Sure, I'd type it myself but I'm a little tired. Here'* a few choice paragraphs:
Other materials which collect in the crankcase, such as water, acids, and unburned fuel, may be in the form of either liquid or vapor. These materials are by-products of the combustion process. For example, when gasoline is burned in the presence of an adequate air supply, water is produced at a rate of about one gallon of water for each gallon of gasoline that is burned.
Most of the water and acids created as vapor in the combustion chamber are carried out of the engine through the exhaust. However, some of these vapors reach the crankcase as blow-by. This means that vapor is forced between the rings and the cylinder walls by combustion pressures which are as high as 700 lbs. per square inch. Blow-by occurs even with engines in good condition, but increases as engines become worn. Unburned fuel resulting from incomplete combustion may also enter the crankcase as blow-by vapor. Additionally, some unburned fuel may be forced into the crankcase on the compression stroke of the engine before the fuel/air mixture is ignited. Pressure in the combustion chamber during the compression stroke may be as high as 200 lbs./psi.
Positive Crankcase Ventilation (PCV) is a system that was developed to remove harmful vapor from the engine and to prevent vapor from reaching the atmosphere. The PCV system accomplishes this by using manifold vacuum to draw vapor from the crankcase into the intake manifold. Vapor is then carried with the fuel/air mixture into the combustion chambers where it is finally burned. Thus, PCV is effective as both a crankcase ventilation system and as a pollution control device.
OPEN PCV SYSTEMS
The open system draws fresh air through a vented oil filler cap.
This presents no problem as long as the vapor volume is nominal. However, when the crankcase vapor becomes excessive it is forced back through the vented oil filler cap and into the open atmosphere. Therefore, the open PCV system though successful at removing contaminated vapors from the crankcase is not 100% effective as a pollution control device.
CLOSED PCV SYSTEMS
The closed PCV system draws fresh air from the air filter housing. The oil filler cap in this system is NOT vented.
Consequently excess vapor will be carried back to the air filter housing where it enters the intake manifold through the carburetor. The closed system prevents vapor, whether normal or excessive, from reaching the open atmosphere. The closed system is very effective as an air pollution control device.
Blow-by gases and vapor should be removed at about the same rate they enter the crankcase. Since blow-by is nominal at idle and increases during high-speed operation the PCV valve must control the flow of vapor accordingly. The PCV valve is designed to compensate for the ventilation needs at varying engine speeds. It is operated by manifold vacuum, which increases or decreases as engine speeds change.