Lesson 4/Learning Event 2
Learning Event 2:
DESCRIBE PURPOSE OF CRANKCASE VENTILATION SYSTEM
The crankcase ventilation system has two important jobs to do. It must get rid of the fuel and water vapors
that collect in the crankcase. It must also remove the "blowby" gases that would otherwise build up
considerable pressure in the crankcase and cause oil seals and gaskets to leak.
We have already discussed how the fuel and water vapors condense in the crankcase and form sludge when
mixed with the oil. Now let's discuss what "blowby" is and what problem it creates.
As you know, the piston rings and the lubricating oil have the job of sealing the high-pressure gases in the
combustion chamber that are present during the compression and power strokes of the piston. The rings
cannot form a perfect seal because they must have a gap so they can be installed on the piston and to allow for
expansion when they become hot. They must also be free to move in the piston ring grooves. Otherwise, they
could not expand out against the cylinder walls. The film of oil on the pistons, cylinder walls, and rings can
help to seal the gases in the combustion chamber, but the film is not 100 percent effective as a seal. So part of
the high-pressure gases in the combustion chamber escape by the rings and oil film into the crankcase. The
gases escaping by the rings and into the crankcase are known as blow by gases.
The gases entering the crankcase as a result of blow by can build up enough pressure to cause oil seals and
gaskets to leak if they are not allowed to escape. In addition, these gases contain unburned fuel, carbon, and
other byproducts of combustion that will mix with and ruin the oil in the crankcase.
Now let's see how one type of crankcase ventilation system works to get rid of the blow by gases and the water
and fuel that get into the crankcase.