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#61
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Preheating engines: Airplane engines versus auto engines
On Dec 22, 6:43 am, Mike Spera wrote:
When we bring the airplanes in after operating in cold weather, oil and water emulsion will be found on the floor under the breather tube the next morning. I suspect that the puddles have more to do with the fact that breather outlets tend to be on the top of the engine and are connected to a 3 foot tube running straight down than any gasses purging out of the crankcase at shutdown (or what the temperature was outside). The tube walls are coated with a water/oil mix from flight and slowly this drips down to cause the puddle. An interesting test would be to remove the breather tubes completely after flight and see if anything accumulates. Good Luck, Mike That's what I meant. It's not any gases coming out after shutdown; it's the thickened oil, containing water, that hangs in that cold breather tube and drips out slowly overnight in the heated hangar. The water in the emulksion came from combustion blowby. Dan |
#62
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Preheating engines: Airplane engines versus auto engines
I suspect that the puddles have more to do with the fact that breather outlets tend to be on the top of the engine and are connected to a 3 foot tube running straight down than any gasses purging out of the crankcase at shutdown (or what the temperature was outside). The tube walls are coated with a water/oil mix from flight and slowly this drips down to cause the puddle. An interesting test would be to remove the breather tubes completely after flight and see if anything accumulates. Good Luck, Mike That's what I meant. It's not any gases coming out after shutdown; it's the thickened oil, containing water, that hangs in that cold breather tube and drips out slowly overnight in the heated hangar. The water in the emulksion came from combustion blowby. Dan Sorry Dan, That thread was so screwed up I could not tell if you were in the "it blows out after shutdown" or the "it sucks in after shutdown" group. I'm not sure the breather tube needs to be in the cold. My Cherokee drips whether it is 0 or 90 outside. Mike |
#63
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Preheating engines: Airplane engines versus auto engines
On Dec 21, 11:38*pm, nrp wrote:
*"As the pressure drops inside, the air outside has to enter the crankcase to equalize the pressure, correct? With the air temp as much as 200 deg higher inside the engine as outside, that means that a volume of about half the crankcase of outside air ENTERS the crankcase during the pressure equalization process. At least that is the way it seems to me." But it isn't air in the crankcase during engine operation. *It is a mixture of CO2 and water vapor. * Well OK. The gas law of PV=nRT is true for all gases. All of them. The point is that the engine does inhale (for the lack of a better word) a significant volume of air from outside as it cools down. This is a well known process. This air contains moisture. The oxidation process (rusting) of the engine parts is galvanic corrosion and all it needs is a molecular thin layer of moisture on the surface of say the cam lobe. The total amount of water needed to cause this process is miniscule. What prevents it from rusting your engine parts is the layer of oil on them. Regular running of the engine replenishes this layer of oil. This is what is important. Outside air will re-enter only when the water vapor condenses after shutdown. The air enters as the pressure drops when the GAS inside the crankcase cools. Condensation has nothing to do with it. If there were no moisture in the gas contained in the crankcase at all, none, at shutdown, the engine would still injest much more than enough moisture than necessary to cause problems as it cools down. Condensation is only only something else that happens along with the heat loss. The amount of water vapor in the comparatively cool outside air being drawn in is one or two orders of magnitude less than that in the hot crankcase. The point is that you can fly all you want and remove all the water from the oil that exists. You will still get plenty of moisture inside the engine as it sits idle just from the outside air it injests as it cools. Even barometric pressure changes as the weather passes by is enough to cause engine corrosion. Many pilots seem to believe that blow-by in the engine is a normal operating condition. My experience( and opinion) and that of many other race engine mechanics that I have talked to, is that once combustion gases begin to leak past the rings, the end of that engine is imminent and soon. Very soon. Blowby totally destroys the lubrication of the piston in the area of the blowby, and it shouldn't take a rocket scientist to know what that means. regards, Bud |
#64
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Preheating engines: Airplane engines versus auto engines
On Dec 22, 6:15 pm, wrote:
Many pilots seem to believe that blow-by in the engine is a normal operating condition. My experience( and opinion) and that of many other race engine mechanics that I have talked to, is that once combustion gases begin to leak past the rings, the end of that engine is imminent and soon. Very soon. Blowby totally destroys the lubrication of the piston in the area of the blowby, and it shouldn't take a rocket scientist to know what that means. regards, Bud My experience is as an aircraft mechanic. Part of the inspection process is the differential compression test on each cylinder, when it's hot after shutdown. All cylinders leak a small amount past the rings, and when the engine cools the leakage is considerably worse. All rings have ring gaps, and unless you have stacked rings (two rings in the same groove) you cannot stop the leakage. Aircraft engines do not have stacked rings. The fact that a frozen-shut breather will cause the front seal to blow out is enough evidence that rings leak. The volume of the crankcase, as noted earlier, is very small. The amount of water in a cubic foot of air, even if it's saturated, is miniscule compared to that which gets past the rings curing combustion, unless the aircraft is parked for a long time and the heating/cooling cycles of day/night pump air in and out repeatedly for a long time. Water, even a small amount, mixes with oil and in the presence of metal, which acts as a catalyst, breaks the oil down and creates acids. The thin film of oil on the parts is the first contributor to this process and is not much protection at all. |
#65
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Preheating engines: Airplane engines versus auto engines
"Mike Spera" wrote in message ... I've been more cautious about rapid heating up of engines since I unfortunately split the engine block of my Jaguar XJ6 4.2 litre that I conclude was caused by immediate rapid driving from -5c being late for an appointment. Guess that expensive incident will always stay in my mind... My guess would be that the very cold temps caused the block to split due to the coolant freezing over night. If it was -5C when you started it, chances are it was much colder at some point in the evening. Common in the upper Midwestern US was to see 70's era cars blow out freeze plugs and/or crack blocks by running no (or too weak) antifreeze. A cold start and immediate run up to high speed should not cause catastrophic failure of a block. Being a Jag, it may have had a badly cast block since new. Their QC ain't the best. Good Luck, Mike Mike, the coolant was at the correct winter dilution so I don't think freezing was a contributory factor. More likely sloppy engineering standards which similarly and sadly led to the downfall of the UK's motorcycle industry, I know I owned many UK produced bikes. At least those industries didn't manufacture aero engines. Cheers, Mike |
#66
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Preheating engines: Airplane engines versus auto engines
More likely sloppy engineering standards
which similarly and sadly led to the downfall of the UK's motorcycle industry, A slight aside: that situation is the end point of all socialized societies... Go along to get along, and all that... denny |
#67
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Preheating engines: Airplane engines versus auto engines
A fun science experiment is to put a small amount of water in the
bottom of a 1 gallon rectangular can. Boil it for a few minutes to displace the air inside, remove it from the heat, and immediately replace the cap and watch. For more excitement, repeat with another can only this time cap it and sprinkle cold water on the outside of the can too. (Hint - don't plan on using the cans again.....!) |
#68
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Preheating engines: Airplane engines versus auto engines
* * * * My experience is as an aircraft mechanic.
Thanks for a great primer, Dan. -- Jay Honeck Iowa City, IA Pathfinder N56993 www.AlexisParkInn.com "Your Aviation Destination" |
#69
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Preheating engines: Airplane engines versus auto engines
On Wed, 19 Dec 2007 12:21:28 -0500, "Peter R."
wrote: This time of year here in the Northeast US I always preheat my Bonanza's IO520 engine with a Tanis heater and an insulated cowling/prop cover as it sits in an unheated t-hangar. The result is that the oil temperature at startup is around 105 degrees F, even if the outside air temperature is as low as -15 degrees F. Monday night I arrived at my t-hangar to discover that at some point during the day the line person accidentally pulled out the plug connecting the Tanis heater to the small extension cord I use to extend the plug to the outside of the cowling cover, so the aircraft had not been preheating. Outside and inside temperatures were both a cold 25 degrees F. Given any other day, I would have plugged the aircraft back in and scrapped the flight but in this case I had an Angel Flight patient waiting in another city for my arrival and I was already late. Thus I made the painful decision to start up the aircraft and allow it to low idle until the oil heated thoroughly. A small consolation is that the engine had been recently filled with fresh Exxon Elite oil. To my relief the aircraft started right up. I know what I did has negative long term repercussions on my engine's health Ahhhh...How do you "know" one cold start is going to have long term, negative effects? I normally preheat mine and when flying a lot the preheater was always on with the cowl double wrapped. No condensation and worked great. OTOH I have gone out on days that were really cold, not 25 degrees and if I could get it to start I'd go flying without worrying about it. I think you'll find that an engine that has set a long time and is really cold takes a while to get oil to the cam. Every thing else gets oiled just fine. I'd not be overly concerned about a few cold starts as long as the engine is given time enough to circulate warm oil. In the Bo you'll know if it wasn't hot enough as the oil will congeal in the oil cooler and the prop governor will let you know right away! :-)) and I have already derived a tool to lock the two cords and prevent this accidental unplugging from happening again. However, this leads me to question the differences between aircraft engines and auto engines: Why is it that here in the Northeast US seemingly no one preheats their automobile engine before start-up in very cold temperatures? Is the long-term damage the same for both autos and aircraft engines? If so, why do you suppose auto owners don't typically do this? Is it because that most auto owners do not keep their cars very long? Nearly all the cars we've had in recent years have gone between 100 and 200 thousand miles. None gave engine problems, none were preheated, and all stayed outside, or in an unheated garage. The engines stood up far better than the bodies did. Roger (K8RI) |
#70
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Preheating engines: Airplane engines versus auto engines
On Wed, 19 Dec 2007 17:35:50 +0000 (UTC),
(Paul Tomblin) wrote: In a previous article, "Peter R." said: Why is it that here in the Northeast US seemingly no one preheats their automobile engine before start-up in very cold temperatures? Is the long-term damage the same for both autos and aircraft engines? If so, why do you suppose auto owners don't typically do this? Is it because that most auto owners do not keep their cars very long? Aircraft engines are air cooled, auto engines are liquid cooled. The following is what I was told when I was driving a Volkswagen Beetle, and the experts were saying that you needed to let the beast idle for at least 5 minutes in the winter: Liquid cooled engines stay in a very narrow temperature range while operating, so are built with very tight tolerances, but air cooled engines Careful here. Don't confuse tight tolerances with tight clearances. Aircraft engines have tighter tolerances than automobile engines. Automobile engines have tighter clearances, but wider tolerances than aircraft engines. If my car engine had the same fit as my IO-470N it'd be considered worn out. Roger (K8RI) have more slop because they get both hotter and colder than liquid cooled engines. Also, they are cooled primarily by the engine oil. Because of that, you need to preheat the engine enough that the oil is spread around and everything has warmed enough that the pistons are making good contact with the cylinder walls. |
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