Crank Case Explosion

       

   Under normal conditions the atmosphere in the crankcase when the engine is running contains a large amount of relatively large oil droplets (200 micron) in warm air. Because of the droplets small surface area to volume ratio, the possibility of ignition by a heat source is very low.

Should overheating occur in the crankcase, say by failure of a bearing, then a hot spot is formed (typically over 400'C although experiments have shown two seperate temperature ranges, the other between 270 - 300'C>.  

It can be seen that following the initial explosion there is a drop in pressure, if the initial explosion is not safely dealt with and damage to the crankcase closure occurs, it is possible that air can be drawn in so creating the environment for a second and possible larger explosion.
The limiting factors for an explosion is the supply of fuel and the supply of oxygen, the air as shown can be drawn in by the slight vacuum created by the primary explosion.
The supply of fuel may be created by the passage of the shockwave shattering the larger oil droplets into the small size that can readily  combust.

By regulation,non returning relief doors must be fitted to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of

Here lub oil falling on to the surface is vaporised ( in addition some is broken down to flammable gasses such as Hydrogen and acetylene), the vapour can then travel away from the hotspot where it will condense.
The condensed droplets, in the form of a dense white mist, are very much smaller (6 to 10 microns) than the original and so have a high surface area to volume ratio

Ignition by a hot spot (generally of the flammable gasses which in turn ignite the fine droplets in the mist), which may be the same on that cause the original vaporisation, is now a possibility.
Oil mists are formed at temperatures of around 350 degreeC
Ignition occurs at under 500 degree C


The white mist will increase in size and density until the lower flammability limit is exceeded (about 50mg/l is generally found in real situations ), the resultant explosion can vary from relatively mild with explosion speeds of a few inches per second and little heat and pressure rise To severe with shock wave and detonation velocities of 1.5 to 2 miles per second and pressures of 30 atmospheres produced.
This is the extreme case with pressures of 1.5 to 3.0 bar more normal raising to a maximum of 7.0 bar.