RobertE Posted July 5, 2013 Report Posted July 5, 2013 A current question on this forum got me thinking about this. We've all heard of how the absence of at least 180 degree oil temps produces acids that we don't exactly want. If, indeed, moisture and the related acid build up is so harmful that it's unwise to occasionally ground run an engine to re-coat everything with oil, how is it that our engines survive this exact condition with every cold weather flight? Every 10 or 20 minute descent I make in the winter reduces oil temp to well below 180, whereupon I put the plane away until the next flight. Does anyone have any actual data that either shows the general harm caused by a few minutes running on the ground or demonstrates that the conditions we experience on winter descents produces diminished moisture and acids than an equivalent duration ground run? I'm suspicious, as you can tell.
jetdriven Posted July 5, 2013 Report Posted July 5, 2013 People who fly every day make TBO easily and the parts are still in limits. People who fly their airplanes without letting them sit longer than 10-14 days usually make TBO. People who dont fly their engines regularly replace them from corrosion. People who run them on the ground and don't fly regularly replace them even faster. I'm a big fan of using the airplane at a minimum every 10-14 days. You get one shot to run it 60 seconds and coat everything with oil if you exceed that. If you are going to go outside that, pickle the engine and put dessicant plugs in it. Some have had great success with aquarium pumps circulating air through a dessicant container then the engine. Perhaps Don Muncy can give his experiences on here about that. 1
Alan Fox Posted July 5, 2013 Report Posted July 5, 2013 The oil temp is much cooler than the air temp in the crankcase , the oil is used to cool as well as lubricate , when the engine is running at a percentage of HP that is significant , not only does it boil out the existing moisture , but it keeps the air in the case from condensing moisture when its running....also the condensate that gets in from sitting , doesn't mix with exhaust gases unless you run it.....So if you run it run it flying for a minimum of 30 minutes...
DonMuncy Posted July 5, 2013 Report Posted July 5, 2013 I have been running my dessicant treated air through my engine 5 minutes daily (and 10 minutes or so after each flight) for a few years now. I have had no problems, but I doubt that proves anything. My engine is a Continental, which isn't supposed to be particularly susceptible to rusting. I don't get to fly as often as I like, and figure precautions can't hurt.
Greg Posted July 6, 2013 Report Posted July 6, 2013 Simply heating the engine compartment to a temperature that is 20 degrees warmer than the current outside (the engine compartment) air temperature will lower the relative humidity around the engine and keep it dry. Inside a hanger the temperature will be more stable than outside on the ramp.
aaronk25 Posted July 6, 2013 Report Posted July 6, 2013 Oil doesn't become more acidic from lack of temp it it becomes more acidic from time in service from combustion by products. The oil becomes more acidic the longer its run.
aviatoreb Posted July 6, 2013 Report Posted July 6, 2013 I have been wondering the following.... Why doesn't ground running work? I mean what is to prevent me from running it on the ground for say 30 min at a power level that cause the oil to hit 180F? I am speaking theoretically since a) I think the neighbors would get mad, and if I am going to do that then I would rather just fly the darned thing!
Jamie Posted July 6, 2013 Report Posted July 6, 2013 It's a complicated problem, no doubt. Lots of variables and worse, you can't easily monitor the engine (inside) during the experiment. I've always wondered about the "people who fly everyday make TBO and beyond". I don't doubt it happens, but the reason isn't obvious... Is it because running an engine every day is better for it, or is it simply that those people will reach TBO sooner than me in terms of elapsed time? 100 hours a year == 20 years to TBO. 1 hour a day == 5.5 years to TBO. Water + Iron + Oxygen == rust. A sitting engine -can- rust. Why? Where does the water come from? Water resulting from combustion, ends up in the oil. Water that condenses out of the air (mostly after engine shutdown) Any other sources? I'm no expert... Anyway. I'm also suspicious of the "get your oil to at least 180F". What's magic about this number? It's below the boiling point of water. Assuming a major source of water for the engine rust is water created during combustion, I don't see how this helps remove it. My current hypothesis is: Rust occurs when metal parts are directly exposed to air (water condenses out, O2 readily available). Oil will eventually (weeks) drain away from metal parts leaving them exposed to the air. Regular running recoats the interior with oil. Other effects mentioned may contribute, but note the use of dessicant plugs when storing an engine. That -isn't- to remove water that didn't boil away in a run to 180F so much as it's to prevent water in the air from condensing out. I would imagine (but do not know) that the preservative oil used is probably more viscous than regular oil (stickier). Are engines in dryer climates (arizona?) any less prone to rusting if allowed to sit? Like so much of what we do to and with our engines, it's really voodoo. Cause and effect are separated in time so much that we're really just guessing.
aaronk25 Posted July 6, 2013 Report Posted July 6, 2013 I I've always wondered about the "people who fly everyday make TBO and beyond". I don't doubt it happens, but the reason isn't obvious... Is it because running an engine every day is better for it, or is it simply that those people will reach TBO sooner than me in terms of elapsed time? 100 hours a year == 20 years to TBO. 1 hour a day == 5.5 years to TBO. Run it every day and I'd bet at 2000 hours the engine would look close to new inside and would be safe to 2-3 times TBO. 4,000-6000hours.
Jamie Posted July 6, 2013 Report Posted July 6, 2013 Sure. No argument. But.... why? The more reading I do, the more this looks like a "simple" problem of keeping water away from iron. If true, then it could be as simple as "Running the engine regularly re-coats the interior with oil." What I don't know is, how much water will be in the oil that coats the engine, and is this worse than letting the oil drain away and expose the metal to the Alabama humidity. The wiki article on rust says that sulfur dioxide and carbon dioxide in combination with water can speed up rusting. Also, that the pH of the environment affects how fast something rusts. It could be that a low temperature run causes more products (other than water) which actually accerlerate rusting. An engine running hotter and leaning probably produces less of these chemicals.
Cruiser Posted July 6, 2013 Report Posted July 6, 2013 lots of statements and opinions but all short on factual data to support. First data must be why (cause) does an engine fail to make TBO?
Jamie Posted July 6, 2013 Report Posted July 6, 2013 lots of statements and opinions but all short on factual data to support. Yup. Short of dissassembling an engine every week and looking at it, I don't know how to get that data either. Anything else is "anecdata".
jetdriven Posted July 6, 2013 Report Posted July 6, 2013 It's a complicated problem, no doubt. Lots of variables and worse, you can't easily monitor the engine (inside) during the experiment. I've always wondered about the "people who fly everyday make TBO and beyond". I don't doubt it happens, but the reason isn't obvious... Is it because running an engine every day is better for it, or is it simply that those people will reach TBO sooner than me in terms of elapsed time? 100 hours a year == 20 years to TBO. 1 hour a day == 5.5 years to TBO. Water + Iron + Oxygen == rust. A sitting engine -can- rust. Why? Where does the water come from? Water resulting from combustion, ends up in the oil. Water that condenses out of the air (mostly after engine shutdown) Any other sources? I'm no expert... Anyway. I'm also suspicious of the "get your oil to at least 180F". What's magic about this number? It's below the boiling point of water. Assuming a major source of water for the engine rust is water created during combustion, I don't see how this helps remove it. My current hypothesis is: Rust occurs when metal parts are directly exposed to air (water condenses out, O2 readily available). Oil will eventually (weeks) drain away from metal parts leaving them exposed to the air. Regular running recoats the interior with oil. Other effects mentioned may contribute, but note the use of dessicant plugs when storing an engine. That -isn't- to remove water that didn't boil away in a run to 180F so much as it's to prevent water in the air from condensing out. I would imagine (but do not know) that the preservative oil used is probably more viscous than regular oil (stickier). Are engines in dryer climates (arizona?) any less prone to rusting if allowed to sit? Like so much of what we do to and with our engines, it's really voodoo. Cause and effect are separated in time so much that we're really just guessing. the highest oil temp is found in the bearings and where the oil exits the engine for the oil cooler. 180F on your oil temp gauge translates to 212F at the bearings and will cause the moisture to boil off. You can't run it on the ground long enough to get the oil hot enough unless you run it at 55% power or more, and that causes serious hot spots on the cylinders which will ruin them. The engine was designed to fly.
aaronk25 Posted July 7, 2013 Report Posted July 7, 2013 lots of statements and opinions but all short on factual data to support. First data must be why (cause) does an engine fail to make TBO? How about you pay for the factual data. A engine fails to make TBO because it doesn't get run enough, maintained properly (oil changes) or abused. Pretty simple. That covers 99% of the issues.
aviatoreb Posted July 7, 2013 Report Posted July 7, 2013 How about you pay for the factual data. A engine fails to make TBO because it doesn't get run enough, maintained properly (oil changes) or abused. Pretty simple. That covers 99% of the issues. What amazes me is how well it seems those engines in the trainers do, like the 172 being rented on our field. I would think they would be severely abused by the 10 hour pilots of the world on first solos. But they seem to make tbo - this is strong anecdotal evidence that running it often is the most important thing if these abused engines that run every day can make the distance.
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