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#41
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Preheating engines: Airplane engines versus auto engines
On 12/20/2007 9:15:15 PM, Dave wrote:
Watch this like a hawk on cold starts of any kind...15 secs is marginal, 10 secs max preferred... Will pay better attention to this. Thanks. -- Peter |
#42
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Preheating engines: Airplane engines versus auto engines
Preheating a car at +5C is just ridiculous if you are using the
proper weight of motor oil. I might use a block heater if the temperatures went below -20C, but not higher than that. Try using a good 10W30 oil between +5 and -10, and perhaps a 5W30 below that. You won't damage anything, and your engine will last longer than the body panels on the car! I use 5W30 year-round in my '01 Chevy. Runs just fine, and it's what the manufacturer recommends for our climate. The tolerances in these auto engines are now so small that anything heavier is not only a waste of time but could be really bad in the cold. Aircraft engines have much larger tolerances, mostly because they're air-cooled and get a lot hotter, with clearances between things like aluminum pistons and steel cylinders getting pretty tight at high temps. Aluminum expands at twice the rate of steel, and while some pistons have steel inserts cast into them to control that expansion, they still expand a lot. Liquid-cooled engines can be built much tighter. And auto engines have much smaller cylinders than aircraft engines of the same HP and so the overall expansion is less. Rings are either chomed steel or cast iron and will expand at around the same rate as the cylinder, but they'll still get tighter from the heat draining off the piston through them. They have a bit of difficulty getting rid of that heat through the microscopic oil film on the cylinder wall. With larger tolerances, more oil escapes. With larger tolerances, the arc of contact area is shorter. With larger tolerances, things tend to strike each other harder. So heavier oils are necessary to slow the oil's escape from bearings and so on, heavier to lubricate the shorter contact arcs, heavier to dampen the shocks of parts banging into other parts. Piston scuffing often happens when an engine is driven to full power too soon. Pistons get hot and cylinders are still cool, clearances disappear. Scuffing can happen if the oil isn't reaching the cylinders, and since it's usually thrown off the rods (some engines have a squirt hole in the rod opposite), a really low idle might leave the cylinders dry. Some two-strokes like the aircooled Rotaxes must be warmed up thoroughly or they'll seize soon after takeoff. Local guy learned that the hard way. Seized it twice before someone clued him in. Best things for folks who fly infrequently include installing a preoiler and don't make short flights. The preoiler will fill all the oil galleries and pressure will come up even before start. The short flights leave water in the oil the eat the engine. The folks who "run it up once in a while to keep it healthy" without flying it are doing the worst damage by far. Dan |
#43
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Preheating engines: Airplane engines versus auto engines
A key to lubrication of any aircraft engine is how soon an oil fog can
be established in a crankcase. This takes a while - especially if the oil is thick, as the bearing journal clearances are not that great and the amount of oil thrown from them is minimal given starting oil viscosity and the comparatively low crankcase activity at low rpm. Most of the oil flow generated by that fixed volume oil pump ends up blowing over the relief valve and dropping directly back to the sump, giving the oil little access to the heat of the engine pistons and cylinder heads. There are no features on the rotating machinery that will centrifugally splatter oil blobs onto the cam surfaces. They have to depend on previous operation for lubrication until the fog gets more generally developed. Oil viscosity can easily vary over a 100:1 range between moderately cold start and normal operating values. The leakage from a journal bearing will correspond inversely as that viscosity and directly as the cube of the bearing clearance - which is also compromised by the differential thermal expansion of aluminum vs steel. Remember it isn't the bearings - it is all the expensive surfaces in your engine that are starved in a cold start. And the operating profile is almost always a full takeoff power after only a few minutes of operation. I do agree though that the OP didn't do any substantial damage to his engine since it had been run so recently and since it has Continental's bottom-of-the-crankcase camshaft. |
#44
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Preheating engines: Airplane engines versus auto engines
On Thu, 20 Dec 2007 21:43:18 +0200, "gpaleo"
wrote: Ο "Matt W. Barrow" έγραψε στο μήνυμα ... "gpaleo" wrote in message news:1198177392.30618@athprx03... "Matt W. Barrow" wrote news "gpaleo" wrote in message news:1198154485.901022@athprx03... "Matt Whiting" wrote news ................................................. ............................... Perhaps with your extensive and pontifical experience you cold summarize the findings of Shell, Chevron, and TCM of the percentage of wear that occurs in the first 30 seconds of cold (ie, below 40 degrees) starts? May I humbly pontificate that proper starting of engines (incorporating the, appropriate for the temps and engine, oil) at 25F will NOT impact their respective TBOs to any statistically significant degree. Well, your holiness, no one said anything about TBO. I see what you mean. I intended to convey the idea that no damage would occur to the engine resulting from the cold-ish starts within the time period to overhaul. Do find some inner peace, my son ;-)) (the holy-ness kicking in). I'm stuffed if I can see how the cold start will have ruined his engine. the traditional reason for warming an engine prior to flight is so that the exhaust muffle is hot enough to deliver carby heat if needed on climb out. now if the engine is fuel injected.... all engines get some wear in the first 30 seconds until all the oil is around the sliding surfaces but for heavens sake the engine us not running under any load so the wear will be minimal! humble little I only run an O-200 in a Wittman Tailwind. I can be airborne in under a minute from a cold start my hangar is so close to the runway end. if I do take off with a cold engine and I dont encounter icing my worst worry is a little blowby spraying some oil mist under the belly which resolves in very short time as the parts come to working temperatures. I just stick some new oil in :-) one of the hardest things for an owner is to develop enough experience with hands on operation to know the *actual* relative importances of the concepts of engine operation. from my experience gpaleo is bang on the money ....and I'm an atheist :-). when it is all said and done... there is more said than done. Stealth Pilot Australia |
#45
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Preheating engines: Airplane engines versus auto engines
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#46
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Preheating engines: Airplane engines versus auto engines
Peter, this thread has grown way too long and ****y for me to go check if
you already have received this info, so please excuse any duplication. Mike Noel hit on an important factor that did not seem to be followed up on about a difference between aircraft and automobile piston engines. See the papers at http://www.tanisaircraft.com/articlesresearch.html . Stan "Peter R." wrote in message ... 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 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? -- Peter |
#47
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Preheating engines: Airplane engines versus auto engines
Stealth Pilot wrote:
I'm stuffed if I can see how the cold start will have ruined his engine. the traditional reason for warming an engine prior to flight is so that the exhaust muffle is hot enough to deliver carby heat if needed on climb out. That's pretty funny. A preheat has so little affect on the temperature of the exhaust muff that it isn't even a consideration. Within 10 seconds of engine start, the heat of the exhaust pipe will be at roughly the same value whether the engine was started at 20 degrees F or 80 degrees F. Matt |
#48
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Preheating engines: Airplane engines versus auto engines
Tauno Voipio wrote:
Matt Whiting wrote: Tauno Voipio wrote: Peter R. wrote: --- clip clip -- 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? Here in the north of Europe We'll pre-heat our cars if possible, if the temperature goes below +5 C (whatever it is in F, around 40?). You can force an engine to start even at -30 C, but it means that the poor thing runs some time practically dry of lubrication. Really? Where does all of the oil disappear to that was there when the engine was shut down? Matt The lubrication is based on fluid between the metal surfaces. When the oil thickens enough, it will not get to the small spaces between the metal surfaces. There was oil in those spaces when the engine was shut down and it doesn't magically disappear. Most engines will run for some time with no oil pressure and without damage. Matt |
#49
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Preheating engines: Airplane engines versus auto engines
Stan Prevost wrote:
Peter, this thread has grown way too long and ****y for me to go check if you already have received this info, so please excuse any duplication. Mike Noel hit on an important factor that did not seem to be followed up on about a difference between aircraft and automobile piston engines. See the papers at http://www.tanisaircraft.com/articlesresearch.html . Even more interesting is that Tanis leaves such obsolete information on their web site. Take a magnet to a few car dealerships and see how many engine blocks are cast iron these days... Matt |
#50
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Preheating engines: Airplane engines versus auto engines
On Dec 21, 4:45 am, Stealth Pilot
wrote: squeezing past the rings probably contributes a millionth of one poofteenth of a percent to the problem. when the crankcase cools there is moist external air sucked into the cavity of the case through the crankcase breather. the moisture condenses onto the cold internal surfaces of the engine. can you please factor that into the alchemy above? it probably does the damage occurring between the poofteenth of your scenario and the 100%. Nope. We have taken rocker covers off engines immediately after a runup of a brand-new engine and found copious amounts of water in them. The blowby of any cold engine is significant. If we briefly run up an engine that has sat all night in a heated hangar and in our very dry winter climate, we will find water on the dipstick every time, with the engine at any point in its life. And the dipstick was dry beforehand. We operate on the western Canadian prairies where the air is drier that where I grew up in south-central BC, which is the northern tip of the Sonora Desert. We get little rain and snow here. Temps reach -40C, more typically -20C, no fog and clear skies most of the winter. It's REALLY dry, and any air sucked into these engines after shutdown doesn't have enough moisture to make a couple of tears. Dan |
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