Glowplugs?

Here is an interesting site I found recently:

http://www.fubarhill.com/GLOW.htm

It seems to say that the ONLY decider on which heat plug to use is the methanol percentage!

Where does this leave the cool engine/low nitro=hot plug,hot engine/high nitro=cool plug rule of thumb?

According to the calculations on this site a Fox 35 running 29% oil/5% nitro (leaving 66% methanol) should use a cool plug!

There is also a matter of compression, whether we are running rich or lean, etc. I don't think I would suddenly throw away all of my hot glow plugs and go out and get cold ones simply because of what someone wrote on the Internet.

Will a hot plug work?
Will a cold plug work just as well or even better?
Did someone make a statement not based on all facts?
Is this guy a stunt flyer?
When was the last time he put his theories to the text?

In other words, what are the "experts" using? Do
you think these guys might have tried a cold plug sometime in his life? You betcha. Now, what are the "experts" using?

The glowplug chart by Cooper Fuels is useful. The rest is poopy. Steve

the basic chemistry described in this page seems correct. But, as Len says, there is more to the plug choice than just heat range. High compression, high performance, lean setting needs a sturdier wire. The heat loss to the plug body also affects things. Why else would a perfectly good plug with the coil pushed to one side sometimes run so bad? And then run OK when you just shove the coil back to the center.

What the hell is SPORT HOT?

Scott(451)Riese

Yes I thought that the tables were the best bit too!

It's a little erroneous to say that with 29% oil (and 5% nitro) the Fox is only running with 66% methanol because the needle is used to regulate the fuel flow which is only the methanol/nitro components. With a blend like this the FUEL is composed of 66 parts methanol and 5 parts nitro so the methanol is 93% (66/71) of the total. If the oil was reduced to 20% then the FUEL part would then be 75 parts methanol and 5 parts nitro or 93.75% methanol (75/80). In both blends the oil is just along for the ride but it's interesting to note that with the greater % of oil the needle has to be opened further which gives an even greater flow of oil through the engine than the higher % would indicate.

The best site I've found for plugs is at http://www.modeltechnics.com/glowplug/glowplug.html and in particular their next page at http://www.modeltechnics.com/glowplug/firepower.html where their tests show the affects of different heat ranges and in a couple of cases different compressions and fuels.

To answer Len's queries, it seems the original link was written by BRISTUNT (correct me if I'm wrong Brian) but slanted more towards the beginner RC flyer and not as an indepth treatise for the top rank stunt flyer

I wonder what Bill Netzeband has to say about this?
Here's the updated version of his original article:
http://www.iroquois.free-online.co.uk/gloplug.htm

Here's a quote from the web page:

>Next you need to understand that a certain surface area of platinum
>is required to generate a sufficient catalytic reaction to keep the
>internal combustion process going. Also it is necessary to allow
>extra surface area for the reaction to be great enough when it
>diminishes with the available methanol dropping as in the case at
>motor idle. Simply put, cold plugs are manufactured using a thicker
>wire to give greater surface area to facilitate a greater reaction
>and thus the required catalytic reaction where less methanol is
>present in the fuel mixture.

>So! More nitro means less methanol which in turn means a greater
>surface area to platinum will be required to generate a sufficient
>catalytic reaction. Suddenly it all makes sense! To work out which
>temperature plug to use, you need to know how much methanol is in
>your fuel, not how much nitro or oil.

This doesn't make sense to me. A hotter plug is one which causes the fuel to ignite earlier. It's easy to tell a hot plug from a colder one when you're turning over a Combat engine by hand - a hot plug will "bump" a lot harder. Fuel pre-ignition leads to loss in power, overheating, and blown plugs.

If what he says is true, then the "colder" plug would cause ignition before a hot plug will, and will, the opposite of the observed behavior.

Like a lot of "common wisdom" in the hobby, I doubt what was presented on the website is anything more than a guess, based on some experience, rather than anything based on actual controlled experiments, so it should be taken with a grain of salt.

Does anyone really use plated elements in their glow plugs? I've always thought all plugs were solid platinum/rhodium/irridium/palladium etc. alloy. Note that he doesn't identify which ones are plated in the tables, either.

This is from Bill Netzeband's article.

So where are these "Belleville washers" that Dirty Dan says make up the seal? Come to think about it, how can you make a seal out of metal, which conducts electricity?

>Here's a quote from the web page:
>
>>Next you need to understand that a certain surface area of platinum
>>is required to generate a sufficient catalytic reaction to keep the
>>internal combustion process going. Also it is necessary to allow
>>extra surface area for the reaction to be great enough when it
>>diminishes with the available methanol dropping as in the case at
>>motor idle. Simply put, cold plugs are manufactured using a thicker
>>wire to give greater surface area to facilitate a greater reaction
>>and thus the required catalytic reaction where less methanol is
>>present in the fuel mixture.
>
>>So! More nitro means less methanol which in turn means a greater
>>surface area to platinum will be required to generate a sufficient
>>catalytic reaction. Suddenly it all makes sense! To work out which
>>temperature plug to use, you need to know how much methanol is in
>>your fuel, not how much nitro or oil.
>
>This doesn't make sense to me. A hotter plug is one which
>causes the fuel to ignite earlier.

I think where our friend has missed the boat on his reverse analysis (always take anything posted on the Internet with a grain of salt) is that the heat range of a plug, if using the same alloy, is determined by the diameter of the wire. A .06 wire will be hotter than a .12 wire. "Yet the .12 wire has twice the surface area and will thus produce a better combustion", he says. "Ah, but we need to look at one thing more. That coil not only has to react with the methanol, it has to get hot in doing so," says I. It is not the reaction, it is the heat.

When we start our engines, we are not using the catalytic reaction of the methanol with the coil wire at all. We heat it with our little batteries. It is only as the engine begins to fire that there is sufficient catalytic action to maintain this heat. The combustion is not taking place because the methanol reacts with the platinum alloy. The combustion takes place because the methanol ignites because of the hot wire which maintains its heat through the catalytic reaction. So the surface area is only a matter of keeping the wire hot, not a matter of igniting the fuel.

If you have two wires, a .06 (hot) and a .12 (cold) the hot plug will generally work better in our applications (like the Fox 35, with its 29% oil, etc.) "But it only has half the surface area!" he cries. "Yes, but it is hotter," I reply, "and heat is what fires the fuel."

The simple answwer is this: The amount of surface area is in direct proportion to the circumferance of the wire. The amount of wire to be heated is in direct proportion to the cross section area of the wire.
Circumferance is calculated as Pi times Diamater.
Area is calculated as Pi times Radius Squared.
Since we are multiplying both by Pi, we can eliminate it from our calculations in determining ratios.

If one wire has a diamaeter of 6 (simpler than saying .06) and one has a diameter of 12 then:

The "6" wire has diameter of 6 and area of 3 x 3 = 9.

The "12" wire has a dimater of 12 and area of 6 x 6 = 36

So the larger wire has twice the area to react with the methanol, but four times the amount of wire to heat. That is why it produces a colder plug, and it is the heat of the wire, not the catalytic reaction that fires the mixture.

End result: The above cited article is all wet. Sorry, but its author just used poor logic.

...and I thought diesel engines were complicated with many variables to get "right"...

So where are these "Belleville washers" that Dirty Dan says make up the seal? Come to think about it, how can you make a seal out of metal, which conducts electricity?

One might try looking at plugs or drawings of plugs which use cited sealing technology instead of a drawing which shows one of numerous other ways to form seal in glow plugs.

Examples? The way you put it the last time, it sounded like you were pretty sure that this was common practice, and not used on just a few plugs.

In the text of the article Bill mentions variations - some plugs omit the top sealing layer, some have the crimp at the top, some don't. So it sounds like the drawing is representative of most, if not all, the plugs he's sectioned, and he's sectioned quite a few. He never mentions washers. I can't believe that plug construction has changed much since the 1950s, either, other than with one or two minor exceptions (Glo-Bees). One of the plugs mentioned in the tables is (or was, until recently) being made - the K&B 1L. The Testors plug may well be, too, as one of the current McCoy line.

Interestingly, the Swanson (Fireball) plugs are missing from the table. Al Kelly mentioned that there was something slightly different about the way the seal on this plug was constructed that caused the seal to eventually blow, and I wish I understood or remember what he was telling me at the time.

I'm still wondering about this "plated element" myth. I've seen it on a couple of web sites. Usually in the context of "our plugs use solid platinum alloy, and are superior to those plugs which use plated elements." What brands use plated elements?

>I'm still wondering about this "plated element" myth. I've
>seen it on a couple of web sites. Usually in the context of
>"our plugs use solid platinum alloy, and are superior to
>those plugs which use plated elements." What brands use
>plated elements?

I am with you on that one. I haven't heard of any, either. It may have to do with the fact that some get coated and pitted looking from the combustion elements (and can fail even though they glow under bettery fire). Under magnification it may look like plating has fallen off, but that is not the case.

Iskandar,

Other than the four-stroke plugs I have been using, it is my belief that every single plug--ooops, keep coming up with exceptions, this time it is the O.S. #8--I have used for many years was assembled by the company founded by Ohlson (sic?). This would cover plugs marketed by K&B, Thunder Tiger, Sonictronics, McCoy and Nelson.

While surely the patent has expired, it is my understanding the principal of this company patented the use of Belleville washers to form the seal in glow plugs and this operation is performed using one heck of a lot more force than would seem reasonable for a small glow plug. It seems to work pretty well; I can recall only a very few instances where any of above named plugs lost a seal. And of course it doesn't take very long to see the results.

The patent, along with a superior method of actually making the seal, resulted in quite a lot of job-shop work for others interested in marketing plugs. This leadership position continues to this day, current or expired patent seems to make little difference.

As I have mentioned before, if one wishes to market his own line of glow plugs, these guys will want to know the type of finish on the body, the style of pin. You buy the wire on the commodities market, send it to them and they assemble the plugs. Do be prepared for a substantial minimum buy.

Yes, I've heard that most of the plugs on the market are made by Ohlsson. That includes K&B and McCoys, but doesn't include Fireball (Swanson) and Fox (and Cox). So that would mean that most (if not all) of the plugs that Wild Bill sectioned were made by Ohlsson. As one can see from the drawing, no washers are involved. If they were ever used, they weren't being used any more when the article was written.

Might be interesting to see if I can find the original patent online. Yes, it surely expired a long time ago, so it's likely everyone's using more or less the same construction in their plugs today. Unless I see evidence to the contrary, however, I don't think that construction method involves washers.

>So where are these "Belleville washers" that Dirty Dan says
>make up the seal? Come to think about it, how can you make a
>seal out of metal, which conducts electricity?
>
>One might try looking at plugs or drawings of plugs which
>use cited sealing technology instead of a drawing which
>shows one of numerous other ways to form seal in glow plugs.
>
>Dan

I have a small jar of "Belleville washers" that I keep in my Flight Box right next to the "Chicken Hopper" fuel tanks. (smiles)

A few weeks ago I increase my caster oil from 20% to 25% now my engines won't run. They are hard to start, won't run very good without the glow plug starter, once the airplane is in the air the engines starts running like its too rich then die about the 3 lap.
I have tried several glow plugs and some are worse than others. The original fuel that I bought was 15% nitro, 16% oil, 69% methanol. Now my fuel is 13% nitro, 25% oil, 62% methanol after adding about 9.5 ounces of caster oil. My glow plugs are in the medium to cold range.

This information seems backwards to me. The flash point of 80% methanol will be less than 65% methanol at the same cylinder pressure and amdient temperature therefor a hotter glow plug would be needed for less methanol. According to the informatio below my glow plugs should all be too hot which would make my engine all fire too soon.

Here is the information from the link you provided.

As a rough rule of thumb;

80% methanol or above, use a hot plug.

70%-75% use a medium plug.

60%-75% use a cold plug.

65% or less use a very cold plug.

Gary, that link is just absolutly wrong. You need a hot plug, and when you run the engine rich with more oil you need a hotter plug. For our purposes, hot plug! Period!

And, for a reference point, see my comment concerning the cross section of the wire on another response in this thread. The fuel does not fire by catalytic action. The fuel fires by the heat of the element and the element gets hot by catalytic action. A thinner wire will get hotter, even though there is less surface area, because there is less cross section area. And the hotter plug element enables the rich mixture to fire even though the extra fuel/oil tends to cool things off.

If you have a problem as you state above, try a hotter plug.

Len, you might want to be a bit less dogmatic that the fuel in a glow engine doesn't ignite from catalytic action, but heat. The two go together, but not much heat is really needed. For example, how can an engine start without a battery? It happens a lot, and 6:1 compression in a Fox Stunt is not enough to significantly heat the charge. But if the mixture is right it is enough, along with the catalysis of the plug, to make the engine start.

For another example, fire up a Cox 049 at night sometime. As you adjust the needle valve you will see the glow in the exhaust change. Lean it out, it gets brighter, richen it up and it gets duller. usually in a fat 4 cycle the light from the cylinder goes out, so the plug has to be a lot colder than when running full bore.

Catalysis from the element is a key factor in making a glow plug work. Without it you would need it keep the battery applied at all times, and it might not work well even then. Catalytic action is why platinum and similar metals are used. Otherwise, Inconel or some other high strength, high temp alloy, which is a lot cheaper, would be used. Catalytic action also explains why sometimes plugs just quit working. The element in what looks to be a perfectly good plug can get poisioned or covered up by a number of contaminants(sulfur, aluminum, maybe silicons from Armorall) and just quit working.

>Len, you might want to be a bit less dogmatic that the fuel
>in a glow engine doesn't ignite from catalytic action, but
>heat. The two go together, but not much heat is really
>needed. For example, how can an engine start without a
>battery? It happens a lot, and 6:1 compression in a Fox
>Stunt is not enough to significantly heat the charge. But
>if the mixture is right it is enough, along with the
>catalysis of the plug, to make the engine start.

Maybe I can answer with one word: "Diesel".
How does a Diesel engine work without catalytic action? There is enough heat generated from compression to...well, you get the idea. If it were simply catalytic action that caused the engine to fire, we wouldn't need a batter at all, would we? And diesel engines, even big rigs have glow plugs to start them on certain occasions. No alcohol here, however, or fancy catalytic action. Just plain old heat. Of course, the glow plug is not the prime source of running once the engine gets up to heat.

>For another example, fire up a Cox 049 at night sometime.
>As you adjust the needle valve you will see the glow in the
>exhaust change. Lean it out, it gets brighter, richen it up
>and it gets duller. usually in a fat 4 cycle the light from
>the cylinder goes out, so the plug has to be a lot colder
>than when running full bore.

Let me offer two possible scenarios (might be more out there, but lets start with these two):
First, if it is catalytic action and more methanol means more catalytic action, then the richer mixture should cause a hotter glow. But it doesn't.

Perhaps, then, the richer mixture is cooling off the element which is why our stunt engines normally need a hot plug. Still, the catalytic action helps to keep the element hot, the heat from the plug keeps the engine running. We all know what happens when our batteries get low. The engine becomes more difficult (if not impossible) to start. Same thing happens when the plug cools too much. Just like turning a switch. "Ignition off."

>Catalysis from the element is a key factor in making a glow
>plug work. Without it you would need it keep the battery
>applied at all times, and it might not work well even then.
>Catalytic action is why platinum and similar metals are
>used. Otherwise, Inconel or some other high strength, high
>temp alloy, which is a lot cheaper, would be used.
>Catalytic action also explains why sometimes plugs just quit
>working. The element in what looks to be a perfectly good
>plug can get poisioned or covered up by a number of
>contaminants(sulfur, aluminum, maybe silicons from Armorall)
>and just quit working.

No argument here. "Catalysis from the element is a key factor in making a glow plug glow. Without it you would need to keep the battery applied at all times" (I cnaged one word here, but only for emphasis. It is the catalytic action that makes the plug do its work. It is also why platinum is used, etc. and why a plug, as you put it, can get "poisoned". You can start the engine with a battery, but have difficulty in keeping it running when the battery is removed. If you remove the battery after starting the engine and there is a significant drop in rpm, either the plug is bad or the plug is too cold (which means the plug is bad for the engine.)

But hey, we are just theorists here. Sometimes it is fun to mention all this nitty gritty stuff, but fact is, all we really need to be concerned about is what works and what doesn't work. I don't always need to know why, but I do need to know what.

Glow engines are actually more akin to model diesels than they are to spark ignition motors, and model diesels are not all that closely related to car/truck/locomotive diesels. In both model diesels and glow engines, ignition occurs when the charge is compressed to a certain pressure (and, due to adiabatic heating, a certain temperature). Model diesels work without a glowplug due to the ether in the fuel, which ignites at a practical pressure. You could probably get glow fuel to ignite, too, but you'd need a higher pressure than what we encounter in model diesels. A glow engine can actually work at a lower pressure than a model diesel because that glow plug helps the fuel ignite at a lower pressure. The plug element is hot, so the fuel/air mixture does not have to be compressed to the pressure at which it will ignite spontaneously.

Whether it's the heat or the catalytic action of the plug that causes the fuel to ignite is moot, I think. Both are necessary to some degree. There's no doubt the plug needs to be hot in the vast majority of cases before the engine will start. Has anyone actually tried running an engine with a plug with an inconel (would even inconel have a higher melting temperature than platinum?) element (with battery attached, if necessary) to see if it'll run? I suspect it will, but it probably won't start as easily as the same engine with a regular platinum element plug, and it will perhaps not idle worth a darn. It'd also be interesting to see if you could get a glow engine to run on some other fuel that has a low flash point, but doesn't react with platinum to the same degree methanol does. I'm pretty sure that the heat of combustion, not just the catalysis, plays a big role in keeping the plug hot, otherwise we could heat up the plug by simply exposing it to methanol vapor. Using an electric starter on an engine would be enough to start the plug glowing and start the engine.

Interestingly enough, reading stories about old inventors, early gasoline engines as built by Gottlieb Daimler et al. often used "glow tubes" - platinum tubes, closed off at one end, heated externally with a bunsen burner as the engine was started. Once the engine was running, the bunsen burner could be turned off, the heat from the burning fuel in the engine being enough to keep the tube hot.

Ron B.
F4Fguy

I have run two K&B 4011's on Davis 1/2A diesel fuel on the bench. One engine would run quite nicely after keeping the glow plug lit for maybe 15-20 seconds. Otherwise It would run on undercompressed and took longer than I was willing to wait to warm up. The other engine would run undercompressed regardless unless the battery was reattached. Havn't checked, but I suspect a thicker head gasket. With an 11 x 5 APC, the RPM was exactly the same as with Sig Champion 10%, with the needle turned in about a turn.

My guess, based on this experience, is that the catalytic reaction with glow fuel is the major source of extra heat necessary to lite up the methanol. Otherwise, why would the glowplug coil be the "hot spot" in the combustion chamber? I also don't think there was any catalytic reaction with the diesel fuel, but I don't know.

Aren't there handwarmers that heat up from the catalytic reaction of a screen of some metal on methanol and air?

All very interesting.

Here are the plugs I sectioned. Contrary to my earlier post,
the Fox plug does not use any form of Belleville washer, but doesn't swage right onto the insulator like the Fireball either. They always seemed to have there own solutions to problems. The washer in the K&B is more of a cup washer than a true Belleville spring washer.
Rather than use a digital camera, I just placed the plugs on a flatbed scanner. Some of the plugs are not ground exactly in half, so don't jump to conclusions about chamber volume based on this.
Pat MacKenzie

>You continue to have an inordinate amount of faith in what
>you see published in toy airplane magazines. Including
>cut-away drawings which may or may not have been drawn by an
>authority on the subject.

It's better than having inordinate faith in what someone else says without supporting evidence other than "I said so". Not even a "I took some plugs apart and this is what I found". Not that the existence or non-existence of these washers is going to affect the way my engines run, one way or another, so no biggie.

>Look, I don't have a dog in this fight. Why don't you just
>call Ohlsson, act like an interested purchaser of their
>production, and ask them about these washers?

>Some effort on your part may be required, I do not have
>their phone number with me at this time.

Even simpler, I'll just write to Al Kelly. Like Wild Bill, he's sectioned a lot of plugs and even manufactures them. And his email address has been posted somewhere on the forum (just saw it a few days ago).

Ah.. found the thread:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=6089&forum=DCForumID1

>It's better than having inordinate faith in what someone
>else says without supporting evidence other than "I said
>so". Not even a "I took some plugs apart and this is what I
>found". Not that the existence or non-existence of these
>washers is going to affect the way my engines run, one way
>or another, so no biggie.

Iskandar,

While you don't upset me in any way, it is getting harder and harder to take you seriously.

My primary "I said so," source was John Brodbeck, Jr. who had more than a few glow plugs assembled by the Ohlssons. As we were in his Corvette at the time, John had a well deserved reputation for hard driving and, well, there may have been some liquor involved, maybe he actually said something quite different from what I remember.

As you say, no biggie.

I didn't make any claims as to having sectioned plugs as I have never done so. You might want to do so yourself. Do be sure to get back to us at your earliest opportunity.


>
>Even simpler, I'll just write to Al Kelly. Like Wild Bill,
>he's sectioned a lot of plugs and even manufactures them.
>And his email address has been posted somewhere on the forum
>(just saw it a few days ago).

I think that's a real good idea. Tell Al I hope he is doing well.

>And so it came to pass that both Mr. Brodbeck the junior and
>Da Dirt had their credibility raised a full point with the
>petty Iskandar dude...
>
>Thanks, Pat.
>
>Dan

Ron B.
F4Fguy

My faith was never shaken,for thy truth is self evident.

> I surface-ground a few different plugs today to get a good
>look at the construction of modern examples. The ones I did
>were:
> Fox, KB1L, Mcoy MC9 , Nelson and Fireball.
> The MC9 and the Nelson plug definitely incorporate a
>belleville washer. The Fox and K&B have a version of a
>Belleville washer.
> The Fireball does not, and this probably explains why
>the seals fail on them so easily. My digital camera is in
>the Caribbean (without me!)
> so I can't post any pictures right now. If anyone is
>interested I can probably post something over the weekend.
> The big difference between a modern plug and the one in
>the article is the insulation. It is very thin, about .020"
>compared to the big chunks in the one in the article.
> Pat MacKenzie
>P.S. if anyone is curious about what a Belleville washer is,
>look here:
>http://www.bellevillesprings.com/disc-springs.html
>Basically it is a very compact short travel spring.

Thanks a bunch for posting the pictures. I was very interested to see what plugs looked like in cross section, but didn't have the equipment (other than a hacksaw) to attempt this myself. Dan said there were two washers squashed together. What I see is one, right under the crimp of the body. The crimp forces the washer down, the washer apparently pushes down on the fat part of the plug post, which seals the plug above the element. At least that's how it looks to me. There has to be some insulation between the washer and the post, otherwise it'll short the post to the body.

Al Kelly mentioned the tapered seal on the Fireball plug, but I couldn't figure out what he meant. Now I see. According to him the seal material gradually erodes from the bottom, and it's due to the shape. No amount of additional sealang (blobs of epoxy, etc.) around the post at the top of the plug will help. I still don't understand why this is the case, but I can now see how the seal differs from the other plugs.

It's also interesting to see the similarities between the K&B 1L and the MC-9 (the thickness of the fat part of the post in particular).

I've had exactly one plug seal fail, and that was a Fox Std. Long. I've used a few Fireballs now and then, and they haven't failed in my engines.

Dan's still wrong about the Honda CVCC and catalytic convertors.

Ron B.
F4Fguy

For some reason,this didn't post properly so I'lltry again.

Chicken,egg,chicken,egg,chicken,egg......................I can remain silent no more!I can't stand it any longer.As usual, everyone is right,to a degree and,in most cases,wrong to a greater degree.

Iskandar is right in that theres a lot in common between glow and our"diesel"model engines.But,the comparison is backward.Our"diesels"are two stroke OTTO cycle engines utilizing compression ignition.The "spark" timing is controlled by changing the point where compression heat is enough to ignite the mixture.Ether is added to the fuel to lower the ignition point of the mixture.In a true Diesel,compression is fixed at a high level and timed injection of fuel controls timing and continues thru most of the power stroke to maintain high pressure in the cylinder.

As usual in modelbiz,the terms are ill defined(for instance measuring pipe length from the center of the cylinder),but,I'll take a shot.In all the rest of the world of IC engines a hot plug is one with a long heat path,causing it to lose heat more slowly.A cold plug,as you have guessed,has a shorter or less difficult heat path,and consequently runs cooler because it can dissipate heat more readily.If you look at the cross sections in a post above you can see this demonstrated.The Fox,K&B,and Fireball are relatively high heat range plugs,The Nelson with its bottom tapered seat and very short heat path is on the cool,low heat range end of the spectrum.The reason for hot/cold plugs is to tailor the plug temperature to the operating conditions.This has little to do with igniting the mixture,except to keep it from occurring before or after you want it to.The idea is to keep the chamber temperature as uniform as possible,to avoid preignition(I did NOT say detonation)due to the plug being a hot spot firing the mixture before the spark.And to avoid fouling or late firing due to the plug quenching the flame initiation if the plug is too cold.

Our model engines are no different.Hot plugs are indicated when:CR is low;when revs are low;when nitro content is low(high methanol).Cooler plugs are required when revs,nitro,and/or CR are high.Timing in glow engines is controlled by the lowest point where the heat of compression and the element temperature will initiate catalysis.Once begun,heat of combustion will keep everything hot until the next cycle.But,if the plug is too hot,ignition will occur too early because if it can't get rid of it's heat fast enough,the coil temp will continue to rise initiating combustion at a lower compression pressure.Obviously if the plug is too cold it will not gain enough heat to sustain catalysis and the timing will be retarded,or ignition will not occur at all.The central point of all this is that its not a matter of the coil being hot,it's rather a problem of keeping the plug at a controlled temp by changing its ability to gain or lose heat to the surrounding structure.Short post large wire fewer coils make heat transfer easier hence cooler plug.Long post,smaller wire more coils,more difficult transfer and hotter plug.

You can argue the catalysis/heating coil thing 'til you're blue in the face.The fact is,you need both to make the system work.If you've ever experienced the "coil coating" mentioned in several other posts,you know it takes more than just a hot coil.The plug can glow almost white hot and the thing won't run without the battery.Most catalytic processes require high catalyst temperatures to start the reaction.The combination of resistance heat and heat of compression accomplish this locally.Heat retained in the coil,added to heat from the previous cycle is enough to maintain catalysis.The catalysis acts as the "spark".Note that all the plug does,in any case,is INITIATE combustion,in the same manner as a spark plug.

Other fuels can be used with glow plugs.in the early days of glow ignition,a gasoline based fuel was marketed.If I recall correctly,it was labeled "Blue Flame" and (I think) was a Testors product.It was basically white gas/oil with nitropropane as an igniter.We used to mix our own,since it was cheap,but it would rapidly"poison"plugs.I suspect any hydrocarbon fuel would run(not necessarily well)if you had the right plug.

All of this is not just opinion or hypothesis.It the result of almost 40 years of training and experience in research and development engineering of IC engines.

Ron,

Thanks, we often need to have terms defined properly. I don't know what it is about modeling, but the misuse of words and terms constantly annoys me.

Well, at least it does when I know a little something about the subject. When dealing with things not well understood I suppose I revert to tossing around words incorrectly as well.

Hey, we measure tail moment from the trailing edge of the wing! And nose moment from the leading edge of wing. I have given up on trying to get people to quit referring to center of gravity when what they are talking about is the balance point.

However, my favorite--and I mean my very favorite--is the old question: "How much tip weight do you have?" I generally respond with something like, "Do you think I need more? Or less?" I mean, those are the real questions, are they not?

But some day, maybe this year, I will be asked that question and I am going to go into full detail as to where the heavy ribs were placed and the weight differential, the amount of hollowing on left tip block, style of adjustable leadout guide and its weight, solid or stranded leadouts, crimps or wraps, whether or not the right tip block is hollowed at all, the materials used for tip weight box, dimensions of tip weight box, etc., etc.

Or maybe not...

>Ron,
>
>Thanks, we often need to have terms defined properly. I
>don't know what it is about modeling, but the misuse of
>words and terms constantly annoys me.
>
>Well, at least it does when I know a little something about
>the subject. When dealing with things not well understood I
>suppose I revert to tossing around words incorrectly as
>well.
>
>Hey, we measure tail moment from the trailing edge of the
>wing! And nose moment from the leading edge of wing. I have
>given up on trying to get people to quit referring to center
>of gravity when what they are talking about is the balance
>point.
>
>However, my favorite--and I mean my very favorite--is the
>old question: "How much tip weight do you have?" I generally
>respond with something like, "Do you think I need more? Or
>less?" I mean, those are the real questions, are they not?
>
>But some day, maybe this year, I will be asked that question
>and I am going to go into full detail as to where the heavy
>ribs were placed and the weight differential, the amount of
>hollowing on left tip block, style of adjustable leadout
>guide and its weight, solid or stranded leadouts, crimps or
>wraps, whether or not the right tip block is hollowed at
>all, the materials used for tip weight box, dimensions of
>tip weight box, etc., etc.
>
>Or maybe not...
>
>Dan

Ron B.
F4Fguy

......No,but seriously Dan,how much tip weight do you have?

I just tell'em I've got a whole drawer full,but I leave most of it at home.

For some real yuks,read the All About Engines series(is two a series?)in MA the last couple of months.I almost fell out of my chair laughing.I hope they do more.

LAST EDITED ON Apr-29-04 AT 05:51 PM (CDT)
 
......No,but seriously Dan,how much tip weight do you have?

>I just tell'em I've got a whole drawer full,but I leave most of it at home.

>For some real yuks,read the All About Engines series(is two a series?)in MA the last couple of months.I almost fell out of my chair laughing.I hope they do more.

>Ron B.

Ron,

While I would love to read something you found to be so funny, I did not renew my AMA membership until a month or so ago.

At times I would wonder what I was missing, this nicely answered when June 2004 issue of Model Aviation showed up a few days ago: Nothing.

Dan

An edit because I did learn something from mentioned magazine: If and when we get suitable electric-powered CL Stunt models, charging the batteries in a van full of models built from balsa, surrounded by gallons and gallons of fuel, will be really exciting.

I have long enjoyed both CL models and fireworks. This will be a chance to combine the best of both worlds. Can't wait...