every engine needs to breathe,
both down in the crank case & up in the combustion chamber
the combustion chamber is basically above the pistons, where your spark plugs, valves, intake & exhaust manifolds are
Fuel & air goes into the intake manifold, then to the top of the pistons, the spark plug fires, the exhaust manifold takes away the unused pressure
below the pistons, there is the crank case
there is nothing there but a load of parts moving around in oil ( there is no gas or air in the crank case ), it is suppose to be a separate sealed off area
from the combustion chamber, but it is only separated completely by pistons, piston rings, hydraulic lifters, & seals on the valves. So with all these parts moving, there is some PRESSURE in the crankcase, because of blow by.
That pressure builds up & it has no where to go
old engines crankcases had a ROAD DRAFT TUBE that allowed the crankcase to release that pressure build up, the excess pressure went down a tube & dumped just in back of the engine on to the road.
Along came the SMOG people, they said that we could no longer dump these oil fumes onto the road, so PVC smog laws were made.
They said that we had to reuse these oil fumes, basically recycle them through the engine again
So now we are putting oil fumes into the combustion chamber, where oil is not suppose to be,
so someone invented a catch can, it allows the dirty air to pass through the filter in the top of the catch can, but the oil in the fumes are trapped & drop into the bottom of the catch can.
all we have to do is dump the contents when we change our oil
look at this drawing, it shows how the road draft tube used to function