One of most frightening phenomenon for operators of diesel engines is crankcase explosion. Although many force-lubricated, totally enclosed, reciprocating machines like diesel engines, steam engines, air compressors and many others had been running for many hours without any such problems, still problem of crankcase explosion is ever present and we cannot forget about it.
In order to understand phenomenon of crankcase explosion, we have to understand nature of fire, because, after all, an explosion is a very rapid propagation of fire.
A fire will only start whenever three conditions are met: fuel, oxygen, and heat.
A fire can very well start inside crankcase of a diesel engine when conditions are just right. For all recorded occurrences of crankcase explosions, one factor is common, namely a hot spot. That is heat source.
Now, let us take a look inside crankcase. It houses crankshaft, connecting rod, piston rod, cylinder liner, piston, drive gears or chains and lubrication oil. To prevent lubrication oil loss, protect persons from moving mechanical parts, prevent contamination of oil, and many other reasons crankcase is totally enclosed and separated from external environment.
First, let us look at moving parts. There is reciprocating mechanism consisting of crankshaft, journal bearings, and connecting rod. If it is a crosshead engine, crosshead sliding and reciprocating mechanism. For trunk type engine, there is piston and small end bearings.
There are also gears or chains to drive camshaft. For reversible engines, reversing drive mechanisms. There is also thrust bearings at end of crankshaft to take up driven load. There may also be cooling oil pipes, or cooling water pipes sliding inside crankcase for piston cooling. The piston rod itself may also be sliding through a gland assembly at under piston space.