Documents in this section

Medium speed diesel engines operating on residual fuel are prone to the formation of deposits within the engine, often called black paint or black sludge. This sludge may also affect auxiliary engine equipment including the lubricating oil centrifuge, filters and fuel pumps. The result is a requirement for more maintenance and a poor cosmetic effect. The deposits may be seen in the rocker box and generally in the sump. Deposits may also affect the piston ring pack to such an extent that they interfere with the correct lubrication of the ring / liner interface.

There are various theories why black paint exists. Some consider it to be a relatively recent problem that has arisen because of the increase in brake mean effective pressure and fuel injection pressure that causes the fuel to leak into the engine crankcase. Once there, the asphaltenes in the residual fuel come out of solution and form the characteristic deposits. Others believe that it is a fuel related problem because fuels have become more viscous and closer to the specification limit. Another theory is that the black paint problem is a fuel combustion problem and can be overcome by the use of a fuel additive, thus providing an economical solution when better fuel quality is not an option.

In some engines and some lubricants black paint has been observed. However, it is interesting to note that other engines and lubricants have operated without any appearance of black paint. Even though these engines have been burning the same type of residual fuel, which on a world-wide basis gets closer to the specification limits.

This would suggest that whilst fuel may be a contributing factor it is not necessarily the dominant one. It is well known that the modern medium speed engines stress the lubricant more than in the past. Indeed some designs are such that the specific lubricant consumption is low and the sump charge is small. Clearly the stresses on the lubricant is a factor that becomes more dominant as engine designs operate at higher and higher mean effective pressures with smaller sumps.

This would suggest that the solution must lie with the lubricant. Trunk piston engine oil has various functions, which include provision of:

• A gas seal and cleaning action for the ring pack.
• A transport medium for the alkaline additive system that neutralises the corrosive acids generated during the combustion process.
• Effective lubrication at the ring pack / liner interface.
• A coolant for the pistons.
• An oil film for the bearings and running gear.

Suitable base oil and additive selection meet these various functions. One of the aspects that are considered is the detergency of the oil. The lubricating oil works to limit black paint by agglomerating the asphaltene deposits so that the centrifuge can remove them. A consequence of this is that the centrifuge has to discharge more frequently but at least the deposits are removed from the engine.