Spark Plug Life Expectancy??

[kcmillin]

I am wondering what to expect out of a homemade spark plug.
I recently made the "Tiny IC" and made a plug for it.

I made it out of leaded steel with acetal, or delrin insulator, and a piano wire electrode.

It has seen about 12 or more hours of operation, and it has been cleaned a bit. I am now having trouble with the spark shorting between the head and the plug end. This means that the spark is not occurring in the cylinder and stopping the engine from running.

Do these plugs loose their ability to work over time like a store bought one?

Seems kinda strange it stops working every now and then. More so now than then.

I had a hunch that it is getting fuel and oil caught between the electrodes, but it worked so fine for the first few hours, maybee its just bad?


I am kinda rambeling here so I will leave it at that.

Kel

 

[gbritnell]

Hi Kel,
When I first started making IC engines there were no commercial plugs available so you had to make your own. At that time people generally made them from Teflon or Corian. A few made some from machinable ceramic. My first multi cylinder engine was my OHV 4 cylinder engine. I made a set of plugs from Teflon and have about 20 hours on them. This engine will rev to some very high rpm's and I have never had a breakdown from the plugs.
When I first started making plugs I never machined a ground strap on them. The spark would jump from the center electrode to the body of the plug. This would cause the Telfon to erode over time but it still worked fine.
If the spark is jumping from the end of the plug to the engine cylinder head it means one of two things, the mixture is too lean or the compression is too high. The reason for my answer is because if it will jump to the head , which probably is at least .250 away but not across a .032 gap there is plenty of spark available. It's just that something inside the combustion chamber is preventing it from sparking.
The best way to check what's going on is to replace the plug and see if that helps.
gbritnell

 

[kcmillin]

Thanks For the insight gbritnell.

I used the acetal because it is all I have. It seems to be working fine. I have not yet seen the engine get hotter than 150 degrees so I am well within the heat range.

The tip of the plug is about 3/32 away from the rocker arm, so that is is pretty small gap.

So today I took a small piece of silicone fuel tubing and put it over the end of the plug and now it works like a champ ;D a pretty simple fix I should have though of earlier.

So now I am going to try and make a tiny spark plug boot out of silicone caulking. Its called OSI vp 275. We use It at work (siding) it is made for vinyl siding, but once cured is mighty strong, and I'm sure it can withstand 150 degrees or so.

Thanks again for the advice.


Kel

 

[putputman]

Kel, I noticed when I had the fuel mixer too rich, it tended to force fuel up the plug and would arc from the head of the plug to the valve linkage. As soon as I leaned it out a little it would quit arcing. It is something you might try.

The problem with Tiny I.C. is that you have to control the speed by setting the fuel mixer a little rich. I am afraid to lean it out too much as it runs so fast it will probably self destruct.

 

[kcmillin]

Arv, I am thinking about a throttle of some sort to vary the speed.

In response to the self destruct comment, I have had mine running tonight for about 3 hours at 6000 rpm average, the only real wear is the connecting rod journal. The main bearings are getting a tiny bit sloppy too.

I really love to run the heck out of my engines.

As it is running it tends to "sputter" or dog down for a half second every now and then. I can see the connecting rod moving in a way in wich it should not be and a loud clinking can be heard.

Kel

 

[WonderousMountain]

Hey machinist,

The project has possibly 35mm diameter cylinder with preferably one 15-20mm tappet valve. Since it's suppose to be functional, long running engine under moderate to heavy load. What are the better commercial models for best TBO and consistent firing? Design Rpm 6,000 common high octane unleaded fuels. Ignition control will be lightweight mechanical-unsophisticated.

One of the Goals is to use off the shelf components whenever possible.

Thx,

Mountain

 

[doc1955]

Kel
That is what I use for my plugs as it is easier for me to get. The last engine I built (the Atkinson) has quite a few hours run time on it now with no problems ans the compression is now pretty good now that the rings have seated real good and still have had no problems with the plug. I have the same material in my odds and ends engine and it has many more hours on it with no failure. I did have to clean it on the odds and ends engine once but I was running her pretty rich since I run her a lot leaner I have not had to clean it. I haven't taken it out in a long time to see what she looks like. I figure its working good don't mess with it. :big:
Anyway I just wanted to say I have had good luck using Delrin material.

 

[kcmillin]

Thanks Doc, it is nice to know that the Delrin is working for you. With my Tiny Inline I used Delrin for the spark plugs. However, with the hard use, high rpm and load, the delrin would burn away. I have had better luck with corian, but I am currently researching ceramic paste material, and thinking I can "Cast" the insulator. I have sent e-mails to a few companies who specialize in ceramic fixing paste, and two part compounds regarding the spark plug situation, I have yet to receive a response.

Kel

 

[gbritnell]

Hi Kel,
McMaster-Carr sells a machinable ceramic but it's pricey. I started out using Teflon and have used Corian and other materials along the way. I've had the best luck with Teflon.
gbritnell

 

[kcmillin]

Thanks George. Teflon id the one thing I have not tried yet. I have thought about the machinable ceramic, but ya, it is quite expensive. A guy at work found a product from http://www.cotronics.com/vo/cotr/ that might do the trick. One of the adhesives I think (I can't find the exact one on their website) but he told me it was around $40 for a 16oz tube. Don't quote me on that though. That would be a whole lot of tiny spark plugs.

BTW, this is one of the companies I sent an e-mail to with no response.

Kel

 

[doc1955]

McMasters have casting materials that maybe stable if you want to cast the insulator.

High Temperature Ceramic Casting Compounds

 

[GailInNM]

I have one Corian (acrylic) based plug that has over 500 hours on it. I also had one fail to an internal low resistance path that developed after about 20 hours. I have run 12(I think) plugs of this design.

I looked at the Cotronics ceramic adhesives, which is also what McMaster is selling, and all of them have relatively low dielectric breakdown voltages. Most are around 200 Volts/mil with the best of them being 270 Volts/mil. On my plugs the insulating electrode holder part is 0,.078 inch diameter with a 0.032 center electrode. 0.078 - 0.032 = 0.046 and dividing that by 2 gives a wall thickness of 0.023, or 23 mils. At 200 volts/mil this is a breakdown voltage of 4600 volts if the electrode is well centered. This is too low to be useful. Delrin (acetal) is about 500 volts / mil and the above calculations gives about 11,500 vollts. Corian (filled acrylic) is about the same. Teflon (PTFE) is about 1500 volts/mil yielding 34,500 volts.

I did the calculations once upon a time for 0.025 gap and 6:1 compression ration and as I remember it come out to about 6000 volts for the spark to start. That's from memory so don't rely on it too much.

Attached is a dielectric breakdown chart for some various materials.
Gail in NM



View attachment dielectric strength.txt

 

[fcheslop]

Hi Kel, i use fuses for the insulator when making rim fire plugs.
best wishes Frazer