from the Tribological Point of View *

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1 Current Design of Cylinder Liner and Piston Ring Package from the Tribological Point of View * Matthias Amoser ** Konrad Räss ** Piston-running behaviour remains one of the key determining factors in the reliability and time between overhauls for the diesel engine. This paper reports on some of development work with Wärtsilä large-bore, low-speed marine diesel engines. This has resulted in much improvement. The work led to better specifications for the cylinder liner surface morphology, standardisation on the chrome-ceramic coating for all piston rings, and improved distribution of cylinder lubricating oil on the surface of the cylinder liner. Shipboard testing dismissed the hypothesis that water droplets were reaching the cylinder liner surface. The overall results were confirmed by long-term trials on a fleet of ships. 1. Introduction Reliable piston running is still one of the key attributes of a diesel engine. The demand for improved component lifetime and further reduced cylinder lubricating oil consumption and thus reduced operational costs is high. During the past years several components of the Wärtsilä two-stroke diesel engines have been updated and newly laid out to fulfil this target. This paper gives an insight into recent development work and covers background information such as: Components that may influence piston running Part modification updates and reasons for them Results gained at different modification stages Latest specification Shipboard service experience Outlook To realise long-term and unchanged good behaviour, all parts involved in a tribological system have to conserve their properties over a long time. Every change in properties necessitates adaptation of the system to the new situation, in which additional transformation energy is used, similar to that of the initial running-in process. Furthermore, it is beneficial that right from the beginning the system shows similar properties to those of very long-term operation, resulting in the desired unchanged tribological condition over the full lifetime. On top of that, such a system needs only a short running-in time. This idea is standard for heavy-duty diesel engines and in four-stroke marine diesel engines and has been introduced for Wärtsilä (Sulzer) two-stroke diesel engines since the beginning of this century. [1] Several publications about this topic have been issued, such as [2], [5]. The present paper describes the latest modifications and improvements to the tribological specification. * 原稿受付平成 2 年 12 月 11 日. ** バルチラスイス Components having an influence on piston running: Table 1 shows the dominant factors influencing piston running. In this paper we concentrate on the running surface of the cylinder liner, piston rings, lubricating oil distribution, cylinder lubrication and scavenge air condensation. Table 1 Factors influencing piston running Design - Concept Water separation Ring package Liner temperature Production & Design Piston geometry Liner geometry Surface roughness Combustion Deposit formation Lubricating System System reliability Oil distribution Oil quality Operation Fuel oil Non-optimal maintenance Lubricating oil dosage Running-in 2. Cylinder Liner The specification for the cylinder liner has been improved to cope with the latest knowledge. The previous specification of deep honing had been defined to Journal of the JIME Vol. 44,No.2(29) 46

2 227 achieve a surface roughness condition which is as close as possible to the condition of well-run liners a long service period. But for a long time it has not been possible to actually measure the surface roughness non-destructively in deep detail. Thanks to the availability of new measurement methods, namely a new non-destructive and non-touching measurement with focused white light, the situation has changed. With this method the surface roughness can be determined in detail from replicas in a rubber compound of the liner surface at any stage of the lifetime of a liner. This measuring method allows a very high resolution in height down to ten nanometres on a height of three to four millimetres, and a larger area can be scanned with a high lateral resolution. The new specifications are: Rk: µm (Core roughness) Rpk: <.3 µm (Peak roughness) Rvk: µm (Valley roughness) Ra: < 1. µm (Average roughness) Fig.2 Different surface morphologies with similar Ra and Rmax values [3] Fig.1 Typical measurement of a replica from a new plateau-honed cylinder surface (Negative values show depths of honing marks on the original cylinder surface.) Measurements were made with this method on numerous liners in the as-machined condition, the shop test and various periods of time in service, including extended operating hours. From these measurements it became obvious that the specification at that time was not fully sufficient for describing the desirable surface condition of a cylinder liner of a two-stroke engine. From literature and from comparison with four-stroke engine specifications, it also became obvious that the surface roughness parameters given in the then valid specification were not sufficient to define satisfactorily a running surface. By providing only average roughness (Ra) and maximum roughness (Rmax) values, very different surface morphologies are possible with one set of values, which are still according to specification; compare the examples in Fig.2. Only the surface shown at the top of this figure represents a desirable running surface. Firstly this surface has very few peaks to contact the sliding counterparts. Secondly the surface includes some valleys to provide a degree of oil retention. The microscopic dimensions of the oil retention volume become more and more important for obtaining a limited disturbance of the oil film which has to cope with increasing mean effective pressures. The additionally roughness parameters Rpk and Rvk have been introduced to describe the amplitudes of the peaks and valleys respectively in the surface morphology. Figures 3 and 4 show that the surface morphology remains close to these initial surface parameters, even for longer running hours because of a short running-in process and a constant but low wear concept. With this concept the surface morphology will remain as it is over a long period of time. Rpk [µm] Rvk [µm] h 715 h 86 h A B C D E F RTA48T RTA84C RTA96C Fig.3 Rpk value of different cylinder liners from new to 86, running hours h 715 h 86 h A B C D E F 4854 h 4854 h RTA48T RTA84C RTA96C Fig.4 Rvk values of the cylinder liners quoted in Fig. 3 The cylinder liners selected for the above investigation were equipped at that time with top piston rings having a h h 1738 h 1738 h Journal of the JIME Vol. 44,No.2(29) 47

3 221 Current Design of Cylinder Liner and Piston Ring Package from the Tribological Point of View chrome-ceramic coating and lower rings with a running-in coating. The wear rates of this specification were at the same low level as with the today s specification where all piston rings have the chrome ceramic coating. We therefore state that these results are comparable. 3. Piston Rings The chrome-ceramic coated ring, which was introduced earlier, has already proved its very low wear capability. An additional benefit of the low wear rate is the stability in the surface shape, which changes only slowly during the lifetime of a piston ring. This led to the idea of using this coating also for the lower rings but with a symmetrical barrel shape. This shape has no scraping effect and leaves a good distribution of oil film on the cylinder liner. It has been observed that when the running-in coating of the lower rings is worn away they may start to disturb the lubricating oil film and consequently the top ring might run under severe conditions when heavily loaded. piston ring pack with all rings chrome-ceramic coated was confirmed in shipboard installations. Low specific diametrical wear rates below.5mm/1 hours and overhaul intervals of more than three years are achievable. Figures 8 and 9 give insights into the typical wear shapes (specific diametrical wear in mm/1 hours) produced by two types of piston ring coatings. Figure 8 shows the high wear of a plasma-coated top ring and Fig.9 the low wear of the top ring with the chrome-ceramic coating. 25% 2% 15% 1% 5% % Relative scuffing resistance [%] Cast Iron Spray Coating Chrome Ceramic Coating Fig.7 Relative scuffing resistance from pin-on-disc tests Fig.5 Profile and coating of top and lower piston rings according to the latest specification Figure 5 shows the shapes of the chrome ceramic piston rings according to the latest specification. The decision to apply the chrome-ceramic coating on the running surfaces of the piston rings was supported by laboratory tests that confirmed the low wear rates and high scuffing resistance under severe conditions. Figure 6 shows the principles of the pin-on-disc wear resistance test machine and figure 7 shows a comparison of the scuffing resistance to the uncoated cast-iron ring specimen. Fig.8 Cylinder liner wear profile for an example with a plasma-coated top ring under normal conditions with no scuffing (9775 running hours) Load Pin holder Spherical loading pin Coating material being tested Ø 8mm Pill shape pin Rotating disc Cross-pin to stop specimen rotation Fig.6 Pin-on-disc test arrangement for determining scuffing resistance The predicted low cylinder liner wear and stable hydrodynamic conditions for long-term running of the Fig.9 Cylinder liner wear profile for an example with a chrome-ceramic coated top ring Journal of the JIME Vol. 44,No.2(29) 41

4 229 The selection of the chrome-ceramic coating in combination with the calculated ring profile for all piston rings led also to a low ring wear. The predicted long-lasting hydrodynamic conditions for a long running period are confirmed on board ships. Figure 1 demonstrates the very low wear of chrome-ceramic coated top piston rings. Each line marks the thickness of chrome-ceramic layer of the top piston ring installed in individual cylinders. Once measured in new condition (left point) and a certain time in operation (right point). The increasing layer thickness measurement of cylinder No. 4 indicates that the measurements were taken at different ring areas (coating thickness tolerance) or involve a wrong measurement. Thickness / um M/V "L B B" Thickness of Top Ring Cr-Ceramics Coating (SCP1CC2) #1 Cyl. #2 Cyl. #3 Cyl. #4 Cyl. #5 Cyl. #6 Cyl. 5' 1' 15' 2' TRH / hrs Fig.1 Wear results for top piston rings with chrome-ceramic coating in an RTA96C engine over time, total running hours Running-in difficulties during shop tests have become practically zero since the new piston ring specification was released. In figure 11 it is clearly visible that the improvement of the oil film with the lower rings could be achieved, since the iron content resulting from mixed friction could be significantly reduced. Furthermore this figure illustrates the fast running-in properties of the selected specification. Iron content in PUS [ppm] previous execution 3 h (max load 75%) 13 h (max load 1%) latest execution Fig.11 Iron content in drain oil (PUS piston underside space) during engine test with different ring executions 4. Lubricating Oil Distribution The Pulse Lubricating System (PLS), which is today standard on Wärtsilä low-speed engines, is designed to deliver the smallest amount of lubricating oil, necessary to secure a stabile lubricating oil film on the running surface of the cylinder liner. Once the oil is delivered onto the cylinder liner running surface, the piston rings distribute the small oil droplets up and downwards. Service experience with some large-bore engines has revealed that the areas between the lubricating oil inlets show a different surface appearance than the areas where fresh oil is available in a higher concentration. This situation could be seen with the latest Pulse Lubricating System as well as the old standard CLU3, accumulator-type lubricating system. Stripes of high friction Fig.12 High friction marks on cylinder liner running surface Various lubricating oil distribution solutions were therefore developed and intensively tested on board ships. All these solutions were designed with the target of intensifying lubricating oil refreshment between the inlet areas on the cylinder liner and to avoid possible local high friction. The solution with zig-zag grooves was chosen based on good service experience with all available types of lubricating oil systems. Figure 13 shows a cylinder liner that was originally produced with only lubricating oil holes. The figure was taken 9 hours the inclined lubricating oil grooves were machined and our Retrofit Pulse Lubricating System (RPLS) was installed. The even oil distribution is well visible and earlier signs of higher friction had started to disappear. Fig.13 Cylinder liner with optimum lubricating oil distribution Journal of the JIME Vol. 44,No.2(29) 49

5 23 Current Design of Cylinder Liner and Piston Ring Package from the Tribological Point of View The introduction of the inclined zig-zag grooves, together with the Pulse Lubricating System, improved remarkably the tribological system. 5. Scavenge Air Condensation Experience has shown that the absence of water droplets in scavenge air is beneficial for achieving stable piston-running behaviour. A hypothesis has been put forward that condensate droplets enter the cylinder liner scavenge air ports and have a direct impact on the lubricating oil film. This hypothesis has been investigated in recent months with theoretical studies being validating with real data. The scavenge air path in an engine was equipped with sight glasses, and sensors for pressure, temperature, relative humidity and absolute humidity. The target was to analyse the scavenge air humidity from the scavenge air cooler towards the scavenge air ports and to confirm if water droplets entered the ports or not. The tests were made in high humidity areas, at low and high loads and in transient conditions. The test vessel was equipped with the older type scavenge air receiver with a vertical, single-piece cooler and the latest type of drain system with independent drain pipes, each having an orifice and a high-level alarm system, leading to a venting vessel for absolute safety against unwanted high volumes of condensate entering the engine. relative Humidity (%) After water separator Scavenge air relative humidity, steady state 12RTA96C After non return flaps Below cylinder liner In front of scavenge air ports Saturation line 25% 4% 5% 75% 9% Fig.14 Scavenge air relative humidity along the scavenge air path at different engine loads from 25 to 9% load Figure 14 shows one of the results of these tests. Relative humidity falls from more than 1% saturation the water separator to approximately 4% in front of the scavenge air ports. After the water separator, some droplets and fog were visible, whereas there was none at all in front of the scavenge air ports. We can state that during these tests no liquid water entered the cylinder liner combustion space and that it is very unlikely that this happens at all, provided the drain system and separation system are in good working order. The hypothesis that water droplets enter the combustion space cannot therefore be supported. Humidity enters combustion space in the form of vapour! With the water separator and drain system in good working order we mean: Water separator elements not bent, not heavily oil contaminated and without large gaps at the periphery that lead to water bypass Scavenge air coolers that do not leak and that have the hot air inlet side ( the turbocharger) sealed well to avoid the bypass of scavenge air preventing a proper drain Open drain lines, orifices not blocked Fully open venting system Perfectly vented drain tanks to avoid any backpressure. 6. Piston-Running Specification The today s tribological specification comprises: Cylinder liner with plateau.honed surface and sufficient hard phase content Pulse Lubricating System Zig-zag lubricating oil distribution grooves Pre-profiled piston rings with chrome-ceramic coating for all rings Anti-Polishing Ring at top of cylinder liner Optimised cylinder liner temperature distribution Effective underslung condensed water separation system with effective drain Today s specification reflects the findings from an intensive investigation programme, and service experience with it is good. The specification is, of course, adapted to new requirements from shipowners but also with respect to exhaust emissions regulations. 7. Shipboard Service Results The results of our development work can be seen in the improved performance achieved in a large number of vessels with various executions of RT-flex96C engines in the fleet of a major North European shipowner. We selected the engines that were delivered with the full chrome-ceramic coated piston ring pack. The types of cylinder liners are various. There are liners that had the old CLU3 lubricating oil system with various types of lubricating oil distribution grooves: horizontal grooves, with no grooves and with zig-zag grooves. The engines with the Pulse Lubricating System are divided between those having liners with no oil distribution grooves and those having liners with zig-zag grooves. The results from this fleet are summarised in figure 15, with a time progression of the development from left to right. It confirms that the current piston-running specification results in considerably improved piston-running behaviour of Wärtsilä large-bore low-speed engines. Figure 15 shows clearly that lubricating oil distribution with grooves has a very dominant effect on reliability. The cylinders with grooves show a low rate of damage compared with the cylinders without grooves. Wear figures are as received and within our tolerance. The few losses can be addressed to influences that are not related to liner or ring design. Journal of the JIME Vol. 44,No.2(29) 5

6 RT-flex96C, performance of one specific fleet Acknowledgements The authors would like to express their thanks to Wärtsilä Corporation for permission to publish this paper and to their colleagues for their support in the theoretical and experimental development work reported. Accumulated cylinder hours mm/1 Hrs, Scuffing / 1 Hrs References [1] R. Demmerle et al., New Insights into the Piston Running Behaviour of Sulzer Large Bore Diesel Engines, CIMAC Conference 21. [2] M. Scherge et al., Fundamental Wear Mechanism of Metals, Wear 255, 23. [3] L. Mummery, Rauheitsmessung Theorie und Praxis, Hommelwerke Gmbh, [4] R. Holtbeckers et al., Taking the Next Steps in Emission Reduction for Large Two Stroke Engines, CIMAC Conference 27, paper 165. [5] K. Räss, et al, Progressive development of two-stroke engine tribology, CIMAC Conference 27, paper 83. CLU3, horizontal l.o.g, full cc ring pack CLU3, no l.o.g., full cc ring pack CLU3, zig zag l.o.g., full cc ring pack Pulse lube, no grooves, full cc ring pack Pulse lube, zig zag l.o.g., full cc ring pack Accumulated cylinder hrs Authors Liner max Wear mm/1 Hrs Scuffing incidents/1 Hrs Fig.15 Progress in piston-running behaviour with various configurations of cylinder liners and piston rings, showing accumulated cylinder running hours, maximum liner wear rate (mm/1 hours) and scuffing incidents per 1 running hours. The configurations involved lubricating oil grooves (l.o.g.) and cylinder lubricating system (CLU3 or Pulse Lubricating System). All piston rings were chrome-ceramic coated (cc). 8. Conclusions There have been important improvements in the tribological performance of Wärtsilä large-bore two-stroke engines. The tribological properties of the parts involved remain, by and large, unchanged for a very long period of time. This includes the morphology of the liner surface as well as the coating of the piston rings which can be left untouched as long as the coating is not worn away. The new Pulse Lubricating System offers, together with the enhanced oil distribution method, a reliable system for cylinder lubrication at low lubricating oil feed rates. The hypothesis that condensate water droplets enter the combustion space could not be seen during any tests. This confirms the efficiency of today s scavenge air treatment system that allows a safe operation at lowest scavenge air temperatures. All this contributes to reliable, economical and efficient operation at low fuel and lubricating oil consumptions. The service results of the engines having the described piston-running specification are good. Wear rates are low and scuffing seldom occurs. Dr. sc.techn. Matthias Amoser Born in 1959 Died November 28 Wärtsilä Switzerland Ltd ETH Zürich Tribology and Piston Running Dipl. Ing. FH. Konrad Räss Born in 1966 Wärtsilä Switzerland Ltd ZHAW Winterthur Tribology and Piston Running Journal of the JIME Vol. 44,No.2(29) 51