Positive Crankcase Ventilation

By Brandon Baldwin

              Have you ever gotten the question at a car show asking “what are the little air filters on the valve covers for?”  Did all of you have an answer?  That’s for crankcase venting.  But, why does it need venting you might ask? There is pressure in the crankcase, especially when the engine is under load.  We need to get rid of that pressure in the crankcase.  The little air filters just vent the crankcase gases to the atmosphere.  But, to pull a small vacuum in the crankcase is even better because we really don’t want any pressure in the crankcase.  Crankcase pressure is the case of many, many oil leaks.  Many oil pan gaskets, valve cover gaskets, and so on have been replaced because crankcase pressure blew them out.  Next we will talk about what this pressure is and then what to do about it.

Crankcase pressure is blow-by.  When a piston is on the compression stroke, there is pressure in the combustion chamber before the combustion even begins.  When a compression test is done, we are checking for leakage of compression pressure past the piston rings.  The rings seal the piston to the cylinder walls.  The other side of the rings, below the piston, is the crankcase.  So, when the piston rings are worn and leak, the pressure leaks into the crankcase, increasing the pressure there.  Now, consider how much more pressure there is in the cylinder on the power stroke.  Yes, you may have figured that this would create even more crankcase pressure.  What leaks past the piston is a combination of combustion gases (exhaust), carbon dioxide, and water vapor.  Of course, these need to be evacuated from the crankcase.  If they aren’t, they dilute the oil and push out the gaskets that can hold the least pressure.  Gaskets, such as the oil pan gasket and valve cover gaskets aren’t designed to hold pressure. 

Speaking of diluting the oil, these blow-by gases are bad for many reasons. Diluting the oil causes break-down of the oil’s lubricating properties.  Think about the ethanol content in fuel now too; it’s a great solvent.  Do you want to dilute the very product that keeps your engine lubricated and cools the crankshaft, camshaft, and so on?  Obviously not.  But, wait, there’s more.  Remember the water vapor of complete combustion?  That’s in blow-by too.  When the engine is cold, and you have warm moist blow-by entering the crankcase, the water vapor condenses on the inside of the engine block walls, then drips down to the oil pan.  If this is excessive (I had a customer who only drove ¼ mile to work, so his oil was always milky=oil and water mix which doesn’t lubricate well), then it reduces engine life significantly.  Yes, short trips do this. 

All this looks terrible, doesn’t it?  There is a solution!  Positive Crankcase Ventilation (PCV) is your cure!  Well, not for short trips, but it still helps.  The PCV valve applies a small vacuum to the crankcase to pull these contaminating gases into the intake to be put back through the combustion chamber with the fresh air/fuel charge.  For those who have taken refrigeration, you know that a vacuum can lower the boiling point of anything.  Therefore, we can lower the boiling point of water too, meaning that we can more effectively use a combination of heat and vacuum to remove the water from the oil.  This combination also helps to remove the other contaminates in the oil.  PCV employs a valve with a tube to the vacuum side and simply a tube on the fresh-air side.  Therefore, we have circulation in the crankcase.

How do I know when any of this is failing?  When you have oil leak problems constantly, when you have oil soaked air filters, or when you have an oil smell around the engine.  Of course, not all oil leaks are a result of the PCV.  Sometimes, gaskets do fail.  But, if you are ever questioning it, replace the valve.  They are usually cheap, unless it’s a Volkswagen.  Theirs is much more complex.  Or, if your car is turbo.  Then there is more tubing and often there is a catch can to capture the oil in vapor form that comes quickly out of the crankcase due to the higher combustion pressures.  Some manufacturers require PCV replacement if the car repair has anything to do with an oil leak or consumption (Subaru).

Enjoy your holidays and keep “combusting along”.

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