2nd piston ring always stuck- what's the story ?

[RFguy]

SPS have come back and said yeah on putting the grooves in no worries.. DO I read it correct that Jabiru went to circlips (weegan) because people couldnt seem to fit their wire clips reliably ? Car racing community seems big on wire clips. 

Best thing to do now is to buy a set of pistons.  I will be interested to know how they measure up. They're obviously fine in a water cooled untapered bore of 97.52mm.

 

Jabiru bore at 150C (125C rise)  in the middle (half way down) on top  (measured) will go to 97.77mm (from 97.6) .  Jab pistons at 97.45mm  and 200C rise on the pin boss region would go to 97.82,.(based on last round of measurements) ....

I will do another round of thermal expansion measurements in the oven at 150C  now I have my fancy external micrometers.
Blast air cooling over the top of the jabiru bores, with no air underneath would certainly distort the shape. 

Danger occurs at top of WOT climb, nose over-  airflow improves, speed goes up 25% = 56% more cooling available = Sudden bore cooling. 
OR backoff from WOT and full rich = cool to cruise throttle  and much leaner off the main jet - now gets hot.........AND / OR  backoff then skirt much hotter than crown now  briefly and crack.. Or go around from glide approach with cool bores and suddenly hot pistons . but it is full rich so fairly cool. lots of guesses there. 
 

 

[facthunter]

The cracks only come after time . (fatigue) The square sectioned clip with eyes for the pliers is a "seeger".  Wire clips can be improved by chamfering the end of the Gudgeon evenly. End pads are another option in non slipper pistons. Unusually short piston skirts can cause rocking and ring face barrelling if they are loose. and power off descents of a long duration are discouraged . Unless pegged (2Strokes) rings rotate during use and the gaps will end up at different places. Nev

[RFguy]

I'll need to consider a test program, also. fortunately, with LCH, I can run at WOT indefinetly on the ground... but that's not really the problem- its the cyclic nature of the flying of course, not just sustained WOT.  Some spring clamp thermocouples on the bores will be interesting  . I do beleive that there is significant room for problems between the bore and the piston half way down... 

With the likely 60 deg C difference in cylinder temps between a water jacket and the cro-mo air cooled bores,  the piston will get much hotter , and despite the bores having a 0.15mm head start (std setup)  the pistons with their 1.4x higher expansion coefficient(of cromo)  will zoom past the bores when things get hot.  I think the commodores get away with the very small clearances because they never really get very (very) hot. With the air cooled bores, while they can get hot(ter) and expand, there is significant extra heat in the higher expansion body (piston)  to cause trouble. 
IE you add another 50 deg C to the pistons and bores and now that 1.4x expansion difference of the piston  is a real problem. 

 

So the bore  temperatures are what needs to be monitored to understand where the clearance might be at  . 

 

I've been reading up on research journal papers on piston temperature modelling, measurement etc. 

 

[facthunter]

Paint's that change colour permanently at a designed temp have been used in the past.. The Blueing of the steel bores will give you a true temp indication if you refer to a "Temper' chart. Basic heat treatment of steel. I've seen gudgeon pins and the small ends of conrods go blue so 150C is way short of the mark for a  max piston temp.  In that area most people are just guessing . Nev

Edited January 21, 2023 by facthunter

[RFguy]

150C was used in the oven  looking at expansion from 25C >> 150C.

Temps can get up to 350C in the crown, 250C in the skirt with sustained high output (IE heat soak) - in engines maintaining stoich mixture . . there is a huge difference as you know between short powers bursts and sustained high output.    The models are very good now., and sophisticated.  Lower specific output (kW/litre) helps alot here...

 

Edited January 21, 2023 by RFguy

[facthunter]

Yes the practical upper limit for HP/ litre for aircooled motors is about 50 HP/Litre. Many aircooled aero motors use the engines mass as a heat sink. Hence the often  Max power for 5 minutes figure.  Use in emergency only. Same with engine overspeeding limits. Piston oil jets are common on supercharged engines.  Checking expansion(thermal) rates over some fixed range is a valid exercise because it's capable of being extrapolated . Nev

[turboplanner]

17 hours ago, RFguy said:

150C was used in the oven  looking at expansion from 25C >> 150C.

Temps can get up to 350C in the crown, 250C in the skirt with sustained high output (IE heat soak) - in engines maintaining stoich mixture . . there is a huge difference as you know between short powers bursts and sustained high output.    The models are very good now., and sophisticated.  Lower specific output (kW/litre) helps alot here...

 

My experience says you're a bit optimistic there. Melting point of Aluminium is 660 deg. C, and steel 1370 deg C, and I've managed to melt the whole crown out of pistons, seen exhaust valve heads looking like they've been attacked by an oxy torch, valve stems stretch at the point of the head and often break off at that point leaving the valve head loose,  somtimes being at 90 deg when the piston embeds it neatly into the very soft piston crown. When the piston has cooled it's like an interference fit.

 

Aircraft engines are continuous power demand at cruise.

On a continuous power demand application, if you put the engine on a dyno set to produce cruise power, the combination of all that compression, decompression and ignition of mixture turns into the front part of a temperature bell curve which rises exponentially, then goes horizontal in the cruise IF the cooling is satisfactory. If it's not the curve goes exponentially up to the place where combustion chamber items are destroyed. EGT and CHT sensors are outside the combustion chamber.

 

 

[RFguy]

My bet Turbs that the melting the piston crown you were looking for more than 36 hp/litre........ 

[RFguy]

and spiral pin retainers... 

[turboplanner]

On a bike at 175 km/hr + once I just notice smoke trailing back off the engine and grabbed the clutch in time. On pulling the motor down I found no circlip and a deep gouge from the pin in one side of the bore. I put that down to the person assembling the motor not checking that he'd put both circlips in, but have never had any other problems with circlips, and no problems with wire.

 

Temperatures:

Horsepower is just a time measurement for a level of work.

 

Horsepower = Torque x rpm/9549.3

 

In most engines if more power is required for an application the application rpm is just increased.

 

FWIW these are just a few figures I pulled together.

As you say Jabiru 2200 and 3300 engines operate at  36 hp/litre.

I had one engine running at 30 hp/litre unreliably until I solved the problems, then zero

I had one at 45 hp/litre unreliable until I solved the problems, then zero

A friend had one at 50 hp/litre with zero problems

A Cat 15 litre diesel truck engine operates at 36 HP/litre with in in-frame rebuild expectancy of 1.4 million km.

[facthunter]

The big limiter is the aircooling. A liquid cooled motor has much higher limits especially where four valves are concerned.  It was considered the Wright J5 was the first air-cooled motor to be successfully well cooled for aircraft use. It was a design Wright took over in the late 20's and a good read for those interested in these matters. These things were tested 50 hours at full throttle. Also have a good look at the big inch radials heads. Forged and then cnc'd. That's what you pay for. Also dynamic counterweights that even the 540 Lycoming has to control torsional vibrations, ALL these big engines have "never use" RPMS specified. in the POH. Nev