Crankcase ventilation system
The blow-by gases would diffuse through the oil and then leak through the seals and gaskets into the atmosphere, causing air pollution and odors.
When the vehicle is moving, airflow across the open end of the tube creates suction (a "draught" or draft) that pulls gases out of the crankcase.
[1] The draught tube could become clogged with snow or ice, in which case crankcase pressure would build and cause oil leaks and gasket failure.
[3] In the early 1950s, Professor Arie Jan Haagen-Smit established that pollution from automobile engines was a major cause of the smog crisis being experienced in Los Angeles, California.
In 1961, California regulations required that all new cars be sold with a PCV system, therefore representing the first implementation of a vehicle emissions control device.
[6] Starting with the 1963 model year, most new cars sold in the U.S. were so equipped by voluntary industry action so as to avoid having to make multiple state-specific versions of vehicles.
PCV quickly became standard equipment on all vehicles worldwide because of its benefits not only in emissions reduction but also in engine internal cleanliness and oil lifespan.
After eighteen months of investigation, the grand jury returned a "no-bill" decision, clearing the AMA, but resulting in a consent decree that all U.S. automobile companies agreed not to work jointly on smog control activities for a period of ten years.
This mixture of air and crankcase gases then exits, often via another simple baffle, screen, or mesh to exclude oil mist, through the PCV valve and into the intake manifold.
At idle, with almost closed throttle, the manifold vacuum is high, which would draw in a large quantity of crankcase gases, causing the engine to run too lean.
The PCV valve closes when the manifold vacuum is high, restricting the quantity of crankcase gases entering the intake system.
A second function of the PCV valve is to act as a flame arrester and to prevent positive pressure from the intake system from entering the crankcase.
As previously mentioned, in vehicles with forced induction systems such as turbochargers or superchargers, the engine's intake manifold experiences positive pressure under load.
In addition to this added role, in boosted applications cylinder pressures are much higher, and consequently, more blow-by gases are pushed into the crankcase thus making a fully functional PCV system all the more important.
[12] Carbon build-up or oil sludge from blow-by gases on intake valves are usually not a problem in port injected engines.
The draught tube routes all blow-by gases back into the intake mixture and is usually located between the air filter and carburetor.
Dry sump engines in some drag racing cars use scavenging pumps to extract oil and gases from the crankcase.
