What does this "air/oil separator" really do?

[rationull]

... well, clearly it's meant to separate air from oil but my question is really more where is that air going?

The current Civic's 1.8l engine has an "air/oil separator" on the back of its block. The best illustration of this I can find is in a TSB I have a copy of (forgive the arrows, the TSB is only tangentially related to this part but it's the best picture I have):

Presumably the actual air/oil separator is the compartment cast into the block with the "maze" of aluminum walls inside. It looks like maybe oily vapor comes in through the hole in the bottom and gets sucked by engine vacuum through the hole in the top, hopefully leaving the oil droplets in the compartment and allowing only the air through.

Where's the air going, though? The top hole is right below the cylinder number 2 intake runner. Is this somehow plumbed into the PCV system even though it's not directly going to the PCV valve as far as I can tell? (The PCV valve is on the block next to cyl 1, to the right of the gray plug in the top right of the top picture).


Maybe I'm being dense or am just uninformed but I've never seen this on the engines of previous cars I've owned (or other cars we have now). Why does the engine have this setup on it?

 

[oggb]

Neat.

 

[Errtt]

Interesting - never seen/heard of it before.

 

[440Magnum]

Interesting. Closest thing I've ever seen, at least as a part of the engine block, is the breather mechanism on an old 8-horsepower Kohler.

 

[expat]

It's not uncommon to fabricate something like this for some older British cars and attach in place of a mechanical fuel pump (on the engine block) when converting to an electric fuel pump.
Many engines did not have valve seals, so it's important to reduce any crankcase pressure as much as possible by venting.
But you don't want to vent your oil with the gasses.

 

[tinmanSC]

I assume this does the job of an "oil catch can" out of PCV valve?

 

[severach]

Oil easily separates from air if the air is slowed down. All you need to do to slow down a constant flow is widen the pipe near the end.

 

[Rick in PA]

Originally Posted By: 440Magnum
Interesting. Closest thing I've ever seen, at least as a part of the engine block, is the breather mechanism on an old 8-horsepower Kohler.


It reminds me of the breather plate on the sides of Tecumseh one cylinders. I always assumed they were there for crankcase pressure relief. The backside of that piston going back and forth has got to be doing funny things to the pressure in the crankcase.

 

[SAJEFFC]

My 2000 Ford ZX2 2.0L DOHC 4 has kind of the same thing on the front of the block below the exhaust manifold. There is just a small hole in the block

and a metal cover over it. The PCV valve actually fits into the cover. It seperates the oil from air and then its sucked into the pcv valve and then on to the bottom of the intake manifold. Simple but effective I guess!

 

[mechtech2]

I have a ZX2 also with a separator.
But they are not uncommon. Yours is more intricate and sophisticated than most, for sure, but many cars have them.
Before blowby gets to the PCV valve and lines, the actual liquids are attenuated by the separator.

 

[rationull]

Interesting. So presuming this really is just before the PCV valve in that circuit, I wonder if this could have something to do with the car's oil consumption...

 

[ddean]

Air/oil separator are an old idea. GM Quad 4 and its later Twin Cam "replacement" used a plastic box separator bolted to the front of the engine. The Quad 4/Twin Cam engines did not have a PCV valve, just the air/oil separator. This is the common reason why these engines will suddenly start to use oil, sometimes at a rate of 1qt/250-300 miles because of clogging up. Having this built into the engine block is a better idea, especially if you can access it 200k miles down the road if you have any problems.

 

[PandaBear]

My understanding is that air oil mixture flow through the maze at high speed and direction change, that oil will hit the wall at the turn and drip down rather than get sucked up.