Jabiru engine cooling

[jetjr]

From other thread

 

Recommend this document,

 

http://www.usjabiru.com/uploads/AirDuctInstall.pdf

 

Can agree with tedious adjustment and test flights, a 2-3 mm change is alot.

 

Im 80% through this i reckon and its looking good - tip is do 1 thing at a time.

 

Interestingly when i put a block in the oil cooler to raise oil temps, it throws out head cooling opposite to what you expect, more block makes heads hotter. Also messes with EGT but I cant work that out.

 

I think jabiru installs in Lightning and bristell have traditional fence style ducting. Think a pic was in Sport pilot a while ago when CWF new Bristell arrived

 

 

[Oscar]

The Jab engine install manual is also worth a good look: http://www.jabiru.net.au/eula/eula.php?u=/Manuals/Engine/JEM2202-6_Inst.pdf and http://www.jabiru.net.au/eula/eula.php?u=/Manuals/Engine/JEM3302-4_Inst.pdf. The general section regarding overall airflow is useful (though some of it is rather basic...)

 

Some other stuff on this site is also worth a read, for general backgrounding if nothing eg: http://www.recreationalflying.com/threads/low-oil-temperature-on-cold-mornings.51270/, which presents a pretty powerful argument for having a thermostatically-controlled oil temp bypass (CAMit are working on one, or there is the reference to the UK one from Steve Rance which has good reports) not just for the better general oil temp. maintenance but also so that once you have the oil cooler airflow worked out you don't change your head cooling characteristics by fiddling with the cooler airflow - as JJ shows is not necessarily without other effects. (Personally I'd prefer to see my oil cooler airflow kept right out of the lower-cowl equation anyway..)

 

Jabiru made their single biggest oopsy in my opinion by not including CHTs on all cylinders from the start (yes, I know the cost argument) but I believe a hell of a lot of grief for everybody could have been avoided, or at least reduced, by that move. Their documentation on cooling certainly promotes the use of CHTs on all cylinders, and given their admission that it's quite usual for CHTs to vary more than 30F, you'd have thought they'd have worked out what risks were being taken by only installing one CHT as standard.

 

If you're going to do any development work, I think it's a really good idea to not just have CHTs on all heads but also go to the trouble of checking that they are reading correctly! One dud splice in the wiring or even a dirty connection can really throw a CHT out of whack. To really get a good picture, a full set of EGT's is highly desirable as well; the somewhat cramped Jab. inlet manifolding can do odd things to the mixture distribution and that can change with different revs, so you might get inexplicable results that you think are an airflow effect that are in fact a mixture distribution effect and go chasing the wrong factor.

 

Something I learned on my recent trip to CAMit is just how intricately the whole engine 'system' is interlinked and how much research and thought it takes to work out how changes to one element can affect other things. This is precisely the reason that simplistic statements about problems are more often than not of very little value; it is very likely that at least some of the problems with Jab engines that get put down to engine faults have in fact been the result of some apparently unrelated change that the operator doesn't connect with its actual effect. JJ's experience with the CHT's and EGT's changing when he changed the airflow through the oil cooler are a very good example of this; sometimes it needs pretty much forensic-level skills to determine the connection.

 

 

[Russ]

Works a treat. Will photo "CHT" gauge , revs etc soon.

 

 

 

[Oscar]

Works a treat. Will photo "CHT" gauge , revs etc soon.

Very schmick - could we have a front view also? Very keen to see the instrument readings.

 

 

[Russ]

Inflite pics to come. ( CHT at cruise is approx 10% in the green! full power climb peaks and holds at 50% in the green )

 

That,s at around 30c ambient, and home strip 2500asl.

 

 

 

[kgwilson]

Now that's what I call a decent lip.

 

 

[Oscar]

Inflite pics to come. ( CHT at cruise is approx 10% in the green! full power climb peaks and holds at 50% in the green )That,s at around 30c ambient, and home strip 2500asl.

That lip is just lovely, it really is, and those figures are nice numbers for sure. If I pat her on the head(s), would she follow me home, do you think? I guess probably not, she would like you lots and lots.. What engine hours and experience of those have you had? - e.g. through bolt and head tension changes, if any?

 

 

[kgwilson]

From other threadRecommend this document,

 

http://www.usjabiru.com/uploads/AirDuctInstall.pdf

 

Can agree with tedious adjustment and test flights, a 2-3 mm change is alot.

 

Im 80% through this i reckon and its looking good - tip is do 1 thing at a time.

 

Interestingly when i put a block in the oil cooler to raise oil temps, it throws out head cooling opposite to what you expect, more block makes heads hotter. Also messes with EGT but I cant work that out.

 

I think jabiru installs in Lightning and bristell have traditional fence style ducting. Think a pic was in Sport pilot a while ago when CWF new Bristell arrived

Good document with far more detail than the standard installation manual. It's strange how Jabiru don't supply something like this to Aus customers.

I had considered installing internal baffles but have not done so. I also have the old style fibreglass ducts with the plug leads on the outside. They have a somewhat lower internal volume than the new style and a single internal baffle over the rear head which had to be cut to allow the duct to sit over the heads properly. Personally I think that there will be a smoother flow of the air in a shorter time frame with the reduced volume. The other benefit is being able to check the plugs without removing the ducts.

 

I was not that impressed when I got these out of the box as the plug lead grommets were originally blanks with a very crudely cut hole in each for the plug lead and some of them had perished. The other thing was the holes were not drilled in exactly the right place as you would expect on a couple of cylinders. To Jabiru's credit I took photos & emailed details & new bits arrived by express mail within a few days. The top cowl on my aircraft is quite low so the new style would almost touch the inside of the cowl. I also have had to cut the front almost completely off & glass on custom made intakes which don't require a fence as they slope up to about the half way point on the cylinder/head. It seems to me that having a big intake & then cutting it in half with a big flat fence will create some turbulence which may reduce airflow.

 

I will be taking some time to get a good seal all the way round the intakes. Pics of custom intakes (unfinished) fibreglassed to the Jabiru ducts & oil cooler intake. Oil cooler is a 7 row Positech attached to the firewall. The cooling air exhaust lip is about 25mm high approx 1 metre wide.

 

 

 

 

[Russ]

Recently installed, was initiated by another 160 here, he has many hrs on his, no problems what so ever, and he don't baby it.

 

Jab sell the lip extension ( small ) you extend their skirt by another 100mm approx. Whole thing weighs as much as a pac of cigs. ( or less )

 

It runs close to the exhaust pipe, so for heat protection of the glass, I just attached a thin piece of stainless sheet directly under the pipe area, about 50mm wide, by 130mm long.

 

Have done a temp check, and the glass just gets "warm" at best.

 

The worst part of the job is the bleedin glass particles attaching to your skin/ clothes etc and causing reeeel discomfort as you sand etc getting ready for painting. For memory jabs lip cost around $30, plus my additionals.

 

 

[jetjr]

Theres an older SB regarding cowl lips and whats available

 

The one i bought included scoop for positech cooler so was whole lower section of bottom cowl

 

 

[Russ]

Inflite pics to come. ( CHT at cruise is approx 10% in the green! full power climb peaks and holds at 50% in the green )That,s at around 30c ambient, and home strip 2500asl.

Cruise 2800, ambient temp 30 , cyl head temp 250f, ( 121c ) that was today.

 

 

[Oscar]

Cool Damn! Did your engine send you a Valentines Day card? - she should have. Would be great to have an ongoing history of your engine in the future. Overheating them kills them quicker than an IED in Afghanistan; a record of being operated within limits would be great to have for all Jab. engine owners.

 

 

[Bruce Tuncks]

Good stuff posted here guys and yes the American air-duct stuff is better. There is a Limbach Technical Bulletin 44.1 ( just google that reference) which is interesting. It explains some of the theory behind what we are trying to achieve here. The Limbach is very similar to the Jabiru engine in layout.

 

I followed this Limbach reference when modifying the inlet-holes to duct flow-path and when I put the gull-wing deflectors on the cylinder undersides. Later I was surprised to see Jabiru reckoned to put the gull-wings on the top. I still think the underside is correct.

 

 

[Oscar]

Excellent find! One of the tricky problem areas for Jab cooling is the airflow past the upper cylinder head bolt nearest the exhaust port and getting decent velocity past that is very important and requires some subtle tweaking. The whole cooling thing is not a simple case of stuffing air at the engine from one side and getting rid of it at the other, it also needs to get to the right places at the right velocities (too slow and it's ineffective, too fast and you can get stagnation from obstructions etc. in critical areas.) The design of air-cooling systems is anything BUT intuitive, and cooling performance testing and adjustment is an area that keeps aero-engineers very busy...

 

 

[jetjr]

I ll support the anti intuitive comment, air inside my cowl flows forwards somehow

 

Had another tried cutting a section off the back of the air duct, letting air bleed OUT of the duct and saw increased cooling??

 

Important here is the monitoring of all cylinders

 

 

[Oscar]

JJ, that reverse airflow - certainly on the downwind duct if sitting cross-wind while warming things up - is a known (and very well documented, in terms of 'proper' engineering practice measurement techniques) phenomenon. I would not be at all surprised if it's cooked a lot of engines while the owners were doing all the right things by the numbers they could see if they didn't have CHT's on all pots.

 

I wonder if anybody has ever done a census of which pots threw through-bolts vs which pots had cht's on them? This is an area where much better recording of all ALL conditions and some research might have provided quite strong indications of problem areas, rather than the simple generalisation of 'engine failed: through bolts / valve guide / valve head failure' etc.

 

I'll also bet that good recording and analysis would demonstrate that there is no absolute linear relationship between effective cooling vs airspeed, i.e. it's not automatic that increased airspeed and lower power setting will throughout the range be better at all points (though obviously it's going to be a better bet to fly a bit faster with slightly lower power than vice-versa..).

 

Anybody who has sailed a yacht competitively knows that when trying to extract the maximum drive when going upwind, you're continually playing with sail settings and relative heading as the breeze changes even by quite small amounts. You are forever adjusting the slot between the jib and mainsail to stop choking, keep the tufts all streaming away, and adjusting the main to stop backwinding from the mast destroying the drive. You'd think a mast, in comparison to the size of the sail behind it, wouldn't make much difference but you can get a really serious area of the main in stagnant/backing air by just a very small change in airflow.

 

 

[jetjr]

Setting up a Dynon to record all engine data is pretty easy yet few do it.

 

Just download after disaster to get an insight into the real cause.

 

 

[Russ]

Just to be more precise.....my cyl head temp is reading from the left hand rear cyl only.

 

( just the one censor fitted )

 

 

[Oscar]

Setting up a Dynon to record all engine data is pretty easy yet few do it.Just download after disaster to get an insight into the real cause.

JJ - recording everything is a great idea (we plan to use an MGL Extreme EMIS for the same purpose, it fits the panel) but it's important to realise this is only one part of what needs to be a two-part exercise.

 

EMIS recording shows the conditions; what needs to be added is analysis of the causes of the conditions. This isn't just semantics, because a condition may be the result of several causes. - which is why I have been arguing that just 'failure' doesn't present the whole picture.

 

Can we examine a completely hypothetical case? Let's say, a through-bolt failure. You have a log of cht's, egt's and revs over time; the MGL Extreme also allows recording of a number of other parameters, including (if a GPS is attached, a flight track, OAT, and fuel flow) These can be downloaded into an Excel spreadsheet for historical record-keeping - I assume a Dynon has a similar capability.

 

Now, if, let's say, your records show conservative use of the engine over a long period of time, plus entirely normal cht's and egt's immediately prior to the failure, the 'cause' is almost certainly a material failure of the through-bolt/ bad machining.

 

If the pot with the bolt failure shows a marked rise in CHT but a normal -ish EGT immediately prior to the failure and the other pots are all in the greens for CHT and egt, then it's likely to be some sort of cooling malfunction for that cylinder head leading to detonation. It may be (and a careful trawl through the historical data may show this), that for some reason, at a particular airspeed maintained over enough time, you get a weird cooling airflow distribution.

 

If the pot with the failure shows abnormal CHT and EGT prior to failure, then the cause is likely to be adverse mixture distribution - leading to detonation. Jabs have inlet manifold quirks at certain fuel mixture flow rates.

 

If ALL pots show abnormal CHTs and EGTs, with that pot being slightly worse, then fuel quality is probably a prime suspect with the pot with the least cooling being the first to reach detonation and let go.

 

All of the above is a pretty crude description but if armed with the information, experienced people can very likely pinpoint a 'most likely' cause from the data - and it's not always the most obvious one.

 

 

[jetjr]

Just to be more precise.....my cyl head temp is reading from the left hand rear cyl only.( just the one censor fitted )

Russ with all respect you dont really know whats going on then, Mine had 50-80 degrees differing CHT accross the engine and rears were coolest. Even after extensive fiddling I still have 100 deg difference in EGT and 20-30 in CHT across 6 cylinders

For arguments sake lets say you have a head running 50 deg hotter then you are well into red zone on that pot

 

This illustrates my point exactly, people are using engines expecting they are all running evenly and they are doing the right thing in operation, when actually a cylinder is regulalrly overheating or running lean at times in the flight. Maybe they are fine but thats where the random nature of failures comes from.

 

THIS is a key failing in the Jabiru system IMO where owners EXPECT that things are setup and running correctly. They arent that well sorted from factory and much of this is Airframe install problems.

 

 

[facthunter]

I cannot believe that you can't expect even cylinder cooling without baffles like used on most other similarly configured motors that must extend to the lower half of the cylinders. The square edged fins on the heads don't help and you would expect some improvement with a thin coat of flat black high temp paint on heads and cylinders. Pressure drop and direction of flow has to be achieved. You can't just blow a bit of wind near a cylinder with a few fins on it and expect it to work well. Nev

 

 

[Oscar]

Absolutely right - and very important. Jab. kicked a monumental own goal, I believe, by not mandating at least CHT reporting for ALL cylinders, on an engine that undeniably requires careful attention to operation. I'll bet London to a brick that there are a considerable number of owners who have been very careful, done all the right things as far as they could tell - and not been rewarded with the engine life they had every right to expect. And they have every right to be angry and feel that the engine has simply let them down, when better information about what was really happening could have made a vast difference to the life they got. Jabiru make strong recommendation about having probes on all cylinders for any non-factory installation, yet they skimped on the factory installs. They make a damn competitive airframe with most things costing way more to buy, yet they don't give it the chance to deliver its best by skimping on instrumentation.

 

The cost of three, or even five extra probes and a suitable instrument (e.g. the MGL Extreme EMS in a package deal with six CHT and 6 EGT probes), is about A$2k. That's a bit more than pocket change, for sure, but if you get just 400 extra hours to first overhaul as a result, it's paid for itself, what - about 4 times?

 

 

[Russ]

I agree completely. I am going to source a 4 in one instrument that will drop into my existing dash, with 4 censors on the engine. Any advice appreciated.

 

 

[jetjr]

Dynon D10 is $2300 USD, inc all std sensors plus 6 sensors CHT and EGT, add $200 for full fuel monitoring - Plus fitting (which isnt cheap in a retrofit but not too difficult either).

 

I have spoken to Jabiru about making these std fitment for their own protection and information gathering.

 

I assume D10 can log data? D180 sure can

 

D10 replaces 1x std large gauge hole, has alarms etcetc.

 

JPI do some cheaper ones I think but much more basic screen, nothing wrong with MGL either

 

 

[cooperplace]

I cannot believe that you can't expect even cylinder cooling without baffles like used on most other similarly configured motors that must extend to the lower half of the cylinders. The square edged fins on the heads don't help and you would expect some improvement with a thin coat of flat black high temp paint on heads and cylinders. Pressure drop and direction of flow has to be achieved. You can't just blow a bit of wind near a cylinder with a few fins on it and expect it to work well. Nev

black paint will affect heat loss via radiation but how much is that? air cooling requires convection as you rightly point out. Might there also be some insulating effect from the paint? Yes there have been some air-cooled engines with black (anodised?) fins, if I recall correctly the RD250/350 had black fins. But I think that if this made a noticeable difference, every air-cooled engine would be black.