Peter wrote:
the compressions have always been around 78/80
If it’s like in MedFlyer’s picture, the oil sweep ring is broken. This does not affect compressions.
My first action would be to find out if the oil is burned in the cylinders or thrown overboard through the crankcase breather. That is best checked by attaching some sort of reservoir at the end of the breather line but in a way that it can not obstruct the airflow — seal the breather line and your engine will stop. If the reservoir has a lot of oil after a flight, then it’s related to your cylinders pressurizing the crankcase. If not, then one or more cylinders are leaky.
If it’s a broken oil sweep ring, there is a chance of fatal engine failure like in MedFlyer’s case. I’d try to get to the bottom of this. Oil consumption does not suddenly increase without a specific reason.
The oily spark plugs aren’t normal either. Should not be like that.
achimha wrote:
If the reservoir has a lot of oil after a flight, then it’s related to your cylinders pressurizing the crankcase. If not, then one or more cylinders are leaky.
Don’t both scenarios indicate leaky cylinders? The first (excessive crankcase pressure, indicated by oil in your temporary “catch tank”) would indicate leaky compression rings (leaking from combustion chamber downwards), the second (no excessive crankcase pressure, no oil in the catch tank) would indicate leaky oil control rings (leaking of oil from crankcase upwards into combustion chamber). Assuming it’s not just a straightforward oil leak somewhere else, then as you imply above, the lost oil can only have been either breathed out the crankcase, or burnt in the combustion chamber.
ortac wrote:
Don’t both scenarios indicate leaky cylinders?
Yes but the scenario with the pressurized crankcase is rather harmless and usually only throws oil overboard up to a certain level in the crankcase. The scenario with the broken oil control ring is very serious and can lead to complete engine failure in a fairly short time.
Yes I agree. One is just a bit of cylinder/ring wear, the other is a broken part rattling around in the engine. I would want to know which it was sooner rather than later.
2 broken piston rings?
Not impossible but an astonishing piece of bad luck.
Why two? Do you mean based on the plugs? You said you had oily plugs in general beforehand anyway so the plug condition could be a red herring.
You could also argue that if one ring broke at X hours, then a second one that has been subject to near identical conditions for the same X hours is liable to fail in a similar-ish time frame. But obviously not at exactly the same time down to the hour.
Peter – I would suspect strongly that what you have is a situation where the oil control ring has become gummed in its groove. This could be happening on more than one cylinder. Compressions and power would be unaffected.
I have had this happen to me twice – most recently a month or so ago, and the other time in about 1997. This is a runaway phenomenon – as more oil blows by, the more gummed the ring becomes. The end game is that you end up burning more than a quart an hour and you need to catch it before then as it is quite possible to empty the sump. In 1997, I was flying an M20J and caught it as the oil pressure started to fall as the pump was sucking in air, diverted and got it on the ground just in time.
I speculate that operating a normally aspirated engine at lowish power for long periods – as I do, and I guess you do too – doesn’t help.
My personal view is once oil consumption is starting to ramp up, it is time to pull the pots and in the process take a look at the cam. If the cam is good, do a top overhaul, if not, a full overhaul.
I log oil consumption as part of normal ops, and keep graphs of consumption between changes. You can usually see the trends pretty clearly.
PS – the “problem” with fine wire plugs is that they don’t tell you which cylinder(s) is oily by fouling!
If the cam is good, do a top overhaul, if not, a full overhaul
Do a full top overhaul if possibly only a single piston ring is broken?? Wow…
That is exactly what I would put money on, Alan.
The “problem” is that I have seen this before many times, so it is possible that high power operation can clear this condition. I will try that next. A 1hr flight at 75%+ power.
