By Jupiter

[Vixen]

Thank you all for calling in, it is always greatly appreciated.

Glazed porcelain ceramic is today's standard insulator material for full size spark plugs, Mica was common early last century when the Bristol Jupiter powered most European aircraft.
Unfortunately porcelain is not a material which lends itself to small batch, small size model plugs.

There are several alternative materials used by model engine builders.
Macor is a machinable ceramic which works well, but it is fabulously expensive and out of reach for the number of plugs that I require. 
Corian, a mineral powder (stone?) filled acrylic plastic is commonly used and works well in small 1/4 " spark plugs. I have made many with this material.
PTFE (Teflon) is another high temperature plastic which works best on small spark plugs. It is a soft waxy material which can flow under pressure.

The model Jupiter is a big engine and will produce a lot of waste heat; which has to go somewhere. I worried that Corian or PTFE insulators (which are both plastics) would become eroded or damaged by the extra heat from the larger capacity cylinder. I looked for an alternative material for the 'hot end' of the plugs. Graham Meek has a cleaver design for model spark plugs which use a ready made ceramic bead at the 'hot end' and a plastic insulator for the cooler outside end. My Jupiter spark plug design was based on Graham Meeks teachings, but modified to meet my design requirements.

Precision molded thermal insulating ceramic beads, often referred to and known as 'fish spines', are frquenty used to insulate the wire of ovens and heat treatment furnaces. These 'fish spine' ceramics are readily available from electrical distributors such as RS Supplies and are available in 4 mm and 5 mm diameter.  The 4mm 'fish spine lends itself to 1/4" plugs, I chose the larger 5mm 'fish spine for the larger 5/16" spark plugs for the Jupiter.

When the design work was completed I started by turning the Corian outer insulation. I was given some Corian offcuts by a friendly posh kitchen installer a few years ago and have been storing them 'lust in case' ever since. I had the choice of white or sky blue material in 12mm strips. I chose the sky blue as being similar to the colour of KLG spark plugs. I made a start by bandsawing the Corian stock into convenient square blocks. Each block was held in the four jaw and turned into round stock. The round stock was then turned and drilled to the finished dimensions while being held accurately in a collet. I found it useful to tape over two of the jaw adjusters of the four jaw, so each piece of square stock was in more or less the same position for turning, this saved a lot of time. Anyone who has machine Corian or any other acrylic (perspex) material will know all about the enormous mess of fluffy swarf it creates. Always best to set up a vacuum cleaner to suck up the dust as you go.







This next image will explain everything, I hope. The top row shows all the parts of the Jupiter spark plug. At the top left, is the cream coloured 5mm ceramic 'fish spine' bead. Next along is a tapered PTFE piece which gas seals the inside of the plug when fully assembled and tightened. Next along is the brass central conductor. the 3mm diameter section in the middle sets the distance between the ceramic 'fish spine' and the Coria outer insulator. The brass conductor is turned to 2.5 mm at either end. The very tip of the conductor is further reduced to provide a 15 thou ( yes I know, I am always jumping between inches and mm) spark gap between itself and the two earth points machined on the steel plug body. At the right end of the top row is a 4mm stainless steel ball which will connect to the plug cap. These stainless steel balls come drilled and taped with a 1.6mm thread. They are available in bulk (30 pieces) from suppliers of body piercing hardware. Buy them in bulk as there is little difference in the price of a handful to the price of a single steel ball.

The second row shows the three parts of the insulator assembled on the brass conductor. A fully assembled spark pug completes the bottom row




There were a lot of pieces to make and assemble for my batch of thirty spark plugs. But will they work? The only way to find out is to test them individually.






Ech plug is being subjected to a internal pressure test to confirm the PTFE seal is fully compressed and doing it's job. I applied 8 Bar (120 PSI) from any shop compressor and checked for air leaks by submerging the plug under water. I found I needed to anneal and soften the copper washers before they would seal. I think they must have become work hardened when the were punched out. Not a very exciting image but it does show the plug surviving an internal pressure of 8 Bar (120 PSI). That test pressure is about equal to the expected Mean Effective Pressure MEP but a lot less than the peak pressure; that needs real engine running conditions



Each plug will then be given a basic electrical insulation test before being mounted in a 14mm adapter and given a test run at full EHT and cylinder pressure on my McCulloch chain saw. It's the only petrol engine I have with a spark plug aperture large enough to take  my 5/16" model plugs. I went over to diesel powered cars many years ago and could not find a single 14mm spark plug in the workshop to be modified to accept the smaller model plug. I had to go and scrounge one from the local garage.

I have still to do the engine running tests but am quietly confident that at least some of them will work. Another item potential crossed of the seemingly endless 'Still to do' list

Special thanks to Graham Meek for his help and support

Stay tuned

Mike

[Johnmcc69]

 
 Very nice work Mike! Great looking plugs!

 John

[Roger B]

Excellent    That's a job that shows the advantages of CNC (I know I need to make the leap one day  ) I hope the test runs are successful     

[Ye-Ole Steam Dude]

Hello Mike,

That work is fantastic.

Have a great day,
Thomas

[Vixen]

Hello Roger, Thomas.

Yes, a small CNC lathe is an excellent addition to any model engineering workshop. Studs, nut and bolts are almost a joy.

My Emco Compact 5 CNC is a standard 'mini lathe' with the addition of two ball lead screws and two stepper motors. My little lathe is coming up to forty years old, I have used it most days for the last 25 years. I completely transformed it into superb little machine, a few years  back, when I interfaced it to a PC running LinuxCNC control software. It will run G-code programs or operate under manual control. The keyboard keys replace the handwheels. I use the manual mode 50% of the time for simple turning.

Provided you chose free cutting materials, it works well and is accurate. Last week we had the hottest days of the year, I was using a ceramic bead as a Go-No Go gauge, I found I had to correct the diameter by a thou every hour or so as the room and the machine temperature climbed higher.

I have to go "Up North" for a few days next week, so the assembly and testing of the plugs will have to wait a while longer.

Mike

[Vixen]

I have returned from my short trip 'Up North' and have been able to complete the assembly and testing of the complete batch of thirty 5/16" sparkplugs.

Each plug was subjected to a compressed air pressure test to 120 psi. This being the maximum pressure my compressor can reach and is also about equal to the expected Mean Effective Pressure (MEP) for the engine. The individual plugs were held underwater to check that the PTFE gland was properly compressed and sealing correctly. Fortunately there were no leaks or bubbles.

Each plug was given a simple test with a multimeter to check that there were no unwanted short circuits in the electrical insulation. The whole batch were then given a high voltage spark test to further check the insulation. I used a piezo spark igniter from a domestic gas fired cooking hob to check each plug in turn. An intense high voltage spark is generated every time the square black button is pressed.

There was an interesting phenomena with the twin spark gap configuration. The spark would arc across to one spark gap until the surrounding air became ionised,  the spark would then favour the other spark gap for a few sparks before returning to the first. The spark would alternate between the two spark gaps, if the piezo igniter was pressed repeatedly.

 


All the plugs passed the pressure and high voltage tests. I thought it would be a good idea to also confirm their operation in a real working engine. The only petrol engine I have with a 14mm spark plug is a small two stroke McCulloch chain saw. I quickly made a 14mm thread adapter for the miniature 5/16"plugs.





This test was inconclusive, therefore not a success. I warmed-up the McCulloch to normal operating temperature using the standard 14mm spark plug, I quickly swapped over to the 5/16" plug. The engine started and ran for about 15 seconds then slowly died. I found the miniature plugs were wet with petrol and oil mix, which killed the spark. The miniature plugs are well hidden in the adapter unlike the 14mm plug which protrudes well into the combustion chamber. This may be the reason the plugs oiled-up so quickly.

I am left with a dilemma, Was the failure to run, due to a failure of my new plugs or was it due to the inherent problem of wet, oily combustion conditions found in all small two stroke engines. I would still like to prove the new plugs will work, so I believe I now need a clean running, single cylinder, four stroke test engine. Anyone got a small motorbike, I could borrow?

Stay tuned

MIke

[steamer]

Lawn Mower......send some over....I'll run them in that and the generator...

Dave

[Vixen]

 

Maybe I will come over with them

Mike

[steamer]

Maybe I will come over with them

Mike

Maybe you should!!!!

[mike mott]

Lovely work on the spark plugs Mike.

Mike

[Admiral_dk]

Mike if it's possible in any way - please consider to modify the adaptor so the small plug has it's thread flush with the adaptor, before running any further tests.

If you do the test on a four-stroke, make sure that the adaptor and plug has the same reach as the original.

Another way would be to make a test chamber with a clear window. that way you can presurize the camber and see if the spark jump as it should.

Best wishes

Per

[Vixen]

Hello Per,

Some good advise there, thank you.

Unfortunately, the diameter of the new 5/16" plug prevents it from being positioned any deeper into the 14 mm spark plug adapter. The model spark plug is already as deep as it can go into the adapter. So I am stuck with the short reach 14 mm plug adapter and therefore will have to use garden machinary for the tests. My neighbour has a four stroke petrol engine lawn mower, I will talk to him over the weekend.

Mike

[michelko]

Hi Mike,
Looks like you are running in the same problems like me.
The bugatti is still sitting on the desk in my shop waiting to see some modification.
Hope you sort out thr plug problem.

Michael

[Vixen]

Hello Michael,

Maybe we do share a common problem.

Both of our designs feature a ceramic insulator at the hot end and corian insulation to the rear. This implies there is a joint between the two insulators hidden inside the spark plug. I have carefully avoided the use of JB Weld in my design, instead I use a PTFE cone to create the pressure seal.

I am hoping to fully test each one of my 5/16" spark plugs, including running each one on a full size 'mule' engine, before I try them on the model. Always best to eliminate as many uncertainties as possible.

So far, I have successfully tested each of the new plugs to confirm they seal. at 120 psi ( 8.25 bar). I have also successfully conducted basic insulation tests including high voltage tests using a piezo gas hob igniter. I am currently trying to find a suitable full size 'mule' engine to prove each of the thirty spark plugs will run consistently. The first full size engine that I tried was a very wet and oily two stroke chain saw. The test plug fouled with oil and stopped working after about ten seconds. I am currently trying to find a 'dry' four stroke engine to repeat the tests.

I will let you know how I get on.

Regards/ Grüße

Mike

[Admiral_dk]

I just "repaired" a lawn sit-on mover last Sunday. Same Thing - nice spark with the plug outside the engine and only splutter and coughs when mounted in the head 

When I look at your otherwise nice parts for the plugs, I can't help thinking that I would suspect them not to work from appearance .... Why ?

Well in order to make sure that the spark can't "fire inside the plug", you will have to have the insulation "overlap" in such a way as to create "a labyrinth" when they match - or put in other words - they must fit inside each other - just meting end to end is not good enough, as the spark will run between the parts to ground (the outside metal part).

Sorry if this isn't a nice answer after all your beautiful work Mike, but I'm afraid that you won't get a satisfying result otherwise and please ask Roger (our resident Very High Voltage expert) for a second opinion.

Best wishes

Per