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Found 1 result

  1. I write to share what I think are results of a pretty cool experiment aimed at preventing engine corrosion in our grounded M20C. Our beloved C is grounded pending piston pin plug replacement on one cylinder. Since buying this aircraft in 2017 I've always flown her for at least an hour every two weeks or so, especially during the steamy parts of the year to keep the oil film on the engine parts and help prevent internal corrosion. Earlier this year increasing amounts of aluminum in the oil filter suggested a possible loose piston pin plug, which was confirmed by endoscopic examination, so we stopped flying in June. Since it is the steamy part of the year here in the deep south U.S., I immediately became concerned about preventing engine corrosion. Searching the internet turned up the most common solution of pumping air through a desiccant and feeding that air into the breather tube. My experience with desiccants in humid environments is that their effectiveness is pretty short lived. And of course there is Camguard(TM) and the like, but I've never been too trusting of magic potions. Call me skeptical. I also see that there is a commercial electric engine air dehumidifier but I have the parts and expertise to design/test a frostless dehumidifier system and have confidence in how it works. My engineer brain kicked in, and I prototyped a closed dehumidifier using a Peltier effect device as an aftercooler. After experimenting with a few configurations, I found a set up that works really well. It consists of a Peltier cooler in a sealed chamber that also contains a piezoelectric positive displacement aquarium air pump that is ridiculously simple and rated at about 10 l/h (~3 gph) flow rate. The intake air to this chamber is filtered using a Fram G-2 just like the one used on Brittain PC vacuum systems. I put an el-cheapo digital temp/r.h. indicator in the cooling chamber just to indicate what's happening in there. I epoxied a type-K thermocouple to the fins of heat sink on the cool side of the Peltier to monitor its temperature. The dehumidified air then passes from the air pump through a hose barb bulkhead fitting and flows into a second measurement chamber that contains a research-grade (Vaisala HMP50) temperature/relative humidity sensor. I also put another el-cheapo digital temp/r.h. sensor in there as an independent check. Testing revealed that the el-cheapo R.H. sensors are qualitatively correct, but not particularly accurate. If they indicate <40% r.h. the dew point is less than 10C at hangar air temps. The air leaves the measurement chamber through a 3/4" hose and flows into the engine breather tube on the aircraft. I sealed off the "ice hole" on the breather tube and unscrewed the dipstick. The slickest part of this system is the programmable controller/data logger. It cycles the Peltier on/off to keep the thermocouple temperature in a range of 11-13C. This prevents icing. I wrote an iterative solver to calculate the dew point temperature from the research-grade r.h. sensor data. If the Peltier temperature exceeds 15C, or if the dew point temperature of the system output air exceeds 15C, the system shuts off the air pump and Peltier, and switches on a red LED on the panel. This puts the system in a safe condition and prevents a failed component from leading to pumping humid air through the engine. That system is now pumping dehumidified air through the engine on a continuous basis, waiting for our date with the repair shop to get 'er fixed. The temperature of the output air is actually a bit warmer than the hangar air because the Peltier doesn't remove much sensible heat, mostly latent heat. Here is a photo of the system in operation, and a data plot showing: (1) horrendous high dew point (humidity) of the ambient air this time of year in Alabama (from ASOS), (2) ridiculous high mid-day hangar air temperatures, (3) nightly low ambient air temperature approaching the ambient air dew point, and (from ASOS), (4) the greatly reduced dew point of the air going into the engine, reduced from 22-24C to below 10C! I over-instrumented the heck out of this thing. The total cost of this system if all parts were bought new would be ridiculous, but I used all surplus measurement/control gear and re-purposed a computer case to serve as a chassis. Oh, and it uses only about 30W of electrical power. A sniff of the air coming out of the dipstick tube confirms flow through the system. A rag around the dipstick opening keeps the bugs out. These things should be common in hyper-humid climates. Engineering rulez.
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