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Standard glow plugs on COX engines - one more time

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Standard glow plugs on COX engines - one more time

Post  balogh on Wed May 03, 2017 2:50 am

Altough discussed here many times in different contexts, I thought I would share my standard glow plug experience on COX engines with you so that some rules of thumb - like additional gaskets put under the glow head with plug conversion -can be drawn.

My 049 size COX engines show the loss of power when run on standard glow plug conversion. IMHO it is not only related to the compression ratio/combustion chamber shape with conversion heads, but standard plugs may emit more heat than stock COX glow head filaments, hence causing premature ignition - pressures working against the piston still moving towards the TDC -, and consuming too much useful energy from the fuel (platina in the filament acts as a catalyst in burning methanol) to keep the filament glowing, that is larger in mass than the ideal filament in a COX stock head. (The energy from the fuel that keeps the plug glowing  is returned by glow heat into the cycle but under sub-optimum conditions...more heat input elongated along the compression cycle etc. The problem is partially mitigated by additional gaskets under the glow head that reduce compression ratio and delay ignition, but reduced compression ratio again reduces engine output for other thermodynamic resons.

The glowplug impact on larger engines is less enhanced, probably because the glow heat returned into the cycle during compression from the standard plug relative to the total heat delivered by the fuel charge is less than with smaller engines.

E.g. with my TD09 driving my Cosmic Wind the power reduction with the Mecoa-supplied, trumpet type glow plug conversion is less but still still sensible, (and the efficiency of my TD09, an SPI engine, is further punished with the muffler that not only allows exhaust gases to enter the crankcase, but keeps the cylinder exterior hot where otherwise - though unfinned - the cylinder would be air-cooled.


The engine runs rather hot - this in itself contributing to early ignition -,and requires more frequent cylinder devarnishing than with a standard COX head. (see the varnish deposited inside the conversion glow head after just a few runs)


For R/C use, however, the more heat emitted by the glow plug is indispensable to maintain a low and smooth idle...the standard COX head will drop the heat easier when in idle and the engine may stop.

So what do I want to say with this boring academic elaboration on something you may find a no-brainer commonsense?

1. Glow plug conversions may cost you loss of power especially on smaller engines... an 09 will already suffer less than a 049
2. The problem can be mitigated by adding extra shims under the glow head - the improvement is expectably  better on larger engines..I tried it during my weekend flights, though did not tach the difference, but relied only on my ears
3. If you fly R/C and want a good idle, you will probably want to look for something different than the stock glow head..even the insert type conversion (https://coxengines.ca/insert-for-.049-head-adapter-3-5-fins-medium-hot.html)  is better than the stock head, in my experience. For COX engines larger than 049/051 the only choice for a good idle is the glow plug..my standard 09 COX head did not maintain low idle in the air.

Sorry if it was too obvious.
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Re: Standard glow plugs on COX engines - one more time

Post  GallopingGhostler on Wed May 03, 2017 6:48 am

Balogh, you bring up an interesting point. A sport model doesn't alway require maximum performance from an engine, if chosen wisely. More important is ease of throttling and reliability throughout its entire RPM range.

Following is a comparison of the Cox Medallion glow head and Mecoa glow plug adapter heads:


They have roughly the same volume. Hand flipping the engine prop, the Mecoa seems to have slightly less compression, about that of my Fuji .099S-II (an Enya .09 clone internally). This .09 is the engine I'm putting into my 1959 Berkeley Impulse. It has a throttle sleeve muffler.

Following is a comparison of the Cox Medallion .09 glow head and stock Queen Bee .074 glow plug adapter head:


It's adapter head gives the .074 considerably more compression than the Medallion with its stock glow head. Reading write ups on the .074, a recommendation is an additional gasket under the head to reduce compression to provide better idling and easier throttling.

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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Wed May 03, 2017 8:34 am

balogh wrote:
My 049 size COX engines show the loss of power when run on standard glow plug conversion. IMHO it is not only related to the compression ratio/combustion chamber shape with conversion heads, but standard plugs may emit more heat than stock COX glow head filaments, hence causing premature ignition - pressures working against the piston still moving towards the TDC -, and consuming too much useful energy from the fuel (platina in the filament acts as a catalyst in burning methanol) to keep the filament glowing, that is larger in mass than the ideal filament in a COX stock head.

I think you are mixing up most of the terms there...
A thinner wire is usually considered to be a hotter plug, and if you want a colder one you simply buy a "colder" plug.

If you want to make some analysis you need to change one thing at time, only then can it be possible to make some conclusions. Running a three-bladed prop and a muffler, plus the silicon extension(!) is already asking a lot, no wonder it is running hot!  affraid
Looking at the piston it also burns very unevenly, I would guess that the hotter region is facing the exhaust port that is obstructed the most, i.e. pointing away from the muffler outlet.

The advantage with a regular glow plug is that they come in different heat ranges, so if you plug acts too hot, just change to a colder one.

The adaptor heads have been discussed at length in the past. There is a good test and write up here; Glow plug run-off

It basically shows that the only problem with a regular plug is the threads, these needs to be sealed with a sealant in order to avoid a power loss. The test also points to a more common/serious problem; the regular glow plugs are all different and there is no good way to ensure a smooth combustion chamber without modding the head for each plug length/style. Hence most adaptors are made too low on the compression ratio and you also need to find a matching glow plug length.

There Turbo plugs are a much better choice, they seal at the right place and they also come in a variety of heat ranges. I wrote a little about that here in past; Turbo Plugs/heads
(that work is based on RCU discussions in the past)


Last edited by Surfer_kris on Wed May 03, 2017 9:04 am; edited 1 time in total
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Re: Standard glow plugs on COX engines - one more time

Post  balogh on Wed May 03, 2017 8:37 am

Surfer_kris wrote:
It basically shows that the only problem with a regular plug is the threads, these needs to be sealed with a sealant in order to avoid a power loss. The test also points to a more common/serious problem; the regular glow plugs are all different and there is no good way to ensure a smooth combustion chamber without modding the head for each plug length/style. Hence most adaptors are made too low on the compression ratio and you also need to find a matching glow plug length.


Thanks Kris

Do you then confirm that in case a standard plug has its threads well sealed, and its length matches the conversion head design, then the output of a 049 engine will be the same as with a stock COX head?
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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Wed May 03, 2017 9:07 am

Not quite the same but fairly close. I think people have come within 500rpm or so using an adapter head that was remachined for a correct compression ratio (usually needs to be raised) and the threads of the glow plug sealed with a gasket silicon sealant (permatex or similar).
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Re: Standard glow plugs on COX engines - one more time

Post  944_Jim on Wed May 03, 2017 10:17 am

Balogh,

The standard Cox heads and the Merlin head conversion seal on the shoulder in the cylinder...That's the only gasketed area with these two types.

For reference, the Merlin glow-plug seals on the cylinder, and the replacement head is actually the clamp that holds the Merlin plug in place.

The other plug conversions use the same gasket between the head and the cylinder, but introduce an additional threaded "potential compromise" between the screw-in plug and the head. It is the additional threaded area that is of concern in the previous couple of posts.

Hope this helps clear up some confusion.
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Re: Standard glow plugs on COX engines - one more time

Post  balogh on Wed May 03, 2017 11:06 am

Thanks gentlemen but no confusion here. I have been using all these conversion options and I am really particular about good thread sealing under the head and plug both.
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Re: Standard glow plugs on COX engines - one more time

Post  GallopingGhostler on Thu May 04, 2017 8:08 am

I'd be curious if anyone has taken a .049 muffled and throttled Tee Dee or Medallion and replaced the piston cylinder set with non-SPI Sure Start set. I know this may seem to be an antithesis to a performance engine, but was wondering that without SPI, perhaps it overall would perform better, considering that these engines already have a more free flowing air-fuel stream to the combustion chamber than the reedies, which contributes to its greater power? Eyebrows

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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Thu May 04, 2017 2:31 pm

The muffled ones are usually non-SPI from the box, like the TD 05RC, not sure what you are after there?
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Re: Standard glow plugs on COX engines - one more time

Post  GallopingGhostler on Thu May 04, 2017 4:57 pm

Surfer_kris wrote:The muffled ones are usually non-SPI from the box, like the TD 05RC, not sure what you are after there?

Not all; my late model Medallion .09 with muffled throttle ring has SPI and so did the earlier 1960's one with sliding throttle baffle have SPI also. I don't know if the pre-Estes Tee Dee R/C's prior to 1995 were SPI or not, hence why I asked. Also, I was interested in how they performed with non-SPI piston cylinder sets.

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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Fri May 05, 2017 12:58 am

I think it depends a lot on the rpm range used, as SPI is more important for higher rpms. With a throttle ring it might not be detrimental at all, as there shouldn't be much restiction at full throttle.

I only have reliable rpm data for the Cox 05 RC and it will spin a graupner 6x3 at around 17200rpm, which is a decent number but I think it looses compared to a regular TD above that. This could also well be due to the carb being more limited than the regular TD venturi.

There are certainly engines that work very well with SPI and mufflers, with the Norvels .049, .061 and .074 perhaps being the best examples. The amount of SPI is a little smaller than on a typical Cox though (as I remember it).
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Re: Standard glow plugs on COX engines - one more time

Post  balogh on Fri May 05, 2017 2:08 am

GallopingGhostler wrote:I'd be curious if anyone has taken a .049 muffled and throttled Tee Dee or Medallion and replaced the piston cylinder set with non-SPI Sure Start set. I know this may seem to be an antithesis to a performance engine, but was wondering that without SPI, perhaps it overall would perform better, considering that these engines already have a more free flowing air-fuel stream to the combustion chamber than the reedies, which contributes to its greater power? Eyebrows

I got your point, I will check it. You may be right: compared to a non-muffled SPI setup, the performance penalty by a muffler on a SPI engine may be more than the performance penalty with a non-SPI setup+muffler, where the exhaust gases from the muffler will not be sucked in by the engine...I think with a TD venturi the performance of the muffled non-SPI is still higher than that of the TD 050 with RC carb, so I expect 18+k speed.

In addition to these component permutations I also will check a No 2 cylinder (single by-pass) on a TD049 that I was sold (without me knowing that the cylinder is not No 4) on ebay..the No 2 cylinder and piston combo are new, but wonder how the TD rotary valve design will improve its performance compared to the stock reedie setup for a No 2 cylinder.
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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Fri May 05, 2017 9:03 am

I have intended to try other piston/cylinder sets on the 05RC, but got distracted by other things at that time. Perhaps I'll give it another go, the spring is finally arriving here and the temps are getting better. We still had frost last night though...
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Re: Standard glow plugs on COX engines - one more time

Post  roddie on Fri May 05, 2017 10:40 pm

Andras, I'd like to see you test the Ace venturi-throttle for the .049 tanked Bee engine, to gather some more data. I've ran my engines with stock Cox glowheads (single head-gasket) 25% nitro fuel.. with favorable results. Favorable meaning; on the bench, running non-SPI cylinders with a muffler. The Ace instructions for their venturi-throttle, stress the use of a muffler.. to hold heat in the cylinder at low-idle speeds. It's not necessarily the recipe for a high-performance engine.. but might be interesting to try on the bench.. followed by a small RC model, if the results are satisfactory. The Ace throttle design is somewhat intrusive to an external tank-bay.. whereas its linkage encompasses the area directly behind the firewall.. but this can be dealt with through creative tank-construction/placement. I've run many tanked bee engines with the Ace-throttle, using external tanks. They all have seemed to draw fuel quite reliably. Props were in the 6 x 3 range. The muffler was my own design though.. and minimally-restrictive. I'd be glad to elaborate on that.. if you're interested.



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Re: Standard glow plugs on COX engines - one more time

Post  Ken Cook on Sat May 06, 2017 9:19 am

I never had success using the Mecoa adapters. They run, performance is poor and they were quickly removed from the engine. Norvel made a few different versions of glow plug adapters. I have one in particular that looks very promising and I haven't tried it yet. Of the adapters I have used, the plug element sits too far back. I have been doing some of my own experimenting. Burnt out Cox heads are plentiful. The main problem is tightening the head, I use a strap wrench to do so because of the wrench flats being lost when turned in the lathe. I've tried to leave them but little to no material is left to support the spanner wrench which generally tears off due to being too thin. The latest attempt is to file flats on the fin for an adjustable. This has also required me to find different length plugs because typically once the 1/4" through hole is drilled, you lose a lot of area internally. The OS plugs are not as long as a long plug and not as short as a short plug. I have one version with 3 fins left and another with 2. I'm trying to get back some performance by lowering the plug and reshaping the combustion chamber. This is just a test of several designs with no known factual evidence of what works, just whatever does prove worthy will be copied. So far, the one with the plug in it, 3 fins seems to be working darn good. I get good starts and fairly good rpm's. It's certainly credible enough to put on a plane and fly it without concerns. My camera is acting funny and for some reason I can't seem to get a good close up of the combustion shape. I tried to make it like a 1702 HC head.  


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Re: Standard glow plugs on COX engines - one more time

Post  balogh on Sat May 06, 2017 12:28 pm

roddie wrote:....The muffler was my own design though.. and minimally-restrictive. I'd be glad to elaborate on that.. if you're interested.




Roddie I sure am interested in your experience with your own muffler that I remember you made from rubber discs with an exhaust manifold pressed in between.

The restriction by the muffler is a trade-off....if too much is the restriction then the heat retention and hence the idle are OK, though the backpressure is too high and the engine runs poor, over-muffled, let alone the additional effect on an SPI cylinder...if the restriction is too small, the engine may run better (still inhaling, if SPI, its own exhaust gases) but then the heat retained is less and the idle is insecure.

I am not sure how restrictive the COX stock mufflers with the slip-rings, that I also use on 049 and 09 engines, are:


Even though the permanent opening on the muffler body has a small bore diameter,but  the slip-ring does not really seal the rectangular slot under it and hence the flue gases find their way around the ring too. So the restriction may be not too much.

The 050 muffler is also not much of a backpressure retainer, because it drops freely on the cylinder with a circumferential gap left in between the cylinder and the muffler body, where much of the gases escape, rather than through the muffler manifold..


So what is your experience with your rubber-muffler? I understand it does not restrict the exhaust too much, but how does it enable a stable, low idle?
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Re: Standard glow plugs on COX engines - one more time

Post  roddie on Sat May 06, 2017 3:03 pm

balogh wrote:

So what is your experience with your rubber-muffler? I understand it does not restrict the exhaust too much, but how does it enable a stable, low idle?

My experience is only with bench-running the muffler/throttle combination. I'm not sure.. but I think the muffler (as minimally-restrictive as it is..) causes the cylinder to run a little hotter. "Ace-RC" states in their throttle instructions "A lower idle can be obtained by using a Cox or Tatone muffler which increases backpressure and holds heat to the plug at low-speeds better".

My muffler's Butyl rubber-disc body seals-off the area above/below the exhaust-ports because the center-holes in the top/bottom discs are a smaller diameter than the cylinder. The holes stretch-open enough to install over the head and also conform to the lower cylinder diameter. No engine disassembly is required.. which is convenient. It's a tight fit on a Tee Dee though. There isn't much clearance between the venturi and the cylinder. The rubber-discs are flexible.. and I think I can make them a bit smaller. Too small results in the inner-aluminum ring fitting too tightly inside.. which stretches-open the center-holes in the rubber. Maybe the center-holes could be punched smaller to compensate. The muffler continues to be a work in progress. The design can be effectively scaled up or down for fitting to a Cox Pee Wee or .09 by sourcing the correct aluminum tubing to clear the glow-head with muffler-pipe(s) installed.

There's probably a few contributing-factors to achieving a stable low-idle. I was able to obtain sub-2K rpm's during the test-run in my video. Compression could have played a part in that. I ran a standard Cox glowhead with a single gasket. I can't remember the cylinder#/porting though. Fuel was Sig Champion 25 and prop was I believe a Cox grey 6 x 3. A well-used engine having less compression may attain a lower idle through less resistance per-stroke, at low speeds.. There is also a "flywheel-effect" if using a heavier prop.. or one at the upper diameter-limit for the engine. Low-pitch propellers may help contribute to lower-idle speeds as well.

When using the Ace throttle for the Bee, I found it best to tune the engine-needle for ultimate operation with the venturi wide-open (as per usual). Adjusting the needle for a lower idle only narrows the rpm-range in my experience. The Bee's back-plate design is such; that Ace's throttle-plunger (needle) seems to do a nice job of metering both fuel and air because the Bee's fuel-jet is immediately inside and perpendicular to the air-intake hole. At lower speeds, the engine continues to 2-stroke/cycle without loading-up too much. Throttle-response is surprisingly good. Maybe not as instantaneous as an exhaust-throttle.. but still very good. The Ace plunger-needle will choke-off the air and stop the engine if advanced into the back-plate past the taper on the plunger. Shutting-down can be done simply and conventionally by using throttle-servo trim.
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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Sat May 06, 2017 3:11 pm

balogh wrote:
The 050 muffler is also not much of a backpressure retainer, because it drops freely on the cylinder with a circumferential gap left in between the cylinder and the muffler body, where much of the gases escape, rather than through the muffler manifold..

Yes, that means the exhaust oil is still going all over the engine....

I have sealed the muffler on my 05RC. Just rapped a thin aluminium foil one full turn around the cylinder and then attached it to the muffler with JB weld. It is now nice and tight all around the cylinder.


Last edited by Surfer_kris on Sat May 06, 2017 3:21 pm; edited 1 time in total
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Re: Standard glow plugs on COX engines - one more time

Post  Surfer_kris on Sat May 06, 2017 3:15 pm

Ken Cook wrote: I have one version with 3 fins left and another with 2. I'm trying to get back some performance by lowering the plug and reshaping the combustion chamber. This is just a test of several designs with no known factual evidence of what works, just whatever does prove worthy will be copied. So far, the one with the plug in it, 3 fins seems to be working darn good. I get good starts and fairly good rpm's.

Have you tried making one for the Turbo plugs?

It is fairly easy once one gets the hang of it. I keep all the head fins and can still use the stock head wrench.




Below is a little comparison of different heads which I've posted the "Turbo-head" thread, in the past. From left to right it shows; regular OEM, Mecoa, regular cox converted to turbo, Valentine dedicated turbo, and the Nelson head by Galbreath:
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Re: Standard glow plugs on COX engines - one more time

Post  Ken Cook on Sat May 06, 2017 3:42 pm

Kris, what you show there with your plug bottom coming right to the high point of the radius is exactly how I did my plug in which my pic doesn't show very well. That's what I was trying to convey. The only reason I haven't tried the Turbo plug is because it's one more style of plug I would need to buy. I try and stay with a commonality but I agree the turbo is the way to go. Similar to Nelson and it seals at the base not at the top of the plug. I should take a picture of this Norvel setup I own. I have 2 styles of them and the one that I have reminds me of a Glo Bee drop in but uses a standard plug. A very good design and I need to try it.
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Re: Standard glow plugs on COX engines - one more time

Post  roddie on Sat May 06, 2017 5:34 pm

Ken Cook wrote:         Kris, what you show there with your plug bottom coming right to the high point of the radius is exactly how I did my plug in which my pic doesn't show very well. That's what I was trying to convey. The only reason I haven't tried the Turbo plug is because it's one more style of plug I would need to buy. I try and stay with a commonality but I agree the turbo is the way to go. Similar to Nelson and it seals at the base not at the top of the plug. I should take a picture of this Norvel setup I own. I have 2 styles of them and the one that I have reminds me of a Glo Bee drop in but uses a standard plug. A very good design and I need to try it.

Ken, you mentioned not being able to get a decent "close-up" image with your camera. Look for a "macro" setting/symbol in the preference menu. It looks like this..



If I disable this function on my digital camera, I can't get a good close-up shot. It took me a while to figure that out.. Laughing
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Re: Standard glow plugs on COX engines - one more time

Post  GallopingGhostler on Sun May 07, 2017 8:07 pm

I just bought off E-bay 14 of the rarer Tornado 7x2 wood props



and 12 harder now to get in half-A size Zinger 6x3 wood props


and an optical digital hand tachometer (appears to be a clone of Hangar Hobbies sold by Tower Hobbies)


I want to bench test how well these props will work with my .074 Queen Bee using its factory plug head adapter, Medallion .09 glow head and Mecoa .09 plug head adapter. Tach was an item that I lacked to give quantitative results.

I also want to repeat this with the Medallion .09 with exhaust restricting throttle muffler on various 7" props including the 7x2. The tach will take about 2 weeks to arrive, ordered from China. I paid a little more for it from a buyer with a very high rating, but less than locally.

I'm thinking that the 7x2 prop will probably perform OK on the Queen Bee. With a slightly larger diameter would provide better thrust for a slower cabin type sport plane. It may or may not have enough pitch for the Medallion. The 7x2 might be also good for the AC Gilbert .11 Thunderhead.

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