Moisture in the oil

[robrien]

Ok, so I'm a new guy to plane ownership, and I see a lot of postings related to drying/moisture - what I can't seem to wrap my head around is the huge quantity of moisture that these (my) engines seem to 'generate' during a flight. I live in Utah and moisture in the winter is in the single digits, and I fly weekly, for about 1.5 hours, give or take. When I put the plane away and open the oil cap there is always water dripping. I noticed this in the summer as well. I scratch my head, as there are no engines that I've ever seen do this - boat, generator, motorcycle, lawn mower, automotive, ... I've read this is normal; my IA says this is normal; still I ask - why so much moisture???

[pdb]

Water vapor is one of the products of combustion and it collects in your oil, frequently turning it into what looks like a gray, brown revolting milkshake.

It occurs most often in winter, especially on short flights, when the oil doesn't warm up hot enough or long enough to cause the water in the oil to evaporate.

Depending on the outside air temperature, you need to fly at least an hour or more at normal operating temperature to burn off the water. That may require the installation of a proper winter kit (a plate in the nose cowl vent and plates partially obstructing the nose cowl grill (see pgs 146 and 147 of the parts catalog) and calibration of your oil temp gauge.

In lieu of the winter kit, some purists like me use duct tape judiciously applied in increments to avoid burning up your engine.

Do a search here on winter kits and oil temps for more information.

[pdb]

These should keep you busy for a while.

http://www.cessna170.org/forums/viewtop ... emp#p82128

http://www.cessna170.org/forums/viewtop ... w+oil+temp

PS: Don't blow off the suggestion about calibrating your oil temp gauge. Its important to get good information from it.

[hilltop170]

Take the oil cap off after every flight and leave it off until the next flight. You can put a loose weave cotton shop rag over the oil filler neck if you are concerned something may get into the engine. If you are in a hangar, leave the cowl open so you know the cap is off. The warm moist vapor will vent out and reduce the moisture content inside the engine but it won't eliminate the engine generating more moisture on the next flight.

[counsellj]

Take the oil cap off after every flight and leave it off until the next flight. You can put a loose weave cotton shop rag over the oil filler neck if you are concerned something may get into the engine. If you are in a hangar, leave the cowl open so you know the cap is off. The warm moist vapor will vent out and reduce the moisture content inside the engine but it won't eliminate the engine generating more moisture on the next flight.

I also recommend a similar technique on your bottle of Whiskey. Though instead of a rag, I often use a straw, to allow the moisture that is trapped deep within the fluid to escape.

Jughead

[blueldr]

If you're parked in a hangar with electric power, a low wattage crancase heater, with the filler cap off, will dry the moisture out of the oil between flights

[flyboy122]

One of the characteristics of air cooled airplane engines is that the pistons are fit relatively loose. I can't remember the exact reason why, but something to do with the air cooling and expansion ratios or along those lines. Anyway, the result is a lot of blow by as compared to a car engine. This is why airplane oil tends to get dirty quick, and probably why you see moisture in the oil. (In a car, more of this moisture would just be sent out the exhaust.)

A case heater (or other undercowl heater) may help, but it is crucial that if you do this the heater keeps the engine and under cowl at least 60-70 deg. This usually requires a lot of blankets and sealing up of holes, so most guys don't bother and just preheat prior to flying. You definitely do not want to leave it plugged in all winter if you can't keep the temp up (or aren't flying everyday). The water will condense, but not cook off, and will rust up the engine FAST. The internet is replete with pictures and stories of people who've done this. The best defense, of course, is just to fly regularly. I only preheat prior to flying, and try to fly at least once every 2 weeks. If nothing else it makes for a convenient excuse.

A word of caution on duct tape winter cowls. Come spring, sometimes more comes of than just the tape. Found this out the hard way!

DEM

[robrien]

Interesting comment regarding blow-by.
That's the first time I've heard this - makes me wonder about fitting then of lawn mower engines, etc - as I've never seen the quantities of water (any water, for that matter) in these crankcases - and assume that the fitting of components is probably not very good? Something for me to noodle over...

[Metal Master]

Back in the not so old days Car engines had what were called road draft tubes. (otherwise known as crankcase vents) The crankcase was vented over board and the drippings were allowed to drop onto the road. Thus we have road draft tube. When the engine cooled it would draw moisture back into the engine and it would form onto the inside of the engine. The oil filler caps were vented and were usually the highest point on the engine crankcase. That helped alleviate the problem to a certain extent. The filler cap was supposed to be replaced at the same time as the filter when you were changing oil but generally did not.The EPA or what ever government acronym they were know by at the time figured this out and started the first of our pollution control systems on automotive engines known as the PCV valve. (Positive Crankcase Ventilation) This nasty device removed the "road draft tube" and replaced it with in its earliest manifestation of an inline PCV valve between the Crankase and the induction system. The PCV valve drew air into the engine while it was running from the Crankcase and eventually gummed up the works under the base of the carburetor. These were installed and eventually along with items such as a bleed off of the exhaust manifold that heated the base of the carburetor and heated air cleaners inlets which helped prevent Carb ice. Car engines used to have Carb heat control. Imagine that? All of this eventually led to the outgrowth of Charcoal canisters that were designed to draw the fumes off of the engine when shut down. This and the massive numbers of vacuum lines that grew to support all of the later coming emission control devises along with automatic chokes we eventually grew automotive engines into what we have today under the hood of out modern (sic) cars.
Aircraft engines are none of these things and still have a road draft tube. Only not called that because we do not drive them over roads at least not in most cases. They do not have a vented oil cap and do not have automatically controlled anti Carb ice prevention devices.
Among many other things that the most modern of reciprocating aircraft engines are is that they are not modern. They still have moisture build up inside the crankcase. And they do not have vented oil fill caps. Thank the aviation gods we do not use paraffin based oil. If you ever had the chance to disassemble an old auto engine that had been ran on paraffin based oil with a PCV valve it was sight to behold. The entire Valve cover would be filled with a waxy oil soaked scum that looked like it came out of a septic tank.
The only thing I think is worse than would be and old PCV valve system installed on an aircraft engine is in my opinion the air oil separator that drains the nasty stuff of of the crankcase vent back into the oil sump. At least the PCV valve was supposed to burn this stuff off in the combustion process as they were vented int the induction system.
The best thing you can do for your aircraft engine it to regularly fly it. I have never seen a regularly flown aircraft engine suffer from what collects in the oil filler cap. For that matter I have never seen what collects in the oil filler cap detrimentally affect any aircraft engine.
All of this is by no means a tirade against what the EPA does or did. Just think how much pollution we might be suffering if we were all driving around in 1955 Chevys like they do in Cuba.

[robrien]

Wow!

[DaveF]

A little MMO should take care of that!

[cessna170bdriver]

...(water boils at 212F, right?). ...

Yes, but it will evaporate at any temperature above the current dewpoint (IF it isn't submerged in the oil); the warmer it is the faster it goes.

[cessna170bdriver]

Got it, that's why they recommend a min of 180F even though that's far below the boiling point. Thanks Miles.

Not at about 17,500 ft...
http://www.engineeringtoolbox.com/boili ... _1344.html