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Difference between 'hot/cold' plugs?

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Difference between 'hot/cold' plugs?

Old 07-09-2022, 07:55 AM
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2W0EPI
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Default Difference between 'hot/cold' plugs?

Hi Folks,

Although I'm going at a steep learning curve with rc, I still don't understand the differences between, for example, my Force .46 cu.in nitro engine manufacturers recommend a cold plug but why, what will happen if I use a hot plug?.

Can anyone help us newbies?

Cheers
Darren

Old 07-09-2022, 08:21 AM
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1967brutus
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It might require a richer setting, and by that reduce a bit in both power and fuel economy, as well as get a bit harder to get the proper needle settings.
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Old 07-09-2022, 10:21 AM
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Originally Posted by 1967brutus
It might require a richer setting, and by that reduce a bit in both power and fuel economy, as well as get a bit harder to get the proper needle settings.
Does this mean a hot plug can be used as an engine 'run in' period and once it is properly run in we can switch to cold plug?
Old 07-09-2022, 11:19 AM
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Originally Posted by 2W0EPI
Does this mean a hot plug can be used as an engine 'run in' period and once it is properly run in we can switch to cold plug?
I would not use it that way. Break in is about temperature mainly, and keeping pressures and forces a bit limited. The rich setting does not serve to directly cool the engine or to lubricate it additionally, but to retard the ignition and keep forces, pressures and burn temperature down. So the rich setting indirectly results in cooler running. A hot plug advances ignition, and increases burn temps and pressures again so that more or less counteracts the richer setting, and in that case you're only wasting fuel.

Best to break the engine in on the same heatgrade and the same fuel that is recommended for normal use, and run it a touch richer according to the break in instructions, with a lighter prop to keep mechanical loads within limits.
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Old 07-09-2022, 11:32 AM
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Originally Posted by 1967brutus
I would not use it that way. Break in is about temperature mainly, and keeping pressures and forces a bit limited. The rich setting does not serve to directly cool the engine or to lubricate it additionally, but to retard the ignition and keep forces, pressures and burn temperature down. So the rich setting indirectly results in cooler running. A hot plug advances ignition, and increases burn temps and pressures again so that more or less counteracts the richer setting, and in that case you're only wasting fuel.

Best to break the engine in on the same heatgrade and the same fuel that is recommended for normal use, and run it a touch richer according to the break in instructions, with a lighter prop to keep mechanical loads within limits.

So, a glow temperature controls the ignition timing glow style?, i.e. we can advance(hot) or retard(cold) as a 3(cold/medium/hot) position ignition timing control?

Last edited by 2W0EPI; 07-09-2022 at 11:36 AM.
Old 07-09-2022, 12:12 PM
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Originally Posted by 2W0EPI
So, a glow temperature controls the ignition timing glow style?, i.e. we can advance(hot) or retard(cold) as a 3(cold/medium/hot) position ignition timing control?
No... it does not work like that.... the complete story is long and complicated, but it basically comes down to that a given plug will, compared to a hotter or colder plug, retard or advance timing, but then the engine won't run right because actual timing required is essentially a given, a "fixed value" so to speak. So you have to retune the needles, because mixture changes ALSO control ignition timing.
That retuning will basically return timing back to where it was before, with the previous plug.
Too lean mixtures lead to thermal runaways, too rich mixtures lead to waste and power loss. Compression ratio also changes timing, as well as Nitro percentages. Compression can be changed by adding or removing headshims (AKA "gaskets") and Nitro, obviously there are various mixes availlable, or you can mix your own if you have access to the ingredients.
These three factors need to be carefully balanced. The manual usually will tell you which fuel composition and which plug to use. You can experiment a little, but you should not stray too far from the manufacturers recommendations unless you have built up a suficient deal of experience with the matter.

Very crudely put: Fuel composition and Compression Ratio should be optimized for full power, and the plug heat grade is a bit of an experience thing: different grades result in differences in throttleability. A different plug will make the engine "run generally nicer" or not, is the best way to put it.

A "hot" plug does not have a higher temperature, that is a misconception. It either has a different alloy (stronger or weaker catalytic action) or a higher or lower heat retention (cooling down or retaining heat between the burn cycles).

Last edited by 1967brutus; 07-09-2022 at 12:31 PM.
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Old 07-09-2022, 01:30 PM
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Well done Brutus, so it's a little bit more complicated than my earlier post, that's fine I have a lot to learn yet, in the meantime I'll be thinking about,.given the complicated nature of glow engines, and I thought your last post was quite good considering factors, cheers for that.

Old 07-09-2022, 01:41 PM
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Yeah, the system is simple, the factors regulating all running parameters are many and their interaction pretty complex. but don't overthink it: just follow the manual and you should be basically OK. Not all engines have the same standard manual. OS and Saito have pretty good manuals, other brands might assume more base knowledge from the user, some brands even have pretty much totally rubbish manuals. Strange enough, the quality of the manual does not necessarily have any direct relation to the quality of the engines (Rossi comes to mind: their engines were the Rolls Royces among 2-strokes, but their manuals.... too small and not absorbent enough to wipe ones hiney.... ).

In general, best to find a club (if you haven't allready) where they can help you hands-on. That is how I learned. The details came later.
Old 07-09-2022, 07:29 PM
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To go one further; there are two functions of the heat range of the plug. How much heat the body retains, and how “hot” the wire is. Some manufacturers make plugs such that there are hot body and cold body plugs, and also hot, medium, and cold elements. You could get a cold body hot element plug, hot body medium element (my preference), etc. Its just more way to fine tune the engine.

Generally, the glow plug heat range is chosen based on the methanol content. The higher the methanol (lower nitro) content, the hotter the plug needed. Methanol itself burns pretty slow, so more heat is needed.
Old 07-09-2022, 08:36 PM
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Originally Posted by 1QwkSport2.5r
To go one further; there are two functions of the heat range of the plug. How much heat the body retains, and how “hot” the wire is. Some manufacturers make plugs such that there are hot body and cold body plugs, and also hot, medium, and cold elements. You could get a cold body hot element plug, hot body medium element (my preference), etc. Its just more way to fine tune the engine.

Generally, the glow plug heat range is chosen based on the methanol content. The higher the methanol (lower nitro) content, the hotter the plug needed. Methanol itself burns pretty slow, so more heat is needed.
Do you have any info on how to differentiate between hot body and hot wire? Or is that just a matter of "this brand such and that brand so"? Because I cannot remember having seen that info listed by any of the name brands to be honest.

Also: to a certain extent, and this is more an "aero-engine" thing AFAIK, engine size (displacement) also does play a role according to some plug manufacturers, larger engines can do with colder plugs. At least, that is what Rossi says on their plug blister packaging. But that is more a rule of thumb.

Last edited by 1967brutus; 07-09-2022 at 08:41 PM.
Old 07-10-2022, 09:41 AM
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Originally Posted by 1967brutus
Do you have any info on how to differentiate between hot body and hot wire? Or is that just a matter of "this brand such and that brand so"? Because I cannot remember having seen that info listed by any of the name brands to be honest.

Also: to a certain extent, and this is more an "aero-engine" thing AFAIK, engine size (displacement) also does play a role according to some plug manufacturers, larger engines can do with colder plugs. At least, that is what Rossi says on their plug blister packaging. But that is more a rule of thumb.
The links I used to use are broken and no longer work. Since the Novarossi factory closed, so did their website which listed what the codes mean for their glow plugs. Picco is similar. I’ll give you what I know.

Picco plugs are coded P6S or P6TH depending on standard (S suffix) or Turbo Hot or Turbo Cold (TH or TC suffix). P for Picco, 6 for the heat range of the element (5 cold, 6 medium, 7 hot). TH or TC is the body of the plug - hot body for off road and cold body for onroad. TC suffixed plugs are significantly colder than TH plugs.

Novarossi - there is some contradictory information in decoding the glow plug designators on the internet, but since Nova closed, the good information is NLA. See attached image. Example C6TGC decoding - C for conical, 6 for wire size (heat range of wire) T for turbo (I was under the impression conical was the same meaning, but they’re Italian. Who knows. ) G for gold plated (they aren’t gold plated. Not sure about this designator either. ) C for hot body (Chaud). F for cold body (Fredda). Some of the standard plugs are gold plated - they will have a G suffix.

Its too bad these companies shut their sites down so quickly. There was good information about selecting glow plugs there, though Nova suggests picking a plug based on ambient air temp and not the fuel used as the primary factor, but that isn’t really how it should be done.

Also - I don’t believe any other companies do this with glow plugs. I have used hot and cold body plugs side by side and there is an immense difference in how they behave. The cold body plug needed the high side needle leaned a full turn from the hot body setting. (Picco plug, Picco engine). Warning: photos on this forum suck. They do not upload right, cannot be viewed fullsize, etc. I use a 3rd party host for images, but some things translate funny because of that. Hopefully you can read the image below.
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Old 07-10-2022, 10:05 AM
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Seems to me, that is exclusevely a thing for Turbo plugs then, that hot body and cold body thing? In that case, albeit true, not really important to TS since Turboplugs are reasonably rare for aero engines.
Old 07-10-2022, 10:20 AM
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I am willing to bet there is some crossover to standard plugs in regard to how the body is shaped for heat retention, there just isn’t any readily available Information available. I have measured numerous plugs comparing the filament diameter and the size of the recess within the plug body. By far, the biggest difference in standard plugs is the size of the recess. Most glow standard plugs I measured had very similar size filaments. I’ll see if I can find my notes on the matter and I’ll post them.
Old 07-10-2022, 10:50 AM
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Originally Posted by 1967brutus
Seems to me, that is exclusevely a thing for Turbo plugs then, that hot body and cold body thing? In that case, albeit true, not really important to TS since Turboplugs are reasonably rare for aero engines.
Hi Brutus,

Please don't go upset when the thread seemingly going the wrong way, just chill.When I made my first rc purchase, bought 5 turbo plugs for £20/5off, I thought ;turbo' was marketing hype, my friend, they do not even fit a normal glow engine, I must admit, I burnt my fingers here?Had I joined this forum 3 months ago ,I would have asked here before making a purchase.

Last edited by 2W0EPI; 07-10-2022 at 11:13 AM.
Old 07-10-2022, 11:08 AM
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Originally Posted by 2W0EPI
Hi Brutus,

Please don't go upset when the thread seemingly going the wrong way, just chill.When I made my first rc purchase, bought 5 turbo plugs for £20/5off, I thought ;turbo' was marketing hype, my friend, they do not even fit a normal glow engine, I must admit, I burnt my finger here?Ebay??
Oh, maybe I expressed myself imperfect, but that was absolutely NOT what I tried to bring across... I just wanted to make it clear that IF the hot/cold body is a thing for Turbo plugs, it is not really relevant for plane engines. Just to make that clear and prevent confusion. No more, no less.

Because of all things, I have a LOT of appreciation for Tim's pretty specific knowledge on this particular matter. More than I'll ever have and I welcome all knowledge.
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Old 07-10-2022, 11:11 AM
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Originally Posted by 1QwkSport2.5r
I am willing to bet there is some crossover to standard plugs in regard to how the body is shaped for heat retention, there just isn’t any readily available Information available. I have measured numerous plugs comparing the filament diameter and the size of the recess within the plug body. By far, the biggest difference in standard plugs is the size of the recess. Most glow standard plugs I measured had very similar size filaments. I’ll see if I can find my notes on the matter and I’ll post them.
I am pretty sure that body design DOES have that effect, but I think those differences probably more likely were "coincidental" rather than intentional, as I have seen those design differences like recess size, and even finned hexagons and centre poles and such, but not often on plugs from the same brand. I mean: I cannot distinguish any of the 6 or 7 different Rossi plugs visually AFAIK, but other brands do have indeed different size recesses and I have seen differences in the filament as well.

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