CHT running too high.

[Wal]

G’day everyone I have had a problem with the CHT running too high in my Jabiru J230 ever since I built it. The following story might interest you.

 

 

My Jabiru J230 was experiencing an assumed heating problem. After having the Jabiru climb with the CHT just in the red (400 deg) and cruise in the yellow (360 deg), I attempted a number of things to reduce the temperature.

 

• Firstly I have babied the aircraft to keep the temp down (which was not good in the run in period).
   
   
  
• I waited to see if the temperature reduced as it ran in as suggested. It did, but only slightly.
   
   
  
• I modified the cowl as suggested by Jabiru by adding the two small deflectors on the bottom (even though the CHT was the only problem, oil temp has always been low).
   
   
  
• I changed the carby jets to enrich the mixture and therefore make the engine run cooler (added an EGT and have reduced EGT with the richer mixture). It is now running the recommended 20 Lt/Hr. fuel use.
   
   
  
• I centred carefully the thermocouple under the spark plug, because evidently if it is not centred then the reading can be high.
   
  

 

 

Considering that the EGT and oil temperature were quite low (oil temp 70 – 80 deg), I surmised that the engine did not seem to be running hot, therefore I should test the CHT gauge itself.

 

 

I removed the thermocouple from the spark plug and placed it in a boiling jug of water. The CHT gauge read about 230 – 240 deg F. Much too high so I removed the whole gauge and thermocouple and sent it to John Press at Australian Aviation Specialists in Tamworth for testing. John also consulted a colleague Keith Jones at Sigma Instruments in Tamworth.

 

 

John and Keith found the gauge and the thermocouple tested okay, using various test equipment, so suggested that it may be a problem with the wiring or connectors as these can affect the resistance, voltages produced at junctions and therefore the voltage that reaches the gauge. Incorrect wiring types can also generate errors as every time the type of wire is changed it forms another thermo couple at the joint. They connected the thermocouple and gauge with the similar copper wires as supplied with the VDO gauge and found that the result was variable and inaccurate. They tried a few different wire types and found that if they used Iron Constantan wire the results were accurate and consistent. They confirmed the results by connecting again with the copper wire and again the results were inaccurate.

 

 

After supplying me with the right length of Iron Constantan wire I was able to test the CHT in the aircraft. I first tested it in boiling water as a comparison. The gauge now indicates about 208 - 210 Deg F with the probe in boiling water which is about right for our altitude of 3500 feet AMSL. In the aircraft it now always runs in the green, mid green (275) in cruise and higher (320) in climb. Great!

 

 

It has been a long road but it is reassuring to know that I had not been overheating the engine prior to the correction.

 

 

My experience suggests a question for other Jabiru owners who are having problems with CHT that may possibly be caused by the wire supplied with the gauge. How many problems are not engine heating problems at all? Are others running engines with CHT under reading?

 

 

Iron Constantan wire is evidently not commonly held in stock because it is quite expensive. John Press at Tamworth now has a supply of this wire and is also available for any questions and advice about what he has found with my aircraft. I have added his contact details for further technical information.

 

 

John Press, Australian Aviation Specialists, Hanger 6, Tamworth Airport

 

E-mail: [email protected] Phone: 0427-681-893

 

I hope this has been of interest.

 

 

[Guest J430]

WAL

 

Probably the post of the decade on this forum . Well reserached and well written and VERY useful info.

 

its a very good point, I have the VDO cable and it is not thrmocouple cable (from memory I used the VDO stuff) however I do have some at work and I am now tempted to test this.

 

I would have assumed like most people that VDO supply the right cable for the job. In my industry we never trust anyone or their supplied goods if we find a bug.....its often the case of the one part you trust lets you down!

 

Guess who might be going to the hangar via the office in the morning!

 

J:wave:

 

 

[Guest Andys@coffs]

I'll second that, a very useful and thought provoking post. Just goes to show that in babying the engine in this case to an incorrect CHT indication you may have been doing more harm than good.

 

Can I ask if you've got EGT thermocouples as well? and if so when the analysis work was done on the low temp CHT sensors was work also done on the EGT's. I suspect the EGT's may not be as sensitive to the wire molecular make up as the temps being measured are much higher.

 

Also, have you passed this back to Jabiru and or the tech manager as I , like J, think this is important enough to be published in the monthly rag as there will be many jabiru drivers who dont come here and they deserve to know this info as well.

 

I wonder what the wiring is made of in the factory built J230's are.

 

Andy

 

 

[Wal]

J430, I guess that VDO produce gauges to an acuracy that would be expected for automotive use, whereas we require a more accurate reading. But this is not an excuse for selling an inacurate gauge.

 

About testing, I guess I should have put it into boilong water sooner, but I assumed that the problem was with something I did in the instalation.

 

Andy, yes the EGT is also a thermocouple, and no I did not get it tested. I am thinking that I should, although it at least had thermocouple cable, what the two metals are I don't know but at least they are different and not just both copper.

 

Yes I have sent a copy to Michael at Jabiru Engines as he has been e-mailing me for some time as we worked on the problem. I have also sent it to any Jabiru owner I know, not that I know many yet, since there is none around here.

 

I guess a reply from Michael might hint to what the factory models have as wiring.

 

Thanks for your replies.

 

 

[Guest J430]

Wal

 

Do you have the CHT ring flat on the head ender the plug and sealing washer, or is it above the sealing washer? The VDO manual says to remove the sealing washer altogether!

 

J

 

 

[Guest brentc]

Personally I've never had an issue with my CHT, but that being said, I didn't cut the wires that came with it, I bunched them up and attached them to the firewall like the instructions said.

 

Others at my field have had problems with the CHT which has usually been resolved by either replacing the probe or the guage itself. I've seen several instances of this and that usually resolves it. Another moderately frequent problem is the Oil pressure sender failing either intermittently or permanently.

 

 

[Wal]

J,

 

Yes I have the Thermocouple against the head. Although the VDO manual says to remove the washer, Jabiru say that they leave the washer intact, but with the thermoucouple between it and the head.

 

I would assume that if you did put the washer under the thermocouple then the reading would be cooler not hotter.

 

Wal

 

 

[Captain]

FYI, my Dynon 180 instructions say the following re the way the compression washer should be handled with their thermocouples:

 

 

 

Quote

 

Jabiru engines require a 12mm ring-terminal CHT probe for each cylinder. First, slide the compression washer off the spark plug. Slide the 12mm ring-terminal probe onto the plug. Now, slide the spark plug compression washer back onto the spark plug. Reinstall the spark plug into the spark plug hole. Please refer to the documentation that came with your engine for more information. Now, plug each thermocouple wire into its corresponding wire on the thermocouple harness. Ensure that you match the wire color pairs on the harness to those on the thermocouples.

 

Unquote

 

 

[Guest J430]

Why did I know this was going to let a few worms out of the can.....

 

Captain, thanks for that post, very interesting.

 

I have the sender between the plug and washer, my temps read the same as Wal's, I have the original wiring and as Brent said, just coiled up not shortened.

 

I do think it will read higher if flat against the head, however maybe the the wire is making a difference and then the positioning is offsetting it.

 

Request to our mate Captain who would seem to have the benefit of the D180. How about some photo's showing CHT (and EGT if you have it) during taxi, climb out at say 90kts and once above 1500AGL and clear of traffic, and then once trimmed out and in the cruise at 120KTAS and stable.

 

I think as Wal pointed out the CHT is mostly OK, its just the gauge, and what we can see is really just a reference of cooler / hotter rather than a definitive temperature.

 

Cheers & thanks in anticipation:thumb_up:

 

J:wave:

 

 

[Guest Macnoz]

I had a heart transplant on my Jab 160c last week at the factory. 404hrs I learnt a lot during “assisting” with this operation some of which would cause me sleepless nights were I of nervous disposition. Some might even encourage a seasoned engineer (non aeronautical) as myself to become a Luddite.

 

Certainly I know now that the chances of any two 2200 engines being identical are much the same as wining the lottery. In say this I am not just referring to the individual hand made components eg exhaust pipes that naturally vary.

 

My “old” engine had been rebuilt with new parts two weeks before. The rebuild was not a success to say the least. On installation of the exchange engine a fortnight later I became aware of the following.

 

The exchange had the “new” finer fin barrels different from the ones I had purchased two weeks earlier

 

The exchange engine has the newer cam ( I forget the specs but have them written down at home one is 265 lift the other 290 I think)

 

I could go on but suffice it to say that I calculated that there were 720 different combinations of finished engine that could exist between the combination of differing parts on my original 404hr (14month old) engine, the parts available and supplied two weeks ago and the parts utilised in an off the shelf factory rebuild exchange engine

 

I asked about the presence / placement of washer as my theory is that leaving it in situ and adding the CHT would result in the plug not being as far into the combustion chamber as the other cylinders. I could be wrong but surely this would change the combustion characteristics of that cylinder.

 

I can’t imagine that the position of the probe relative to a retained washer would cause much or any variance in reading given that heat conduction over such a small distance should be “efficient”

 

However I seem to think / remember that the washer with plug is there for two reasons, to provide a seal and to act as a compression washers maintaining an even loading force of the plug.

 

Did I get an answer – yes – several of them and all different depending of who I asked!

 

 

[Captain]

Captain, thanks for that post, very interesting.Cheers & thanks in anticipation:thumb_up: J:wave:

Whacko J430

 

 

 

All of this cooperation between us is making me ill (mate). This could well mean that we haven't criticised each other for a few days, which must be a record, and Ian will be quite worried seeing the kids playing so well out in the yard. With my kids that has always been a sign that a big bust-up is just over the hill.

 

 

 

Yes, I have 6 CHT's and 6 EGT's and I'll try to take some photos or will see if I can download the stored figures from the data logger. Either way I'll post what I can here within the next 7 days or so if this south westerly buggers off and I can get into the air.

 

 

 

Just 2 other comments if I can ...... in response to Macnoz's comment the Dynon thermocouples are very thin and I don't believe they would adversely effect combustion characteristics due to plug position (and as an indication, the revs drop to the same figure on the digital tacho with each mag run-up test) ..... and the other is that Jabiru recommend that you need to have the thermocouple on the plug nearest the exhaust valve.

 

 

 

Regards Geoff:yuk::clown::pig:

 

 

[Guest J430]

Gooday Captain

 

Not sure what you are referring to mate! I have not been sticking it into you at all, and I went back through all my posts for a month or so, the last few reprinted here for your general interest and I can not see my lack of co-operation. Maybe you have me confused with someone else. That has happened of late!

 

Captain

 

 

Like all things....the more stuff you have the more complicated it all gets!

 

 

 

The way I will do the elevator servo will be different. Mounting over the back behind the cabin bulkhead and using a straight cable rather than a tightly bent one will mean less drag and less clutter in the foot well of the rear seats.

 

 

 

The downside is the servo's will not be available for a few months from my friends in WA of the US of A!

 

 

 

Geeeeez Captain

 

 

 

Is that the best you could muster, could barely hear that shout from here!

 

 

 

On a scaled spectrum from Ballons to say a drifter.......the chute is much closer to the ballons!

 

 

 

A Drifter can at least be navigated accurately in a breeze of more than 10 knots with some degree of certainty!

 

 

 

J

 

 

 

Captain

 

 

 

I do receive and read almost cover to cover every RAA mag, and I have seen barely a handful of reports. I understand that fatal accidents have a different course to follow and its not the RAA that control it. All others though are fair game and while some are adequate in coverage with a single paragraph about a nose wheel failure etc, many could be far better done.

 

 

 

The few accidents written up in the last 5 years you can count on one hand.

 

 

 

I am not being hard on the RAA in my other post at all. Its the plain facts. I have never said anything like they are a bunch of secretive usless etc etc or other slanderous type remarks that I have heard in the past. I do believe they do a good job overall, however, good reporting on accidents and incidents is good educational material, and I stand by my earlier comments about the lack of it.

 

 

 

 

Hey Captain...........ssssshhhhhhhhhhhhh!!!!!!!!!

 

 

 

I had the first D10 in a Jab, (they offered me a dealership....should have taken it in hindsight) and I might be about to have their first AP in a Jab!

 

 

 

Need to send my D10 back to the factory for an upgrade though. Will be the latest spec D10A then.

 

 

 

Just need to make sure I order the correct servo is why I am asking. When I know what works well I will happily advise others.

So how about some nice pic's from your sexy D180 then?

 

J:wave:

 

 

[Captain]

Request to our mate Captain who would seem to have the benefit of the D180. How about some photo's showing CHT (and EGT if you have it) during taxi, climb out at say 90kts and once above 1500AGL and clear of traffic, and then once trimmed out and in the cruise at 120KTAS and stable. J:wave:

J

 

 

 

This thread looks like it is running parallel with the one I started on EGT sensor location on the factory 3300's, but the following roughly addresses J430's request as above.

 

 

 

Data is a tiny excerpt from 5 hours of data logging from a recent flight with hops from Gawler/Swan Hill/Tocumwal/Wagga and then a couple locally.

 

 

 

Taxiing @ 965 rpm, Cylinder#/EGT/CHT's 1/522/99 2/527/106 3/548/137 4/564/132 5/526/114 6/545/116.

 

 

 

Run-up @ 1973 rpm, 1/579/91, 2/594/95, 3/609/117, 4/627/121, 5/630/102, 6/621/96.

 

 

 

Cruise @ 2877 rpm, 1/686/118, 2/702/123, 3/623/129, 4/657/131, 5/662/113, 6/663/101.

 

 

 

The next time I fly I'll put markers in the data logger where you asked for them & at full throttle 75 knot climb, full throttle 95 knots fast climb and 120 knots true cruise and will also note the OAT for each. Is there any other operating condition you would like?

 

 

 

Hope this is of interest.

 

 

 

Regards Geoff

 

 

[Guest J430]

Hey Geoff, I think that is good data for anyone to compare to, well done!!!!

 

Funny thing is that I was at HBA with Bevan and col and others (new project) and Bevan who is a fussy old fella hasa the same probes etc into a different instrument however he has found that No 4. is the hottest also.

 

You should see the new plenum chamber he has made in lieu of the seperate ducts. His comment is the engine runs better and smoother with the better cooling. The whoe barrell gets air flow rather than the top half, and the crank case gets ariflow too! Special gull wings in the bottom of the cylinders also. A piece of art!

 

I will be on a trip to the cape in July with him and a few others, will be interesting...... even Mr Jabiru is coming with us so many yarns over a beer or three I think.

 

thanks again

 

J:wave:

 

 

[Wal]

Great to read the different replies.

 

I guess the only point I am making is to check the thermocouple in boiling water whether or not it seems to be in the right range, just to check and be sure. It only takes a few minutes at the next annual.

 

Whether the thermocouple should be above or below the plug washer is a good question and can be easily checked by shifting it and seeing if there is a significant difference. I intend doing this at my next annual.

 

Thanks for the interest, I have learned heaps.

 

 

[Guest J430]

Seems that Cylinder 4 CHT is the hottest. Contrary to Jabiru saying No.6!!!

 

Not 100% sure yet but after fitting a second CHT sensor, it is likely to be 25-50F higher!

 

Have seen a full plennum chamber cooling system on a J230.........and very good temps!!! Will let you know after a FNQ safari!

 

J:wave:

 

 

[Captain]

Seems that Cylinder 4 CHT is the hottest. Contrary to Jabiru saying No.6!!!Not 100% sure yet but after fitting a second CHT sensor, it is likely to be 25-50F higher!

 

Have seen a full plennum chamber cooling system on a J230.........and very good temps!!! Will let you know after a FNQ safari!

 

J:wave:

 

 

Thanks for that J.

 

 

 

What FNQ Safari is that?

 

 

 

Re improving the CHT of #3 & #4, (or perhaps better described as balancing them all up a bit more) I think I'll try a small f.glass deflector above and ahead of both and see what it does.

 

 

 

However with the deflector below the back of the lower cowl, it may be just as important to ensure that the resultant negative pressure below the engine needs to be able to "drag" the cool air down thru the fins on those heads. Perhaps the -ve pressure below is just as/maybe more important as/than the +ve pressure in the cooling duct.

 

 

 

One other point is that when reducing power on descent with low OAT, #'s 1,2,5 & 6 can get very cool and below 65C, such that shock cooling needs to be considered as a possible issue.

 

 

 

One of the problems with having lots of temp sensors is that we may be tempted to chase differences that perhaps do not matter and are "normal". Maybe if we went to the same trouble on our bikes or cars we would be staggered by the results.

 

 

 

Hope that makes sense & contributes a little to the debate. Regards Geoff

 

 

 

PS Just one more little point. When taxiing and doing engine run-ups it makes a HUGE difference to CHT's whether the aircraft is oriented into the wind or X-wind. With a X-wind it is easy to get up near or above 155 C on #'s 3 & 4 when taxiiing or running up. It needs to be watched.

 

 

 

[Guest J430]

The safari is just a couple of us going for a bit of a tour of the North and doing some fishing etc....not like a Massive Tour or anything.

 

Good point about chasing things you can't or dont want to know about! Ignorance is bliss some times. And one of them hasa to be the hottest, just so long as its not way over the limit:ah_oh:. I suspect it will get to about 350F in the climb and be 330 in the cruise. Will see.

 

As for pointing into wind for runnups, good airmanship! And its also good to then stop and look up at the base/ finals for a/c......and this is likely tobe the opposite direction!

 

J

 

 

[Guest J430]

Talking to Jabiru this morning, the question was asked about the gull wing baffles and whether I had them installed, which I do. They discovered that removing them actually allowed more air out and more IN, and the lower engine compartment ran cooler, around the inlet tubes etc.

 

So out with the Gull wings and see what happens.

 

Having said all that, we have had 600 hours with no valve or other problems, it runs like a new one.

 

Cheers

 

J

 

 

[Guest J430]

News to those with GULL WING Baffles.............. remove them!!!

 

Temps dropped around 10-20 degrees F

 

J:thumb_up:

 

 

[Captain]

J

 

Which temps dropped? CHT? EGT? Or Oil?

 

 

[Guest J430]

Ahhhh Captain...... I was trying to not thread drift.........

 

For a change

 

J

 

PS....twas the CHT!

 

I might add that with 620 hours on the Jab and not in a training school environment working every day, she is running as well now as the day we built her, in fact I would say even better and with no serious issues along the way. When the time comes shortly to sell her to make way for the next project this will be one well sorted and reliable machine. Leak downs are excellent after 140 hrs since new rings and the valve train is in excellent health. It pays to be a good maintainer and not a back yard fiddler.

 

Dynon said to fit the CHT ring above the washer, Jabiru (Rod Stiff) says between washer and the head! Seems to make arund 40 deg F difference on the gauge!

 

 

[jetboy]

Hi J430 do you mean to say changing the position of your CHT thermocouples has made a difference or removing the gull wing baffles?

 

I think its Jab USA that describes placing the thermocouple lug between the spark plug and washer and the reason they cite is to stop hot gas leaking past the lug and causing a 50 deg inrease in indications.

 

I no longer use the lug on the plugs as its CHT I want to monitor not the temperature of the external part of the plug and the cooling air flowing over the end of the lug.

 

For same cyl at same time, I get 50 deg F more reading by correctly fitting a bare thermocouple to the lower part of a fin nearest the exhaust port. That is I clamp the thermocouple to the fin using a ssteel clip with teflon (PTFE) insulation on the clamped side.

 

I'm told current engines have a hole drilled in the heads for alternate fitting of CHTs, so this error of measurement may now be gone. Another error is the statement in the manual that the cold junction at 50 degs is best. Well its not, the cold junction ideally should be within the gauge and most electronic gauges compensate for this.

 

If you were to make the cold junction in an area that was 50 deg, ie. connection of the thermocouple leads to the normal wires is sitting in an area that is at 50 degs, the gauge reading will be 50 degs lower than actual CHT.

 

Actually most of the above is academic when referring to a Jabiru engine, because the Max and min. temps and pressures are referenced to a standard placement of probes. Another example is measuring the oil pressure at the lower port, which is the oil at the main gallery. I have 10 psi less oil presuure here, than at the standard port located upsrteam of the cooler and filter.

 

Ralph

 

 

[Guest J430]

Jetboy

 

I think its Dynon also that say to do that with the lug between the plug and washer. Aand they may be correct about escaping gases. Jabiru say just get it centred!

 

The spacing it out with the washer allows the lug/ring to be cooled more by the airflow so I feel flat against the head is better.

 

I now have two CHT lugs on 4 & 6 and it would seem that 4 is hotter. No matter what you do one will always be hotter. After removing the gull wing baffles the temperature dropped.

 

I will be interested to see what the Dynon CHT/EGT kits come with for the IO-540.

 

J:thumb_up: