Hot rear cylinder example Jabiru 3300A-2575 and plenums

[facthunter]

Maybe somethings changed with the Avgas. I've always stuck to it for the extra  detonation protection. Never recommended it on the Rotax as definitely know of compression loss with those motors when 100 L used. How effective is the air filtration? It still may be dust.. Try analysing it.  It's more build up than I  would like by far. Looks like it won't scratch off easily either.  Install new seats by freezing with dry nitrogen. Some Porsches were done that way.  Test the hardness of the head first. Don't increase the OD of the inserts any more than you have to. THICK is not good. Nev

Edited January 25, 2021 by facthunter

[RFguy]

Going to also get a oil sample chem analysis .

Will check air filtration...... 

There are a couple of options- the filter looked clean. 20 hours TIS  . will look at it under a microscope. 

The Jab filter box - I think you need to be careful not to overtighten the four screws at trhe corners as you will deform the box and lose seal in the middle. Anyone familiar with SEALING boxes and lids will know this problem. Mark K ?  needs an extra screw in the middles, or care with not over tightening.  

 

There is a little extra hint...... Carb heat  does NOTHING ( OAT always >32C at all tests so far though) 

For my annual check last week , I looked at this - and carb heat vane is verified swinging. and blocking....

which together with your thoughts Nev...  maybe a tear in a scat hose  to the carb ?

 

 

Edited January 25, 2021 by RFguy

[facthunter]

IF it is working it definitely makes a go around unlikely when you leave it on. Nev

[RFguy]

either way, something for me to check.  carb heat non functional (no drop in RPM at run up, and no change in EGTs CHTs when pulled in in cruise.... ) and the hot air switch box is verified working... will do a pressure and vac test on the induction system.....including the air box, scat hose to carb, and other hoses...

 

bores not scored.  no hone pattern anywhere, very very light fine in-line lines in the bores, uniform on all cylinders.

BTW aircraft meets performance targets in POH, approximately. 

Edited January 25, 2021 by RFguy

[Bruce Tuncks]

Sorry Rf guy, I have been at the farm since well before Xmas and I don't know much about the new gen 4 engine but I am happy with Kensla's report that it is running clean.

My old engine ( 700 hours 2200A of 1998 vintage ) has just run a few hours on mogas and I hope it will look cleaner inside. Yenn is trying mogas too for the same reason.

My engine has always had ok leakdown results, but not as good as jab 7252's which all went about 78/80 to my amazement.

[kgwilson]

The engine will have to be hot to get any carb heat use change. I had to go around when an aircraft cut in front of me on base & I instantly went full power but the engine would not develop any extra power & ran rough. Carb heat off & power was instant.

[Old Koreelah]

13 hours ago, RFguy said:

OldK - remember wrapping the bores has to be tight otherwise the air wont go in the fins...

Advice appreciated but unneccessary, RF.

As posted: “I’ve also installed close-fitting sheet aluminium ducts around and under the cylinders to direct air thru the fins.”

[Old Koreelah]

4 minutes ago, kgwilson said:

The engine will have to be hot to get any carb heat use change. I had to go around when an aircraft cut in front of me on base & I instantly went full power but the engine would not develop any extra power & ran rough. Carb heat off & power was instant.

KG where does your carb heat draw its hot air from?

I wrapped my whole muffler but it’s not really hot enough to achieve the rapid temperature rise expected of a proper carb heat setup. Even with a hot engine, it takes about minute for the intake air to rise to 50C, so I’ve got in the habit of opening the CH valve as a precaution well before icing is likely to occur.

I didn’t want to wrap a hot header pipe to supply hot air because most of the time that pipe would have no cooling and thus conduct extreme heat back to the head.

[Kenlsa]

RF,

latest leak down on the 100 hour engine is 78 and 79 /80 and these are cold readings! 
leak downs on the avgas went from 50 to 60 hot. So lap the valves and back to high 70’s. Hence our move to Mogas .

Detonation......starting to be an urban myth. Maybe before modern fuels but never heard of it with 95/98. It’s a different world to 30 years ago. The factory says it’s good.......so it’s good. As with anything, ensure the fire/heat sleeves are good.

I have never talked to anybody who had detonation. Starting to get tired of hearing it quite frankly.

Ken

[facthunter]

Not sure you can just wish it away. It caves the tops of pistons in. Engines on gas can shorten rods or split blocks in Halves  Put some diesel or avtur in your Aztec and see how it goes .Grossly overheat any motor on load or put hot plugs in it.. The way mogas is rated is not the same as Aviation rated Octanes. The aviation rating has a range like 130/145(long since unavailable) Engines requiring those fuels have to run de rated. The two figures are for rich and lean conditions. Lean is the lower. Nev

[Bruce Tuncks]

I would have thought that detonation was a problem more for high compression engines, by which I mean 10 to one. The Jabiru is 8.3 to one.

I also reckon that hot spots could cause pre-ignition, which Mike Busch says is worse than detonation.

SO...  running an engine which is internally clean and is not overheating should be fine on high-octane mogas . Your car engine ( admittedly liquid cooled ) is higher compression and runs on lower octane, surely there is some margin there for the hotter air-cooled engine .

 

[Bruce Tuncks]

Thanks Kensla for the update. That's great news about the gen 4 leakdowns on mogas.

[RFguy]

They are good numbers, Ken.  (Bruce jabs are 8:1  except for very early engines 8.3)

 

Maybe having an engine full of crud after operating on AVGAS , perhaps having a higher possibility of hot spots, then going onto MoGas is potentially trouble.  More difficult to run exclusively MoGas in most places. 135 litres is alot of jerry cans for the jab.. need drums. 

 

I'll have to revisit what can be put into LL100 to decrud  them. How long has 100LL compared to 100 been common place ?

 

Anyway, pistons are coming out, to be de-cruded also.  Will take a look at the valves under a microscope and recut / replace if necessary.    
 

 

[Bruce Tuncks]

I've run on avgas for 19 years.  100LL because that is what was in the bowser. It is only since the gen4 engines had problems with lead clogging that I thought to experiment. Maybe they have changed the avgas chemistry like Nev suspects.

Yes my engine had deposits but they seemed to flake off and not increase over time. I would prefer it clean though.

So far, it has about 4 hours on mogas and it runs just the same.

[RFguy]

Bruce, Jab told me that (one person at the planet) thought the higher incidence of fouling and clogging with AVGAS with Gen4 was due to the far lower cylinder, piston and head temperatures.

Maybe less Ethylene Dibromide C₂H₄Br₂ added (lead scavenger) to current LL100 ?

reading about it, they only add enought To take care of the TEL amount,. IE falling TEL = falling C₂H₄Br₂ 
(I AM GUESSING) so that's probably not it.

?

 

Lower TEL ? 0.35 is spec for the Shell LL100, --others is 0.53, 0.55 etc.  it varies......... (old 100 ~ 1gPb/L )

According to Viva (Shelll) LL100 : 
Tetraethyl Lead ASTM D5059 0.35 gPb/L
Total Aromatics ASTM D1319 16%

Edited January 26, 2021 by RFguy

[facthunter]

The only thing the lead lubricates is the Valve seat. It's no help anywhere else. The Rotax runs too cold for it , hence the known problem with it. A red hot exhaust valve is a pretty good glo plug so is a loose spark plug or one of the wrong heat range. (Running absolutely white on the insulator) Idling while taxiing will spoil your plug as an indication of mixture. Nev

[Kenlsa]

RF,

 I was wondering about the far cooler temps with the G4 may have caused a problem but have not followed it up.

 

With mogas, ensure that there aren’t any 90degree bends in any fuel line, or elbow, where you need a change of direction of the line. Under certain conditions (extreme heat) it MAY lead to a low pressure zone on the inside of the elbow causing a potential bubble at that point.

 

The Jab has gentle bends so is not a problem. This advice is mentioned on STCs for certified planes converting to mogas, for example my Colt. Any 90 deg elbows have to be replace with gentler ones.

 

Ken

[RFguy]

Hi Ken
So 90 deg is enough to cause trouble eh ?  Yes, the Jab is gentle, all bends I think also have tube springs, and the manuals go to some trouble to point that out. If I was MOGAS and hot and high regularly, I probably would emply a tank return setup.

A colleague pointed out the Lycoming runs 170C - 190C, and they have mixture control to lean, both possibly reducing buildup.  Like I said, my hot cylinders have less buildup.
Again, wonder just how hot it is really is deep in the head. that topic is discussed on other jab engine forums.

-glen.

[Bruce Tuncks]

Aluminium alloy is such a good conductor of heat that there cannot be much of a temperature gradient through it.  But Rf is right in that the inside will be hotter than the outside.

And, radiant heat is small in comparison. You can calculate it quite accurately and see that a perfect black surface will radiate about one kilowatt per square meter at the sort of temperature reached.

Nev said that radiation was not much and he was right.

[RFguy]

You'd be suprised of the head material  temperature gradient if being forced air cooled.
 

the top screw mounted CHT thermocouples cop a real cooling blast.  reality is much hotter.

 

radiation cooling- for anyone interested :

q = k . (Tobj^4 - Ttarget^4) . A

k = 5.7x10^-8    temps in kelvin
for 180C and 50C target,  = 1.8kW per square meter

not much as Bruce says, for this application.  Better than nothing.  But not much.
 

[kgwilson]

8 hours ago, Old Koreelah said:

KG where does your carb heat draw its hot air from?

I wrapped my whole muffler but it’s not really hot enough to achieve the rapid temperature rise expected of a proper carb heat setup. Even with a hot engine, it takes about minute for the intake air to rise to 50C, so I’ve got in the habit of opening the CH valve as a precaution well before icing is likely to occur.

I didn’t want to wrap a hot header pipe to supply hot air because most of the time that pipe would have no cooling and thus conduct extreme heat back to the head.

It is the standard Jabiru setup with a shroud around the exhaust before it enters the muffler & scat hose to the airbox so the air hot or cold is always filtered unlike Lycomings where hot air is unfiltered.

[RFguy]

YES

 

[facthunter]

By the way the chromium traces in the oil is probably from the Stainless steel oil rings. So is the cylinder fine scratching. These two materials are not compatible.  Nev

[Thruster88]

2 hours ago, kgwilson said:

It is the standard Jabiru setup with a shroud around the exhaust before it enters the muffler & scat hose to the airbox so the air hot or cold is always filtered unlike Lycomings where hot air is unfiltered.

Lycomings are tuff enough to eat some dirt, just kidding.  The reason it is done this way is because the carb heat circuit is also an alternate way to get air into the engine in the case of say the air cleaner became blocked with snow. They think of everything on those certified aircraft. Fuel injected lycomings have a spring loaded alternate air door in the intake.

Edited January 26, 2021 by Thruster88

[facthunter]

With nitrided Cylinders you are pretty close to right there. Not all of them are nitrided though.  Nev