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Vol. 4 May 2006
Velocity's Quarterly Newsletter

Article Header Four

Air/Oil Seperators





Duane Swing

Should I or should I not use an air/oil separator on my engine. Naturally all of us want to keep the bottom side of our cowling and prop free of the nasty oil that comes from the vent tube extending out through the bottom of the fuselage. One solution is to route the vent along the side of the exhaust and let it burn off with the exhaust. Keep in mind that this outlet tube needs to be equal in size to the breather outlet in the engine. If you reduce the size, it could cause excessive back pressure and force oil out through the crankshaft seal. I have had the breather tube freeze up in excessively cold weather to the point that the crank seal was pushed out and oil immediately coated the windshield. On the Velocity we would not even see this oil. What caused the freeze was the water vapor that exits along with the oil. The oil didn’t freeze but the moisture did. What happens when we separate the oil from this “breather” air? The oil, along with the moisture, is returned to the sump. Moisture in the sump can cause all kinds of problems with rusting and pitting,  including the camshaft and other rotating parts. Several years ago I read a paper regarding this subject and decided to condense it here for you. It was authored by Richard E. Barrans Jr., Ph.D.

“An aircraft engine is essentially a distillery in which we add water, combustion byproducts (such as unburned fuel, lead oxides, and nitrogen oxides) into a oil bath; agitate and aerate with the crankshaft and other rotating parts; heat until the more volatile products, principally water, vaporizes into a gas and flows through a pipe called the crankcase breather into the cooler atmosphere. What comes out the engine breather is often a milky colored exudates formed from steam distillation of oil. You sometimes see this on the dipstick or other condensing surface. In this form you don’t want to pump it back into the engine as it consists of a mixture of oil and water. There is also some concern that the NOX emissions have combined with the water to form nitric acid.

Corrosion pitting is the most common reason camshaft lobs and followers are damaged (2nd might be stuck valves). Making sure your distillery is functioning properly by removing water from the engine crankcase is important if you want to prevent corrosion damage to your engine. Normally, you need not worry; operate the distillery often and make sure you completely warm it up. Ground running the distillery just adds water and doesn’t produce steam – you need to fly your distillery to fully heat it up.

Every distillery has a condenser that turns the distilled vapors back into liquid form. With engines we prefer that condensation occurs in the atmosphere or in a vent line that slopes downward. Now lets modify the distillery and add what is commonly called an “air/oil separator” to our system. I use the more accurate term “gas/liquid separator” since our separator cannot tell the difference between air and any other gas, or oil and any other liquid. If our gas/liquid separator is cooler than our oil sump, then our gas/liquid separator also becomes a condenser with a return line back to the engine. Oil, along with any distilled liquids, are returned to the engine.

Take for example a worst case scenario. You mount our gas/liquid separator in a cold area of the engine compartment. You attach it to a large heat sink called the firewall. Everything you have done has increased your condenser’s efficiency at condensing water and other vapors into a liquid. Now your engine pumps the condensate back into the engine along with some engine oil. You couldn’t design a better way of trapping water in your engine. 

The warmer our gas/liquid separator, the less efficient our distillery is at condensing water and pumping it back into the engine”.

Does what Dr. Barrans Jr. say make sense to you? It does to me. I would say he has all his facts correct and the major information you need to take home with you is the frequency of flight in your airplane. Short warm-ups on the ramp are sure to create the moisture he talks about. Fly more often and the use of the air/oil separator. I personally would prefer to route the vent line (no air/oil separator) so that any moisture, oil or water, exits along the exhaust stack and is thus burned up as it passes the exhaust gasses. If you don’t care about a little oil coating the bottom of the cowling and running back into the propeller, just exit out through the bottom of the cowling flange. Remember to angle cut the vent line so that forward motion will tend to force air and oil up through the vent. This will minimize the oil coming out the vent pipe. If you do this, please clean up the mess after every flight. There is nothing that will turn your passengers off more than seeing oil dripping from the bottom of your engine and running off the tips of the prop blades.