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

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torque capacity for 14 cylinders, but the material was upgraded for this appli-

cation to enable an increased shrink ﬁt for a greater design margin. The thrust

bearing structure in RT-ﬂex96C engines with a mid-gear drive was also revised

to reduce deformations and stresses even with the increased thrust in a 14-cyl-

inder model. A common rail fuel injection system is beneﬁcial to the 14-cylin-

der engine, which has a mid-gear drive for the fuel and servo oil supply pumps.

Two identical rail units are thus installed, each serving seven cylinders.

A 14RTA96C engine measures 27.31-m long overall to the ﬂywheel ﬂange

 10.93-m high over the shaft centreline (or 13.54-m overall) and weighs 2300

tonnes dry (compared with 2050 tonnes for a 12RTA96C). The individual ele-

ments, such as the two crankshaft sections, are close to the maximum lifting

capacities of contemporary enginebuilders, which impose a limit on engines

with more than 14 cylinders. The camshaft for engines with more than 14 cyl-

inders would need to be split into three parts, and at least two camshaft drives

would become necessary. Such engines, if needed, would therefore have to

adopt the RT-ﬂex common rail systems for fuel injection and exhaust valve

actuation, which eliminate the camshaft (see p. 431).

With a length 15 per cent longer than the established 12-cylinder engine,

the longitudinal and torsional rigidity of the 14RTA96C model is considered

adequate for the expected ship structures. The torsional vibration characteris-

tics are also considered acceptable for the envisaged ﬁring order.

RTA96C engine uprating data

Original Rating Revised Rating

Bore (mm) 960 960

Stroke (mm) 2500 2500

Stroke–bore ratio 2.6 2.6

power (mcr) (kW/cyl) 5490 5720 (6030)

Speed (rev/min) 100 102

Cylinders 6–12 6–12, 14

Mean eective pressure (bar) 18.2 18.6 (19.6)

Mean piston speed (m/s) 8.3 8.5

Maximum cylinder pressure (bar) 142 145

Maximum output, 12-cycle (kW)

65 880 68 640

Maximum output, 14-cyl (kW) – 80 080 (84 420)

Specic fuel consumption, full load (g/kW h) 163–171 163–171

Note. A subsequent power uprating raised the output of the rTA96C/rT-ex96C designs to 6030 kW/

cylinder at 102 rev/min on a mean eective pressure of 19.6 bar, enabling a 14-cylinder model to oer

84 420 kW. The 13- and 14-cylinder models in the programme are only available in electronically con-

trolled rT-ex versions

rTA design developments 421

422 Wärtsilä (Sulzer) Low-Speed Engines

rTA96C Service Experience

Some cases of main bearing damage were restricted to a speciﬁc series of con-

tainer ships and involved the local breaking-out of white metal in the lower

shells and some local fretting. The fretting problem was solved by applying a

so-called key-slot solution with the aim of increasing the radial forces pressing

the bearing shell into the girder, and by some other small design modiﬁcations.

With the exception of one particular main bearing, no connection could be found

with the bearing load and shaft orbit that could explain the breaking-out prob-

lem. As a countermeasure, the lower shell of the main bearing next to the thrust

bearing is now machined together with its respective bearing cap to ensure bet-

ter bearing geometry. The more precise alignment of the bearing cap and shell,

along with a key between the cap and the shell to secure tight ﬁtting of the shell

in the bearing girder when tightening down, gradually yielded results.

Some reports of cracks in A-frame columns were ﬁrst received in mid-1999

after some 5000–12 000 running hours. A project team was established to inves-

tigate the matter and all other engines in service were inspected. Although the

cracks occurred in seven of the eight engines, they were in only some of the dou-

ble-walled A-frame columns manufactured to the original design and were of

various degrees of severity. The cracks were initiated at the inboard ends of two

pairs of horizontal ribs on the columns and propagated vertically and in the direc-

tion of the crosshead guide rails. The root of the problem was a combination of

lack of welding quality and somewhat higher than anticipated local stress level.

All cracked A-frame columns were repaired by welding. At the same time,

the inboard ends of all horizontal ribs in the engines concerned were cut back

to limit the stress concentration and thereby eliminate the possibility of crack-

ing. Subsequent measurements showed that the modiﬁed shape of the ribs

reduces the stress concentrations in the columns down to around half, and thus

is not as sensitive to imperfect welding. The same shape of horizontal rib is

now applied on all RTA96C engines, whether repaired or built as new; the

shape was adapted from the modiﬁed rib design introduced in December 1997

but slightly revised for ease of manufacture.

Some leaks were experienced in the cylinder pressure relief valve located

in the cylinder cover. These were caused by corrosion, which arose because

the shortened valve was too cold through being too far from the combustion

space. An improved valve design replaced those in existing engines, while later

engines were ﬁtted with valves of a traditional design which operate at higher

temperatures and hence avoid the original problem.

A number of scavenge air receivers suffered cracks in the form of rupturing

of the upper and lower longitudinal welded seams; these were found to have

been caused by resonant vibration in the dividing wall. As a remedy, addi-

tional stiffening was applied to the dividing walls of the affected receivers (and

adopted as standard for new engines) to change the resonant frequency and

thereby reduce vibration amplitudes.

A stiffer turbocharger support was developed for new engines after vibra-

tion had caused problems in the supports of one of the turbochargers of the ﬁrst

10-cylinder RTA96C engine. Excessive transverse movement of the exhaust

manifolds on the ﬁrst such models was also recorded. This was remedied by

stiffening the manifold support in the transverse direction, thus halving the

manifold movements. Excessive movements of the manifolds also led to the

breakage of cooling water discharge pipes; the adoption of ﬂexible pipes of a

proprietary make as standard solved the problem.

A number of broken cylinder cover studs have been noted, mostly broken

at the ﬁrst thread at the top but some fractured at the bottom. The lower frac-

tures were initiated by corrosion because of improper sealing; the upper frac-

tures resulted from materials and thread qualities beyond the speciﬁcation. The

design was changed to a wasted stud and a special nut to reduce stress levels

and ensure a good safety margin.

Tribopack for Extended TBo

The TBO of low-speed marine diesel engines is largely determined by the

piston running behaviour and its effect on the wear of piston rings and cylin-

der liners. Addressing this, Sulzer introduced a package of design measures

in 1999, which are now standard on all new RTA engines and retroﬁttable to

existing engines. The TriboPack technology enables the TBO of cylinder com-

ponents, including piston ring renewal, to be extended to at least 3 years and

also allows a further reduction in the cylinder lubricating oil feed rate. The

design measures (Figure 12.29) are as follows:

l Multi-level cylinder lubrication

l Fully and deep-honed cylinder liner with sufﬁcient hard phase

l Careful turning of the liner running surface and deep honing of the liner

over the full length of the running surface

rTA design developments 423

Liner

insulation

Multi-level

lubrication

Mid-stroke

insulation

Liner

fully

deep

honed

Anti-polishing

ring

Cr-ceramic

pre-profiled

top piston ring

Lower rings

preprofiled and

RC-coated

Thick

chromium

layer

FigurE 12.29 Tribopack design measures for improving piston-running behaviour

424 Wärtsilä (Sulzer) Low-Speed Engines

l Mid-stroke liner insulation and, where necessary, insulating tubes in the

cooling bores in the upper part of the liner

l Pre-proﬁled piston rings in all piston grooves

l Chromium–ceramic coating on the top piston ring

l Running-in coating (RC) piston rings in all lower piston grooves

l Anti-polishing ring at the top of the cylinder liner

l Increased thickness of chromium layer in the piston ring grooves.

A key element of TriboPack is the cylinder liner manufactured in cast iron,

which needs a controlled hard-phase content and the best grain structure in the

running surface for both good strength and running behaviour. Careful machin-

ing followed by full deep honing to remove all damaged hard phase from the

liner surface reportedly delivers an ideal running surface for the piston rings,

together with an optimum surface microstructure. Wärtsilä says that deep hon-

ing of the full liner running surface is a prerequisite for maximizing the ben-

eﬁts of TriboPack. Its experience has shown that plateau honing of a wave-cut

liner is not adequate because, once the plateau is worn down, the rings run on

liner metal whose hard phase structure was damaged during machining. This

damaged hard phase must be removed by deep honing.

Pistons have four rings, all having the same thickness. The chrome–ceramic

top ring, proven in Wärtsilä four-stroke engine practice, has a cast iron base

material. The running face is proﬁled and coated with a layer of chromium as

a matrix into which a ceramic material is trapped. High operational safety and

low liner and ring wear have been demonstrated, with a much better resist-

ance to scufﬁng than any other ring material. Good performance is conditional,

however, on using the chrome–ceramic rings in conjunction with a deep-honed

liner. The other piston rings have a running-in and anti-scufﬁng coating, which

fosters a safe and swift running-in of the engine when the liners are deep honed.

The APR prevents the build-up of deposits on the top land of the pis-

ton, which can damage the lube oil ﬁlm on the liner and cause bore polish-

ing. Deposit build-up can be heavy in some engines, especially those running

on very low sulphur content fuel oil (1 per cent sulphur) combined with an

excessive cylinder lube oil feed rate. If such deposits are allowed to accumulate,

they inevitably touch the liner running surface over a large part of the piston

stroke. The lube oil ﬁlm can then be wiped off, allowing metal-to-metal contact

between the piston rings and the liner; in the worst case there can be scufﬁng.

Applied as standard for some years on Wärtsilä four-stroke engines, the thin

alloy steel APR is located in a recess at the top of the liner and has an internal

diameter less than the cylinder bore to reduce the clearance to the piston top

land. It does not need to be speciﬁcally ﬁxed, as the thermal expansion of the

hot ring keeps it tightly in place. The steel material was selected to ensure and

maintain a high safety margin against thermal yielding. Excessive deposits are

scraped off the piston top land at every stroke while they are still soft, thus pre-

venting hard contact between the deposit and the liner wall surface. The oil ﬁlm

on the liner wall remains undisturbed and can fulﬁl its function. The APR also

stops the upward transportation of new lube oil by the layer of deposits to the

top of the liner where it is burned instead of being used for lubrication. The

ring is thus effective in allowing the lube oil feed rate to be kept down to rec-

ommended values.

Load-dependent cylinder lubrication is provided by a multi-level accumu-

lator system, the lubricating pumps driven by frequency-controlled electric

motors. On the cylinder liner, oil distributors bring oil to the different oil accu-

mulators. For ease of access, the quills are positioned in dry spaces instead of

in the way of cooling water spaces.

It is also important that the liner wall temperature is adapted to keep the

liner surface above the dew point temperature over the whole of the piston

stroke to avoid cold corrosion and maintain good piston-running conditions.

The upper part of the liner is bore cooled with cooling water passing through

tangential drillings in the liner collar. The mid-stroke region of the liner is

cooled by a water jacket, and only the lower part is uncooled. There is often

a tendency for liner temperatures to be too low, thus leading to corrosive wear

from the sulphuric acid formed during combustion. Wärtsilä applies two insu-

lating techniques to secure better temperature distributions. For some years,

PTFE insulating tubes have been ﬁtted in the cooling bores of the liner. As

part of TriboPack, the liner is now also insulated in the mid-stroke region by

a Teﬂon band on the water side. The insulating tubes are adapted according to

the engine rating to ensure that the temperature of the liner running surface is

kept above the dew point temperature of water over the full length of the stroke

and over a wide load range.

Mid-stroke insulation and, where necessary, insulating tubes are therefore

used to optimize liner temperatures over the piston stroke. An insulation band-

age in the form of Teﬂon bands with an outer stainless steel shell is arranged

around the outside of the liner to raise liner wall temperatures in the mid-stroke

region. Mid-stroke insulation is known to be particularly useful for sustained

engine operation at low power outputs, while the TriboPack gives an additional

safety margin in abnormal operating conditions (e.g., against excessive carbon

deposits built up on the piston crown).

While trying to avoid corrosive wear by optimizing liner wall tempera-

tures, it is necessary to keep as much water as possible out of the cylinders.

Highly efﬁcient vane-type water separators ﬁtted after the scavenge air cooler

and effective water drain arrangements are thus vital for good piston running

behaviour. Load-dependent cylinder lubrication is provided by the multi-level

accumulator system, which ensures the timely quantity of lube oil for good

piston running. The lube oil feed rate is controlled according to the engine load

and can also be adjusted according to the engine condition.

Large Bore Liner Wear problems

TriboPack measures resulted in cylinder liner wear rates generally 0.05 mm/

1000 h while using lube oil feed rates of 1–1.4 g/kW h, and the wear rates of

both top and other piston rings were satisfactory for a 3-year TBO. The lower

rTA design developments 425

426 Wärtsilä (Sulzer) Low-Speed Engines

wear rates allowed the guide rates for cylinder lubrication to be reduced from

1.4 g/kW h to 1.1 g/kW h for all sizes of RTA and RT-ﬂex engines built to the

TriboPack standard. The excellent results did not extend to the largest bore

RTA96C and RT-ﬂex96C models, however, which suffered what Wärtsilä

described as ‘unsatisfactory numbers’ of liner and piston ring failures through

sudden severe wear.

Further R&D resulted in other design measures, which demonstrated that

piston-running behaviour could be improved in these large bore engines as

well. One measure was to introduce an underslung scavenge air receiver design

as standard for the RTA96C and RT-ﬂex96C models. With its high efﬁciency

separator and more efﬁcient drain system, this arrangement considerably

improves the efﬁciency of condensate water removal from the scavenge air-

ﬂow between the air cooler and the engine cylinders. The receiver design has

reportedly proved successful, with virtually no water being carried over to the

cylinders, even under high humidity tropical conditions.

In the underslung air receiver, the scavenge air coolers are arranged hori-

zontally beneath the turbochargers. Scavenge air is then swung round 180° in

its ﬂow to the cylinders, in the process passing through the vertically arranged

water separators in the underslung trunks from the air coolers to the scavenge

air receiver. There are ample drainage provisions to completely remove the

condensed water collected at the bottom of the separator.

In the cylinder itself, the new design measures for the largest bore engines

include lower liner wall temperatures, chromium–ceramic pre-proﬁled rings

in all piston grooves, plateau honing of the liner, omission of oil distribution

grooves on the liner surface, bronze rubbing bands on a shorter piston skirt and

improved geometry of the APR. Features such as multi-level cylinder lubrica-

tion and chromium plating of piston ring grooves are retained. Attention was

given to the stability of the lube oil ﬁlm on the liner running surface. The lube

oil distribution grooves were omitted because it was found that they disturbed

the oil ﬁlm more than it beneﬁts from the oil distribution.

It had become clear that polishing of the liner surface by deposits on the

top land of the piston crown must not occur if liner scufﬁng is to be avoided.

An improved APR geometry introduced as standard features a smaller clear-

ance between the ring and the piston crown, and two edges to clean the piston

top land during both up and down strokes of the piston.

Another source of disturbance for the oil ﬁlm is rubbing of the piston

skirt on the liner’s running surface. A new shortened skirt with a ground sur-

face was therefore introduced, equipped as standard with two bronze rubbing

bands. Such features contribute to reduced friction and hence lower risk of oil

ﬁlm disturbance. A reduced liner wall temperature is possible with chromium–

ceramic piston rings, with a positive effect found on the lube oil ﬁlm stability

in RTA96C and RT-ﬂex96C engines. Wärtsilä reports that extended shipboard

tests with cylinder liners having lower wall temperatures returned excellent

results. Liner wear of 0.05 mm/1000 h conﬁrmed that wear is not increased

with the above-mentioned ring pack at the lower temperatures.

pulse Jet Cylinder Lubrication

Piston-running behaviour was further enhanced from 2006 with the availability

of Wärtsilä’s new pulse jet lubricating system (Figures 12.30 and 12.31),

designed to ensure improved cylinder oil distribution on the liner’s running

surface through precise feed timing and dosage. The system, which can be

speciﬁed for new RTA and RT-ﬂex engines or retroﬁtted to engines in service,

allows much lower lube oil feed rates to be adopted. Cylinder oil is sprayed

onto the liner surface from a single row of quills arranged around the liner,

each quill having a number of nozzle holes.

Eight quills (simple and reliable non-return valves) are speciﬁed for

RTA96C and RT-ﬂex96C engines, for example, each quill having ﬁve oil jets.

A total of 40 lubricating points is thus provided on the liner surface, the jets

individually directed to separate points on the surface. There is no atomization

and no loss of lube oil to the scavenge air. Cylinder oil is delivered to the quills

by a lubricator pump powered by the engine’s pressurized system oil. A lubri-

cating module is provided for each cylinder with the associated dosage pump

and monitoring electronics.

Metered quantities of oil are delivered under pressure into the piston ring

package, from where it is evenly distributed around the liner circumference.

rTA design developments 427

FigurE 12.30 Schematic diagram of Wärtsilä’s pulse jet lubricating system, whereby

pressurized cylinder lube oil is delivered to multiple jets around the liner

428 Wärtsilä (Sulzer) Low-Speed Engines

The feed rate and timing are electronically controlled via a solenoid valve at the

lubricator pump. Full ﬂexibility is provided in the setting of the lubricator tim-

ing point, and volumetric metering ensures constant spray patterns across the

engine load range. The dosage is precisely regulated even for low feed rates.

piston rod glands

TBOs of crosshead engines are partly deﬁned by the piston rod glands, in the

sense that their removal for the exchange of elements is often connected to a

withdrawal of the piston and piston rod assembly. The gland elements and pis-

ton rods therefore need to have a long life expectation (TBO of 3 years or more).

At the same time, they have to assure sealing of the crankcase from the piston

underside, limit contamination of crankcase system oil by combustion residues

and keep the oil consumption at a reasonable level for maintaining oil quality.

Recent improvements have introduced additional gas-tight top scraper

rings, stronger springs for the other scraper rings, enlarged drain channels for

the scraped-off oil and the exclusive application of bronze scraper rings on fully

hardened rods. The drain quantities from the neutral space were reduced by a

factor of three. Additionally, the scraped-off oil is reusable without any treat-

ment and therefore can be directly fed back internally in the gland box to the

crankcase. System oil consumption ﬁgures were signiﬁcantly reduced. The

design of the gland box housing was modiﬁed, allowing it to be dismantled

either upwards during piston overhaul or downwards without pulling the piston.

FigurE 12.31 Arrangement of multiple lube oil jets in the cylinder liner, indicating

the pattern of distribution with Wärtsilä’s pulse jet lubricating system

Complete retroﬁt packages available for all RTA engines in service com-

prise the newly designed upper scraper, new middle sealing and new lower

scraper groups, and some modiﬁcations on the gland box housing. The upper

scraper group consists of a two-piece housing with newly designed oil scraper

rings made of bronze, newly designed gas-tight sealing rings and modiﬁed ten-

sion springs. The oil scraper rings consist of four segments conforming better

to the piston rod. Two new seal rings in three parts and adapted tension springs

were introduced for the middle seal group. For the lower scraper group, all

rings are made of bronze, since Teﬂon has an inferior performance when there

is an increased amount of hard particles in the oil residues coming from the

piston underside. Here, the new scraper rings comprise three slotted segments

for adaptability to the piston rod; they are provided with grooves at the top to

promote draining of the scraped oil.

Wärtsilä stresses that the actual surface condition and shape of the piston

rod is of paramount importance. Ideally, the new glands should be used with

hardened piston rods. Existing rods can be retained, however, providing their

surface condition and geometry are acceptable, before introducing any new

stufﬁng box elements. If the rod is worn down, roughened or otherwise surface

damaged, it can be ground to standard diameters of 2 mm or 4 mm undersize

and then surface hardened.

Exhaust valve Behaviour

The exhaust valve is subjected to hot gases, and the temperature resistance of

its seat and body is therefore crucial. Nimonic valves combined with proper

seat cooling have yielded excellent service behaviour and long lifetimes. When

the RTA96C engine was introduced, and its shallow combustion space created

difﬁcult conditions for combustion chamber components, some exhaust valves

were additionally coated with Inconel alloy. After limited running times, how-

ever, there was some cracking of the coating originating from the centre hole,

with loosened material, making removal by grinding necessary. Non-coated

valves, on the contrary, showed excellent performance, remaining free of

cracks after over 14 000 running hours, without any loss of material. Today’s

standard therefore is the non-coated valve.

piston Crowns

Burning-off of material on piston crowns is very dangerous as hole formation

leads to direct contact of the combustion ﬂame with the piston cooling oil sys-

tem and dire consequences. The use of the combined shaker and jet cooling

system in RTA engines assures piston crown temperatures below 400°C and

thus eliminates such burning.

Condition-Based Maintenance

A new tool for managing condition-based maintenance of two-stroke engines

combines the expert knowledge and wide experience of Wärtsilä Corporation

rTA design developments 429

430 Wärtsilä (Sulzer) Low-Speed Engines

engineers. The CBM Management tool is offered in three different solutions,

depending on the extent that customers want to outsource this activity.

A wide range of products for diesel engine monitoring, trend analysis, diag-

nosis and management already support condition-based maintenance. CBM

Management takes these a step further, with the aim of optimizing the balance

between long TBOs, low spare part costs, less time off-hire and high reliability

from the engine. Wärtsilä appreciated that, although customers use the same

engine types in their ﬂeet, they experience quite different maintenance costs

and varying engine reliability. This arises from various factors that inﬂuence

the condition of the engine, such as the quality of fuel, lube oil and spare parts

and the quality of routine inspections and overhauls.

Analysis of cases of extensive damage to engine components had shown

that long before the damage occurred it would have been possible to foresee

that something was going wrong. With the appropriate expert knowledge and

a systematic analysis of available information, it is often possible to predict

problems and to prevent expensive maintenance costs. Wartsila’s solution was

to collect the expert knowledge of engineers from its technical service organi-

zation worldwide and store it in a common database.

Some data are speciﬁc to an individual engine: shop trial results, sea trial

results, electronically stored information from general service reports made by

Wärtsilä and performance data collected onboard. All of these were combined

with the database of expert knowledge, enabling the resulting expert system

to automatically generate advice on the condition of the speciﬁc engine. The

CBM Management tool comprises a data collector (hardware and software)

and an expert system (hardware and software).

The data collector is a hand-held computer whose main purpose is to ena-

ble the ship’s engineers to collect data in a standardized way. It is automatically

loaded with engine performance data from the control system through a standard

interface but manually fed with maintenance and inspection data. For system-

atic and efﬁcient manual input, the data collector contains a structured inspec-

tion guide for the engine, supported by interactive templates for measurement

records and performance sheets. This guide exists for the following 12 function

groups: piston performance, piston rod gland condition, scavenge airﬂow, com-

bustion performance, fuel injection pumps, camshaft, engine structure, gears and

wheels, bearings, crankshaft, engine control system and vibration dampers.

The data collector takes into account different conditions of important

components such as piston rings, cylinder liners and bearings. A picture gal-

lery helps the ship’s engineers to understand the terminology used. The data

collector reduces inspection time and partly replaces the necessary paperwork

through the automatically created templates.

The expert system is the heart of the tool and runs on a PC using data

from the data collector. There are three steps from collected data to real expert

advice: calculate trends, assess engine condition by comparing the data with

a weighted set of reference cases based on the expert knowledge of Wärtsilä

engineers and assess the condition of the engine components and parts and