I have gone through that doc and cannot find anything applicable to this topic. Is it in there somewhere?
Sorry, Peter, had no time to read the whole thing at work, i just thought it might be of interest, ring flutter or not
The oil rings need to move up and down in their groove, apparently, in order to stop the oil getting past them. So, if they get contaminated heavily (lead deposits are the usual thing) and stick in place, you get a high oil consumption. That seems to be well known in our industry.
Another seemingly well known thing is that if there is oil leaking past the oil ring(s) due to the above, and the top ring has more gas pressure on it (engine running at a higher MP) that will reduce the oil consumption – because the top ring is now acting partly as an oil ring. This effect is only partial because obviously the higher gas pressure is present only during a small % of the time, whereas oil is trying to work its way upwards past the rings on the whole of each downward stroke.
But none of the above is “flutter”. It’s just crap behind the oil ring(s). There seems little doubt that you can achieve this condition by long periods at very low power (say 40%) but that still doesn’t make it “flutter” 
When googling on “flutter” the only apparently credible information relates to racing engines and other high revving stuff. It seems to involve the top ring oscillating (the normal meaning of “flutter” especially in aviation) and damaging the piston groove and maybe the cylinder bore.
The thing which makes this oscillation a non-credible proposition (as a damaging effect in our engines) is that the conditions which allegedly lead to it are frequently present during normal operation (e.g. descents and landings) whereas the amount of time one spends avoiding wx at FL200 or whatever is a tiny % of the engine’s operating time.
Is it a good time to mention German engine engineering?
As a VW diesel owner I have to …
The only Q is how big this will eventually become.
On the one hand we are told car engines are so durable because they are run at low power / low manifold pressure. On the other hand we are told running an engine at low MP does all sorts of bad things on the rings and cylinder bores.
Conundrum.
On the one hand we are told car engines are so durable because they are run at low power / low manifold pressure. On the other hand we are told running an engine at low MP does all sorts of bad things on the rings and cylinder bores.
I think the key difference is that our engines make 2000hrs, more or less, at a high power, whereas car engines need to be designed only for mostly low power. Nobody can operate them at say 75% power and keep their driving license for more than 5 minutes. This, together with water cooling, enables them to be made to tight tolerances which in turn makes a lot of stuff possible. There is also a vastly bigger investment in production automation. Volume manufacturing is all about making stuff that works in practice.
Another thing is that automotive oils seem to contain detergents which keep the oil rings clean.
Don’t compare air cooled engines with water cooled engines. Car engines, being water cooler have way tighter tolerances because the temperature variation is slight. Aircraft and other air cooled engines require slacker tolerances so are more susceptible to problems.
STOLman wrote:
Is it a good time to mention German engine engineering?
I don’t want to pull that topic in here, but that has really nothing to do with engineering, more with ethics.
Shorrick_Mk2 wrote:
On the one hand we are told car engines are so durable because they are run at low power / low manifold pressure.
I don’t think light load and low rpm do car engines any good. I have seen how performance slowly degrades when an engine is run this way and how it improves when you start running it harder again (as we say, you burn the cobwebs). Among the car guys I know, there is a belief that you should, at least from time to time, give it a boot. Truck engines last too and those are not run lightly loaded. But car engines are quite ahead and unbelievable sums of money are spent on their development. And certainly not all car engines are designed to run hard for long (they don’t have enough oil, large enough radiators etc.). Emphasis is on consumption, light load driving as that is what’s tested and what people actually do quite a lot – you don’t need much power to maintain even 130 km/h.
What exactly is ring flutter in the first place? And why is it bad? And why should manifold pressure have anything to do with it? Normally aspirated diesels runs with max MP all the time. Interesting discussion though.
With an air cooled engine, running it hard, the “only” thing you do is to rise the temperature. Running it light means low temperatures, thicker oil, different clearances due to different materials used. This is my first thought about this. But then I have never heard of ring flutter before.
Peter wrote:
I think the key difference is that our engines make 2000hrs, more or less, at a high power, whereas car engines need to be designed only for mostly low power.
That’s another OWT. Car engines are durable under constant high load as evidenced by the test stands. I have repeatedly asked that specific question to my car engine development friends. It’s all about getting rid of the head so the cooling system needs to be designed for the high load which it might not be in many car installations but that’s not directly related to the engine.
Lycontosauri just suck in every regard
so the cooling system needs to be designed for the high load which it might not be in many car installations but that’s not directly related to the engine.
Saying that water cooled car engines not having enough of a cooling system is not related to the engine is like saying that air cooling in our engines is not directly related to the engine 
Lycontosauri just suck in every regard
I would say they suck but not necessarily in the same areas in which some people say they suck.
Peter wrote:
Saying that water cooled car engines not having enough of a cooling system is not related to the engine is like saying that air cooling in our engines is not directly related to the engine
The cooling can be designed independently from the engine block depending on the application. If an engine is limited in its continuous power (perhaps depending on the outside temperature), then it might actually be a limit of the cooling system. In order to reduce fuel consumption, car makers try to reduce drag which can be done effectively by limiting the air intakes.
Newer Mercedes have variable air cooling intakes which get opened electrically when needed. That is still very rare in the mainstream car industry. Cessnas have a manual system — cowl flaps — which have a significant impact on speed (I get about 3-4 knots).
We’d be much better off with mixed air and liquid cooling like found on Rotax and SMA. A small modification that was actually done by Continental (TSIOL-550) but too revolutionary for our 1950 mindset.
