Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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just above the uppermost piston ring at top dead centre). This means that the

greater part of the heat load is absorbed by the cylinder cover, which is made of

steel and thus more resistant to high heat loads. In addition, the cylinder cover

is water cooled, making it relatively easy to control the temperature level.

The piston is cooled by system oil, which means a lower cooling efﬁ-

ciency compared with the cooling of the cylinder cover. Oil cooling of the pis-

ton, however, offers a number of advantages. The optimum way of reducing

the temperature level on the piston is to reduce the heat load on it, this being

secured by redesigning the shape of the combustion chamber, including the

piston, to provide more space around the fuel valve nozzles. The new piston

shape was termed Oros (Greek for ‘small mountain’).

The result of the increased distance from the fuel valve nozzles to the

piston surface was simulated by computational ﬂuid dynamics (CFD) analy-

ses, and the optimum shapes of piston crown and cylinder cover determined

from these simulations. Tests on several engine types veriﬁed the simulations.

A signiﬁcant reduction in temperature was obtained after development tests on

K90MC engines with various layouts of fuel oil spray pattern.

Temperature measurements on the piston crown and exhaust valve are

shown in Figure 10.16. The reduction in maximum piston temperature was

approximately 90°C, this result being attained without impairing the tempera-

ture level on the oil side of the piston or the temperature on the exhaust valve.

l Piston ring pack. The CPR top ring with relief grooves is now standard

on all MC engines and has proved very effective in protecting the

cylinder liner surface as well as the lower piston rings against excessive

heat load. The CPR ring has a double lap joint, and an optimum

pressure drop across the top piston ring is ensured by relief grooves

(Figure 10.17). With increasing mean indicated pressures, the traditional

angle-cut ring gap may result in higher thermal load on the cylinder

liner; this load is signiﬁcantly reduced by the CPR ring as no gas will

pass through its double lap joint. The relief grooves ensure an almost

even distribution of the thermal load from the combustion gases over

the circumference of the liner, resulting in a reduced load on the liner as

well as on the second piston ring.

Measurements conﬁrmed that the peak temperature on No. 2 piston ring

was reduced from 300°C in association with an oblique cut top ring to 150°C

with the CPR top ring. No. 2 ring retains its spring force, and TBOs are consid-

erably extended. Furthermore, the pressure drop across the top piston ring has

been optimized with respect to wear on the liner, piston rings and ring grooves.

Thanks to the double lap joint, the pressure drop will be almost constant irre-

spective of the wear on the liner and rings. This contrasts with the traditional

angle-cut ring, with which the cylinder condition slowly deteriorates as the

liner wears. MAN Diesel asserts that with the CPR rings, a continually good

cylinder condition and low wear rate can be expected over the whole lifetime

of the liner.

Large bore engines 311

312 MAN B&W Low-Speed Engines

New Oros design

Conventional design

100% load

Piston

crown

temperature

Gas side

Cooling oil side

Mean 499°C, max 509°C

Mean 197°C, max 209°C

Mean 409°C, max 421°C

Mean 185°C, max 216°C

Exhaust

valve

temperature

Section

M-E

H G

Valve seat

Underside

Mean 439°C, max 456°C

Mean 563°C, max 564°C

Mean 448°C, max 457°C

Mean 577°C, max 577°C

Section

M-E

FigurE 10.16 New oros-shaped combustion chamber geometry (right) has contributed to reduced temperature measurements

Al-coating of the sliding surface of piston rings was introduced to ensure

safe running-in. The aluminium–bronze alloy coating, a type of bearing

material, has proved effective in protecting ring and liner surfaces during the

running-in period. Al-coated rings make it possible to load up a completely

new engine on the testbed within only 5 h, fostering time and fuel savings;

running-in can also be performed with reduced cylinder lubrication. The lifetime

of the coating is 1000–2000 h, depending on such factors as the cylinder oil feed

rate and surface roughness of the liner. The technical explanation is that no scuff-

ing occurs with the combination of aluminium–bronze/grey cast iron as the wear

components in the cylinder. Al-coated rings allow the normal increase in cylin-

der oil feed rate after changing rings and/or liners to be dispensed with.

l Piston ring groove. In order to extend the interval before recondition-

ing of the piston ring grooves is required, the chromium layer has been

increased to 0.5 mm and induction hardening of the grooves before

chrome plating introduced. The chromium layer is thus better sup-

ported by the base material and the risk of cracking of the brittle chrome

reduced, fostering a longer service life.

l Piston cleaning (PC) ring. Incorporated in the top of the cylinder liner,

the PC-ring has a slightly smaller inner diameter than the liner and hence

scrapes off ash and carbon deposits built up on the piston topland (Figure

10.18). Without such a ring, contact between the topland and the liner

wall could wipe off the injected cylinder oil, preventing the lubricant

from performing its optimized role. In some cases, deposit formation on

the topland could cause bore polishing of the liner wall, contributing to

deterioration of the cylinder condition. Introducing the PC-ring elimi-

nates contact between deposits on the topland and the liner, promoting an

enhanced cylinder condition and lube oil performance.

Cylinder Liner

The liners of the large bore engines are bore cooled; the cooling intensity

is adjusted to maintain an optimum temperature level and to ensure optimum

Large bore engines 313

Upper piston ring with double-lap

S-seal and Controlled Pressure

Relief (CPR) gaps

Even heat distribution on 2nd

piston ring

2nd, 3rd and 4th piston rings with

oblique cut ring gaps

New piston ring material: RVK-C

for 50–26 and RVK with plasma

coating on 98–60

•

FigurE 10.17 Conguration of CPr top piston ring

314 MAN B&W Low-Speed Engines

tribological conditions for the cylinder lube oil. For years MAN Diesel used

a wave-cut liner surface, which was modiﬁed a few years ago to a semi-honed

surface to facilitate running-in of the highly loaded engines. The original

wave-cut had a depth of approximately 0.02 mm. Experience showed that a

deeper cut is advantageous in increasing the lifetime of the oil pockets, which,

together with the Al-coated piston rings, leads to very safe running-in. An

improved semi-honed wave-cut liner surface was therefore introduced.

Fuel Valve

The fuel valve design was changed some years ago from a conventional type

to the mini-sac type. The aim was to reduce the sac volume in the fuel nozzle

and curb dripping, thus improving combustion; introducing the mini-sac valves

reduced the sac volume to approximately one-third of the original (see Chapter

8). To improve combustion even further, however, a new slide-type valve was

introduced for all large bore engines, completely eliminating the sac volume

(see Chapter 3). A signiﬁcant improvement in combustion is accompanied by

reduced NOx, smoke and particulate emissions. The reduced particulates also

improve the cylinder condition, and the wear rates of cylinder liner, piston

rings and ring grooves are also generally lower with slide-type fuel valves.

Slide-type valves were introduced on the K98MC engine from the begin-

ning, their positive effect conﬁrmed on the testbed. With fuel nozzles optimized

with respect to heat distribution and sfc, the NOx emission values associated

with this engine are described as very satisfactory.

Electronic High-Pressure Cylinder Lubricator

Cutting cylinder lube oil consumption—which represents a signiﬁcant annual

cost, especially for large bore engines—has been an important R&D target,

resulting in the development of a computer-controlled high-pressure cylinder

Piston – high topland

Cylinder cover

Cylinder liner

Piston cleaning ring

FigurE 10.18 Location of PC ring in cylinder liner

lubricator. Development of the electronic Alpha Lubricator started in 1997 and

the prototype was installed on a 7S35MC engine in the following year; reﬁne-

ments were carried out on MAN Diesel’s 4T50MX research engine.

System ﬂexibility makes it possible to choose any number of engine revo-

lutions (four, ﬁve, six, etc.) between injections of a speciﬁc amount of oil into

the cylinder. For example, lubrication can be effected every ﬁfth revolution in

the compression stroke and every tenth in the expansion stroke if that turns out

to be the optimal. The aim is to inject the cylinder oil exactly where and when

it is needed: in the piston ring pack as it passes the lube oil quills. Thanks to

the high pressure, it is possible to establish an injection period that starts just

when the uppermost piston ring is passing the quills and ends exactly when

the lowermost ring is passing. Furthermore, injection is in the tangential direc-

tion, ensuring optimal distribution of the oil in the complete ring pack and ring

grooves.

The

Alpha Lubricator features a small piston for each lubrication quill in theAlpha Lubricator features a small piston for each lubrication quill in the features a small piston for each lubrication quill in the

cylinder liner. Power for injecting the oil is derived from the system pressure,

supplied by a pump station. A conventional common rail system is used on the

driving side and a high-pressure positive displacement system on the injection

side. Equal amounts of oil are supplied to each quill and the highest possible

safety margin against clogging of individual quills is secured. The basic oil feed

rate can be set by a screw, which limits the stroke of the main lubricator piston.

All MC/MC-C/ME/ME-C engines (new and in service) can beneﬁt from

the system; large bore engines are provided with two Alpha Lubricators per

cylinder and small bore engines with one. The actual savings from the reduced

cylinder oil feed rates achieved depend on the engine size, but MAN Diesel

has suggested 20 per cent-plus cost reductions as possible. Further reductions

are promised by applying the ‘sulphur handle’ to dose oil in proportion to

the amount of sulphur entering the cylinder in the fuel; this Alpha Adaptive

Cylinder oil control (ACC) is now standard on Alpha Lubricators (see Chapter

4 for more details).

WATEr MiST CATCHEr

Water droplets entering the cylinder can have a negative effect on the cylinder

condition by disturbing the lube oil ﬁlm on the cylinder liner surface; all of

the two-stroke engines are therefore equipped with a water mist catcher. With

increasing pressures and air amounts, however, it has become more difﬁcult to

obtain adequate efﬁciency.

Seeking the optimum method of separating water droplets from the scav-

enge air, MAN Diesel carried out advanced CFD calculations to ﬁnd the

trajectories of the droplets as they leave the cooler. An analysis of the inﬂu-

ence of different guide vanes on the air ﬂow made it possible, by calculation,

to predict that all droplets larger than 0.1 mm would be separated before the air

entered the traditional water mist catcher element.

A new air ﬂow reversing chamber was designed with a pre-catcher guide

plate diverting the ﬂow in front of the water mist catcher element and securing

Water mist catcher 315

316 MAN B&W Low-Speed Engines

80 per cent water separation before the normal element. In addition, inspection

covers were ﬁtted before and after the air cooler element, and improved drains

(to prevent the re-entry of water) applied.

MC DESigN rEFiNEMENTS

Feedback from MAN B&W MC engines in service has shown average liner

wear rates as low as 0.05 mm/1000 h and cylinder overhauling intervals of

6000–8000 h. R&D targeting enhanced durability, and extended mean TBOs

has focused on:

l Engine structure. Experience has shown that the jacket cooling water

around the lower part of the cylinder frame may in some cases result in

a corrosion attack and thus lead to higher wear rates. Cooling water in

the lower part of the liner has therefore been omitted, thus increasing the

temperature of the liner surface. The modiﬁcation has been introduced

on the latest engine versions (Figure 10.19). Cracks were experienced in

the horizontal support for the forward main bearing in the thrust bearing

housing. Investigations revealed that, under special service conditions,

Cooling

water inlet

A-A B-B

Cooling

water inlet

Previous design New design

FigurE 10.19 Simplied cylinder frame (right) alongside previous design

the design seemed to leave too narrow a margin; improper welding qual-

ity was also a contributory factor. While such rib cracks do not imply

any risk with respect to the service of the ship in the short term, they

should be rectiﬁed at the ﬁrst available opportunity. For engines in

service, the designer has evolved a rectiﬁcation method, which, in addi-

tion to remedying the cracks, also increases the margins. Repairs have

been successfully carried out using this procedure. The standard design

has also been changed to address the above conditions.

l Piston rings. Investigation showed that the overhauling interval could

be prolonged by introducing approximately 30 per cent higher piston

rings in the two uppermost ring grooves. Such rings of a special alloyed

grey cast iron were therefore introduced as standard on new engines.

A further extension of maintenance intervals can be secured by plasma

(ceramic) coating or chrome plating the surface of the uppermost piston

ring. These coated rings are available as options. Experience showed

that in some cases the normal oblique-cut piston ring gap led to a high

heat input to the liner when all cuts coincided. An improved design of

S-seal ring (double-lap joint) was developed and tested to achieve a

completely tight upper ring and then reduce the pressure drop across this

ring by introducing a number of small oblique slots to secure a control-

led leakage (CL). The temperature distribution and pressure drop meas-

ured across the ring were promising and very clean ring lands resulted.

Rings of vermicular iron have also shown promising results. The higher

piston topland and higher top rings (the uppermost featuring a pressure-

balanced design) were introduced on engines with higher mean effective

pressures to enhance reliability and extend TBOs (Figure 10.5).

l Cylinder liner. Factors to be considered when designing cylinder liners

include material composition, strength, ductility, heat transfer coef-

ﬁcient and wear properties. The original MC-type liners with cast in

pipes unfortunately suffered from cracks, but the introduction of the

bore-cooled liner solved the problem (Figure 10.20). Such liners have

been ﬁtted to all new engines produced in recent years. The change to a

bore-cooled liner made of grey cast iron for engines originally supplied

with liners of the cast-in cooling pipe design, however, could not be

easily effected. Various parallel developments were therefore initiated,

based on the use of stronger materials, and improvements to the original

design introduced. A modiﬁed design of the cast-in pipes, in combina-

tion with a tightening-up of the production and quality speciﬁcations,

has led to considerably improved reliability from this type of liner. The

modiﬁed liner is the standard spare part supply for older MC engines.

The standard speciﬁcation for new engines is a bore-cooled type made

of Tarkalloy C grey cast iron.

The inside surface temperature of the liner greatly inﬂuences the general

cylinder condition. Traditionally, the cooling system has been laid out to match

MC design renements 317

318 MAN B&W Low-Speed Engines

the mcr load, but there is an advantage in controlling the inside liner surface

temperature in relation to the load. MAN Diesel has investigated and tested

different solutions for load-dependent cylinder liner cooling. One system sim-

ply adjusted the cooling water ﬂow through the original cooling ducts in the

liner but the results were not promising.

Another system features different sets of cooling ducts in the bore-cooled

liner, the set deployed depending on the engine load. At nominal power and

high loads the inner row of ducts is used to cool the liner, yielding the highest

cooling intensity. In the intermediate load range, the cooling function is shifted

to the next set of ducts, which are located further away from the inner surface;

this means that the cooling intensity is reduced and the liner surface tempera-

ture is kept at the optimum level. At very low loads both rows of cooling ducts

are bypassed in order to further reduce the cooling intensity. Tests showed

that the optimum liner temperature could be maintained over a very wide load

range and that this system was feasible, but the added complexity had to be

weighed against the service advantages.

The operating condition of cylinder liners and piston rings is to a great extent

a function of the temperature along the liner. The upper part is particularly

important, and a triple-fuel valve conﬁguration (see the next section) reduces

thermal load while, at the same time, the pressure-balanced piston ring and the

high topland ensure an appropriate pressure drop across the ring pack and con-

trol the temperature regime for the individual piston rings. For monitoring the

temperature of the upper part of the liners, MAN Diesel offers embedded tem-

perature sensors and a recorder.

Alarm and slowdown temperature settings allow the operator to take proper

action to restore proper running conditions if, for example, a piston ring or fuel

valve is temporarily or permanently out of order. Other features, such as an

uncooled cylinder frame, serve to increase slightly the wall temperature on the

lower part of the cylinder liner while, at the same time, reducing production

Previous design of

cast-in cooling pipes

Changed position and shape

of cast-in cooling pipes

FigurE 10.20 MC engine cylinder liner modications. A current bore-cooled stand-

ard is shown (right)

costs (Figure 10.19). The rise in wall temperature is aimed at counteracting the

tendency towards cold corrosion in the lower part of the liner.

l Exhaust valves. A high degree of reliability is claimed for the MC

exhaust valve design, and the recommended mean TBOs have generally

been achieved. The average time between seat grinding can also be con-

siderably increased for a large number of engines. Examples of more

than 25 000 running hours without overhauls have been logged with both

conventional and Nimonic-type exhaust valves. The Nimonic spindle is

now standard for the 600-mm bore engines and above; and a steel bot-

tom piece with a surface-hardened seat is speciﬁed to match the greater

hardness of the Nimonic spindle at high temperatures. The combination

of spindle and bottom piece fosters a mean TBO of a minimum 14 000 h.

Some cases of wear of the spindle stem chromium plating may be related to the

sealing air arrangement. A new sealing air system was therefore designed, incorpo-

rating oil mist and air supply from the exhaust valve’s air spring (Figure 10.21).

MC design renements 319

Control air

Non-return

Orifice

Sealing air/oil

mist inlet

Stop

cylinder

Filter

Damped closing

FigurE 10.21 Modied exhaust spindle guide sealing air arrangement (schematic)

320 MAN B&W Low-Speed Engines

In-service testing gave promising results: completely clean sealing air cham-

bers and virtually no wear of either the spindle stem or the sealing rings. The

system is now standard for new engines and can be easily retroﬁtted to those in

service.

Cold corrosion of the exhaust valve housing gas duct led to lower-than-

expected lifetimes for a number of valves, particularly those installed in large

bore engines. The corrosion attack occurs adjacent to the spindle guide boss

and in the duct areas at the cooling water inlet positions (Figure 10.22). The

problem has been addressed by new housings designed with thicker gas walls,

which are now standard ﬁtments for new engines and spares (Figure 10.23).

For engines in service, the following repair methods and countermeasures have

proved effective in dealing with corrosion attacks in the exhaust valve housing:

high-velocity sprayed Diamalloy 1005 coating in the gas duct; and repair weld-

ing with gas metal arc welding (MIG-type), preferably in conjunction with the

sprayed coating.

l Bearings. There should be no problems related to production or mate-

rials since the manufacture of white metal–type main and crankpin

bearings is well controlled (Figures 10.24 and 10.25). Some cases of

production-related problems were traced to the use of copper- or lead-

polluted white metal or the lack of proper surface treatment of the steel

back before casting the white metal. Based on service feedback, the

main bearing design has been modiﬁed to secure a wider safety margin.

The modiﬁcations include a larger bore-relief in order to prevent the

Corrosion

FigurE 10.22 Exhaust valve housing, indicating where corrosion problems were

encountered and remedied