Before the invention of Crankcase Ventilation in 1928 the engine oil seals were designed to withstand this pressure, oil leaking to the ground was accepted and the dipstick was screwed in. The hydrocarbon rich gas would then diffuse through the oil in the seals into the atmosphere. It is therefore an emissions requirement as well as a functional necessity that the crankcase has a ventilation system. This must maintain the crankcase at slightly less than atmospheric pressure and recycle the blow-by gas back into the engine intake. However, due to the constant circulation of the oil within the engine, along with the high speed movement of the crankshaft, an oil mist is also passed through the PCV system and into the intake. The oil is then either burnt during combustion or settles along the intake tract, causing a gradual build-up of residue inside the inlet path. For this reason many engine tuners choose to replace the PCV system with an oil catch can and breather filter which vents the blow-by gases directly to atmosphere and retains the oil in a small tank (or returns it to the sump), although this technically fails to meet most engine emission legislation.
In 1948-49, Arthur R. Glines, an Air Force ROTC mechanical engineering student studying at the University of New Hampshire, recognized the need for a system of this type on current gasoline engines. Having spent many years servicing and repairing cars in the Town of Carroll, New Hampshire, as he worked his way through college, during an advanced automotive engineering class, he conceived of the PCV valve in the same form as was commercially developed in the early 1960s. He was called to fulfill his ROTC commitment at graduation so he did not have an opportunity to develop the PCV valve further for commercial production. His skills and ingenuity were not lost, though, as he went on to work on the first surveillance satellite to photograph the Soviet Union (Corona). He retired in the mid 1980s, after working on the Hubble Space Telescope and the Keyhole series of surveillance satellites that were instrumental in ending the Cold War with the Soviet Union.
In 1952, Professor A. J. Haagen-Smit, of the California Institute of Technology at Pasadena, postulated that unburned hydrocarbons were a primary constituent of smog, and that gasoline powered automobiles were a major source of those hydrocarbons. After some investigation by the GM Research Laboratory (Dr. LLoyd L. Withrow) it was discovered in 1958 that the road draft tube was a major source, about half, of the hydrocarbons coming from the automobile. GM's Cadillac Division, which had built many tanks during WWII, recognized that the simple PCV valve could be used to become the first major reduction in automotive hydrocarbon emissions. After confirming the PCV valves' effectiveness at hydrocarbon reduction, GM offered the PCV solution to the entire U.S. automobile industry, royalty free, through its trade association, the Automobile Manufacturers Association (AMA). In the absence of any legislated requirement, the AMA members agreed to put it on all California cars voluntarily in the early 1960s, with national application following one year later.
Following its introduction into production, several years later the PCV became the subject of a Federal grand jury investigation in 1967, when it was alleged by some industry critics that the AMA was conspiring to keep several such smog reduction devices like the PCV on the shelf to delay smog control. After eighteen months of investigation by U.S. Attorney Samuel Flatow, 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.
The system relies on the fact that, while the engine is running, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.
The PCV system consists of the breather tube and the PCV valve. The breather tube connects the crankcase to a clean source of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after passing through a screen, baffle, or other simple system to arrest a flame front, to prevent a potentially explosive atmosphere within the engine crank case from being ignited from a back-fire in to the intake manifold. The baffle, filter, or screen also traps oil mist, and keeps it inside the engine.
Once inside the engine, the air circulates around the interior of the engine, picking up and clearing away combustion byproduct gases, including a large amount of water vapor, then exits through a simple baffle, screen or mesh to trap oil droplets before being drawn out through the PCV valve, and into the intake manifold.
The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.
It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will be insufficient. A plugged or malfunctioning PCV system will eventually damage an engine. PCV problems are primarily due to neglect or poor maintenance, typically engine oil change intervals that are inadequate for the engine's driving conditions. A poorly-maintained engine's PCV system will eventually become contaminated with sludge, causing serious problems. If the engine's lubricating oil is changed with adequate frequency, the PCV system will remain clear practically for the life of the engine. However, since the valve is operating continuously as one operates the vehicle, it will fail over time. Typical maintenance schedules for gasoline engines include PCV valve replacement whenever the air filter or spark plugs are replaced. The long life of the valve despite the harsh operating environment is due to the trace amount of oil droplets suspended in the air that flows through the valve that keep it lubricated.
Not all gasoline engines have PCV valves. Engines not subject to emission controls, such as certain off-road engines, retain road draft tubes. Dragsters use a scavenger system and venturi tube in the exhaust to draw out combustion gases and maintain a small amount of vacuum in the crankcase to prevent oil leaks on to the race track. Small gasoline two cycle engines use the crank case to compress incoming air. All blow by in these engines is burned in the regular flow of air and fuel through the engine. Many small four-cycle engines such as lawn mower engines and small gasoline generators, simply use a draft tube connected to the intake, between the air filter and carburetor, to route all blow by back into the intake mixture. The higher operating temperature of these small engines has a side effect of preventing large amounts of water vapor and light hydrocarbons from condensing in the engine oil.