Development of Piston Ring Packs and Cylinder Liners

Development of Piston Ring Packs and Cylinder Liners -Viewpoint of Tribology

― 52 ―Journal of the JIME　Vol. 44， No.２（2009）日本マリンエンジニアリング学会誌　第44巻　第２号（2009）

Development of Piston Ring Packs and Cylinder Liners

-Viewpoint of Tribology

Henrik Rolsted　　Jesper Weis Fogh

Journal of the JIME Vol. 44 No． 3 (2009) －1－日本マリンエンジニアリング学会誌第 44 巻第 3 号 (2009)

1. Introduction

The present design of piston ring packs and cylinder liners represents a development driven by environmental regulations, reliability issues and operating costs of our two-stroke engines. The use of low-sulphur fuels, now a reality in several SECA areas, an increasing demand from shipowners to be able to operate the engines without major overhauls between dockings and the cost of lube oil are some of the elements which have led to the present design for cylinder condition. This development started in the mid-1990ies, resulting in the introduction of a new design in year 2000. This design incorporates: The top piston ring design is CPR (Controlled Pressure Relief) The piston crown is of the Oros design with High TopLand (HTL). The new design has a significantly higher topland compared with the previous design, see Fig. 6.

The liner surface is of the semi-honed type, Fig. 8. This ensures a good combination of oil retaining grooves and sufficient area in relation to the radial load. The PC ring provides safety against excessive deposit build-up on the topland, ensuring that topland deposits cannot get in contact with the liner. Development in the area of lubrication has led to a new lubricator, the Alpha Lubricator. As a result of research, it has been possible to lower the feed rate significantly over the past 5 years.

2. Combustion chamber

The combustion chamber for our medium and large bore engines is shown in Fig. 1. To cope with the increasing temperatures and pressures, the use of low-sulphur fuels and development of the combustion chamber parts has led to the present design.

Development of Piston Ring Packs and Cylinder Liners

- Viewpoint of Tribology

Henrik Rolsted Jesper Weis Fogh

The present design of piston ring packs and cylinder liners represents a development driven by environmental regulations, reliability and operating costs of two-stroke engines. The use of low-sulphur fuels, now a reality in several SECA (Sulphur Emission Control Area), an increasing demand from shipowners to be able to operate the engines without major overhauls between dockings and the cost of lube oil are some of the elements which have led to the present design for cylinder condition. This development started in the mid-1990s, resulting in the introduction of a new design in year 2000. Development in the area of lubrication has led to a new lubricator, the Alpha Lubricator. As a result of research, it has been possible to lower the feed rate significantly over the past 5 years.



233Development of Piston Ring Packs and Cylinder Liners as seen from the Viewpoint of Tribology


Fig.1 Combustion Chamber for

MAN B&W Engines

A detailed insight in the different fields will be given in the following.

3. Piston Rings The piston ring design has been developed from a simple oblique-cut piston ring into an advanced design with a gas-tight lock and controlled leakage grooves. This has led to a strong reduction of the maximum temperatures experienced by the rings. Thus the risk of ring collapse or ring breakage due to thermal overload has been eliminated. The mechanical load of the piston ring is determined by the forces acting on the ring. One key factor in this regard is the differential pressure over the rings. The ring pack seals against the combustion pressure. In order to share the load between the rings, a certain gas flow is required. With regard

to the oblique cut ring, this gas flow results in a relatively high thermal load around the ring gap, ref. Fig. 2. This has various negative effects:

a. Lube oil in the position of the ring gap experiences a high thermal input, resulting in a lack of oil locally on the liner surface.

b. High surface pressure between the liner and the ring at the ring gap due to uneven thermal expansion of the ring.

c. Gas jet from the top ring gap leads to local thermal overload of ring No. 2.

The local effect of the above is strongly reduced by the CPR ring design, where the ring gap is gas tight and the gas flow is divided into 4 or 6 CL grooves, ref Fig. 3 and 4.

Fig. 2 Thermal load with open ring gap

Fig. 3 Thermal load with gas tight gap

Fig.4 CPR piston ring, with gas tight lock and 4 CL-grooves (Two CL-grooves symmetrical around the lock)



234 Development of Piston Ring Packs and Cylinder Liners as seen from the Viewpoint of Tribology


Fig. 5 Latest Ring Pack Design The running-in properties were strongly improved with the implementation of a copper-based running-in layer, enhanced with aluminium oxides. The brand name is Alu-coat. In 2003, chrome-plated ring undersides on rings 1 and 2 were introduced. This finally concluded the ring groove wear issue. Up to 2003, we had experienced a number of cases with too short time between overhauls, due to the ring groove wear. The latest update of the ring pack relates to the running surface, which has been upgraded by applying a cermet coating. A cermet is a composite material consisting of ceramic and metal. This gives the optimal properties of ceramic, such as high temperature resistance and hardness, combined with the metal properties, such as the ability to undergo plastic deformation. The cermet used consists of chromium carbides as the ceramic part, and molybdenum, nickel and chrome as the metallic part. The high melting temperature and the high hardness of the chromium carbides leads to an increased margin against scuffing, and low wear of the coating. The low wear of the coating ensures long time between overhaul, Ref. 1. The above development of the ring pack design is illustrated in Fig. 5.

For marine engines, this ring pack is recommended for 80, 90 and 98 bore of the latest design. This ring pack is recommended for all engine types when operating on LSF and ULSF (low-sulphur fuel and ultra low sulphur fuel).

4. Piston Crown In order to reduce the corrosive wear and thermal load of the ring pack, the rings have been moved down as shown in Fig. 6. This new design is designated High TopLand (HTL) Service tests clearly confirm that the lower position of the rings lead to lower thermal load, as the rings did not lose their pre-tension over time, after moving down the rings. The liner temperature was lowered slightly after the introduction of the high topland. Service tests confirmed our expectations regarding a reduced level of acid condensation with HTL. The low level of acid condensation is caused by the lower pressure in the combustion chamber at the time when the liner surface is exposed to the exhaust gases. As can be seen from Fig. 6, using the K90MC as an example, the cylinder pressure is 118 bar when the liner running surface is initially exposed to the combustion gases in case of LTL. Regarding HTL, this pressure is reduced to 88 bar. Part of the condensation rate is governed by the partial pressure of





water vapour and SOx. The reduction of these partial pressures is proportional to the cylinder pressure reduction, thus the remarkable difference in acid condensation in the HTL design. High TopLand Low TopLand

Fig. 6 Oros Design Previous Design The level of acid condensation between LTL and HTL can be calculated and is illustrated relative to LTL, as shown in Fig. 7. (Ref. 2).

Fig. 7 Theoretical relative difference in acid condensation This made it possible to lower the cylinder oil feed rate as the requirement for acid neutralisation additives was reduced considerably and, at the same time, the liner wear rate could be reduced.

5. Liner The liner material is grey cast iron with flake graphite in a lamellar pearlite matrix. In addition to a hard phase, mainly consisting of cementite and steadite, this material ensures good tribological properties, with good thermal conductivity and good wear properties. The latest development on the liner has been with focus on the running-in properties. Experiments have led to a semi-honed surface as shown in Fig. 8.

Fig. 8 Liner Surface The semi-honed surface ensures good oil retaining properties, as oil is accumulated between the plateaus. The plateau itself is sufficient to ensure the formation of a hydrodynamic oil film. The Piston Cleaning ring (PC-ring) is an integrated part of the liner, Fig. 1 and 9. This ring has solved the problems caused by excessive deposits on the piston crown topland. Such excessive deposits are very harmful to the wear condition of the piston ring and liner, as the contact between the topland deposits and the liner surface lead to bore polish, resulting in poor oil retaining properties. The PC ring diameter is smaller than the liner diameter. In case of excessive deposits, this leads to scraping of the topland every time the piston is travelling around TDC. In this way, contact between the topland deposits and the liner running surface is avoided.





Fig. 9 PC-ring

6. Cylinder lubrication

The “cylinder condition”, i.e. the wear of liners and rings, the gas sealing of the piston rings, the time between overhauls, etc. will always be an interaction between the design of the combustion chamber, the load pattern of the engine, the fuel being used and the cylinder oil applied. Our policy is that excessive loading of the engine is prevented by limiters integrated in the engine control system. Thereby the loading of the engine is part of the engine design too. The fuel oil is controlled by specifications, however having a relatively wide range. Finally, the cylinder lubrication becomes the decisive control of the cylinder condition. This final adoption of cylinder oil to the actual need for one particular engine is, and always has been, the most difficult in the operation of two-stroke engines with forced cylinder lubrication. What complicates this process further is the fact that the result of a faulty lubrication mode is often not realised until months after the wrong adjustment has been introduced. A further complication is the widespread belief that too much oil cannot harm the cylinder condition. That is not the case! Cylinder lubrication guidelines must rely on service experience from a higher number of similar engines being operated under various conditions. When we started getting service experience from the new generation of engines with high topland pistons, shortly after the year 2000, it was confirmed that the corrosive wear level of liners and piston rings was considerably lower than before. We have learned from former engine types that the need for cylinder oil preferably should follow the sulphur content in

the fuel being burned proportionally. By introducing the so-called ACC cylinder lubrication mode, we could ensure enough alkalinity to neutralise the sulphuric acid at high sulphur contents, and decrease the amount of oil at lower sulphur contents. Thereby, a considerable overall saving of cylinder oil was obtained. The first sulphur proportional factor was set at 0.34 g/kWh x S% with an absolute minimum at 0.7g/kWh. When this algorithm was used on the new engine generation in connection with the introduction of the new Alpha Cylinder Lube system, the result was initially very satisfactory. Very low wear rates and a very low cylinder oil consumption compared to earlier experience. However, new tests revealed that both the feed rate factor and the minimum figure could be reduced further. Today, after testing many steps, we have approved a feed rate factor of 0.20 g/kWh and a minimum of 0.6 g/kWh while still keeping very low wear rates of both liners and rings. Thanks to the new combustion chamber, our new piston ring packs, the PC ring and the slide fuel valve, we are able to keep the very low wear rates of liners and rings and, at the same time, to lower the cylinder oil dosages to very low levels. This condition has made it possible to increase the time between overhauls significantly, previously set at 12-16,000 running hours and now at 32,000 running hours as a possibility, when condition-based overhaul is performed, Ref. 1.

7. Abnormal Wear A “healthy” liner surface is necessary to maintain a good cylinder condition. A healthy liner surface means a running surface with good oil-retaining properties. From new, this is ensured by the “oil-pockets” formed by the wave-cut/semi-honing pattern. As these will naturally be worn away after a certain running time, the oil-retaining ability now depends on the cast iron itself having an open structure, i.e. that the graphite lamellas are not closed by smearing. This is ensured by having a certain degree of acid corrosion taking place continuously, which will be fulfilled following the ACC factor 0.20 g/kWh x S%.





Fig.10 Example of ACC factor optimisation test

Fig.11 ACC Guidelines





Bore polish is often the result of alkali over-additivation or simply over-lubrication. If more additives are applied than needed in relation to the sulphur, the additives have a tendency to accumulate on the piston topland leading to mechanical bore polish, and excess additives will suppress the corrosion completely leading to “chemical” bore polish. Both are seen leading to scuffing of rings and liners.

8. Low sulphur fuel operation Lubricating proportional to the sulphur at the factor 0.34 g/kWh x S% on the older low topland engines and 0.20 g/kWh x S% on our newer high topland engines will give more or less the ideal match between sulphur and neutralising alkali additives. However, at lower sulphur percentiles, below the breakpoint where the minimum dosages at 0.6 g/kWh must be followed instead, an excess alkalinity will be the result with the risk of getting bore-polish problems. Lower BN cylinder oils are needed for that reason. For today’s SECA zones where 1.5% S is maximum, BN40 oil is close to the ideal match. However, for the coming new SECA and CARB rules, where the sulphur will be limited to a maximum of 0.5% and lower, we need lower BN cylinder oils, BN20 or BN12, with today’s high detergency level.

9. Concluding Remarks The development of piston rings, cylinder liners and piston crowns, in connection with the Alpha Lubricator and the ACC algorithm, has lowered the cylinder lubricating oil costs significantly in the past five years. At the same time, long time between overhauls has become possible, applying the condition-based overhaul recommendation. Furthermore, the latest design of ring packs and cylinder liners, is well prepared to meet today’s requirements of international emission regulations in respect to low sulphur operation. Lube oils, cylinder liners and piston rings need to be further developed to meet the requirements of the coming SECA and CARB rules. This development is in the direction of low BN cylinder oils and a ring pack with hard coating on all four rings.

References [1] Service letter SL07-483/HRR, August 2007 [2] VDI Forshungsheft, Nr. 626/1984. Christoffer Teetz; Beitrag zur verminderung der Nasskorrsion im Dieselmotor

Authors •Rolsted Henrik (1) •Born in 1949 •MAN Diesel •graduated from Technical University of Denmark •Combustion Chamber and the Cylinder Condition

•Fogh Jesper Weis (2) •Born in 1957 •MAN Diesel •graduated from Technical University

in Odense •Research Area

Documents

Development of Piston Ring Packs and Cylinder Liners