First you entered the control room, a small space with a lot of screens. Communication with the bridge is done from here and the engineers can control the different machinery of each engine. Then with proper protection for your ears, you reach the place where the 4 engines of the ship are. Generally, two of them are running at the same time. It is hot and very noisy - not an easy place to work in - but clean!
Each engine has 9 cylinders. Injectors and exhaust valves are quite impressive in size particularly compared to car engines.
The ship tanks contain 800 cubic meters of diesel plus there is a dedicated low sulphur-diesel tank.
The engines convert thermal energy from the combustion of the fuel to electricity which drives the two shafts to which the two propellers of the ship are attached.
This is how the ship moves, but a lot of other things happen down here: the desalination of seawater to freshwater (for the showers and sinks), the treatment of sewage before its discharge in the sea, the cooling of the water used to cool the engines, the cleaning of the engine oil, … etc.