Q I have always assumed that blue
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smoke is produced by lubricating oil being discharged as a mist with the exhaust gases. A colleague states that it can be caused by incomplete combustion. Is this possible?
AYou do not state whether the type of unit
under consideration is a petrol engine or a diesel. In a diesel engine blue smoke cannot be caused by incomplete combustion (which produces black smoke), but it can be caused by non-combustion of part of the fuel which vaporizes and is discharged with the exhaust gases. It can also be produced by vaporization of unburnt lubricating oil in the way you describe, which is the only cause of blue smoke in the case of a petrol engine.
Non-combustion of part of the fuel in a diesel may result from retarded injection timing, cold running under light load, the use of a poor quality fuel or from faulty injectors that permit the fuel to impinge on the combustion chamber walls.