PCV valve
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The Positive Crankcase Ventilation valve, or PCV valve, is a one-way valve that ensures continual refreshment of the air inside a gasoline internal combustion engine's crankcase.
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[edit] Explanation
As an engine runs, the crankcase (containing the crankshaft and other parts) begins to collect combustion chamber gases that leak past the seals around the pistons. These combustion gases are sometimes referred to as "blow by" because the combustion pressure "blows" them "by" the pistons. These gases contain compounds harmful to an engine, particularly hydrocarbons, which are just unburned fuel. It also contains a significant amount of water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds begin to condense out of the air within the crankcase and form sludge on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.
[edit] History
Prior to the early 1960s, automobile gasoline engines ventilated directly to the atmosphere through a simple vent tube. Frequently this consisted of a pipe (the "road draft tube") that extended out from the crankcase down to the bottom of the engine compartment. The bottom of the pipe was open to the atmosphere, and was placed such that when the car was in motion a slight vacuum would be hopefully obtained, helping to extract combustion gases as they collected in the crankcase. The system was not positive though, as gases could travel both ways, or move not at all, if conditions were just right. Modern diesel engines still use this type of system to dispose of crankcase fumes.
In 1952, Professor A. J. Haagen-Smit, of the California Institute of Technology at Pasadena, revealed that hydrocarbons were a primary constituent of smog, and that gasoline powered automobiles were a major source of those hydrocarbons. After some investigation, it became obvious that the road draft tube was allowing considerable amounts of hydrocarbons to enter the atmosphere. The solution was the PCV valve. California was the first US state to mandate installation of a PCV valve around the early 1960s, and the system slowly went nationwide by about 1966.
[edit] PCV system
The PCV valve is only one part of the PCV system, which is essentially a closed-loop calibrated air leak, whereby the engine returns its crankcase combustion gases. Instead of the gases being vented to the atmosphere, gases are fed back into the intake manifold, to re-enter the combustion chamber as part of a fresh charge of air and fuel. The PCV system is not a classical "vacuum leak." Remember that all the air collected by the air cleaner (and metered by the mass air flow sensor, on a fuel injected engine) goes through the intake manifold anyway. The PCV system just diverts a small percentage of this air via the breather to the crankcase before allowing it to be drawn back in to the intake tract again.
The system relies on the fact that the intake manifold's air pressure is usually less than crankcase air pressure. The lower pressure of the manifold forces air to flow 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: 1) The breather tube , and 2) 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 backfire 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.
[edit] PCV valve
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. 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 restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the PCV valve opens further 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.
[edit] Operation
Should the intake manifold's pressure be higher that of the crankcase (which can happen under certain conditions, such as an intake backfire), the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again. This is where the positive comes from in the name. Positive is basically a synonym for one-way.
It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will not be correct. 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 constantly changing its resistance to flow by opening and closing proportionally as one drives a car, it is subject to eventual wear out over time. Typical maintenance schedules for gasoline engines are to replace the PVC valve whenever 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. These droplets keep the valve 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 2-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 4-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 in to the intake combustion air. 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 lube oil.
[1] - pcv image taken - others available
