Piston Rings

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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.
Pressure acting on rings

Faults leading to ring collapse
Ring faults
Material History
Improvements to ring longevity before the 1970's were mainly concerned with design changes to improve lubrication.

Piston ring sealing and collapse.

Faults leading to ring collapse

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 Combustion,

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Fuel oil is a hydrocarbon consisting of hydrogen and carbon, together with other elements most of which are unwanted.

Hydrogen has a higher calorific value than carbon, therefore, more heat may be obtained from fuels containing higher Hydrogen/Carbon ratios.

The lower specific gravity of hydrogen than carbon allows a rough rule of thumb to be; the higher the Specific Gravity, the lower the Calorific Value (and quality) of the fuel. The presence of impurities clouds the issue slightly

For efficient combustion an ignition source and sufficient oxygen need be present to completely oxidise the Hydrogen to water vapour and the carbon to carbon-dioxide.

    The combustion is required to occur in a short period of time in an internal combustion engine, there are five essential requirements to ensure this;
        Correct Air/fuel ratio-There must be sufficient oxygen to burn not only the hydrogen and oxygen present but also any other combustibles, such as sulphur. To be effective and efficient all the fuel must be burnt in the cylinder i.e. all the hydrogen must be burnt to water and all the carbon must be burnt to carbon dioxide. As the time for combustion is short excess air must be supplied to increase the possibility of the fuel being in close proximity to the oxygen molecules. The correct maintenance of the scavenge system including turbocharger suction filters is therefore essential.
        Atomisation-To ensure that the fuel breaks down into its constituent elements as quickly as possible it is atomised, which means it is injected into the cylinder under pressure through a small orifice (high surface area/volume ratio allowing rapid oxidation ).
        Mixing-Atomised fuel made up of fine droplets does not penetrate well into the cylinder combustion space , mixing with the air is promoted by giving the a swirling motion.
        Injection Timing-As the fuel burns it creates a pressure wave which acts against the piston.
        If the injection is too late, the piston is travelling down the liner. The pressure wave created by ignition moves rapidly down to meet the piston causes excessive shock loading on the top of the crown (this is the characteristic 'Diesel knock' of engines when started from cold).Less power is derived as the correct pressure does not act on the piston during the early stages of the stroke.
        If the injection is too early then very high temperatures and high peak pressures can be generated caused by the rapid combustion period occurring when the space available is very small. This can lead to increased engine efficiency but also to overloading of the bearings, particularly the top end bearings.
        Compression temperature-The diesel engine is a compression ignition engine , this means that the ignition of the fuel is reliant on the temperatures generated by the compression of the combustion air.
        The compression ratio is set at the design stage to give the correct temperature. However, loss of compression, say by a leaky exhaust valve or piston rings can lead to a late timing of ignition. A similar effect can occur if the cylinder parts are not kept at the correct temperature

Cylinder mixing

Cylinder mixing
Combustion chamber pressure curve.

Cylinder mixing

Phase one Ignition delay-Fuel injection does not start immediately the pump plunger begins to lift, there is a delay due to compression of the fuel and expansion of the pipework. Although liquids are often classed as being incompressible, they can be compressed to some extent at the pressures involved. Pipework will expand at these pressures and a certain amount of oil must be delivered in order to take account of these factors. Pump timing can be adjusted to take account of this because the amount remains the same at all engine speeds. When oil pressure reaches a high enough value the injector needle will lift and injection commences.

Ignition lag-The duration of this period is set as a definite period of time, irrespective as to how fast the engine turns, and that period depends upon the chemical structure of the fuel. Basically, the lag period depends upon the number upon the number of molecular bonds which must be broken in order to release atoms of hydrogen and carbon from the fuel molecule. The longer and more complex the molecular chain, the greater will the amount of heat energy required to release the atoms and the longer will be the amount of heat energy required to release the atoms and the longer will be the ignition lag period. Because modern residual fuels result from complex blends of crude oil of many different types, they are complex structures and the ignition quality may be very variable between nominally the same grade of fuel. Formerly the cetane number was used to define ignition quality but cetane is a single element fuel and relating this to the complex nature of residual fuels is not realistic. The general term ignition quality is now used.
Ignition lag is the preparation period of the fuel within the cylinder for spontaneous ignition and beginning of combustion. The physical and chemical processes occurring during this period are characterised by weak ABSORPTION and liberation of heat. Thus there is little if any deviation from the compression curve. The length of the lag period depends on the fuels ignition quality and nothing else. The higher the ignition quality, the shorter will be the lag period, and the lower the ignition quality, the longer the lag period.
The constant nature of the lag period has litle effect in the marine slow speed engine. For an automobile engine operating at much higher speeds this period is a significant proportion of crank angle. As the revs of the engine increase ignition of the fuel will occur later leading to a possibility of 'pinking', a timing retard is therefore required.

Phase two- Uncontrolled or rapid combustion period over a short period (5 to 10 degrees). Initially considerable heat is given off. This causes violent chemical reactions in the air vapour mix which has built up during the first phase. Between 40 to 70% of available energy is released during this phase

Phase three-Controlled burning period. Characterised by a slower pressure rise at the end of the injection period. The physical and chemical processes occurring during this phase are identical to those in the previous phase. The rate of pressure rise reduces as the piston sweeps down the liner.
The time available for combustion is relatively small with higher soeed short stroke engines, but is greater for slow speed long stroke engines. These can ten burn lower quality fuels with higher carbon content.
Heating of residual fuel
When burning residual fuel, heating is required in order to reduce the viscosity at the injectors to approximately that of diesel oil. This ensures good atomisation and brings the temperature of the.fuel closer to the ignition point. Heating the fuel helps separate solid and liquid contaminants in tanks and in centrifuges, and allows it to flow readily from the tanks to fuel manifold where the final heating for injection takes place. Fuel lines are provided with booster or surcharge pumps on order to force fuel from the tanks through final heaters to the fuel injection pumps, thus ensuring that oil is always available at the pumps. If oil is heated to high temperature it is essential that it is kept under pressure to prevent gassing up of the HP pumps. Heating requires the fuel pump and injector clearances to be increased.
Atomisation
For good combustion the oil droplet size in the combustion space should be at a minimum, and so have a maximum surface area to volume ratio. This ensures rapid heating and an increase in the percentage of fuel molecules in contact with the combustion air. Droplet size should be about 10mm dia. However, as the droplet size reduces so it ability to penetrate into the combustion space reduces. This is because the droplet has little mass so has little momentum and will be quickly slowed by friction of the dense combustion air. This will produce poor combustion due to the inefficient mixing with the air.
This size must be balanced with the problems of oversized droplets. This is not only with the surface area to volume ratio, also, large droplets can have too great a penetration, still burning fuel can contact with the liners and cylinder wall causing erosion and burnaway. Unburnt fuel can pass down the liner walls where it can mix with the unburnt cylinder liner oil and accumulate in the scavenge risking a potential fire. On trunk piston engine fuel dilution of the crankcase oil can result.

Effect on oil droplet after injection

effect on fuel droplet

Fuel injector tip
High pressure fuel is forced through small holes in the injector tip and this produces a high velocity jet of fuel. Friction between the fuel jet and the compressed air causes the fuel jet to break down into droplets, the size of which depend upon the density od the compressed air and the velocity of the jet. In order to achieve the optimum jet, fuel pressure and hole diameter must be within well defined limits. In general the length/hole ratio should be about 4:1.

Larger droplets may be produced by enlarging the hole or reducing the fuel pressure whilst smaller droplets may be formed by using smaller diameter holes or higher fuel pressure. Slow running results in larger droplets because fuel rail pressure falls as there is a longer period of time for injection to take place. Slow running for short periods is not a problem, for longer period 'slow steaming' nozzles with reduced diameter holes are used. Over a period of time injector nozzles will wear increasing hole diameter and require their replacement.
Power Cards

A power card is a graph of cylinder pressure against time, it was originally drawn using a mechanically driven pen onto graph paper mounted on a drum. The drum was rotated by string, via a cam on the camshaft and pushrod. As the drum rotated the pen mounted on the linkages was pressed up to the paper. For clarity the pen is released once a single cycle has passed otherwise slight fluctuations in power demand could lead to several cycles being superimposed on one another blurring the image.

power card meter
The indicator is a sensitive piece of equipment which can malfunction and so it must be treated with care. It can only be used effectively on an engine operating below 200 rpm due to the difficulty involved in getting only a single line on the card. In addition the inertia in the drum can lead to delays distorting the shape. For higher speed diesels either peak pressure indicators are used, or sophisticated electronic monitoring equipment is required with oscilloscope type displays. The time base for these is off transducers mounted on the flywheel.
It is important that the indicator is kept well lubricated with a light high quality oil . Prior to mounting the indicator the indicator cock is blown through to ensure it is clear. Compression cards are then first taken to check for errors caused by wear or friction/stiction in the instrument.
Compression curves
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Two stroke cycle power card
two stroke power card

        Bottom dead centre
        scavenge port closed
        exhaust port shut-commence of compression
        fuel injection
        top dead centre
        7post combustion expansion
        exhaust port opens

Four stroke cycle
Shown above are typical power cards for 4 stroke engine. The lower one shows the effect of improving turbocharger efficiency. That is some mechanical effort is made by the charge air pressure lowering fuel consumption. Poor timing can negate this effect.

Shown below is a power card drawing taken from an exercise book. it should be noted that the 3rd and 4th stroke indicate power is being abosrbed. It is probable that this drawing was made for a non turbocharged engine although the source is forgotten. The atmospheric line would split the 3rd and 4th stroke
4 stroke power card

        3-4-5 fuel injection and combustion
        5-6 expansion
        6-7-8-Exhaust valve open
        8-9-10 overlap, exhaust remains open whilst air enters
        10-1 aspiration and exhaust valve closes

Power calculation
The area swept out by the power stroke will give the power developed by the engine. It should be noted on a four stroke most of the non-power stroke occurs below atmospheric on a naturally aspirated engine and so gives a net loss of power. Power = p.A.L.n p - mean average pressure in the cylinder
A-area of piston[m3]
L-stroke [m]
n-revolutions per second
From a power card this is altered to

Power = area of diagram/length of diagram x Indicator spring constant
By use of an instrument called a Planimeter the area scribed out by the pen could be measured giving the power generated by the cylinder. In addition, through experience, certain problems could be diagnosed by looking at the shape drawn.

Fault diagnosis

As indicated there are practical difficulties with use of the power indicator instrument on a high speed four stroke engine. Therefore the following is based around the two stroke

The light spring diagram For this, the spring is replaced with one of much lower spring constant. In this way the operation at the lower pressures, i.e. around bottom dead, may be examined. In particular this gives indication of blocked or restricted scavenge and exhausts. To further clarify, the motive effort for rotating the drum is often by hand so only a small part at the end of the stroke is covered.

Light spring diagram
Draw card (90o out of phase)
Scavenge port opens at 140 degrees after top dead and closes 140 degrees before top dead.

Early injection

Early injection can be caused by incorrect fuel timing, broken or wrongly set up fuel injector, incorrect fuel condition, overheating of parts around the combustion space.
Its effect is to increase the maximum cylinder pressure. There will be an increase in combustion efficiency but the increased peak pressure leads to overload of the bearings and shock to pressure parts.

Late injection

Late injection can be caused by loss of compression, insufficient scavenging, delayed timing, incorrect fuel condition and atomisation, undercooled parts around the combustion space. It results in a condition called diesel knock where the flame front travels rapidly down the liner to strike the receding piston. In addition, leads to afterburning and high exhausts

Afterburning

Causes loss of power, smoke and high exhaust temperatures. Can lead to damage to exhaust valves and seats as well as piston crowns. Fouled turbocharger and waste heat recovery units. High cylinder temperatures causes problems with lubrication

Leaking fuel injector

Detected by loss of power, smoky exhaust and high temperatures. A knock can be heard on the fuel supply system. Can lead to after burning

Crankshafts

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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;
• Gas loads on the crankpin which produces alternating tangential bending of the webs alternating bending of the crankpin and on elements of shearing of the crankpin at the inner web faces
• Torsional vibrations producing alternating twisting of the crankshaft, the journal of which is in any event torsionally loaded by the gas loads via the web
• Axial vibrations in conjunction with the alternating lengthening and shortening of the shaft and in conjunction with local bending. Crankshafts may, in addition be subject to misalignment due to bearing wear or poor chocking. This produces and alternating bending of the crankshaft

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.

Vibration

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.

Semi-built up

No dowels are fitted as these can act as stress raisers.

Welded crankshaft

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 5^o 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 5^o 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 180^oC. 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.

Post machining

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 reliabililty 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 bearinf 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 hardeness and due to changes in the molecular structure will crack). In both these cases grinding is generally nnot an option for repair.
For modern material cranks subject to normal wear grinding may be carried out.

Cylinder covers

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 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.< class ="noindent">

Bedplate

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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 tighteness.

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

M=Bending moment
I=2nd moment of area of the cross section
s =Stress
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.

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Chain drives

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 p[umps 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

Stress changes on the link as it passes from the driving side to the driven

Tensioning of the chain dependent on the number of links between the sprockets varying i.e. related to the pitch

Centrifugal forces acting on the links

cam shaft

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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 rsing edge, the dwell period at the op, and the trailing or falling edge are all profiled to give the corect 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.

author note: 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.