8 Engine Terms Every Marine Engineer Should Know – Part 2

In the previous article, 8 Engine Terms every marine engineer should know, we talked about marine engine terms which marine engineers should know without fail.

In this article, there are eight more engine terms which deals with the characteristics of fuel and lubricating oils used on ships. These terms are used extensively on ship and therefore they are of great importance.

8 More Engine Terms Every Marine Engineer Should Know

1.       Viscosity of Oils

Viscosity of oil is defined as the ability of the oil to flow. It is the property of the liquid which tends to prevent relative movement between adjacent parts within itself.

Generally, thicker the fluid, higher is its viscosity; whereas thinner liquids have lower viscosity.

2.       Viscosity Index

Viscosity Index is a term which is mainly related to marine lubricating oils. It can be defined as the change in viscosity of the oil which takes place as a result of change of temperature.

Higher the viscosity index of the lubricating oil better is the quality. This means that lubricating oil with higher viscosity index has only a small change as a result of large temperature difference.

As a general practice, various additives are added in the oil to improve the viscosity index of the lubricating oil.

Viscosity index is a dimensionless number.

3.       Cloud Point

Cloud point is the term which is related to the wax formation in the oil.

Cloud point indicates the temperature at which waxes begin to form in the oil.

Gradually, the wax formed crystallizes and clogs the filters.  The cloud point helps in finding out the tendency of the oil to form wax.

4.       Pour Point

Pour point of the oil can be defined as the temperature at which the oil stops to flow.

Pour point is lowered using additives known as pour point depressants.

5.       Flash Point

Flash point of oil can be defined as the lowest temperature at which the oil will give off sufficient inflammable vapour to produce a flash when a small flame is brought to the surface of the oil.

6.       SAE Number

SAE number of the oil indicates its viscosity based on classification involving two temperatures. Every lubricating oil comes with a specific SAE number. The Society of Automotive Engineers is responsible for the classification of SAE numbers.

7.       Total Base Number (TBN)

Total Base Number (TBN) can be defined as the measure of reserve alkaline additives that are put into the lubricating oil to neutralize the acids.  It determines how effectively the acids formed during the combustion process can be controlled.

Higher the TBN better is the capability to fight oxidisation and corrosion, and to improve viscosity characteristics. It also allows longer operating period between lubricant changes under harsh operating conditions.

Diesel engines burning low grades of fuel show high rate of liner wear as low grade fuel have higher sulphur content. This high sulphur content leads to corrosive wear to the liner surface. Thus, alkaline lubricating oil is used to protect the liner surface against corrosive attack by neutralizing the sulphur derivative compounds.

The TBN is generally between 8-10 for marine lubricating oils.

8.       Total Acid Number (TAN)

Technically, the total acid number (TAN) of the oil indicates the deteriorating condition of the lubricating oil. Higher the TAN, more acidic is the lubricant, and further are its chances of getting more deteriorated.

TAN also indicates the potential of the oil to cause corrosion problems, leading to component failure. The TAN should not be more than 2 for marine lubricating oils.

8 Engine Terms Every Marine Engineer Should Know – Part 1

If you are a marine engineer or aspiring to be one, then it’s obvious that you would eat, breath, and sleep marine engines. The study of marine engines forms an integral part of marine engineering syllabus. Needless to say, it is imperative for every marine engineer to know this important machine inside out.

Marine engine is a complex machine, which requires years of experience and knowledge for understanding and handling the same. Moreover, there are several engineering terms which an engineer should know like the back of his hand.

In this article, we have enumerated eight important definitions of marine engine terms which are extremely important and which will be used throughout the professional life of a marine engineer.

1.       Swept Volume

Swept volume can be defined as the volume swept by the engine piston during one stroke.

Swept volume is also the product of piston area and stroke.

2.       Clearance Volume

Clearance volume can be defined as the volume that remains in the cylinder when the engine piston is in the top-centre position.

Clearance volume can also be defined as the difference between the total cylinder volume and the swept volume. The space covered by the clearance volume also forms the combustion chamber.

3.       Compression Ratio

Compression ratio can be defined as the value obtained by dividing the total cylinder volume by the clearance volume.

Compression ratio is generally between 12 and 18; however, it depends on the design of the engine. Compression ratio outside this ration would either prevent engine from starting or lead to other problems.

Marine engines with smaller cylinders will have higher compression ratio.

4.       Volumetric efficiency

Volumetric efficiency can be defined as the ratio of the volume of air drawn in to the cylinder to the swept volume.

In marine engines, the volumetric efficiency generally falls between 0.85-0.95.

5.       Scavenge Efficiency

Scavenge efficiency can be defined as the ratio of the volume of air in the cylinder at the start of the compression to the volume swept by the piston from the top edge of the ports to the top of the strokes.

6.       Air Charge Ratio

Air charge ratio can be defined as the ratio of the air contained in the cylinder at the start of the compression to the swept volume of the piston. It is also known as air mass ratio or air supply ratio.

In four stroke marine engines, the value of air charge ratio will fall in the range of 0.85 to 4.

In two stroke engines, the value will be in the range of 0.85 to 2.5

7.       Natural aspiration

Natural aspiration is a term which mainly applies to four stroke engines and is defined as the process by which air charge is brought in to the engine cylinder by only the downward movement of the piston without using other aids.

8.       Supercharging

Supercharging is a term used to indicate that the weight of the air supplied to the engine has been considerably increased for greater fuel usage and power production per stroke.

It is also noted that supercharged engines produce more power as compared to non supercharged engines having the same stroke and speed.