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John_F wrote:
On Thu, 02 Sep 2004 16:56:03 -0400, Matt Whiting wrote: John_F wrote: There are two terms in this temperature equation. One is steady state thermal transfer rate. If you generate a quantity of heat say one BTU then it will raise one quart of oil X degrees F. If you add two times as much heat to the same oil it will raise the temperature of the quart of oil 2X degrees F. At any given RPM the oil pump will pump "Y" quarts of oil per minute whether you have 2 quarts or 10 quarts in the sump since the pump is a gear positive displacement pump. This means that the oil will carry of the SAME amount of heat per minute if the temperature delta is the same. If you want to get rid of more heat then you have to pump the oil faster or heat the oil hotter to get a larger delta temperature difference. This is the steady state condition. This is the condition the engine is in when the temperature gage quits moving up. Or you start with cooler oil in the sump. This is what, I believe, will happen when you have more oil in the engine. The oil has a longer residence time in the sump and contacts more surface area of the sump through which it may dissipate heat. You always START with COOL oil. It just does not stay that way long. I didn't mean start as in first starting the engine. I meant start as in the oil pump is starting out with cooler oil at its intake if there is more oil in the sump and it has resided there longer to dissipate its heat to the sump. Let's run a "thought" experiment at the limits. Let's assume that the oil level is so low that no oil is ever in the sump. The oil pump pulls it out just as fast as it comes in, just short of the point of sucking air. I realize this isn't possible in the real world, but that is why this is a thought experiment. This is done all the time. It is called a DRY sump and is used on many aircraft. As far as I know ALL radial use a dry sump. You are really showing your ignorance now. A dry sump system doesn't mean that there is no reservoir of oil, it just isn't in the sump, but is remotely located. This actually gives better oil cooling typically as the oil reservoir often isn't even connected to the engine. You may want to read a little at the following link. It not only explains different oiling systems, but is also states that oil serves a cooling function in an engine. http://www.geocities.com/zuuba_2000/Aeromat.html In this case, the oil will get very hot as it is constantly being circulated through the heads which are one of the hottest parts of most engines. The oil has very little opportunity to reject heat in the coolest part of the engine, the sump. The equilibrium temperature will be rather high. Nope! That is what the oil cooler is for. Heat doesn't care where it's at, it always moves from a hotter location to a cooler location. Sure, the oil cooler is made to cool the oil, but the oil will still lose heat anywhere in which the oil temperature is higher than the surroundings. This can be in the sump or can be in a remote oil tank as with dry sump systems. Most oil sumps on Lycoming engines are flat bottomed with vertical sides. Look at the ratio of the surface area of the bottom to the sides. It is at least two to one. Aluminum conducts heat MUCH better than oil so any oil that runs across the bottom of the sump will get cooling via the bottom and by conduction through the aluminum up the sides. Most of the heat does NOT go out through the sump but through the oil COOLER. If you have enough oil in the sump to cover the oil intake it will be cooled by the bottom surface of the sump. If you do not believe that cover up your oil cooler and see what will happen to the oil temperature. I never said that most of the heat goes out through the sump. I said that SOME heat does. And I said that MORE heat will be rejected as the contact area between the oil and the sump increases. Sure the bottom has a lot of area in contact, but if you raise the oil level it will lose even more heat due to contact with the sides of the sump. More heat loss equals lower oil temperature and greater engine cooling by the oil. Now take the other extreme. The oil sump has infinite capacity so the oil starts out at the same temperature regardless of how hot the hot parts of the engine are. The oil will enter the oil pump relatively cool and pick up heat, but will never again get circulated through the engine so it has "forever" to dissipate its heat. If you have an infinite oil cooler that will cool the oil back to the original temperature it still does not matter how much oil is in the sump. True, what is your point? A real engine is somewhere in between these to limit cases, therefore it is reasonably logical to expect some slope that connects the steady-state oil temperature of the one limit with the other. I don't think it reasonable to believe that both steady-state temperatures will be the same and thus have a zero-slope line in between. This is what would have to be the case for the oil temperature to be completely independent of the amount of oil in the engine. There may be a small slope but it may be positive not negative since the extra oil gets tossed around more and heats up due to mechanical splashing. Wrong. If any significant splashing occurs due to contact with the crank, the oil will aerate, the oil pump will cavitate and the engine will fail shortly thereafter. Applying some simple math: Since 90 % of the heat that is carried by the oil is lost in the oil cooler not the sump you would expect that if the quantity of heat carried away by the sump doubled the oil temperature would only drop 50% of 10% which is 5%. What is your source for the 90% figure? 5% is still 5% and will result in increased engine cooling as the oil level increases. It may well be a small number, but any extra cooling at all is significant as the life expectancy of oil drops dramatically with increasing temperature. Matt |
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