1. The Importance of effective Cylinder Oil condition monitoring in
Two-Stroke, Slow Speed, Diesel engines.
Cristiano Garau (V)
Lead Application Engineer, Parker Kittiwake, Littlehampton, United Kingdom.
The maritime industry is currently going through a significant number of changes due to the introduction of tighter emission
regulations. A stronger awareness in preserving the environment has pushed forward more stringent IMO (International Maritime
Organization) legislation that imposes on ship owners and managers the use of new technologies which affect the day by day running
of the vessel, starting with the choice of fuel, through changes in the engine operational parameters, and culminating in a severe
reduction in allowable exhaust emissions. These changes combined with a volatile fuel market, high competition in cargo rates, the
pressure to reduce operating costs and the introduction of new technological advancements have brought the industry into uncharted
operational territories, abandoning the ‘comfort zone’ that has been enjoyed in the last twenty years or so. The present changeable
environment has a significant impact in the way two-stroke, slow speed, diesel engines are managed, introducing new challenges for
different fuel types, different lubricants and ancillary equipment required to meet the new requirements. Field experience has shown
that all these factors can lead to unintended consequences, including engine damage caused by poor fuel quality, lack of
training/knowledge of the operators, incorrect lubrication choice and poor set up. This paper discusses how the combination of
offline and online condition monitoring techniques, both on-board and on-shore, can be successfully used to prevent engine damage
and avoid unplanned maintenance costs due to downtime.
INTRODUCTION
Two-stroke, slow speed, diesel engines are used in the marine
industry to power the largest commercial ships currently sailing
on the seas. These internal combustion engines are used for
their high thermal efficiency (up to 60% 1
), exceptional
reliability, and ability to use a variety of fuel types including
residual oils. These fuels are, broadly speaking, the very end-
product of the crude oil refining process and are commonly
referred to in the marine industry as HFO (Heavy Fuel Oil) or
Residual Fuel Oil (RFO). These fuels are regarded to be the
most cost effective available (around 350 USD per ton in July
2015) and for this reason are the preferred choice to power main
engines and generators in large, ocean going, vessels. HFO
comes in several grades, broadly classified by high or low
Sulphur content: the best and most expensive grades of HFO
have the lowest Sulphur content (less than 0.1% by mass).
Choosing HFO however presents challenges in dealing with the
varying Sulphur contents in available fuels (from 0.1 to 4.5%2
),
the high viscosity (up to 700 cSt3
at 50°C), significant water
content (0.5%) and the (potential) presence of abrasive
aluminum silicate compounds. These latter materials are
carried over from catalytic cracking during the crude oil
refining process and are referred to as catalytic fines or simply,
‘cat-fines’. The International Standards Organization has
published a specification for marine HFO (ISO 8217:2010)
which imposes upper limits on these, and other, fuel parameters
to provide consistency in the market. Nevertheless, bunkered
HFO, even when conforming to these specifications, requires
further onboard processing to reduce the water and solids
contents to levels deemed acceptable for engine operation. This
purification process is achieved by centrifugal separation and
the treated fuel is stored in several day-tanks to provide a
1
MAN Diesel & Turbo:
http://powerplants.man.eu/docs/librariesprovider7/technical-
papers/two-stroke-low-speed-diesel-engines-for-independent-
power-producers-and-captive-plants.pdf?sfvrsn=10
2
Note that since Jan 2012, the maximum allowable sulphur
content for open ocean sailing is 3.5% without exhaust
emission after treatment.
continuous supply to the engine. Any breakdown or
malfunction in the purification process can lead to fuel quality
outside of the engine manufacturer’s specification, potentially
resulting in severe damage.
Two-stroke, slow speed, diesel engines have two independent
lubrication systems; the crankshaft and other rotating parts
within the crankcase are isolated from the combustion chamber
and are lubricated separately from the cylinder liner / piston
ring interface which is served by a second, single-shot,
arrangement. This design choice is driven by the need to
prevent high levels of contaminants and HFO combustion by-
products from entering the crankcase oil. A single lubrication
system, similar to conventional internal combustion engine
layouts, would require complex, expensive and very difficult to
manage filtration in order to maintain oil cleanliness. Moreover,
the use of two separate systems allows different oil
formulations appropriate for the individual applications.
The main, crankcase, lubrication system is of a closed loop
design with external filtration and purification that ensures very
long service life. The lubricant used has a fairly low base
number4
(typically below 10 BN) as there is no requirement to
deal with any acidic combustion by-products. System oil
volumes are in the tens of thousands of liters range and this oil
is very rarely changed out – topping up or partial refreshing is
the most common form of service maintenance.
Conversely, cylinder liner lubrication is designed as an open
loop system using a high BN oil (from 25 to 110 BN) in order
to neutralize the acidic combustion by-products from fuels with
high Sulphur contents. According to the engine design,
3
cSt or centistokes, a unit of kinematic viscosity, equivalent
to the SI unit of mm2
/s.
4
Base Number is a measure of a reserve alkalinity of a
lubricant expressed expressed in terms of the equivalent
number of milligrams of potassium hydroxide required to
neutralize all basic constituents present in 1 gram of oil
sample (mg KOH/g).
2. specifications and operational conditions, this lubricant is
injected directly onto the piston rings and/or liner interface at a
certain cycle frequency and with a particular ‘feed-rate’5
. The
cylinder oil is then evenly distributed over the entire liner in
order to lubricate and cool down the two sliding surfaces
(piston rings and liner). The exhausted cylinder lubricant, also
called ‘scrapedown oil’ due to the scraping action of the rings
on the cylinder liner, is then collected in the scavenge drain area
and flows through dedicated piping to a collection tank for
subsequent disposal.
The scrapedown oil is normally analyzed onboard, using test
kits and instruments, as well as on-shore by sending the oil
samples to specialized laboratories. The parameters monitored
are used to assess the engine condition and the effectiveness of
the lubricant. Typical parameters measured include remaining
base number, metal content as elemental analysis, soot content,
viscosity etc.
In the marine industry, environmental regulation is on the
increase. With the recent introduction by the International
Maritime Organization (IMO) of the 0.1% sulphur limit6
in
fuels on vessel navigating in Emission Controlled Areas
(ECAs), operators are facing new challenges that threaten their
cash-tight budgets. The Vessel General Permit (VGP)
regulation is another example of environmental legislation that
impacts the marine market, affecting most vessels operating
within three nautical miles of the coast of North America. This
regulation requires the use of more costly7
environmentally
acceptable lubricants (EALs) in vessels operating in the
exclusion area, again affecting the running costs of the vessels.
In vessels spending a considerable portion of time in ECAs and
areas covered by the VGP, these regulations present numerous
challenges to operators. But it is not just the added cost of more
expensive alternative fuels or lubricants that can impact
operators, critically it is also the effect that these changes have
on the operating conditions of the vessel, leading to unexpected
damage and causing unplanned downtime.
With such stringent and widespread regulations, compliance to
the rules becomes even more challenging. New operating
methods and procedures for fuel changeover, oils and
equipment required for compliance can indeed lead to
unintended consequences such as damage caused by out-of-
specification fuel or incorrect/insufficient cylinder lubrication.
Moreover, the fuel saving technique of slow steaming can
introduce new technical issues such as ‘acid wear’ of the liner
or piston rings due to the combined effect of low operational
temperatures, inadequate lubricant choice leading to sulphuric
acid build-up on the exposed parts of the combustion chamber
(also referred to as ‘cold corrosion’). With the scope and rigor
of regulations only increasing, compliance solutions should be
a consideration from the outset and even at the earliest stages
of vessel design in order to effectively manage costs. Amidst
the omnipresent drive for safety and operational efficiency,
effective condition monitoring tools and techniques have never
been more valuable in helping operators manage, avoid or
mitigate these costly issues.
The use of on-shore laboratories for in-depth scrapedown oil
analysis is not new. Samples have been collected and sent for
analysis for many years as recommended by the engine
builders. Specific actions adopted in response to the results are
most often left to the vessel’s crew, drawing on the chief
engineer’s experience and discretion. Although providing the
most in-depth results, shore based laboratory analyses suffer
from significant reporting delays, since the samples can only be
sent to the laboratory during port visits. In the time elapsed
from initial sampling to reporting, considerable damage could
have been sustained by the engine if the operating parameters
were out of specification. The use of on-board testing, either
offline or online, can alert the crew to any critical issues by
trending the condition of the spent lubricant. Quick action can
be taken to prevent damage whilst more comprehensive results,
from on-shore laboratory analysis, can be subsequently used for
confirmation and fine tuning of operational parameters.
Solutions come in many shapes and sizes, from simple, two-
minute hand held test kits to state-of-the-art online sensor
technology. This paper will demonstrate that a combination of
these tools can deliver real savings by; (i) preventing
accelerated wear in liners, piston rings and pistons, (ii) reducing
lubricant costs by optimising feed rates, (iii) avoiding
catastrophic engine damage, and (iv) enabling proactive
maintenance scheduling and eliminating costly, unexpected,
downtime.
ON-BOARD TESTING OF SCRAPEDOWN OIL
– COMPARISON WITH LABORATORY
METHODS.
Routine on-shore oil analysis has been carried out on used
scrapedown oil samples from a 6 cylinder two-stroke, slow
speed, diesel engine powering a bulk carrier cargo vessel.
The results from the samples sent to laboratory are presented in
Table 1.
Table 1 - Laboratory oil analysis results
5
Expressed as weight/energy ratio (g/kWh) often set
automatically by systems like the MAN Alpha Lubricator or
the Hans Jensen Lubtronic SIP
6
For example compliance to the EU Sulphur directive
2005/33/EC
7
As compared to similar mineral based oils
3. In the elemental analysis section, high levels of iron can be seen
across all 6 cylinders. These results were obtained from
Inductively Coupled Plasma (ICP) Spectroscopy, which is by
far the most commonly8
utilised technique by laboratories for
quantifying elemental composition.
The ICP method is based on atomisation of the oil sample in a
plasma and subsequent observation of the spectrum of emitted
light by each element. Different wavelengths of light are
produced by different elements, and the intensity of the emitted
light is proportional to the concentration of the particular
element. In this way, the proportion of various metals and some
other elements can be quantified. Figure 1 shows a schematic
of a typical instrument.
8
Another technique used for scrapedown oil analysis is XRFs
(X-Ray Fluorescence Spectroscopy), however the high cost
per sample, footprint of the machinery and the fact that uses a
Figure 1 - Principal of Inductively Coupled Plasma (ICP)
analysis
source of x-rays makes this technique less usable as a routine
test.
4. However, a known 9
limitation of ICP spectroscopy is that
conventional instruments cannot accurately measure metallic
particles larger than 5 µm10
to 8 µm in size, as these are only
partially vaporised in the plasma. This leads to under reporting
by the instrument and significantly lower readings than the
actual levels present in the oil. This is particularly
disadvantageous for the iron levels associated with mechanical
wear events of the piston ring-liner surfaces since they often
consist of debris larger than 5 µm.
The same samples have also been analysed using an on-board
magnetometry based instrument (Figure 2) that is able to
quantify the ferrous11
mass contained in the scrapedown. This
instrument uses a measurement technique where the oil sample
is immersed in an alternating magnetic field generated by an
excitation coil, the presence of any ferrous material will cause
variations in the field strength which can be sensed by a second,
detection coil. Magnetometry based instruments are highly
sensitive and can resolve ferrous masses down to the mg/kg, or
parts per million (ppm) level. Furthermore, magnetometers do
not suffer from any debris size limitations such as ICP.
Discrepancies were observed between the readings received for
iron content from the laboratory (via ICP) and this on-board
ferrous wear meter. In order to further investigate these
discrepancies the oil samples were subjected to a number of
additional test measurements on other magnetometry based
instruments.
The measurement devices used for testing of the oil samples
and used for comparison here consisted of the following units:
1. The on-board results obtained using the model 1,
(referred to as field model 1)
2. Factory reference standard model 1 (referred to as
gold model 1)
3. Factory reference standard model 2 (referred to as
gold model 2). The model 2 is well regarded as a top
end ferrous wear instrument that has been produced
for approximately 8 years with proven field track
record and is used in the scrapedown oil analysis
program of a major oil supplier.
The samples were prepared by shaking the oil pots to ensure a
homogeneous mix, 5 millilitres of each oil sample was then
decanted into measuring test tubes suitable for use in the model
1 and model 2 instruments.
9
http://www.machinerylubrication.com/Read/1384/ferrous-
density
10
1 µm = 1 x 10-6
metres
Figure 2 - Ferrous wear meter (model 1) in operation.
Prior to making measurements, the samples were again shaken,
to ensure that there was no settling of any iron debris. Each
sample was measured three times in each instrument and
the average of the results is presented in Table 2. Little variance
was observed between the measurements of the three
instruments. The full results can be found in Appendix A.
Table 1 — Laboratory oil sample results
Cylinder
Laboratory
(ICP)
Field
model 1
Gold
model 1
Gold
model 2
1 830 1848.3 1901.3 1799
2 536 543.3 464 432.7
3 595 446.7 477 418
4 752 965 858 759.6
5 809 836.7 814 740.3
6 373 310 290.3 275.7
These results are presented graphically in Figure 3.
Figure 3 - Graphical representation of results.
11
The meaning of ferrous iron here is that of iron in an
oxidation state of 0, in other words metallic iron which
exhibits ferromagnetism.
5. It can be seen that there is a good correlation between the
laboratory results and the re-tested values for measured oil
samples for cylinders 2, 5 and 6. It is further noticeable that
there is a large discrepancy in the results for cylinder 1 and a
smaller discrepancy for cylinders 3 and 4.
However, the magnetometry results only give an indication of
the ferrous iron content of the oil samples, iron in any other
oxidation state – e.g. Iron (II), so-called ferrous, or Iron (III),
so-called ferric, compounds such as ferric oxide (rust) or
ferrous sulphate (an acid corrosion by-product) will not be
measured in these type of instruments.
The scrapedown oil samples were then cross-tested using a
chemical kit designed to quantify the amount of corroded iron
present in the oil due to the cold corrosion phenomenon. This
test kit is referred as cold corrosion test kit (CCTK). As
indicated previously, the magnetometry based instruments are
designed to only measure the iron contained in the sample in a
metallic (ferromagnetic) form, also denoted in chemistry as
Iron (0). The CCTK is designed to measure iron compounds
(Iron (II) and Iron (III)) that can be found in a scrapedown oil
sample subject to the cold corrosion phenomenon. This test
gives an indication of the ‘non-metallic’ iron content in the oil
samples using a measurement technique based on
colourimetry12
. Any Iron (II) and Iron (III) compounds present
are first extracted from the oil by adding a combined reagent to
the sample that initiates a separation into two phases with an
aqueous phase containing these Iron compounds forming below
an oil phase (see Figures in Appendix B). The reagent further
binds the Iron (II) and Iron (III) to a dye and leads to a
colouration of the aqueous layer (see for reference appendix B).
Measuring the intensity of the separated phase colour13
using a
simple visual comparator with a calibrated scale will give a
quantitative measure of the Iron (II) and Iron (III) compounds
present in the oil.
The sum of the CCTK results (Table 3) and those from the
magnetometer tests represents the “total” Iron content and can
be compared directly to the laboratory data (Table 4 and Figure
5). However, by evaluating the individual Iron contents in these
two separate tests, it is possible to distinguish between Iron
resulting from mechanical wear or from corrosion. This is
valuable information that the ship’s crew can use to adjust the
engine operating parameters accordingly. Total Iron content
alone would not give this level of detail.
12
Colourimetry: measurement of the wavelength and the
intensity of electromagnetic radiation in the visible region of
the spectrum. It is used extensively for identification and
Figure 4 - Cold corrosion test kit (CCTK)
Table 2 – Iron (II) and Iron (III) compounds by CCTK
Cylinder CCTK (PPM)
1 75
2 80
3 70
4 80
5 180
6 70
The measurement samples from these tests are pictured in
Appendix B.
Table 3 - Combined Iron (0) and IRON (II) / IRON (III)
measurements
Cyl-
inder
Laboratory
measure-
ment (ICP)
Field
model 1
+ CCTK
Gold
model 1
+ CCTK
Gold
model 2
+ CCTK
1 830 1923.3 1976.3 1874
2 536 623.3 544 512.7
3 595 516.7 547 488
4 752 1045 938 839.6
5 809 1016.7 994 920.3
6 373 380 360.3 345.7
determination of concentrations of substances that absorb
light. Source http://www.britannica.com/science/colorimetry
13
The sample is compared to a blank (0 ppm) sample used as
colour baseline.
6. Figure 5 - Graphical representation of combined results14
In Figure 5 a much closer correlation can be seen here between
the laboratory results and those obtained using on-board
measurement techniques for cylinders 2, 3 and 6, indicating that
a portion of the iron in the sample is in corroded form (Iron (II)
and Iron (III) compounds).
However, there are still discrepancies in the results obtained on
cylinders 1, 4 and 5, with the combined field results indicating
a higher amount of Iron than that obtained from the laboratory
results. This is indicative of the samples containing larger
particles of metallic iron (0) and thus not being fully quantified
during the laboratory ICP measurement. Further microscopic
analysis via analytic ferrography15
confirmed the presence of
larger metallic particles in these samples.
The results obtained between the three magnetometry based
instruments combined with the cold corrosion test kit show a
good consistency across all samples. Small variations are
expected due to the nature of the instruments and the fact that
the measurements were conducted months apart from each
other. It was also found, that consistent sample preparation is
important to obtain accurate, repeatable results.
With the possible exception of cylinder 6, all samples indicate
high total Iron ((0) + (II) + (III)) levels. This is consistent in
both the field instrument and the laboratory results, providing
confidence that a combination of the field instrument and
CCTK test is indeed suitable for field measurement indication
of liner wear levels, comparable with those obtained by
laboratory analysis.
Both the on-board test results and the laboratory data indicate
that cylinder 1 is experiencing significant wear. Increased
14
Please note the change in scale of the Y axis compared to
figure 3.
15
This is a laboratory technique that provides microscopic
examination and analysis of debris found in lubricant.
These particles consist of metallic and non-metallic solid
substances. The metallic particle is normally associated to a
wear condition that separates different size and shapes. A non-
metallic particle is normally associated to contaminant ingress
like dirt, sand or corroded metallic particle.
16
Scuffing is a wear mechanism due to a breakdown in the
liner-piston ring lubrication which results in localised welding
levels of viscosity are also reported in the laboratory results, but
the very high iron levels measured by the on-board instrument
model 1 and CCTK give rise to serious concerns that a severe
wear process such as scuffing16
has occurred. Due to the larger
size of these wear particles, they are not fully quantified in the
laboratory ICP analysis, however cross testing the sample with
different test methods shows the wear events more evidently
and allows the ship’s chief engineer to take prompt corrective
action.
ONLINE MEASUREMENTS OF SCRAPEDOWN
OIL TO DETECT IN REAL TIME WEAR
EVENTS.
On-line ferrous wear sensors are also another tool available to
ship owners and operators in order to monitor in real time
mechanical wear17
liner conditions on a 24/7 basis. One such
tool is the base for the system from which data will be shown
in this section. Based on similar magnetometry based
measurement technology used in both laboratory instruments
and on-board, offline devices as described previously, the
system uses sensors installed on the drain pipe that samples
continuously the used scrapedown oil during the engine
operation. Such drains can be found on each cylinder in a two-
stroke, slow speed, diesel engine below the scavenge box. By
placing a single sensor on each cylinder drain, any issues
pertaining the single individual units can be observed, allowing
corrective action to be taken immediately whenever required.
This online liner wear monitoring system, referred in this paper
as system 1 was initially developed to allow operators to
optimise cylinder oil feed-rates by controlling in real time the
wear level. Feed-rates could be safely lowered monitoring the
liner wear iron measurements hence reducing the operational
costs of the cylinder lube and effectively lubricate the
combustion chamber parts. However, over the years the system
has also been shown to give indications of other liner
mechanical wear issues, such as scuffing incidents, adhesive
wear and abrasive wear due to off specification fuel such as
high cat-fines contents18
.
With online monitoring, instant warnings can be obtained to
highlight any issues as they progress. The results below in
Figure 6 are presented from a 713 ft, 27,100 LT Max
Deadweight container vessel. In this instance, these results
were obtained three days after the initial fitting of the System1.
The X axis indicates engine running hours, the Y axis the
amount of metallic iron (in ppm) in the individual cylinder
scrapedown oils, cylinders 1 through 8.
between points on the rings and liner surface with subsequent
tearing of microscopic particles. This is regarded to be a very
severe and a fast propagating form of wear.
17
Normally associated to incorrect liner lubrication set up or
due to the presence of contaminants in the fuel that disrupt the
oil film.
18
Max allowed content of cat fines in the post treated fuel is
15 ppm, however the bunkered fuel can contain up to 80 ppm
of these silica-aluminium based compounds.
7. Figure 6 - Output from System1 showing increased levels of
wear.
At point A on the graph, the vessel’s engine was started using
MGO19
. The sharp rise in wear is typically observed on engine
start up and rapidly falls as a film of cylinder oil is formed on
the internal surfaces of the liner and piston rings.
At Point B, the vessel engine was switched to HFO and, almost
instantaneously, the wear levels across all cylinders can be seen
to rise to significant levels (>450 PPM) and remain high for a
prolonged period of time, not falling rapidly as per at start up,
point C.
In order to verify the results observed, the operator switched the
vessels engine back to MGO in order to check if the recorded
wear event was due to the fuel changeover. To verify the results,
the operating loads applied to the engine were also kept
constant. Increases in wear can be seen during these
occurrences, but once switched over to MGO the same rapid
decay in wear back down to <50 PPM levels can be seen after
such event.
To ensure that the results previously observed were correct, the
vessel was then switched back to HFO at point D. The same
increased wear levels trends are the seen immediately as
previously in the graph linking the wear event to the use of the
HFO.
Further investigations on the vessel were carried out on the fuel
system and high levels of cat fines were found on one of the
day tanks, possibly due to a fuel oil purifier malfunction. It was
also found that the last chance filter elements on the fuel line
were of an incorrect specification (32 µm rather than the
specified 10 µm). These filters would eventually have
minimised the severity of the wear events in the case of off-
specification fuel was burned.
Once the fuel supply was changed over to another HFO day
tank the wear event recorded in the graph in point D rapidly
disappeared showing an acceptable liner wear rate. Following
the information provided by the real time data supplied by
system 1 the crew was also able to track down the incorrect
filter and replace it with a correctly specified one.
If the wear levels on the cylinders had not been monitored with
this online system, continued operation of the engine would
have caused a rapid deterioration of the liners across all 6
cylinders. The cost of replacement is typically around USD
$40000 per liner. This unexpected expenditure coupled with the
associated overhaul downtime was thus avoided and no off-hire
of the vessel occurred, saving the operator significant expenses,
inconvenience and loss of reputation.
CONCLUSIONS
In this paper, the results of both on-board off-line and on-line
analysis of scrapedown oil have been presented with field data
obtained from seagoing vessels. The data shown demonstrates
that the on-shore scrapedown oil analysis can be used in
conjunctions with on-board offline and online condition
monitoring solutions. The combination of these different
techniques shown to provide a great wealth of information,
allowing ship owners and operators to take immediate
corrective actions to mitigate any damage, to allow continued,
efficient operation and prevented the high costs associated to
undetected liner wear events. The savings associated with this
early warnings in both presented cases far outweighed the cost
of the condition monitoring tools used to detect them. The
prevention of liner damage to a 6 cylinder two stroke slow
speed diesel engine as a result of data provided by the System
1, just 3 days after it was installed, convinced the ship owner of
the high value of the provided data. The above shown case
actually convinced the shipping company management to roll
out the system across the remaining vessels of the fleet.
ACKNOWLEDGMENTS.
Dr. Steve Dye contributed heavily to the writing of this paper
and his work is acknowledged. We are indebted to Dr Stuart
Lunt for the help provided in proofreading this document.
19
MGO: Marine Gas Oil. Distillate fuel roughly equivalent to
a No. 2 fuel oil.
