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Friday, September 15, 2000



Longer times between overhauls for Sulzer low-speed engines

Wärtsilä Corporation has introduced design modifications for Sulzer RTA-series two-stroke diesels. They provide improved piston-running behavior, longer times between overhauls, and lower cylinder lubricating oil consumption.

These "TriboPack" modifications are now standard on all newly-built Sulzer RTA-T and RTA-C engines. Although primarily intended for new engines, TriboPack is also offered for retrofit to existing RTA engines. The exact modifications necessary for the retrofit depend on the particular engine type, and the condition of cylinder liners and piston crowns.

TriboPack comprises several design improvements:

  • Fully- and deep-honed cylinder liner with sufficient
    hard phase;
  • Multi-level cylinder lubrication;
  • Chromium-ceramic top piston rings;
  • Pre-profiled rings in all four grooves;
  • An anti-polishing ring;
  • Mid-stroke liner insulation;
  • Insulating tubes in the cooling bores in the upper part of the liner according to the rating;
  • Re-adjusted cylinder lubricating oil feed rate where necessary.

 

Together, these measures increase safety against scuffing, reduce wear of cylinder liners, and extend the time between overhauls (TBO). It is expected that, with TriboPack, times between overhauls can be extended to some 18,000 running hours, or about three years operation, compared with about two to two-and-a-half years with the previous design standard.

Each individual TriboPack measure offers some benefit , but, says Wärtsilä, only application of the complete package will give the full advantage to the shipowner.

The better piston-running behaviour, avoidance of cold corrosion and the benefits of the anti-polishing ring mean that TriboPack would also allow the feed rate of cylinder lubricating oil to be reduced. However, at present, Wärtsilä continues to recommend that the cylinder oil feed rate is set at the existing level of 1.3 g/kWh for normal operation of TriboPack-equipped engines.

The benefits of TriboPack have already been demonstrated in a number of Sulzer RTA engines with individually up to some 10,000 running hours. These include two 12-cylinder RTA84C engines in a land power station, an eight-cylinder RTA84C engine propelling a container ship, two seven-cylinder RTA84T-D engines and two seven-cylinder RTA84T-B engines in tankers.

Background to TriboPack
TriboPack is the result of an extensive research and development program at Wärtsilä over the past ten years or so to improve piston-running behavior, to reduce wear rates and to reach even better reliability in Sulzer RTA two-stroke diesel engines.

Cylinder liners are manufactured in cast iron which needs a controlled hard-phase content and the best grain structure in the running surface for both good strength and running behavior. Careful machining of the liner's running surface is followed by full deep honing to remove all damaged hard phase from the surface of the liner. Only then can running-in be expected to be free of problems. Deep honing of the full liner running surface is a prerequisite for obtaining the benefits of TriboPack. By experience, plateau honing of a wave-cut liner is not adequate as, once the "plateau" is worn down, the rings run on liner metal whose hard-phase structure was damaged during machining. This damaged hard phase must be removed by deep honing.

Pistons now have four rings, all of the same thickness. The piston ring pack has been standardized to include pre-profiled rings in all grooves, with chromium-ceramic rings in the top groove.

Although new to two-stroke engines, chromium-ceramic rings
have been used in the company's four-stroke engines for more than six years. These rings have cast iron as the base material. The running face is profiled and coated with a layer of chromium as a matrix into which a ceramic material is trapped. These rings have demonstrated a high operational safety and give excellent low liner and piston ring wear. They have proven to resist scuffing much better than any other known ring material. However, for good performance, the chromium-ceramic rings must be used in conjunction with a deep-honed liner.

The other piston rings have a running-in and anti-scuffing coating which enables a safe and fast running in of the engine when the liners are deep honed.

The most visible feature of TriboPack is the anti-polishing ring (APR) at the top of the cylinder liner. Used as standard in Wärtsilä four-stroke engines for some years, the anti-polishing ring helps maintain the liner surface in the best condition for piston running. With an internal diameter less than the cylinder bore to reduce the clearance to the top land of the piston, the anti-polishing ring prevents the build up of deposits on the top land of the piston. Such deposits can be heavy in some engines, especially those running on very low-sulfur fuel oil (less than one per cent sulfur) combined with an excessive cylinder lubricating oil feed rate. If the deposits are allowed to build up, they inevitably touch the liner running surface over a large part of the piston stroke. They can then wipe off the lubricating oil film and so allow metal-to-metal contact between the piston rings and liner. In the worst case, there can even be scuffing.

The anti-polishing ring also stops the layer of deposits from causing upward transportation of new lubricating oil to the top of the liner where it is burned
instead of being used for lubrication. Thus the anti-polishing ring is effective in allowing the lubricating oil feed rate to be kept down to recommended values.

For effective but economic lubrication, the cylinder oil has to be delivered to the right place at the right moment and in the right quantity. This is obtained by measured delivery of cylinder oil to the liner surface through quills at two levels in the liner (multi-level lubrication).

The correct temperature distribution on the liner surface over the whole of the piston stroke is also critical for good piston-running conditions. The upper part of the liner is bore cooled with cooling water passing through tangential drillings in the liner collar. The mid stroke region of the liner is cooled by a water jacket, and only the lower part is uncooled. Often there is a tendency for liner temperatures to be too low thereby leading to corrosive wear from the sulfuric acid formed during combustion.

Wärtsilä uses two insulating techniques to allow better temperature distributions.

For some years PTFE insulating tubes have been fitted in the cooling bores of the liner. As part of TriboPack, the liner is now also insulated in the mid-stroke region by a Teflon band on the water side. The insulating tubes are adapted according to the engine rating to ensure that the temperature of the liner running surface is kept above the dew point temperature of water over the full length of the stroke, and over a wide load range
of the engine.

Although not part of TriboPack, the water separator after the scavenge air cooler plays a key role in achieving trouble-free piston running. Experience shows that any water entering the cylinder with the scavenge air may be harmful to piston running.

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