2-Stroke Slow Speed Diesel Engines are the main propulsion unit for merchant ships. They are very efficient, run slowly enough to be direct coupled to the propeller and are able to burn residual fuel oil grades. However these benefits also impose some lubricant related problems, espcially challenging is the increasing need to fuel switch on entering ECA (Emission Control Areas).

Cylinder Oil in 2-Stroke Diesel Engines
Cylinder oil is a high base number, high viscosity, high VI oil that is injected into the cylinder liner. The oil serves three roles:

lubricating the liner
neutralising acids from combustion
removing debris from the rings and liner surface to drain via the scavenge space.

The oil is used only once and not recovered. IMO MARPOL 73 / 78 Annex VI and EU Directives aim to control the sulphur content ofmarine fuel used in sensitive areas. This places some interesting demands on the cylinder oil, in that there may be a necessity to change the base number of the oil to better match the fuel sulphur content.

Additionally, there is a move to reduce the feed rate of cylinder oil resulting in significant savings in operating costs. The problem is that it is difficult to predict just what the minimum should be, as it changes with sea state, fuel, cylinder number etc. The average insurance claim for a lost liner is over $100,000 and so its risky to play with feedrates without good feedback. Did you know that:

LinerScan is a system developed in co-operation with ExxonMobil and approved by MAN B&W for monitoring the iron content in scavenge oil drains. It the only system available for this application and has an impressive reference list.
Kittiwake also make the On-Site Scrapedown Analyser used by ExxonMobil in their FRO (Feed Rate Optimisation) programme
and use a parallel technology for Shell in their RLA feed rate reduction programme.

Above: LinerSCAN Feedrate Optimisation graphs (click to enlarge)

Crankcase Oil in 2-Stroke Diesel Engines

System oil is used in the crankcase. It is a low BN oil with a primary purpose of lubrication, as it does not contact the combustion spaces. Water is a major contaminant and is easily identified through testing. Base number (BN) and insolubles levels can also rise, indicating leaks around the piston gland. Again, both are easy to test for using Kittiwake equipment On-Line or On-Site. Viscosity only really changes if the engine is topped up with the wrong oil grade but testing is still recommended.

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