Crankcase ventilation systems 1

[Deividas]

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?

 

[Deividas]

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?

 

[Tmoose]

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 -

http://speedtalk.com/forum/viewtopic.php?f=1&t=44510&hilit=vacuum+ring+seal&start=15

 

[Deividas]

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/crankcase_vacuum_facts.html

 

[gruntguru]

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

 

[tbuelna]

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.

 

[Deividas]

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

 

[dicer]

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.