, but all types will minimise vibration and reduce noise level.
purpose of Geisinger coupling
The main function of a Geislinger coupling is to assist in the damping out of torsional vibrations. This is accomplished by connecting the engine crankshaft to the load via flexible steel leaf springs arranged radially in the coupling, which is also filled with oil. As torsional fluctuations occur they are absorbed by the leaf springs which deflect and displace oil to adjacent chambers, slowing down the relative movement between the inner and outer components of the coupling. The makers claim that this effective damping is achieved without problems of wear because of the absence of friction. Damping oil is supplied from the engine oil system through the centre of the coupling. It is returned to the engine through hollow coupling bolts. Maintenance is limited to cleaning, inspection and the replacement of ‘O’ rings.
Explain gearboxes?
Gear boxes are very interesting and need a great deal of care in both manufacture and on-going care. They have to transmit sometimes large forces through relatively small areas of contact. The metal obviously transmits that power but metal-to-metal contact would mean that the component parts would not last for long. This means that the quality of the oil and the oil supply is vital to the on-going success of the gearbox. Although the gears are said to be meshing they are actually sliding over one another and the oil needs to be in the right place to ensure that the gear teeth perform correctly. Another major consideration for the ship’s engineering staff is to ensure that no metal, tools or other foreign bodies are allowed to enter the casing of the gearbox. The effect is catastrophic if metal gets between the teeth on the gearwheel due to the small clearance between the gears. Gearboxes transmit power through a drive train. The gears can be arranged to increase or decrease the speed of rotation of input and output shafts or they can be used to transmit power through an angle so that the output shaft is pointing in a different direction to the original shaft. The basic arrangement is to have straight gears around the outside of a wheel, fixed to the end of an input shaft, linking with a second wheel, of a different diameter, fixed to the start of an output shaft. The dimensions of the gear wheels will determine the different input and output speeds to and from the gearbox. Any combination of speeds can be chosen to suit the designer’s needs. Straight cut gears present a problem in so much as they have the minimum surface-to-surface contact area through which to transmit the power. Therefore they have to be sized accordingly. If the teeth are set at an angle across the end of the wheel to form a helical gear then the area for transmitting power in increased and the gear wheel can be more compact than a gear transmitting the same power using straight teeth. The problem here is that because the teeth are set at an angle there will be forces transmitted at different angles. One component of the force will be transmitted through the gear as required and another will be transmitted along the shaft as a vector component of the total force from the input shaft. This will result in a lateral thrust being transmitted along the shaft. The value of the thrust will depend upon the angle of the teeth and the total power from the input shaft. This means that with a single helical gearwheel a thrust block of some sort will be needed to counteract the thrust from the gearwheel. Another answer to this problem is to arrange for half the width of the gearwheel to have helical teeth set in one direction and the other half of the wheel to have teeth set in the opposite direction. This means that the thrust from one set of teeth is offset by the thrust from the other set of teeth and the need of a thrust block has been overcome. The profile of the teeth is very important to the smooth operation of the gearbox because for the teeth at the end of the input shaft to mesh and transmit power they have to slide into the space in-between the gears on the output shaft. It is also important for the tip of the gear not to make any contact with the root of the opposing gear wheel as this will also impose forces on the gears resulting in gearbox failure. Smaller gearboxes and low-power gearboxes might be lubricated by relying on the oil splashing onto the gears as they operate. However this means that at start-up the gears are not so well protected and wear can occur during this time. Larger or more powerful arrangements will have the oil pumped into the gearbox where it is arranged to spray directly onto the meshing gears ensuring the even at the start-up stage the gear teeth are well lubricated. Gearboxes do need to be checked and looked after. Any unusual noises must be investigated and routine inspections must be made at the appropriate intervals. It is not a good idea to make frequent visual inspections because there is more chance of introducing foreign materials inside the casing.
When an inspection is made the engineer should be looking out for the following:
- Broken teeth on the gearwheels
- Discolouration anywhere on the gearwheel of teeth (indicating overheating)
- Excessive wear on the faces of the gear teeth (indicating a lack of lubrication)
- Condition of the oil.
A sample of oil can be sent away for further analysis. This will be checked for any metal or water content and from this analysis a picture of the condition of the gearbox can be formed. This process obviously takes some time therefore some companies such as Kittywake International are now supplying analysis kits that can be used on-board. The next step is to offer online real-time testing of lubricating oil. This will then start to move the industry towards a CBM approach.
In order to minimise maintenance and to prolong valve life, bearing in mind that burning of high viscosity oil is essential due to the higher cost of light diesel oil, certain design parameters and operating procedures must be followed. explain what these are?
These are:
1.Separately caged exhaust valves are preferred even though they increase the initial cost. If they are made integral with the cylinder head and used with poor quality fuel then there will be an increased frequency of valve replacement and overhaul. Cylinder head removal each time becomes a tedious time-consuming operation and the caged valves save a lot of time. However part load or short trip operation can be a problem as the exhaust valves could be running at a temperature where the dew point of the gasses is reached. Some cross-channel operators have in the past had problems with acid erosion of exhaust valves spindles on uprated Pielstick PC 2.5 because they had water-cooled exhaust valve cages. The previous version of the engine running on the short voyages did not have the same problem.
2.All connections to the valves, cooling, exhaust, etc. should be capable of easy disconnection and re-assembly.
3.Materials that have to operate at elevated temperatures must be capable of withstanding the erosive and corrosive effects of the exhaust gas. When burning oils of high viscosity which contain sodium and vanadium deposits can form on the valve seats which, at high temperatures (in excess of 530°C at the valve seat), become strongly corrosive sticky compounds which lead to burnt valves. Hence, the need for materials that can withstand the corrosion and for intense cooling arrangements for valve seats.
4.Stellite valve seats have started the quest for improved durability of exhaust valves. Stellite is a mixture of cobalt, chromium and tungsten extremely hard and corrosion resistant that is fused on to the operating surfaces.
Low temperature corrosion due to sulphur compounds can occur during prolonged periods of running under low load conditions. The valve spindle and guide, which would be at a relatively low temperature, are the principal places of attack due to the effective cooling in this region. Ideally, valve cooling should be a function of engine load with the valve being maintained at a uniform temperature at all times, as stated this could prove complicated and expensive to arrange for part load and low load conditions.Further to the use of Stellite, a nickel-chromium alloy, strengthened by additions of titanium, aluminium and carbon called Nimonic 80A, has gained favour for use in exhaust valve construction. Recently MAN have found that welding a high-temperature resilient Ni-Cr alloy onto a stainless steel spindle would dramatically improve the hardness and ductility of the valve seat as well as its resistance to cracking when compared to chromium- and nickel-based hard facings including Nimonic 80A.In the first stage of the process, the stainless steel DuraSpindle is placed through a new robotic welding procedure where Inconel, an alloy traditionally used in gas turbines, is welded into the groove of an exhaust spindle valve seat.Once the alloy has been welded in place, the DuraSpindle is then machined after which more than 10 tonnes of force is used during the special rolling process to work harden the Inconel weld to 500 HV. While the spindle is being rolled and rotated three or four concentric grooves, depending on the spindle size, are etched into the seat at a depth of several millimetres. This further hardens a relatively ductile material.The rolling process provides compressive stresses into the component, as opposed to tensile stresses which may cause cracking in the seat area. Compressive stressing significantly reduces the probability of cracking even in the advent of welding defects.
The hard facing on the spindle seat is further hardened by heating the material up to 600–700°C. The metallurgical reaction, called precipitation hardening, further hardens the seat to 600 HV.Compared with an Alloy 50-type hard facing material DuraSpindle is 20% harder and 50% harder if compared to a spindle with Stellite hard facing or Nimonic 80A.
- Effective lubrication of the valve spindle is necessary to avoid risk of seizure and possible mechanical damage due to a valve ‘hanging up’. In order to minimise lubricating oil usage the lubrication system for the valves would be similar to that used for cylinder lubrication and since the amount of oil used would therefore be in small quantities, any contamination of the oil by combustion products and water, etc. would be minimal, and this would also increase the life of crankcase lubricating oil.
explain what the methane slip is?
Methane (CH4) slip is where some of the methane from the fuel moves through the engine and out of the exhaust without being burnt. Some manufacturers are keen to point out that this occurs more on the engines that operate on the Otto cycle rather than the Diesel cycle. However, the industry is confident that as the mechanics of the methane slip become better understood, so changes in combustion design will reduce the problem. Some suggestions for how methane can by-pass the combustion process include being injected early or late in the combustion cycle and the gas is therefore caught in the scavenge port and gets sucked though during the overlap period. Another possibility is that the air/gas mix in the Otto cycle can be caught just above the piston ring where it remains unburnt and escapes with the exhaust. It then follows that older, fuel oil, combustion space designs could be more prone to these imperfections than would new engines that are designed with methane slip in mind. It also follows that any reduction in fuel injection performance could make the situation worse. Methane slip occurs when there is a small amount of LNG that is unburned due to incomplete combustion cycle. Without any action being taken to reduce methane slip, methane can escape into the atmosphere. Methane which escapes combustion and is released by the engine exhaust or through the crankcase ventilation is referred to as methane slip. Methane is a Green House Gas that has a greater negative impact on the environment than CO2. For the methane slip for my dual fuel engine, i must keep it as low as possible, as well as for any bunkering operations that will be carried out, otherwise the benefit from reduction in CO2 due to the use of LNG will be non-beneficial. The dual fuel 4 stroke engine i am using in this project operates in an Otto cycle, with gas supply pressure being low. In this cycle fuel is mixed with air and introduced into the cylinder before the compression process starts. The Otto cycle combustion is started by injection of pilot oil. The pilot oil for this dual fuel engine is LNG. Since this engine operates in an Otto cycle, which uses premixed air to fuel ratio, the issue that can occur is methane slip. This wouldn’t occur in a 4-stroke engine because this type of engine doesn’t work in an Otto cycle.
what are the principle design parameters for medium speed engines
The principal design parameters for medium-speed diesel engines are:
1.High power/weight ratio.
2.Simple, strong, compact and space saving.
3.High reliability.
4.Able to burn a wide range of fuels.
5.Easy to maintain, the fact that components are smaller and lighter than those for slow-speed diesels makes for easier handling, but accessibility and simple to understand arrangements are inherent features of good design
6.Easily capable of adaption to unmanned operation.
7.Low fuel and lubricating oil consumption.
8.High thermal efficiency.
9.Low cost and simple to install.
10. Four-stroke design leads to electronic control and use of advanced environmental techniques such as the Miller cycle
do a comparison and explain some of the differences between the following coolants:
FW
Distilled water
LO
Comparison of coolants
Fresh water
Inexpensive, high specific heat, low viscosity. Contains salts which can deposit, obstruct flow and cause corrosion. Requires treatment. Leakages could contaminate lubricating oil system leading to loss of lubrication, possible overheating of bearings and bearing corrosion. Requires a separate pumping system. It is important that water should not be changed very often as this can lead to increased deposits. Leakages from the system must be kept to an absolute minimum, so a regular check on the replenishing-expansion tank contents level is necessary.
If the engine has to stand inoperative for a long period and there is a danger of frost,
(a)drain the coolant out of the system,
(b)heat up the engine room or
(c)circulate the system with heating on.
It may become necessary to remove scale from the cooling spaces and the following method could be used. Circulate, with a pump, a dilute hydrochloric acid solution. A hose should be attached to the cooling water outlet pipe to remove gases. Gas emission can be checked by immersing the open end of the hose occasionally into a bucket of water. Keep compartment well ventilated as the gases given off can be dangerous. Acid solution strength in the system can be tested from time to time by putting some onto a piece of lime. When the acid solution still has some strength and no more gas is being given off then the system is scale free. The system should now be drained and flushed out with fresh water, then neutralised with a soda solution and pressure tested to see that the seals do not leak.
Distilled water
More expensive than fresh water, high specific heat, low viscosity. If produced from evaporated salt water it would be acidic. No scale-forming salts. Requires separate pumping system. Leakages could contaminate the lubricating oil system, causing loss of lubrication and possible overheating and failure of bearings, etc.
Lubricating oil
This is expensive and generally there is no separate pumping system required since the same oil is normally used for lubrication and cooling. Leakages from cooling system to lubrication system are relatively unimportant provided they are not too large; otherwise one piston may be partly deprived of coolant with subsequent overheating. Due to reciprocating action of pistons some relative motion between parts in contact with the coolant supply and return system must occur; oil will lubricate these parts more effectively than water. No chemical treatment required. Lower specific heat than water, hence a greater quantity of oil must be circulated per unit time to give the same cooling effect. If the lubricating oil is subject to a high temperature it can burn leaving carbon deposit as it does so. This deposit on the underside of a piston crown could lead to impairment of heat transfer, overheating and failure of the metal. Generally the only effective method of dealing with the carbon deposit is to dismantle the piston and physically remove it. Since oil can burn in this way a lower mean outlet and inlet temperature of the oil has to be maintained. In order to achieve this more oil must be circulated per unit time.
how to prepare engine for sea service?
contact the bridge and chief engineer to inform them the engine is being prepared for sea service. test the steering gear to ensure its operational. prepare the jacket water system by first checking the expansion tank level and replenishing it if required. then check the jacket water temperature use preheater if necessary to warm up the gradually to operating temperature (60-65degrees) to allow for sufficient cooling and not raise the internal temperature which would lead to thermal stress and component failure. start the jacket water pump to begin circulating jacket water through the main engine. prepare the LO system by checking LO sump tank level and fill up if necessary. start the LO purifier to begin removing any impurities in LO. start the main engine LO pump to to circulate LO around the main engine for cooling and lubricating components and for jacket water to warm up the LO to operating temperature. continue to monitor all pressures and temperatures for J/W and LO system. prepare the fuel oil system and steam tracing system, by starting FO purifier and ensuring fuel is heated up by the steam system until fuel oil service operating temperature (90degrees). drain any water from fuel oil service tank and settling tank. prepare the air starting system by checking the air reservoir pressure and having compressors on auto so air reservoirs can be filled up automatically when necessary. drain any oil/ water moisture from air starting system from the air compressors and air bottles and open main air starting valve. contact bridge to inform the engine is going to be turned over. manually activate cylinder lubrication and engage the turning gear. ensure engine is turned for at least 15minmum to allow for 2revs and sufficient lubrication. open indicator cocks, start an additional generator for electrical power. kick the engine on air and observe indicator cocks to ensure cylinder are purged of debris/oil and water moisture. then disengage turning gear and close indicator cocks. test engine on fuel by opening necessary valves and starting fuel pump. switch auxiliary blowers to auto position. once all is confirmed okay contact the bridge to let them know the engine is ready and finally fill up the ER logbook.
what’s the purpose of a lambda controller?
The purpose with the lambda controller is to preventinjection of more fuel in the combustion chamber of an auxiliary engine on the ship, thancan be burned during a momentary load increase.This is carried out by controlling the relation betweenthe fuel index and the charge air pressure.The Lambda controller is also used as stop cylinder.
what are the advantages of a lambda controller?
The lambda controller has the following advantages:
1.Reduction of visible smoke in case of suddenmomentary load increases on auxiliary engines.
2.Improved load ability.
3.Less fouling of the engine’s exhaust gas ways.
4.Limitation of fuel oil index during startingprocedure.
with the aid of a drawing explain the working principle of a lambda controller?
see motor sketch pack for drawing
Figure above illustrates the controller’s operation mode. Incase of a momentary load increase, the regulatingdevice will increase the index on the injection pumpsand hereby the regulator arm (1) is turned, the switch(2) will touch the piston arm (3) and be pusheddownwards, whereby the electrical circuit will beclosed.
Thus the solenoid valve (4) opens. This valve is supplied with compressed air and the same is supplied to assist the turbocharger. When this jet system isactivated, the turbocharger accelerates and increasesthe charge air pressure, thereby pressing the piston(3) backwards in the lambda cylinder (5). When thelambda ratio is satisfactory, the jet system will be deactivated.At a 50% load change the system will be activated forabout 3-8 seconds.
If the system is activated more than 10 seconds, thesolenoid valve will be shut off and there will be aremote signal alarm for “jet system failure”.
what are the causes of top end bearing failure and how is it possible to assess what caused the top end bearing failure
Causes ofTop End Bearing Failure
Following are the possible causes of top end bearing failure:
1.Wiping of the bearing due to high bearing loads caused by excessive cylinderpressure being developed
2.Insufficient lubricating oil supply due to supply pump failure, failure of the oilpiping linkage, oil filter blockage
3.Impurities within the lubricating oil, causing abrasion of the bearing and pinsurface
4.Corrosion of the bearing and pin due to oil contamination with acidic productsand/or water
5.Wiping of the bearing due to low viscosity of the oil caused by excessive oiltemperatures and/or water
6.Insufficient bearing clearances within the bearing, causing excessive oiltemperatures and hence low oil viscosity
7.Excessive bearing clearances within the bearing, causing low oil generatedpressures due to excessive bearing end leakage
Assessment ofTop End Bearing Failure
The following information would be useful in assessing the possible causes of thefailure:
History of all work carried out on the bearings, to try and establish if any possiblepattern or links exist.
History of bearing clearances, to investigate whether the clearances have beenmaintained at the correct values.
Readings of the power developed by the engine, to establish if the engine has beenoperating at overload.
Readings of the maximum combustion pressures developed by each cylinder, toestablish if the load on the top end bearing has been excessive.
Readings of the lubricating oil analysis, to determine if the oil condition isacceptable.
what are ways to prevent top end bearing (cross head) failure
Prevention of Top End Bearing Failure
1.Bi- monthly monitoring of all bearing clearances, to ensure these are withinnormal limits
2.Bi -monthly oil analysis of the oil by the oil manufacturer, to ensure oil quality isensured
3.Weekly oil tests on-board for water contamination, dirt levels, viscosity and BNlevels, to ensure that the oil condition is acceptable.
4.Monthly checks of the lubricating oil low pressure alarm and trip, to ensure thatthe engine is protected at all times
5.Three times a day recording of the supply oil pressure and temperature, to monitorthe supply oil
6.Monthly monitoring of the cylinder pressures using indicator cards, to preventbearing overload
7.Closely monitor any overhaul and repair work carried out on the bearings toensure that the correct procedure was being followed, and that the re-assemblywas correct
8.Monitor any replacement parts that have been used to ensure they are the correctspecification
9.Monthly checks of the general crankcase to ensure all locking devices are still inplace
what is ignition quality and ignition delay? and how is good ignition quality and engine performance achieved?
Ignition delay is the time between fuel injection and fuel ignition. During this time the fuel get mixed with hot compressed air and vaporizes. After the ignition delay, spontaneous ignition of the fuel occurs. The longer the ignition delay, more fuel will be injected and vaporizes inside the combustion chamber. This results in a rapid explosion or combustion causing shock waves and high surface temperatures. This may lead to excessive loading of piston crown, breaking piston rings weakening of the material due to erosion by hot gas flow, etc. the higher temperatures inside the combustion space also cause an increased NOx emissions.
The ignition quality is a measure of the relative ease by which the fuel will ignite. It is measured by the cetane number for distillate fuels. The higher the number, the more easily will the fuel ignite inside the engine.For residual fuel, the ignition quality is measured by the Calculated Carbon Aromaticity Index (CCAI). It is an empirical equation based on the density and viscosity of the fuel. CCAI is normally in the range of 800-870. The higher the CCAI, the longer fuel takes before it starts ignition in the engine. In other words, ignition delay for a fuel with low CCAI is minimal.
Changing to a fuel with a higher ignition quality or lower CCAI index means early ignition of fuel, higher peak pressures, excessive load on the bearings (especially cross head bearings) and loss of engine power. Similarly, using a fuel with higher CCAI index or lower ignition quality will cause late ignition, causing after burning which damage exhaust valve, fouling of turbocharger, burning of piston crown and liner, and loss of engine power.
Two stroke slow speed engines and some medium speed engine on ships uses variable injection timing (VIT) or fuel quality setting (FQS) levers to control the starting of injection to take account of the ignition delay. These procedures to be done as per maker’s instructions and calculating the CCAI index of the fuel to be used.
sketch and briefly describe a single reduction gearbox suitable for speed reductions from 750to 150 rev/min reduction
The main wheel would contain two journal bearings for shaft support, as well as a thrustbearing to absorb the propeller thrust. The input shaft would only require simple journalbearing supports. The gearings and bearings would be lubricated by a self contained lube oil system, with thegears being supplied by sprayers.
