why the ME engine accumulator is filled with nitrogen?
This question is marked "community wiki".
Hydraulic accumulators act as pressure surges compensators in hydraulic systems.
Gas chamber of accumulator changes its volume in response to change of system pressure. All accumulator's gas chambers should be pre-charged with gas to some pressure (which should be regularly checked every 4 months).
Air is not suitable as a compressed gas medium as it contains oxygen. Nitrogen is the cheapest inert gas and perfectly suitable as compressible medium in technical systems.
