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.

A 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.

engine room


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.

Swept Volume


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

The 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 ratio would either prevent the 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 into 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.

Scavenge Efficiency

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 into the engine cylinder by only the downward movement of the piston without using other aids.

Natural aspiration

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.


You may also like to read:  Capacity Control   Power Balancing   Energy Recovery Systems   Vibrations

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  1. Good morning MI.

    Many thanks for sharing useful articles. Those are indeed helful in my field of work right now.

    Best regards,

    Capt. Dante Albano

  2. Seems to me that scavenge efficiency, volumetric efficiency, air charge ratio having the same meaning.. ratio of volume of air drawn in during suction stroke to the volume swept by the piston by the end of compression stroke. Isnt it?

  3. Volumetric efficiency is ratio of volume
    Of air taken by cylinder to the swept volume but air will spread all over cylinder n it will occupy cylinders volume and swept volume is always less then cylinders volume so volumetric efficiency should be>1

  4. Thanks for the knowledge given. I always follow your articles. Please post some articles about control circuit of boilers. Thanks

  5. This site is boosting my performance in the engine room.you’re helping us good job.

  6. Hi Marine Insight,

    It seems that the scavenge efficiency and the air charge ratio are the same. It this OK?

    P.S: thanks for all the interesting articles

  7. Sir,thank you for enlighttening me,you imspeech me some area about different department.

  8. What does it mean by engine capacity(which usually expressed in cc)???
    Is it the total cylinder volume or only displacement or swept volume??

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