Lubricating Oil – BN for Medium Speed Engines using Low Sulphur Fuels.

The usage of Ultra Low sulphur fuels in the ECAs have made a huge impact in terms of fuel changeover and associated machinery failures.  Though the especial emphasis is on the 2-stroke main propulsion engines, it is equally essential to safeguard the 4-stroke engines either for main propulsion or for the power generation.  One main question that many ponder over is about the BN of lubricating oil to be used for a 4-Stroke medium speed engines when it burns ultra low sulphur fuels.

In general:

1. Medium-speed engine oils used for the lubrication of cylinders, liners and bearings are available with various BN levels. The BN requirements are specified by the engine manufacturer.
2. The information provided in the below table should only be used as a guideline. For specific cases, please refer to the engine manufacturer’s documents.
3. Many engine manufacturers specify the minimum required BN level of the oil in service in relation to the fuel in use. Their limits should be considered after judging the used oil analysis results.  As a guideline, the BN of used diesel engine oil should be greater than 50% of the fresh oil value.

 Theory & Concept:

Engines burning residual fuel typically show deposits on the inlet valves.  These deposits consist of soot and fuel ash that act as a lubricant when the valve closes, which is done with an additional rotational movement.  When burning distillate fuels, the inlet valve seat deposits are significantly less during residual fuel operation, as distillate produces fewer combustion deposits.  To add additional lubricant to the valve seats during operation some engine types are equipped with a valve seat lubrication system.  By injecting a small amount of lube oil into the charged air a small ash and coke layer is produced on the valve seat which prevents excessive wear.

Most of the residual fuel operated engines are equipped with water-cooled injection nozzles.  This reduces the temperature at the nozzle tip avoiding coking of the fuel which would cause deposits, sometimes called trumpets, on the nozzles.  Distillate fuel has a much lower fuel pump and injection temperature compared to residual fuel.  Therefore additional cooling with water might cause overcooling with the associated risk that the nozzle temperature may fall below the dew point of the sulphuric acid during the gas combustion, causing corrosion of the nozzle.  Consequently nozzle cooling water has to be switched off during distillate operation (Check with individual engine maker’s manual).

Fuel leakage from the pump into the lubricant is low during residual fuel operation.  The amount of fuel leakage increases with a decrease in viscosity and can be significant during distillate fuel operation leading to accelerated lubricant degradation.  To avoid leakage of distillate fuel into the oil, some engines are equipped with sealing oil (lubricating oil sealing) at the fuel pump that has to be used during operation on distillate fuel.  Due to potential incompatibility between lubricating oil and some residual fuels the sealing oil should, if possible, be switched-off while operating on residual fuel.  This incompatibility can cause deposits in the fuel pump resulting in fuel pump sticking.

Liner lacquering is a rare phenomenon generally only observed with operation on low sulphur distillate fuel.  These deposits form a very smooth surface on the liner with the effect that adhesion of the oil film to the surface is compromised.  A significant increase in lubricating oil consumption (factor 2-4) can be a result of this lacquer.  This phenomenon is mainly known from applications with a load pattern running intermittently on full load followed by idling (e. g. fishing vessels).  However, engine design and aromatic fuels with a large proportion of heavy components can also promote lacquer formation.  Operation on low sulphur distillate while still using the lubricant for HFO with a BN of 30 to 55 is not critical for a limited period of time (typically 1000 hours but refer to OEM recommendations).

For long term operation on low sulphur distillate a lubricant with a suitably lower base number is recommended.  If the engine is going to be operated on distillate fuel for over 1000 hours before reverting back to residual fuel at a later stage, then a low BN version (20 or 30, in accordance with OEM recommendations) of a “residual fuel lubricant” should be used while operating on distillate fuel.  Specific OEM requirements need to be considered when making the decision to switch to a different lubricant BN.  Lubricants designed for engines operating continuously on distillate fuel typically have a BN in the range of 10 to 16 mg KOH/g. Operation on residual fuel is not permitted with such a lubricant.  Mixing of a “distillate diesel engine lubricant” with a “residual fuel type lubricating oil” is also not allowed due to compatibility problems.

Engine maker must have specified the limits for BN for low Sulphur operation and if not please consult engine maker for exact requirements.  Ensure that the engine is designed to run on Ultra Low Sulphur fuels.  Further Generic Guideline table is attached below for your reference.

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