An Investigation into the Mechanism of Oil Loss past Pistons

Abstract

The mechanism of oil loss past pistons was investigated in a laboratory test rig in which a piston was both reciprocated and subjected to gas pressures. The investigation brought to light the following facts and it is believed that this is the first time that they have been demonstrated: (1) Oil control is a two-way and not a one-way problem. The only reason why the rate of oil consumption is reasonably small in most engines is that, although a large amount of oil flows towards the combustion chamber past the rings during part of the engine cycle, nearly all of it returns to the sump during the remainder of the cycle. (2) The various routes permit surprisingly large rates of oil flow in both directions past rings. This applies even with good rings. (3) If oil is added to the combustion chamber, a good ring arrangement will pass it to the sump at a rate up to many thousand times larger than the normal rate of oil consumption with the same arrangement. (4) The piston lands, and the top land in particular, play an important role in deciding the rate of oil consumption. If some oil has passed the rings it may be thrown into the combustion chamber and be lost; alternatively, an effective top land may retain this oil until by one method or another it is returned to the sump.