By: Thomas Yoon

One of the most frightening phenomenon for operators of diesel engines is the 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 the problem of crankcase explosion is ever present and we cannot forget about it.

In order to understand the phenomenon of crankcase explosion, we have to understand the 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 the crankcase of a diesel engine when the conditions are just right. For all the recorded occurrences of crankcase explosions, one factor is common, namely a hot spot. That is the heat source.

Now, let us take a look inside the crankcase. It houses the crankshaft, connecting rod, piston rod, cylinder liner, piston, drive gears or chains and the lubrication oil. To prevent lubrication oil loss, protect persons from the moving mechanical parts, prevent contamination of the oil, and many other reasons the crankcase is totally enclosed and separated from the external environment.

First, let us look at the moving parts. There is the reciprocating mechanism consisting of the crankshaft, journal bearings, and connecting rod. If it is a crosshead engine, the crosshead sliding and reciprocating mechanism. For the trunk type engine, there is the piston and the small end bearings.

There are also gears or chains to drive the camshaft. For reversible engines, the reversing drive mechanisms. There is also the thrust bearings at the end of the crankshaft to take up the driven load. There may also be cooling oil pipes, or cooling water pipes sliding inside the crankcase for piston cooling. The piston rod itself may also be sliding through a gland assembly at the under piston space.

All the sliding parts can become a hotspot whenever there is insufficient lubrication to reduce the friction, probably during starting of the engine or any abnormal interruption in the force lubrication oil system.

Next, the use of fuel oil in the engine. Dripping fuel injectors will result in incomplete combustion of the fuel above the piston top. If the dripping is serious, the liquid fuel can find their way through the piston rings aided by the high pressure of the combustion process to eventually find its way into the crankcase. For trunk type engines, this is a direct path to the crankcase. It can become a source of low flash point fuel when mixed with the lubricating oil!

For dripping fuel injectors, it is common to find unburnt carbon that will cause the piston rings to stick within their grooves, thus destroying the sealing function of the piston rings. This will not only cause the fuel to drip through, but can also cause fire from the combustion space to blow past through the gaps between the piston rings and the cylinder bore. This can be a source of heat!

There are other conditions favorable to cause an explosion, for example the quality of the oil and the effects of oxidation after a long period of use, the generation of oil mist due to agitation of the oil spray, the property of the oil, etc.

It is impossible to guarantee that any engine will operate for the whole of its life without, somewhere, at some time, a hot spot appearing. Therefore it is essential that should a hot spot occur and if it goes undetected, an explosion will not result.