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It was once explained to me that the FAA has a rather interesting view about oil capacity of aircraft engines. Each engine manufacturer establishes an acceptable range of oil consumption. The manufacturer also establishes minimum oil level for proper functioning. When an airframe manufacturer designs an aircraft, one of the considerations is fuel capacity and endurance with a particular engine. Here’s where it gets interesting. For the sake of explaining the concept, I'm going to make some assumptions about consumptions and levels. I’m not stating numbers to be used as fact.... illustration only.
Let’s say the IO-540 on the new Redbird airplane needs a minimum of 4 qts oil, to stay lubed - provide acceptable cooling and maintain propellor function. That IO-540 can use up to 1 qt of oil per hour, as acceptable oil consumption. The Redbird is a long range animal and has enough gas capacity to fly 6 hours. If it needs 4 qts to operate and could burn 1qt per hour for 6 hours, then that IO-540 has to have minimum 10 qts oil capacity. A different version of the Redbird might have shorter range tanks, yielding 4 hours range, thus that one needs only 8 qts to be acceptable. Seemingly similar engines might indicate different oil capacities due to such issues.
It's my understanding that sometimes, the only difference between the 10qt engine and the 8 qt engine, is the dipstick.. Just because you can put 10 qts in the engine doesn’t mean it’ll stay in the engine. Automotive engines employ 'windage trays' that attempt to isolate oil in the sump from the spinning crankshaft and the rods and pistons flinging wildly in the engine. If that oil gets too involved with the rotating mass, it’ll whip it up to a virtual froth, full of air. If the oil gets too frothy, it doesn’t pump well, it requires power to keep it whipped and a lot of it blows out the vent, or through the PCV valve which isn’t good. Actually, none of this is good. Airplane engines are simpler in that they don’t have windage trays. They don’t rev as fast as high performance auto engines either, but airplane engines bounce around a good bit. Airplane engines are pretty 'loose', too. Air cooled engines do a lot of expanding and contracting, so they have to be kinda loose. Loose means blow by pushing gases out the vent.
In a perfect world, the 10 qt engine would have a nice big deep oil sump that would provide lots of distance from the oil to the crankshaft. But, nice big deep oil sumps are heavy and big, which doesn’t lend itself to streamlining cowls. So the sump might be a compromise. Not too big, but big enough to hold 10 qts.
No matter what the dipstick says, there’s a point where enough separation exists, so that the oil doesn’t get flung around and blown out the vent. If one consistently puts more than that amount into the engine, it will consistently blow it out or burn it.
The dipstick in my Patrol says 8qts at the top line. Filled to 8 qts, it’ll happily puke out 2 qts. Down to 6, it uses very little oil.
Whew! That’s a lot of words......
Bill
Let’s say the IO-540 on the new Redbird airplane needs a minimum of 4 qts oil, to stay lubed - provide acceptable cooling and maintain propellor function. That IO-540 can use up to 1 qt of oil per hour, as acceptable oil consumption. The Redbird is a long range animal and has enough gas capacity to fly 6 hours. If it needs 4 qts to operate and could burn 1qt per hour for 6 hours, then that IO-540 has to have minimum 10 qts oil capacity. A different version of the Redbird might have shorter range tanks, yielding 4 hours range, thus that one needs only 8 qts to be acceptable. Seemingly similar engines might indicate different oil capacities due to such issues.
It's my understanding that sometimes, the only difference between the 10qt engine and the 8 qt engine, is the dipstick.. Just because you can put 10 qts in the engine doesn’t mean it’ll stay in the engine. Automotive engines employ 'windage trays' that attempt to isolate oil in the sump from the spinning crankshaft and the rods and pistons flinging wildly in the engine. If that oil gets too involved with the rotating mass, it’ll whip it up to a virtual froth, full of air. If the oil gets too frothy, it doesn’t pump well, it requires power to keep it whipped and a lot of it blows out the vent, or through the PCV valve which isn’t good. Actually, none of this is good. Airplane engines are simpler in that they don’t have windage trays. They don’t rev as fast as high performance auto engines either, but airplane engines bounce around a good bit. Airplane engines are pretty 'loose', too. Air cooled engines do a lot of expanding and contracting, so they have to be kinda loose. Loose means blow by pushing gases out the vent.
In a perfect world, the 10 qt engine would have a nice big deep oil sump that would provide lots of distance from the oil to the crankshaft. But, nice big deep oil sumps are heavy and big, which doesn’t lend itself to streamlining cowls. So the sump might be a compromise. Not too big, but big enough to hold 10 qts.
No matter what the dipstick says, there’s a point where enough separation exists, so that the oil doesn’t get flung around and blown out the vent. If one consistently puts more than that amount into the engine, it will consistently blow it out or burn it.
The dipstick in my Patrol says 8qts at the top line. Filled to 8 qts, it’ll happily puke out 2 qts. Down to 6, it uses very little oil.
Whew! That’s a lot of words......
Bill
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