Internal Engine Humidity

[Chris Zavatson]

A non-Lancair pilot friend asked for assistance in determining what the environment is like inside his engine from a humidity perspective. He was concerned about corrosion, in particular the camshaft. Lots has been written about flying an engine for 30+ minutes to boil moisture out of the oil. He was interested to what happens in the engine after the flight. I helped make a temperature and humidity probe that could be lowered into the dip-stick tube. Another set of sensors remained outside the plane to record the hangar environment. All recording was done on my data logger.

A few interesting trends were revealed.

After a flight, the engine is initially hot and the relative humidity very low. As the engine cools however, the relative humidity climbs. How much depends on the actual moisture content, but it turns out that after the engine has fully cooled (~11 hours), the humidity inside the engine is much higher than ambient. We also found there is very little communication to the outside through the crankcase breather. Crankcase temperatures will cycle daily with a lag, but the transport of moisture in and out of the engine is very slow. Equilibrium takes days.

Part two of this testing was to see how effective a home-made dehumidifier would be in reducing the internal engine humidity. An aquarium pump was rigged to pull air from the exhaust pipes of the engine, push it through to container with a gallon of desiccant and then feed it back to the crankcase breather.
The drop in internal humidity was substantial. After a few hours the relative humidity dropped to ~3% even when the hangar was shrouded in fog.

If the engine parts see regular use and is coated with oil, there is probably little need to worry about the internal humidity. For an engine that sits for awhile in a humid climate, drying the air may be beneficial.

[Ryan Riley]

Chris, this is great information. I really like how you back your experiments with data. That takes the guesswork out of proposed solutions.

[BobMackey]

Here's a relatively inexpensive desiccator.
I've been using it for a couple of years now.

http://www.rbaviation.com/mojave/