Shop built sparkplugs for model engines

[Brian Rupnow]

And here we have the finished sparkplug. It works, just as I figured it would. I'm not quick enough on the trigger to get a single picture of the spark when it occurs. That is why I made the attached video.---Brian

 

[Carbuilder]

I have yet to make a sparkplug, but it's on my list of things to do. Having read about different types of insulator materials, I am wondering if anyone has tried PEEK or some of the other high temperature plastics? PEEK in particular seems to be a good candidate, and while not cheap seems affordable in the small quantities required.

I made 1 spark plug for my Little Demon so far and used PEEK. Bought it from McMaster Carr; 1/4" diameter rod is pretty cheap. It machines nicely, is slightly flexible, not brittle at all. No idea how it will work, but I think it should be OK.

 

[Carbuilder]

Nice sparks there Brian!

 

[Steamchick]

Unless you are running "car" distances, there should be no need for a tungsten tipped electrode. Steel is OK: Piano wire is high tensile steel. A side electrode allows the spark to be located at a particular point, and closes the gap where it would otherwise be too large (and cause flash-over on the insulator - not a good place to try and ignite the gas!). A centre earth (like on a car plug) may project too far into the combustion chamber on some models.
It is a combination of geometry and getting the optimum spark location for firing the mixture.
The insulator shape is quite critical on car engines - etc. - for whatever purpose, as the shape determines the heating (to burn off combustibles) and cooling (so it doesn't melt!). Get that wrong and suffer problems of running or a holed piston.
I know a CZ engine (2-stroke motorcycle) that would start with ANY spark plug, but stopped in seconds to minutes, depe3nding on which plug of 6 was fitted - until the correct heat of plug was fitted, when it ran for thousands of miles.... The owner thought it "should be OK" with a similar plug - just a single grade "cooler running" often called a "hotter" plug, as it runs in "hotter-running" engines!). - lesson - do as the book says... I solved it for him by cleaning the original (rusty) plug - as it was the correct grade according to the book... He then bought a new plug of the correct grade, instead of trying everything else in his junk box!
So get the HEAT RANGE shape correct.
K2

 

[Steamchick]

PEEK may not withstand combustion temperature, at tip temperatures of the insulator that are high enough to burn off any fouling. IE. around 400deg.C...? Try domestic-fuse-body ceramics?
Pure PTFE may work, as it ablates when it gets hot. I just can't believe it will be durable though. And must be clamped into the steel body, as it does not stick to any adhesive (chemical adhesion). - Only fried egg in the frying pan...
K2

 

[Carbuilder]

Corian, from the specs I can find, is not good for anywhere near 400C (212F seem to be the max recommended temperature), but seems to work just fine. I suppose the heat is being removed from the plug faster than the combustion temperatures can heat it up. PEEK should resist higher temperatures than Corian.

 

[Brian Rupnow]

And, as a fitting end to this post, the engine that this sparkplug was intended for is up and running very well.

 

[almega]

That is spectacular. Just out of curiosity, what prompted you to make the "$300" spark plug for this engine as opposed to purchasing one as you had suggested?

 

[ranger]

Found these in a ’Tin’ I kept ‘glow’ plugs in. The ‘NGK’ plug is obviously a purchased one, the other is one made from the body of a burnt out glow plug, a piece of piano, (music) wire And either a piece of Epoxy ‘Stick’ or ‘JB Weld’. I can’t remember if I made a mould for the JB Weld, or used the epoxy stick and ‘Finger’ formed it. The insulator is flush with the end of the body, and sanded flush after curing. This ‘Surface Discharge’ plug worked ok in a modified two stroke aero model engine (Methanol and 20% Castor oil without fouling). The NGK plug would foul. Another possible benefit of The surface discharge plug in a very small engine is no excess volume inside the plug to affect compression etc. If you know people who are into ‘Proper’ RC aircraft, (not electric), or cars, you shouldn’t have a problem obtaining a regular supply of ’burnt out’ glow plugs, saves making the bodies!

 

[Brian Rupnow]

I did first buy a sparkplug from gentleman who sells them on the internet. He makes good quality sparkplugs and will make "specials" on request. I could get the engine to fire and run with one of these purchased plugs in it, if I was running a Viton o-ring on the piston, but not with cast iron rings. After making and installing my cast iron rings, the engine wouldn't fire. I checked everything I could on the engine, and compression was good, ignition timing was good, gas tank was full---everything was perfect---except that the engine wouldn't fire. The only thing left that it could possibly be was the purchased sparkplug.-So---I built a sparkplug. And I still couldn't get the engine to run--and so I decided one more time to reset the valve timing. That fixed it. The engine ran.---Probably if I had checked the valve timing before building the sparkplug, it would have ran with the purchased sparkplug.

 

[stewart drummond]

surface discharge , no separate earth electrode tip , good luck

 

[chevell69]

So, now we have all of the parts for a sparkplug except for the Corian. I will turn the piece of Corian later today or tomorrow.

do you have to use corian can u use delrin

 

[Oldiron]

If you want tungsten how about a TIG electrode?

 

[Brian Rupnow]

I didn't want to use a tig electrode. I could have, I have a package of them for my tig welder. I also have two 36" lengths of music wire, which is considerably cheaper than tig electrode. As far as using Delrin is concerned, I know two people making and selling sparkplugs, and they use Corian. Why would I stick my neck out and use something that I had never seen anybody else use?

 

[Brian Rupnow]

Okay kids and turtles--Here is a strange observation. From the brass cap to the base on this spark plug is 1/2". When the engine is running, it runs good for a while, then begins arcing between the brass cap and the steel body along the outside of the Corian. The gap between the center electrode and the steel body is only 1/16" at the tip, down inside the cylinder. It will arc two or three times in a row, and the engine falters and stumbles when that is happening. Then it will stop arcing and the engine will resume running normally again. I guess the solution to that would be to make another sparkplug 1/4" longer . I doubt that it would arc thru a 3/4" air gap.

 

[Gordon]

Does the Corian become slightly conductive when it gets hot? Regular spark plugs are porcelain or similar.

 

[almega]

Once it did its arcing thing did it repeat that? If not, is it possible there was some contamination on the outside of the insulator that acted as a conductor until it was burned off and then the engine performed as expected?

 

[Jasonb]

If you look at all the other spark plugs pictured in this thread and the other commercial ones like Rimfire none have the brass endcap overhanging the insulator. Spark is less likely to want to jump around a corner. Look at your design again rather than just making it longer.

It is always harder for a spark to jump in the compressed gas than outside the engine plus if the plug is oiling up then the spark will take the easy route. Your straight wire discharging to the plug body will also mean a larger gap than a bent wire or the usual bar on the body so spark has more difficulty jumping the bigger gap in the combustion chamber so more likely to jump outside.

 

[stevehuckss396]

1/16 is a huge gap. Gap should be about half that or less. Under compression it is very difficult to get a spark to jump a gap. With a gap that big it must become easier to jump the half inch gap. Electricity always takes the path of least resistance.

 

[Stefan-K]

Jasonb i totally agree. That's exactly what happens here. I would also remove the overhanging brasscap. Instead i would drill a small piece of threaded rod, maybe M3 and solder the music wire into it and then glue the whole Thing into the plug.
Most People forget that you need much more energy to make a spark jump a gap in a compressed Gas than outside in atmospheric pressure. High voltage will always take the easiest way. And half an inch is no Problem for a couple thousand volts.
Stefan