Today's Message Index:
----------------------
1. 12:17 AM - Re: Jabiru 3300 CHT Disparity (Rob Turk)
2. 02:22 AM - Re: Re: Jabiru 3300 CHT Disparity (FLYaDIVE)
3. 03:11 AM - Re: Jabiru 3300 CHT Disparity (Rob Turk)
4. 03:02 PM - Re: Jabiru 3300 CHT Disparity (BobP)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: 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:
> 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.
>
Message 2
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: 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@panth.co.uk> wrote:
>
> 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
>
>
Message 3
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: 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:
> 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@panth.co.uk
> <mailto:matronics@panth.co.uk>> wrote:
>
> <matronics@panth.co.uk <mailto:matronics@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
>
>
> ==========
> -List"
> target="_blank">http://www.matronics.com/Navigator?JabiruEngine-List
> ==========
> http://forums.matronics.com
> ==========
> le, List Admin.
> ="_blank">http://www.matronics.com/contribution
> ==========
>
>
> *
>
>
> *
Message 4
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| 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 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 reference 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 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!
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 (shouldnt 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
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=407069#407069
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|