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