What is Positive Crankcase Ventilation (PCV)? What is a PCV Plate?
Before we go over the variety of oil catch can set-ups, it’s best to explain why you need a positive crankcase ventilation (PCV) system, how it works, and the role it plays on your car.
If you aren't familiar with how a four stroke engine works:
A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
- Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum (negative pressure) in the cylinder through its downward motion.
- Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
- Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
- Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
Depending on you specific engine, the pressure inside the combustion chamber during the compression, and power strokes can be thousands of PSI. Though the piston rings do a relatively good job at sealing this pressure into the combustion chamber, some of the combustion gasses make their way past the rings into the crankcase. This process, and the gases, is called blow-by.
The PCV system is in place on your vehicle to allow the blow-by gasses from the combustion process to evacuate the crankcase. Engine performance, emissions, and reliability are all impacted by how your PCV system functions. Improper release of blow-by gases can result in poor efficiency, erratic behavior, and even mechanical issues like blowing out a dipstick, or rear main seal.
So how does a PCV system work? A stock PCV system will take the blow-by from the combustion process, filter it, and recirculate it back into the intake tract. It does this under all engine conditions, idle, cruising, heavy load, boost, engine braking, etc. All components of the PCV system need to work as one, to ensure the recirculation of the blow-by goes smoothly.
The main PCV components include:
-An oil separator or two: A means of separating the oil from the air. Many Mazda and Ford engines are equipped with two; the oil separator on the engine block, which our PCV plate replaces, and the valve cover.
-A PCV valve: A means to control the flow of air from the crankcase.
-Intake manifold port: A means to pull a vacuum.
-Intake tract port, pre-turbo: A means to pull a vacuum under boost.
The PCV system uses the PCV valve to ensure proper operation of the system under various conditions. This valve’s job is to strictly manage the flow of air through the crankcase under any vacuum condition, and to prevent intake backfires or boost from entering the crankcase. Under various increments of intake vacuum, the PCV valve adjusts to allow only the amount of flow needed to properly evacuate the blow-by.
First under high vacuum, such as idling or engine braking, the plunger inside the valve is fully opened from the high vacuum in the intake manifold. This restricts the amount of air pulled from the crankcase into the manifold, as under these conditions blow-by is quite low. If the vacuum is high enough, and blow-by low enough, fresh air from the intake tract may come into the valve cover, and the crankcase.
Next we see a cruising condition. When cruising at mid load, there is not as much vacuum from the manifold, but blow-by is more prominent. The plunger inside the valve is pulled open to a middle position. The middle position of this valve allows a larger flow area than the fully open position. This allows the additional blow-by gases to escape into the manifold, despite having lower vacuum to pull them.
Lastly we see a no vacuum condition, an intake backfire or boost, where the valve closes to prevent additional pressure from entering the crankcase. With a closed PCV valve, the valve cover to intake tract is the main means of evacuating blow-by under boost conditions. Under boost is when the most blow-by can occur, so it has to be evacuated from the crankcase efficiently to avoid excess crankcase pressure. The intake helps evacuate blow-by gasses by pulling a slight vacuum on the crankcase. Since the PCV valve is closed on the other end of the system, there is no fresh air source. This is generally enough to properly evacuate the crankcase of blow-by.
By now, you hopefully understand the importance of the PCV system, and how the pieces work together to keep crankcase pressures under control under all conditions the OEM designed for. However, this system isn’t perfect.
The most common concern with a PCV system is that the blow-by gases that come out of the crankcase, and into the intake side of your engine, have entrained oil that comes along with them. This oil can coat intakes, intercoolers, even valves with oil buildup. It can lead to loss of efficiency, power, or even reliability, among other things. A solution to this is to add in a filter, or a means to further separate the entrained oil in the blow-by. This is where an oil catch can comes into play, as an OCC is a means of filtering.
Another concern is increased crankcase pressure from engine modifications, and tuning. Tuned cars running higher than stock boost levels will see an increase in cylinder pressures, and more blow-by because of it. This leads to higher crankcase pressure, and this added flow may be more than the OEM PCV system can relieve. If you’ve ever seen a dipstick pop out, and spew engine oil all over an engine bay, it’s because the excess crankcase pressure was not allowed to evacuate fast enough. Excess crankcase pressure can also harm performance, and engine efficiency. With lower crankcase pressure you prevent oil from pushing past the rings making it’s way into the combustion chamber. This makes for a cleaner, more efficient air and fuel burn.
In contrast from crankcase pressure, you also do not want is excess vacuum on the crankcase, as excess crankcase vacuum can be harmful to your engine. This vacuum can pull oil away from important components such as the oil control rings, piston wrist pins, and even the camshaft oiling surfaces. Instead of blowing out seals with excess pressure, excess vacuum can pull on seals like the rear main and front crankshaft seals, causing leaks. This is why incorporating a PCV valve is beneficial for controlling the amount of vacuum flow on the crankcase.
An internal combustion engine is built around a series of hollow cylinders, in each of which is a moveable piston designed to glide up and down inside it. A mixture of air and gasoline is pumped through a system of tubes called the intake manifold through each cylinder's intake valve (or valves), where a spark from a spark plug causes the mixture to explode in the open space at the top of the cylinder called the combustion chamber. The pressure from this explosion drives the piston in the cylinder downward, where it causes the crankshaft to rotate. The rotation of the crankshaft not only pushes the piston back up into the cylinder so it can do all this again, but it also turns the gears within the car's transmission that eventually make the car move. Meanwhile, the rising piston pushes the air and gas left over from the explosion back out of the cylinder through an exhaust valve.
However -- and this is where crankcase ventilation comes in -- a certain amount of that mixture of air and gasoline is pulled down by the piston and slips through the piston rings into the crankcase, which is the protective cover that insulates the crankshaft. This escaping gas is called blow-by and it's unavoidable. It's also undesirable because the unburned gasoline in it can gunk up the system and produce problems in the crankcase. Until the early 1960s, these blow-by gases were removed simply by letting air circulate freely through the crankcase, wafting away the gases and venting them as emissions. Then, in the early 1960s, positive crankshaft ventilation (PCV) was invented. This is now considered the beginning of automobile emission control.
Positive crankcase ventilation involves recycling these gases through a valve (called, appropriately, the PCV valve) to the intake manifold, where they're pumped back into the cylinders for another shot at combustion. It isn't always desirable to have these gases in the cylinders because they tend to be mostly air and can make the gas-air mixture in the cylinders a little too lean -- that is, too low on gasoline -- for effective combustion. So the blow-by gases should only be recycled when the car is traveling at slow speeds or idling. Fortunately, when the engine is idling the air pressure in the intake manifold is lower than the air pressure in the crankcase, and it's this lower pressure (which sometimes approaches pure vacuum) that sucks the blow-by gases through the PCV valve and back into the intake. When the engine speeds up, the air pressure in the intake manifold increases and the suction slows down, reducing the amount of blow-by gas recycled to the cylinders. This is good, because the blow-by gases aren't needed when the engine speeds up. In fact, when the car is up to speed, the pressure in the intake manifold can actually become higher than the pressure in the crankcase, potentially forcing the blow-by gases back into the crankcase. Since the whole point of positive crankcase ventilation is to keep these gases out of the crankcase, the PCV valve is designed to close off when this happens and block the backflow of gases.
PCV System Oil and Air Separator
The crankcase in a car is used as a storage place for oil, usually in a pan located below the crankshaft. While the crankshaft and the oil aren't intended to come into contact (because if they did the oil would get frothed up like a thick, black milkshake), oil vapors can still find their way into the blow-by gases. It's not a good idea for these oil vapors to be recirculated back into the cylinders along with the blow-by gases because they make the gas-air mixture too combustible, equivalent to lowering the octane of the gasoline, which in some engines can degrade performance slightly and in older engines can even cause backfire when the gas-air mixture combusts prematurely. The oil vapors can also coat the air intake with an oily film, gradually clogging the air flow over time. If you don't drive a high performance vehicle, these problems aren't exactly crucial to your car's operation and the oil build-up can be scrubbed out periodically during maintenance, but some people (and some car manufacturers) prefer to have something that will scrub the oil out of the blow-by gases before they're recirculated in the first place. Enter the oil and air separator.
The idea of an oil and air separator is to extract the oil from the air before it's sent back to the intake manifold and put it someplace where it won't cause a problem, either back in the crankcase or in a small receptacle called a catch can. Not all cars come with built-in oil separators and not all cars necessarily need them, but they can be purchased as aftermarket items. And if you have the necessary DIY skills, you can even make one yourself. There are actually a number of different ways in which these oil and air separators can work. Probably the most common kind blows the oily air through a mesh filter. The oil droplets are trapped in the mesh while the air passes through. The most effective such filters are made up of microfibers, which can trap very small particles of oil. Alternatively, the air and oil filter may require the recycled gases to go down a tube with holes in its side. The lighter air molecules escape through the holes, while the heavier oil droplets fall all the way to the bottom, where they can be removed. And some advanced systems use a centrifuge to drive the heavier oil droplets out of the air. The oil coalesces on the sides of the centrifuge and can be channeled back into the crankcase.
How does a PCV system work?
What happens when a PCV valve goes bad?
How much does it cost to replace a PCV valve?
What are the signs of a bad PCV valve?
What is the most common problem with PCV valves?
Intake manifold vacuum is applied to the crankcase via the PCV valve. The airflow through the crankcase and engine interior sweeps away combustion byproduct gases. 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. On some PCV systems, this oil baffling takes place in a discrete replaceable part called the 'oil separator'. Aftermarket products sold to add an external oil baffling system to vehicles, which were not originally installed with them, are commonly known as "oil catch tanks".
The PCV valve controls the flow of crankcase gases entering the intake system. 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.
When the engine is under load or operating at higher RPM, a higher quantity of blow-by gases are produced. The intake manifold vacuum, with wide open throttle, is lower in these conditions, which causes the PCV valve to open and the crankcase gases flow to the intake system. The greater flow rate of intake air during these conditions means that a greater quantity of blow-by gases can be added to the intake system without compromising the operation of the engine. The opening of the PCV valve during these conditions also compensates for the intake system being less effective at drawing crankcase gases into 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. This can happen on turbocharged engines or when a backfire takes place, and the positive pressure could damage the crankcase seals and gaskets, or even cause a crankcase explosion. The PCV valve therefore closes when positive pressure is present, to prevent it from reaching the crankcase.
The crankcase air outlet, where the PCV valve is located, is generally placed as far as possible from the crankcase breather. For example, the breather and outlet are frequently on opposite valve covers on a V engine, or on opposite ends of the valve cover on an inline engine. The PCV valve is often, but not always, placed at the valve cover; it may be located anywhere between the crankcase air outlet and the inlet manifold.
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