Questions and Answers on Marine Diesel Engine

Lamb's Questions and Answers on the ---Marine Diesel Engine Eighth E!Jition Fi I published in Great Britain l9 1 S1:uh edrtion 1951) Sc-·t'lllh tdi111011 \sity (cSt) = 14.9 at 100°c The viscosity of distillate fuel is quoted in cSt at a temperature of 40°C and the viscosity of residual fuel is quoted in cSt at a temperature of 80°C or 100°C. Note Viscometers giving a digital readout of viscosity based on electronic circuitry are now available. •

3.9 State how pure mineral lubricating oil may be improved for use in diesel engines.

Oils produced from various types of crude-oil stock possc5s difrering characteristics. Oil from paraffinic base stocks has generally a lower s.g. and a

Blending of oils from different base stocks makes for some improvement, but to obtain improve ments necessary for the commercial success of the newer generation of high output engines, additives must be used in the lubricants. The use of additives in internal combustion engine lubricating oils commenced in the 1920s and gained some impetus in the early 1940s from research carried out earlier. With the gain of knowledge from the field of organic chemistry the results from the use of additives have been spectacular in the last few years. Crankcase oils in both crosshead and trunk-piston engines deteriorate in use through changes brought about by oxidation. When this occurs the viscosity of

the oil increases, corrosive material in the form of organic...acids is created, and sludges may form. Additives in the form of complex compounds of sulphur, phosphorus or both may be used. Another class of additives obtained from modified compounds of ammonia or derivatives of aromatic organic com pounds is also used. These additives are referred to as antioxidants. Oils used in the crankcases of slow-speed engines where large amounts are in circula tion do not suffer from oxidation at the same rate as high-rated high-speed aux- · iliary engines. In these engines the temperatures are higher, and much smaller amounts of oil are in circulation. The use.of antioxidants reduces or prevents the breakdown of crankcase lubricating oil due to oxidation. Different addi tives act in different ways. Cylinder oils used in crosshead engines and the crankcase oils used in trunk piston engines, if untreated, allow carbon to build up in the pistonring grooves. This is caused by the high temperatures involved. Combustion residues also form sludges which mix with the crankcase oil in trunk-piston engines. A group of additives referred to as detergentdispersants are used to prevent carbon build-up in piston rings and to prevent the formation of deposits from sludges. The detergent-dispersants are generally complicated compounds of barium or calcium. Some types of engine develop lacquers on the pistons and cylinder liners from untreated lubricants; the use of detergent-dispersants prevents lacquer formation. Certain engine parts and gearing may at times work very near boundary lu rication conditions. To r.educe wear rates under these conditions, fatty acids, or compounds of sulphur, phosphorus or chlorine may be used. The additive selected will depend on the severity of operating conditions and possible temperatures. Additives to improve the resistance of oils to very heavy loads are called extreme pressure (E.P.) additives. Additives may also be used to improve the viscosity index or-illl" oif; i.e. to give an oil a higher viscosity at higher temperatures. They are usually polymers of high molecular weight. High-molecular-weight polymers may also be used as additives in paraffinic base lubricants to lower the poUL point. Th additives arc referred to as pour point depressants. Oil used for lubrication of reduction gearing and in the crankcase of high speed engines becomes aerated and may allow large volumes of foam to build

PCM. ZJMIZJhiJ - I PMJMMJ DJ §JZMJJ ll/ up. Foam formation is prevented by using small quantities of an organic"silicon compound in the form of a polymer. - . ·Most of the treated oils in common use today are fairly stable in normal,., storage conditions. •

3.10 Name the properties or constituents that may be found in a blended fuel having a high viscosity and a high carbon. content. Explain how they may cause problems in engine operation.

Density. The ability of a centrifugal separator to function correctly and

remove water and other foreign matter from fuel oil is dependent on the differences between the density of the oil, the water, and the foreign matter. As the density of the oil increases the difference in the separating forces between the oil, the foreign matter, apd the water is reduced; the ability of the cen f gal purifier t? function correctly is then impaired. LIDllts on the density of the fuel oil are fixed by the density it will have at the operating temperature of the centrifugal separator. The operating temperature of the separator must be less than the boiling point of water due to the problem of losing the water seal.

Viscosity. High viscosity values have a similar effect as high fuel densities on the action of a centrifugal separator. . If the viscosity of the fuel is such that it cannot be reduced by heating to the requirements of the fuel injection system, problems may arise with combustion and failure of parts in the injection system due to the high pressures that are created. ·· Pour point. Fuels having a pour point higher than the expected ambient

storage te pcratures must be maintained at some safe temperature above the pour potnt to prevent waxes coming out of suspension, or the oil congealing. If some form of solidification occurs the action is often irreversible and causes serious technical problems and a heavy financial commitment to remove the solidified fuel. As the pour point increases above the ambient temperature the demand for heating steam increases and may reach a point where the exhaust gas boiler cannot meet the total steam demand. In such cases it may be necessary to supplement the heat from the exhaust gases by firing the auxiliary boiler or shutting down the steam turbine generating set if one is fitted and running an auxiliary diesel set for electrical power. Whatever is done, some of the saving frQm utilizing lower-cost heavier fuel is lost when the pour point is higher than the ambient storage temperatures.

Carbon residue. Fl,tels with a high carbOn residue value often run into problems with combustion and the build up of carbon and other materials in

Asphalt content. When a fuel is high in asphalt or asphaltenes and low

the combu5- tion chamber and exhaust system. This may affect exhaust valves the exhaust gas section of turbochargers, the heating surfaces within exhaust' boilers, and spaces in silencers or mufflers. 48· Questions and Answers on the Marine Diesel Engine

on neutral. The measured amount of acidic solution required to neutralize the · alkaline solution can be then used as a measure of the strength of the alkali in the alkaline solution. Jn a similar manner acidic solutions can be tested for the strength of the acid content by carefully measuring the amount of alkaline solution to make the resulting solution neutral. Acidic reagents such as standard solutions of hydrochloric acid can be titrated into alkaline oil solutions to measure their alkalinity; alkaline reagents

certain aromatics some of the asphaltenes will not be held in solution and trouble may be experienced with fouling of filters and separators. Fuels with a high asphaltene content bum relatively slowly when compared· with fuels from a paraffinic base crude. . Generally the effect of high asphaltene content is similar to the effects of high carbon residue content. Note The paraffinic series of hydrocarbons have a chain-type molecular structure while the napthenic series of hydrocarbons have a ring structure. Asphaltic base crudes and napthenic base crudes are synonymous terms. Sulphur is known to ·cause corrosive wear in cylinder liners but the problem of high sulphur content fuels has been overcome with alkaline cylinder oils. When burnt, sulphur forms gases having various combinations of sulphur and oxygen. Hydrogen when burnt creates H20 in the form of steam vapour. If at any point in the exhaust system the exhaust gases fall in temperature below their dew point, corrosive.acids are formed. These acids cause corrosion damage at the place where coodensation of the acid vapours occurs. Tr .manifested-by rates of cylinder· liner wear, has been experienced with very low sulphur fuels when used with some of the high

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¥t

alkalinity

cylinder

lubricants

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available.

ISilica

and alumina. These fuel contaminants are very abrasive. If they are not removed when the fuel is cleaned in the separator and clarifier they may cause extensive wear of the fuel injection equipment in a very short space of time.

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Sodium and vanadium. These eontaminants are chemically combined with the

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fuel and cannot be removed by centrifuging. In conjunction with each other after combustion they are highly corrosive in the liquid sta e. If the exhaust valves cannot be operated at a sufficiently low temperature the corrosive products in the liquid state stick to the valve seating surfaces and lead to early problems with gas leakage. This results in burnt valves and low compression pressures leading to a loss of efficiency. 3.11

How can lubricating oils be tested for alkalinity or acidity?

Lubricating oils can be tested for alkalinity or acidity by a method known as titration in much the same way as the water taken from a boiler is, tested for alkalinity. Titration is an old experimental method of volumetric analysis used by chemists. It is known that if an increasing amount of an acidic solution is added to an alkaline solution the resulting solution will eventually be made

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Fuels, Lubricants - Treatment and Storage 49 such as standard solutions of potassium hydroxide can be titrated into acidic oil solutions to measure their acidity. Colour indicators (e.g., methyl orange, litmus, phenolphthalein, etc.) are used to find the end point during titration as in boiler water testing. These indicators are not, however, suitable for finding the end ,point when testing solutions of lubricating oils. Potentiometry is then used to find the end point of the titration. This involves placing electrodes or half cells in a solution of the oil under test and measuring any potential difference across them with a millivoltmeter. When the reagent is being added to the oil solution the potential difference measured on the millivoltmeter is recorded together with the amount of reagent added to the oil solution. The results are plotted on a graph with the end point shown by the midpoint where the plot changes from concave to convex curvature. It takes considerable skill to operate this type of analytical apparatus·and interpret the results. The final results are calculated and then reported as the, weight of reagent --required to neutr8lize some standard mass of the oil sample. Common units are milligrams of reagent to 1gram of sample. The numbers corresponding to these units are then expressed as Total A