How does a PCV valve work VS. how it's supposed to

[ueberooo]

How does a PCV valve actually work ??? ( VS. how it's supposed to work.)

So, I know what the function of this valve is and what it's supposed to do.

However, when I inspect the mechanical device it shouldn't seem to work (my explanation below). I also tested it, by blowing through it from the (crankase end)--the valve did not open, as expected from my understanding of the mechanical workings of this device.

I would expect the device to work, if it were seated upside down from its current orientation into the intake manifold.

The device seems to have a plug. With no pressure, gravity would pull the plug into a closed position. Pressure from the crankase would push the plug even deeper into the closed position.

I'll attempt some ASCII sketch.


||| [ ]
[ ]
[ ]
|*| \ /_____________
intake manifold


I would understand how the valve would function, if things were upside down, in that case, pressure--if it was large enough-- from the crankcase would overcome the force of gravity and push the plug upwards into an open position. However, as it is oriented, pressure from the crankcase air and gravity work along the same direction, and will never cancel another out.

Now the above ascii sketch isn't 100% accurate, in reality the valve is seated at about a 45 degree angle, but still with gravity seating the plug in the closed position.

Can someone explain this mystery to me please??


Also, why is there a need for a valve at all?? Why not a straight-thru hose The intake manifold is always vaccuum-like, or below atmospheric pressure (at worst at most at atmosheric pressure when engine is at WOT and low rpm) and the crankcase always is greater or equal to atmospheric pressure; so therefore the flow (if there is a flow at all) would always go from crankcase into the intake manifold; then why is there a need for a valve at all?

What would happen if I replaced the valve with just a hose connection?

 

[Astro14]

The valve is there to prevent excessive flow at idle, when another air source would be essentially a vacuum leak (and there is vey little blowby that requires ventilation).

 

[joegreen]

They used to have a hose vented straight to the atmosphere many many years ago but it was polluting so they vented it back into the engine to be burned.

 

[EvanD]

pvc is emission control. Fresh air usually after MAF then into intake manifold. Where it just causes oil and junk to collect. Change an intake manifold after 100K you will get oil spill out of one.

I just freely vent mine.

 

[ueberooo]

thanks.

I've found one sketch so far indicating the plug has passages. Still, the principle by which the plug opens up further from high crankcase pressure eludes me

If gravity here matters, the pic would be oriented almost upside down relative to how the pcv screws into my intake housing.

 

[Clevy]

Ive read that the pcv lessens crankcase pressure and if you create enough vacuum it can help increase power.
I can't say for sure where I read it. I'm thinking it was a performance car magazine. They had an engine on a dyno and were toying with varying crankcase pressures and their effect on horsepower and torque. Move vacuum equaled more power however there was a trade off though I can't remember exactly what it was.
The pcv helps the oil last longer by re-ingestion of crankcase gasses. We used to just put a filter on the hole and leave it however vacuum is a better idea.

 

[chrisri]

quote:
Also, why is there a need for a valve at all?? Why not a straight-thru hose The intake manifold is always vaccuum-like, or below atmospheric pressure (at worst at most at atmosheric pressure when engine is at WOT and low rpm) and the crankcase always is greater or equal to atmospheric pressure; so therefore the flow (if there is a flow at all) would always go from crankcase into the intake manifold; then why is there a need for a valve at all?

What would happen if I replaced the valve with just a hose connection?
[/quote]





Actually there are lot of cars without PCV valve. I have seen more cars without it to be honest. Currently own two that only have pipe breather connected to manifold.

 

[zray]

Check this out.





http://mewagner.com

I have one of these, modified to work well with my 4 Weber 48 IDA carbs.

Z

 

[Rick in PA]

The other part of the equation is where the make-up air comes from for the crankcase.

There's a larger diameter tube (A) connected from the clean air side of the intake system to the crankcase. i.e. downstream of the air filter

When the engine has enough vacuum, the PCV valve opens and passes blow-by directly into the intake manifold. Filtered make-up air comes in through the tube "A". This way a vacuum cannot develop in the crankcase. (the arrows show this operation)

When the engine doesn't have enough vacuum, the PCV valve (mostly) closes and any blow-by is directed through the above tube into the intake system. (reverse flow through tube "A")

As was mentioned above, before the PCV system came into being, engines had a simple "road draft tube" that dumped blow-by directly into the atmosphere. It polluted the atmosphere and actually wasn't very good for the engine. In a cold engine there wasn't significant air flow over the oil to remove blow-by and a lot of it condensed in the oil.

 

[Rick in PA]

Just to note: PCV = Positive Crankcase Ventilation

 

[zray]

Originally Posted By: Rick in PA

"....As was mentioned above, before the PCV system came into being, engines had a simple "road draft tube" that dumped blow-by directly into the atmosphere. It polluted the atmosphere and actually wasn't very good for the engine. In a cold engine there wasn't significant air flow over the oil to remove blow-by and a lot of it condensed in the oil.


Sort of true, there wasn't much air flow when the car was moving slowly, but at speed it worked very well in that regard. but you didn't mention the stink inside the car when stopped. Whew, now that was unbearable at times. I have a early '65 Mustang that came with a road draft tube (since removed).

They did have one advantage over a PCV system however. The road draft tube was very effective in reducing crankcase pressure when under full throttle. The bottom end of it was cut off at a steep angle, and the effect of the wind rushing over that end was a substantial sucking, and thereby a reduction in crankcase pressure when it is really needed. In contrast, a PCV does nothing at full throttle since there is very little to no engine vacuum to activate it. The "reverse flow" mentioned previously doesn't relieve much pressure. That's why drag racers, among others whose engines are under full throttle constantly, utilize an exhaust driven crankcase evacuation system. That's a vacuum signal that just gets stronger as the engine winds up.

Z

 

[Astro14]

No PCV is illegal in most states here if the car came with PCV.

My Packard has a road tube to vent combustion blow by.

You can't just hook up a hose to the intake manifold, because this is connected to the manifold after the throttle plate (vacuum), so that hose will provide unmetered air entering the manifold. Air that didn't go past the MAF, or the carb jets, will cause the mixture to be off and because the flow rate will vary, it will never idle right.

The valve closes at high vacuum (idle) then opens at lower vacuum so that the engine idles correctly. Otherwise that hose is just a giant vacuum leak.

 

[artificialist]

Some modern cars use a fixed orifice instead of a PCV valve. The GM Ecotec and Jeep 4.0L are the first 2 examples I can think of.

 

[TiredTrucker]

All sounds nice, except the 5.3L in my 2013 Silverado doesn't have a PCV valve. Only an orifice at the rear of the left side valve cover. No internal component to it and a non replaceable item. Hose straight to intake. This is one of the reasons I put on a oil catch can in the line. And glad I did. It captures about 1 oz of oil every 1000 miles on average. The pickup "uses" roughly 1/4 quart (8 ox) of oil in 6000 miles. Of that, the catch can captures about 6 oz of oil in the same time frame. Some say a catch can is not necessary. Could be, but until they are paying the bills, they have no standing to decide what is right or wrong for my vehicle. The results convince me it was worth it.