Marine diesel engine is a kind of diesel engine used on ships.

Its working principle is as follows:

A fresh air is pumped in or pumped into the engine cylinder, which is then compressed to a high pressure by the moving piston.
When the air is compressed, its temperature rises so that it can ignite a fine mist of fuel injected into the cylinder.
The burning of the fuel adds more heat to the filling air, causes the expansion and forces the piston to work on the crankshaft, which in turn drives the ship's propeller through other shafts.
  

The operation between two fuel jets is called a working cycle.
In a four-stroke diesel engine, this cycle needs to be completed by four different strokes of the piston, namely, inhalation, compression, expansion and exhaust.
If we combine inspiration and exhaust with compression and expansion, the four-stroke engine becomes a two-stroke generator.
  

The two-stroke cycle begins at the bottom of the piston from the bottom of its stroke (the bottom point), when the air inlet of the cylinder is open.
At this point, the exhaust valve is opened, fresh air is filled into the cylinder, and the exhaust gas from the last stroke is blown out through the open exhaust valve.
The valve blew out.
  

When the piston moves up to about a fifth of the morning, it closes the air inlet and the exhaust valve closes, so the temperature and pressure rise to a high value.
When the piston reaches the top of its stroke (i.e., the upper stop), the fuel valve is sprayed into the high temperature air in the cylinder, and the fuel burns immediately, causing the pressure to rise quickly.
Thus, the expanding gas forces the piston to move downward in the work stroke.
 

When the piston moves down to half of the trip of a place, the exhaust valve opens, the high temperature of the gas pressure from its beginning through the exhaust valve to drain out, helped by the pressure through the inlet into the fresh air.
The inlet is opened as the piston moves further down.
Then another cycle begins.
  

In a two-stroke engine, the crankshaft on a business trip for a power stroke, and four stroke engines, need the second revolution of the crankshaft to do work of a stroke, that is why the two-stroke engine under the same size can do about two times the four-stroke engine by the reason of doing work.
In the current actual use, the engine with the same cylinder diameter and the same speed, the output of the two-stroke engine is about 80 percent higher than the four-stroke engine.
The increase of the power of the engine makes the two-stroke engine widely used as a large ship host.
   

There are two differences between Marine diesel engine and ordinary diesel engine.

For one, the ship's oil is usually high in oil.
Due to the high sulphur content of Marine fuel (generally within the range of 0.5% to 3.5%), the lubricating oil must have sufficient alkali retention to neutralize the acid generated by the combustion of fuel.
      

Second, the ship's oil resistance is good.
It is inevitable that the ship will encounter water pollution at sea. Therefore, it is required that the Marine lubricating oil must have good anti-emulsifying performance and water separation performance, while the diesel oil of the land use diesel engine has no such working condition and no such requirement.
In addition, ship oil has all the other properties of diesel engine oil.