Maximum safe cylinder head temperature (CHT)

[HeadSmess]

ok, jaguar, if thats your argument about ceramic/porcelain insulators....

hey, my coffee mug gets pretty hot on the outside, and the handle, but its "ceramic". its porcelain. its the same stuff they make plugs from.

using your logic, thats just the heat of my hand, unable to be channeled through the handle. from your statement, thats whats happening. my hand provides heat that cant escape, handle gets hotter and hotter.

does anyone else see the slight FLAW in this logic?


the porcelain conducts heat. it does have a bad thermal coefficient, but it will conduct heat. all porcelain/ceramic will. some, with more or less efficiency, than others. it is not as thermally conductive as metal. it is completely nonconductive to electricity, unlike metal. as it needs to be insulated from hi voltages, whilst conducting heat, porcelain is ideal.


yes, they coat pistons to reduce heat transfer. one, aluminum is a great conductor. two ceramic is much harder than aluminium.

the thermal conductivity of any joint is determined by contact. so gas in contact with aluminium will cool down faster than gas in contact with a ceramic coated piston/cylinder. plus it doesnt tend to score so easily, and lasts longer. a layer of carbon will reduce thermal transfer to much the same as the ceramic has when new.... its the increased service life that wins out.

a completely different ceramic to the sparkplug.


oh, wait! they machine tungsten carbide with, wait for it...ceramic inserts. this ceramic does not conduct heat very well and suffers failure if it is not supplied with adequate coolant in use. and fails instantly when used on mild steel regardless. a sandblaster nozzle is invariably ceramic these days. that one even conducts electricity so they dont build up huge static charges. just like the rubber hoses are now conductive. but rubbers an insulator? so is air, but once the pressure is reduced it becomes conductive? how else do neon, xenon and other discharge tubes work?

superconductors are ceramic. they only start conducting electricity when the things are dipped in liquid nitrogen or something. at which point the elctricity being conducted isnt exactly electricity anymore there are a million and one varieties of ceramics in use today, not counting the ones being developed.

what the ceramic consists of is the critical factor. white natural sieved bentonite clay, no grain over 0.01mm? or lets say its man made boron nitride? or maybe carborundum? wait, diamond is technically a "ceramic". artificial ones are definitely a ceramic. subject a diamond to the heat inside a cylinder and the thing will revert to basic carbon, catch on fire, and burn. maybe its titanium nitride?



maybe what you need, jaggy baby, is a dictionary?


A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling.[1] Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass). Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the noncrystalline glasses, a distinction followed here.


every source of information on spark plugs says EXACTLY what KCvale has said. narrow tips enhance spark, more volume of ceramic makes for a larger conductive path to transfer heat through so cooler plugs, a plug has to be hot enough to clean but not melt or it will foul up,


and then, every logical examination of a plug just corroborates this concept that is not theory, it is cold hard fact!


except the platinum bit.... i wont buy platinum plugs. that has nothing to do with the properties of platinum/iridium. rather, the added cost

 

[Deleted member 12676]

The thermal conductivity of aluminum is 205, stainless steel 16, porcelain 1.5, rubber .13 source: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Champion Spark Plugs says:

The spark plug design determines its ability to remove heat from the combustion chamber. The primary method used to do this is by altering the internal length of the core nose. In addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges.

*When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, keeping the firing tip cooler.

*A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.
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What heat is it talking about? The heat of the "firing tip", not of the whole combustion area. find any spark plug manufacturers info that says the heat range of the plug affects head temperature. I dare you.
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NGK says:

When the spark plug firing end (tip) temperature exceeds 800°C, pre-ignition originating from the overheated insulator ceramic can occur. Pre-ignition will dramatically raise the cylinder temperature and pressure and can cause serious and expensive engine damage.
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This is the only exception, pre-ignition.

 

[Deleted member 12676]

Here are some good quotes.
from http://matchlessclueless.com/mechanical/ignition/spark-plug-temperature/
"Firstly, lets get rid of a common misunderstanding about spark plug heat ranges and say what this does NOT mean. The spark plug heat range does NOT control the temperature at which the engine runs; fitting a 'cold' plug will not make an overheating engine run colder, nor will fitting a 'hot' plug make a cool-running engine run warmer.
Engine temperature is governed by factors such as the timing and fuel-air mixture settings, running conditions, lubrication quality, compression and cooling system design to name a few, but certainly NOT by the spark plugs heat range. The spark plug gets its heat from the burning fuel in the combustion chamber, and not the other way round!"

from https://en.wikipedia.org/wiki/Spark_plug
"The heat range of a spark plug has only a minute effect on combustion chamber and overall engine temperature. A cold plug will not materially cool down an engine's running temperature. (Too hot of a plug may, however, indirectly lead to a runaway pre-ignition condition that can increase engine temperature.) Rather, the main effect of a "hot" or "cold" plug is to affect the temperature of the tip of the spark plug."

Here's a thread about the same topic: http://www.thumpertalk.com/topic/963222-spark-plug-heat-range-question-im-confused/

 

[KCvale]

I am not sure what part of the crystal clear diagram of how a spark plug dissipates heat to the head or holds it in but heavens sake this in NGK's diagram and explanation.

Please, for all of us Jag look at that diagram again because the heat it transferred to the head with the WHITE part in contact with metal threads (see center parts with the big red arrows showing heat dissipation), the pink tip parts is what keeps heat in and as mentioned.

Then again, maybe you should just contact one of the worlds biggest spark plug makers and tell them they are wrong?

 

[Deleted member 12676]

People have taken the wording of that NGK statement and made it mean something it didn't mean to say. Yes, the plug tip does remove some of the combustion temperature, a small insignificant amount which NGK did not specify. It is like saying that getting a cup of water from a lake removes lake water. Yes it does remove lake water, but an insignificant amount. Let me do the math for you here:

The equivalent circle diameter of my slant 55cc head is around 50mm. That gives a square millimeter area of 1963.
The circle diameter of the plug tip is 2mm and its height is 2mm which gives 25 square millimeters of exposed surface area.
25 divided by 1963 is .013 which means the percentage of head area the plug tip is exposed to combustion is 1.3%.
Since the thermal conductivity of ceramic is only .7% that of aluminum I won't include it in the calculation.

So 1.3% of the exposed area (not even counting the piston top) is plug tip. Most tip cores are copper with similar thermal conductivity as aluminum and so there is no way 1.3% of the surface can magically wisp away more than its percentage of heat touching the head area.

 

[Deleted member 12676]

OK I want to know how much heat increase (measured with a spark plug washer temperature probe with readout at the gauge on the handlebars) you think there will be from a cool NGK B7HS to a hot BR4HS. Wide open throttle run till the temperature stops rising or the engine blows up.
HS, KC, Grim, anyone else?

1.3% of 425 is 5.5 degrees farenheit by the way.

 

[Deleted member 12676]

C'mon guys. Put your guesses where your mouth is. Tell me how much hotter you think it will run with the hotter plug.
I don't doubt it will be a little bit hotter. How much is the question.
Place your bets and I will run the test.

 

[Deleted member 12676]

I promise not to criticize or make fun of any guesses that are far off. I just want to know what the general feeling is on the possible temperature change. We are all in this to learn.

Also, even if the reading is higher (even much higher than what I would guess) consider this: the thermocouple is right there at the base of the plug, so of course it will be more a reading of the plug heat than the whole head. Only if I can figure out how to connect the thermocouple away from the plug will it give a true reading of head temperature change. Or figure out a way to get a quick infrared gauge readout before it begins to cool. I do have an infrared heat gauge.

 

[HeadSmess]

i got yelled at oh this is fun

to quote, if i may...

"Default CHT increase

Put your guess where your mouth is. Tell me how much hotter you think it will run with the hotter plug."


this topic really gets up your goat, dont it?


well, ok. the hotter plug dissipates less heat or holds more, so theoretically the head would be slightly cooler. not hotter at all. of course, the couple location will play a large part in the test. maybe you need more sensors for more reliable results, like in labs?


so, given that all over variables are listed, ie airpressure, air speed and wind direction at all times during the test, comprehensive listing of temp versus rpm/throttle settings. theres a few more as well. humidity. exact altitude. your weight, bikes weight, fuel temp and density...

i want all these results for both tests, using say, a 5 and a 7.


i note you list a b7hs and br4hs.... well, they have to be identical except the heat range, so please get two plugs that are either resistored or not! be scientific and pedantic.

ngk,champion, bosch, they all claim that the heat ranges are mere "comparisons" and have no corelation between brands, so they have to be the same brand plugs, obviously.

the same companies state theres also no clear division between each heat range, being an "approximate".



so, im going to make a rough guess that the 4 will run, say... 15F COOLER than with a 7.


thats my guess im actually interested to see the results.

both jennings and bell and most high end two stroke tuners recomend taking "plug chops" literally and chop the threads off to get a good idea of whats actually happening down inside that plug chamber. reading the ENTIRE PLUG.


small "FLAW" in your mathematics. you measure the tip, the centre electrode from the sound of your measurements. the entire porcelain is in contact with the combustion gasses. its invariably tapered and actually has quite a large surface area. its also internally in contact with the centre electrode.

and theres the steel section itself. thats also part of the spark plug. it didnt just suddenly exist in a vacuum.

so these areas must be included in the calculations.

and the thermal transfer through the threads.

why else would people actually cut threads off and look at the base of the insulator?


"mensuration"- measuring volumes and areas of solids.

for a cone(to a point)
1/3pi r^2 h = volume
area= pi r l where l is the length of side (or hypotenuse if projected as a triangle)

frustrum of a cone (a section)

volume = pi*h/3(R^2 + r^2 + R*r)
area=pi*l(R+r)

l is side length of cone.
h is height.
R is larger radius.
r is smaller radius.


(i will also point out another flaw. MY OWN!

recently i said something about keeping hemispherical chambers and 50% squish areas. did all the maths and proved that my vague feeling wrong, no, the CR would remain constant with varying cylinder diameters.
well i was wrong! my guts knew better!

i used 4/3 pi r^2, not 4/3pir^3.
redone. the smaller a cylinder, the higher the CR when squish area is kept at 50% with perfect hemi chambers. so you simply cant get a good squish band in small engines without CR going out the roof and on a 48 it ended up being around 9.5:1 with 0.6mmm clearance)



apparently ive been offensive, yet i havent out right called anyone anything.... im waiting for my infraction notice still

 

[Deleted member 12676]

Interesting. you predict a colder head with a hotter plug.
yeah, I just read somewhere that the porcelain they use is fairly thermally conductive so that greatly alters my prediction. There should be more heat transfered to the head than I initially thought.
I rummaged thru my stuff and found two NGKs just one heat range apart. I will test them tomorrow.
I was able to put the thermocouple under one of the forward head studs with a fiber washer somewhat insulating it from the stud/washer/nut.