The Cylinder Liner (Diesel Engines)

Source: MAN B&W

Cylinder Liner

• The function of cylinder liner is to form part of the combustion chamber which is compression and combustion of fuel/air mixture take place

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

•The liner can be manufactured using a superior material to the

cylinder block. While the cylinder block is made from a grey cast

iron, the liner is manufactured from a cast iron alloyed with

chromium, vanadium and molybdenum. (cast iron contains graphite,

a lubricant. The alloying elements help resist corrosion and improve

the wear resistance at high temperatures.)

•The cylinder liner will wear with use, and therefore may have to be

replaced. The cylinder jacket lasts the life of the engine.

•At working temperature, the liner is a lot hotter than the jacket. The

liner will expand more and is free to expand diametrically and

lengthwise. If they were cast as one piece, then unacceptable thermal

stresses would be set up, causing fracture of the material.

•Less risk of defects. The more complex the casting, the more

difficult to produce a homogenous casting with low residual stresses.

Cylinder Liner - Types

• Wet liners – usually used in medium and slow speed engines and normally cooling by water

• Dry liner – used for small engine like life boat engine etc which is the engine block built with fins and the cooling agent will be an air

3 –PIECES LINER (DOXFORD’P’)

EXHAUST BELT

EXHAUST PORTS

UPPER LUBRICATION

LOWER LUBRICATION

SCAVENGE PORTS

COMBUSTION BELT

UPPER LINER

LOWER LINER

STEEL SHRUNK RING

VALVE POCKET

COOLING WATER IN

COOLING WATER OUT

COOPER RING

UPPER LINER

COMBUSTION BELT

TRIPARTITE (3 PARTS) CYLINDER LINER AND JACKET (DOXFORD ‘P’TYPE

CYLINDER LUBRICATION OIL 8 POINTS

CYLINDER LUBRICATION OIL 2 POINTS

WAVE JOINT

LOWER

UNCOOLED

PART BOLTED

TO JACKET

UPPER COOLED PART

SOME COOLING OUT

COOLING WATER IN

COOLING WATER OUT

TO COVER

EXHAUST PORTS

SCAVENGE PORTS

JACKET

SUPPLEMENTARY

CYLINDER OIL SUPPLY

CYLINDER OIL SUPPLY

GROUND FACE

ABESTOS PACKING

Copper ring TELL TALE LEAK OFF

MAN KZ LINER (2

PIECES)

ANGLED C/W

DRILLINGS

DRILLING PASSES CLOSE TO

LINER SURFACE

BORE COOLING

VIEW

C/W OUT TO CYL HEAD

GROUND FACE

SECOND ADDITIONAL

CYLINDER OIL SUPPLY

PLUG

C/W INLET

LEAK OFF LINE /

TELL TALE HOLE

LEAK OFF LINE /

TELL TALE HOLE

LEAK OFF LINE /

TELL TALE HOLE

CYLINDER OIL SUPPLY

JACKET

RUBBER RINGS

EXHAUST PORTS

SCAVENGE PORTS

FREE

THERMAL

EXPANSION

THICK COLLAR

COMBUSTION SPACE ABOVE

JACKET

RND SULZER (ONE PIECE) HYPERBOLIC COOLING

PASSAGES

Stresses on liners

• Mechanical stress – pressure

• Thermal stress - temperature

Mechanical stress

• In supercharged engines, maximum firing pressure is about 90 – 100 Bar (non supercharged is about 75-85 Bar).

• This pressure produces circumferential (hoop) stress and longitudinal stress, but hoop stress is twice longitudinal stress so only hoop stress is considered where:

• HOOP STRESS, h = P x D

WHERE, P = GAS PRESSURE

D = LINER DIAMETER

t = LINER THICKNESS

• Thus hoop stress will increase if bore size and firing pressure increase

2t

CYLINDER HEAD

CRACK DUE

TO HOOP

STRESS

THERMAL STRESS

• Resistance to heat flow through liner metal produces a temperature gradient across liner hence, the inner wall expands more relative to outer wall, where:

thermal stress, T = T

where T is temperature gradient

Liner-Stresses

• Thicker liner will increase temperature gradient hence thermal stress but on the other hand, thicker liner have good resistance to mechanical stress. Thus, liner design becomes complex ,

• Also, inner liner surface temperature should be sufficiently low to retain oil film and high enough to avoid acid-dew (sulphur)

Estimated temperature in liner

- Next to liner wall 500oC

- On liner wall 140oC due to oil film, carbon deposits and stagnant

- Outer liner wall 75oC due to thickness of liner wall and cooling water

- Lower part 40oC due to expansion

Cylinder cover

Cylinder

jacket

Piston

Cylinder

liner

Cooling

water

space

1650 oC 500 oC

140 oC

75 oC

40 oC

Combination of stresses (plain liner)

METAL THICKNESS

THERMAL STRESS

HOOP or MECHANICAL STRESS

OPTIMUM

METAL

THICKNESS

Liner-temperatures

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

Liner-Temperatures & Cooling

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

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

Liner-Temperatures

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

Liner-Temperatures

• The solution is to bring the cooling water closer to the liner wall, and one method of doing this without compromising the strength of the liner is to use tangential bore cooling.

TANGENTIAL BORE COOLING

TANGENTIAL BORE COOLING

• Holes are bored from the underside of the flange formed by the increase in liner diameter. The holes are bored upwards and at an angle so that they approach the internal surface of the liner at a tangent. Holes are then bored radially around the top of the liner so that they join with the tangentially bored holes.

Liner-Bore Cooling

• On some large bore, long stroke engines it was found that the undercooling further down the liner was taking place. Why is this a problem? Well, the hydrogen in the fuel combines with the oxygen and burns to form water. Normally this is in the form of steam, but if it is cooled it will condense on the liner surface and wash away the lube oil film. Fuels also contain sulphur.

Liner-Cooling

• This burns in the oxygen and the products combine with the water to form sulphuric acid. If this condenses on the liner surface (below 140º) then corrosion can take place.

• Once the oil film has been destroyed then wear will take place at an alarming rate. One solution was to insulate the outside of the liner so that there was a reduction in the cooling effect. On The latest engines the liner is only cooled at the very top.

Cylinder liner-Loads

• The inside surface is subjected to the rubbing action of the piston rings as the piston is moving reciprocate in the bore of liner.

• Subjected to a very high combustion pressure and temperature, particularly at upper end.

• Takes the side thrust of the piston caused by the connecting rod acting at an angle

Liner-Lubrication

• The oil is of a high alkalinity which combats the acid attack from the sulphur in the fuel. The latest engines time the injection of oil using a computer which has inputs from the crankshaft position, engine load and engine speed. The correct quantity of oil can be injected by opening valves from a pressurized system, just as the piston ring pack is passing the injection point.

4 stroke Cylinder Liner

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

• Modern liners employ bore cooling at the top of the liner where the pressure stress is high and therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits so that there is not a breakdown in lubrication or excessive thermal stressing.

• Although the liner is splash lubricated from the revolving crankshaft, cylinder lubricators may be provided on the larger engines.

4S Liners

• On the example shown, the lubricator drillings are bored from the bottom of the liner circumferentially around the liner wall. Another set of holes are drilled to meet up with these vertically bored holes at the point where the oil is required at the liner surface.

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

Other engines may utilise axial drillings as in a two stroke engine.

Cylinder Liners-Cooling Water

• Where the cooling water space is formed between the engine frame and the jacket, there is a danger that water could leak down and contaminate the crankcase if the sealing O rings were to fail. As a warning, "tell tale" holes are led from between the O rings to the outside of the engine.

MAN-B&W L58/64 Liner

Cylinder Liners-Cooling Water

• Modern engines tend not to use this space for cooling water. Instead a separate water jacket is mounted above the cylinder frame. This stops any risk of leakage of water from the cooling space into the crankcase (or oil into the cooling water space), and provides the cooling at the hottest part of the cylinder liner.

Cylinder Liner-AP Ring

• Note that the liner opposite is fitted with a fireband. This is sometimes known as an antipolishing ring.

• It is slightly smaller in diameter than the liner, and its purpose is to remove the carbon which builds up on the piston above the top ring. If this carbon is allowed to build up it will eventually rub against the liner wall, polishing it and destroying its oil retention properties.

Liner-Materials

• Cast iron is used for the following reason:

- Castibility is good for intricate shapes

- Good wear resistance due to large surface of irregular shaped graphite flakes and semi-porous surface to retain oil

- Good thermal conductivity

- Good internal damping properties (vibration)

- Cost less relatively

MATERIAL

• Nodular or spheroidal cast iron is used for the following reason:

- tougher

- More resistance to crack formation (less stress raising matrix)

- Less self lubrication properties

* For higher power, shock loading due to combustion pressure

MATERIAL

• Alloy element

- Specify alloying elements namely nickel, chromium, copper or molybdenum – wear resistance to corrosion

MATERIAL

• For large bore cylinder liners generally the cast iron contains:

Carbon - 3.00% Silicon - 0.70%

Manganese - 1.00% Sulphur - 0.10%

Phosphorus - 0.25% Vanadium - 0.15%

Manufacture

• Two methods

- Sand casting

- Centrifugal casting

Liners-Manufacture

• After casting, liners are rough- machined and hydraulically pressure tested approximately 7 Bar

• Ports are formed in casting (old practice),however nowadays they are marked, drilled shaped and machined finish

• Then outside and inside surface is final-machined, sometime inside surface is honed to improved surface finish approximately (3.5m) or surface treatment is given

Surface finish treatment

• Normally used an electrolytic deposition of hard chrome sometime nickel

• Chrome layer is approximately 0.2 to 1.0 mm

• Chrome surface must be porous for oil retention and this can be achieved by etching with acid or a patented “porus-krome”

Advantages:

• Hard surface and improve wear-resistance, corrosion resistance, uniform surface finish

Disadvantages

• Expansive since reduction in wear rate cannot offset cost, plating can peel-off, must use only with cast iron piston rings, running in action may be delayed ,leading to rings collapse or scoring of liner

Sand casting

• Better wearing properties due to better grain flow produced in cast material

• Better graphitisation, thus lubricating properties, due to slow cooling rate

• Normally used for large, slow speed engines

Centrifugal casting

• Stronger liner

• Homogeneous in structure

• Poor wearing quality (fast cooling rate)

• Normally used for medium and high speed engine

Porosity manufactured

• The plated cylinder liner is immersed into a special bath and the current (for electroplating) is reversed. This forms minute pits and channels in the chromium plating

• After plating, liner is ground or honed. Then it is replaced in the bath together with a screen attached to its surface. The current is then reversed and small-hemispherical cavities are produced

Liner fault - crack 1 Crack across liner flange due to uneven or excessive

tightening of cylinder head studs

2 Hoop stress crack due to poor liner support

3 Circumferential crack along wear ridge due to stress concentration or more likely new rings hitting the ridge

4 Star or crazy cracking caused by flame impingement

5 Star cracks around lubricator quill due to water leakage

6 Cracks across port bars due to overloading, poor cooling, scavenge fire, poor fitting of rubbing sealing ring etc

1

2 3

4

5

6

Cylinder liner wear

There are three main cause of damage to

the liner material;

• Abrasion-caused by solid particles breaking through the lubricant film

• Corrosion-caused by the acidic products of combustion

• Friction or scuffing-Break down of the lubricating oil film leading to metal to metal contact

Abrasion

• Hard particles combined with cylinder lubricant to form a light abrasive paste causing liner wear

• In crosshead engine, cylinder lubricant is limited and thus flushing of abrasive paste is not effective compared to trunk engine having a good flushing action. Thus in crosshead engine, abrasive wear is the major contributor for liner wear.

Liner Wear

• Source of hard particles can originate from

- Air borne dirt,

- Ash in fuel,

- Carbon from combustion

- Piston rings wear

Air borne dirt

• Dirty air filter

• Dirty scavenge ports which is should be keep degree of dirtiness to minimum

• Scavenge manifold dirt

Ash in fuel

• Fuel consists of vanadium, sodium silica and scale (iron rust) which cannot be avoided, however it can be reduced by effective centrifugal separation

Carbon from combustion

• Combustion can never be perfect to form a hard carbon particles

• Means that if bad combustion occurs, more carbon will be produced even abrasive matter

• Therefore keep purification and combustion good apart from injection and pump timing even fuel temperature to be maintained.

Piston rings wear

• Produces wear dust from rubbing and this increases wear of both liner and rings itself

• Therefore should used good quality ring material and lubrication apart from good fitting

Corrosion

• Marine fuel contain sulphur to form sulphur dioxide and sulphur trioxide when the fuel burns as shown:

SO2 + H2O --------- SULPHUROUS ACID

SO3 + H2O --------- SULPHURIC ACID

UNBURNED FUEL (SULPHUR) + UNPERFECT COMBUSTION (O2 + H2O) -------- SULPHURIC ACID

Pre

ssu

re (

bar

)

H2S

O4 d

ewp

oin

t te

mper

ature

oC

) RELATION BETWEEN H2SO4 DEWPOINT

TEMPERATURE & FUEL SULPHUR CONTENT AT

DIFFERENT PRESSURES

110

120

130

140

150

160

170

180

1.0 2.0 3.0.

Sulphur in fuel %wt

1

40

80

Liner wear

• The dew point of the above acid is around 110oC to 180oC depending upon concentration, hence acid is always present due to liner temperature around that boundary

• Thus to combat or reduces this type of wear, the following should be made:

Remedies of corrosion

• Used alkaline oil for cylinder lubrication

• Control condensation in air cooler

• Keep cooling water temperature in jacket as high as practicable

• Keep quantity of starting air minimum

Note: to protect waste heat system from corrosion, by pass such system when engine is on light load

Scuffing or friction

• This is occur when lubricant failed to separate rings and liner surfaces efficiently and subsequent contact caused microwelding or microseizure.

• Reasons why these can occur on the liner surfaces:

Liner Wear-Scuffing

• Too smooth liner surface resulting in too little lube oil retention

• Water on the liner surface repelling oil film

• Blow past and thus removing oil film

• Poor or inadequate oil distribution around the liner surfaces

• Deposit on the piston absorbed oil film

Liner wear pattern

• Maximum normal liner wear occurs at top of the liner, in port-starboard direction and around scavenge and exhaust ports

• The reason for the above matter are as follow; – High temperature region reduces oil viscosity and

thus oil thickness

– High gas pressure increases ring loading causing penetration of oil film

– Slow movement of piston results in ‘oil wedge’ to breakedown (reversal of piston movement)

– Movement of ship maximum in port- starboard direction than forward-aft direction (thus causing more wear here)

– High temperature makes oil film less resistance to acid penetration (acid more active when hot)

– Tiny particles of carbonaceous matter are formed by combustion process, some are abrasive, but some accumulates in grooves around ring leading to wear promoting condition

– At higher temperature, cast iron has less resistance to wear

– At scavenge and exhaust ports bars, oil film will be blown-off when topmost ring opens to the ports

– Also due to the ports opening, relative pressure on ports bars increased resulting in increase wear another form of liner wear pattern is called cloverleafing

CLOVERLEAFING

• Alkali in cylinder oil is used to neutralize acid.

• For fuel with 4-5% sulphur -cylinder oil with TBN (Total Base Number) of 70

• For fuel with 1% sulphur, used cylinder oil TBN of 20 or 30

• To obtained perfect distribution of cylinder oil is difficult so the surfaces get either more alkalinity or less depending on the position from cylinder lubrications quill and the TBN used

Measured by

profilograph

an instrument

Lube oil Lube oil

Lube oil

Lube oil

Wear due to

excessive alkalinity

in cylinder oil

Wear due to exhausted

alkalinity before

cylinder oil spreads

across liner surfaces

CLOVERLEAFING

Liner wear-TBN

• If TBN use is more, surfaces near quills will get excessive alkalinity leading to hard calcium compounds formed.

• Alkaline compounds are burnt and formed heavy deposits which caused abrasive wear.

• Surfaces further from quills will have alkali neutralized by then and thus minimum wear is experienced

Liner Wear-TBN

• If TBN used is less, surfaces near quills will have minimum wear but surfaces furthest from quills will be starved of alkaline compounds when reaches there.

• This will lead to acidic corrosion (since insufficient alkalis to neutralize acid) and thus experienced maximum wear.

• If insufficient oil used, the effect will be exchanged (TBN)

MICROSEIZURE – OIL FILM

RING

LINER

WALL

Asperities meet

RING

LINER

WALL

RING

LINER

WALL

Frictional heat and

welding

Tearing out,

cooling, hardening

OIL - WEDGE

LINER

WALL

LINER

WALL

PISTON

SKIRT PISTON

SKIRT

OIL

FILM

OIL

FILM

OIL –

WEDGE

(1:150

TAPPER)

WEAR RATE Cyl No Date

Date fitted Date of last

gauging

Total hours Hours since

last gauging

Position Forward and After Port and Starboard

A

B

C

D

E

F

G

H

Max wear

Mean Max Wear

Wear rate since new

Wear rate since last gauging

Remarks

A

B

C

D

E

F

G

H

LINER

EXHAUST

PORT

SCAVENGE

PORT

TEMPLETE

LINER WEAR RECORD SHEET

TOP RING

POSITION

WEAR

RESULTS

Liner wear-Analysis

• Wear rates vary, but as a general rule, for a large bore engine a wear rate of 0.05 - 0.1mm/1000 hours is acceptable. The liner should be replaced as the wear approaches 0.8 - 1% of liner diameter. The liner is gauged at regular intervals to ascertain the wear rate.

• It has been known for ships to go for scrap after 20 + years of operation with some of the original liners in the engine.

• The wear rate for a medium speed liner should be below 0.015mm/1000hrs.

Gauging a Liner

Wear rate

• It is related to time in order to put these figures into useful form of comparison, in the form of diameter increase per thousand running hours. Thus :-

• Wear rate since last recorded measurement

Increase in O since last record

Running horus since last record

X 1000 =

= mm/1000 hrs

Liner Wear-Analysis

• Wear rate since new :

Total increase in O

Total running hours since new

X 1000 =

= mm/1000 hrs

Maxi. Liner wear rate mm/1000

RUNNING-IN

PERIOD

GRAUDALWEAR

STARTS

WEAR RATE INCREASES

DUE TO WORN RINGS

AND LINERS

MAXIMUM CYLINDER LINER WEAR

RATE PER RUNNING HOURS

ENGINE RUNNING HOURS x 1000

0.1

0.2

0.3

0.4

2 4 6 8 10 12 14 16 18 20

Causes of excessive wear • Improper running in – smoothing and geometry

• Misalignment of piston or distortion of cylinder – thermal stress and uneven tightening

• Inadequate oil supply or unsatisfactory oil supply

• Lube oil too low in viscosity or too low in alkalinity

• Incorrect piston ring clearances

• Unstable cylinder liner material – phosphorous / silicon

• Contamination of lube oil by extraneous abrasive material – 4stroke engine

• Cylinder wall temperature too high or too low – oil film breakdown or corrosive wear

• Overloading of engine – overheated,distorted and lube oil destroyed

• Scavenge air temperature too low – wash oil film, form acid, rusting

• Inefficient combustion – carbon

• Use of low sulphur fuel (less than 1% sulphur) in conjuntion with high alkaline cylinder oil and vise versa

DETAIL of SEALING RINGS • Depth of groove = 0.7 to 0.8d where “d” is diameter

of rings section. This is because the sealing effect is best when ring is in deformed state.

• Rubber ring cross-sectional area is 75 to 90% of grooves area. (due to rubber being flexible but incompressible)

• Di (inside Ø of rubber rings) is 2% less than DG (diameter of the bottom of groove) – for prestressing effect when fitted in liner.

• Sealing parts smooth and guiding edges tapered and rounded. Rubber rings and sealing parts applied with tallow or soft soap.

DG

Di

d

0.7 to 0.8d

Di = 0.98 DG

Cylinder liner lubrication

Cylinder lubrication

•Because the cylinder is separate from the crankcase there is no splash lubrication as on a trunk piston engine. Oil is supplied through drillings in the liner. Grooves machined in the liner from the injection points spread the oil circumferentially around the liner and the piston rings assist in spreading the oil up and down the length of the liner.

LUBRICATOR QUILLS

DISTRIBUTOR

DISTRIBUTOR

CYLINDER OIL SUPPLY

REGULATING VALVE

CYLINDER OIL PUMP

PURPOSE CYLINDER

LUBRICATION • Reduce friction between liner and ring – scuffing

• Assist in sealing of combustion gas – blow-past,burnt oil film etc

• Insulate liner against high gas temperature – viscosity and fluidity

• Guard against corrosive attack – as above

• Neutralizing combustion acid – TBN and rate of reaction

• Removing carbon or oxidation deposits – in ring zones and ports

TO CYLINDER

LUBRICATOR

QUILLS

LUBRICATOR QUILL OR

INJECTOR

CYLINDER

LINER

HYDRAULIC

PLUNGER ROTATING

RATCHET

ROTATING DISTRIBUTOR

DISCHARGE FLOW

INDICATOR AND

ALARM

SUCTION FLOW

INDICATOR

FROM TANK

FILTER

INJECTOR BARREL WITH HELICAL

SLOT

SPRING RETURN PLUNGER WITH

SPILL PORT

CAMSHAFT

FORK LEVER ROLLER ACTUATOR BARREL

WITH PORTS

OIL PULSE TO

OPERATE RATCHET

MECHANISM

DOXFORD ‘J’ CYL LUB. SYSTEM

FILLING

FILTER

GAUGE

GLASS

CAM (POSITION

ADJUSTABLE)

CAMSHAFT

OIL IN

DUAL SUCTION

VALVE

FLUSHING

TRIGGER

NUT

QUANTITY

REGULATES

MOVING

PLUNGER

WITHOUT

CAMSHAFT

TURNING

DIFFERENT

PUMP

LUNGER

AIR VENT

SCREW DUAL DELIVERY

VALVE

SIGHT GLASS

FILLED WITH

FLUID

NUT

BUBLER GAUGE

WIRE

NON-RETURN

VALVE

TO LUBRICATOR

QUILL

TYPICAL LUBRICATOR

NON-RETURN

VALVE OIL INLET

FILLING PIN

RUBBER PACKING

JOINT PACKING

CYLINDER

JACKET COOLING

WATER

SPACE

COOLING

WATER

SPACE

CYLINDER

LINER

GROUN FACE JOINT

OR COPPER GASKET

TYPICAL QUILL FOR LUBRICATOR (OLD TYPE)

CYLINDER

LINER

NON-RETURN

VALVE

LUBRICATOR

QUILL

ACCUMULATOR

FLOW CONTROLLER

CYLINDER LUBRICATING PUMP OIL QUANTITY ADJUSTING LEVER

AUTOMATIC LOAD

DEPENDENT FEED

RATE REGULATION

ECENTRIC EXCENTRE

DRIVE

CAMSHAFT

SULZER RND - M

CYLINDER LUBRICATION

(ACCUMULATOR SYSTEM

NEW TYPE SULZER RND-M LUBRICATOR QUILL

CYLINDER

LINER

COOLING

WATER

SPACE

CYLINDER

JACKET

FILLING PIN

JOINT PACKING

ACCUMULATOR

ACCUMULATOR PISTON &

DIAPHRAGM

OIL INLET

SEALING RING

SLEEVE

GROUND FACE

JOINT

NON-RETURN VALVE

PISTON &

RINGS

UPWARD SLANTING

BORE

ACCUMULATOR operation

• Through pipe oil is supplied by the cylinder lubricating pump (at about every 10 – 15 engine turns)into space under piston and diaphragm.

• The accumulator piston which is sealed off by a diaphragm is against the force of the spring pushed upwards.

• Through this a pressure builds up in the system which is higher than the scavenge air pressure of the engine

• If the pressure at the delivery point drops below the accumulating pressure, the oil will then flow through the upward slanting bore into the cylinder.

• As soon as the pressure at the lubricating point on the cylinder higher than the accumulating pressure, the lubrication is stopped (non- return valve closes)

• At which moment, in relation to the piston position respectively the pressure ratio, the lubrication is carried out as shown in schematic illustration below:-

1

1

2

2

3

3

4

4

5 6

PRESSURE (BAR)

40

30

20

10

0

BDC TDC BDC 90o 270o

CRANK ANGLE

LUBRICATING QUILL

CYLINDER LINER

PISTON

COMPRESSION EXPANSION

Oil pressure in accumulator

QUILL PRESSURE

FLUCTUATION

• In the range of BDC, between the position 5 + 6 and in the range of TDC, between the position 2 + 3 the pressure in the accumulator is higher than the pressure at the lubricating point. Consequently the oil flows in the said range to the cylinder liner running surface.

• So the running surface is lubricated twice per one engine turn.

• Between positions 6 + 2 as well as 3 + 5 the pressure is higher at the lubricating point than in the lubricating quill. The oil supply to the cylinder liner is stopped (non –return valve closes)

• Cylinder oil feed rates

– Uniflow scavenge - 0.54g/kWh

– Loop / Cross scavenge - 0.8 - 0.9g/kWh

– Trunk engine - 1.0 - 1.6 g/kWh

POSITION OF CYLINDER LUBRICATORS

• Not to be near the ports – oil can be scraped and blown away

• Not to be situated too near the high temperature zone or the oil will burn easily

• There should be sufficient points to ensure as even and as complete a coverage as possible

• Oil is injected between 1st and 2nd rings at the outer end of stroke (upper piston) also oil is injected between 1st and 2nd rings during early part of compression stroke (lower piston)

A

B

C

A

B + C C

Oil film thickness Hmin

Hmin

V ring

A C B+C

Oil refreshing rate

near TDC

Multilevel

lubrication

B

MULTILEVEL LUBRICATION

SULZER RTA 52/62/72/76/84/84M

Difficulty on achieving

• Piston direction changes every stroke

• In 2-stroke engine, no non-working stroke available for oil film to reform

• In diaphragm engine, no oil is returned, therefore supply is limited to control consumption and no cooling effect.

• Piston and ring distorted due to gas pressure and temperature

• All fuel contain abrasive contaminants

• Liner temperature varies causing change in viscosity

Difficulty in Timing • Piston speed may be high (approximately 2o of crank angle only)-

slow movement will destroy oil wedge

• Only very small quantity of oil being pumped

• Gas pressure resists delivery charge

• Cylinder oil viscosity changes due to temperature even will impede (prevent) oil formation

• A long, small bore pipe is needed to connect pump and quill(delivery lag)

• Carbon deposit may foul the quill

• A non-return valve is needed in quill which may become sluggish

• Delay between pumping and delivery may represent up to 30o of crank movement

• Oil groove in liner may be filled with carbon deposits

Gauging a cylinder liner

Liner Gauging

• Gauging a liner is carried out for two reasons: To establish the wear rate of the liner, and to predict if and when the liner will require changing.

• Although on a 2 stroke engine the condition of the liner can be established by inspection through the scavenge ports (evidence of blowby, scuffing etc.), the liner is gauged during the routine unit overhaul (15000 hrs), or if the unit has to be opened up for any reason

Source: www.marinediesels.info

Liner Gauging

• Because of the action of the piston rings, the varying gas pressure and temperature in the cylinder, the wear will not be even down the length of the liner. Consider the piston just beginning the power stroke. The gas pressure pushing the piston rings against the liner wall is at its highest; The liner surface temperature up at this part of the liner is about 200°C, so the viscosity of the lubricating oil is low. The relative speed of the piston is low, and so the lubrication is only boundary.

Liner Gauging

• Because of these factors wear at the top of a liner increases to a maximum a few centimetres below the position of the top ring at TDC, and then decreases as the ring pressure and liner wall temperature decreases and the piston speed increases building up a hydrodynamic film between liner and ring surfaces. Then as the piston slows down and the rings pass over the port bars, the wear will increase due to boundary lubrication, a reduction in surface area, and oil being blown out into the scavenge space.

Liner Gauging

• A liner is gauged by measuring the diameter of the liner at fixed points down its length. It is measured from port to stbd (athwartships) and fwd to aft. An internal micrometer is used because of its accuracy (within 0.01mm). To ensure that the liner is always measured in the same place, so that accurate comparisons may be made, a flat bar is hung down the side of the liner with holes drilled through where the measurements are to be taken.

Gauging a liner on a large bore RTA engine.

Source: www.marinediesels.info

Liner Gauging

• Measurements are taken at more frequent intervals at the top of the liner where wear rate is expected to be highest.

• To ensure accuracy, the micrometer gauge is checked against a standard, and the liner and micrometer should be at ambient temperature. If the temperature is higher then a correction factor can be applied. To ensure micrometer and liner are at the same temperature, lay the micrometer on the entablature for a few minutes before starting.

Liner Gauging

• The readings can be recorded in tabular form, and from the data obtained the wear rate/1000 hours can be calculated. Wear rate varies, but on a large 2 stroke crosshead engine ideally should be about 0.05mm/1000 hours. On a medium speed trunk piston engine where the procedure for gauging is similar, the wear rate is around 0.015mm/1000 hours.

Cylinder Number: 1

Nominal Dia: 840mm

Total Running hours:

60000

Running hours since last calibration:

15000

Gauging

point

P - S

F - A

Wear rate

(average)

P - S

Wear rate

(average)

F - A

last

calib.

P - S

wear

rate

P - S

last

calib.

F - A

wear

rate

F - A

1

841.2

841.26

0.02

0.021

840.95

0.017

841

0.017

2

841.38

841.44

0.023

0.024

841.1

0.019

841.17

0.018

Etc

Figures are for illustration only.

Manufacturers quote max wear for a cylinder liner at about 0.8%

of original diameter. If the wear rate is kept to a minimum, then

the liner may last the life of the engine.

Replacing cylinder liner

Diesel Engine

The photograph above shows clearly the evidence

of the leaking liner. The cooling water has

evaporated leaving white deposits of the cooling

water treatment chemical

• The oil mist detector had activated during a day at sea and the engine had shut down automatically due to high oil mist content in the crankcase. After a period of about 30 minutes the crankcase doors were removed and the crankcase partitions inspected. The reason for the activation was not an overheated bearing, but water vapour due to the failure of a cylinder liner lower “O” ring seal on one cylinder that was allowing cooling water from the jacket cooling space into the crankcase. This meant that the whole running gear for that cylinder was required to be removed for replacement of the liner.

Liner Replacement

• The engine was isolated from main cooling water systems and drained, the compressed air system was isolated, the pre-lubrication pump was stopped, and the turning gear engaged. The expansion tank levels were monitored during maintenance in case a valve was passing on the engine, and the engine refilling with water. The drain for the engine was kept open and monitored closely for signs of leakage.

Liner Replacement

• The cylinder head and the piston were removed. The cylinder lubrication pipes at the bottom of the cylinder liner were disconnected by undoing and removing the banjo bolts. The bolt securing the centering piece which locates the liner in the correct position in the cylinder bore is removed.

Liner Replacement

• The liner must be jacked off its seating using a hydraulic jack. In the case of the ZA40 the jacking device is bolted to the crankpin bearing. (Left in place when removing the con rod, which is normally bolted to the bearing by means of a marine palm

After attaching the jacking device to the bottom end bearing the

bearing was turned through 90° and the crankpin turned to

TDC. The hydraulic pump connected to the jacks was operated

so that the jack locates in the bottom of the liner. The liner was

then jacked upwards until the liner moved off its seat. (the jack

has only a 54mm lift).

The liner lifting tool was

then bolted on to the top of

the cylinder liner and hooked

up to the engine room crane.

The liner was then carefully

removed from the engine

(mass of liner 450kg).

Liner Replacement The new assemblies were inspected prior to

fitting.

While the liner was removed from the engine, the jacket cooling space around the liner was inspected for overall condition that can indicate the effectiveness of the cooling water treatment.

The guiding bores in the entablature and O ring seating were cleaned and examined for evidence of corrosion /erosion and the landing face for the cylinder liner was cleaned and examined.

Fig: View looking down through engine frame with cylinder liner removed. Note: Although this is a ZA40 engine it is not the one being described. This is a Vee engine: You can see the side by side bottom end arrangement.

The new liner was cleaned, inspected and gauged to ensure it was within limits specified by Wartsila.

Landing and sealing faces were inspected to ensure they were free from damage.

Lubrication drillings were blown through with compressed air to ensure they were clear.

The lifting gear was attached and the liner tried in the entablature without O rings to ensure that it fitted without binding.

Liner Replacement The liner was withdrawn and heat resistant Viton O

rings fitted which were well lubricated with engine oil.

The landing face was smeared with a sealing compound.

The liner was fitted into the engine ensuring that the centering piece was correctly lined up.

Once the liner was in position the centering piece location bolt was fitted and the cylinder lubricators connected and checked by operating the pumps by hand and ensuring that oil issued from the lubrication points.

The Liner was gauged and the readings recorded.

The running gear was reassembled, fitting new piston rings.

Once the cylinder running gear was all in place, the engine was refilled with water, and the crankcase checked to ensure no leakage was occurring.

The engine was then prepared for running. All equipment used was accounted for and the crankcase was checked clear of tools, rags and personnel. The cylinder lubricators were wound 15 times to ensure lubrication of the piston and liners, and the engine was turned a minimum of 2 revolutions with the turning gear to check for correct operation of the running gear, the running gear is observed through the crankcase doors and the rocker cover.

The ammeter on the turning gear panel is monitored so

that if a partial seizure was to occur, the current drawn by the motor would have increased, indicating a problem.

The engine crankcase was monitored for water leakage

from the liners during the warming through procedure. The lubricating oil, which had been circulating through the

purifier was checked for water content, and the oil pumps switched on and oil flow through the bearings checked.

Once the engine was ready for starting the normal routine for checking the engine was followed:

- After a very short run (30 seconds) the engine was stopped and the bottom end bearing on the overhauled unit checked. - The engine was then run for increasing intervals of time; 2, 10, 30 minutes, checking the bearing in between. Finally the load on the engine was slowly increased

and the unit run in as per manufacturers instructions; reduced load and increased cylinder lubrication whilst the rings bedded into the liner.

References:

1. www.marinediesels.info

2. The Running and Maintenance of Marine Machinery – Cowley

3. Reeds Marine Engineering Series, Vol. 12 – Motor Engineering Knowledge for Marine Engineers

4. Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen

5. Principles and Practice of Marine Diesel Engines – Sanyal

6. www.dieselduck.info (Martin’s Marine Engineering Page)