These are ships designed and built to carry crude oil or the refined products from where they are extracted to the refineries mostly done between countries. These tankers are highly specialized and are usually constructed according to specification under strict government or regulatory agency supervision. They are classified according to their sizes ranging from Small Range product tankers(10,000 to 60,000 tons) to Ultra Large Crude Carriers (320,000 to 550,000 tons). These vessels require a huge amount of power generation for ship propulsion. Since these vessels are very large, slow speed marine diesel engines are used to power these vessels. These are two- stroke diesel engines that require two strokes of the engine to complete one cycle of operation. They are suitable for large vessels that do not require high speed cruises. They operate in the cycle of compression, combustion,power stroke, exhaust. The fuel is injected by the fuel injector into the combustion chamber which contains the piston. In the combustion chamber is hot air which has been compressed to a very high temperature. As the fuel is injected and mixes with the hot air, enormous amounts of energy are released which pushes the piston downwards. The downward motion of the piston transmits motion through the connecting rod to the crankshaft. The crankshaft then transmits rotary motion to the propeller shafts which then rotates and provides the thrust that propels the ship forward. There are a series of gears which connect the crankshaft to the propeller shaft and reduces or increase the RPM of the ship as required during voyage.