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Leaving the oil filler cap ?Open? to vent moisture ? (For a bit)

Go back and look at the plot.

Temperature goes up, relative humidity goes down. Temperature goes down, relative humidity goes up. Perfectly normal.

Quantify it. Assume 100F and 11% (middle of screen). Calculate water quantity. Now re-calculate using the averages, 53F and 53%. Same quantity of water. Nothing changed.
 
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below 60% RH seems to be a safer region. the nice thing about purging with dry air is that it goes to every nook and cranny inside the engine.

extract from a paper.

http://resource.npl.co.uk/docs/science_technology/materials/life_management_of_materials/publications/online_guides/pdf/atmospheric_corrosion.pdf

Control of RH -- Most atmospheric corrosion can be prevented by maintaining RH below 60%. Desiccators and dehumidified stores can therefore be used for storage. In storerooms and warehouses it is important to maintain the air temperature at a reasonable level and to avoid large variations in temperature; a fall in temperature overnight or at the weekend may lead to heavy condensation of moisture. Condensation may also occur if massive metal parts are placed while cold into a warm room if the air is not saturated at the prevailing temperature. Stoves and gas heaters must be provided with adequate flues. The air in store cupboards may be dried by the use of desiccants or by refrigerating plant. For special purposes, including display cabinets, where it is essential to ensure immediate access to complex equipment refrigerated surfaces may be the most practical means of protection.
 
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if the engine is kept dry by a dehydrator, would it be possible to increase the interval between the normal 50 hr oil change if the oil is clean?
 
if the engine is kept dry by a dehydrator, would it be possible to increase the interval between the normal 50 hr oil change if the oil is clean?

no .


Lycoming put a vent on the engine to remove moisture and relieve pressure in the crankcase. Why do you need to open up another hole? If you have moisture in the crankcase after flying, you didn't fly long enough to fully atomize the moisture. Fly more!

I’m with you Ron. I’m wondering if this discussion would be happening if we weren’t laying low because of corona virus. Can’t fly (well, some of us can, and do), and we’re bored. It was a nice day yesterday here in OH and I flew twice yesterday - no moisture from the tube either time.

Gentlemen: We cannot argue with the combustion equation. Exhaust, and therefore blow-by, is rich in water and is in the crankcase at shut down whether seen or not.

BTW - I was experimenting (Before Covid) for a year with a peltier device for chilling and condensing the water using 12 vdc and had to think about this whole humidity thing and what was really important. It is dew point. Dew point. Dew point.
 
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condition inspection for me today. all day effort and 4 more hrs tomorrow for the wheels. no findings except I replaced a couple zip ties. I did change the oil and did a 5 minute run as a leak check. I wondered what the humidity in the engine was so I connected the purge system. to my surprise it registered 80% humidity. so, operating the engine on the ground for 5 minutes lubricates all the components but also spikes the internal humidity.
 
I am no expert, and please feel free to correct me if I am talking rubbish, but I think that:

You get about 1 litre of water generated for every 1 litre of fuel you burn. Most of this is expelled in the exhaust, but some will get into the crankcase as blow-by. Oil tends to absorb water, so there will be a tendency for the oil to retain some of the moisture, and because the crankcase is a semi sealed vessel with no air movement inside, as the dew point fluctuates with daily temperature changes, the water will evaporate and then condense inside the crankcase causing the dreaded corrosion. Combustion byproducts in the presence of water in the oil and crankcase will also form acids that will also contribute to engine damage.

I am thinking of doing an experiment the next time I do an oil change. My plan is to weigh the old oil on a digital scale, then heat it up to boil the water off on a stove and then re weighing to see the difference after the water is removed.

Warm desiccated air flow is ideal, but even basic pressurised air flow into the crankcase using a cheap aquarium pump will eventually equalise the moisture content of the oil in the crankcase with the outside environment. Thats ok and an easy low tech and very cheap solution if your outside humidity is relatively low.
 
I am no expert, and please feel free to correct me if I am talking rubbish, but I think that:

You get about 1 litre of water generated for every 1 litre of fuel you burn. Most of this is expelled in the exhaust, but some will get into the crankcase as blow-by. Oil tends to absorb water, so there will be a tendency for the oil to retain some of the moisture, and because the crankcase is a semi sealed vessel with no air movement inside, as the dew point fluctuates with daily temperature changes, the water will evaporate and then condense inside the crankcase causing the dreaded corrosion. Combustion byproducts in the presence of water in the oil and crankcase will also form acids that will also contribute to engine damage.

I am thinking of doing an experiment the next time I do an oil change. My plan is to weigh the old oil on a digital scale, then heat it up to boil the water off on a stove and then re weighing to see the difference after the water is removed.

Warm desiccated air flow is ideal, but even basic pressurised air flow into the crankcase using a cheap aquarium pump will eventually equalise the moisture content of the oil in the crankcase with the outside environment. Thats ok and an easy low tech and very cheap solution if your outside humidity is relatively low.

Warming the air increases it's capacity to absorb moisture, which it then releases onto a colder surface as condensation. It is actually better just to dessicate the air and circulate it that way. Temperature differences are not your friend.
 
Thanks Don!!!

Thanks for the data Bill, based on your description, I'm invisioning your doing the following.
Are you using a 120 volt blower?
I have used Duck Bill valves in the past and they may work well in this application as check valves.

oCv.jpg

Don, thanks for pushing me to investigate the check valves further. I did not go with the duck bill valves as packaging was more machining than I wanted, but did find a Minnesota company that makes check valves and they kindly waived the minimum fees and sold me 5 prototypes. It had more cracking pressure than I liked but it seems the pressure drop after opening was low enough. I was able to continue the development and ended up with a 5 min and 60 min timer. The purge provides 20 l/min and dryer 3 l/min. The purge clears 99% of crankcase air in 4 min followed by 60 min with dry are that reaches 99.9% replacement in 30-40 min. So, some extra time to spare.

I went with mechanical timers for reliability and shorter development time. Component development with a reliable, cost effective, low labor investment a variable electronic timer would be in order for an updated version. But it will have to work on 12 volts for the portable purge version.

If any are interested in the high flow check valves, I can post the detailed information. The company was eager to sell more. They were less than $6 w/o shipping.
 
Another thought, somewhat outside the box.... buy a second oil cap that seals 100%, and plumb it with a fitting and hose to a vacuum compressor pump. Run the vacuum pump trying to suck down the remaining pressure in your engine and crankcase. Lower pressure lowers the boiling point of water. The water will evaporate right out with the vacuum pump running.

This is how we purge all air conditions and refrigeration units before adding more freon. The whole plumbing system has to be completely moisture free. A vacuum pump gets the job done.

Be aware it might pull some less than atmospheric pressure on all the seals on your motor. I've no idea if that's detrimental to the life of the seals.
 
Another thought, somewhat outside the box.... buy a second oil cap that seals 100%, and plumb it with a fitting and hose to a vacuum compressor pump. Run the vacuum pump trying to suck down the remaining pressure in your engine and crankcase. Lower pressure lowers the boiling point of water. The water will evaporate right out with the vacuum pump running.

This is how we purge all air conditions and refrigeration units before adding more freon. The whole plumbing system has to be completely moisture free.

Small problem with that theory is that an a/c system in sealed while an engine isn't. You're just sucking moist ambient air into the engine through, for example the exhaust pipes, crankcase breather tube, etc, to replace the air you're sucking out through the oil filler tube.
 
Small problem with that theory is that an a/c system in sealed while an engine isn't. You're just sucking moist ambient air into the engine through, for example the exhaust pipes, crankcase breather tube, etc, to replace the air you're sucking out through the oil filler tube.


Yes, but if the total pressure pulled out is less than the outside atmospheric pressure, wouldn't you still be pulling moisture out of the system?

Other option is to fly at very high elevations, which lowers the boiling point of water.
 
Yes, but if the total pressure pulled out is less than the outside atmospheric pressure, wouldn't you still be pulling moisture out of the system?

Other option is to fly at very high elevations, which lowers the boiling point of water.

You must be punking us. If you remove any moisture it means you're removing air volume which has to be replaced because the engine isn't sealed.

Regarding flying higher, just to keep this simple, you eventually have to descend and land and cool down the engine.
 
black max engine dryer

I just plug in my black max when I'm in the hangar - looks like a tiny GPU. When I stop for a while outside the hangar I remove the dipstick cap and leave the filler door open to vent both hot air and (theoretically) let moisture out of the engine.

Over 25 hours on the hobbs and no problems with corrosion yet! :D
 
You must be punking us. If you remove any moisture it means you're removing air volume which has to be replaced because the engine isn't sealed.

Regarding flying higher, just to keep this simple, you eventually have to descend and land and cool down the engine.

Maybe then, someone can explain to me, then, why I'm not reading about the Rotax 912 ULS in my RV-12 having corrosion problems in the engine.

Why do they promote it as a sealed crankcase system, with a dry sump that relies on blow by from the engine to pump the oil in the crankcase back in to the sump tank on the firewall?

I've never seen any white goo or condensation in my motor oil or on the cap. Matter of fact, have to remove that sealed oil cap and turn the prop by hand numerous times until the compression blow by "burps" all the oil back into the dry sump container, so that you can check the oil level in the sump tank.

It's plenty humid here, I don't live that far away from the pacific ocean here in Los Angeles, see the onshore fog regularly, plus Baja Hurricane season in the summer too makes the heat and air nice and unbearably humid.

Yes I understand it's not a Lycoming or a Conti... but still, it's an internal combustion engine burning gasoline making water as a by product, and the motor has and relies upon combustion blowby to get the oil back into the sump tank.
 
I like the concept of a dehydrator to aid in corrosion prevention by getting rid of crankcase moisture...less condensation sitting around in the upper engine, so I'm using an aquarium pump to push dehydrated air from the crankcase vent tube back through the dipstick tube.

I don't know if it has an ultimate benefical effect, but relative to corrosion, getting rid of standing moisture in the crankcase and upper cylinders can't be a bad thing, and the device only cost about $25 to build.

https://www.aircraftspruce.com/catalog/pdf/2039test.pdf

https://www.aviationconsumer.com/maintenance/engine-dehydrators-engine-saver-prevails/
 
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So I can’t find any thoughts on an open oil filler cap to let engine moisture escape from a hot engine ??

So after a nice one hour plus flight today , after a fresh oil change, on a 300 hour well maintained engine (oil temp running at 185F). I pulled the aircraft is back in hanger .. I pulled the dip stick and again saw the milky white oily mixture on the filler cap / filler neck (clearly water in the oil) and watch the vapors pour out..

Try increasing your oil temp by 20 degrees to get rid of the milky white oil mixture. Take the temp of your oil now. I doubt it is reaching 185*.

Venting the crankcase so moisture won't condense on an engine that is cooling down at a slower rate than the air passing through..... You need a colder surface for the moisture to condense on.

Water does not float on top of oil. So how does it get from the oil to the cold surface of the cam, case etc.?

If the oil temp during flight was high enough to to keep the combustion moisture suspended in the crankcase and is venting during flight, the balance of the EXTRA moisture in the crankcase will balance out with the ambient humidity as the engine cools down.

You are more likely to get condensation on, and inside your motor when it is stone cold, parked in a non insulated metal hangar that has a dirt floor, during sunrise.

So when you shut down after a nice flight, the entire inside of the motor is coated with oil. It is not going to start rusting until the film thins out enough for moisture to contact the metal surface. How many days does that take?

Take a piece of metal (4130) etc.and sand it to bare metal, wipe it down with a thin coat of your motor oil and place it with your RV. How long did it take to rust?

Where is the first place to rust inside a Lyc?? I would say it would be in a cylinder that has the piston at the bottom of the stroke and the rust would start at the bottom where moisture would collect.

Keep your oil hot, and fly way before your test metal shows any rust.
 
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Maybe then, someone can explain to me, then, why I'm not reading about the Rotax 912 ULS in my RV-12 having corrosion problems in the engine.

Why do they promote it as a sealed crankcase system, with a dry sump that relies on blow by from the engine to pump the oil in the crankcase back in to the sump tank on the firewall?

I've never seen any white goo or condensation in my motor oil or on the cap. Matter of fact, have to remove that sealed oil cap and turn the prop by hand numerous times until the compression blow by "burps" all the oil back into the dry sump container, so that you can check the oil level in the sump tank.

It's plenty humid here, I don't live that far away from the pacific ocean here in Los Angeles, see the onshore fog regularly, plus Baja Hurricane season in the summer too makes the heat and air nice and unbearably humid.

Yes I understand it's not a Lycoming or a Conti... but still, it's an internal combustion engine burning gasoline making water as a by product, and the motor has and relies upon combustion blowby to get the oil back into the sump tank.
Fair question on the Rotax. Are we sure they don't have cam/follower issues? We know the cam location is under the crank, and possibly they have a cam/follower design contact stress design that allows use of materials more tolerant of condensation. The entire valve train is a "system"

Different engines have different problems. Designs, materials, vary widely between manufacturers based on their experience with customer base, field problems that have shaped their internal design practices.

The "water" is the result of combustion, it leaks past the rings into the crankcase. The resulting H2O/cubic foot of air in the crankcase has little relationship to the ambient air as the combustion air/exhaust/blowby has a dew point well above ambient already.

Unfortunately, this is not a good definitive answer, but you should see that the true answer is quantified in the details and beyond simple logic.
 
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