He said 'emulsified'
You just don't get a chance to say emulsified every day.
Doha!
PCV was for emissions on cars in the late 50's early 60's and really is no benefit for planes. Well I'm lying a little. Positive Crank Case Ventilation has some benefits, like possibly cleaner oil. If you ever worked on old car engines, you find lots of gook in them, pre-PVC. But we change our oil every 25 hours anyway. Air-cooled engines do have more blow-by, so there's plenty of positive pressure to blow out the vent to take the gook with it. It does not NEED more suck on the vent to vent in other words.
Time honored consensus is a neutral or slight positive pressure at the vent exit is good. Suction or negative pressure at the vent exit just means sucking out more oil than would normally vent. Just let it vent naturally is the advice I give. Many a builder have accidentally put his vent in the air-stream with a square cut exit. They found oil got sucked out their engine at an alarming rate. You don't want to increase oil use.
USING THE EXHAUST FOR A VENT?
To open "da udder can-O-worms", some guys are enamored with the IDEA of routing the crank case vent to the exhaust. I did a lot of research on using the exhaust for crank venting. In my opinion there are too many unknowns. A few possible failure scenarios could mean no vent at all. Also pressure in the exhaust of a plane varies wildly. In drag car's, it's just, Waaaaa, wide open...shut down, which is another story.
Some drag cars use exhaust vented crankcases set-ups or belt driven vacuum pumps to pull down the crank pressure, which increases piston seating, thus making more power. The exhaust set-up often has some kind of reverse flow valve. Would you put that on a plane? What if the valve stuck closed? Keep in mind these are supercharged drag engines turning 9,000 rpm. The piston and rings get changed every race or week. Do we need to change or increase RING seating on a Lycoming? Oil use is needed in air cooled engines. Oil use is normal and encouraged. This is why I say unknowns.
My biggest worry is a BLOCKED VENT. Blowing the front seal and massive oil loss means all kinds of hurt. If some one came up with some standard components to vent through the exhaust, flight tested, sized, redundant and fail-safe, great. However getting hot rod parts for a V8 or home brewed exhaust vent is a big if. Some concerns are:
Excessive loss of oil
Too effective at high power
Not effective (especially at altitude or low power)
Exhaust blown back (pos pressure) into crankcase (need valve?)
If reverse flow valve is used, could it fail closed and kill all venting
Fitting inside exhaust blocks exhaust flow (neg affect on performance)
More hoses, weight, cost and fab time welding on exhaust, for what?
The time honored vent hose end in the cowl, just above the hot exhaust, with the angle cut facet facing fwd works. Why re-invent the wheel. It's in the RV plans. Of course a whistle slot is a good idea as always.
Regarding air/oil separator, ditto as asav8tor. If you are blowing so much oil out the vent you need to collect it, you have an engine issue. A healthy Lyc flown regularly should not burn more than say one qt in 15 hours, min 8, max over +20 hours per qt. A lot of that oil is being burned and goes out the tail pipes anyway, not the vent. I've run both w/ and with out air/oil separator. There's was no difference on the belly. I collected to return line in a bottle, not returning it to the engine. As asav8tor said its an emulsified mess. So I had the hassle of draining the bottle from time to time. Keep it light and save your money.
