Lube Oil for Diesel Engines

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Crankcase

The oil has to serve two purposes;
1. reduce friction
2. Cool bearings

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 oils 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 the viscosity. Additives cannot change an oils susceptibility to this degradation. The refining process can remove compounds which effect the thermal stability of the oil and also those that lower oxidation resistance. Most of the chemicals found in an oil will react more or less with oxygen, The effects of this oxidation is always undesirable. Hence, a major objective of the refining process of a mineral oil is to remove those hydrocarbons i.e. the aromatics, the small amount of unsaturates together with molecules containing sulphur, oxygen and nitrogen.
Unfortunately these same molecules are those that improve the boundary lubrication performance. Hence, 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 via coolers
4. leaky seals or washing in purifiers
5. Condensation

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

May 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.O.
• reduces flashpoint
• Introduces impurities such as sulphur
• dilutes lub oil when in large quantities

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. Straight mineral oils hold 1% carbon in suspension, dispersant oils hold about 5%.

Bacterial attack

Certain bacteria will attack oil but water must be present. The bacteria may exist in a dormant state in the oil but water is required if they are to reproduce.. The bacteria digest the oil causing breakdown 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 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 (pH 5 or 6) and a slightly raised temperature (20 to 40^oC) which can lead to rapid growth.

Biocide 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. If mild attack takes place the oil may be heated in the renovating tank to above 90^oC for 24hrs before being returned to the sump via the centrifugal separator. For a severe attack the only solution is complete replacement of the charge followed by sterilisation of the system. It may be noted that on replenishment the bacteria may be present in a dormant state in the new charge.

Test results of crankcase oils

Viscosity-Increases due to thermal degradation or hfo contamination, reduces with diesel oil contamination, corrective action needed if it increases by 25% from new oil.

Water content-Corrective action required at 1%

Insoluble Sediments-basically the result of wear and oxidation, corrective action at 1% by weight

Ash-a measure of incombustibles in the oil sample, corrective action at 0.13% by weight

TAN-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.

SAN-Strong acid number, the oil should be renewed if any is detected

TBN-Total base number indicates the alkaline reserve particulary important for trunk piston engines

Closed flash point-highlightd fuel contamination, corrective action if reduces by 30^oC 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 piston. However, the property requirements are basically the same but will vary in degree depending upon the fuel and operating conditions.

Normal properties required are;
1. adequate viscosity at working temperature so that the oil spreads over the liner surface to provide a tough film which resists the scrapper action of the piston rings
2. the oil must provide an effective seal between the rings and liner
3. only a soft deposit must be formed when the oil burns
4. alkalintiy level (total base number or TBN) must match the acidity of the oil being burnt
5. detergent and dispersant properties are required in order to hold deposits in suspension and thus keep surfaces clean

Additives

All oils for all purposes can be designed to give particular properties through the careful use of additives to the base mineral oil stock.

Common additives are;
• Antioxidants-these are used in all oils to reduce the rate at which oxidation occurs and are especially useful were the lub oil cools the piston
• Extreme pressure agents these are compounds of phosphorus, Sulphur or Chlorine 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.
• Viscosity index improvers- these prevent excessive changes in viscosity with change in temperature
• Other additives can be defined by name such as anti-wear, anti-corrosion, anti-bacteria, anti-foaming etc.

When running in, the cylinder lube oil injector pumps may be filled with a a straight 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.

Problems caused by stuffing box leakage oil entering crankcase

Low 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 Base number of crankcase oils over recent years particularly for engines built since the early 1980's. Increased 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.

Deterioration of the crankcase oil has led to the expensive necessity of replacing up to 50% of the sump, this is particularly of concern as it is often only a temporary measure.

Four causes are put forward,
1. New crankcase oil contaminated with new cylinder oil-unlikely
2. Cylinder oil drainings being recycled and returned to the sump-very likely as it is a common practice to purify oil leaking through the gland, tests done on this purified oil found high amounts of insolubles
3. Leakage past rod gland- very likely, high pressure scavenge air can blow cylinder oil and dirt past the top scrapper ring and sealing rings into the piston rod drain tank, and even possibly directly into the sump. A problem that worsens with age and wear.
4. leakage of exhaust valve lubrication system-unlikely

From above the suggestion is the most likely cause for contamination is leakage past the piston rod. It 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