crosshead engions,

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Simplified diagram of cross section

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.
Bedplate
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.

Welded bedplate

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.

Modern welded bedplate

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.

Forces acting on transverse girder

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.

Sulzer amin bearing arrangements .
Superlong stokes
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.

Staying of tall engines
Camshaftless engines
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 control
Schematic of camshaftless fuel system

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
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
Modern developments
The modern trend has followed the smaler and lighter rule with an ever increaseing power to weight ratio. In addition to this simplified maintenance and production proceddures 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.

Liners

    Modern requirements for the design and contruction of a cylinder liner include;
        Material with sufficient hard phase and ductility
        Smooth surface finish
        Full honing of running surface
        Bore cooling of liner and components surrounding combustion chanmber- the insertion of insulating tubes into the bore cooling holes of liners is sometimes necessary to prevent undercooling and allow a standard liner to be used with different ratings.
        Critical profiling of liner wall thickness to stabilise temperatures and prevent corosion attack
        Multi-level cylinder lubrication with variable speed (load dependant) pumps
        Efficient water removal from scavenge air

Pistons

High topland ( the 1st piston ring is positioned will below the upper surface of the piston) with asociated 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 depoisits.

0 Cross section showing piston cleaning ring

Comparisons of cross head and trunk piston engines,

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    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

Crosshead construction
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
        guide faces take side thrust, this is easily lubricated, wears little and takes side forces off the piston and liner running surfaces.
        Uniform clearance around piston allows for better lubricating oil distribution reducing wear
        simplified piston construction designed for maximum strength and cooling. Extended load bearing skirts found on trunk pistons unnecessary
        due to gland lubricating oil may be optimised for crankcase and cylinder. High alkalinity oils used in cylinder allow poorer quality fuels to be burnt.

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.

    Advantages are;
        Perfect primary balance by balancing
            upper reciprocating masses and lower velocity side cranks against
            lower reciprocating mass and the higher velocity centre crank
        No gas loading transvered to bed plate (normally via head and tie rods) on engine meaning that construction could be lighter

Exhaust valves,

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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;
        a tappet clearance must be allowed to suit thermal expansion of the valve stem. Insufficient clearance may result in the valve not fully closing whilst excessive clearance can effect timing and the period the valve is open. Hammering will also take place at the tappet face. (The use of self adjusting hydraulic tappets went some way to alleviating these problems)
        A side thrust is imparted to the valve stem when the valve opens. Thus causing wear at the stem bushing and subsequent leakage
        wear takes place at the pushrod and valve contact face, additionally the rocker bearings require constant lubrication.

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.

Cross section through modern exhaust valve

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 dmage 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
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. Thisi both eliminates the problems of valve bounce , spring brakage 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.
Rotocap
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.
Valve closing    Valve opening
rotor cap in unloaded position     rotor cap in loaded position

Action of rotor cap

Crosshead

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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.

Crosshead guides

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.

Flexible mounts relieves stress points

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.

Modern cross head design

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 distruction 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.

Tie bolts,

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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 center 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.
Modern trends
The traditional through tie bolt is being supersceeded by shorter twin stay bolts which have the advantage of reducing distortion of the main bearing keep

Piston

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Manufacturing and materials
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.

Annealing

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 stresse as well as refining the grain structure.

Cooling

Water Cooled    Oil Cooled
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

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

Uniflow scavenged 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.

Sulzer watercooled piston

The sulzer water cooled piston differs from that of the Oil cooled variety by the method it uses for distributing the cooling medium. In tis 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.

Water trasition pipe for water cooled piston

Composite pistons
Composite piston

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
rotating mechanism of 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
Piston distorting under load 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
piston distorting due to thermal load
Anti-Polishing rings
High topland ( the 1st piston ring is positioned will below the upper surface of the piston) with asociated 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 depoisits.

Cross section showing piston cleaning ring
Modern Design

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

Stuffing box

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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
Top ring 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.

Scrapper ring section
Bottom section ring 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.

assembly

A clearance is given at the ends of the parts to ensure contact with the piston rod as the rubbing face wears.
Author note:

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.

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,

Add caption

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
.

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