Diesel engine is generally classified as follows according to operation speed : 
 
Low Speed: Below 350RPM / Intermediate Speed: 350~1000RPM / High Speed: 1000RPM or more  
 

The low speed engine is used for the propulsion of container vessels or tankers more than 10,000 tons. 
 Intermediate speed or high speed engine is used for car ferries or fishing boats in general or widely used as auxiliary power.
 A recent trend shows use of intermediate engines of larger cylinders for large vessels, while smaller-dimension and lower speed engine offers beneficial points in propelling transport ships. 
 All low-speed engines are designed to use residual fuel, while intermediate-speed engine can use residual fuel or distillate fuel or residual oil depending on operation types and a high-speed engine always uses distillate fuel.
 All the engines today are the single acting type, having only one combustor at the top of the piston. The main driving stroke is as follows:
 

 
 4 Stroke Engine
 
While the crank shaft rotates twice in a four stroke engine, each cylinder will have one power stroke.  
 
First, the suction stroke sucks the air into the cylinder. 
Second, the compression stroke increases temperature by compressing air. is injected when the piston reaches the top dead center of the compression stroke. 
Third, in the power stroke, fuel combusts when it contacts with heated air and the increased air pressure pushes the piston to the bottom. 
Fourth, in the exhaust stroke, the piston discharges combustibles through mechanic action of the exhaust valves. 
 
 

Four-stroke engine is usually vertical or v-type. 
 Some four-stroke cross head-type engines were made many years ago, but four strokes is now manufactured as a trunk piston type without seal between cylinder and crank.  
  
 

2 Stroke Engine 
 
It refers to the internal friction that resists to the flow of liquid. Theoretically, absolute viscosity and dynamic viscosity demonstrate the following correlation : 
 
Dynamic viscosity (absolute viscosity / density)
 In general, viscosity refers to Kinematic viscosity in lubricant, and the unit cSt is used mostly. Besides the unit of Kinematic viscosity are as follows : 
 
① cSt (Kinematic Viscosity)
     The Kinematic viscosity displayed in the C. G.S units is Stocks and uses 1/100 of it to show Centistoke(cSt). 
     Measurement temperature is 40℃ and 100℃ in accordance with ISO (International Standard Organization) viscosity classification and
      is used worldwide.  
 
②°E(Engler Viscosity) 
     The outflow time of the sample fuel of 200cc is divided into ration. The temperatures measured were 20℃, 50℃, and 100℃, and mostly
     used by European countries.  
 
③ SUS or SSU(Saybolt Universal Viscosity) 
     Calculation of the ratio of the outflow time of the target sample fuel of 60cc, and the measured temperature are 100℉, 130℉, 210℉,
     used mostly in the U.S.