Hello, List...
I hate to admit my age but feel I must, because I note some confusion among
the group about "POSITIVE" crankcase ventilation.
Apparently, some are understanding the "positive" part to mean "Positive", or
"Above atmospheric" pressure, and therefore feel that the crankcase should be
sealed and pressurized. This is exactly the opposite of the true meaning of
the term. Correctly, "Positive" ventilation means "Reliable".
We all know that when an engine runs it creates lots of nasty things inside,
such as acids and water, etc. If allowed to remain in the engine, these
create "sludge", and eventually ruin an engine. This eventually closes oil
passages and if you have ever opened up one of the old "Pre-detergent-oil"
engines you know that it is a real mess.
This condition can be lessened by ventilating, or creating a movement of air
through the crankcase to allow the byproducts to evaporate as they are
generated.
In past, this was done by having a filter of steel wool in the oil filler cap
so air could get into the crankcase, and a "Road draft tube" hanging down
under the bottom of the car to move air out of the crankcase. This tube was
typically about 1" - 1 1/4" in diameter and cut at a 45 degree angle on the
end so that air flowing over it as the car moved down the highway pulled air
from the crankcase and into the atmosphere. (Oh, nooooooo, the
ATMOSPHERE!!!!)
The system worked just wonderfully except that it had two flaws: On short
runs, the engine temperature didn't get high enough to vaporize the
contaminants, and some service such as delivery trucks and city police cars,
the vehicle didn't move down the road fast enough to create any airflow
through the engine. These engines literally "stewed in their own juices".
The early Plymouth V-8's were bad for this. The oil supply to the valves was
poor to begin with and sludging rapidly closed it off completely in a short
time.
The cure for the problem was fairly straightforward. You could buy a kit
that replaced the oil filler cap with a small squirrel-cage blower that blew
air through the engine whenever the ignition was turned on. It provided an
airflow that could be relied on to happen... a "Positive" crankcase
ventilation system.
I'm sure you've all seen old cars that had oil vapors blowing out underneath
them. This is one sign of a worn engine. It is called "Blowby". Obviously
not good for breathing.
Enter the EPA (Or their forbears.) This was not acceptable any longer. I
was working at the Chrysler Engineering Carburetor Lab about that time in the
late 50's. Someone, somewhere, came up with the idea of feeding the Blowby
back through the carburetor airstream and burning it. That worked OK except
that it dirtied the carburetor and still didn't move air at low roadspeeds.
Finally, the first practical antismog device was developed: The PCV valve.
It entered the system below the carburetor so created fewer problems.
At high manifold vacuum conditions (idle) the valve allowed a small but
"Positive" amount of airflow through the crankcase without overwhelming the
idle system. At lower manifold pressures it allowed a larger flow of air to
compensate for more Blowby. It continues to be used the same way today.
Please pardon the rambling.... I can't sleep!
Joe Savard
Lake Orion, MI
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