Pcv how does it work

Until the early s, 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 s, 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.

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.

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. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. This can lead to the gases mixing with motor oil to create corrosive, engine-clogging sludge. Additionally, the gases that exit through the exhaust system damage the environment by polluting the atmosphere.

While gases can exit the crankcase through the PCV valve, they cannot return into it. This prevents the crankcase from developing engine-damaging oil sludge. Unlike changing your battery or oil, there is no set time frame to replace your PCV valve.

Over time, however, the gases flowing through your PCV valve leave traces of oil that can clog it up. Be sure to look out for symptoms of a damaged PCV valve, including:. You can typically find it attached on a valve cover at the end of a hose or tube in your engine. If you shake your PCV valve and hear the metallic rattling noise of it opening and closing, it is likely working well. Additionally, if your vehicle is idling roughly, your PCV valve could be to blame. Rough idling due to a damaged PCV valve, unfortunately, is often misdiagnosed as more expensive issues.

Save yourself time and frustration and contact Kennedy Transmission to get help from the experts.