MARINE DIESEL ENGINE STRUCTURE
THE BEDPLATE: The Bedplate is the foundation on which the 2 stroke engine is built. It must be rigid enough to support the weight of the rest of the engine, and maintain the crankshaft, which sits in the bearing housings in the transverse girders, in alignment. At the same time it must be flexible enough to hog and sag with the foundation plate to which it is attached and which forms part of the ships structure.
If the bedplate was too rigid, then as the hull flexed, the holding down bolts, which secure the engine into the ship would be likely to break, and there would be a danger of the bedplate cracking.Basically the bedplate consists of two longitudinal girders which run the length of the engine. Connecting these longitudinal girders are the transverse girders which are positioned between each crankshaft throw, and either side of the thrust collar. Built into the transverse girders are the main bearing pockets for the crankshaft to run in.
On the small bore engines, the bedplate can be made from cast iron as a single casting. Larger engines have a fabricated bedplate. This means it is welded together from steel sections, steel castings and plate. The steel is to Classification Society specifications and is a low carbon steel with a maximum carbon content of 0.23%.
Earlier fabricated bedplates had box section longitudinal girders and box section fabricated transverse girders. Problems were encountered with cracking of the transverse girders, which increased as engine powers and crankshaft throws got larger.
The modern bedplate is constructed from fabricated longitudinal girders with cast steel transverse sections containing the bearing pockets and tie bolt holes welded into place. After manufacture, the bedplate is stress relieved, the bearing pockets are line bored and landing surfaces machined.
THE ENTABLATURE: The entablature is the name given to the cylinder block which incorporates the scavenge air space and the cooling water spaces. It forms the housing to take the cylinder liner and is made of cast iron. The castings are either for individual cylinders which after machining on the mating surfaces are bolted together to form the cylinder beam, or they may be cast in multi - cylinder units, which are then bolted together. The underside of the cylinder beam is machined and then it is aligned on the A frames and fastened in position using fitted bolts.
It is important to remember that the fitted bolts used to bolt the entablature, A frames and Bedplate together are for alignment and location purposes only. They are not designed to resist the firing forces which will tend to separate the three components. This is the job of the tie bolts.
A-FRAME: These carry the crosshead guides and support the engine entablature (the cylinder block). On large engines, the A frames are individually erected on the bedplate directly above the transverse girders. When boxed in with plating they form the crankcase. The trend nowadays is to build the frame box as a separate fabricated construction and then, after stress relieving and machining the mating surfaces, to mount it on the bedplate. This has the advantage of saving weight.
When the frames are aligned on the bed plate they are secured together by drilling and reaming and using fitted bolts.
Cracking in A frames can occur leading to misalignment and excessive wear of the running gear. Cracks can start from welds, sharp changes in section and where strengthening stringers are terminated sharply. Repairs can involve cutting the crack out, grinding and re-welding. The danger is that after repair there may still be misalignment.
A-FRAME: These carry the crosshead guides and support the engine entablature (the cylinder block). On large engines, the A frames are individually erected on the bedplate directly above the transverse girders. When boxed in with plating they form the crankcase. The trend nowadays is to build the frame box as a separate fabricated construction and then, after stress relieving and machining the mating surfaces, to mount it on the bedplate. This has the advantage of saving weight.
When the frames are aligned on the bed plate they are secured together by drilling and reaming and using fitted bolts.
Cracking in A frames can occur leading to misalignment and excessive wear of the running gear. Cracks can start from welds, sharp changes in section and where strengthening stringers are terminated sharply. Repairs can involve cutting the crack out, grinding and re-welding. The danger is that after repair there may still be misalignment.
HOLDING DOWN BOLTS: The engine must be securely fixed into the ship. As the engine turns the propeller, the propeller tries to push or thrust the propeller shaft and engine crankshaft forward into the ship. The thrust bearing which is situated at the aft end of the engine transmits this thrust from the crankshaft to the bedplate.
The bedplate is mounted on chocks and is securely bolted to the engine foundation plate on which it sits and which forms part of the structure of the hull. It is the friction between the bedplate, chocks and foundation plate that transmits the thrust, not the bolts themselves.
The Engine must also be lined up with the propeller shaft. If the engine output driving flange was higher or lower, or to port or stbd of the propeller shaft, then it is easy to visualise that trying to connect them would cause bending stresses to be set up.
The engine must also be bolted to a flat surface. If the surface was uneven, then when the bolts were tightened the bedplate would be distorted, which in turn would distort the crankshaft, causing unacceptable stresses to be set up when the engine was running.
Before the engine is bolted down it is supported on jacks whilst it is aligned with the tailshaft bearing. This used to be done by stretching a wire above the tailshaft, and measuring the distance from the wire to the crankshaft bearing centres. Modern methods use a laser.
When the bedplate is in perfect alignment, cast iron chocks are hand fitted between the machined underside of the bedplate and machined spots on the foundation plate. This is a skilled task and 80% contact is the aim.
Once the engine is supported by the chocks the jacks are removed and the holding down bolts are tightened using a hydraulic jack to stretch the bolts. Holding down bolts should be checked regularly for tightness. If they are allowed to come loose, then the mating surfaces will rub against each other and wear away in a process known as fretting. If this continues and the bolts are subsequently tightened down, the bedplate (and main bearings) will be pulled out of alignment.
TIE BOLTS: To understand the importance of the role played by the tie bolts, it is necessary to appreciate what is happening inside the cylinder of the engine.
When the piston is just after top dead centre the pressure inside the cylinder can rise as high as 140 bar (14000kN/m2). This acts downwards through the piston rod and con-rod, pushing the crankshaft down into the bearing pockets. At the same time, the pressure acts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into the entablature prevent this happening and so this upward acting force tries to lift the entablature from the frames and the frames from the bedplate, putting the fitted location bolts into tension.
As the piston moves down the cylinder the pressure in the cylinder falls, and then rises again as the piston changes direction and moves upwards on the compression stroke. This means that the fitted bolts are under are cyclic stress. Because they are not designed to withstand such stresses they would soon fail with disastrous consequences.
To hold the bedplate, frames and entablature firmly together in compression, and to transmit the firing forces back to the bedplate, long tie bolts are fitted through these three components and then tightened hydraulically. To prevent excessive bending moments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to hold the crankshaft main bearing cap in position instead of conventional studs and nuts.
Operating the engine with loose tie bolts will cause the fitted bolts holding the bedplate, frame and entablature in alignment to stretch and break. The machined mating surfaces will rub together and wear away ( this is known as fretting). Once this has happened the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause the transverse girders to bend which could lead to cracking, and main bearing misalignment. Once fretting between the mating surfaces has occurred, then tightening of the tie bolts will pull the engine out of alignment. The crosshead guides, the cylinder liner, and the stuffing box will no longer be in line and excessive wear will occur. Because the tie bolts will no longer be pulled down squarely they will be subject to forces which may lead to them breaking. If fretting has occurred, then the only solution is to remove the entablature or/and frame and machine the fretted mating surfaces (a very costly exercise).
Tie bolts can break in service. To reduce the risk of this happening they must be checked for tightness; not over-tightened; and the engine not overloaded. If a breakage does occur, this is not disastrous, as the engine can be operated with care for a limited period (the load on the engine may have to be reduced). The position of the fracture will dictate how the broken pieces are removed. However in the worst possible scenario where the bolt is broken at mid length, then the solution is to lift out the top half, remove the bottom nut, and then feed a loop of braided wire cable (about 7mm diameter) down the tie bolt tube, down the side of the broken tie bolt and once it emerges at the bottom a supporting piece can be fitted to the wire enabling the broken tie bolt to be withdrawn.
THE MARINE DIESEL ENGINE COMPONENTS
THE CRANKSHAFT: The crankshafts on the large modern 2 stroke crosshead engines can weigh over 300 tonnes. They are too big to make as a single unit and so are constructed by joining together individual forgings. On older engines the so called fully built method was used. This consisted of forging separate webs, crankpins and main journals. The crankpins and journals were machined and matching holes bored in the webs, which were slightly smaller in diameter. The webs were heated up and the crankpins and journals fitted into the holes (which due to the heat had expanded in size). As the webs cooled down, so the diameter of the bored holes would try and shrink back to their original size. In doing so, the crankpins and journals would be gripped tightly enough to stop them being able to slip when the engine was being operated normally.
Today, crankshafts for large 2-stroke crosshead type engines, are of the semi built type. In this method of construction the crankshaft "throws" consisting of two webs and the crankpin are made from a single forging of a 0.4% carbon steel. The webs are bored to take the separately forged and machined main journals which are fitted into the webs using the shrink fitting method described above. The shrink fit allowance is between 1/570 and 1/660 of the diameter.
The advantages of this method of construction is that by making the two webs and crankpin from a single forging the grain flow in the steel follows the web round into the crankpin and back down the other web. Because the crankpin and webs are a single forging, the webs can be reduced in thickness and a hole is sometimes bored through the crankpin, reducing the weight without compromising strength.
THE CONNECTING ROD: The Connecting Rod is fitted between the crosshead and the crankshaft. It transmits the firing force, and together with the crankshaft converts the reciprocating motion to a rotary motion. Made from drop forged steel, on the older engines the bottom of the con rod terminates in a flange known as a Marine Palm which is bolted to the split bottom end (Crankpin) bearing, whilst at the top another flange is formed on which is bolted the two crosshead bearings.
Connecting Rods on the later engines are produced as a single drop forging incorporating the top half of the crankpin bearing housing and the bottom half of the solid crosshead pin bearing housing.
Connecting Rods on the later engines are produced as a single drop forging incorporating the top half of the crankpin bearing housing and the bottom half of the solid crosshead pin bearing housing.
On older engines the bearings were white metal thick wall bearings, scraped to fit. Clearances were adjusted by inserting or removing shims between the bearing halves. Modern bearings are of the "thinwall" type, where a thin layer of white metal or a tin aluminium alloy is bonded to a steel shell backing. The clearance on these bearings is non adjustable; When the clearance reaches a maximum the bearing is changed.
Oil to lubricate the crankpin bearing is supplied down a drilling in the con rod from the crosshead. When inspecting the crankpin bearing and journal it is good practise to check the journal for ovality because if this is excessive, a failure in the hydrodynamic lubrication can occur.
THE CAMSHAFT: The Camshaft carries the cams which operate the fuel pumps and exhaust valves. Because these operate once every cycle of the engine, the camshaft on a two stroke engine rotates at the same speed as the crankshaft.
It is also very important that the fuel pump and exhaust valve operate at exactly the right time, so the camshaft is driven by the crankshaft. Two methods are used, a geared drive and a chain drive.
Chain drives are relatively light, narrow in width and flexible; however they elongate in service due to wear, which will affect the camshaft timing, and they have a limited life - 15 years.
Gear drives should last the life of the engine; However, a gear train is heavier, and more expensive. If the gear wheels are misaligned, shock loading will result, which can lead to broken teeth.
The Camshaft is made in sections, each section spanning one or two cylinders. The individual cams are manufactured from steel which are heat treated to give them a very hard surface, but retaining a tough interior. The cams are expanded onto the camshaft in the correct position using either heat or hydraulic means. The couplings for joining the camshaft segments together are also expanded hydraulically, allowing for adjustment and to facilitate the replacement of a cam if necessary.
The camshaft runs in underslung white metal lined bearings, lubricated in most cases by the main engine LO system. Older B&W engines used a separate LO system for the camshaft because of the possibility of contamination by fuel oil leaking past the fuel pumps.
On engines with chain driven camshafts, as the chain elongates, the timing of the fuel pumps and exhaust valves is retarded. When this retardation reaches a certain point, the camshaft must be re-timed. This is done by expanding the coupling between camshaft and drive using high pressure oil, and turning the camshaft to the correct position using a large spanner and chain block.
THE CYLINDER HEAD: Cylinder heads are exposed to maximum gas pressures and temperatures. They must therefore have adequate strength and cooling. This results in complex structures of strengthening ribs and cooling water passages. The design of heads is further complicated by the need to house various valves, fuel, air start, relief etc.
Where exhaust valves are situated in the head the structure design has to take into account the relatively high local temperatures around the valve which can cause thermal stressing. The combustion chamber may be formed by either shaping the cylinder cover or the piston crown. A flat piston crown is usually used with a shaped cover further complicating design and construction.
The original Sulzer engines employed single piece cylinder covers, but thermal stress cracks developed in relatively uncooled section where the conical part of the combustion chamber changed to the flat top. In order to avoid this problem some allowance was required for thermal expansion, and this was provided by having a two part cover with an inner and outer section.
THE CROSSHEAD: The purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the con rod and so bearings are required. It is also necessary to provide guides in order to ensure that the side thrust due to the conrod is not transmitted to the piston. This also ensure the piston remains central in the cylinder thus limiting wear in the liner.
Two faces are required as the thrust acts in opposite directions during power and compression stroke. Guide shoes positioned at the extreme ends of the crosshead pin provided a large area and minimise risk of twisting.
The usual way of checking guide clearance is by means of a feeler gauge with the piston forced hard against one face and the total clearance taken at the other face. This gives a reasonable estimation as wear should be approximately the same in the ahead and astern faces.
Guide clearances are usually adjusted by means of shims between the hardened steel guide bars and the mounting points. Bolts are slackened off allowing slotted shims to be inserted or removed. Note, care must be taken when handling these shims.
Crosshead pins are supported in bearings and the traditional way has been to mount the piston rod at the centre of the pin with a large nut and having two bearings alongside. This arrangement is like a simply supported beam and the pin will bend when under load. This gives rise to edge pressures which break through the oil film resulting in bearing failure. The Sulzer solution is to mount the bearings on flexible supports. When the pin bends the supports flex allowing normal bearing contact to be maintained.
In order to minimise the risk of bearing failure the actual force on the oil within the bearing should be kept within reasonable limits this can be achieved by having as large a bearing area as possible. Increasing the diameter of the pin and hence the bearing will minimise the problems as this not only allows for a large bearing area but it also avoids the problem of pin bending. Pin bending is further prevented by means of a continuous bearing. This also avoids the loss of oil which can take place with short bearings. Most modern engines tend to have single continuous bearings. Oil loss from the ends of bearings is prevented by means of restrictor plates. Some engine builders provide booster pumps which increase the oil pressure to the crosshead during the critical firing period. Cross heads do not have complete rotary motion and so a complete oil wedge does not form. The use of means for preventing oil loss are therefore useful in maintaining an oil film between pin and bearings.
THE STUFFING BOX: Since the crankcases is separated from the cylinder and scavenge space by the diaphragm plate on a two stroke crosshead engine, provision must be made for the piston rod to pass through the plate without oil from the crankcase being carried upwards, or used cylinder oil contaminated from products of combustion being carried downwards. It is also highly undesirable to allow the pressurized air in the scavenge space to leak into the crankcase.
As the piston passes down through the stuffing box, the top set of scraper rings will scrape off the contaminated oil into the bottom of the scavenge space, where it is drained away via the scavenge drains. However if these rings are faulty, then the oil may drain into the recycling tank.
By observing the open ended tell tale referred to above, a guide to the condition of the rings can be ascertained. If a large quantity of oil is draining out, then the lower set of rings are faulty. If air is blowing out, then the upper rings are worn.
Regular maintenance of the stuffing box will keep it in good condition. Checking garter spring tension, ring butt and axial clearances, and replacing worn rings are all part of the overhaul procedure.
Excessive wear will take place if the crosshead guides are out of alignment or if the guide clearances are excessive. Worn stuffing boxes and excessive leakage can exacerbate the incidence of scavenge fires and increase the risk of a crankcase explosion.
THE PISTON:The Piston comprises of two pieces; the crown and the skirt. The crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures. Steel, alloyed with chromium and molybdenum is used, and some pistons have a special alloy welded onto the hottest part of the crown to try and reduce the erosion caused by the burning fuel. The crown also carries the 4 or 5 piston ring grooves which may be chrome plated.
The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides).
A forged steel piston rod is bolted to the underside of the piston. The other end of the piston rod is attached to the crosshead pin.
Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.
Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.
An alternative method of cooling uses a nozzle plate and nozzles. Note that the oil goes up the annular space formed between the oil tube and the bore in the piston rod, and returns down the centre.
The oil is sprayed up matching bores onto the underside of the crown. This allows the crown to be made as thin as possible, to allow for maximum heat transfer while maintaining strength, and combined with the "cocktail" shaker effect caused by the reciprocating motion, gives efficient cooling.
When overhauling the piston it is important to check the thinning of the piston crown due to burning/erosion/corrosion. The piston should be dismantled to check the cooling space. If this is subject to a build up of carbon (in an oil cooled piston) or scale (in the case of a water cooled piston) then this may have led to thermal stressing of the piston, which in its turn can lead to cracking of the piston crown. If the cooling oil is allowed to leak into the combustion space then the consequences could be disastrous.
PISTON RINGS: The Piston Rings are made of alloying cast iron with chromium, molybdenum, vanadium, titanium, nickel and copper. They are harder than the cylinder liner in which they run to give them a maximum life.
Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls.
When overhauling the piston it is important to check the ring grooves for wear and the piston ring condition. The axial and butt clearances should be measured and recorded.
Rings must have sufficient spring so that they will provide an initial seal with the liner. As pressure builds up gas acting on the back face of the ring increase the sealing effect. The spring must be retained under normal operating temperatures. They must not crack under high temperature and pressure ranges.
With modern long stroke engines the rings do considerably more rubbing than equivalent sections of the liner and so the rubbing faces are usually made slightly harder. This is achieved by a case hardening process (usually Nitriding) some rings are contoured on the rubbing face in order to promote faster running in. Copper or carbon coatings are sometimes provided for the same purpose. When running-in, cylinder L.O. is increased to provide an additional flow to carry away metallic particles and a straight mineral oil without anti-wear properties is used.
The ring axial depth must be sufficient to provide a good seal against the liner but it must not be so great so that an oil wedge does not form. The ring actually distorts in the groove to form the wedge but if they are too deep they cannot do so. Thin rings will distort easily and scrape the oil from the surface. Radial depth must be sufficient to allow adequate support for the ring in the groove when the ring is on max. normal wear for its self and the liner.
Rings must be free in their grooves and the correct clearance is required. Excessive clearance can allow rings to twist while insufficient clearance can cause jamming and prevent the gas pressure from acting behind the rings. Also the rings may tend to twist excessively. Radial clearance must be sufficient between groove and ring back to allow a gas cushion to build up. The butt clearance must be sufficient to allow for thermal expansion. If insufficient the rings may seize and if excessive can lead to excessive blowpast
Grooves are sometimes coated with chromium to restrict deposit build up. For reconditioning the bottom face of the groove is generally provided with a replaceable steel wear ring. As the rings maintain the gas seal there is a desire to position the top or firing ring as close to the piston crown as possible. However ,since the crown is highly stressed, thermally, this results in distortion of that zone. There is thus a desire to position the ring a long distance away from the crown. A compromise position is decided upon in each engine design.
In order to minimise wear, a film of lubricating oil must be maintained between the moving parts i.e. the rings and liner, and rings and groove. Also the lubricating oil must spread over the liner surface by the rings, this helps to combat acidic products of combustion.
Skirts fitted to pistons on some designs perform the function of sealing the exhaust ports at T.D.C. these extended skirts have bronze rubbing rings inset to provide a bearing surface during the running in period.
Faults leading to collapse of piston rings:
The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides).
A forged steel piston rod is bolted to the underside of the piston. The other end of the piston rod is attached to the crosshead pin.
Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.
Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.
An alternative method of cooling uses a nozzle plate and nozzles. Note that the oil goes up the annular space formed between the oil tube and the bore in the piston rod, and returns down the centre.
The oil is sprayed up matching bores onto the underside of the crown. This allows the crown to be made as thin as possible, to allow for maximum heat transfer while maintaining strength, and combined with the "cocktail" shaker effect caused by the reciprocating motion, gives efficient cooling.
When overhauling the piston it is important to check the thinning of the piston crown due to burning/erosion/corrosion. The piston should be dismantled to check the cooling space. If this is subject to a build up of carbon (in an oil cooled piston) or scale (in the case of a water cooled piston) then this may have led to thermal stressing of the piston, which in its turn can lead to cracking of the piston crown. If the cooling oil is allowed to leak into the combustion space then the consequences could be disastrous.
PISTON RINGS: The Piston Rings are made of alloying cast iron with chromium, molybdenum, vanadium, titanium, nickel and copper. They are harder than the cylinder liner in which they run to give them a maximum life.
Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls.
When overhauling the piston it is important to check the ring grooves for wear and the piston ring condition. The axial and butt clearances should be measured and recorded.
Rings must have sufficient spring so that they will provide an initial seal with the liner. As pressure builds up gas acting on the back face of the ring increase the sealing effect. The spring must be retained under normal operating temperatures. They must not crack under high temperature and pressure ranges.
With modern long stroke engines the rings do considerably more rubbing than equivalent sections of the liner and so the rubbing faces are usually made slightly harder. This is achieved by a case hardening process (usually Nitriding) some rings are contoured on the rubbing face in order to promote faster running in. Copper or carbon coatings are sometimes provided for the same purpose. When running-in, cylinder L.O. is increased to provide an additional flow to carry away metallic particles and a straight mineral oil without anti-wear properties is used.
The ring axial depth must be sufficient to provide a good seal against the liner but it must not be so great so that an oil wedge does not form. The ring actually distorts in the groove to form the wedge but if they are too deep they cannot do so. Thin rings will distort easily and scrape the oil from the surface. Radial depth must be sufficient to allow adequate support for the ring in the groove when the ring is on max. normal wear for its self and the liner.
Rings must be free in their grooves and the correct clearance is required. Excessive clearance can allow rings to twist while insufficient clearance can cause jamming and prevent the gas pressure from acting behind the rings. Also the rings may tend to twist excessively. Radial clearance must be sufficient between groove and ring back to allow a gas cushion to build up. The butt clearance must be sufficient to allow for thermal expansion. If insufficient the rings may seize and if excessive can lead to excessive blowpast
Grooves are sometimes coated with chromium to restrict deposit build up. For reconditioning the bottom face of the groove is generally provided with a replaceable steel wear ring. As the rings maintain the gas seal there is a desire to position the top or firing ring as close to the piston crown as possible. However ,since the crown is highly stressed, thermally, this results in distortion of that zone. There is thus a desire to position the ring a long distance away from the crown. A compromise position is decided upon in each engine design.
In order to minimise wear, a film of lubricating oil must be maintained between the moving parts i.e. the rings and liner, and rings and groove. Also the lubricating oil must spread over the liner surface by the rings, this helps to combat acidic products of combustion.
Skirts fitted to pistons on some designs perform the function of sealing the exhaust ports at T.D.C. these extended skirts have bronze rubbing rings inset to provide a bearing surface during the running in period.
Faults leading to collapse of piston rings:
THE 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 homogenous 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 has to be cooled.
• 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 homogenous casting with low residual stresses.
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 undercooling further down the liner was taking place. Why is this a problem? Well, 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 (below 140ºc) then corrosion can take place. Once the oil film has been destroyed then wear will take place at an alarming rate. One solution was 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 at the very top.
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 0.05 - 0.1mm/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.
Besides corrosive attack, wear is also 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.
THE EXHAUST VALVE: Exhaust valves open inwards into the cylinder, so that the gas pressure in the cylinder will ensure positive closing and help dislodge any build up of carbon on the valve seat.
Two stroke crosshead engines have a single exhaust valve mounted in the centre of the cylinder head. The opening and closing of the valve is controlled by a cam mounted on the camshaft. On older engines the cam follower lifts a push rod, which operates a rocker arm and opens the valve.
This has disadvantages- the push rod and rocker arm is heavy and the engine must overcome the inertia of these heavy parts. The motion of the rocker arm is an arc of a circle, which will tend to move the exhaust valve sideways, causing wear on the exhaust valve guide which locates the exhaust valve spindle. Exhaust gas can then leak up the spindle, causing overheating and accelerating wear. The springs which ensure the valve closes will weaken with use and are liable to break.
Modern two stroke crosshead engines have a hydraulically operated air sprung exhaust valve. The cam operates a hydraulic pump instead of a push rod. Oil (from the engine LO system) displaced by the pump operates a piston in the exhaust valve which pushes the valve open.
Instead of mechanical springs, the valve has an "air spring". Air at 7 bar is led via a non return valve to the underside of a piston attached to the valve spindle. As the valve opens, the air underneath the piston is compressed. The expansion of this compressed air, when the hydraulic pressure is relieved assists in the closing of the valve. The air is supplied with a small amount of oil for lubrication purposes. Air is also led down the exhaust valve guide. This keeps the guide cool and lubricated, and prevents the exhaust gas leaking up the guide. Excess oil which collects at the bottom of the air spring cylinder is drained to a collecting tank.
To prevent the possibility of an air lock, the hydraulic system has a small leak off at the top of the exhaust valve hydraulic cylinder. Oil is made up via a non return valve. A relief valve is also fitted (not shown). A damping arrangement on top of the piston in the exhaust valve prevents hammering of the valve seating.
The valve spindle is fitted with a winged valve rotator. The kinetic energy in the exhaust gas rotates the valve a small amount as it passes. This keeps the valve at an even temperature and helps reduce the build up of deposits on the valve seat.
The cage of the exhaust valve is of cast iron as is the guide. The renewable valve seat is a hardened molybdenum steel and the valve spindle can be a molybdenum chrome alloy with a layer of stellite welded onto the seating face, or alternatively a heat resistant nimonic alloy valve head, friction welded to an alloy steel shaft.
When the valves are overhauled, the valves and seats are not lapped together. Instead special grinding equipment is used to grind the seat and spindle to the correct angles.
THE FUEL PUMP: Fuel has to be injected into the engine at a high pressure so that it atomizes correctly. Injection takes place over a short period of time and this period of time must be accurately controlled; late or early injection will lead to a lack of power and damage to the engine. Because the timing of injection is crucial, cams mounted on the camshaft, which is driven by the crankshaft are used to operate the fuel pumps, one of which is provided for each cylinder.
As the cam rotates it operates a spring loaded ram (the plunger) which moves up and down in a cylinder (the barrel). As the plunger moves up the barrel, the pressure of the fuel in the barrel above the plunger rises very quickly. The high pressure fuel then opens the fuel valve (injector) and is sprayed into the cylinder in tiny droplets known as atomization. It is important to note that the injection only takes place when the plunger is moving up the cam slope.
This is the principle behind the operation of the fuel pump. However, the pump illustrated opposite could not be used because it will always deliver the same amount of fuel. Once started, the engine would over speed. A method which will infinitely vary the amount of fuel injected into the engine controlled by the governor must be utilised. Two different methods are used. In the first, the plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The plunger reciprocates in a barrel, located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor. As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should therefore be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, then no injection will take place and the engine will stop.
The plunger is machined to very fine tolerances, as is the matched barrel in which it reciprocates. Wear due to abrasive particles in the fuel will mean that the pump will take longer to build up the injection pressure required. Wear due to erosion also takes place on the top edge of the plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.
On the scroll or helical fuel pump previously described, although the end of injection can be varied, the start of injection (i.e. when the top of the plunger covers the spill ports) is fixed. Fuels of different qualities may require advancing or retarding the injection timing, in addition to which if the injection timing is advanced when the engine is running at loads below the maximum continuous rating, then a saving in fuel can be achieved. This method of varying the injection timing (known as Variable Injection Timing) can be achieved by the method shown. The bottom of the barrel has a coarse screw thread cut into it. This is located in a threaded sleeve which is turned by a rack and pinion. The barrel is free to move up and down in the pump casing but cannot rotate. This means that as the threaded sleeve is rotated by the VIT rack the position of the spill ports relative to the barrel is changed, thus altering the start of injection.
The second method of controlling the quantity of fuel is by using suction and spill valves operated by push rods. A plain plunger reciprocates in a barrel. As the plunger moves up and down, two pivoted levers operate push rods which open the suction and spill valves. When the cam follower is on the base circle of the cam, the suction valve is open and the spill valve is closed. As the plunger moves up the barrel, the suction push rod moves downwards and the suction valve closes. Injection then commences and fuel is delivered via a non return valve to the injectors. As the plunger continues upwards so the spill push rod will open the spill valve, the pressure above the plunger will fall and injection will cease.
The quantity of fuel delivered can be controlled by altering the position of the eccentric pivot for the spill valve operating lever. This will cause the spill valve to open earlier or later. By altering the position of the suction valve pivot, the start of injection can be similarly controlled, and therefore it can be seen that the pump utilises VIT. This pump will not suffer the erosion problems that affect the scroll type pump. However wear due to abrasive particles in the fuel will still affect performance. Regular maintenance will include overhaul of the valves and seats.
THE FUEL INJECTOR: The fuel is delivered by the fuel pumps to the fuel injectors, also known as fuel valves. For the fuel to burn completely at the correct time it must be broken up into tiny droplets in a process known as atomisation. These tiny droplets should penetrate far enough into the combustion space so that they mix with the oxygen. The temperature of the droplets rise rapidly as they absorb the heat energy from the hot air in the cylinder, and they ignite and burn before they can hit the relatively cold surface of the liner and piston.
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".
At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.
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 540 bar.
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 engines do not have internal 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.
As the cam rotates it operates a spring loaded ram (the plunger) which moves up and down in a cylinder (the barrel). As the plunger moves up the barrel, the pressure of the fuel in the barrel above the plunger rises very quickly. The high pressure fuel then opens the fuel valve (injector) and is sprayed into the cylinder in tiny droplets known as atomization. It is important to note that the injection only takes place when the plunger is moving up the cam slope.
This is the principle behind the operation of the fuel pump. However, the pump illustrated opposite could not be used because it will always deliver the same amount of fuel. Once started, the engine would over speed. A method which will infinitely vary the amount of fuel injected into the engine controlled by the governor must be utilised. Two different methods are used. In the first, the plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The plunger reciprocates in a barrel, located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor. As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should therefore be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, then no injection will take place and the engine will stop.
The plunger is machined to very fine tolerances, as is the matched barrel in which it reciprocates. Wear due to abrasive particles in the fuel will mean that the pump will take longer to build up the injection pressure required. Wear due to erosion also takes place on the top edge of the plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.
On the scroll or helical fuel pump previously described, although the end of injection can be varied, the start of injection (i.e. when the top of the plunger covers the spill ports) is fixed. Fuels of different qualities may require advancing or retarding the injection timing, in addition to which if the injection timing is advanced when the engine is running at loads below the maximum continuous rating, then a saving in fuel can be achieved. This method of varying the injection timing (known as Variable Injection Timing) can be achieved by the method shown. The bottom of the barrel has a coarse screw thread cut into it. This is located in a threaded sleeve which is turned by a rack and pinion. The barrel is free to move up and down in the pump casing but cannot rotate. This means that as the threaded sleeve is rotated by the VIT rack the position of the spill ports relative to the barrel is changed, thus altering the start of injection.
The second method of controlling the quantity of fuel is by using suction and spill valves operated by push rods. A plain plunger reciprocates in a barrel. As the plunger moves up and down, two pivoted levers operate push rods which open the suction and spill valves. When the cam follower is on the base circle of the cam, the suction valve is open and the spill valve is closed. As the plunger moves up the barrel, the suction push rod moves downwards and the suction valve closes. Injection then commences and fuel is delivered via a non return valve to the injectors. As the plunger continues upwards so the spill push rod will open the spill valve, the pressure above the plunger will fall and injection will cease.
The quantity of fuel delivered can be controlled by altering the position of the eccentric pivot for the spill valve operating lever. This will cause the spill valve to open earlier or later. By altering the position of the suction valve pivot, the start of injection can be similarly controlled, and therefore it can be seen that the pump utilises VIT. This pump will not suffer the erosion problems that affect the scroll type pump. However wear due to abrasive particles in the fuel will still affect performance. Regular maintenance will include overhaul of the valves and seats.
THE FUEL INJECTOR: The fuel is delivered by the fuel pumps to the fuel injectors, also known as fuel valves. For the fuel to burn completely at the correct time it must be broken up into tiny droplets in a process known as atomisation. These tiny droplets should penetrate far enough into the combustion space so that they mix with the oxygen. The temperature of the droplets rise rapidly as they absorb the heat energy from the hot air in the cylinder, and they ignite and burn before they can hit the relatively cold surface of the liner and piston.
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".
At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.
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 540 bar.
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.
THE AIR STARTING VALVE: The valve is fitted into the cylinder head. It is opened by control air from the starting air distributor. Main starting air at about 30 bar from the manifold enters the chamber above the valve via the circumferential ports in the valve body.
When the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.
The air pressure will not open the valve because a spring is holding the valve shut, an the area of the balance piston is the same as that of the valve lid so the valve is pneumatically balanced.
When the valve is required to open, air at 30 bar from the air start distributor enters the the top of the valve body and acts on a piston. This force overcomes the spring force holding the valve shut, and the valve opens. When the air signal from the air start distributor is vented, the spring closes the valveWhen the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.
The Sulzer air start valve uses air on both sides of the operating piston to maintain positive closing. The piston is stepped. The reason for this is so the starting air valve will not open when the gas pressure in the cylinder is higher than the starting air pressure; i.e. when the cylinder is firing. Once the valve starts to open then the opening is accelerated when the larger diameter piston has the opening air acting on it.
The stepped piston also means that closing of the valve is damped as air gets trapped in the annular space formed when the smaller diameter piston enters the upper part of the cylinder.The air to operate the valve comes from the main air start supply. The distributor pilot air operates the pneumatic change over valve.
After certain periods of service starting air valves are changed and overhauled. If piston rings are fitted, care must be taken to ensure that they are free in their grooves. Should it be necessary to fit new rings, the butt clearances of the rings must be carefully checked by placing the ring into the operating cylinder and measuring the clearance. This is especially important if they are usually made of brass which has a larger coefficient of expansion than the other parts of the valve. The valve and valve seat are ground with grinding paste and finished to a fine surface with lapping paste. It is essential to ensure that all parts of the valve are scrupulously clean before reassembly. Lubricate all sliding surfaces sparingly with a molybdenum disulphide grease.
THE TURBOCHARGER: By turbocharging an engine, the following advantages are obtained:
• Increased power for an engine of the same size OR reduction in size for an engine with the same power output.
The turbocharger consists of a single stage impulse turbine connected to a centrifugal impeller via a shaft. The turbine is driven by the engine exhaust gas, which enters via the gas inlet casing. The gas expands through a nozzle ring where the pressure energy of the gas is converted to kinetic energy. This high velocity gas is directed onto the turbine blades where it drives the turbine wheel, and thus the compressor at high speeds (10 -15000 rpm). The exhaust gas then passes through the outlet casing to the exhaust uptakes.
On the air side air is drawn in through filters, and enters the compressor wheel axially where it is accelerated to high velocity. The air exits the impeller radially and passes through a diffuser, where some of the kinetic energy gets converted to pressure energy. The air passes to the volute casing where a further energy conversion takes place. The air is cooled before passing to the engine inlet manifold or scavenge air receiver.
The nozzle ring is where the energy in the exhaust gas is converted into kinetic energy.
It is fabricated from a creep resistant chromium nickel alloy, heat resisting moly-chrome nickel steel or a nimonic alloy which will withstand the high temperatures and be resistant to corrosion.
Turbine blades are usually a nickel chrome alloy or a nimonic material (a nickel alloy containing chrome, titanium, aluminium, molybdenum and tungsten) which has good resistance to creep, fatigue and corrosion. Manufactured using the investment casting process. Blade roots are of fir tree shape which give positive fixing and minimum stress concentration at the conjunction of root and blade. The root is usually a slack fit to allow for differential expansion of the rotor and blade and to assist damping vibration. On small turbochargers and the latest designs of modern turbochargers the blades are a tight fit in the wheel.
Lacing wire is used to dampen vibration, which can be a problem. The wire passes through holes in the blades and damps the vibration due to friction between the wire and blade. It is not fixed to each individual blade. The wire can pass through all the blades, crimped between individual blades to keep it located, or it can be fitted in shorter sections, fixed at one end, joining groups of about six blades.
A problem with lacing wire is that it can be damaged by foreign matter, it can be subject to corrosion, and can accelerate fouling by products of combustion when burning residual fuels. Failure of blading due to cracks emanating from lacing wire holes can also be a problem. All the above can cause imbalance of the rotor.
The turbine casing is of cast iron. Some casings are water cooled which complicates the casting. Water cooled casings are necessary for turbochargers with ball and roller bearings with their own integral LO supply (to keep the LO cool). Modern turbochargers with externally lubricated journal bearings have uncooled casings. This leads to greater overall efficiency as less heat energy is rejected to cooling water and is available for the exhaust gas boiler.
The compressor impeller is of aluminium alloy or the more expensive titanium. Manufactured from a single casting it is located on the rotor shaft by splines. Aluminium impellers have a limited life, due to creep, which is dictated by the final air temperature. Often the temperature of air leaving the impeller can be as high as 200°C. The life of the impeller under these circumstances may be limited to about 70000 hours. To extend the life, air temperatures must be reduced. One way of achieving this is to draw the air from outside where the ambient air temperature is below that of the engine room. Efficient filtration and separation to remove water droplets is essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of salt water.
The air casing is also of aluminium alloy and is in two parts. Bearings are either of the ball or roller type or plain white metal journals. The ball and roller bearings are mounted in resilient mountings incorporating spring damping to prevent damage due to vibration. These bearings have their own integral oil pumps and oil supply, and have a limited life (8000 hrs). Plain journal bearings are lubricated from the main engine oil supply or from a separate system incorporating drain tank, cooler and pumps. Oil is supplied in sufficient quantity to cool as well as lubricate. The system may incorporate a header tank arrangement to supply oil to the bearings whilst the turbocharger comes to rest should the oil supply fail. A thrust arrangement is required to locate and hold the rotor axially in the casing. In normal operation the thrust is towards the compressor end.
Labyrinth seals or glands are fitted to the shaft and casing to prevent the leakage of exhaust gas into the turbine end bearing, or to prevent oil being drawn into the compressor. To assist in the sealing effect, air from the compressor volute casing is led into a space within the gland. A vent to atmosphere at the end of the labyrinth gives a guide to the efficiency of the turbine end gland. Discoloring of the oil on a rotor fitted with a roller bearing will also indicate a failure in the turbine end gland.
A labyrinth arrangement is also fitted to the back of the compressor impeller to restrict the leakage of air to the gas side
THE TURBOCHARGER: By turbocharging an engine, the following advantages are obtained:
• Increased power for an engine of the same size OR reduction in size for an engine with the same power output.
• Reduced specific fuel oil consumption - mechanical, thermal and scavenge efficiencies are improved due to less cylinders, greater air supply and use of exhaust gasses.
• Thermal loading is reduced due to shorter more efficient burning period for the fuel leading to less exacting cylinder conditions.
The turbocharger consists of a single stage impulse turbine connected to a centrifugal impeller via a shaft. The turbine is driven by the engine exhaust gas, which enters via the gas inlet casing. The gas expands through a nozzle ring where the pressure energy of the gas is converted to kinetic energy. This high velocity gas is directed onto the turbine blades where it drives the turbine wheel, and thus the compressor at high speeds (10 -15000 rpm). The exhaust gas then passes through the outlet casing to the exhaust uptakes.
On the air side air is drawn in through filters, and enters the compressor wheel axially where it is accelerated to high velocity. The air exits the impeller radially and passes through a diffuser, where some of the kinetic energy gets converted to pressure energy. The air passes to the volute casing where a further energy conversion takes place. The air is cooled before passing to the engine inlet manifold or scavenge air receiver.
The nozzle ring is where the energy in the exhaust gas is converted into kinetic energy.
It is fabricated from a creep resistant chromium nickel alloy, heat resisting moly-chrome nickel steel or a nimonic alloy which will withstand the high temperatures and be resistant to corrosion.
Turbine blades are usually a nickel chrome alloy or a nimonic material (a nickel alloy containing chrome, titanium, aluminium, molybdenum and tungsten) which has good resistance to creep, fatigue and corrosion. Manufactured using the investment casting process. Blade roots are of fir tree shape which give positive fixing and minimum stress concentration at the conjunction of root and blade. The root is usually a slack fit to allow for differential expansion of the rotor and blade and to assist damping vibration. On small turbochargers and the latest designs of modern turbochargers the blades are a tight fit in the wheel.
Lacing wire is used to dampen vibration, which can be a problem. The wire passes through holes in the blades and damps the vibration due to friction between the wire and blade. It is not fixed to each individual blade. The wire can pass through all the blades, crimped between individual blades to keep it located, or it can be fitted in shorter sections, fixed at one end, joining groups of about six blades.
A problem with lacing wire is that it can be damaged by foreign matter, it can be subject to corrosion, and can accelerate fouling by products of combustion when burning residual fuels. Failure of blading due to cracks emanating from lacing wire holes can also be a problem. All the above can cause imbalance of the rotor.
The turbine casing is of cast iron. Some casings are water cooled which complicates the casting. Water cooled casings are necessary for turbochargers with ball and roller bearings with their own integral LO supply (to keep the LO cool). Modern turbochargers with externally lubricated journal bearings have uncooled casings. This leads to greater overall efficiency as less heat energy is rejected to cooling water and is available for the exhaust gas boiler.
The compressor impeller is of aluminium alloy or the more expensive titanium. Manufactured from a single casting it is located on the rotor shaft by splines. Aluminium impellers have a limited life, due to creep, which is dictated by the final air temperature. Often the temperature of air leaving the impeller can be as high as 200°C. The life of the impeller under these circumstances may be limited to about 70000 hours. To extend the life, air temperatures must be reduced. One way of achieving this is to draw the air from outside where the ambient air temperature is below that of the engine room. Efficient filtration and separation to remove water droplets is essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of salt water.
The air casing is also of aluminium alloy and is in two parts. Bearings are either of the ball or roller type or plain white metal journals. The ball and roller bearings are mounted in resilient mountings incorporating spring damping to prevent damage due to vibration. These bearings have their own integral oil pumps and oil supply, and have a limited life (8000 hrs). Plain journal bearings are lubricated from the main engine oil supply or from a separate system incorporating drain tank, cooler and pumps. Oil is supplied in sufficient quantity to cool as well as lubricate. The system may incorporate a header tank arrangement to supply oil to the bearings whilst the turbocharger comes to rest should the oil supply fail. A thrust arrangement is required to locate and hold the rotor axially in the casing. In normal operation the thrust is towards the compressor end.
Labyrinth seals or glands are fitted to the shaft and casing to prevent the leakage of exhaust gas into the turbine end bearing, or to prevent oil being drawn into the compressor. To assist in the sealing effect, air from the compressor volute casing is led into a space within the gland. A vent to atmosphere at the end of the labyrinth gives a guide to the efficiency of the turbine end gland. Discoloring of the oil on a rotor fitted with a roller bearing will also indicate a failure in the turbine end gland.
A labyrinth arrangement is also fitted to the back of the compressor impeller to restrict the leakage of air to the gas side
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