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Oil consumption versus RPM?


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Interesting question.  I've never seen it addressed.

Assuming consumption is due to blow-by combustion, not leaks, it would make sense that the slower the engine turns, the less it burns.

However, it may not be a measurable, or significant difference.

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Someone was saying operating at 2500 for takeoff was better for oil consumption.  The decreased time distance downrange/ getting high in case the fan quit/ fuel vs getting out of the heat all came into play.  My engine leaks oil.....  but not $30K worth of oil

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The challenge I encountered with the O360...

high power settings and high angle of attitude climb were responsible for most of my oil loss out the case vent...

Made it look like pattern work was burning oil... but it was more venting more oil...

good luck with measuring the details with any precision...

Long flights, with careful climbs may reveal some pretty good oil use numbers...

Best regards,

-a-

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In my opinion, and from my observations, I think I use less oil when running at higher RPM/lower MP.  I try not to pull the RPM below 2600 unless my MP is below 20" and usually closer to 15".  Before we decided to do a top overhaul, our plane started burning about 1 quart every 3 hours.  By keeping the RPM up and minimizing the time at lower RPM, I was able to increase that to about a quart every 4.5 or 5 hours.  Now that we have done the top, we get about 10 - 12 hours/quart.

Here's my theory.  I think most of my oil consumption is due to blow by pressuring the crankcase and blowing oil out the breather.  My engine is a "D" engine so it is timed at 25 BTDC.  High RPM combined with low MP moves the peak pressure point further past TDC.  This reduces the pressure which keeps my CHT's cooler and reduces blow by.  In addition to lower peak pressure, it also has less time to act before the exhaust valve opens.  While there will be more strokes/minute, I don't think it is as important as the reduction in pressure.

If your engine is timed at 20 BTDC you may get different results.

Try it both ways and see which works better for you.

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8 hours ago, Mooneymite said:

Any correlation between oil pressure and oil consumption?

Bottom of the green band versus top of the green band?

I cant give you scientific info but my 95 PSI engine in cruise burns 1qt in about 20 hours. 

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I think it has more to do more with internal cylinder pressure than RPM.  A bit of anecdotal evidence for you.

I used to treat my engine "nice."  Cruise 65%, some LOP ops, lower MAP with 2450 RPM etc.  I discovered my oil was getting dirty fast, I had a good amount of lead deposits in the cylinders, #3 compression was beginning to decrease (72 vs 77-78 for the rest) and I was beginning to use oil at a pretty good rate; around 1qt every 6ish hours.  Obviously the engine was unhappy with me. I decided to change my M.O. when it came to engine operation.  

After dumping a qt of MMO in the crankcase, I ran the engine no less than 75% and 2500 RPM and 100 ROP.  I left the MMO in the system for 10 hours then changed oil and ran another 25+ hours.  

The results were AMAZING and somewhat unexpected...

The oil stayed clean. Like, read the numbers through the oil on the dipstick clean.  Oil consumption went to 1qt every 16 hours.  Compression on #3 came back to 77.  

My conclusion is this:

1. These engines like to work, much like a diesel.  If you try to be too nice to it, it won't like it. 

2. A happy medium between being abusive and maintaining good ICP keeps the rings seated.

3. A periodic dose of MMO seems to cheer the engine up.

Side note:  For a short time, I tried low RPM (2000) with high MAP.  The plane was quiet and the cylinders were VERY cool.  The engine used NO oil when I was operating like this so I think it lends a little proof that somewhat higher ICP may not be a bad thing.  

These are just my observations with my own plane.  



 

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