On each compression and firing stroke, some combustion gases inevitably escape past the pistons into the crankcase. This is known as piston blow- by. The fumes consist mostly of unburned fuel (hydrocarbons). As an engine wears, piston blow-by increases.

Because unburned hydro- carbons form an explosive mixture, dilute the sump oil and form sludge, car makers ventilate the crankcase to let them out. Until emission control regulations took effect, the ‘road draught’ system was used, in which forward motion of the car created a vacuum at the outer end of a ventilation tube from the crankcase. Fresh air was drawn in—usually through the oil filler—to replace the vented crankcase fumes.

 Unfortunately ‘road draught’ ventilation was ineffective below 25mph and oil contamination was high in engines used mostly in town traffic.

Positive crankcase ventilation was introduced to control emissions (unburned hydrocarbons are poisonous) and provide ventilation regardless of road speed. A positive system uses the vacuum in the inlet manifold to suck fumes from the crankcase and pass them to the combustion chamber where they are burned.

The system contains a positive crankcase valve (PCV) that acts as a fire precaution and adjusts the flow of fumes taken in by the engine. If a back-fire occurs in the manifold, the back-flow pushes the valve down, blocking the route to the crankcase. When manifold vacuum is high, at idling speed or small throttle openings, the valve lifts against spring pressure, reducing the flow of fumes and air to the manifold. This prevents an over-weak mixture from stalling the engine.

Open ventilation

This works by taking in fresh air through a ventilated oil filler cap and feeding crankcase fumes into the engine air-intake system.

On new engines it works well, but worn engines can overload the system by producing more piston blow-by at full throttle than the inlet manifold vacuum can cope with. If this occurs, surplus fumes escape to the atmosphere through the filler cap.

Closed ventilation

Fresh air is taken from inside the carburettor air cleaner body to a sealed oil filler cap or direct to a sealed valve cover. If the crankcase of a worn engine cannot be cleared by inlet vacuum, the surplus fumes are vented into the air cleaner where they enter the carburettor and eventually are conveyed to the combustion chambers. Carburettor settings are arranged to allow for this extra air.