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How it works:
The 4-stroke cycle:
The four stroke cycle is the main process in a 4 stroke diesel engine, like the one in the Queen Elizabeth 2 after her 1986 refit. The process consists of 4 parts: intake, compression, combustion, exhaust. Intake is when air comes into the cylinder through a valve on the top of the cylinder. Compression is when this air is compressed to very high amounts of pressure. This creates heat. To understand the combustion stage, we need to know the fire triangle. The fire triangle is a model which shows what three things you need to start a fire: heat, oxygen and fuel. If you remove one of these, a fire can't start. So far, in the cylinder, we have heat due to the compression and oxygen. To start the combustion, we need fuel. A small injector in the top of the cylinder injects diesel into the cylinder completing the fire triangle. Combustion takeas place in the cylinder and pushes the piston down. This is what creates power in the engine. The cylinder then goes back up and the exhaust valve opens to push out exhaust gases. You can see this process in motion on the diagram on the left.
Starting and stopping the engine:
To start an engine, compressed air has to be blown into the cylinder in the correct sequence so that the pistons are moved down and back up. The pistons will then compress the air causing combustion. Once combustion has taken place, the engine can work on its own and does not need help from compressed air. It should continue running until you cut off the fuel supply by stopping the fuel injectors.
Coolant:
Cooling the engine is very important as a lot of heat is produced in the 4 stroke cycle. If the engine is not properly cooled, the components can warp and crack ultimately causing total engine faliure. On a marine engine, there are two types of cooling systems: closed and open loop. Open loop is when water is pulled directly from the sea. This water is pulled through a strainer to get rid of all the sea weed and fish etc. This strainer is routinly cleaned. The water then goes into the engine and cools the components that need to be cooled. The closed loop system does the same but the coolant does not leave the system. It goes into the engine, cools the component, leaves the engine and is then cooled itself.
This strainer is from a smaller engine
The Crank and Propeller shaft:
Each piston is connected to the crankshaft. This is how the pistons reciprocating motion is turned into circular motion. On the moving diagram above, the crankshaft is labelled N and you can see how the piston moves the crankshaft. The crankshaft is directly connected to the propeller shaft and at the end of the propeller shaft is the propeller. This then spins and propells the ship forward. Each propeller will have a seperate engine in most cases.
CAM shaft and valves:
The CAM shaft is connected to the crankshaft via the timing belt or gear trains (most larger ships will use gear trains as a timing belt is not strong enough). The CAM shaft makes sure the valves in the cylinder open at the correct time. They will make sure that: on the first stroke, the fresh air valve is open; on the second stroke, both valves are shut; on the third stroke, both valves are shut and on the last stroke, the exhaust valve is open. If a valve is open at the wrong time, major damage could take place. For example, a piston could slam into a valve and render the cylinder inaffective. For this reason, the timing belt / gear trains must be tight enough and keep the CAM and crank shaft in time.
Image of diesel engines on ocean liners
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