A Helpful Guide On Lube Oil Properties For Marine Engine

Having correct lube oil properties is critical for the successful operation of machinery. They are used as a coolant, reduce friction, provide cushioning, act as a cleaning agent, protect components against corrosion, and more.

There are various grades and types of lube oil available; based on engine speed, working conditions, operations, specific requirement or regulations.

Choosing the right lube oil for your engine is important as it put a significant impact on a maintenance schedule, cost of repairs and overall well being of the machinery.

When it comes to two-stroke marine engines; we have two sperate lube oils types in the form of cylinder lube oil and system/main lube oil. Which then have their dedicated lube oil systems.

The system lube oil is responsible for crankcase lubrication and cooling under piston space. On the other hand, cylinder lube oil is more alkaline in nature to counter the acidic effects of combustion products.

In general, the lube oil properties depend on their function in machinery. For example in four-stroke engines lube oil have combined properties of both cylinder lube oil and system oil as in two-stroke marine engine.

It is so because in 4 stroke engine cylinder and crankcase are lubricated through splash lubrication.

Functions Of Lubricating Oil In A Machinery

The main function of lubricating oil in a piece of machinery or system is to avoid metal to metal contact/sperate two surfaces under varying conditions of load, speed, and temperature.

All other functions of the lube oil system are; acting as a protecting agent, protect against ill effects of oil stress, good film strength, prevent scuffing, reduce wear, neutralize acidic combustion by-products, etc.

Some other secondary functions involve role such as protection against corrosion, act as a noise damper, hold dirt/residue/debris with itself to filters or purifier, acts as a cooling fluid and act as a sealing agent.

Many times additional additives such as anti-wear additive, extreme pressure additive, friction modifier, corrosion inhibitor, and viscosity index improver are added to further improve these functions.

S.No
Engine Component
Lube Oil Type
Primary Functions of Lube Oil
1
Crosshead
System lube oil
Lubrication + Cooling
2
Under piston Space
System lube oil
Cooling + Cleaning
3
Thrust bearing
System lube Oil
Lubrication
4
Main bearing
System lube Oil
Lubrication + Cooling
5
Crank Pin
System lube Oil
Lubrication + Cooling
6
Camshaft Drive Gear
System lube Oil
Lubrication
7
Cam & Cam Roller
System lube Oil
Lubrication
8
Stuffing Box
System lube Oil
Lubrication + Sealing
9
Cylinder Liner
Cylinder lube Oil
Sealing + Lubrication + Neutralization + Wear Control
10
Crosshead Guides
System lube Oil
Lubrication
11
Exhaust valve
System Oil Or Cylinder oil
Lubrication + Sealing + Hydraulic Action + Neutralization
12
Fuel injection pump
System lube Oil
Lubrication + sealing
Important Lube Oil Properties

Important Lube Oil Properties For Marine Engines

1 ) Viscosity

The term viscosity refers to the ability of fluid to resist its movement or flow. It is one of the most important lube oil properties that determine which lube oil to use for different machinery based on its physical property.

A less viscous fluid can easily pass through an area with small clearances while the lube oil with higher viscosity cannot. On the contrary viscous fluids are good at boundary lubrication while less viscous does not.

It is thus required to have a lube oil which has nither too high or too low viscosity. Since the viscosity of fluids changes with temperature; the viscosity of an oil is classified on either of these three temperatures -18, 40, and 100-degree Celcius.

2 ) Alkalinity

In the trunk type or four-stroke marine engine, it is the role of the lube oil to neutralize the acidic effects of combustion products such as sulphuric acid. if remained uncheck they can cause acidic corrosion.

When the fuel burns it forms sulfur oxides which then reach with present water vapor to form sulphuric acid. The alkalinity of the lube oil then balances these acidic effects.

It is even more important because in four-stroke engines; liner lubrication is carried by lube oil. The alkalinity of the lube oil depends on the sulfur content in the fuel oil.

3 ) Carbon Residue

Carbon residue is the tendency of oil to burn with a carbon residue under inert or elevated temperatures. These are generally gummy in nature and cause the piston rings to stick within their groves.

It is further expressed as Micro Carbon Residue or MCR in technical terms. An oil with a high carbon residue number does not burn easily and even if they burn they end up leaving a huge shoot deposit.

For cylinder lubrication, such lube oils that have a lower value of carbon residue is preferred. The carbon forming ability of these oils can be tested by destructive distillation under periodic heating.

4 ) Oiliness

The main job of lube oil is to reduce friction between the two sliding bodies. Lubricating a surface significantly reduces the coefficient of friction acting between the two surfaces.

This is achieved mainly due to the property of an oil to stick to a surface. It is this property that is responsible for boundary lubrication and is commonly known as oiliness.

Naturally, vegetable oils have higher oiliness than mineral oils. Thus additional additives in the form of antiwear agents are added to lube oil to further improve the oiliness property and improve their efficiency under boundary lubrication.

5 ) Stability

Lube oil needs to be thermally stable and is so one of the major lube oil properties to consider for the marine engine. It should not undergo cracking under elevated temperature conditions. This means the oil must not break down and show a significant change in properties with increased deposits/sludge.

The thermal stability of the oil is important to sustain mechanical processes. Having a high thermal conductivity not just helps with cooling but also avoids localized thermal stresses.

The operating temperature of a lube oil depends upon the thermal stability of that oil. Having lower thermal stability means the operating temperature for lubricant will be narrow and thus impractical for use.

6 ) Sediments

Sediments are one such lube oil properties that are applicable mostly to lube oil in operations. Sediments are generally micro carbon particles and metal parts from wear and tear.

Ideally, a lube oil should have no sediments but they always do have around 0.01 to 0.1% in running oil. Thus lube oils are required to be purified or changed over periodically.

7 ) Oxidation Resistant

Oil oxidation stability or oxidation resistant to lube oil is critical for maintaining the effectiveness of lube oil. Since lube oil is always in contact with air; there is a constant risk of oxidation and subsequent degradation of lube oil.

Furthermore, they are subjected to high temperatures in a marine engine. This further increases the risk of oxidation; as the rate of oxidation increase with the increase in temperature.

Oxidation of lube oil degrades oil, increases oil’s viscosity, produces sludge and sometimes microbial contamination. If not checked it can reduce your oil life by half or even more.

8 ) Load Carrying Capacity ( LCC )

The load-carrying capacity of oil is one of other lube oil properties; that plays a vital role under load applications. Film strength and load-carrying capacity of a lube oil determine their main application. It is the ability of the oil to maintain film and its properties under extreme pressure.

In a marine diesel engine, there are many internal parts in which the lube oil has to undergo/withstand extreme pressure. Now if the load-carrying capacity of oil is too low; they will be wiped out by pressure resulting in the metal to metal contact.

The load-carrying capacity of a fluid is directly proportional to its viscosity. Thus high viscous fluid performs better under high-pressure. Extreme pressure (EP) additives are added to increase LCC of lube oil.

9 ) Base Number

Total base number or base number is the measure of the alkalinity of lube oil. This plays a critical role in neutralizing acidic effects of sulphuric by-products of fuel oil ( Marine fuel have high sulfur content ).

The higher the base number of lube oil is; the more acid it will be able to neutralize. This helps improve the viscosity coefficient, fight corrosion and resist oxidation.

Thus Base number is one of such lube oil properties that directly affects the engine performance and its life.

Modern marine engines run on BN 30/40/50 for low sulfur oil; while for higher sulfur oil they use to run on BN70/100. The TBN for lube oil decreases with use and needs to be changed when it falls below a limit.

10 ) Total Acid Number

The total acid number is another lube oil properties that indicate degradation or contamination of lube oil. The total acid number or TAN represents the total amount of acid present in the oil.

This can be there due to organic acids as a byproduct of microbial contamination, the byproduct of oxidation or from mixing of acidic sulphuric compound produced during combustion.

11 ) Detergency

It is the property of lube oil by which a lube oil resists deposit formation at key positions in a system. This is mainly achieved by adding specific additives to keep the system clean.

These metallic additives while allowing microparticles of metal or carbon to flow with oil but resist them from depositing at any certain location. This plays a critical role in combustion space; as it flows them away from the combustion space, piston rings and its groves.

12 ) Volatility

Oil is said to be volatile if it is highly reactive, has lower flash points and self-ignition temperature. The self-ignition temperature or flash point helps avoid unwanted fire or damage to machinery.

The oil volatility of a lubricant must be as low as practically possible. If the volatility is high it evaporates too fast and may catch fire below 200-degree Celcius. On the contrary, if it is too low it’s stable, does not evaporate or catch fire easily.

13 ) Pour Point

Pour point of a lubricant is the lowest temperature at which it flows and thus can be used in a piece of machinery. If the temperature goes below its pour point wax form and the oil turns into grease.

The pour point of a lubricant depends on its chemical structure and physical property such as viscosity. Additives are then added to the oil to avoid crystal or wax formation.

14 ) De-Emulsivity & Water Content

Demulsibility is the tendency of an oil to easily separate from water. The greater the value easier it is to separate them in a purifier. If not it will lead to emulsification and may damage the engine parts.

This water can come to lube oils from a number of sources but not limited to; condensation, jacket water leaks, piston blow-by, lube oil coo.ler leakage, gland seal leaks, piston cooling water, etc.

These are some of the lube oil properties ones need to check before ordering lube. A higher water content with less de-emulsivity can leads to higher level of purification.

This not just degrades the quality of oil breaking oil film under boundary lubrication; causing the two opposing surfaces to make direct contact. Furthermore, it helps in microbial growth that further contaminates the oil.

15 ) Dropping Point

For high-pressure applications, grease is the preferred lubricant type. Ideally, grease is under semi solid-state; but at elevated temperatures, it turns into liquid. The temperature at which this phase change happens is known as the dropping point.

The dropping point of a lubricant or grease provides its heat resistance capability and indicates how it will behave at very high temperatures. Thus it indicates its suitability for a particular application.

Note: This article is produced on request from Aman Garg.

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