What’s the story on air-oil separators? Do they keep oil off the belly? Do they put water and acids back into the engine? Are they worth having?
As with most products, some work better than others. What follows has to be qualified as my opinion, as opposed to my having purchased a copy of all separator models and slicing them in half to “have a look”. I have seen inside some of them, but not all.
The older style separators were there primarily due to the “wet” (oil mist lubricated) vacuum pumps. Those pumps generated a rather large volume of oil fog, most of which had to be returned to the engine. If it wasn’t, a lot of oil ended up out of the engine and on the belly of the plane. Just like the old auto engine “crankcase breathers”, most of those separators were simply large cans with a metal wool inside. The theory was that the metal wool would cause the oil to condense on it, and from there the oil would drain back into the engine. Under some circumstances, it probably was possible for this type to also condense some water and other combustion byproducts, and return them to the engine along with the oil. I have never personally witnessed this on an aircraft separator, but I certainly saw it happen on auto breathers.
Unfortunately, most of the separators on today’s market are still much the same design. I believe, however, that there is one exception; the M20 Separator made by Bill Sandman’s outfit down in South Florida. This unit is much more compact, is sized for the engine, and I suspect that it works very differently inside. I believe that it probably works more on centrifugal principles rather than condensation principles. The water vapor in the engine will be both hot and light (that’s why clouds rise). The oil vapor in the engine is less a true vapor than it is more of a mist; it is the result of “churning” and throw-off in the case area and accessory gear train. My suspicion is that the M20 design creates rotation inside the separator. The heavier mist (which has more mass) gets thrown against the inside walls and collects (more so than “condenses”), then drains back into the engine. The much hotter and lighter vapor doesn’t have the same moving mass, and simply makes the turns up, over into the standpipe, and out the vent. Without the metal foil surfaces, there is little opportunity for the water vapor to condense inside the separator. I have an M20 on my Sierra, and I have never found any trace of moisture in the drain line back to the engine.
Having said all this, I need to emphasize that the quality of Separator installation is critical. It needs to be mounted as described, the hoses must be routed (and have the angles) as described, the drain tube connection must be as described, and the overboard vent tube must be in the proper area (and be properly shaped). At least in my case, when I have looked at the installations that the owners said “didn’t help”, I was able to readily spot installation errors. In my case, when I put mine on, it was three multi-hour flights before the very first trace of oil even showed up at the bottom of the new vent tube. It is working perfectly on my Sierra.
Note that there is a new wet vacuum pump on the market again. If you look into this, I would talk to the M20 people about using their separator with it, rather than the traditional condensing-type separator. And no, I have no financial or social interests in the M20 producers. This is just a report on a product that works as advertised, if properly installed.
Here is an older note that I previously wrote on this subject, in response to a posting on the Beech Owners List:
“I have heard others make this same argument (i.e., problems caused by an air/oil separator). What are your thoughts?”
As is stated in the article, in a worst case scenario this is technically possible. In realistic application I suspect it rarely happens, though it might have been more prevalent on the “golden age” engines (for a number of reasons). Just as one small example, the article describes a “milky liquid” consisting of emulsified oil and water draining from the vent tube on a non-separator equipped engine (vent tube only). When was the last time you saw anything but a drop of oil on the bottom of a vent tube? The only time I have ever seen emulsion was on old car engines (pre-PCV days), when they were run for only short distances in cold weather. Even then, I saw it only inside the vent cap on the filter, not at the vent tube drain.
It’s been my experience that the following circumstances make distilling water back into the engine highly improbable on most current aircraft engines:
1. In nearly all cases, an aircraft engine warms up very fast (5-10 minutes), and is usually run long enough to dry out the oil (30 minutes or more). The gases exiting the engine get hot within a relatively few minutes.
2. A good separator (like the M20 model from Bill Sandman) does not work through distillation; it works through centrifugal separation (like big diesel air cleaners). That’s why Sandman’s is so small compared to the big old traditional separators.
3. A centrifugal separator is designed to “spin out” the oil fog. The oil is not in a true vapor form, the way water can be a vapor due to evaporation. The oil doesn’t evaporate inside the engine; it just gets whipped into small particles. Vaporized oil could be an explosion risk, just like vaporized gasoline. The oil is more like a mist, or a heavy fog.
4. Water vapor, especially hot water vapor, is lighter than air. The oil fog is heavier than air. The blow-by gases and water vapor, being the lighter components, simply pass through the separator virtually unimpeded. They aren’t in there long enough to condense, and the smaller M20 separator quickly gets too warm to act as a condenser. The heavier oil mist gets “spun out”, collects (as opposed to “condenses”) on the walls, and drains back into the engine.
That’s my take on it. I considered this possibility when I was pondering my separator purchase and installation. I concluded it was not a realistic risk, and I have never seen any indication of water in my engine.