I was emptying my catch can today, and had a bit of a noob question. Why does this even work? My understanding is, if you didn't have the catch can, the gunk pools up in the intake manifold, where it gets sucked back into the engine for cruddy combustion. But... even with the catch can, doesn't it just pool up in the catch can, where it can get sucked back into the engine for cruddy combustion? What's the difference? The only answer I can think of, is the CC is more accessible, so it gets cleaned out more(monthly) than the IM (every couple years). And you know at least the bit you clean out isn't going into the engine. Is that really all it comes down to? It's in a place easier to empty/clean?
Yeah pretty much. The stock system burns the vast majority of whatever makes it through the PCV valve. With a catch-can, you're intercepting it before it can be recycled. You get a cleaner manifold, but you're dramatically increasing the hydrocarbon emissions for the vehicle. Much cheaper than cleaning out the manifold. I'm fond of saying that Priuses are for cheapskates; environmentalists don't have cars.
An oil catch can does not increase emissions. Emptying the can be recycled with regular oil changes. The oil and water caught in the catch can is not emitted to the air.
It is emitted from the car without benefit of combustion that might provide energy to the crankshaft and also without secondary combustion/reaction in the catalytic converter. The fact that it leaves in a liquid state carried by hand matters little. Remember, PCV valves feeding the intake replaced draft tubes.
An Oil Catch Can that continually returned it's content to the crankcase would be an improvement? If so, if you're draining it with every oil change and dumping it into the drained oil, that's not so bad? That's my scenario btw: I've got mine mounted on the lower cross beam, and it's got a drain spigot and tap. The only practical/quick access is from below, typically at oil change, so I just do the two together. And this is a few tablespoons of "brew", more at the spring oil change, and a cafe au lait appearance. At the fall oil change it appears to be mostly oil, and a lot less. No, it's trapped in the oil catch can.
If you mount the catchcan low like @Mendel Leisk and I have it is the true “low point” knock out pot. The advantage here is it takes a very deep vacuum to suck liquid through the plumbing back through the intake and our engines can’t do that. So it sits there in the catch can waiting for you to drain it and never getting back to the intake manifold where the process of passing through the system is easier. I check our catch cam every so often (once a month it seems anymore) as it has a dipstick that tells me the level and if too high (which it never has been) or at the 10 k oil change interval, it gets dumped into the used oil container . I’ve only caught a couple of ounces each time I’ve changed the oil. Hope that helps.
How is it trapped? The only thing standing between the gunk and the crankcase is a pathway of air and opportunity. Which gets back to my original question. Why is the vacuum not strong enough to suck it from the catch can, but it *is* strong enough to suck it from the intake manifold? Both ways you have liquid just sitting about 1 inch below the main airflow. I'm not trying to be argumentative, I'm genuinely curious about how the physics of this works. Haha, I had very little in the summer, but since winter has started, I'm up to a full quart I've pulled of carbon cappuccino.
Ever try to suck a liquid through a straw that isn’t in your drink? Since there is no diptube in the catch can, the vacuum has to completely overwhelm the pressure and cause liquid to be forced up. Going against gravity ain’t easy. Which is why going low ensures the best odds of no liquid in the intake or back pressure against the PCV valve . Hope that helps.
This: Plus most Oil Catch Cans have some sort of labyrinth. Incoming oil-laden air travels through it, the oil droplets condensing and dripping down to the bottom, the air passing through. I had a perineal oil swimming pool below the throttle body. One year post-install of my Morosso can, it's stayed gone.
Ive also added steel wool to the catch can to add additional residence time to the vapors. Definitely ensures all the vapors condense and stay in the can. Since the can is low and away from the engine or inverter, there is plenty of cooling to allow for condensation . Excellent point to add.
Here's the top half of mine: Air comes in one side, goes out the other. Here's the inside, an "exploded" view: The central wall means all the air wanders through a real labyrinth, out though the holes in the plate, then back up the other side. All of this intended to waylay the airborne oil particles.
Google: Moroso 85474 Air-oil separator Be prepared for sticker shock though. Besides Amazon, check on Jegs too, they will have it, and similar but cheaper alternatives. One technical issue: I found the supplied bolts that screw into clamping collar near-impossibly tight. They have a dab of loctite applied. Even with that wire brushed off, they were very stubborn. I took the collar down to a hardware store and found (and trial fit) similar off-the-shelf bolts, maybe 1/4" dia National Coarse?
Amazon has a $40 version that is what I bought and has worked well. And it’s not either gold plated or made of unobtainium like @Mendel Leisk ’s. Works well .
Just to up the ante: here's a Moroso vs RX Performance test. He puts the 2 in series, to see which will scavenge better, and the RX comes out way ahead:
