Crankcase

The crankcase is the lower structural section of a diesel engine that houses the crankshaft, main bearings, connecting rods, and the lubricating oil sump. In a two-stroke crosshead engine the crankcase is separated from the combustion space by a diaphragm plate and stuffing box around the piston rod, preventing combustion products from contaminating the crankcase lubricating oil. In a four-stroke trunk-piston engine the crankcase communicates with the underside of the pistons, requiring the lubricating oil to have adequate detergency and alkalinity to handle combustion blowby. The crankcase must be kept under slight negative pressure by a crankcase ventilation system to prevent oil mist from being expelled around covers and creating a deck-level fire hazard. Opening crankcase doors while the engine is running or immediately after stopping is strictly prohibited because the rush of air into a hot, oil-mist-laden atmosphere can trigger an explosion. SOLAS II-1/40.3 requires crankcase doors on large engines to be fitted with relief valves that open under explosion overpressure and direct the blast away from personnel. Engineers access the crankcase only when the engine is stopped, cooled, and confirmed safe by elapsed time and temperature measurement.