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Crankcase ventilation systems

Crankcase ventilation systems

Crankcase ventilation systems

Hi everyone! I have some questions about crankcase vent systems with pcv valves.

there is one system

there is another system

and there is pcv valve
My question is, when and how fresh air from intake before throttle plate could flow INTO crankcase? Like in second picture. If at idle pcv highly restricts air coming from crankcase, then pressure in crankcase should be less than atmospheric to draw air from intake before throttle plate via breather tube.
If engine running at WOT, then pcv also closes, because pressure after throttle plate is about atmospheric and all blow-by gases should go out via breather tube, which is before throttle plate, but only if crankcase has higher pressure than atmospheric. (Or via pcv, if crankcase pressure eovercome pcv spring force).
So pcv is open only at mid engine load (like in 3rd pic) and then in crankcase maybe is slightly vacuum, and in that situation can be, that air comes IN crankcase from breather tube which is before throttle plate (like in 2nd pic)? OR all system should work like in first picture, when fresh air never comes into crankcase? Or maybe i don't understand something?

RE: Crankcase ventilation systems

Link to 3rd picture is broken.

Passenger car engines need ventilation. That is, at one remote region fresh (clean) air comes in that has to migrate thru all sections of the crankcase, cylinder head etc and then go into the intake to be burned.

Just venting off pressure ( no fresh air inlet ) would leave stagnant regions.

At full throttle WOT the PCV valve is wide opent I believe.

RE: Crankcase ventilation systems

Link to 3rd pic: http://www.stealth316.com/images/pcv-operation.gif
Yes, i was wrong, at WOT pcv valve should be open, sorry, it closes only when manifold pressure overcomes spring force (turbocharger or backfire). So air comes into crankcase via breather only then, when crankcase has lower pressure than intake before throttle plate, and crankcase has lower pressure than intake before throttle plate only at mid engine loads? Because at idle, pcv is closed, and at WOT, pressure through all intake system is about atmospheric, so vacuum in crankcase can not be, or i'm wrong? And at WOT all blow-by gases should go into intake via pcv and also via breather, yes?

RE: Crankcase ventilation systems

"Because at idle, pcv is closed......."

I believe when the PCV is "closed" at idle it still is a small calibrated leak which lets "air" ( plus blowby ) into the carb.
There is still blowby at idle, so if unvented, there would be pressure in the crankcase. There is ( or should be) vacuum in the crankcase at idle because the PCV uses the high manifold vacuum.

RE: Crankcase ventilation systems

Then how about diesels, when in intake is always atmospheric pressure? Or BMW Valvetronic systems, where also in intake is always atmospheric pressure?

RE: Crankcase ventilation systems

Never dealt with a deezul.
This description at the autozone sight is pretty specific for Isuzu engines , gas, diesel and turbo diese types. It appears for neither diesel there is no "fresh air in".


This discussion over on a Dodge truck Cummins news group discusses a

These are supposedly quotes from Cummins service manuals. They suggest that at high loads the unthrottled air intake does still have enough pressure drop to require regulation/limitation by the CDR, but there is no mention of "fresh air" into the crankcase.
I don't picture diesel blowby being daisy fresh so as to not benefit from a fresh air intake, but I guess that may be the case.
"The CCV valve is used to vent crankcase gases back into intake of engine. If crankcase ventilation filter becomes clogged, under high intake vacuum situations, the CCV will prevent the engine from siphoning crankcase gases and oil from the engine crankcase."

"The CDR, Crankcase Depression Regulator, is designed to function with low-level vacuum. The tuna-can size is to accomodate the large silicone rubber\synthetic diaphragm. Vacuum on the diaphragm actuates the valve against a low-pressure spring, calibrated for turbo or non-turbo applications, which also functions to ensure that the valve opens as vacuum decreases.

Fully open at idle, it will begin to close as power demand and vacuum increases, regulating the 'depression' in the crankcase such that excessive oil is not drawn out of the engine, and blow-by pressure in the crankcase does not increase."

Some diesels have intake throttles, presumably to create some manifold for EGR or something.
Look at the complicated centrifugal oil vapor filter and heateres used on some Mercedes diesels

RE: Crankcase ventilation systems

Every single motorcycle engine that I have ever owned or dealt with, has a simple vent tube from the crankcase to the airbox. No PCV valve, no flow-through ventilation.

RE: Crankcase ventilation systems

Hello. I am new here. I am not an engineer, but I try real hard.

I have a '59 Chevrolet Apache truck where the previous owner replaced the 6-cylinder with a 305 V8. I noticed when driving at high rpm (highway driving) oil accumulates on top of the engine at the junction of the valve cover/head and the intake manifold. One pcv port on the passenger valve cover was blocked, so I inserted new pcv valve and ported it to the air cleaner. The driver side valve cover has pcv valve ported directed to intake manifold.

Anyone have any suggestions as to why oil pools on top of intake at the valleys with the heads? Only other thing I could think of is blow-by past the valve cover gaskets. Perhaps blow-by or gassing between the intake and heads pushing oil up and through intake manifold gasket. There is no vent from the oil pan or lower end of the block.

I welcome your thoughts.


RE: Crankcase ventilation systems

Racing cars from that era had multiple vents/ breathers all over the valve covers.
... but the engine in the vehicle now is not that old, so you can't do it the 1959 way.
... which by the way, did allow/cause oil pooling on the engine, but only hot rodders cared, hence the vents.

Try to figure out the age of the current engine, and try to approximate the PCV plumbing for a vehicle of that era.

Mike Halloran
Pembroke Pines, FL, USA

RE: Crankcase ventilation systems

I can't imagine any GM small block V8 having a problem with lube oil accumulating in the cylinder head. The flow of lube oil into the cylinder head is quite modest, and the size of the return drain areas to the crankcase is fairly large.

RE: Crankcase ventilation systems

I had a 305 that did similar, and started to smoke at higher speeds. The drain holes in the heads were full of carbonized oil flakes. Opened them up and it quit.

"One pcv port on the passenger valve cover was blocked, so I inserted new pcv valve and ported it to the air cleaner. The driver side valve cover has pcv valve ported directed to intake manifold."

No, if the PCV valve is on the driver side, the passenger side valve cover should be connected to the air cleaner WITHOUT a PCV valve.

RE: Crankcase ventilation systems

Since a 305 does not have the port for the road draft tube like the 283 did, it needs a pcv port in a valve cover.
The oil is likely from some bad valve cover gaskets, they really didn't do a very good job with the amount and spacing of the bolts to help them seal.

RE: Crankcase ventilation systems

Maybe someone can explain to me one thing. I was looking at my father's Ford 1.8L diesel engine, and it has one thing, that i can't understand
[img http://workshop-manuals.com/ford/Mondeo_2007.5_02....]
it has same crank vent system like in this picture, all info of that vent system i can find is:
"The PCV system consists of a crankcase vent oil separator connected to the crankcase and valve cover by hoses.

The crankcase pressure is controlled by a diaphragm operated PCV valve built into the valve cover and cannot be removed. As the pressure in the crankcase vent oil separator reduces, the diaphragm valve closes against the force of the PCV valve spring, so closing the PCV valve. As the crankcase pressure rises, the PCV valve is pushed open assisted by the PCV valve spring. This allows the positive crankcase gasses to be scavenged by the air intake system."
So, on the right side is oil separator which is connected to the head and to the crankcase, in the head is pcv valve, and in other side of head is hose, which connects head with engine intake system (not shown in this picture). To hose, which connects oil separator and crankcase, is connected another hose (left one), which goes to engine head, maybe someone knows that hose (left one) purpose?

RE: Crankcase ventilation systems

I'm asking, because if pcv valve in the head is fitted to stop air/gases from going to crankcase from head, then all that air/gases can go to crankcase through another (left) hose and pcv valve is useless. And if in crankcase is higher pressure than in head, some pressure from crankcase can go to head and then to intake through left hose, bypassing oil separator. Or i'm missing something?

RE: Crankcase ventilation systems

well - if gases do go directly into the head area, perhaps past the valve guides, perhaps it is better to extract them from the head rather than having them flow down and out through the crankcase?
Also, you have to recall that any explanation in a workshop manual could be misleading or wrong.

Jay Maechtlen

RE: Crankcase ventilation systems

JayMaechtlen - all gases should go into intake through pipe in other side of head, if in head or crankcase is excessive pressure. I wonder, if in one port in head cover is pcv valve, which should stop gases from going into crankcase, and then hose, connected to pcv valve, which goes to oil separator and then to crankcase, then what purpose is to fit another hose, which goes from another port in head cover to that hose, which goes from oil separator to crankcase :)

RE: Crankcase ventilation systems

The key word in PCV is "positive". The PCV valve is designed to prevent excess pressure in the crankcase volume. The air being displaced from the crankcase contains a fair amount of hydrocarbons and oil vapor so it cannot be discharged into the atmosphere. Instead it gets recycled into the intake airflow, and combusted by the engine. The reduced air pressure level in the intake manifold downstream of the throttle makes it easier to generate a flow out of the crankcase.

RE: Crankcase ventilation systems

tbuelna - i know that, but look at picture in my previous post, and you see, that in this system (Ford Endura diesel engines use it) there is hose, through which all gases can bypass oil separator and pcv valve :)

RE: Crankcase ventilation systems

Air goes IN through one of those hoses. It goes OUT through the oil separator and valve. The other end of those hoses ends up in a place where the pressure is different while the engine is running, which encourages the flow to be in this direction.

RE: Crankcase ventilation systems

Thank You very much :)

RE: Crankcase ventilation systems

Yesterday i heard, that high crankcase pressure can keep piston rings from sealing, and low pressure, or vacuum, in crankcase can help rings seal better, is it realy true? If it's true, then how vacuum can help rings seal better? :)

RE: Crankcase ventilation systems

I think crankcase seals and gaskets would suffer before the ring sealing changed much in passenger car service.

Piston rings, like o-rings, rely on the pressurized fluid enhancing and even creating their seal. The sealing relies on the difference in pressure.
IN the extreme if the pressure wer equal on both sides there would be no seal.

At TDC the inertia of the top compression ring can overcome the compression pressure and cause ring flutter etc.

Current thinking is the second ring gap should be larger than the top, specifically to be sure the pressure between the rings is as low as possible to enhance seal.

Here is one discussion -

RE: Crankcase ventilation systems



Piston rings, like o-rings, rely on the pressurized fluid enhancing and even creating their seal. The sealing relies on the difference in pressure.
IN the extreme if the pressure wer equal on both sides there would be no seal.

But how that pressure difference affects rings sealing? It helps reduce rings flutter and keep rings on the bottom of the ring groove? I heard, that if there is enough vacuum in crankcase, it's posible to use lower tension rings and get less friction from them

Source: http://nutterracingengines.com/racing_oil_pumps/cr...

RE: Crankcase ventilation systems

The gas force pushing the rings against the cylinder is a function of the pressure difference (combustion - crankcase) not combustion pressure alone.

je suis charlie

RE: Crankcase ventilation systems

The aspect ratio of the compression ring (width/height) and static friction coefficient at the ring/groove contact have a significant effect on the radial force produced at the cylinder wall.

RE: Crankcase ventilation systems

So this is one of the main reasons why some engine builders use crankcase vacuum pumps?

RE: Crankcase ventilation systems

Positive------> no possibility of doubt. Unlike an old road draft tube system, no draft then no ventilation.
That's what the key word Positive in PCV means.

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