Lubricating Oils

Significantly improved thermal efficiency, fuel economy and ability to burn poor quality bunkers have resulted from the intensive development of low speed crosshead and high/medium speed trunk piston engine designs in the past 20 years. Further progress in performance and enhanced lifetimes can be expected from the exploitation of higher firing pressures and better combustion characteristics.

Such advances pose a continuing challenge to engine lubricant formulators, however: higher pressures and temperatures make it more difficult to provide an oil film in the critical zones, and longer strokes (another design trend) lead to spreadability problems. The success of new generations of engines will therefore continue to depend on the quality of their lubricants.

Lubricants are complex blends of base oils and chemical additives. Base oils are derived from the refining of specific crudes, and their properties—such as stability at high temperature and viscosity—depend on the nature of the crude and the production process. Additives either improve the basic properties of oils or underwrite new properties. Marine lubricants exploit many types of additive:

• Detergent and dispersant additives are used to clean engines. Detergents are most effective in the hot areas (pistons, for example) where they destroy carbon deposits or prevent their formation; dispersants help to maintain particles (such as combustion residues) in suspension in the oil until they are eliminated via filters and centrifuges.

• Anti-wear and high pressure additives contribute to the maintenance of satisfactory lubrication under the most severe wear conditions.

• Anti-oxidation additives retard damage due to thermal and oxidation phenomena at high lubricant pressures and hence extend the life of the lubricant.

Other specific roles are performed by anti-foaming, anti-corrosion, anti-rust and anti-freeze agents.

Environmental legislation has also impacted on the composition of lubricants. A reduction in the sulphur content of fuels, for example, influences Total Base Number (TBN) levels and calls for a new balance of detergent and dispersant additives. Measures taken to reduce NOx emissions from engines—such as delayed fuel timing, the injection of water into the cylinder, emulsified fuel and selective catalytic reduction (SCR) systems based on ammonia—also dictate attention to lubricant detergency, resistance to hydrolysis and compatibility with catalysts.

All additives are created from a number of ingredients performing diverse functions. One part of the chemical molecule of 'overbased' detergents, for example, cleans the engine and another neutralizes acids produced by combustion. Additives can interact with each other either positively (synergy) or negatively (antagonism). The art of the lubricant formulator is to select the best base oils and optimize the proportions of the various additives for the specific duty.

Whether or not the engine is a two-stroke or a four-stroke design, operating on distillate or residual fuel, the function of the cylinder lubricant is the same:

• To assist in providing a gas seal between the piston rings and cylinder liner.

• To eliminate or minimize metal-to-metal contact between piston rings, piston and liner.

• To act as a carrier fluid for the functional alkaline additive systems, particularly that which neutralizes the corrosive acids generated during the combustion process.

• To provide a medium by which combustion deposits can be transported away from the piston ring pack to keep rings free in grooves.

• To minimize deposit build-up on all piston and liner surfaces.

Formulating an 'ideal' lubricant to perform all these functions for all engines on the market is difficult and will always be subject to experience and judgement. First, there are varying requirements depending on engine type (single or dual level lubrication, oil injection position in the liner and oil feed rate applied), operating condition (power and rotational speed set points) and fuel quality used. Secondly, in many cases the requirements are contradictory; for example, the high viscosity necessary to yield good load-carrying performance will adversely affect spreadability.

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