Author Topic: Which glow plug?  (Read 4080 times)

Offline RayW

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Which glow plug?
« on: May 02, 2016, 07:38:15 PM »
I am intending to use continuously connected glow plug ignition for my 1895 Otto as it is the nearest thing to the original hot tube ignition, but I have no experience of using glow plugs, so have no idea what sort I need.
I know that they come in various heat settings but can anyone give me some guidance please? The building notes translated from the German simply say "use a glow plug without an idle bar".
Ray

Offline Flyboy Jim

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Re: Which glow plug?
« Reply #1 on: May 02, 2016, 08:15:13 PM »
I don't know if these are the type of glow plugs you're referring to, but here's a couple of links that might be useful:

http://www.osengines.com/glowplugs/index.html

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCB26&P=ML

Jim
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Offline RayW

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Re: Which glow plug?
« Reply #2 on: May 02, 2016, 09:07:29 PM »
Thanks Jim. I know that I will have to have the glow plug continuously connected to a battery as the engine is quite slow running and will not keep the plug hot enough otherwise, but the different heat ranges are pretty confusing.

Ray
Ray

Online Vixen

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Re: Which glow plug?
« Reply #3 on: May 02, 2016, 09:25:19 PM »
The 'idle bar' glow plugs are designed for the specific needs of four stoke model aircraft engines. The 'idle bar' is a a small steel bar welded across the end of the glow plug and shields the platinum glow plug coil and helps maintain temperature during slow running, an alternative name is 'idling bar'. So you need to look for an 'idle bar' glow plug in the shop or catalog. The OS Type F may be a good starting point.
The platinum coil of the glow plug maintains it's temperature by a catalytic reaction between the platinum wire and the methanol fuel. Petrol (gasoline) does not heat the glow plug in the same way, so you will need  external current to keep it hot.
A 2.0 volt battery is usually required to make the glow plug glow at yellow heat to start the engine. The voltage can probably be reduced to about 1.5 volts (via a dropper resistor)  when the engine is running happily,  Red heat should be enough to sustain the engine.
Mike
« Last Edit: May 02, 2016, 09:34:23 PM by Vixen »
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Offline RayW

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Re: Which glow plug?
« Reply #4 on: May 03, 2016, 09:02:59 AM »
Thanks Mike, that's very helpful. I just wonder why Heinz Kornmuller, the designer of the model, specifically says to use a glow plug without an idle bar?
Ray

Online Vixen

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Re: Which glow plug?
« Reply #5 on: May 03, 2016, 09:50:43 AM »
No two engine designs are the same. What works for one design may not work in another. There is a lot of difference between a modern commercial four stroke aero-engine and your 1895 Otto hot bulb engine.
Heinz Kornmuller will have experimented with several types of glow plug and has obviously found the non idle bar plugs work best in the Otto. Follow his teachings and conduct your own experiments with different plugs and voltages until you find your own solution to the Otto's unique ignition requirements.
Have fun doing it
Mike
It is the journey that matters, not the destination

Online Jasonb

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Re: Which glow plug?
« Reply #6 on: May 03, 2016, 04:55:01 PM »
I've not use one permanently wired but if I was building this engine I would start with a middle of the road 2-stroke plug, something like an OS No 8.

J

Offline Allen Smithee

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Re: Which glow plug?
« Reply #7 on: May 03, 2016, 05:39:47 PM »
The 'idle bar' glow plugs are designed for the specific needs of four stoke model aircraft engines.

To be pedantic - the idlebar plugs were originally introduced for the early throttled engines, whose porting and carbs weren't ideally suited to throttled use. The only specific "four stroke" plugs were the original OS "touch hole" plugs which had a restricted front oriface ahead of the coil to create a sort of "pre-combustion chamber" to keep it warm through the dead cycle. Users soon found that conventional plugs worked just as well, and I'm not sure OS have actually made the touch hole plugs for over a decade.

I would suggest that for your use you'd want a plug that is "hot" (ie one which retains heat rather than dissipating it to the cylinder head) if you want it to run with the power off, or a physically robust one if you want it to withstand extended periods with the power on. The physically robust plugs are the 1.5v ones rather than the 2v ones as the latter need a higher resistance (smaller wire cross section). If you're keeping the power on then don't worry about the heat rating - it's irrelevant.

If you're nearby (Surrey/Hants border, UK) pop round and I'll happily give you a few to play with because I have quite a lot that I'll never use. I could also let you have some Nelson Heavy Duty plugs which, whilst being "cold" racing plugs have elements like bits of barbed wire - they count as "robust" :D The Nelson plugs have an 11/32"x32tpi thread rather than the more usual 1/4"x32tpi, and are intended to seat on a front taper. But for your use you don't need the extra sealing this offers so you could just seat them the usual way with a copper washer. I have literally hundreds of "used" Nelson and Glowbee (similar concept, but with a flat-coil element) plugs because in my racing days we always fitted a new plug for every flight, and set the used ones aside for "testing".

Regards,

AS
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Offline Dave Otto

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Re: Which glow plug?
« Reply #8 on: May 03, 2016, 05:57:54 PM »
This is a low speed hit and miss engine; I don't think there is any possibility of the engine continuing to run with the power removed. I think what is needed would be a plug that will generate enough heat to fire the mixture, have low power consumption, and able to be powered for extended periods with out burning out.

Is there a plug that would fill these requirements?

Just my thoughts,
Dave

Offline RayW

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Re: Which glow plug?
« Reply #9 on: May 04, 2016, 05:06:52 PM »
Thanks Allen for that very helpful information. I live near Maidstone in Kent, so not exactly local to you. If you were willing to put one or two plugs in the post I would happily pay you for them and for the postage. If that is OK, please PM me with your address and let me know how much I owe you and I will put a cheque in the post to you. I have already made provision for a 1/4" x 32TPI plug so will stick with that size to start with.
As Mike pointed out, Heinz Kornmuller probably played around with different plugs and I will do the same until I find which gives the best results.
As Dave comments,there is no way the engine would generate enough heat to run with the power removed, hence my intention to power it continuously. As for durability of the battery, I am only likely to be running the engine for comparatively short periods at a time so do not anticipate an issue there.
One thing I forgot to mention in my initial thread is that I plan to run on propane. I don't know whether that is relevant to the type of plug I need?
Ray

Offline littlelocos

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Re: Which glow plug?
« Reply #10 on: May 05, 2016, 02:12:06 AM »
Hello RayW,
Sorry to be coming into this thread late, but you may want to give SonicTronics a try for the glow igniter.  They use a pulsed igniter that can actually be flown.  It uses much less than the 1.5-2Amps you would be looking at with a standard driver.    Here's a link to what I'm talking about.  No relation to them, just familiar with their offerings.  This type of system is used with an on-board starter and can also be used at low throttle when the glow-plug is likely to go out.

Regarding fuel, my understanding is that the wire element is a specific alloy that reacts catalytically with the methanol / nitro fuel.  Your use is much closer to a glow plug used to start an automotive Diesel engine rather than one that stays lit on its own.

Hoping this helps,
Todd.

http://www.sonictronics.com/xcart/product.php?productid=16479&cat=0&page=1


Offline RayW

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Re: Which glow plug?
« Reply #11 on: May 05, 2016, 04:52:25 PM »
Thanks Todd. An interesting option but perhaps a bit costly and over complicated for what I need. Another forum member has kindly offered to give me some glow plugs to try so will see how they perform first.

Regards

Ray
Ray

Offline petertha

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Re: Which glow plug?
« Reply #12 on: May 05, 2016, 08:24:49 PM »
I'm actually not familiar with the 1895 Otto engine, but what fuel will it run on and at what compression ratio? Pretty much everything which has been posted thus far regarding glow plugs & igniters pertains to plugs designed around methanol fuel (typically methanol + oil + nitro methane) to various recipe percentages based on application. At CR's of  typical RC engines, they just need to be ignited for starting & then they sufficiently 'glow' thereafter to sustain ignition through (2 or 4 stroke) cycles. Where the constant glow electric assist comes into play is when one or more parameters start to vary: the rpm, the CR, fuel composition, air/fuel mixture... I suspect the plug which the 1895 engine designer is recommending is probably one he discovered by trial and error. Plugs vary in a number of ways, but what is loosely called heat rating is a function of the wire coil & related properties. I guess what I'm trying to say is: pay just as much attention to replicate these other mechanical/fuel type properties as the plug itself. There is some wiggle room, but you can't be too far off in any one parameter before glow plugs don't work as well.

Of potential interest, OS (and maybe others) have introduced glow plugs intended for gasoline (as opposed to methanol) engines.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXEJRG
http://www.osengines.com/engines-airplane/osmg1513/index.html

Offline petertha

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Re: Which glow plug?
« Reply #13 on: May 05, 2016, 08:41:26 PM »
...there is no way the engine would generate enough heat to run with the power removed, hence my intention to power it continuously.... One thing I forgot to mention in my initial thread is that I plan to run on propane.

oops, I just re-read & now caught the propane comment. Sorry, can't help you there. No experience.

Regarding the 'generate enough heat' comment. That's one side of the equation to sustain glow unassisted. The other, maybe more important, is how much cooling between the strokes. Constant glow (meaning applied power to the plug) doesn't really hurt the element but I suspect it shortens the lifespan a bit. Whether that is of any consequence to your application, I really cant say.

Offline RayW

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Re: Which glow plug?
« Reply #14 on: May 06, 2016, 12:02:02 PM »
Thanks for all the advice. As I said in a previous post, I think it will just be a matter of "try it and see".
Ray