Electronic headaches

[Parksy]

Hi all

If I have 2 dual output coils running four spark plugs, both coils being run by a driver each but all share the same common ground and power supply, is it possible that while one coil fires, a stray voltage could be sent to the non firing coil?
One of my coils has stopped working and it's possible the second is also damaged and I'm trying to work out why.

If these stray voltages are occurring, is there anything I can do to prevent them?
A diode on each driver possibly to keep the power source 'independent'?

Thanks all

Andy

 

[Blogwitch]

Hi Andy,

I would have a chat with Julian at Minimag as you have already proved that one of the coils is u/s, and it looks like it has been from the get go because of your original starting problems.

He is a very understanding person and I am sure he will sort it out for you.

Ask him the same question you have shown above as well, if he doesn't know, I doubt if anyone will.


John

 

[Parksy]

Good idea john

I've sent him an email.

 

[Brian Rupnow]

I have a dual spark coil from a snowmobile on one of my twin cylinder engines. It is a "waste spark" set-up, but the path the electricity takes is very strange if somebody doesn't make you aware of it. Each time it fires, the electricity comes out of one coil wire, travels down the center electrode of the sparkplug, jumps the gap to the outer grounded shell of the sparkplug, travels thru the engine block to the grounded outer shell of the second sparkplug, then jumps from the outer grounded shell of the second sparkplug to the center electrode of that plug, then follows the second spark-plug wire back into the coil. If you remove one plug wire to see if the engine will run on only one cylinder, then the engine won't run at all because neither plug will fire.

 

[Parksy]

Thanks for explaining that Brian. The coils do come with a schematic of the inside windings and I noticed that both high tension leads are open. I had a suspicion that what you just explained above was the case, but I wasn't sure.

Cheers!

 

[mcostello]

Why must things be so complicated?

 

[Parksy]

i got a response from Minimag and basically there will not be any arcing or any problems running two coils with the same earth and power supply.

Maybe it's something simple like a battery that can't supply enough current? The spark plugs fire fine when outside of the cylinders, but I'm led to believe under compression, they can behave differently?

 

[Brian Rupnow]

I have seen that, but it is quite unusual. If they are new plugs, and you have the electrics set up right, they should fire under compression. Personally, I don't care for Hall effect switches triggered by a magnet. It is far too easy to turn the engine over with a plug laying out on the cylinder head to see that the spark is coming at the right time and if the sparkplug doesn't have a really good ground, it burns out the hall effect switch. Old style automotive points systems don't care if the sparkplug has a good ground or not.

 

[mayhugh1]

Parksy,
A sparkplug in a cylinder under compression will always require a higher voltage to fire than the same plug with the same gap in a cylinder with no compression. Here is a link to an example worked out using some elementary physics which deals with the issue.

http://www.homemodelenginemachinist.com/showthread.php?p=216940&highlight=Paschen#post216940

In this example, a .018" gapped plug requires 8 kV to fire at about 75 psi but only 2.5 kV at atmospheric pressure. These numbers would come from an engine with about 5:1 c.r. or so.

Your waste spark system takes advantage of this phenomenon. The two plugs are in series with the coil(s). The plug that's not under compression fires at a much lower voltage than the one under compression leaving a majority of the coil's energy available to the plug under compression. - Terry