Scavenge space fire

INTRODUCTION: 

For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present a combustible material, oxygen or air to support combustion and a source of heat at a temperature high enough to start combustion.

Source: www.marinediesels.info

In the case of scavenge fires:
 the combustible material is oil. The oil can be cylinder oil which has drained down from the cylinder spaces, or crankcase oil carried upwards on the piston rod because of a faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinders and other similar causes.
 The oxygen necessary for combustion comes from the scavenge air which is in plentiful supply for the operation of the engines.
 The source of heat for ignition comes from piston blow-by, slow ignition and afterburning, or excessive exhaust back pressure, which causes a blowback through the scavenge ports.

• A scavenge fire can cause serious damage to the piston rod diaphragm gland as well as leading to possible distortion of the air box and cracking of the liner. Tie rod tension will almost certainly be affected.
• The worst case scenario for a scavenge fire is it leading to a crankcase explosion
• The fire may also spread outside the scavenge box due to relief doors leaking or oil deposits on the hot casing igniting. For these reasons a scavenge fire should be dealt with as quickly as possible.

INDICATION

 Loss in power and irregular running of the engine,
 High exhaust temperatures of corresponding units,
 High local temperature in scavenge trunk,
 Surging of turbocharger,
 Sparks and smoke emitted from scavenge drains.
 External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles.
 If the scavenge trunk is oily the fire may spread back-from the space around or adjacent to the cylinders where the fire started and will show itself as very hot spots or areas of the scavenge trunk surfaces.
 In ships where the engine room is designed as UMS, temperature sensors are fitted at critical points within the scavenge spaces. So, activation would cause automatic slow down of the engine.

ACTION TO BE TAKEN WHEN SCAVENGE FIRE OCCURRED

 In the event of scavenge fire the engine must be put to dead slow ahead as soon as possible and the fuel must be taken off the cylinders affected by the fire or preferably stopped.
 The turning gear should be put in and the engine continuously turned with increased cylinder oil to prevent seizure (jam).
 All scavenge drains must be shut to prevent the discharge of sparks and burning oil from the drains into the engine room.
 Air supply should be cut off by enclosing the turbocharger inlets, for mechanically operated exhaust valves the gas side should also be operated, (hydraulically operated exhaust valves will self close after a few minutes).

For a minor scavenge fire:
–  A minor fire may shortly burn out without damage, and conditions will gradually return to normal. The affected units should be run on reduced power until inspection of the scavenge trunking and overhaul of the cylinder and piston can be carried out at the earliest safe opportunity.
–  Once navigational circumstances allow it, the engine should be stopped and the whole of the scavenge trunk examined and any oil residues found round other cylinders removed.
–  The actual cause of the initiation of the fire should be investigated

For a major scavenge fire:

–  If the scavenge fire is of a more major nature, if there is a risk of the fire extending or if the scavenge trunk is adjacent to the crankcase with risk of a hot spot developing it sometimes becomes necessary to stop the engine.
–  Normal cooling is maintained, and the turning gear engaged and operated. Fire extinguishing medium should be applied through fittings in the scavenge trunk: these may inject carbon dioxide, dry powder or smothering steam.

 The fire is then extinguished before it can spread to surfaces of the scavenge trunk where it may cause the paint to start burning if special non inflammable paint has not been used.
 Boundary cooling of the scavenge trunk may be necessary. Keep clear of scavenge relief valves, and do not open up for inspection until the engine has cooled down.

After extinguishing scavenge fire:
 After extinguishing the fire and cooling down, the scavenge trunking and scavenge ports should be cleaned and the trunking together with cylinder liner and water seals, piston, piston rings, piston skirt, piston rod and gland must be inspected.
 Heat causes distortion and therefore checks for binding of piston rod in stuffing box and piston in liner must be carried out.
 Tightness of tie bolts should be checked before restarting the engine.
 Inspect reed valves if fitted, and scavenge relief valve springs.
 Fire extinguishers should be recharged at the first opportunity and faults diagnosed as having caused the fire must be rectified.

SAFETY FITTING

1. Scavenge belt relief door
2. Fire Fighting Media

1. SCAVENGE BELT RELIEF DOOR:

Scavenge belt relief door Fitted to both ends of the scavenge belt and set to lift slightly above the maximum normal working scavenge air pressure.

2.  FIRE FIGHTING MEDIA
 Carbon dioxide- will put out a fire but supply is limited. Susceptible to loss if dampers do not effective prevent air flow
 Water spray- perhaps the ideal solution giving quick effective cooling effect to the fire.
 Dry powder- will cover the burning carbon and oil but is messy. As the fire may still smoulder below the powder care must be taken when the scavenge doors are removed as the powder layer may be blown away.
 Steam smothering-plentiful and effective

Source: www.marinediesels.info

PREVENTION

 Good maintenance and correct adjustment must be carried out
 Scavenge trunking must be periodically inspected and cleaned and any buildup of contamination noted and remedied.
 The drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludges which do not drain down so easily and which are a common cause of choked drain pipes
 Scavenge drains should be blown regularly and any passage of oil from them noted.
 The piston rings must be properly maintained and lubricated adequately so that ring blow-by is prevented.
 At the same time one must guard against excess cylinder oil usage.
 With timed cylinder oil injection the timing should be periodically checked.
 Scavenge ports must be kept cleared
 The piston-rod packing rings and scraper rings should also be regularly adjusted so that oil is prevented from entering the scavenge space because of butted ring segments.
 This may and does occur irrespective of the positive pressure difference between the scavenge trunk and the crankcase space.
 Fuel injection equipment must be kept in good condition, timed correctly, and the mean indicated pressure in each cylinder must also be carefully balanced so that individual cylinders are not overloaded.
 If cylinder liner wear is up to maximum limits the possibility of scavenge fires will not be materially reduced until the liners are renewed

REFERENCES:
1. www.marineengineering.co.uk
2. The Running and Maintenance of Marine Machinery – Cowley
3. Reeds Marine Engineering Series, Vol. 12 – Motor Engineering Knowledge for Marine Engineers
4. Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen
5. Diesel Engines – A J Wharton
6. www.marinediesels.info

Written by Mohammud Hanif Dewan, IEng, IMarEng, MIMarEST, MRINA

Fuel injector

Image Credit: www.riceweightloss.com

Older loop scavenged engines may have a single injector mounted centrally in the cylinder head. Because the exhaust valve is in the centre of the cylinder head on modern uniflow scavenged engines the fuel valves (2 or 3) are arranged around the periphery of the head.
The pressure at which the injector operates can be adjusted by adjusting the loading on the spring. The pressure at which the injectors operate vary depending on the engine, but can be as high as 540bar.

OPERATION

– Fuel injectors achieve this by making use of a spring loaded needle valve.
– The fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where the needle valve is held hard against its seat by a strong spring.
– As the fuel pump plunger rises in the barrel, pressure builds up in the chamber, acting on the underside of the needle as shown. When this force overcomes the downward force exerted by the spring, the needle valve starts to open.
– The fuel now acts on the seating area of the valve, and increases the lift.
– As this happens fuel flows into the space under the needle and is forced through the small holes in the nozzle where it emerges as an “atomised spray”.

Image Credit: www.marinediesels.co.uk

At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.

ATOMIZATION

It is the break-up of the fuel change into a very small particles when it is injected into the cylinder
Proper atomization facilitates the starting of the burning and ensures that each minute particle of fuel is surrounded by oxygen particles which it can combine

Image Credit: www.marineinsight.com

PENETRATION

It refers to the distance that the fuel particles travel or penetrate into combustion chamber

TURBULENCE or SWIRL

– It refers to the aim movement pattern within the combustion chamber at the end of compression.
The spray pattern of the fuel is cone-shaped.

– These occurs when there is an excess velocity of fuel spray during injection, causing contact with metallic engine parts and one result is flame burning

INJECTOR NOZZLE:

The body of a fuel injector valve is normally flanged at the upper end and the lower end is threaded to accommodate the nozzle body and nozzle cap nut
The nozzle body contains four holes. One is for the fuel inlet and another for the fuel priming valve, these two holes are connected through a common space within the fuel nozzle or by annular space

Image Credit: DieselNet

The valve needle which has been lapped into a very accurately machine guide into the nozzle body, is held on the conical seat immediately above the atomization holes
Slightly clearance between needle and nozzle body to allow for temperature changes when working with heated fuel.

COOLING OF FUEL INJECTION VALVE:

Some injectors have internal cooling passages in them extending into the nozzle through which cooling water is circulated. This is to prevent overheating and burning of the nozzle tip.
Injectors on modern 2 stroke crosshead engines do not have internal water cooling passages. They are cooled by a combination of the intensive bore cooling in the cylinder head being close to the valve pockets and by the fuel which is recirculated through the injector when the follower is on the base of the cam or when the engine is stopped.

As well as cooling the injector, recirculating the fuel when the engine is stopped keeps the fuel at the correct viscosity for injection by preventing it from cooling down.
The animation opposite shows the principle on which one system operates.
Fuel injectors must be kept in good condition to maintain optimum efficiency, and to prevent conditions arising which could lead to damage within the cylinder. Injectors should be changed in line with manufacturers recommendations, overhauled and tested. Springs can weaken with repeated operation leading to the injector opening at a lower pressure than designed. The needle valve and seat can wear which together with worn nozzle holes will lead to incorrect atomisation and dribbling

FAULTS OF FUEL INJECTORS:

1. Over heating OR under cooling:
If cooling of the injector is reduced, either by fuel valve cooling system or poor heat transfer to the cylinder head, then the working temperature of the injector will rise. This can cause:-
– Softening of the needle and seat which increases the possibility of nozzle leakage and/or,
– Fuel to expand/boil out of the fuel sac, leading to carbon trumpet formation, and increased levels of HC and smoke in the exhaust gases.

2. Over cooling:
More common on older vessels with separate fuel valve water cooling systems. When the injector is over cooled, the tip of the injector falls below the condensation temperature and acid corrosion due to the sulphur in the fuel oil occurs. This can severely corrode the injector tip, causing the spray pattern to be affected.

3. leakage from Nozzle:
This fault will produce carbon trumpets as the dribble of fuel burns close to the tip and the carbon deposits remain. The formation of the trumpets will have a progressive affect by influencing the spray pattern of the fuel, and this can be detected in the increased exhaust gas temps and smoke levels.
Nozzle leakage can sometimes be identified by a seat defect(the seat is no longer narrow in appearance, and is caused by):-
– Insufficient cooling,
– Dirt within the fuel damaging/abrading the seating area,
– Excessive needle valve hammering, due to excessive time in service, excessive needle lift or spring force.

4. Weak spring:
This will cause the injector to open and close at a lower pressure. Thus the size of the fuel droplets will increase during these injection periods.
Increased droplet size at the start of combustion will decrease the maximum cylinder pressure (late combustion), whilst increased droplet size at the end of combustion will increase the exhaust temperature and smoke (afterburning).
Causes of a weak spring are usually metal fatigue, due to an excessive number of operations.

5. Slack needle:
Slight leakage between the needle valve and its body is required to provide lubrication of the moving parts. However excess leakage due to a slack needle will allow a greater quantity, and larger size of fuel particle to pass between the valve and body.
The quantity of leakage should not influence injector performance unless excessive, but dirt particles between the needle and body can increase friction and make the needle action sluggish.
The cause of a slack needle is usually poor filtration of the fuel causing wear between needle and body.

6. Poor atomisation:
This will increase the size of the fuel droplets, which will increase the time required for combustion. Thus engine noise, exhaust smoke, exhaust temperatures, etc will increase. Poor atomisation can be caused by low injection pressure (fuel pump wear), high fuel viscosity and nozzle hole obstruction such as carbon trumpets.

7. Poor penetration
This will reduce the mixing which occurs between the fuel and air, and will increase the over-rich areas in the centre area of the cylinder. Thus only following combustion in the centre area will the expanding gases move the fuel charge into the air rich outer ring of the cylinder where the greatest mass of air is present.
This will increase the time required for combustion as the fuel/air mixture is not correct in many areas, and hence afterburning, exhaust temps, and smoke will increase.
Causes of poor penetration is reduced injection pressure, and nozzle hole blockage such as trumpets or sac deposits.

8. Over penetration
This will occur when the air density within the cylinder is reduced, or with over-size holes. The liquid stream travels too far into the cylinder, so that a high level of liquid impingement on the liner wall takes place. This will remove the liner lubrication, and once burning will greatly increase the liner wall temperature, and its thermal stress.
If this over penetration is caused by prolonged low power operations, then “slow speed” nozzles should be fitted.

Slow steaming nozzles can be used when regular and prolonged engine operation is required between 20-50% power.
The nozzle hole diameter is reduced to
i. Reduce the penetration that will occur into the less dense cylinder air
ii. Keep the atomisation level and injection pressure sufficient, as mass flow rate is reduced.

If the engine is operated for long period on low levels of power/speed with `normal’ size injector nozzles, then the atomisation will reduce, thus engine noise, mechanical loading, exhaust smoke, exhaust temps, and fuel consumption will increase.

EFFECT OF FAULTY FUEL INJECTORS:

1. Greatly enlarged holes cause overheating, perhaps burning of piston upper surface, also cause carbon deposits in the piston cooling space, if oil cooled. It may also cause increased cylinder and piston ring wear

2. If the holes are chocked, the fuel sprays will be effected to the extent that imperfect combustion will result. This in turn may reduce the power output quite considerably and bring about all the mechanical troubles usually associated with after burning.
3. If the injectors leaky or spring is damaged, burning of piston upper surface, also cause carbon deposits in the piston cooling space, if oil cooled. It may also cause increased cylinder and piston ring wear and can lead to scavenge fire.

INDICATION OF FAULTS:

1. Early injection is usually evidenced by knocking in the cylinder. On the power diagram the maximum pressure will be considerably in excess. Exhaust temperature will be low.

2. Leaky valve can be detected through indicator diagram, which show reduced combustion pressure. This will be some reduction in power output, increasing in exhaust temperature about 10oC and smoky gases. Chocking of atomizer and exhaust ports. Surging in turbo-blower are also some of the indication

3. After burning will cause higher exhaust temperature and pressure. The maximum height of both the power and draw diagram would be reduced. Other indications are smoky exhaust, possible fires in uptake, fouling of exhaust system, surging of turbo-blower

4. Choked fuel injectors – combustion efficiency of an engine depends on fuel atomization, shape and direction of the fuel sprays. So the holes should be clear and clean. First outward indication of accumulation of carbon deposits will be increase in the exhaust temperature due to fuel not mixing properly with the air, consequently not burning completely in the allocated time. Power output is reduced and the exhaust is smoky.

MAINTENANCE

• Fuel injectors must be kept in good condition to maintain optimum efficiency, and to prevent conditions arising which could lead to damage within the cylinder.
• Injectors should be changed in line with manufacturers recommendations, overhauled and tested.
• Springs can weaken with repeated operation leading to the injector opening at a lower pressure than designed.
• The needle valve and seat can wear which together with worn nozzle holes will lead to incorrect atomization and dribbling.
• Proper cooling should be made during operation. Cooling passages to be cleaned during overhaul.
• Proper grade of fuel oil should be used and it should be used after proper purification to prevent atomized holes become enlarged, conical and oval due to abrasive materials.
• The valve body and valve needle should always be considered as a unit, not as two separate pieces and they should be renewed together.
• The holes should be cleaned and cleared properly without damaging by blown with compressed air.
• The valve needle must be perfectly fluid tight when in the closed position and must open and close smartly.
• The cam operating the fuel valves or the fuel pump, as the case may be, should effect opening and closing in the shortest time practicable.

References:

1. www.marineengineering.co.uk
2. The Running and Maintenance of Marine Machinery – Cowley
3. Reeds Marine Engineering Series, Vol. 12 – Motor Engineering Knowledge for Marine Engineers
4. Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen
5. Principles and Practice of Marine Diesel Engines – Sanyal

Written by Mohammud Hanif Dewan, IEng, IMarEng, MIMarEST, MRINA

construction of cylinder liner 2- stroke and 4 - stroke

Operational Information of Two Stroke Cross-head Engine  Cylinder Liner:

The cylinder liner forms the cylindrical space in which the piston reciprocates. The reasons for manufacturing the liner separately from the cylinder block (jacket) in which it is located are as follows;

• The liner can be manufactured using a superior material to the cylinder block. While the cylinder block is made from a grey cast iron, the liner is manufactured from a cast iron alloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)
• The cylinder liner will wear with use, and therefore may have to be replaced. The cylinder jacket lasts the life of the engine.
• At working temperature, the liner is a lot hotter than the jacket. The liner will expand more and is free to expand diametrically and lengthwise. If they were cast as one piece, then unacceptable thermal stresses would be set up, causing fracture of the material.
• Less risk of defects. The more complex the casting, the more difficult to produce a homogeneous casting with low residual stresses.

The Liner will get tend to get very hot during engine operation as the heat energy from the burning fuel is transferred to the cylinder wall. So that the temperature can be kept within acceptable limits the liner is cooled.

Cylinder liners from older lower powered engines had a uniform wall thickness and the cooling was achieved by circulating cooling water through a space formed between liner and jacket. The cooling water space was sealed from the scavenge space using 'O' rings and a telltale passage between the 'O' rings led to the outside of the cylinder block to show a leakage.

To increase the power of the engine for a given number of cylinders, either the efficiency of the engine must be increased or more fuel must be burnt per cycle. To burn more fuel, the volume of the combustion space must be increased, and the mass of air for combustion must be increased. Because of the resulting higher pressures in the cylinder from the combustion of this greater mass of fuel, and the larger diameters, the liner must be made thicker at the top to accommodate the higher hoop stresses, and prevent cracking of the material. 

If the thickness of the material is increased, then it stands to reason that the working surface of the liner is going to increase in temperature because the cooling water is now further away. Increased surface temperature means that the material strength is reduced, and the oil film burnt away, resulting in excessive wear and increased thermal stressing.

The solution is to bring the cooling water closer to the liner wall, and one method of doing this without compromising the strength of the liner is to use tangential bore cooling. Holes are bored from the underside of the flange formed by the increase in liner diameter. The holes are bored upwards and at an angle so that they  approach the internal surface of the liner at  a tangent. Holes are then bored radially around the top of the liner so that they join with the tangentially bored holes.

On some large bore, long stroke engines it was found that the under cooling further down the liner was taking place. Why is this a problem? The hydrogen in the fuel combines with the oxygen and burns to form water. Normally this is in the form of steam, but if it is cooled it will condense on the liner surface and wash away the lube oil film. Fuels also contain sulphur. This burns in the oxygen and the products combine with the water to form sulphuric acid. If this condenses on the liner surface, then corrosion can take place. Once the oil film has been destroyed then wear will take place at an alarming rate. One solution is to insulate the outside of the liner so that there was a reduction in the cooling effect. On the latest engines, the liner is only cooled by water at the very top, relying on the air in the scavenge space to cool the lower part of the liner.

The photo shows a cylinder liner with the upper and mid insulation bands known as "Haramaki" Although Haramaki is a type of Japanese Armour, the word also means literally " Stomach or Body Warmer". i.e an insulator.

Cylinder lubrication: Because the cylinder is separate from the crankcase there is no splash lubrication as on a trunk piston engine. Oil is supplied through drillings in the liner. Grooves machined in the liner from the injection points spread the oil circumferentially around the liner and the piston rings assist in spreading the oil up and down the length of the liner. The oil is of a high alkalinity which combats the acid attack from the sulphur in the fuel. The latest engines time the injection of oil using a computer which has inputs from the crankshaft position, engine load and engine speed. The correct quantity of oil can be injected by opening  valves from a pressurized system, just as the piston ring pack is passing the injection point.

As mentioned earlier, cylinder liners will wear in service. Correct operation of the engine (not overloading, maintaining correct operating temperatures) and using the correct grade and quantity of cylinder oil will all help to extend the life of a cylinder liner. Wear rates vary, but as a general rule, for a large bore engine a wear rate of 0.05mm/1000 hours is acceptable. The liner should be replaced as the wear approaches 0.8 - 1% of liner diameter. The liner is gauged at regular intervals to ascertain the wear rate. It has been known for ships to go for scrap after 20 + years of operation with some of the original liners in the engine.

Gauging a Liner

As well as corrosive attack, wear is caused by abrasive particles in the cylinder (from bad filtration/purification of fuel or from particles in the air), and scuffing (also known as micro seizure or adhesive wear). Scuffing is due to a breakdown in lubrication which results in localised  welding between points on the rings and liner surface with subsequent tearing of microscopic particles . This is a very severe form of wear.

Medium Speed 4 Stroke Trunk Piston Engine:

The cylinder liner is cast separately from the main cylinder frame for the same reasons as given for the 2 stroke engine which are:

• The liner can be manufactured using a superior material to the cylinder block. While the cylinder block is made from a grey cast iron, the liner is manufactured from a nodular cast iron alloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)
• The cylinder liner will wear with use, and therefore may have to be replaced. The cylinder jacket lasts the life of the engine.
• At working temperature, the liner is a lot hotter than the jacket. The liner will expand more and is free to expand diametrically and lengthwise. If they were cast as one piece, then unacceptable thermal stresses would be set up, causing fracture of the material.
• Less risk of defects. The more complex the casting, the more difficult to produce a homogeneous casting with low residual stresses.

Modern liners employ bore cooling at the top of the liner where the pressure stress is high and therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits so that there is not a breakdown in lubrication or excessive thermal stressing. Although the liner is splash lubricated from the revolving crankshaft, cylinder lubricators may be provided on the larger engines. On the example shown opposite, the lubricator drilling are bored from the bottom of the liner circumstantially around the liner wall. Another set of holes are drilled to meet up with these vertically bored holes at the point where the oil is required at the liner surface. Other engines may utilize axial drilling as in a two stroke engine.

Sulzer ZA40 Liner (vee engine; The straight engine is similar)

MAN-B&W L58/64 Liner

Where the cooling water space is formed between the engine frame and the jacket, there is a danger that water could leak down and contaminate the crankcase if the sealing O rings were to fail. As a warning, "tell tale" holes are led from between the O rings to the outside of the engine. modern engines tend not to use this space for cooling water. Instead a separate water jacket is mounted above the cylinder frame. This stops any risk of leakage of water from the cooling space into the crankcase (or oil into the cooling water space), and provides the cooling at the hottest part of the cylinder liner. Note that the liner opposite is fitted with a fire band. This is sometimes known as an anti-polishing ring. It is slightly smaller in diameter than the liner, and its purpose is to remove the carbon which builds up on the piston above the top ring. If this carbon is allowed to build up it will eventually rub against the liner wall, polishing it and destroying its oil retention properties.

The liner must be gauged regularly to establish the wear rate and check that it is within manufacturers tolerances. The wear rate for a medium speed liner should be below 0.015mm/1000hrs. Excessive wear is caused by lack of lubrication, impurities in fuel air or Lubricating oil, bad combustion and acid attack.

cylinder liner, construction, lubrication, stresses, replacing!!