positive crankcase ventilation

[jpeter]

Hey guys, I hate to admit it but my 4-stroke 8 cylinder 44cc engine burned a lot of oil, spit it out the exhaust pipe and smoked up the shop. Recently I tore into it making new oil rings, changing the piston design, re-honing the cylinders all which helped but it still smoked. The next fix which helped a lot was increasing the crankcase ventilation, but the downside of that was having to deal with the oil that spit out the large vent, and I still had some smoke. To solve that problem I directed the vent to the intake manifold with a vacuum hose through a bottle for oil separation. Now after 10 minutes of running I collect a couple of thimbles full of oil in the bottle but the exhaust is completely devoid of smoke and oil and the engine has never run so good.

My question is how do I apply positive crankcase ventilation without having to collect oil in a bottle. There must be a way to return oil the the crankcase?

 

[gbritnell]

Hi Jim,
Normally on an engine with multiple cylinders the crankcase pulses from the pistons going up and down is fairly well balanced, meaning when one is going up another is going down so there wouldn't be much crankcase pressure from this. If you have a single cylinder or twin with both pistons going up and down at the same time there would be more crankcase pressure.
Automotive practice up until pollution control took over was to put a metal material like a coarse scouring pad inside the breather cap. This would allow the oil to be separated from the vented vapors and would drip back down the filler tube. Another way of doing this was to put baffles into the filler cap so the airflow would have to make right hand bends and being that the oil was heavier than the air flow it wouldn't make the bend and would drip back down into the engine.
When pollution controls took over the crankcase fumes were redirected into the crankcase through the PCV system.
Now onto small engine venting. I have vents on all my engines irregardless of the number of cylinders. On some I suspect they need it on others like my V-8 probably not. The culprit in your question is too much crankcase pressure and the source would be too much blowby from the piston rings. While most builders just use compression type rings in their engines the best way would be to also use an oil control ring. I realize that when you have engines with bores smaller than 1.00 diameter these rings are very hard to make, or should I say install on the pistons without breaking so most builders don't make them. My 302 V-8 has oil rings and it doesn't pass any oil through the exhaust. My Holt doesn't have oil rings but runs very clean also, oil smoke wise.
Here's something else to look at. If I overfill my splash oil system engines with too much oil they will smoke. The reason being there's just too much oil slinging around in the crankcase and without an oil control ring the oil gets sucked into the combustion chamber during the intake stroke through the ring end gaps. For these engines I make sure that the oil level is just high enough for the rods to dip into. My Holt has cover plates on the cankcase and at one time I made a clear cover for one of them just for grins to see what was going on. The oil that gets thrown around in there is tremendous.
Here's another thing to consider. A 2 cycle engine has oil mixed with the fuel for lubrication at anywhere from 32:1 to upwards of 60:1 and they run fine with just that thin film of oil so not a great amount of oil is needed in a splash oil system.
If you are getting that much blowby I suspect that even if you made new rings you are still getting too much cankcase pressure.
gbritnell

 

[BronxFigs]

Thanks George B. for the explanations, and suggested fixes. Very informative.

Frank

 

[t.l.a.r. eng]

Just my humble observations from my last three engines, the small piston rings really don't seat in until after about two hours running time. My best efforts with good materials and the Trimble way of making rings still take time to seat in.
Two hours seems like an eternity for model run times, but when the rings seat, the compression gets dramatically better and the oil control seems better as well.
My Kinner radial now has snappy compression, and the Cole's small hit n miss has bouncy compression after approx. 20 hours of running. My home made 4-stroke single with splash lube has lost its lust for drinking oil after about 3 hours running. The rings just seem to get better with running time.
Perhaps more run time is needed before changing anything?

 

[tezoost]

I hope to find a way of producing castings at a sensible price so others can make this model too. It is about 3/4 complete.