There are hundreds of commercial crankcase oils marketed today. Obviously, engine manufacturers or users cannot completely evaluate the numerous commercial oils. The performance of a lubricant, ...
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Lube Oil requirements for Diesel Engines Crankcase The oil has to serve two purposes; 1. redu reduce ce fric fricti tion on 2. Coo Cool bear bearin ing gs
A good quality mineral oil will serve the purpose of reducing friction to an acceptable level depending upon the metals involved and other conditions such as temperature. All o ils will oxidise and this reduces its effectiveness as a lubricant. Oxidation will also cause deposits which can block passage ways and coat working parts. The rate of oxidation will depend upon temperature, the higher the temperature the more rapid the rate. Anti oxidants are available which reduce the rate, also additional properties can be achieved by the use of additives. Under high temperatures an oil is liable to thermal degradation which causes discoloration and changes changes the viscosity viscosity.. Additives Additives cannot cannot change change an oils susceptibilit susceptibility y to this degradation. degradation. The refining refining process can remove compounds which effect the thermal stability of the oil and also those that lower oxidation resistance. ost of the chemicals found in an oil will react more or less with oxygen, The effects of this oxidation is always undesirable. !ence, a ma"or ob"ective of the refining process of a mineral oil is to remove remove those those hydroc hydrocarb arbons ons i.e. i.e. the aromat aromatics ics,, the small small amount amount of unsatu unsatura rates tes togeth together er with with molecules containing sulphur, oxygen and nitrogen. Unfort Unfortuna unatel tely y these these same same molecu molecules les are those those that that improv improve e the bounda boundary ry lubric lubricati ation on performance. !ence, a careful balance must be struck. The use of anti#oxidants make a slightly better balance although there usefulness is limited. Tin based whitemetal is susceptible to hardening as an oxide layers from on the surface. These tin oxides are a grey#black in appearance and are extremely hard. There formation reduces the bearing clearance as the oxide layer is thicker than the original white metal material from which it formed. The oxide has a lower coefficient of friction than the original white metal but it will cause problems if it brakes up as fragments will become embedded edge on in the white metal and can score the pin.
Contamination Water Water from, 1. bilge's 2. Jackets 3. Sea Sea via via coo coole lers rs 4. leaky leaky seals seals or or washin washing g in purif purifier iers s . Con Condens ensation ion Problems caused by water contamination, Water leads to corrosion especially if there is sulphur present due to fuel contamination forms emulsions which are not capable of withstanding high loads removes water soluble additives when centrifuged out leads to possible bacterial attack • • • •
Fuel ay be heavy residual or light diesel$gas oil and can be sourced to faulty to cylinder combustion or faulty seals on fuel p$ps. Problems Increases viscosity for hfo, reduces viscosity for D.. reduces flashpoint Introduces impurities such as sulphur dilutes lub oil when in large !uantities • • • •
Solid impurities i.e. carbon from the cylinder combustion process, particularly of importance with trunk piston engines but also for crosshead engines with inefficient diaphragm. The carbon can lead to restrictions and blockages of oil ways causing bearing failure. %traight mineral oils hold &' carbon in suspension, dispersant oils hold about ('.
Bacterial attack )ertain bacteria will attack oil but water must be present. The bacteria may exist in a dormant state in the oil but water water is require required d if they they are to reprodu reproduce. ce... The bacteria bacteria digest digest the oil causin causing g breakdo breakdown wn emulsions to be formed, acidity increases, dead bacteria block filters and corrosive films form on working surfaces. In summary their must be three essential conditions for microbiological growth; There must be a source of carbon" present in the oil There must be some bacteria or fungal spores present"these present"these are almost universally present in the atmosphere There must be free water present • •
Two other factors which encourage the growth are a slight acidity in the water *p! ( or + and a slightly raised temperature *- to /o) which can lead to rapid growth. grow th. 0iocide additives are available but they are not always compatible with other desired additives and can lead to large organic blockages if treated in the machinery. The best solution is to avoid the presence of water. 1f mild attack takes place the oil may be heated in the renovating tank to above 2 o) for -/hrs before being returned to the sump via the centrifugal separator. 3or a severe attack the only solution is complete replacement of the charge followed by sterilisation of the system. 1t may be noted that on replenishment the bacteria may be present in a dormant state in the new charge.
Test results of crankcase oils #iscosity #1ncr 1ncrea ease ses s due due to ther therma mall degr degra adati dation on or hfo hfo cont contam amin inat atio ion, n, contamination, corrective action needed if it increases by -(' from new oil.
redu reduce ces s
with with dies diesel el oil oil
Water content #)orrective action required at &' Insoluble $ediments #basically the result of wear and oxidation, corrective action at &' by weight %sh#a measure of incombustibles in the oil sample, corrective action at .&4' by weight T%& T%Total acid number consists of the strong acids *mainly sulphuric acid formed in the combustion process and weak acids resulting form oxidation of the lub oil. $%%trong acid number, the oil should be re renewed newed if any is detected T'Total #Total base number indicates the alkaline re serve particulary important for trunk piston engines (losed flash point #highlightd fuel contamination, corrective action if reduces by 4 o) from new
Cylinder lub oil The type of cyl l.o. required will depend upon the cylinder conditions and the engine design e.g crosshead or trunk trunk piston piston.. !oweve !oweverr, the propert property y requir requireme ements nts are basica basically lly the same same but will will vary vary in degree degree depending upon the fuel and operating conditions. &ormal properties re!uired are; a. ade!uat ade!uate e viscosity viscosity at working working temperat temperature ure so that the the oil spreads spreads over the liner surfac surface e to provide a tough film which resists the scrapper action of the piston rings b. the oil oil must provid provide e an effectiv effective e seal betwee between n the rings rings and liner liner c. only a soft soft deposit deposit must must be be formed formed when when the the oil burns burns d. alkalintiy level )total )total base number or or T'&* must must match match the acidity acidity of the oil being being burnt burnt
e. deterge detergent nt and dispersa dispersant nt propertie properties s are re!uired re!uired in order to hold deposits deposits in suspensio suspension n and thus keep surfaces clean
Addities All oils for all purposes can be designed to give particular properties through the careful use of additives to the base mineral oil stock. (ommon additives are; %ntio+idants"these are used in all oils to reduce the rate at which o+idation occurs and are especially useful were the lub oil cools the piston +treme pressure agents these are compounds of phosphorus, $ulphur or (hlorine which increase the strength of the oil film under conditions of high temperature or pressure. ,Dispersants or detergents"found in trunk piston engine oils and cyl l.o. these keep surfaces clean by holding deposits in suspension. #iscos #iscosity ity inde+ inde+ improver improvers" s" these these prevent prevent e+cessi e+cessive ve changes changes in viscosi viscosity ty with change change in temperature ther additives can be defined by name such as anti"wear, anti"corrosion, anti"bacteria, anti" foaming etc. •
5hen running in, the cylinder cylinder lube oil in"ector in"ector pumps may be filled with a a straight straight mineral mineral oil without anti#wear properties# typically the crankcase oil# once this small reserve of oil is exhauted, running in carries on with normal cylinder lube oil. The flow of oil is increased to carry away metallic particles.
!roblems caused by stuffing bo" leakage oil entering crankcase 6ow speed engines are particularly at risk from crankcase lubricant contamination caused by cylinder oil drainage past the piston rod gland and combustion products. This can lead to severe damage of engine crankcase components and reduction of life of oil which is normally expected to last the lifetime. There has been a general increase in the viscosity and 0ase number of crankcase oils over recent years particularly for engines built since the early &278s. 1ncreased alkalinity, viscosity and insolubles, fuel derived elements such as vanadium and oil additive derived elements such as calcium, suggest that the contamination is from the cylinder oil drainage. 9eterioration of the crankcase oil has led to the expensive necessity of replacing up to (' of the sump, this is particularly of concern as it is often only a temporary measure. -our causes are put forward, 1. !ew crank crankcas case e oil conta"in conta"inated ated with with new cylin cylinder der oil#unl oil#unlikel ikely y 2. Cylinder Cylinder oil draining drainings s being recyc recycled led and return returned ed to the su"p#ve su"p#very ry likely likely as it is a co""on practice to purify oil leaking through the gland$ tests done on this purified oil found high a"ounts of insolubles 3. %eakag %eakage e past rod gland# gland# very likely$ likely$ high high pressure pressure scaven scavenge ge air can blow cylinde cylinderr oil and dirt past the top scrapper ring and sealing rings into the piston rod drain tank$ and even possibly directly into the su"p. & proble" that worsens with age and wear. 4. leakage leakage of of ehaust ehaust valve valve lubric lubricatio ation n syste"#u syste"#unlik nlikely ely
3rom above the suggestion is the most likely cause for contamination is leakage past the piston rod. 1t is seen that maintenance of the stuffing box is of the utmost importance. Tell tales and drainage lines should be proved free and use of oil drained from the uppermost drain should not be allowed even after purification due to the high level of contamination which can destroy the properties of the oil in the sump
I know of a case where due to the increased viscosity of the oil a 50% charge of hydraulic oil was added to the sump of a very large slow speed engine under advice from the manufacturer