Pres Rosgren

Compiled for the Cimac Circle

at SMM in Hamburg, 26 September

by Carl-Erik Rösgren

th

Interaction between

and

Design,

ConditionCylinder

Engine

CylinderDesign

Lube Oil

The introduction of the so called Antipolishing Ring (APR) has revolutionized the 4-stroke engine industry to the

benefit of the users.

The main effect caused by the APR is that the carbon deposits on the piston top land cannot touch the cylinder liner

surface. Thus bore polishing and rubbing off lube oil from the liner surface, as illustrated above, is avoided.

Wärtsilä introduced already before the introduction of the APR the pressurized piston skirt lubrication. Its main

function was and still is to guarantee a controlled lube oil supply to the piston rings, evenly distributed all around the

liner running surface. Together these two features have virtually eliminated piston ring scuffing in 4-stroke engines.

The figure below also shows the evolution of piston design from traditional 4-ring crowns with inner/outer support

to 3-ring crowns with inner support and box type skirt design. Thanks to the 3-ring design the crown can be lower

which can be used for lowering the piston height or increasing the guiding length above the gudgeon pin.

4-stroke engines 2

Function of antipolishing ring

Cylinder liner

without Antipolishing Ring

Cylinder liner

with Antipolishing Ring

Fig. 1

Piston technologies

State-of-the-art Composite piston

Steel/Nodular cast iron or

Steel/Steel Pfmax 210-250 bar

“Traditional” Composite piston

(Steel/Nodular cast iron) Pfmax 155 bar

Pressurized

skirt lubrication

Pressurized

skirt lubrication

Fig. 2

To increase the cooling surface the inner cooling gallery is provided with waves. This also contributes to less carbon

build up in the cooling gallery, especially when running with deteriorated thermal stability of the lube oil. The ridges

remain free from deposits over long running periods and thus maintains the cooling efficiency also in adverse running

conditions.

3 4-stroke engines

Wave shaped and long hole design to reduce undercrown deposit formation

Piston crown cooling galleryFig. 4

Piston crown cooling gallery

Wave shaped and long hole design to increase the cooling surface area

Fig. 3

The piston ring pack has developed from a total of four rings to only three (below). In the 4-ring pack (above) the third

compression ring was also given a certain lube oil scraping function to assist the oil control ring. The top ring was

provided with increased height and thicker chromium plating to give it a longer lifetime and increase time between

overhauls.

In today's engines with stable ring pack performance over long periods of time the oil control ring can very well do

its job "alone" and even with a lower contact pressure against the liner. The chromium layer thickness of the top ring

could also be reduced inspite the cylinder overhauls are frequently increased to 18000 hours.

4-stroke engines 4

Chrome ceramic

coated asymmetric

and chamfered I-ring

with side coating

Chrome coated

asymmetric II-ring

Spring loaded

oil control ring

“APR” Piston and Piston ring technologyFig. 6

“Pre-APR” Piston and Piston ring technology

Chrome coated

asymmetrical barrel

shaped compression

rings I and II

Taper faced

compression ring

Spring loaded

oil control ring

Fig. 5

The main purpose of the ring pack is quite naturally to minimize the blow-by of combustion gases down to the

crankcase and to control the amount of lube oil passing up to the combustion chamber. Some details which are of

importance for a well working ring pack are shown in the figure above.

The asymmetric barrel faced top piston ring provides the best running in conditions thanks to the pressure balance.

To maintain the pressure balance over its entire lifetime and to avoid upward scraping of the lube oil the top ring is

provided with a chamfer (below).

5 4-stroke engines

Top piston ring profiles

WornNew

Fig. 8

Piston ring optimizationPiston ring pack for

a) minimum blow-by of combustion gases

b) optimum lube oil consumption

- Minimized residence time for combustiongases and lube oil, 3 rings only

- Chamfered top piston ring

- Minimized gas leak areas (ring gaps and profiles)and volume, V1

- Minimized ring back volumes

- Profile ground oil control ring

- Ring stability, Pc > P 1> P 2

- Effective oil draining

- Typical blow-by for a well working ring pack,1 - 1.5 ‰ of air consumption

PT

P2

V2

PC

P1

V1

H

•

•

•

PT

P2

V2

PC

P1

V1

H

•

•

•

PT

P2

V2

PC

P1

V1

HH

•

•

•

Fig. 7

The cylinder liners have generally not required big changes when integrating the APR. However, some small

readjustment of the cooling arrangement might be necessary to maintain the liner running surface temperatures within

the limits. Too high temperature might cause increased ring and liner wear and too low the well-known cold corrosion

(above).

Before the introduction of the APR Wärtsilä tested a numerous amount of concepts to achieve longer liner and

piston ring lifetimes (below). Most of them did not reach production engines but some of them were promising.

The need for further increased wear resistance for cylinder liners and piston rings lies probably quite far away in the

future regarding standard engines.

When operating engines under more severe conditions, e.g. at elevated temperatures (Hot combustion) or on

"exotic" fuels like Orimulsion might, however, trigger a need for more advanced solutions.

4-stroke engines 6

Cylinder liner technologies

“Pre-APR”

Surface temperatureat 25 bar BMEP

Optimal temperaturerange

°C250 200 150 100 50

°C250 200 150 100 50

with APR

°C250 200 150 100 50

with APR

°C250 200 150 100 50

°C250 200 150 100 50

Fig. 9

Pre-APR

- Steadite network (phosphorus)

- Carbide formers (Cr, Mo, B, V, Ti)

- Nitriding

- Laser hardening

- Plasma spraying

- Plateau honing

- Bimetal combinations

With APR

- Steadite network (phosphorus)

- Plateau honing

Tested measures to increasecylinder liner performance

Fig. 10

The APR has a remarkable influence on the cleanliness of the engine, especially the piston ring zone (above). The lube

oil filter exchange intervals have also significantly increased on engines still running with cartridge filters (below).

7 4-stroke engines

Without Antipolishing Ring With Antipolishing Ring

HFO Operation 500-h Endurance Test without Oil Centrifuging

Antipolishing Ring Effecton Piston Cleanliness

Fig. 11

Antipolishing ring effecton fine filter change intervals

WITHOUT

ANTIPOLISHING RING

WITHANTIPOLISHING RING

Ave

rag

ec

ha

ng

ein

terv

als

(h)

460

1760

Fig. 12

Lube oil consumption has been significantly reduced when introducing the APR. This is valid for all types of 4-stroke

engines running on any type of fuel from natural gas to heavy fuel. The engine blow-by has also become dramatically

stable indicating that the performance of the piston ring pack is not changing over time (below).

4-stroke engines 8

Examples of lubricating oil consumption figures

0.0

0.3

0.6

0.9

1.2

1.5

1

Gro

ss

Lu

bri

cati

ng

Oil

Co

nsu

mp

tio

n[g

/kW

h] MINIMUM AVERAGE MAXIMUM

LFO / HFO Natural Gas Light Fuel Oil Heavy Fuel Oil

without anti- with antipolishing ring

polishing ring

Fig. 13

Antipolishing Ring Effect on Engine blow-by

Crankcase pressure, Vasa 8R32 M/V Transnordica

0

10

20

30

40

50

60

70

80

29000 31500 34000 36500 39000 41500 44000 46500 49000 51500

Running hours

Hgmm

With APR

Without APR

*

*

*

Overhaul*

Fig. 14

The wear rates of the top piston ring has been reduced and is no longer a limiting factor for extending the cylinder unit

TBO (above). Chromium plated side faces of the piston rings provide an optimal counterpart to the hardened piston

ring grooves prolonging piston crown lifetime.

Also the cylinder liner lifetime has been extended thanks to the APR (below) . The APR itself is an additional wear

part which needs to be changed at certain interval, which can vary a lot depending on operating conditions etc.

However, a minimum lifetime of 18000 hours can easily be achieved.

9 4-stroke engines

Top piston ring and groove wear

Top piston ring Top piston ring groove

lifetime

(1000 h)

Pre-APR with APR

wear

(µm/1000 h) lifetime

(1000 h)

Pre-APR with APR+ side chrome

20

18

16

14

12

10

8

6

4

2

50

40

30

20

10

20

18

16

14

12

10

8

6

4

2

100

80

60

40

20

wear

(µm/1000 h)

Fig. 15

Cylinder liner wear

Cylinder Liner Cylinder liner ovality

Pre-APR with APR Pre-APR with APR

wear

(µm/1000 h)

lifetime

(1000 h)

20

18

16

14

12

10

8

6

4

2

6

5

4

3

2

1

oval wear

(µm/1000 h)

125

100

75

50

25

lifetime

(1000 h)150

125

100

75

50

25

Fig. 16

Engine mounted lube oil filters have for years been the standard for Wärtsilä 4-stroke engines. For environmental

reasons all new engines are designed with engine mounted automatic backflush filters (above), either one or two in

parallel.

For extracting dirt particles from the system instead of circulating them back to the oil sump the backflush line is

provided with a lube oil driven centrifugal filter boosted by a separate drive oil supply (below).

4-stroke engines 10

Modern engine mountedlube oil filtration system

Fig. 17

Principle of back-flushing oil filterwith centrifugal filter

Fig. 18

Downsizing of the lube oil filter sizes is made possible thanks to the APR and introduction of automatic backflush

filters. Further downsizing will be possible through introduction of double backflush filters (above). The principle of

double backflushing is shown below.

11 4-stroke engines

Development of engine mountedlube oil filters

Automatic back-flushing

Filter area: 2375 cm2 (2.4) Filter area: 1530 cm2 (1.57)

Filter oil volume: 18 dm3 (30) Filter oil volume: 13 dm3 (20)

Filter area: 97500 cm2 (100)

Filter oil volume: 61 dm3 (100)

Cartridge type

Single back-flushing Double back-flushing

Fig. 19

Back-

flushing

Single back-flushing Double back-flushing

Clean oil

to engine

Clean oil

to engine

Oil inlet

from sump

Back-flushingBack-flushing

from sump

Oil inlet

Principle of back-flushingFig. 20

After 6 years of running experiences it can be said that this technology has even exceeded the expectations (above).

The time and the reasons to reach the condemning limits for the lube oil have changed quite dramatically along with

the introduction of the APR (below).

4-stroke engines 12

Maintenance needsfor automatic lube oil filtration

• Recommended TBO, 8000 hours->12000 h-> 18000 h?

• Expected candle lifetime, 4 years

• Time for replacing the candles, 30-40 min

Fig. 21

Lubricating oil properties causing oil change

75 75

20 20

5 5

0

20

40

60

80

100

Viscosity Insolubles Other Base Number Viscosity Other

Without antipolishing ring With antipolishing ring

Fig. 22

Since the need for replenishment as well as the rate of deterioration of the lube oil has changed dramatically also the cost

structure of the engine lubrication has changed. An example of the saving possibilities is shown above. It also has an

influence on in which direction the lube oils need to be developed for the future, especially to extend the lube oil change

intervals.

13 4-stroke engines

Lubrication Oil Economics In Diesel Operation10 000 Operating Hours 4 x Wärtsilä 9L46

Fig. 23

Lubricating oil properties causing oil changeFig. 24

The occurance of sudden, severe wear rates of cylinder liners in 2-stroke low-speed engines has been a disturbing and

costly issue in some cases during the recent years. The difficulty to predict it is well illustrated by the picture above. To

remedy this situation to the satisfaction of all the involved parties, Wärtsilä set up a program to systematically improve

the whole cylinder unit and lift its performance to another level. The targets set for this package of measures are listed

below.

2-stroke engines 14

TriboPack

Time

Wear

rate

Test bedServiceSea trial

Severe wear rate categories of the past are

improved with the TriboPack

Fig. 1

TriboPack

Target

� Give the customer the best standard to achieve an extended timebetween overhauls [TBO] of up to three years for hot parts

� Allow the customer to reduce the lub oil feed rate to values ofapproximate 0.9 g/BHPh or 1.2 g/kWh with no risk of excessive wear

� Reduce the wear rate for the liner to 0.05 mm/1000h or less over awide load range of the engine

� Reduce the risk to fail during running-in and seatrial and to generallyreduce the running-in time to 10 h and less

Fig. 2

The program resulted in a number of improvements, which altogether formulate a technical concept named TriboPack.

In essence the TriboPack consists of the features listed above. The AntiPolishing Ring (APR) as described in the

4-stroke part plays an important role also in the 2-stroke engines. However, it was obvious from the outset that the APR

alone would not give all the targetted improvements since the basic function of the APR would not change the running

behaviour for the first hours of operation. Firstly the machining of the liner running surface was calling for a much more

consistant quality (below).

15 2-stroke engines

TriboPack

Cr-ceramic

pre-profiled

Top Piston Ring

Cr-ceramic

pre-profiled

Top Piston Ring

Multilevel

Lubrication

Multilevel

Lubrication

Anti-polishing

Ring

Anti-polishing

Ring

Liner

fully

deep

honed

Liner

fully

deep

honed

Mid-stroke

Insulation

Mid-stroke

Insulation

Liner

Insulation

Liner

Insulation

Lower ringspre-profiled

and RC-coated

Lower ringspre-profiled

and RC-coated

Thick

chromium

layer

Thick

chromium

layer

Fig. 3

TriboPack - Liner Machining

Good surface� No broken hardphase visible as

carefully machined

Poor surface� Broken hardphase as deep as

0.7mm well visible

� Destroyed and broken hard

phase through improper

machining

� No reasonable piston running

will ever be possible under

such conditions of uncontrolled

breaking-out of particlesDepth

Running Surface

Status of liner surface after turning

Images inverted in colour

Fig. 4

There are many ways to create detrimental surface properties for a machined surface (above). One of the most common

in the engine industry are the so called shatter marks, normally a result of machine tool vibrations caused by

mismatching machining parameters. There geometrical magnitude ranges from only a few microns and thus stays well

within normal drawing tolerances. However, the counter surface, the piston rings in this context, feels them with

sometimes severe consequences. By honing the shatter marks can be reduced and by deep honing even eliminated.

Additionally the deep honing leaves the wear resistant hard phases unbroken (below).

2-stroke engines 16

TriboPack - Liner Machining

Improper machining

1 Plateau honed liner surface

2 Plateau honed liner surface with heavy shatter

marks. No running hours

3 Wide honed liner surface with scratches

through broken-out hard phase

4 Technovit print of a liner with heavy friction

flash marks.

The liner was machined on a vibrating lathe,

producing the regular friction traces

Insufficient honing process

5 Piston ring running on a surface according to

3 or 4

1

2

3

4

5

Fig. 5

TriboPack - Liner Machining

Honing

�Neither plateau honing nor widehoning showed the desiredimprovements in piston running

�Deep honing over the full strokewas successful

�Deep honing results in a smooth

surface without broken hard phase

�Building-up of a proper

hydrodynamic lubrication is only

possible on a plane surface

�Every disturbance on the surface

leads to increased friction

between protruding materials

�Deep honing is standard forSulzer engines since 1997

Fig. 6

The sulphuric acid or low temperature corrosion is a well understood problem which is all about temperature control of

the liner running surface. The cylinder lube oil does a part of the job by neutralizing the sulphuric acid but the rest has to

be done by the engine design. In this sense both 4-stroke and 2-stroke engines are in a similar position. Because of its

longer stroke and the longer residence time of the corrosive combustion residues on the liner surface the 2-stroke needs

a more accurate temperature control (above). By proper tools it is possible to quite accurably predict the liner surface

temperature as well as the critical temperature line below which the corrosion may take place for different engine ratings

(below). Based on these results the liner surface temperature can be adjusted by means of insulated cooling bores and

midstroke insulation of the cylinder liner.

17 2-stroke engines

TriboPack - Liner Insulation

Controlled liner surface temperature� Cold liner surface leads to cold

corrosion� Cold corrosion leads to breaking out of

corroded hard phase� Increased friction and scuffing� High wear rate

� Insulation prevents condensation ofsulphuric acid and corrosion� Improved wear rate

RTA84T and RTA84T -B

Fig. 7

Optimized liner temperatureRTA84T-Engine, 100%R1: Measured Cylinder

Liner Temperature with and without Insulation

-10

0

10

20

30

40

50

60

70

80

90

100

110

50 100 150 200 250 300

Dew Point and Liner Surface Temperature [°C]

Po

sit

ion

be

low

TD

C(R

ing

A)

[%o

fE

ng

ine

Str

ok

e]

RTA84T-MK1

RTA84T, FullyInsulated

Dew PointTemperature

Effect of RTA84T liner wall T on wear

Explanation to above field test

� Original liner execution

no insulation and high wear

� Improved liner execution with insulated

bore cooling and mid stroke insulation

wear below 0.05 mm/1000h

(in diameter)

� Calculated dew point of H2O on liner

wall

All considerations for 100% MCR

Fig. 8

With appropriate running conditions ensured for the liner it is a lot easier to work out a piston ring pack, consisting of a

chrome-ceramic coated top ring and lower rings with running-in and anti-scuffing coating, for fast running-in and a

stable running behaviour over a long period of time. The complete TriboPack piston ring pack is prescribed above.

And with appropriate liner and piston ring specification the AntiPolishing Ring will further stabilize the performance

of the cylinder unit over prolonged running time (below). Thus forming a good basis for achieving the 3 years or 18000

hours between overhauls.

2-stroke engines 18

TriboPack - Piston Ring Specification

•

•

•

•

•

Piston ringsAll piston rings are profiled forstable, hydrodynamic lubricationChrome-ceramic coated top ring forimproved wear resistanceLower rings with running-in and anti-scuffing coating for ease of running-in.

• The coating is worn off after a fewhundred hours only

All rings with straight cut gap forease of manufacturingTransient period of mixed executionsduring introduction

Fig. 9

TriboPack - AntiPolishing Ring

Function of the AntiPolishing ring APR

�Reduced liner top diameter with APR,

where the piston crown gets in and out

�Continuous scraping-off of deposits built

up on the piston crown

�Avoidance of contact with the liner wall

along the stroke through increased

clearance due to missing coke on crown

�Undisturbed oil film mid stroke

�Avoidance of liner polishing

�APR consisting of an alloyed steel with a

high yield point under elevated

temperature to remain in shape

Fig. 10

The benefit of the APR can be easily judged from the superior cleanliness of the piston top land and piston ring area.

Cleanliness is a prerequisite to stable operation over long running periods.

19 2-stroke engines

TriboPack Status

MT Eli Maersk 7RTA84T-D (HYU)7406 rhs

same vessel

Antipolishing Ring Piston

Fig. 11

Comparison of Carbon Deposits with /without Anti Polishing Ring

MV "Schieborg" 7 RTA 52 U APL Ringafter 2512 running hoursCylinder Lube Oil Feed Rate 1.1 g/kWh

•

With Anti Polishing Ring

MV "Spaarneborg" 7 RTA 52 Uafter 797 running hours (unit # 6)Cylinder Lube Oil Feed Rate 1.1 g/kWhWithout Anti Polishing Ring

Fig. 12

Extensive field testing of the wear rate of different piston ring materials has clearly shown the superiority of the

chromium ceramic ring (above). Fortunately the one which gives the lowest ring wear also gives the lowest liner wear

(below).

2-stroke engines 20

0.00

0.05

0.10

0.15

0.20

GG spez 3 Plasma Chromium ceramic

mm

/10

00

hrs

Max Min

Influence of piston ring coating on piston ring wear

TriboPack - Piston Ring WearFig. 13

Influence of piston ring coating on liner wear

0

0.01

0.02

0.03

0.04

0.05

GG spez 3 Plasma Chromium ceramic

mm

/10

00

hrs

Max Min

TriboPack - Liner WearFig. 14

A correct lube oil feed rate to the cylinders is a crucial factor for stable operation. Presently the recommendation stands

at 1,3 g/kWh but the TriboPack provides the prerequisite for significant reductions in the future. Latest experience from

engines in service has shown that even with a reduced lube oil feed rate below 1 g/kWh the piston running behaviour

with TriboPack is still excellent.

21 2-stroke engines

TriboPack - Lub Oil Feed Rate

Excessive lubrication is dangerous

�Below statements are based on intensive Oil

Film Thickness Measurements [OFTM] on

the test engine 4RT-flex58T-B during 1999

�Feed rates above 1.6 g/kWh clearly and

repeatedly lead to unstable oil films.

This was reproducibly measured in test and

experienced in service

�Feed rates of 0.8 g/kWh still showed a safe

oil film appearance in test and are applied on

field test engines in service

�The recommended feed rate at this stage of

experience is 1.3 g/kWh with the clear

intention for further reduction.

All conditions to do so result from the

consequent application of the TriboPack.

Fig. 15

Development of cylinder lubricationFig. 16

46 engines have up to now been equipped with the full TriboPack concept and most of the RTA engines are represented

(above). Scuffing of any kind has not occurred so far and the cylinder liner wear has been thoroughly followed in several

engines (below). The recorded wear rates clearly undercut the target of 0.05 mm/1000 hrs, indicating cylinder lifetimes

of more than 50000 running hours.

2-stroke engines 22

TriboPack StatusFig. 17

Liner wear with TriboPackFig. 18