Semi-surface discharge spark plugs

[Rink]

I’m building a 4-cylinder, air-cooled, twin-plug, ~1.9 liter autocross engine. The cylinders are
Nikasill with JE Pistons. The pistons will be TB coated on the top and have anti-friction coatings on the
lands and skirts. The combustion chambers have been reworked to unshroud the valves
and the intake passages cleaned up. The intake valve is 42mm and the exhaust is 34mm,
both with 8mm stems. The CR target is 10:1 and rev limit will be 6500.

Unfortunately, there is some interference between the 10mm "extra" plugs and the piston
domes. I know how to accurately mark and machine the domes to solve this. However,
I have been wondering if a semi-surface discharge plug could solve the problem without
the extra machine work.

The NGK racing plug catalog- <

http://www.NGKsparkplugs.com/docs/racing_catalog.pdf>

shows several 10mm plugs that might work, including the R0045G-x and
R0045J-x, which come in a wide variety of heat ranges. I plan to use a Crane
Hi-6 ignition system with LX-91 coil (360mA) so I think I have enough spark.
I’m thinking that the plug ‘face’ will be flush or project 0.010" into
the chamber.

Questions: How will a surface discharge plug handle the 10:1 compression ratio?
Anyone have experience with these plugs. Or an opinion about their use in my application?
Any other issues to consider? Am I going to be singing "come on baby, light my fire?"

TIA

Rink

 

[reidh]

I have tested NGK surface discharge spark plugs in a lab environment before, and didn't have any problem with them. I don't have the report in front of me, but from memory I believe we found that a slightly greater ignition energy was required to fire the spark plug compared to a traditional plug with a ground electrode in the same engine, most likely due to the larger gap of the surface discharge plug.

-Reidh

 

[patprimmer]

So called "racing plugs" had a side ground electrode and both electrodes were contained within the tip of the plug. They may still be available.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Rink]

reidh.

Did your test conditions approximate higher compression ratios? And, tip proximity close to pistons? If not, any ideas about this?


patprimmer,

Yes, I'm aware of the Side or Wire electrodes. In fact some are shown on the NGK page that I referenced. Not as many heat-range choices, IIRC.

Thanks to both of you for your comments. Anyone else?

Rink

 

[thundair]

If my memory serves that the gap and voltage played against the dielectric strength of the mixture are all calculable.
Then again my collage courses were slide rule, drafting standards and beer.



I don't know anything but the people that do.

 

[kenre]

Hmm, you could try something i seen a few years ago that Saab were looking into. No earth electrode, the piston had a protusion for the spark to act on.

With enough ignition voltage, and assuming the piston top does earth out via the rings, cut the earth straps off and try it. By using different reach plugs you can set the "gap" by grinding down the centre electrode.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[FoMoCoMoFo]

Quote "The pistons will be TB coated on the top and have anti-friction coatings on the lands and skirts." could adversely effect the coatings. not that trying to weld the piston/ring/cylinder wall would otherwise be a good idea.

 

[reidh]

Rink:

We did experiment with slightly higher compression ratios, and our results logically pointed to an increase in required ignition energy as CR was increased, since it is harder for the spark to jump at a higher pressure. After the official test was over, one of our mechanics for fun increased the CR to the point that the spark plug actually suffered from flashover (where the spark travels from the top of the plug under the cap down the outside to the plug to ground). He then put in a standard spark plug and was unable to replicate the phenomenon.

We did not change the set-up with regards to tip proximity.

FoMoCo: I completely agree. Maybe that is one of the good reasons that Saab never put this into production.

-Reidh

 

[Lou Scannon]

One could imagine a flexible ground strap going from each piston to the block, but it's a little more difficult to imagine such a setup lasting 100,000 miles or more!

 

[Rink]

Thanks for all the comments.

I spoke to an NGK technical representative about 10mm surface discharge and fine wire plugs. It turn out there are a couple of problems that will prevent me from using these type of plugs. The most difficult one is the heat range available. NGK claims about 50deg/C tip temperature for each step of heat range. The rep. also said the tip need to be above 450dec/C to self-clean. The NGK surface discharge plug R0045J-9 is the hottest but still too cold. It’s also a 3/4” reach plug and I need 1/2”. It is possible that I could make a 1/4” spacer that would allow the use of the plug. I could then adjust that dimension to allow me to precisely place the “tip” in the chamber and change the heat range as well (making it a little hotter). But I would have to run the engine to determine that and if not suitable would have to disassemble and machine the piston domes as I was trying to avoid. Too many variables for me. I’ll just notch the pistons and run a conventional plug, NGK C7HSA, which is 2 steps or ~100deg/C hotter, and only $2 ea., and be done with it. Also, those surface discharge plugs are $20 each.

The fire wire plugs (R0373A-9) have some of the same problems but also project more into the chamber and cost $81 each. No thanks.

The suggestion to fire plugs to the pistons is silly in my opinion. Maybe okay for a 1500 RPM engine with very little ignition advance. In any case I would not consider trying this for this particular application.

Rink

 

[Rink]

Whoops, meant to say fine wire plugs, not fire wire plugs.

Sorry,

Rink

 

[thruthefence]

Regarding the piston providing the ground, Wouldn't the timing be very retarded on such a set up?

 

[FoMoCoMoFo]

or a big gap that gets bigger as timing advances.