How does a positive crankcase ventilation (PCV) system work?

A certain amount of air and fuel mix is pulled down by the piston and slips through the piston rings into the crankcase. This escaping gas is called blow-by -- and it's unavoidable. See more pictures of engines.
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Unless you're a real gear head, just seeing the phrase "positive crankcase ventilation" probably makes your head hurt, because it sounds, well, complicated. But it really isn't all that complicated. Or at least it shouldn't seem complicated after we've finished explaining it to you. But to do that, we're going to have to give you a quick refresher course in how the internal combustion engines found in most automobiles work. Okay -- one, two, three, go!

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.

Author's Note

It sometimes amazes me how much thought has gone over the years into the ways that cars work and how some of our ideas about automobile construction have changed over time. Today, emission control is an extremely important part of automobile design, because it minimizes the amount of pollutants that escape into the atmosphere and degrade the environment. While researching this article I was impressed to learn that the idea of emission control began almost exactly half a century ago, with the invention of positive crankcase ventilation and the PCV valve. Of course, there are much more advanced emissions control systems available today and cars with zero emissions are already possible -- electric cars have no emissions at the tailpipe, though emissions may be produced when the electricity is initially generated -- and within a few decades, when internal combustion engines in cars have become obsolete, automotive emissions may be a thing of the past. When that happens, we can thank the inventors of positive crankcase ventilation for leading the way.

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  • Bastias, Dr. Pedro, et al. "Air/Oil Separator with Minimal Space Requirements in the Crankcase Venting System." (May 15, 2012)
  • Blackwood, Jim. "Positive Crankcase Ventilation." British V8. (May 15, 2012)
  • Conceptual Polymer. "PCV Line Removal 102." (May 15, 2012)
  • "Positive Crankcase Ventilation." (May 15, 2012)
  • University of Missouri. "Positive Crankcase Ventilation System." (May 15, 2012)
  • Yahoo Autos. "What is the PCV valve and what does it do?" (May 15, 2012)