Diesel Engine Construction
Description of the salient parts
Modern engines designed for increasing power to weight ratios are reliant on not only stronger materials but also on careful design. Each component design must not only be optimised for its own purpose, but also in some cases, to provide strength to the overall structure.
An example of this is how the bedplate and A-frame combine to create a strong rigid box able to resist the forces of combustion and maintain essential alignment for the crankshaft and over moving parts.
The entablature not only supports the cylinder liner and head it also creates areas for cooling water and scavenging air.
Fabrication techniques are extensively used simplifying castings and speeding assembly times by reducing the number of fastening.In order to obtain ideal strength transfer between components the fastenings must have intermit contact with the surfaces of the components and hence fitted bolts are used.
Cylinder blocks must be cast, due to the difficulties in casting large components generally single cylinder blocks are created joined to each other and to a common fabricated A-frame/bedplate box.
The bedplate acts as the main strength member, maintains correct alignment and supports the weight of the components. it must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure.
In addition it may also collect lubricating oil.
In slow speed engine design, it consists of a deep longitudinal box section with stiffening in the form of members and webs.
Transverse members are fitted between each throw of the crankshaft. These support the main bearing saddles and Tie -rod connection. They are attached to the structure by substantial butt welds.
To reduce the engine height the sump of the bedplate may be sunken allowing it to fitted into a recess in the ships structure.
Plate and weld preparation is required with welds of the double butt type if possible. Regular internal inspection of the parts especially the transverse girder is required for fatigue cracking. Tie bolts should be checked for tightness.
Box girders-A box girder is stronger and more rigid then I or H section girder of the same c.s.a.
From the simple beam bending equation we have;
M /I = s /y = E/R
I=2nd moment of area of the cross section
y=distance from the axis of bending to the outer face
E= modulus of elasticity
R-radius of curvature of the bending.
This can be arranged into
s = (M/I) . Y
It can be seen that for the same bending moment on a symmetrical shape of same size, the stress is reduced on the increasing 2nd moment of area. The second moment of area increase with moving of material away from the axis of bending towards the extremes of the section.
Because of this the commonest way of construction a fabricated bedplate is by creating two box section girders and tie them using transverse girders.
The advent of the small bore slow speed has seen the use of single side bedplates. A box section is then created by using a box section crankcase structure rather than the more traditional A-frame.This has the advantages of reducing width as well as weight and increasing the amount of fabrication so reducing assembly times.
Due to the weight penalty, the use of cast iron is generally limited to smaller units where fabrication becomes impractical. However, cast iron has internal resilience allowing it to dampen down vibrations, this has led to its usage on some medium speed installations, especially passenger carriers, where noise and vibration suppression is important.
The most highly loaded pat of a bedplate is the transverse girder. Classification societies require that residual stress is removed after construction.
The transverse girder acts as a simple beam with the forces of combustion acting on the piston passing down through the bearing. The forces acting on the head are passed through the Tie rods.
It can be seen that to reduce the bending moment the tie rods have to be brought closer to the crankshaft. The limit to this is the securing arrangement required for the main bearing keep. One method is to use two instead of one bolts which can be made of smaller diameter. Sulzer use an alternative and very successful method in the form of jacking bolts. These jack against the bottom of the A-frame.
The increasing stroke to bore ratio has led to several problems not least of which is cylinder lubrication, thermal gradients over stroke, starting etc. I addition there is a necessity to dampen rotational vibrations over the engine height by means of hydraulic stays attached to the ships structure. These reduce the movement of the engine without allowing excessive vibration to be transmitted to the hull.
The camshaftless two stroke crosshead engine has two main advantages;
Firstly it simplifies engine design in particular negating the need for chain or gear driven camshafts. This allows a net reduction in weight, simplifies engine erection and removes some physical constraints for future engine design.
Secondly, it allows for finite control of parameters such as fuel delivery volume and timing, and exhaust valve opening and closing times.
Traditionally fuel, exhaust valve opening, starting air and cylinder lube oil delivery are all controlled by camshaft lobe design. It is possible now to control these using high reliability solenoid valves. This method is used on the sulzer RT-flex engine
Fuel is delivered to a common rail by a high efficiency fuel pump operated by a multilobe cam driven off the main engine crankshaft.
A control unit activates electric solenoid valves to deliver fuel to the appropriate cylinder injectors.
This allows control of fuel volume, fuel delivery progression ( that is the shape of fuel delivery- e.g slow than fast) and precise timing. It means that fuel ignition quality and condition at delivery can be taken into account.
Exhaust valves are operated in the normal way via hydraulic pushrods. However hydraulic oil delivery is not by individual pump but by common rail supplied by a high capacity, high pressure servo pump. The engine management control unit operates the exhaust valves by energising the appropriate solenoid valve. Other starting air valves and cylinder lube oil are all similarly controlled by the engine management control unit via solenoid valves
The modern trend has followed the smaller and lighter rule with an ever increasing power to weight ratio. In addition to this simplified maintenance and production procedures have been incorporated.
Intelligent engines without camshafts are being introduced enabling increased efficiency with reduced fuel quality. Intelligent engines also allow for increased efficiency when running at part load.
Modern requirements for the design and construction of a cylinder liner include;
High topland ( the 1st piston ring is positioned will below the upper surface of the piston) with associated reduced ring heat load has given better ring pack performance by improving working conditions for the cylinder lube oil. The disadvantage of this system is that a coke build up can occur above the piston which leads to 'bore polishing'. This polishing reduces the ability of the cylinder lube oil to 'key' into the liner therefore increased cylinder lube oil consumption/increased liner wear can result. To combat this piston cleaning rings are incorporated into the liner. These slightly reduce the bore removing the deposits.
Comparisons of cross head and trunk piston engines
There are two basic types of connecting a piston to a crankshaft;
Crosshead construction-used by all slow speed two stroke engine manufactures
Trunk piston construction- used in smaller four stroke engines
The piston is rigidly fixed to the piston rod. The rod passes through a gland to a cross head to which it is attached via a flange , or shoulder through bolt and nut. The crosshead consists of an rotating element which is attached to the connecting rod. It is through this bearing that the vertical load is transferred from the combustion space and converted into the rotary motion of the crankshaft via the connecting rod and big end bearing. Horizontal thrust generated at the crosshead are absorbed by white metal surfaced shoes which run up vertical athwartships mounted guides.
The advantages of the crosshead design are
Trunk piston construction
The piston is directly attached to the connecting rod by a small end rotating bearing. Side thrust is absorbed by extended skirts on piston.
The main advantage is reduced engine height
Opposed piston engines
Mainly built by doxford and consisted of two opposing piston moving in a common liner. Fuel injection occurred at the centre where the piston met. Construction is of the crosshead design with the upper piston connected to the crankshaft via two side rods and transverse beam. Timing was approximately 180oC except for a small angle of advance for exhaust timing.
Holding down arrangements
The engine must be firmly in position in the ship in order to preserve alignment between the crankshaft and the propeller shaft.
Vibration, rolling and pitching will tend to move the engine from its original set position.
The bedplate is aligned and individual chocks are used to maintain that alignment. Holding down studs which are preferred to bolts because they are cheaper are then used to hold the engine firmly to the tank top.
The bedplate must be as rigid as possible in order to keep the crankshaft straight. The ships structure, which includes the tank tops will distort as the ship rolls and pitches. Modern bulk carriers are constructed of high tensile steel because thinner plates may be used, thus reducing the deadweight and allowing more cargo to be carried. Thinner plates are more flexible and this increases the relative movement between tank top and bedplate.
This relative movement between the rigid bedplate and flexing tanktop leads to bending of the stud and possible fatigue failure.
One solution is to use spherical washes but these are expensive. A better solution is to use longer studs. When these bend the radius of curvature is grater and so reducing the bending moment for the same relative movement. A large radius of curvature means a lower stress and the risk of fatigue failure is reduced.
M /I = s/y = E/R
I=2nd moment of area of the cross section
y=distance from the axis of bending to the outer face
E= modulus of elasticity
R-radius of curvature of the bending.
This can be arranged into
s = E . y /R
or s = k. 1 /R
where k is a constant
that is, stress is inversely proportional to the radius of curvature.
End Chocks (aft end only)
Steel chocking has the disadvantages that each block must be individually fitted, a time consuming process, and after fitting are susceptible to fretting and wear.
Resin chocks are poured and therefore are much quicker to apply. They form into the shape of the clearance and key into surface imperfections. This much reduces damage due to fretting and removes bending moment on the holding down bolts.
The disadvantage is that the resin creation must be precise and that it is less straight forward to replace in the event of damage of misalignment
The material used for the resin chocking is Class tested to ensure minimum standards.
A sample cured in the correct way is tested for the following;
These are positioned at each transverse girder. They are intended to keep the transverse girder in compression at all times thus minimising risk of fatigue cracking. Correct tension is therefore important and this should be checked regularly in accordance with the engine manufacturers instructions, this normally means retensioning the bolts in pairs from the centre of the engine. alternately for'd and aft.
Tie-rods are often in two parts for ease of manufacture and fitting when head room is restricted. This also makes changing the bolt in the event of breakage simpler
Pinch bolts are fitted at certain points to prevent vibration which can induce stress and cause fatigue. These must be released before the bolts are retensioned
Tension should be checked at set intervals, following a scavenge fire, after application of an excessive load, following grounding or collision, or where the landing face have become suspect. Tiebolts are susceptible to fretting, often indicated by the presence of red dust (sometimes called cocoa) around the nut. In the event of this it is important to check the condition of the nut landing and to ensure before retightening that the surface is clean and free from moisture.
The most common method for applying the correct tension to the bolt is by use of hydraulic jacks. These are mounted on the tiebolt thread above teh nut. The jack stretches the bolt by acting on a removable sleeve surrounding the nut. Once the bolt has been extended the nut may be rotated via slots cut into the sleeve allowing access. Pressure is applied as per manufacturers requirements which extends the bolt within its elastic limit, the nut is screwed down hand tight and the pressure released. A second method involves the nut turning to handtight, then by use of a gauge the nut is rotated a further angle.
Tie-rods are nor required on medium speed engines generally because the relatively thick sections used means that stress is lower.
Opposed piston engines do not require tie-rods because combustion load is transmitted from the crankshaft to the bedplate is very low.
The traditional through tie bolt is being superceded by shorter twin stay bolts which have the advantage of reducing distortion of the main bearing keep
Bending results in tensile, compressive and shear stresses in the material of the crank web.
Twisting results in shear stresses.
Crankshafts are subject to a complex form of loading which varies with time. In addition shrink fits, oil holes and fillet radii add to the complexity. Pure stress analysis and rules governing crankshaft dimensions are based upon a combination of theory and experience.
The three main loading stresses are;
All the above alternating stress patterns produce fatigue and so the material must have a built in resistance to it- this is of equal importance to its U.T.S. (Ultimate Tensile Stress). Mild steel is usually the material used but in some cases alloying the steel with a small percentage of nickel, Chromium, Vanadium may take place.
Crankshafts fail usually because of cracks propagating from a stress concentration point.
All components vibrate e.g. a weight on a spring, rotating components such as crankshafts can vibrate in a torsional manner. The systems will differ but the principals are the same. The operating frequency caused by the operating speed is known as the forcing frequency. All systems have natural frequencies were the vibration amplitude is excessive (consider out of balanced wheels on a car). Resonance occurs when the forcing frequency and natural frequency coincide and the result is excessive vibration. If it is required to keep the vibration amplitude below a certain value in order to limit stress to prevent fatigue, then speeds coinciding to the natural frequency orders of it must be avoided. These speeds are referred to as the barred speeds (or critical speed ranges).
If the barred speed is located where it is required to operate the engine, say at half ahead, it will be necessary to fit a detuner or vibration damper. These lower the vibration peak and move it slightly higher in the range. The barred speed is either removed or moved away from the area in which the engine is operated. A vibration damper consists essentially of an additional rotating mass driven by the crankshaft and connected to it by a spring or a hydraulic fluid. The energy of vibration is used up in distorting the spring or shearing the fluid.
With constant speed engines employing a CPP propeller, vibration dampers are sometimes required because natural frequencies of the engine and shaft system changes with load due to the pitch of the propeller. In some cases there may even be a barred pitch.
Methods of forming a crankshaft
The ideal arrangement is that of the solid forged structure because there is continuity of material grain flow which allows for smooth transmission of stress.
Unfortunately, such crankshafts are limited to the smaller engines because there is a limit to the size of forging equipment and the size of steel bar which can be produced.
Built up crankshafts with shrink fits or welded sections allow very large units to be produced, but they tend to be heavier and less rigid than an equivalent solid forged.
The grain flow method allows solid forged crankshafts to be produced with minimum energy and minimum need for post machining. A heated section of bar is held by three clamps which can be moved hydraulically. The three stages for forming the crank throws are shown. When one throw has been formed the next section of bar is heated, the shaft is held in the clamps again and the next throw formed.
A form of crankshaft construction recently developed is that of welding. Cast web crank pin and half journal units are connected at the half journals by welding. These welds are stress relieved and the pins ground to give the correct finish. This form of construction is suitable for large direct drive engines and it provides strength close to that of the solid forged crankshaft. Any number of units may be connected
The usual form of construction for direct drive engine crankshafts is the semi-built up type. This makes use of shrink fits between the journals and webs. Careful design is required to ensure the shrink fit is strong enough but does not impose excessive shrinkage stress.
The shrink fit must provide sufficient strength to allow necessary torque to be transmitted. The actual allowance is about 1/500-1/600 of the diameter. Too large an allowance produces a high stress which can result in yielding when the working stress is added. Too small an allowance can lead to slippage.
In order to provide for large torque transmission without high stress the area of contact at the shrink fit should be increased.
This is usually by means of an increased diameter (over increase length as this increase the engine length) which allows the fillet radius to be used, as the journal part of the pin does not need to be of the same large diameter. The fillet allows a smooth transmission and is rolled because this produces a compressive stress which provides safe guard against fatigue. The fillet is undercut allowing the web to be positioned against the bearing reducing the engine length and oil loss from the ends of the bearing.
Slippage of shrink fits
Slippage can occur at the shrink fits and this can be noticed by consideration of the reference mark at the end of the web and pin.
For Slippage upto about 5o retiming of the effected cylinder can take place so long as oil holes passing through the shrink fit do not become obstructed.
For slippage above 5o there may be problems of loading on the crankshaft due to firing angles and the relative position of the cranks, this can lead to excessive vibrations and stress. The ideal solution is the replacement of the effected parts, a temporary repair may be carried out. This consists of cooling the pin with liquid nitrogen and heating the web to give a temperature difference of about 180oC. The web may then be jacked back into position. In both cases the slip fit will have been damaged, the contact faces which originally should be as smooth as possible to give maximum contact area. The engine should be run at below the max. rating until the parts can be replaced.
Most slipped fits are caused by starting the engine with water in the cylinder. But any overload can result in this problem.
Modern engines designed for high power and weight should have a well balanced crankshaft with a minimum of material. Post machining allows the tapering and chamfering of webs and the counter boring of pins, thereby removing all unnecessary metal. A modern well balanced engine using higher strength steels can avoid the use of balance weights.
Crankshaft alignment check
If a main bearing has suffered wear then the journal supported by the bearing will take up a lower position, if adjacent bearings have not worn to the same degree then the shaft will take on a bent attitude causing the crankwebs to be subjected to an oscillatory bending action and so fatigue.
It is therefor necessary to check the alignment of crankshafts by the use of special gauges.
The crankweb will often have a light center punch mark to ensure that the gauge is fitted in the same position at each reading. The trim of the ship, whether loaded or unloaded, whether hogged or sagged are all important factors which can effect the reliability of the readings. Ideally the readings should be taken when the ship is drydocked
Medium speed vee-type crankshaft layouts
With vee-type engines it is necessary to connect two con rods too each bottom end. Three basic arrangements are available as shown. The side by side is the simplest with each bottom end being positioned alongside each neighbour on the crankpin. This requires cylinders to be offset across the engine thus giving a slight increase in length. The fork and blade type allows cylinders to be in line across the engine but the bottom end arrangement is more complicated. The fork may have two bottom end shells with the blade positioned between them.
Alternately the arrangement as shown may be used. But in this case the fork shell runs the whole length of the crankpin and the blade shell runs on specially ground outer face of the fork shell.
The articulated arrangement has cylinders in line across the engine and a single bottom end is used. On con rod is connected rigidly but because of piston motions the other rod is connected by means of a gudgeon pin arrangement. Both pistons and con rods can be removed without disturbing the bottom ends.
Modern trends in materials
For a long period most crankshafts were made out of a material known as CK40. This
had very good ability to withstand the damage caused by bearing failure such as localised
hardening and cracking. Undersizing by grinding was possible.
The modern trend is to move the chrome-molybdenum alloyed steel of high tensile stress. These may be non-surface hardened ( which tend to bend and have localised hardening when reacting to an overheated bearing) or hardened ( tends to loose its hardness and due to changes in the molecular structure will crack). In both these cases grinding is generally not an option for repair.
For modern material cranks subject to normal wear grinding may be carried out
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 doxford engine uses a centrally positioned shoe because there is no room at the ends of the pin due to the side rod crossheads.
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. A more accurate idea can be gained by chocking the piston centrally in its bore than measuring the clearance at each face. This will also give the athwartships alignment. The edges of the guide shoes are also white metal faced and these run against rubbing strips. Clearance at these faces can be checked with feelers and this gives the fore and aft alignment.
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 crosshead pin is fitted with a loose fitting pin. This pin allows a small degree of movement (about 1mm) between the guide shoe and the pin giving better alignment.
Types of damage associated with the crosshead bearing
There are two possible types of damage which may be sustained;
wiping-this is where part of the white metal contact faces are wiped out so that machining marks and oil grooves disappear, the material is displaced into the lubrication grooves where it forms 'stubble' or may fill them completely. Providing adequate lubrication this may be caused by two high a degree of roughness of the crosshead journal. Possibly due, if occurring after trouble free operation, to particles in the lubricating oil. Roughness may also occur due to corrosion by weak acids forming in the lubricating oil. A water content above 1% can attack the white metal and cause formation of SnO which has the appearance of dark smudges on the surface. This must be removed whenever possible as the tin oxide can become harder than the metal of the journal causing obvious destruction of surface finish.
cracking- these may appear as individual cracks, hair line cracks, or densely cracked
or crackled areas.
The latter may be so dense as to give the appearance of segregated grains. This can lead to scratching on the journal. The reasons for cracking may be insufficient bonding of white metal to the steel.
Densely nested networks of cracks is due to fatigue fractures.
The basic purpose of a cam is to convert rotary motion into reciprocating motion in order to actuate some mechanism. For an engine this usually means the operation of a valve or pump. A cam must be hard enough to withstand the considerable forces exerted upon it but it must also be reasonable resilient. For these reasons cams are generally made from surface hardened steel. The exception is the indicator cam which is usually made from cast iron as the loading are small.
Couplings are provided at each cylinder section of camshaft, these couplings being shrink fits with hydraulic adjustment capability. The advantage of having sections of camshaft is that it allows cams, couplings, to be removed and replaced more easily then would be the case with longer shaft sections.
The Sulzer engines employ a different method of cam fitment. A hub is keyed and shrunk onto the camshaft and the cam fits onto this hub being held in place axially with a nut. The cam is secured against rotation by means of radial teeth on both hub and cam, and since there are 360 of these teeth the cams may be altered in one degree steps. The profile of a cam, including the leading or rising edge, the dwell period at the op, and the trailing or falling edge are all profiled to give the correct rate and duration of movement for the equipment they are operating .The rate of rise of the leading edge of the cam governs the speed at which the valve or pump operates. Too slight and operation may not be crisp, too steep and undue loading may occur.
Critically profiled cams , especially fitted for operating the fuel pump, may be used. In this the leading edge of the cam is critically profiled to give a requisite flow variation to suit engine makers fuel delivery requirements.
In the case of mechanically operated fuel valves on the Doxford timing block the lift only needs to be small and the cam profile may be designed to suit the rate of change required. With such a system there is no need to provide a usual cam needed. This insert is generally held into place by set-screws and slotted holes in the insert allows the cam to be adjusted.
Some followers do not run on the base circle of the cam, stops being used hold the follower clear. This is said to minimise wear and avoids problems due to the screw holding the cam insert in place.
By far the most common method for fixing cams is by hydraulically floating the cams onto the shaft. o-rings seals being provided for that purpose with the high pressure oil supplied from an external pump. When hydraulically floated the cam may be rotated into position.
on a large bore B&W one of the exhaust valve gear operating cams slipped causing severe engine running problems.No gear was on board for hydraulically floating the cam so the engineers managed to rig a system of chain blocks whereby they where able to drag the cam back into position as an emergency repair. Next port a makers representative oversaw proper repair. He never did accept that it was possible to move the cam by this method!
On a valve operated by direct contact with the cam or via a pushrod and rocker, there must always be tappet clearance in order to allow for thermal expansion of the valve during engine operation. That tappet clearance must be correct, too much and the opening period and timing can be altered, too little and the valve might not fully close.
Camshaft bearings for most large engines are of the white metal type. This not only allows for more convenient replacement and adjustment but also allows an oil wedge to build up, that oil wedge restricting the hammering effect on the bearing. Ball or roller races would be subject to considerable brinelling damage. Bearing weardown reduces the effective lift of both valves and pump plungers and so weardown must be corrected as soon as it reaches recommended limits.
Author note: Spalling damage was noted on what was believed to be the leading edge of cams on a daihatsu medium speed engine. Correspondence with the makers regarding the possibility of damage being caused by the follower slamming down on the trailing edge of the cam drew denials.
It was later found that the damage was actually on the leading edge of the cam. As the damage was so severe as to alter the profile of the cams repair was by replacement. On this engine the cams where mounted on individually sized tapers increasing in diameter away from the end the cams where fitted on. The cams where locked into position and jacked off by nuts fitted on threads located either side of the taper. An excellent system making adjustment to timing very simple.
Rotation of camshafts in an engine may be by gears or by chain turned by the main crank. The disadvantage of using gears is difficulty in alignment, lubrication and disadvantage to wear from foreign materials as well as their increased cost.The disadvantage of chains is the requirement for tensioning and their finite life. Although for large installations this can be very long.
Wear on the chain pins, bushes as well as the chain sprockets can all lead to a slackening off of the chain. This can lead to 'slap' and changing of cam timing.This alters the leads of the fuel pumps and exhaust valves.. The degree of angular displacement by checked using a manufacturer supplied poker gauge.
Chain damage occurs if the chain is too tight or too slack and the result is fatigue cracking of the links. If the tension is too tight, then this adds to the working stress of the chain. Insufficient tension leads to 'slap' with resultant damage to chain and rubbing strips. Vertical misalignment of the sprockets means rubbing at the side plates resulting in reduction of thickness and possible failure.
Chain stretch and hence reduction in tension can be accounted for by movement of a tensioning wheel. The tension usually being checked by movement to and fro at the centre of the longest free length.
Max. is about 1 chain pitch.
Recommended limit on stretch is about 1.5 to 2%, if max. movement of the tensioned is reached before the chain has reached its max. stretch then a pair of links may be removed. When max. stretch is reached, or if the chain shows signs of damage then the chain should be replaced.
The simplest method is to break the old chain and attach the new chain to it. The engine is then turned and as the old chain is paid off, the new chain can be paid in. This maintains approximately the correct timing, the tension of the chain can then be set.
Final adjustment of the timing can be made following manufacturers instructions, this generally means turning the engine until No1 is at top dead, then checking by us of pointer gauges the position of the cam.
The cam drive is adjustable and can be slackened off, by hydraulic means on large modern engines, the section of cams can then be turned relative to the crankshaft angle and the timing restored.
The chains are lubricated by the injection of a jey of oil between the chain wheels and the chain rollers just before the rollers are about to engage the wheel. Thereby an oil cushion is formed to dampen the impact
A question asked by an examiner was to explain the polygon of forces with respect to chain drive. This refers to the forces acting on the chain links as they pass over the chain wheel
Some of these forces are; Bending moment on the link as it travels around the sprocket
In the bore for the piston rod in the bottom of the scavenge air box a stuffing box is mounted to prevent lubricating oil from being drawn up the crankcase into the scavengeing air space. The stuffing box also prevents scavenge air from leaking into the crankcase.
The stuffing box is mounted on a ring which is bolted onto the underside of the scavenge air box. The stuffing box is taken out together with the piston rod during overhaul of the piston, but also can be disassembled for inspection in the crankcase with the piston remaining in position.
The stuffing box housing is in two parts, assembled by a flanged joint. In the housing five ring grooves have been machined out of which the two uppermost ones accommodate sealing rings that prevent scavenge air from blowing down along the piston rod. In the lowermost grooves scraper rings are fitted which scrape the lubricating oil of the piston rod. The oil is led through bores in the housing and back to the crankcase.
Between the two uppermost ring grooves, for the sealing rings, and the three lowermost grooves, for the scraper rings, a cofferdam has been machined out which, through a bore in the housing and a connecting pipe, communicates with a control cock on the outside of the engine. It can be checked by opening this control cock that the scraper and sealing rings are functioning correctly.
Sealing ring section
The two sealing rings each consist of a four piece brass ring which accommodates eight brass sealing segments, two per base, guided by four cylindrical pins. The parts are pressed onto the piston rod by a helical garter spring
The three scraper rings are made up of three steel base parts into which two lamellas are fitted into a grooves machined in each part. A garter spring keeps the ring in contact with the piston rod. Scraped off oil is led through ports in the base ring back to the sump
A clearance is given at the ends of the parts to ensure contact with the piston rod as the rubbing face wears.
Extremely high wear was noted on a class of vessels with B&W gfca engines. Balls of wire wool where removed from between the segments at overhaul.
Repair was to send the piston rods for machining from their cloverleaf shape back to circular. When fitting new lamellas emery paper was wrapped around the rod and the lamellas 'bedded' in. This prevented the segments from canting and the ends of the lamellas digging in.
Cylinder liners are generally made from grey cast iron because it is easily cast and has self lubricating properties due to the graphite flakes for, some modern engines spheroidal graphite or nodular graphite is used. This has greater mechanical strength, but has the same self lubricating properties.
The critical part of any liner is the upper section were the temperature and pressure conditions are at their most difficult. Cooling is required to maintain strength and the temperature variations must be maintained within set limits in order to avoid cracking. Rapid change of temperature due to the rapid variation in cylinder condition or cooling water temperature can result in cracking.
Early engines e.g. Sulzer R's were lightly loaded and thin section liners could withstand the pressure , the thin sections avoided any problems of thermal stresses. Fire rings were often fitted to protect the inner face of the liner from impingement by the combustion flame.
With the advent of turbocharging e.g. Sulzer RD, it was necessary to provide strengthening in order to withstand mechanical stress increasing the wall thickness would have resulted in thermal stress.
Shrink rings or support rings were used to strengthen the upper section of the liner and the cooling space was provided , the support ring took about 50% of the load, between the liner and the strengthening ring.
For modern highly rated engines support or shrink rings are not suitable and thick section bore cooled liners are employed
A typical cast iron as used in liner construction begins to lose its strength at a surface temperature of about 340oC . A liner must therefore be either alloyed with expensive elements or cooled to about 80oC below this temperature.
A typical cylinder lubricating oil forms a lacquer at about 220oC . A liner must therefore be cooled to about 40oC below this temperature in service, to reduce formation of carbon deposits.
A liner must therefore have a maximum temperature in the thickened region, of about 260oC and a max. temperature in the thinned section of about 180 oC . This produces large temperature gradients axially in the liner and also across the walls of the liner. This could produce component failure due to high thermal stress if the material was too thick or failure by low metal strength if the material was too thin.
The design that has been adopted is to have the cooling surface around the combustion zone formed by a large number of hole drilled at an angle to the vertical axis of the liner. This produces a fully machined cooling water surface close to the combustion side of the liner, thus keeping thermal stresses low.
It is usual to allow the liner to expand freely in the axial direction away from the combustion zone. The cooling spaces may be sealed by neoprene rubber rings fitted in the grooves in the liner. The rings and grooves being closely matched to ensure a positive seal. Alternately copper rings may be fitted
Wear of cylinder liners
There are three main cause of damage to the liner material;
Corrosion-caused by the acidic products of combustion
Abrasion-caused by solid particles breaking through the lubricant film
Friction-Break down of the lubricating oil film leading to metal to metal contact
Normal liner wear exists for the reasons given above. Wear rates are greatest towards the top of the stroke due to the high temperatures thinning out the oil film and high gas pressure behind the piston rings forces the land into contact with the liner wall. In addition, piston is moving slowly at the end of its stroke and a good oil wedge cannot be formed.
Wear rates reduce lower down the stroke because pressure and temperature conditions are less arduous and piston speed has increases. At the bottom end of the stroke wear rate increases again due to reduce piston speed, but also due to the scouring effect of the in coming scavenging air. The reduced temperature increases the viscosity of the oil so reducing its ability to spread evenly. Long stroke engines are sometimes provided with quills at the bottom of the stroke.
Cylinder oil is injected by means of quills positioned in the liner, the number of which is governed by the diameter of the liner and ensures sufficient oil to be injected. The use of grooves in the liner helps spread and retain the oil film.
Vertical positioning of the quills is important and the oil should be injected so that it is spread upwards by the top two piston rings. If injected too early the top ring will scrape the oil upwards to be burnt. If too late the oil will be scraped off the liner by the next downstroke. Injection timing is therefore critical, too much so as experiments to inject the oil precisely have failed. The remedy has been to over supply the quantity of oil and provide extra quills at the bottom of the stroke
Cylinder Lubrication quill
Abnormal Liner Wear
a,Scuffing- This occurs if the cylinder lo quantity is insufficient. A complete oil film is not obtained and rings contact the liner surface. Local seizures takes place producing a hardened glassy surface on the rings and liner and as the rings rotate in their grooves scuffing speeds around the liner. If scuffing is extensive the only solution is to replace rings and liner. Minor scuffing may be corrected by replacing the rings and braking don the scuffing area on the liner with a rough stone to provide a key for the cylinder l.o.
It is necessary to determine the cause of scuffing and correct it. As stated the most likely cause is insufficient quantity of l.o.
b, Cloverleafing-if the cyl l.o. has inadequate acid neutralising properties for the fuel being burnt or if there is insufficient quantity of oil injected then cloveleafing can occur.. This is basically regions of corrosive wear midway between the quills and upwards towards the top of the liner. These areas may be visible due to the corrosive effect and they are cloverleaf shaped. Eventually the rings become unsupported in these areas, gas builds up on the front face and the ring is subject to collapse.
There are consequences of over lubrication, particularly with sticking rings and choked ports due to carbon build up. Excess unburnt oil can also accumulate in the scavenge space risking fire.
Ships which operate for long periods in the 'down by the stern' trim may exhibit an increased wear in the for'd to aft direction over the athwartships direction. Athwartship wear is aggravated by the reaction forces from the piston and rotation of the crankshaft. Although the bulk of this is removed by the crosshead on slow speed engines, this resultant force still causes the athwartship direction to have the greatest wear rate.
Maximum allowable liner wear is determined by the manufacturer but generally is between 0.7 to 1.0%.
Manufacturing and materials
Piston crowns attain a running temperature of about 450oC and in this zone there is a need for high strength and minimum distortion in order to maintain resistance to gas loads and maintain the attitude to the rings in relation to the liner. The heat flow path from the crown must be uniform otherwise thermal distortion will cause a non-circular piston resulting in reduced running clearance or even possible contact with the liner wall.
In addition to this thermal stress they are also subject to compressive stress from combustion and compression loads, as well as inertial loads.
Materials such as pearlitic, flake and spheroidal cast iron, alloy cast irons containing Nickel and chromium, and aluminium alloys may be used.
The determining factor is the design criteria for the engine.
For a modern slow speed engine steel forging or castings of nickel-chrome steel or molybdenum steel are common. The weight of the material is not normally a governing factor in this type of engine although resistance to thermal stress and distortion is. Efficient cooling is a required to ensure the piston retains sufficient strength to prevent distortion.
For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into the crown, although this may be omitted on smaller engines.Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface.
Hard landings are inserted into the ring groves to keep wear rated down. Composite pistons may be used consisting of an cast alloy steel crown with an aluminium-alloy or cast iron body.
After casting or forging the component is formed of different material thicknesses. The thinner parts will cool more quickly thereby setting up internal stresses. Annealing removes or reduces these stresses as well as refining the grain structure.
High specific heat capacity therefore removes more heat per unit volume
Low specific heat capacity
Requires chemical conditioning treatment to prevent scaling
Does not require chemical treatment but requires increased separate and purification plant
Larger capacity cooling water pump or separate piston cooling pump and coolers although less so than with oil
Larger capacity Lube oil pump, sump quantity and coolers
Special piping required to get coolant to and from piston without leak
No special means required and leakage not a problem with less risk of hammering and bubble impingement.
Coolant drains tank required to collect water if engine has to be drained.
Increased capacity sump tank required
Pistons often of more complicated design
Thermal stresses in piston generally less in oil cooled pistons
Cooling pumps may be stopped more quickly after engine stopped
Large volumes of oil required to keep oxidation down and extended cooling period required after engine stopped to prevent coking of oil
Wear rings are found on some slow speed engines employing loop or cross flow scavenging although they may be found in most designs. They are made of a low coefficient of friction material and serves two main purposes. To provide a rubbing surface and to prevent contact between the hot upper surfaces of the piston and the liner wall.In trunk piston engines wear rings to negate the distortion effect caused by the interference fit of the gudgeon pin .
The ring may be inserted in two pieces into the groove then lightly caulked in with good clearance between the ends.
B&W LMC oil cooled piston
The piston has a concave top. This is near self supporting and reduces the need for internal ribbing. It prevents the cyclic distortion of the top when under firing load. This distortion can lead to fatigue and cracking
Pistons may be cooled by oil or water. Oil has the advantage that it may be supplied simply from the lubrication system up the piston rod. Its disadvantage are that maximum temperatures is relatively low in order to avoid oxidised deposits which build up on the surfaces. In addition the heat capacity of oil is much lower than that of water thus a greater flow is required and so pumps and pipework must be larger. Also, if the bearing supply oil is used as is mainly the case a greater capacity sump is required with more oil in use.
Water does not have these problems, but leakage into the crankcase can cause problems with the oil (such as Micro Biol-Degradation). The concave or dished piston profile is used for most pistons because it is stronger than the flat top for the same section thickness
Sulzer watercooled piston (rnd)
Increasing section thickness would result in higher thermal stress.
Sulzer piston require a flat top because of the scavengeing and exhaust flow arrangement (loop scavengeing of RND etc). in order to avoid thicker sections internal support ribs are used. However these ribs cause problems in that coolant flow is restricted. The flow of water with an RD piston is directed to and from the piston by telescopic pipes. The outlet is positioned higher than the inlet within the cooling cavity and on the opposite side of the support rib in order to ensure positive circulation.
With highly rated engines overheating occurred in stagnant flow areas between the ribs and so a different form of cooling was required. The cocktail shaker effect has air as well as water in the cooling cavity as the piston reciprocates water washes over the entire inner surface of the piston just as in a cocktail shaker. Unfortunately air bubbles become trapped in the water and flow to outlet reducing the air content and removing the cocktail shaker effect. To avoid this problem air must be supplied to the piston some engine builders use air pumps feeding air to the inlet flow. The sulzer engine allows air to be drawn into the flow at a specially designed telescopic transfer system.
The telescopic arrangement is designed to prevent leakage and allows air to be drawn into the coolant flow to maintain the cocktail shaker effect. Consider the inlet telescopic, a double nozzle unit is fitted to the top of the standpipe. Small holes allow connection from the main seal to the space between the nozzles. Water flowing through the lower nozzle is subject to pressure reduction and a velocity increase. The space between the nozzles is therefore at a lower pressure than other parts of the system. Any water which leaks past the main seal is drawn through the radial holes into the low pressure region and hence back into the coolant flow.
The pumping action of the telescopic draws air past the lower seal and this is also drawn through the radial holes into the coolant flow. This maintains the air quantity in the piston and so maintains the cocktail shaker effect.
The sulzer water cooled piston differs from that of the Oil cooled variety by the method it uses for distributing the cooling medium. In this case the piston is not continually flooded but instead contains a level governed by the outlet weir. Cooling of the crown occurs during change of direction at the top of the stroke by so called 'Cocktail shaker' action
With medium speed and higher speed engines considerable inertia forces are placed on the conn rod and bearings as the piston changes direction at the ends of the stroke. The amount of force is a factor of the speed and rotating mass. To reduce this force whilst maintaining the same engine speed it is necessary to reduce this rotating mass.
Aluminium, with its lower density than steel is used when alloyed with silicon for extra strength. Even alloyed the aluminium has less mechanical strength than the steel, therefore damage is possible due to gas pressure acting on crown and piston rings. The piston could deform sufficiently to prevent proper operation of the rings in their grooves. Some engine manufacturers fit cast iron inserts into the grooves but more generally the piston is made in two parts with a cast steel crown containing two grooves.
Aluminium has a better coefficient of heat transfer than steel thus overheating is not a problem. Its lower coefficient of friction avoids the problems of fitting bushes for the gudgeon pin, thus a floating gudgeon pin may be used. The higher coefficient of expansion could lead to the need for greater piston/liner clearance. However, as the main body is not subject to the high temperatures of combustion this expansion is not a problem.
Sulzer rotating piston
This piston rotates as it reciprocates. The rotation being brought about by the swing of the con rod. This causes two spring loaded palls located in the spherical top end to oscillate. These palls engage with a toothed rim which is connected to the piston by means of a compensating spring. As the conrod swings the palls act on the toothed rim causing it, and hence the piston, to rotate. The amount of rotation is limited to one tooth pitch every engine rev and the action is similar to that of a ratchet mechanism. The advantage of this is that local overheating of the piston or the liner due to blow past is prevented. Running in characteristics are improved and liner wear are improved. There is a better spread of oil brought about by the piston rotation. A spherical top end is required but this provides better support for the piston which does not distort as much as one fitted with a gudgeon pin. Piston to liner clearance may therefore be reduced.
Transfer of gas loads from crown to piston rod
Is usually transmitted from the reinforced crown to the piston rod by internal mechanism avoiding possible distortion of the ring belt.
The tops of pistons are made dome shaped or have strong internal ribbing.
Thermal distortion of Piston
High topland ( the 1st piston ring is positioned will below the upper surface of the piston) with associated reduced ring heat load has given better ring pack performance by improving working conditions for the cylinder lube oil. The disadvantage of this system is that a coke build up can occur aboth the piston which leads to 'bore polishing'. This polishing reduces the ability of the cylinder lube oil to 'key' into the liner therefore increased cylinder lube oil consumption/increased liner wear can result. To combat this piston cleaning rings are incorporated into the liner. These slightly reduce the bore removing the deposits.
The top piston ring is moved further down the piston. This allows the crown to enter deeper into the crown reducing temperature and pressure on the liner. The top piston ring is a 'Controlled Pressure relief' (CPR) ring. This design has several oblique shallow grooves in the piston ring face allowing some gas presure to pass through to the 2nd ring thereby reducing load on the top ring. To reduce blowpast an 'S' type joint is formed n the ring ends
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. Rings are generally of spherical graphite cast iron because of the strength and limited self lubricating properties.
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 antiwear 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.
Piston ring sealing and collapse.
Faults leading to ring collapse
Improvements to ring longevity before the 1970's were mainly concerned with design changes to improve lubrication.
Chromium plated rings running in unhardened liners were brought in but found to be susceptible to seize and burn marking with above average loading.
At the end of the 70's very hard plasma jet weld coatings were applied to the rings which gave excellent wear rates and resistance to burn marks. However running in unhardened liners gave high liner wear rates. Laser hardening of the liners gave improved life with acceptable maximum cylinder pressures of 145bar for medium speed engines. With increasing pressure requirements modern designs utilise a ceramic coating which gives excellent wear characteristics negating the need for laser hardening of the liner.
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.
As the head runs at a fairly high temperature the cooling water must also be at a reasonably high temperature. This further thermal stressing. It is therefore usual to have the cooling water for the head in series with the jacket. The covers are attached to the cylinder block by means of large diameter bolts. The gas loads acting on the head are thus transferred to the cylinder block from which the tie bolts transfer it to the bedplate and then to the hull of the ship.
The original Sulzer engines employed single piece cylinder covers, but thermal stress cracks developed in relatively uncooled section were 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 inner section was of cast iron due to the complicated shape and the outer section cast steel for strength, a soft iron ring provided the joint between inner and outer sections. When the two parts are bolted together the head may be treated as a single unit.
For recent engines the single piece bore cooled steel cylinder cover has been developed and presents no particular problems.
Sulzer RD cylinder cover
B & W cylinder safety valve
This type of valve is suitable for use with the special long studs used in modern engines. The safety valve is small an indicates the onset of over load. To release excessive pressure from the cylinder the cover is able to stretch the studs release the pressure and reseat. Hopefully cleanly
Large exhaust valves are provided with detachable seats made form molybdenum steel. The main parts of the valve casing is of cast iron and water cooled, there being no particular strength requirement for this part. It is the seat area which is subject to high temperatures and wear, hence the use of better materials. The seat is detachable in order to allow removable for machining and replacement.
Rocker operation of valves presents certain problems;
To avoid these problems hydraulic valve actuation is used there is no tappet and no tappet clearance to set. Thermal expansion is accounted for by allowing the oil to escape at a relief valve on the pump unit. Oil loss is made up at the pump unit from the cam lube oil supply system. The opening face is always axial. Note: The hydraulic pipe must be sheathed to avoid the risk of fire in the event of pipe failure.
With modern fuels, vanadium and other deposits can build up on valve faces leading to damage. These deposits can be hammered into the seating faces. If the valve is rotated and reseats in a different place then the same damage does not occur. Rotating the valve also prevents localised overheating due to a faulty atomiser.. If the valve is set spinning and is still rotating as it reseats a light grinding action takes place. This removes deposits and ensures a good seal. Such rotation is induced by spinners on the valve stem upon which the escaping exhaust gas acts.. To allow for this effect the frictional effect of the springs and valve/cover must be removed. The removal of springs means that a closing force by some other means is required. Air springing can be used. This consists of a piston fitted to the valve stem below the hydraulic unit. As the valve opens air below the piston is compressed and this compression provides the upward force of closing the valve. The space above the piston is vented to atmosphere and the pressure below the piston maintained at 5 bar from an air supply via a non-return valve.
An additional advantage with this system is that when the engine is stopped the valves will all close after a short delay. This prevents the flow of cool scavenged air through units which with a rocker system would otherwise be open. Preventing this allows all cylinder to be equally warm and stops the rotation of the turboblower which can occur.
Springs of sufficient force must be provided in order to ensure that the valve closes when the tappet force is removed. Once the valve is closed, the pressure in the cylinder will increase the sealing force on the valve seat.
Springs have natural frequencies and if the engine operating frequency is close to the natural frequency of the spring then vibration will take place and valve bounce will occur. Springs also twist when they are compressed and this causes wear at the landing faces. To avoid problems, double springs may be fitted one inside the other in parallel. These springs must be of different size and so have different natural frequencies. Valve bounce due to spring vibration is thus avoided.
The springs are wound in different directions to prevent twist and also to prevent one coil entering the other in the event of breakage, thereby locking it up.
Long springs tend to bow out when they are compressed and this increases the risk of stress failure. A solution is to have two springs in series, one above the other and separated by a centre disc which is located via a pivoted arm arrangement so that only vertical movement is allowed. Series /parallel arrangements are available.
Modern engines use pneumatic springs. This both eliminates the problems of valve bounce , spring breakage and also the need for rotor caps. As the valve is free floating spinner vanes fitted on the spindle allow the valve to be rotated by the flow of exhaust gas.
The rotocap is a mechanical device which produces valve rotation by a small amount as the valve opens. The valve rotation is about 8o when the unit is in good condition.
Rotation to a new position avoids deposits from being hammered into the seat and repositions the valve thus preventing local overheating. Frictional contact is provided through the springs to the valve cover via the belleville washer which contacts at point A and C. As the tappet force increases to open the valve, the belleville washer is collapsed thus removing that frictional contact. Further increase in tappet force acts on the spring loaded ball bearings and the ramped slots tend to slide over the ball bearings. These slots are in the valve cover which is connected to the stem thus as the cover moves it rotates the valve.
As the tappet force is removed when the valve closes the belleville washer restores frictional contact and prevents further rotation. Springs return the ball bearings to their original position ready for the next stroke.