I’m sharing my latest winter aircraft improvement project, hoping that it might be useful to others. I’m not the first one to have this idea so I don’t take credit for it.
Background
The biggest threat to our gasoline aircraft engines is corrosion. It is not only an issue in corrosive environments such as coastal regions but generally an issue because there is an awful lot of water inside these engines. When burning gasoline, you get two major products: CO2 and H2O, i.e. water. The water mostly escapes as vapor through the exhaust but parts of it enter the crankcase through the piston rings. Compared to more modern liquid cooled gasoline engines, our aircraft engines provide a very poor seal between piston and crankcase which is why your oil turns black in matter of hours whereas your car oil still looks new after 20km and 2 years. In addition to that, the crankcases of the typical aviation engines are very “open”, i.e. they have a large crankcase breather tube through which moist air can enter. Car engines are practically sealed. The reason for this is again the leaky piston rings. This can lead to the moving cylinders pressurizing the crankcase for which the breather tube provides an escape. An oil film provides some protection from corrosion but this needs frequent use of the engine and is generally a problem with Lycoming engines where the camshaft is at the top and therefore not well protected. This is where corrosion typically hits you first, even on frequently flown engines (I’ve had that case myself last year).
Goal
We cannot do anything against water accumulating in the crankshaft, our engines are primitive products from decades ago. You should make sure your oil temperature in flight is over 100°C so that water boils (and thus vaporizes) and you can open the oil filler cap after landing to let the water vapor escape (you can see it very well) but this will only partially address the issue. Therefore we have to find a way to remove the moisture. This is where a dehydrator comes into play. If the air inside the crankcase has a very low relative humidity, it will bind water and through air exchange remove it from the crankcase. The device needs to provide dried air and feed it into the engine at a constant flow rate. Of course the device should not costs thousands of Euros.
Construction
Dry air can be produced by running air by a desiccant. One of the best is silica gel which everybody knows from packaging: those little white bags that say “do not eat”. Silica gel used to be rather expensive in larger quantities and collecting those little bags is tedious. Luckily, a few years ago a new field of use for silica gel was found which turned it into a mass market and drove down the cost: cat litter. For a few Euros, bags of 5kg can be purchased, some of them even have a color dye indicator showing blue grains when it is still capable of taking moisture. Make sure to take one without fragrance or anything else, it must be pure silica gel. When it is soaked up, you can either replace it or bake it for some time to remove the moisture.
Now a device containing the silica gel through which ambient air is run through at slow speed is required. I went to the local home depot and bought a few standard parts. Gray plastic waste water pipes with 100mm diameter (HT 100 in German nomenclature) are perfect for the job. Those are incredibly stable and you find all kinds of parts providing good seals. A 150cm pipe with a sealing cap, a service Y adapter with a screw on cap (to fill in the gel), a double sleeve and another cap complete it. The pipe will be filled with silica gel and air blown into it at the bottom and taken out at the top. Simple plastic hoses and threaded adapters from the compressor section of your home depot do fine. Last but not least, a gasoline filter for lawn mowers was added as a safety precaution so that no dirt can enter the engine.
To operate the device, a little compressor is required. I found a nice little aquarium pump with a flow rate of 5l/min for 10 €. That is perfect because you want a low flow rate to get really dry air (the desiccant’s effectiveness is a function of time of exposure).
Everything was assembled in less than 30 minutes. The hose adapters got some teflon around the threads and were screwed into the the pipe with a matching screw nut from the inside. These HT pipes are so sturdy that you can guarantee a good seal. Make sure you got everything right before you put on the caps because they are not designed to be removable. A quick pressurization test showed that the whole assembly was super tight. Then the silica gel was filled in (about 10kg) and a piece of foam plastic inserted on top of it to act as a first air filter.
Putting it to Use
There are three main ways to get to your crankshaft:
What we need is an easy way to provide an air tight fit of the dry air hose to one of them. In my aircraft with a Lycoming 540, the oil dipstick tube was ideal. Dry air can be pushed inside it and it will overflow through the crankcase breather. To my surprise, I could just insert the hose into the dipstick tube and it would have a snug fit without any adapter. The filter was attached to the tube and a hook added to it to hold the oil dipstick (checklist needs to include checking to not forget to insert it back in!).
What remains to be done is measure the drop in relative humidity under different humidity conditions. It will be significant but I’d like to know how much and this requires an exact hygrometer which is hard to come by (the cheap ones don’t work < 20% relH). I have seen reports by people that a notable difference in oil color after ca. 10 hours can be observed.
The area this device does not address is the cylinders. A bit of dry air might leak through piston rings but this will be negligible because air flows the path of least resistance which is going to be the breather tube. I believe the cylinder walls are much less of a problem compared to camshaft / crankshaft / con rod / piston / piston plug corrosion and can be easily inspected and also fixed at relatively low cost by re-honing the cylinders and replacing piston rings.
Total cost was 50 € and about 45 minutes of labor.
That’s an ingenious project, do you have any information on the effectiveness from other people who made or bought similar devices?
There is a report from a commercial system here showing that the air coming out of the device is very dry. I decided against a closed loop system like shown in that article because I think the oil vapors would eventually poison the silica gel. With the low cost of cat litter gel and the ability to regenerate it (ca. 8h at just over 100°C), an open system seems to be a much better solution. Compare the size of my system to the commercial one in the article or this one from Tempest or the $500 solution from Tanis. Tanis claim they can reduce humidity to 10% which I don’t believe but which isn’t that important either.
I think that is a superbly well executed solution.
I started building something like that but a lot smaller and was going to hang it off the exhaust pipe.
Sure the dry air will take the easiest path but it will completely fill the crankcase cavity first and that is where e.g. the camshaft sits. And, over hours, it will find its way past all the piston rings, valves, etc.
My concern would be about theft, so you really need a hangar. And in my case I could not even use a free standing device because the plane gets moved around (it’s a busy maintenance hangar for turboprops). It would have to be something attached to the plane.
Unfortunately if the aircraft gets moved a lot the power cable becomes a problem, so it’s not for everyone. I would have the same issue at Gamston, but I still think it’s a great idea that is well worth doing if you can use it. A perfect example of what is good about EuroGA.
It occurs to me that restrictors across the air filter and exhaust pipe outlets might help keep the cylinders filled with dry air coming past rings and valve guides.
Even in a relatively wet climate I believe the main issue is the cam and lifters on Lycoming engines specifically, during periods of storage. Dry air supplied through the breather will help. Where the climate itself is dryer this is still a consideration although less so: last weekend some local guys were flying a Lycoming powered Piper Pacer that was last flown in 2002. No problems so far, they annualed it and flew it.
A guy I know has two freshly overhauled radial engines for his Travel Air (a 1920s biplane), each in a pressurized plastic bag fed by dried air. I wonder if something like that couldn’t be done for longer term aircraft storage, sealed around the fuselage aft of the firewall and containing the prop.
Unfortunately if the aircraft gets moved a lot the power cable becomes a problem
My aquarium pump is specified at 2 watts and 50l/h. This means it should be possible to find a battery powered model that can be operated for a very long time with a battery.
It occurs to me that restrictors across the air filter and exhaust pipe outlets might help keep the cylinders filled with dry air coming past rings and valve guides.
You’d also have to block the crankcase breather pipe. I didn’t want to do that because forgetting to unblock it could be fatal. I’d have to fabricate a rubber cap that is physically tied to my dehydrator so it couldn’t possibly stay on. I’ll give this some thought but I fear it wouldn’t be very effective as the air resistance will vary greatly among cylinder rings. Also blocking the air inlet and exhaust is not easily done, only practical for long term storage.
Where the climate is dryer this is still a consideration although less so
The source of humidity in the crankcase isn’t really ambient air in most cases but combustion. Water enters the crankcase from the cylinders through the cylinder rings. Even on a very dry day with a large spread between temperature and dew point, I can hold a glass over the oil filler and water will condense inside the glass. There’s an awful lot of water in the crankcase. Even in a dry climate, that water will only very slowly move out the crankcase because there is no air flow between the crankcase and ambience.
I pull the dipstick like you when I shut down the engine and replace it when the engine is cold. After that, and with the engine not running, any humidity in the crankcase is coming from the atmosphere.
It would be interesting to look at designing a steel piston for existing Lycomings, using modern design and manufacturing tools and running tighter clearances. There are reasons why aircraft engines have steel cylinders, and a steel piston would better match their thermal expansion. I have no idea if it is practical.
Hi Achim,
I think about building a similar device, are you still using your dehydrator? How often did you replace (bake?) the silica gel within the last year?
I still have it connected every time the aircraft is not flying. I have replaced it only once, I think drying the outside air is not the most important aspect, it’s pushing out the humid air inside the engine, resulting from the water in the crankcase after a flight.
