1
1.1 INTRODUCTION
High temperatures, in excess of 200
o
C that occur on bearing surfaces under initial failure conditions, can
lead to a rapid generation of oil vapour When the hot vapour contacts the relatively cooler atmosphere
of the crankcase it condenses into a fine mist, with typical particle sizes of around 0.5 to 5 microns in
diameter. When the density of these particles reaches between 30 to 50 mg/l (Milligrams per litre),
depending upon the type of oil, an explosive condition exists.
Figure 2 Areas of Failure 4 Stroke Engine
A fire or explosion needs three constituents: fuel, oxygen and an ignition source. Remove one of these
and no explosion will occur. Similarly, within the crankcase, the three constituents which could cause
oil mist density can be measured at levels as low as 0.05 mg/l and give early warning of a rise in oil mist
density.
Oil Mist Detection (OMD) techniques have been used to monitor diesel engine crankcases for potential
explosive conditions and early detection of bearing failures. The systems available rely mainly on
analysing the optical density of oil mist samples drawn from the crankcase compartments, through pipes
to the detector. While these systems proved successful in the past, engine design has improved
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