Why is maintaining crankcase vacuum critical in large two-stroke diesel engines?

Maintaining crankcase vacuum in large two-stroke diesel engines is critical primarily for two reasons: controlling oil mist and detecting scavenge fires early. Large two-stroke engines have a significant amount of lubricating oil splashed around inside the crankcase to lubricate the moving parts such as the crankshaft, connecting rods, and bearings. This splashing creates a fine mist of oil particles. By maintaining a slight vacuum, typically a few millibars below atmospheric pressure, this oil mist is drawn towards the suction points of the vacuum system. This prevents the oil mist from escaping into the engine room, which would otherwise create a hazardous and dirty environment. The crankcase vacuum system typically includes oil mist separators to remove the oil particles from the extracted air before venting it to the atmosphere or back into the engine's air intake. The second crucial reason is early detection of scavenge fires. In two-stroke engines, the scavenge space is used to force air into the cylinders to expel exhaust gases. Oil can accumulate in this space over time. If hot combustion gases leak past the piston rings, they can ignite this oil, leading to a scavenge fire. A crankcase vacuum helps prevent the rapid spread of such a fire by limiting the oxygen available. Also, smoke detectors connected to the crankcase vacuum system can quickly detect the presence of smoke from a small fire, allowing for prompt intervention before it escalates into a major engine room fire. The vacuum also helps to contain any explosive pressures that might build up from such a fire, directing them through the controlled venting system.