Estimating check valve cracking pressure

[moonie223]

Hello!

I am trying to make a fast acting check valve for use in the PCV system in a car with a turbo. Normally only half of the crankcase breather vents are open to the air under power, as the PCV closes in absence of vacuum. This works fine for a stock car, but I have increased blowby with the added output.

Instead of removing the PCV system and venting both halves of the valve cover, I would like to install a oil catch can with a check valve that opens with even a tiny bit of differential pressure while maintaining the PCV valve. I am thinking that in most cases the catch can will be under vacuum holding my catch can check valve closed and the system will work like normal, but in turbo boost the PCV will close, the catch can will pressurize a little and pop the check valve open so I have ventilation out both factory ports in my valve cover.

I think a check valve could be made using a 3/8NPT Male to 1/2NPT Female adapter, a 1/2NPT sintered vent, a 1/2" rubber ball, and a 1.6lb/in conical spring. Mcmaster has all this, for around what a commercial valve costs. I would mount all this vertical in the catch can, so gravity is helping seat the valve.

https://files.engineering.com/getfi...af-4751-b3d6-255962a68ca3&file=checkvalve.png

If Mcmaster solid models are halfway accurate, I expect 0.206in^2 of check ball exposed to pressure, or the acting piston area I guess.

And here's where I hit a wall! If we assume the valve "cracks" open at a incredibly small value, like a tenth of an inch, then the valve has to open with nearly no pressure differential? Otherwise I would think the math is simple, 1.61lb/in / 0.206in2 = 7.8PSI? That doesn't make sense, plus the valve can't even open an inch...

And so, I ask for any advise you are willing to give. Will this work like I expect it to? Do you think using rubber under ~1/2lb of spring preload against a seat like this is a good idea? I was planning on adding a small 90 degree chamfer to make an actual seat.

Thank you in advance!

 

[RVAmeche]

Just use the stock PCV (a check valve itself) with a catch can between it and the intake manifold; many turbo cars are set up this way. A separate vacuum connection should be connected to the valve cover(s) on the inlet/vacuum side of the turbo (after your MAF if you have one).

I don't think you need to re-engineer the wheel here.

 

[kjoiner]

Hello,

You might want to re-post this in the "engine and fuel engineering" group. There has historically been a lot of discussion on turbocharged engines there.

Kyle

 

[moonie223]

The problem with using the stock PCV system like you describe is under power half of the vents in the block are closed. This does not work, the crankcase will pressurize.

https://www.youtube.com/watch?time_continue=12&v=rsYK_PwlzHk

Most people go ahead and drill out the ports in the oil catch can built into the valve cover to fix this...

https://www.miataturbo.net/diy-turb...em-crankcase-pressure-test-80353/#post1259695

But this has a problem of increasing air flow through the internal baffles too much, or doing so decreases air speed so much that the oil extracting chambers in the valve cover don't work. Many people who drill out the valve cover ports have problems with catch cans filling with oil in no time at all. See the comment on the youtube video I posted, someone drained a quart of oil out the PCV in a single 3-4-5 pull. I believe that's because they have way too much airflow through a modified valve cover.

To try and avoid all this I would prefer to use both internal oil baffles in their stock configuration, without drilling them out. This was the reason for putting a check valve on the catch can in addition to the stock PCV valve. When the PCV is closed from boost the catch can is closed off unless I add a check valve that opens to atmospheric. If I do, then I should theoretically gain about double the flow through the stock ports in the valve cover without touching the port sizes. Hopefully this keeps the air speed in each set of internal baffles down, so that the oil has a second to condense and drain back down to the sump instead of being pulled through to a catch can.

I run speed density, no MAF, so that part is easy!

Maybe I will move my post over to the other group, thank you for the advise!

 

[RVAmeche]

The PCV is only there to relieve crankcase pressure when it becomes positive. If you're running boost, yes, it won't open; that's why you have the other connection to the turbo inlet which is always drawing vacuum.

 

[hydtools]

Should you be concerned that bounce and vibration would open your ball check valve if the opening force it too light? The weight of the ball + spring force divided by the ball seat area would be an estimate of force to open. Look at a duckbill valve, that might work.


Ted

 

[moonie223]

Hydtools, I actually considered no spring for a while. The internal geometry of the brass fittings plus the fact that most of the time the catch can will be in partial vacuum and holding the ball in place.

Looks like I was figuring wrong, I only need to know the springrate to calculate the preload. That does make more sense! In this case I'd swap to a shorter, higher rate spring. 3.57lb/in, 0.625" long with ~0.045" preload. The seat area is still 0.206in^2, so I'd expect a cracking pressure around 0.7 PSI. Only 0.15lb of preload, that is kind of light and assumes I can crank those brass fittings to full thread depth. I might have to look elsewhere for springs, or shim it a little. I could use a 0.75" 1.94lb/in spring, but with 0.16" preload I'd have a cracking pressure ~1.5 PSI. Guess I could order both springs.

If I wanted to design a PCV valve in a similar way, the seat area would be a hemisphere's surface area, right? The area being acted upon by the "pulling" force? Or is it still (differential) pressure pushing the valve open, therefore it'd be the much smaller area at the seat?

Most stock PCVs are crap, the sealing valve is a stamped or even just sheared off coil stock. It works 99% of the time for N/A cars, besides a backfire they'd never even really need PCV valves anyway. While I can find a factory PCV valve, probably off a turbo car, that isn't total crap that will work I'd rather try making my own.

A duckbill valve does look like it could work, although it would need stiff reeds to handle vac. Plus, it might give me an audible clue that my setup is working! Either way, never heard of such a thing. Thanks for the tip!