New Discovery (for me) Glo Plug vs Ignition

[cfellows]

So this is one of the more exciting things I've discovered.  I made an adapter for my Plumbing Parts engine, my test bed for new ideas and such, for a glo-plug.  I'm really excited about how well it works.

http://www.youtube.com/watch?v=AXydzBWrHEY

The secret to getting a glo plug to work properly is the timing, which is achieved by the distance the fuel mixture has to travel on the compression stroke to reach the glo plug.  My initial length was too long, so I put the adapter in the lathe and shortened it by turning off 1/16" from the end.  Put it back in the engine, installed the glo plug and surprise, surprise, it took right off.  The good news, I got rid of the troublesome misfire I was having with the spark ignition.

Now a glo plug and a large battery set me back less than $10, compared to almost $100 for the CDI ignition module, the spark plug, batteries and case.  The other nice thing, is, this pretty much demonstrates that the engine would operate equally as well with a hot tube set up, which would be more "period correct" for antique model IC engines.  The glo plug (and I assume hot tube) seems to be less sensitive to fuel mixture as well.

One additional note, in this video, I have the air bleed completely off.  I actually put a piece of tape over the air bleed hole and readjusted the main mixture for it to run right.  Not sure at this point that air bleed adds any value to the carburetor.

Chuck

[PatJ]

My experience with glow plugs comes from flying model airplanes.

I had a module that would vary the voltage to the glow plug when starting the engine, and as I recall, the glow plugs lasted a lot longer at the lowest setting possible that would still start the engine.

At the higher voltage settings, the glow plug did not last very long.

Pat J

[Alan Haisley]

Depending upon engine speed, you should be able to run the engine with no power to the plug except to start it. Consistent slow speed running (what these engines are typically doing anyway) needs a glow plug with a bar. The bar keeps the plug from flooding out at low speeds. It may be that low speed with this engine is so low that its not possible to operate the glow plug without power at any achievable speed though.

Alan

[cfellows]

Depending upon engine speed, you should be able to run the engine with no power to the plug except to start it. Consistent slow speed running (what these engines are typically doing anyway) needs a glow plug with a bar. The bar keeps the plug from flooding out at low speeds. It may be that low speed with this engine is so low that its not possible to operate the glow plug without power at any achievable speed though.

Alan

Yeah, the engine won't run if I disconnect the battery.  I've tried it at low and high RPM with no luck.

Chuck

[Ramon Wilson]

Yeah, the engine won't run if I disconnect the battery.  I've tried it at low and high RPM with no luck.

Chuck

Chuck - What fuel are you running on? Though most visualise - including myself -  further ignition as keeping the glo pug hot for subsequent self sustaining ignition I believe it is the catalytic action of methanol acting on the platinum element that actually does this. No methanol .....

The other thing is the type of glo plug as there are many to chose from but a basic rule of thumb is a 'hot' plug goes in a 'cold' engine and vice versa.  'Hot' and 'Cold' engines basically referring to the type  - ie high and low performance. Hot plugs usually have a greater air volume around the element. On a very slow speed set up I would advise using a plain plug rather than the R/C bar type -

Control line aerobatic model aircraft engines of the traditional type were set to run on a slow, rich 'four stroke' in level flight breaking into a faster two stroke during manoevres. (These engines had no throttle, speed being directly set on the needle) R/C type plugs were tried to help keep the plug hot during this slow phase but proved for the most part unsuccessful - the bar apparently shielding the element. One of the most favoured plugs for this was the plain Enya #4

I'm not meaning to teaching granny here but if that's not the case then I hope this may shed a little light.

Regards - Ramon

[AOG]

I'm sorry Ramon but I must disagree with you about not using a bar type plug. Let me explain why.  This is a classic thermodynamics problem. To sum things up we have a thermal storage device that is charged with heat during the ignition cycle. (The glow plug). The the temperature in the plug decays at a fixed rate. The object is to complete the cycle and be ready for ignition before the plug cools down below ignition temperature. The missing element here is the length of the stroke. The longer the stroke the longer the time it takes to to complete a cycle. That means the longer the stroke the faster you have to run the engine to complete the cycle before the plug cools off below ignition temperature. Adding the bar to the plug acts as a heat reservoir that lengthens the time it takes for the plug to cool off. The impact of the stroke is negligible in model airplane engines because they run so fast. Please note that this is relevant to 2 cycle engines. 4 cycle is a whole different ball of wax.

Tony

[Jasonb]

I always thought that the idle bar was to protect the wire from the incomming mixture at low revs and help prevent it cooling, never seen one get hot enough to imagine it would pass that heat back to the wire and make it glow.

I would say Chucks engine too slow reving to keep any plug hot enough.

J

[AOG]

To be strictly technical the bar adds to the thermal inertia of the plug. It makes it slower to heat up and cool down.

Tony

[Ramon Wilson]

I understand what you are saying Tony but before anything can be established I think we have to know what fuel Chuck is using.

I have always been of the belief that glow plugs are designed to be used with Methanol based fuel for the reason stated - the catalytic action is what keeps the plug glowing - they are not, as I have ever understood it, a replacement for a hot bulb/hot wire ignition source on other type fuels. I'm quite happy to  to be corrected if I'm wrong though .

I perhaps have not explained my previous use carefully enough either. When I talk of setting a control line aerobatic engine to four stroke I am not talking of a four stroke engine per se but a conventional two stroke glow engine albeit one with the correct timing for this specific method.

I agree that used in this context the term four stroke is a misnomer but it is the term used throughout the CLA fraternity to describe a steady, rich but consistent RPM. This method of setting a two stroke glow has been used for many years to enable a steady but relatively slow level flight, the engine breaking into a clear two stroke to increase RPM and power as manouevres are begun. It is regularly referred to as a 4-2-4 run. Most commercial engines however simply will not run in this manner without modification to the timing. Those that will 'straight out of the box' are few and are well favoured. 

With the engine running rich in this fashion then the glow plug is cooled and can easily stop working if not the correct type. Hence the use of 'hot' plugs however despite what one would think the RC bar type plugs did not prove to help in this matter. Given this and  the relatively slow speed of Chucks engine then personally I do not think a bar type plug is the answer.

As already said I believe if Chuck is not using a methanol based fuel then I think that what ever type of glow plug is used it is unlikely that the element will self sustain. In that case a constant current would be required to keep the glow alight in which case then surely it would it not be acting as a glow plug but a hot wire.

A few years back I was dealing with an old 10cc spark engine that simply would not run well. A knowledgeable friend recommended fitting a glow plug - "if it will run on glow then it will more than likely run on spark" - the fuel had to be changed of course 

Regards - Ramon

[PatJ]

Not to diverge too far from the initial post, but I had a control line airplane as a kid (I still have that plane), and now that it is mentioned, I do recall that it was set to run rich in level flight, not firing on every stroke, and then in a climb it would fire on every stroke.

I loved control line flying.  The planes were super light and manuverable, and would fly just as well inverted as upright.

And I recall some people outfitting a battery and servo-controlled switch to their glow plug to turn it on during landing, when the engine was prone to stop at slow speed without the glowplug operating.

It is interesting that Chuck got the glowplug to run an engine without alcohol based fuel, but as he says, I guess it is like the hot bulb engines.

My dad had a John Deere model "H", and it had two fuel tanks.
The larger tank was for kerosene, and the smaller one was for the more expensive gasoline.
I tried to run it on kerosene, and indeed it did run very well on that fuel, if the engine was hot, and if you did not try to idle the engine on kerosene.

Pat J

[dieselpilot]

Platinum and several other metals use catalytic reaction with methanol to keep the coil hot. This is the concept on which glow engines work. However, methanol is not the only fuel that works and sometimes the reaction is not needed. There are examples of methanol fueled engines which use nichrome wires in glow plugs and platinum plugs running engine burning gasoline fuel without the need for current to heat the element.

Methanol is a great fuel, but it doesn't mix with most oils. Fundamentally ignition timing of a glow engine is controlled by air/fuel ratio, compression ratio, and RPM. You can add glow plug type(heating element dimensions) to the mix to fine tune things. In a hit n miss the condition will vary a lot between combustion cycles. I think current will be needed to help stabilize things, even with methanol fuel.

[Jasonb]

From the other site it looks like Chuck was using Colemans as the fuel.

J

[cfellows]

Thanks, all, for the discussion and good information.  As Jason said, the fuel I'm using is Coleman Fuel, which is petroleum naphtha.  That fact that I have to keep the battery connected isn't a big deal.  The engine is obviously stationary and I don't run it very long at a time.  The only concerns I would have are how often will I have to replace the battery and the glow plug. 

Chuck

[Noitoen]

There are some regulator circuits that help start glowplug engines and save the plugs. When started you can lower the voltage/current to increase it's life.

[Rustkolector]

Chuck,
It runs nicely on glow Chuck. but you better get a rechargeable battery. I have an engine that ran fine on a Coleman fuel, points, TIM6, hot coil, and rechargeable wet battery. It also ran fine on gasoline. I switched to a CDI with Hall sensor and AA cells to simplify the ignition and it would start and then run poorly and then quit. After much frustration, I went back to original set up and it runs fine again. Go figure! The quality of the spark evidently matters to some engines. I am still a fan of CDI since most of my other engines run fine on CDI systems. I know you like to experiment so you might want to try switching fuels. Maybe gasoline or propane. Or, try an old fashion buzz coil ignition. The cam wiper contact can be set to whatever spark duration you like.

Jeff