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Jabiru 3300 CHT Disparity

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Tim Juhl

Joined: 21 Mar 2006
Posts: 488
Location: "Thumb" of Michigan

Posted: Tue Aug 13, 2013 10:14 am Post subject: Jabiru 3300 CHT Disparity

Also posted in Zenith 601 section



I attended a forum at Oshkosh where a fellow discussed his research on cooling a Jabiru 3300.  He had compared the spark plug washer type CHT sensors to ones directly in the head and showed that they pretty consistently indicated CHT's about 70Â° cooler than actual temperatures!  If this is the case, Jab operators could be cooking their engines while thinking that their CHT's were within safe limits. He ended up designing a cooling baffling system that is nothing like what comes from the factory and claimed that was the only way he was able to get the CHT's under control.



I wonder what the rest of you Jabiru 3300 operators have to say about this? I'm not flying mine yet but will be getting ready to hang it soon.



For a summary on the fellow who made the presentation check out http://www.eaaapps.org/presenterinfo.aspx?id=1973



Tim

_________________
______________
CFII
Champ L16A flying
Zodiac XL - Jabiru 3300A
Almost done! It'll fly in spring!

jabfox(at)hotmail.com
Guest

Posted: Tue Aug 13, 2013 10:36 am Post subject: Jabiru 3300 CHT Disparity

I was there also, that guy was trying to reinvent the wheel 

 > Subject: Jabiru 3300 CHT Disparity

 	  Quote: 	
		   From: timjuhl(at)frontier.com

 Date: Tue, 13 Aug 2013 11:14:04 -0700

 To: jabiruengine-list(at)matronics.com

 --> JabiruEngine-List message posted by: "Tim Juhl" <timjuhl(at)frontier.com>

 Also posted in Zenith 601 section

 I attended a forum at Oshkosh where a fellow discussed his research on cooling a Jabiru 3300. He had compared the spark plug washer type CHT sensors to ones directly in the head and showed that they pretty consistently indicated CHT's about 70Â° cooler than actual temperatures! If this is the case, Jab operators could be cooking their engines while thinking that their CHT's were within safe limits. He ended up designing a cooling baffling system that is nothing like what comes from the factory and claimed that was the only way he was able to get the CHT's under control.

 I wonder what the rest of you Jabiru 3300 operators have to say about this? I'm not flying mine yet but will be getting ready to hang it soon.

 For a summary on the fellow who made the presentation check out http://www.eaaapps.org/presenterinfo.aspx?id=1973

 Tim

 --------

 ______________

 CFII

 Champ L16A flying

 Zodiac XL - Jabiru 3300A

 Wing modifications done - back working on the fuselage

 Read this topic online here:

 http://forums.matronics.com/viewtopic.php?p=406639#406639

 > 

  [quote][b]

nick(at)flylightning.net
Guest

Posted: Tue Aug 13, 2013 10:45 am Post subject: Jabiru 3300 CHT Disparity

 	  Quote: 	
		  From what I understand jabirus CHT limits are based on the CHT probe being

under the spark plug. Regardless of what is going on else where in the head,
	


this is where their testing comes from. 

I would put the CHT probe under the exhaust spark plug as noted in the

installation manual, and use the limits set by jabiru.  

We have been flying the 3300 in the Lightning design now since early 2006

and never have "cooked" and engine while operating inside the limitation set

forth by jabiru. We have over 130 aircraft flying with this engine and the

stock jabiru provided baffle system all over the world with no issues. 

Nick Otterback

Arion Aircraft LLC

R&D Technical Support

931-680-1781

nick(at)flylightning.net





--

JohnDRead(at)aol.com
Guest

Posted: Tue Aug 13, 2013 11:20 am Post subject: Jabiru 3300 CHT Disparity

I was there also. I was very skeptical too about his opinions. I too would  use the baffling as provided from Jabiru USA and work from there.

 Regards,  John

CH701 - Colorado - Jabiru 3300

Cell: 719-494-4567

Home:  303-648-3261   

  In a message dated 8/13/2013 12:36:20 P.M. Mountain Daylight Time,  jabfox(at)hotmail.com writes:

  	  Quote: 	
		        I was there also, that guy was trying to reinvent the wheel    

   > Subject: Jabiru 3300 CHT Disparity

 	  Quote: 	
		   From:    timjuhl(at)frontier.com

 Date: Tue, 13 Aug 2013 11:14:04 -0700

 To:    jabiruengine-list(at)matronics.com

 --> JabiruEngine-List    message posted by: "Tim Juhl" <timjuhl(at)frontier.com>

    Also posted in Zenith 601 section

 I attended a forum at    Oshkosh where a fellow discussed his research on cooling a Jabiru 3300. He had    compared the spark plug washer type CHT sensors to ones directly in the head    and showed that they pretty consistently indicated CHT's about 70Â° cooler    than actual temperatures! If this is the case, Jab operators could be cooking    their engines while thinking that their CHT's were within safe limits. He    ended up designing a cooling baffling system that is nothing like what comes    from the factory and claimed that was the only way he was able to get the    CHT's under control.

 I wonder what the rest of you Jabiru    3300 operators have to say about this? I'm not flying mine yet but will be    getting ready to hang it soon.

 For a summary on the fellow    who made the presentation check out    http://www.eaaapps.org/presenterinfo.aspx?id=1973

 Tim

 --------

 ______________

 CFII

 Champ L16A    flying

 Zodiac XL - Jabiru 3300A

 Wing modifications done - back    working on the fuselage

 Read this    topic online here:

    http://forums.matronics.com/viewtopic.php?p=406639#406639

 > 

 	  Quote: 	

ist href="http://www.matronics.com/Navigator?JabiruEngine-List">http://www.matronics.com/Navigator?JabiruEngine-List

s.matronics.com/">http://forums.matronics.com

p://www.matronics.com/contribution">http://www.matronics.com/contribution

 [quote][b]

pete(at)usjabiru.com
Guest

Posted: Tue Aug 13, 2013 12:28 pm Post subject: Jabiru 3300 CHT Disparity

I did attend that forum.  This man spent a great deal of time creating a

solution that is looking for a problem to solve.  He had a Jabiru J series

aircraft which are not hard to cool.  Very early cowls on that aircraft did

not cool the best but there was an easily fix that Jabiru put out in a

service bulletin.  My company has built about 125 of these aircraft and I

can say without hesitation that cooling is not much of an issue.



As far as putting the sensor some other place that indicated by Jabiru -

we've tested a few.  Under a head bolt on the exhaust side of the head

produces a reading that is about 20 degrees cooler than under the plug.  If

you create a new scale with lower maximum limits and operate your engine to

those cooler indications I don't see much of a problem.



The gentleman at the forum however was promoting a probe location in the 3mm

hole that is drilled into the head between the spark plugs.  I have found

this location to be unpredictable and unreliable.  We had some bayonet

probes made that matched the hole diameter and flew for more than 50 hours

with a probe under the plug and another down the hole on the same head.  We

did not find that temps were consistently higher indications down the hole.

Sometimes they were lower and sometimes higher.    We could easily make the

temps "down the hole" rise quickly to very high levels while the indications

under the plug began to cool.  Other times indications under the plug were

hotter than "down the hole".  We decided that there was not enough

consistency to recommend operators employ that method.



We then tried a small ring terminal probe fastened to the hole with w small

self tapping screw.  We got basically the same result with the being

variation from under the plug being less.  Still not predictable to use.



The Jabiru fleet now numbers some 2500 planes.  Fleet hours must be

somewhere between 1/2 to 1 million flight hours.  Enough that is had been

proven that the under the plug probe location and the limits published by

Jabiru will predictably keep the heads from suffering heat damage.  We can't

say that for the "down the hole" location.



Pete



--

Tim Juhl

Joined: 21 Mar 2006
Posts: 488
Location: "Thumb" of Michigan

Posted: Tue Aug 13, 2013 2:47 pm Post subject: Re: Jabiru 3300 CHT Disparity

Wow, what great information!  I'm glad I made this post.  Thanks to all for responding.



Tim

_________________
______________
CFII
Champ L16A flying
Zodiac XL - Jabiru 3300A
Almost done! It'll fly in spring!

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Tue Aug 13, 2013 4:56 pm Post subject: Jabiru 3300 CHT Disparity

Guys:

 Am I reading this correctly - The Ring / under the sparkplug read LOWER in CHT than the probes that are screwed into the cylinder???  If so, that does NOT sound correctly at all.  

 Two examples:

1 - On Lycoming or Continental engines the Ring/under the plug thermocouple always reads HIGHER than screwed in or bayonet probes.

 2 - The heat range of a plug affects the reading of the Ring/under the plug thermocouple...  YES, it will also affect the standard reading CHT but the Ring type raises the CHT greatly - As ANY changes to the seating of the plug to the head will.

I can't comment on the baffling, so I won't.

Barry

 PS

Once again GOOGLE changed things without my permission.

Barry   

“Chop’d Liver”

"The reason Benjamin Franklin was such a great inventor was everything lay before him.  The reason why we don't have great inventors today is, everyone is trying to reinvent the wheel" 

On Tue, Aug 13, 2013 at 2:35 PM, David McCormick <jabfox(at)hotmail.com (jabfox(at)hotmail.com)> wrote:

[quote]    I was there also, that guy was trying to reinvent the wheel 

 > Subject: Jabiru 3300 CHT Disparity

 	  Quote: 	
		   From: timjuhl(at)frontier.com (timjuhl(at)frontier.com)

 Date: Tue, 13 Aug 2013 11:14:04 -0700

 To: jabiruengine-list(at)matronics.com (jabiruengine-list(at)matronics.com) 

 --> JabiruEngine-List message posted by: "Tim Juhl" <timjuhl(at)frontier.com (timjuhl(at)frontier.com)>

 Also posted in Zenith 601 section

 > 

 	  Quote: 	
		   I attended a forum at Oshkosh where a fellow discussed his research on cooling a Jabiru 3300. He had compared the spark plug washer type CHT sensors to ones directly in the head and showed that they pretty consistently indicated CHT's about 70Â° cooler than actual temperatures! If this is the case, Jab operators could be cooking their engines while thinking that their CHT's were within safe limits. He ended up designing a cooling baffling system that is nothing like what comes from the factory and claimed that was the only way he was able to get the CHT's under control. 

 I wonder what the rest of you Jabiru 3300 operators have to say about this? I'm not flying mine yet but will be getting ready to hang it soon.

 For a summary on the fellow who made the presentation check out http://www.eaaapps.org/presenterinfo.aspx?id=1973

 > 

 	  Quote: 	
		   Tim

 --------

 ______________

 CFII

 Champ L16A flying

 Zodiac XL - Jabiru 3300A

 Wing modifications done - back working on the fuselage

 > 

 	  Quote: 	

 Read this topic online here:

 http://forums.matronics.com/viewtopic.php?p=406639#406639

 > 

 	  Quote: 	

 > 

   	  Quote: 	

ist" target="_blank">http://www.matronics.com/Navigator?JabiruEngine-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution

[b]

BobP

Joined: 16 Aug 2013
Posts: 10
Location: UK

Posted: Sat Aug 17, 2013 1:27 pm Post subject: Re: Jabiru 3300 CHT Disparity

I don't see how a CHT thermostat properly mounted in a small hole between the plugs can do anything but record the true CHT temperature.  



I can see how a thermostat crimped into an aluminium tag that is waving in the breeze might not accurately display  the temperature at the other end of the tag (ie at the plug/head interface).

 If the air blasting over this tab were to be increased (eg by fitting a baffle that deflects and concentrates air onto the sensor tag) I would expect the thermocouple temperature would reduce - even if the baffle restricted the total airflow over the head so far that  true head temperature increased. 



If the above is true it would explain why the readings of sensors in the two different positions give differing readings. 



BobP



PS is there a text  summary of the presentation that started this thread?

_________________
BobP

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Sat Aug 17, 2013 6:16 pm Post subject: Jabiru 3300 CHT Disparity

Bob:

 Anything that changes the conductivity of heat between the spark plug and the engine/cylinder will effect the CHT.

A spark plug is designated as a 'cold' or 'hot' plug because of the amount of heat it holds.  The LESS heat that is conducted off the plug makes the plug a hotter plug.  A hotter plug will cause the CHT's to increase.  Putting s Ring thermocouple under the spark plug cause LESS heat to be conducted away from the plug which raises the heat range of the plug and therefore the CHT's.

 Also - Ring thermocouples read HIGHER because the heat range of the plug is now increased.  



 



I am not sure of what you are saying about a crimped aluminum tag?  Would you have a part number that can be researched online with a picture?

 



Barry



On Sat, Aug 17, 2013 at 5:27 PM, BobP <matronics(at)panth.co.uk (matronics(at)panth.co.uk)> wrote:

 [quote]--> JabiruEngine-List message posted by: "BobP" <matronics(at)panth.co.uk (matronics(at)panth.co.uk)>

  

 I don't see how a CHT thermostat properly mounted in a small hole between the plugs can do anything but record the true CHT temperature.

 

 I can see how a thermostat crimped into an aluminium tag that is waving in the breeze might not accurately display  the temperature at the other end of the tag (ie at the plug/head interface).

  If the air blasting over this tab were to be increased (eg by fitting a baffle that deflects and concentrates air onto the sensor tag) I would expect the thermocouple temperature would reduce - even if the baffle restricted the total airflow over the head so far that  true head temperature increased.

  

 If the above is true it would explain why the readings of sensors in the two different positions give differing readings.

 

 

 

 

 

 BobP

 

 PS is there a text  summary of the presentation that started this thread?

 

 --------

 BobP

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=406929#406929

 

 

 

 

 

 

 

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 ="_blank">http://www.matronics.com/contribution

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 [b]

BobP

Joined: 16 Aug 2013
Posts: 10
Location: UK

Posted: Sun Aug 18, 2013 4:01 am Post subject: Re: Jabiru 3300 CHT Disparity

Hi Barry,

I can't see that a plug  temperature range will affect the (true) cylinder head temperature. My understanding is that the terms hotter and colder plugs refer to the  thermal coupling between a plug's centre electrode/ceramic nose, and the main plug body. The plug  temperature range needs to be selected so the plug nose gets hot enough to burn off oil and carbon that might otherwise kill the spark, and cool enough not to act as a glow plug resulting in pre-ignition.  



All the plug type thermocouples I have seen consist of an aluminium washer that is squashed between head and cylinder, the washer has a tag extending from it that "waves in the breeze". The thermocouple junction is crimped into that tag and so reads the tag temperature rather than the cylinder head temperature, or even the temperature at the plug/head interface.

_________________
BobP

guccidude

Joined: 11 Aug 2013
Posts: 1

Posted: Sun Aug 18, 2013 12:52 pm Post subject: Re: Jabiru 3300 CHT Disparity

I agree with BobP on the first paragraph.  The second paragraph may need a little explanation.  The ring does all that he mentioned but I think it does not read the "temperature"directly.  The ring is the ground.  The head temp changes the resistance value in the ring(ground) and that is what the temp indicator is displaying.  The temp you read off the gauge is a resistance value displayed as a temp.  That is my understanding of temp probes(rings).  Dan

BobHaas

Joined: 22 Mar 2013
Posts: 17

Posted: Sun Aug 18, 2013 1:39 pm Post subject: Jabiru 3300 CHT Disparity

I concur, But I don't think the heat range is overly affected by the 

thermocouple.  Bob Haas N380BH N3300BH.



--

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Sun Aug 18, 2013 3:27 pm Post subject: Jabiru 3300 CHT Disparity

Dan & Robert H.:

 To answer the question correctly you have to tell us what is the Make and Model of Temperature Gauge.  Most gauges use a Thermocouple.  It is more accurate and can handle all sorts of harsh environments.  The other option is a Thermal Resistor not as accurate and environmentally challenged.

 



In either case GROUND is not used as part of the temperature acquiring circuit, it is used for noise isolation and and RF isolation.  Different companies have different ideas on how to handle these issues. The three types of thermocouples are: Open element, Sealed Un-Grounded and Sealed Grounded.  They each have their own advantages and disadvantages. 

 



As for the Ring type thermocouple it is a THERMOCOUPLE not a Thermal Resistor.  The thermal resistor would be destroyed by torquing down the spark plug.  And your idea, you could not count on the contact resistance being uniform from spark plug to spark plug or from day to day.

 



Robert - Opinions do not count here, only the facts count. The ring type thermocouple most definitely does affect the heat range of the spark plug.  It can take the heat range UP two or more range levels.  Here is a simple check you can perform from the comfort of your computer chair:

 Ask around of owners of the SAME engine and SAME installation - What their CHT's are.  Also ask which thermocouple they are using...  Ring type or Screwed in type.  There is also a bayonet type but not to many owners use it.  You will find Ring thermocouples will always be a Higher Temp.

 



Hope this information enlightens those gray areas.

 



Barry

 Test Engineer for the past 20 years, developing and doing exactly this type of testing.

 





On Sun, Aug 18, 2013 at 4:52 PM, guccidude <dcr(at)fdltownhomes.com (dcr(at)fdltownhomes.com)> wrote:

 [quote]--> JabiruEngine-List message posted by: "guccidude" <dcr(at)fdltownhomes.com (dcr(at)fdltownhomes.com)>

  

 I agree with BobP on the first paragraph.  The second paragraph may need a little explanation.  The ring does all that he mentioned but I think it does not read the "temperature"directly.  The ring is the ground.  The head temp changes the resistance value in the ring(ground) and that is what the temp indicator is displaying.  The temp you read off the gauge is a resistance value displayed as a temp.  That is my understanding of temp probes(rings).  Dan

  

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=406988#406988

 

 

 

 

 

 

 

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 [b]

matronics(at)rtist.nl
Guest

Posted: Sun Aug 18, 2013 11:17 pm Post subject: Jabiru 3300 CHT Disparity

Barry,

     

     Just to be pedantic, the ring is not part of the thermocouple. The     ring is no more than a convenient mechanical shape that conducts     heat to the tiny thermocouple that is soldered/welded into the crimp     end of the ring.

     

     The entire concept of CHT is funny. The cylinder head is a big lump     of metal. One end is the hottest (the inside of the combustion     chamber), the other end the coolest (the fins that we try to blast     with cold air). The metal of the cylinder head conducts heat from     the hot side to the colder side, and anywhere in the head you will     find different temperatures. Therefor there's no such thing as 'the     right CHT'. You will find temperature gradients throughout the head.

     

     This said, there is a predictable relationship between all     temperatures. When you know the highest temperature at the hottest     spot that the material will withstand, thermo analysis allows you to     calculate what temperature each other location in the head will be.     There will be variations such as airflow, cooling from fuel mixture,     ambient temperature etc.

     

     The manufacturer picks a convenient location where they attach a     thermocouple, then tell you what temperature that particular spot     can be without the hottest spots in the head getting damaged. That's     the goal, prevent damage.

     

     Now, the issue with Jabiru is not that it allows for such low CHT.     The problem is that they picked a spot (the crimp of that CHT ring     sensor) that gets a *lot* of influence from the outside. Both     ambient temperature and the airflow we use to cool the engine also     influence the actual thermocouple temperature. This makes it much     harder to be certain that if the thermocouple indicates 350F, the     hottest spot in the head is not going to be damaged. Why does an air     dam inside the airduct appear to be so effective? Because it directs     the airflow down, placing that thermocouple directly into the cold     airstream. Now wrap that thermocouple with a few layers of HT rubber     and your readings go up. Not because the head get hotter, but     because the thermocouple no longer gets direct cool air.

     

     So, long story short, the max CHT is whatever reading ensures no     damage to the head. Unfortunately, Jabiru's design choices have made     it really hard to predict what that reading is.

     

     Rob

     

     On 8/19/2013 1:26 AM, FLYaDIVE wrote:

     

     [quote]                Dan & Robert H.:

         

         

                    To answer the question correctly you have to tell us what is           the Make and Model of Temperature Gauge.  Most gauges use a           Thermocouple.  It is more accurate and can handle all sorts of           harsh environments.  The other option is a Thermal Resistor           not as accurate and environmentally challenged.

         

         

         In either case GROUND           is not used as part of the temperature acquiring circuit, it           is used for noise isolation and and RF isolation.  Different           companies have different ideas on how to handle these issues.           The three types of thermocouples are: Open element, Sealed           Un-Grounded and Sealed Grounded.  They each have their own           advantages and disadvantages. 

         

         

         As for the Ring type           thermocouple it is a THERMOCOUPLE not a Thermal Resistor.  The           thermal resistor would be destroyed by torquing down the spark           plug.  And your idea, you could not count on the contact           resistance being uniform from spark plug to spark plug or from           day to day.

         

         

         Robert - Opinions do           not count here, only the facts count. The ring type           thermocouple most definitely does affect the heat range of the           spark plug.  It can take the heat range UP two or more range           levels.  Here is a simple check you can perform from the           comfort of your computer chair:

         Ask around of owners           of the SAME engine and SAME installation - What their CHT's           are.  Also ask which thermocouple they are using...  Ring type           or Screwed in type.  There is also a bayonet type but not to           many owners use it.  You will find Ring thermocouples will           always be a Higher Temp.

         

         

         Hope this information           enlightens those gray areas.

                    

         

         Barry

         

         

                    Test Engineer for the past 20 years, developing and doing           exactly this type of testing.

         

       

     [b]

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Mon Aug 19, 2013 1:22 am Post subject: Jabiru 3300 CHT Disparity

Bob P:

 There is more to it than just "burn off oil and carbon".  The spark plug heat range also affects the internal gas burning temperatures within the cylinder.  If you don't believe me get a set of plugs that are one or two heat ranges higher and watch your CHT - EGT and even your oil temps. They WILL get higher.  Another way to think about it is:  Why have different heat range spark plugs if heat range was such a constant?

 



NO TWO SAME ENGINES WILL READ THE SAME TEMPS!  This has been discussed and notes over many, many years and millions of hours flown.  The idea of the thermocouple dangling in the breeze makes as much a difference as spitting in the ocean does to the water level.  Forget About IT!  

 



What does make a difference - AND  - As I said, do the test, ask those on the group, ask other groups - - - Is Ring Thermocouple Temps Vs Threaded Thermocouple Temps.  You will see the Ring temps are Higher.  The REAL question you should be asking is:  ARE the CHT's REALLY HIGHER?  NO!  They are NOT!  

 The next question you should be asking yourself is: WHY are they NOT higher?

Well, let me explain it this way:- 

 If you take 10 people in good health...

Take the Same - Standard (not medical) Thermometer ...

 And take their body temperature at the following locations: Mouth, Armpit, Forehead, Rectal and bottom of the Foot...  Would you expect to see the same temperatures?

 NO!  Of course not!  

YET!  All 10 of these people are in good health!!!

So what is the difference?

 The difference is Location, Location, Location.  We all know Standard 'internal' body temperature is 98.6F.  It is just where and how and with what, you take the reading.

 AND!  Each person in good health has their own range of 'good health'.  

 The bottom line is:  No matter how much you try to reinvent the wheel a wheel is still going to be round.  Ring thermocouples will read higher, they will raise the heat range of the spark plug and ALL thermocouples and temperature readings are a reference point.

 FLYING is NOT an EXACT SCIENCE! Only because Pilots are involved. There are way too many variables.  

 



Oh, the Rings are NOT made of aluminum... If they are they should NOT be.  Aluminum is a POOR conductor of heat. Yes, a poor conductor.  I know I have you scratching your head with that statement. They should be made from copper or brass.  Other than heat reasons, malleability. Thermodynamics 101.

 



 Barry

PS:

 Capitals do not mean I'm yelling.  That is a dumb Internet definition of capitals, I am just bring attention to particular words.  I do this since you can not hear the inflection in my voice.

 

On Sun, Aug 18, 2013 at 8:01 AM, BobP <matronics(at)panth.co.uk (matronics(at)panth.co.uk)> wrote:

 [quote]--> JabiruEngine-List message posted by: "BobP" <matronics(at)panth.co.uk (matronics(at)panth.co.uk)>

  

 Hi Barry,

 I can't see that a plug  temperature range will affect the (true) cylinder head temperature. My understanding is that the terms hotter and colder plugs refer to the  thermal coupling between a plug's centre electrode/ceramic nose, and the main plug body. The plug  temperature range needs to be selected so the plug nose gets hot enough to burn off oil and carbon that might otherwise kill the spark, and cool enough not to act as a glow plug resulting in pre-ignition.

  

 All the plug type thermocouples I have seen consist of an aluminium washer that is squashed between head and cylinder, the washer has a tag extending from it that "waves in the breeze". The thermocouple junction is crimped into that tag and so reads the tag temperature rather than the cylinder head temperature, or even the temperature at the plug/head interface.

  

 --------

 BobP

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=406955#406955

 

 

 

 

 

 

 

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 [b]

matronics(at)rtist.nl
Guest

Posted: Mon Aug 19, 2013 2:11 am Post subject: Jabiru 3300 CHT Disparity

Barry,

     

     BobP is right, spark plug heat ranges have very little to do with     CHT. The different ranges are there to ensure the spark plug     continues to operate well within a particular engine. It has zero     effect on the actual operation of the engine. A hot plug produces     exactly the same sparks as a cold plug for the same amount of     electrical energy.

     

     A picture tells more than 1000 words. Here's a video that explains     very well what spark plug heat ranges mean:     http://www.youtube.com/watch?v=LbR75wq9nmM

     

     Rob

     P.S. Capitals *do* mean yelling on the Internet. Just like EULA's     and insurance policies they cause people to ignore the entire text.     There are better ways to emphasize. 

     

     On 8/19/2013 11:21 AM, FLYaDIVE wrote:

     

     [quote]                Bob P:

         

         

                    There is more to it than just "burn off oil and carbon".  The           spark plug heat range also affects the internal gas burning           temperatures within the cylinder.  If you don't believe me get           a set of plugs that are one or two heat ranges higher and           watch your CHT - EGT and even your oil temps. They WILL get           higher.  Another way to think about it is:  Why have different           heat range spark plugs if heat range was such a constant?

         

         

         NO TWO SAME ENGINES           WILL READ THE SAME TEMPS!  This has been discussed and notes           over many, many years and millions of hours flown.  The idea           of the thermocouple dangling in the breeze makes as much a           difference as spitting in the ocean does to the water level.            Forget About IT!  

         

         

         What does make a           difference - AND  - As I said, do the test, ask those on the           group, ask other groups - - - Is Ring Thermocouple Temps Vs           Threaded Thermocouple Temps.  You will see the Ring temps are           Higher.  The REAL question you should be asking is:  ARE the           CHT's REALLY HIGHER?  NO!  They are NOT!  

         The next question you           should be asking yourself is: WHY are they NOT higher?

         Well, let me explain           it this way:- 

           If you take 10 people in good health...

         Take the Same -           Standard (not medical) Thermometer ...

                    And take their body temperature at the following locations:           Mouth, Armpit, Forehead, Rectal and bottom of the Foot...            Would you expect to see the same temperatures?

                    NO!  Of course not!  

         YET!  All 10 of these           people are in good health!!!

         So what is the           difference?

         The difference is           Location, Location, Location.  We all know Standard 'internal'           body temperature is 98.6F.  It is just where and how and with           what, you take the reading.

         AND!  Each person in           good health has their own range of 'good health'.  

                    The bottom line is:  No matter how much you try to reinvent           the wheel a wheel is still going to be round.  Ring           thermocouples will read higher, they will raise the heat range           of the spark plug and ALL thermocouples and temperature           readings are a reference point.

         FLYING is NOT an           EXACT SCIENCE! Only because Pilots are involved. There are way           too many variables.  

                    

         

         Oh, the Rings are NOT           made of aluminum... If they are they should NOT be.  Aluminum           is a POOR conductor of heat. Yes, a poor conductor.  I know I           have you scratching your head with that statement. They should           be made from copper or brass.  Other than heat reasons,           malleability. Thermodynamics 101.

         

         

         

         

                    Barry

         

         

         PS:

                    Capitals do not mean I'm yelling.  That is a dumb Internet           definition of capitals, I am just bring attention to           particular words.  I do this since you can not hear the           inflection in my voice.

                    

         

         

         

       

       

         

         On Sun, Aug 18, 2013 at 8:01 AM, BobP <matronics(at)panth.co.uk (matronics(at)panth.co.uk)>           wrote:

            	  Quote: 	
		  -->             JabiruEngine-List message posted by: "BobP" <matronics(at)panth.co.uk (matronics(at)panth.co.uk)>

             

             Hi Barry,

             I can't see that a plug  temperature range will affect the             (true) cylinder head temperature. My understanding is that             the terms hotter and colder plugs refer to the  thermal             coupling between a plug's centre electrode/ceramic nose, and             the main plug body. The plug  temperature range needs to be             selected so the plug nose gets hot enough to burn off oil             and carbon that might otherwise kill the spark, and cool             enough not to act as a glow plug resulting in pre-ignition.

             

             All the plug type thermocouples I have seen consist of an             aluminium washer that is squashed between head and cylinder,             the washer has a tag extending from it that "waves in the             breeze". The thermocouple junction is crimped into that tag             and so reads the tag temperature rather than the cylinder             head temperature, or even the temperature at the plug/head             interface.

             

             --------

             BobP

             

             

             

             

             Read this topic online here:

             

             http://forums.matronics.com/viewtopic.php?p=406955#406955

             

             

             

             

             

             

             

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     [b]

BobP

Joined: 16 Aug 2013
Posts: 10
Location: UK

Posted: Mon Aug 19, 2013 2:00 pm Post subject: Re: Jabiru 3300 CHT Disparity

Hi Barry

I'm replying to your email in serious mode just in case you were being serious and not "pulling my leg"



Plug manufacturer NGK make no reference to combustion temperatures being affected by heat range - except in the abnormal condition of incorrect plug selection causing (dangerous) pre-ignition (see link below)



http://www.ngksparkplugs.com/tech_support/spark_plugs/p2.asp

I thought my comments relating to a "spark plug ring and tag" CHT sensor giving misleading CHT readings  as compared to a sensor buried in the head self-evident. This assertion has been confirmed independently by the observations  of a number of Jabiru pilots who have (before fully switching to "buried" sensors) run their engines with a plug ring and a buried CHT sensor AT THE SAME TIME and on the same cylinder and noted the temperature difference. They invariably report that the buried sensor reads hotter â€“ typically by 20 degrees centigrade (not cooler as you suggest). 



The reason for my original posting in this thread was because it had been suggested (by others) that "in head" sensors gave inconsistent CHT measurements. The point I was making was that perhaps (I believe)  the â€œreferenceâ€ plug ring and tag CHT sensor is giving inconsistent measurements (because of heat loss from the tag) and the buried sensor is giving accurate (or at least more accurate) measurements. 



You made the assertion  â€œThe idea of the thermocouple dangling in the breeze makes as much a difference as spitting in the ocean does to the water level. Forget About IT!â€

Barry,  the only thing that stops the cylinder heads going into melt down is that same cooling breeze!!!!   Clearly the airflow has much more effect than you suggested (and the baffle location maximizes that airflow exactly over the sensor tag!).  



If you still think I'm wrong could you please back up any further  advice by conducting the following tests on your aircraft and reporting back:



1)	Totally block off your CHT cooling air ducts (shouldnâ€™t be a problem if the "cooling air makes as much a difference as spitting in the ocean does to the water level").



2)	If you think 1) might lead to some overheating  fit some super cool spark plugs (you claim CHT, EGT and even oil temperature move in sympathy with the plug temperature range).



3)	Do a WOT  power check and advise how many seconds before the engine seizes up (don't even think about taking off).



With regard your observation that "aluminium  is a bad conductor of heat" you might wish to note that  aluminium(pure) conducts heat twice as well as brass (admiralty) - a material you claim to be a good conductor of heat! 



I do agree that copper would be a better material for ring tabs  - subject to age/work hardening not being  an issue.

_________________
BobP

jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Thu Aug 29, 2013 3:44 pm Post subject: Re: Jabiru 3300 CHT Disparity

Jabiru now have an alternate CHT thermocouple location



http://www.jabiru.net.au/images/AVDALSR085-1_CHT_sensor_mod.pdf

I dont use either method because they only read the temperature of the lug, in free air, or the sparkplug body temp..



putting the thermocouple junction -with or without the existing crimped area of the lug - into the hole and sealing over with RTV is a more accurate way.



just make allowance for the higher readings obtained, when referring to CHT operating limits specified by Jabiru.

_________________
Ralph - CH701 / 2200a

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