Air / Oil Separators - The Whole Story

by Michael Leighton

At the recent Oshkosh show, I joined Mr. Bill Sandman, owner of M-20 Turbos, in the M-20 Air/Oil Separators booth. I had the chance to talk to thousands of M-20 owners and interested future customers.  Many show attendees asked how M-20 Air/Oil separators work, who needs them, and how are they installed.

How Does It Work?

Unlike other brands, the M-20 engine breather air/oil separators use scientific principles to separate the oil from the air and water. The actions are instantaneous and clean so they need no filters—the separators can be small. Significantly, the size is too small to cause condensation of the water vapor in the air. The water vapor and the air flow straight through, but the oil clings to a variety of collecting surfaces, forms into droplets and returns to the engine.  By using gravity alone, the risk of adding pressure to the crankcase from the vacuum pump is avoided

Moisture return

Many plane owners asked if the M-20 separators return moisture to the engine.  No, this cannot happen because of the scientific principles mentioned earlier.  As engine temperature rises, the water in the crankcase boils off into water vapor.  While it is in vapor (gaseous) form, it will not separate from the air. The risk of condensing the vapor into liquid is avoided by the small diameter and short length of the M-20 design.  If it were larger, or if the flow was slowed down by a filter, condensation would occur.  Then, the water and the oil together would make sludge.  Most significantly, the actual separation of oil from air would be degraded.

Freezing

Another question concerned freezing when using a separator. The freezing issue comes up because everybody remembers that an engine experienced freeze-up of its breather line and blew a prop gasket it flight.  After that, the FAA mandated that a hole should be made in the breather line 6” to 9” up from the point of exit from the cowling. This provides an alternate exit for the air in case an icicle formed at the tip. It had nothing to do with the engine having or not having a separator. There is no more chance of a breather line icing with a separator or without.

What’s the best installation method?

A lot of customers asked about installation.  On most engines the best method of returning oil to the crankcase is to remove the pipe plug provided by the engine manufacturer and substituting a hose fitting.  Small engines generally have only these plugs on the accessory case.  Some larger ones also have one on each side of the crankcase. 

On Cubs, Champs, and Cessna 120-140’s which use the Continental C-65/85 series engines or the O-200 as used on the Cessna 150, there may be no engine plug.  In those installations M-20 Turbos recommend welding a hose connection into either the base of the oil filler tube or the top of the oil “kidney” tank.

On virtually any engine, inserting a hose fitting into the rocker cover is an option.  Installers should use care that their entry point is centered on the cover and is above the oil flood level in flight.  This level is higher in the Franklin engines, so check it out before drilling.

The multitude of installation options available, make it easy to route the oil back to the engine.  If the installation instructions and principles are followed and the oil return line runs downhill all the way, the units perform as expected.  Improper installation can result in less than desirable results.

Mounting the M-20

What is the best mounting location?  Can the M-20 Separator be suspended in the breather line? Because the M-20 works with gravity, mounting it as high in the engine compartment as possible is recommended. On engines where the breather comes from the front of the engine over the top and through the baffling for in-line mounting, splice into the breather hose anyplace where convenient behind the engine baffle.  Weighing only 5.5 ounces, it needs only an Adel clamp to hold its position. The M-20 Separator can also be mounted to the firewall, an engine mount or any available bracket. But it should be firmly secured.

Who Needs An M-20 Separator?

Anyone who wants a clean, oil free aircraft belly and a clean, cool-running engine needs an M-20.  In addition, the M-20 allows the plane to run a full oil sump. If an engine has an eight-quart sump, with the M-20 Separator, it runs eight quarts; a 12-quart sum, 12 quarts. Owners of  Turbo-charged engines find the M-20 of particular interest because the longevity of a turbo is directly related not only to temperature but the quantity and quality of the engine oil. Running a full sump means more oil to suspend contaminants and dissipate heat.

Does the M-20 300 work on Wet Pumps?

Here I found more confusion than elsewhere.  The M-20 engine breather separators, Model 300 Series and Model 400-XL work on all piston engines, flat or radial, from 45 horse power to 950 horse power, having either dry or wet vacuum pumps or no vacuum system at all.  Wet vacuum pumps need an additional separator for their unique need for separating the high pressure oil stream from the high pressure air put out by the vacuum pump. In other words, if your engine has a wet (oil lubricated) vacuum pump, you need two different M-20 Air/Oil separators.  The wet pump requires the Model 600-WP. The engine needs whichever Model 300 or 400 is appropriate to the horsepower or compression.

Additional Answers

The M-20 installation and operation have no effect on either the engine or airframe manufacturer warranties.  Interestingly, the FAA approval is an airframe STC, so if the engine is replaced or upgraded, the M-20 can be moved to the new engine.  If the upgrade moves the horsepower from 315 horsepower or less to more than 315 horsepower, the Model 300 would have to be replaced with a Model 400XL.  Similarly, if an airplane’s standard pistons were changed to helicopter model high compression pistons, the Model 300 or 300-B would have to be changed to the Model 300-H or 300-HB.

For more information on the M-20 Model 300 Air/Oil Separator, call 1-800-421-1316.

Home | About Us | Search | Contact | Lifetime Vacuum Pumps
Breather Separators | Vacuum Pump Separators | Turbo Kits