Blowby, Breathing and Oil Slobbering from Small Engines

Breathing flow is the sum of blowby and pumping effect by the piston reciprocating in the crankcase. In a single cylinder engine or in an engine where the volume of the crankcase changes violently due to piston motion (e.g., a two cylinder engine where the two pistons reciprocate in phase) the fluctuating air flow through the breather would be high irrespective of engine speed but for a non-return valve. Oil slobbering through the engine breather is a function of breathing rate, oil loading and separation in the breather.

Experiments showed three components of the crankcase pressure: high frequency acoustics due to the blowby jets, cyclic pressure due to the change in volume, and the pressure due to the mass flow from the cylinders and to the atmosphere through the breather valve. A model for the adiabatic pulsating flow is developed using the first law of thermodynamics. Leakage through engine openings present inadvertently in the engine is modelled as a parallel path to the atmosphere. With leakage through the openings, the air flow through the breather increases greatly and oil slobbering (oil throw) ensues.