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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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 Posted: Sun May 27, 2012 11:18 pm Post subject: Spark duration effect on 'wasted' cylinder? |
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Hi, I've been reading a lot about the Electromotive ignition / coil setup...
What I've seen is that the typical spark duration time is 2mS.
I understand the benefits of having a longer spark duration for the primary cylinder, however, it seems to me that the long spark duration also applies to the wasted cylinder. As such, what happens when the 'wasted' cylinder starts its intake cycle, and the spark plug is still firing?
For example, if my engine is running 30° ignition advance at 6000rpm, then the 2mS of spark duration is 72°. So this means that the 'wasted' cylinder would be firing until 42°ATDC... Wouldn't this ignite the incoming intake charge?
Furthermore, what if I have an aggressive camshaft, which starts the intake event well before TDC - wouldn't this cause even more issue with the 'wasted' cylinder?
Thanks, |
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stevieturbo
Joined: 29 Aug 2006
Posts: 869
Location: Northern Ireland
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| Posted: Mon May 28, 2012 3:24 am Post subject: |
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It fires on the exhaust stroke.. There would be no intake valves open.
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Mon May 28, 2012 3:48 am Post subject: |
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I guess you didn't really read through my post...
Yes, when the ignition first starts, on the primary cylinder it will fire during compression (as I gave an example of 30° BTDC), the secondary cylinder is in the exhaust stroke. The secondary cylinder is also 30° BTDC (because the cylinder pairs are on the same throw of the crank). So once the piston is at TDC, the primary cylinder is on power stroke and the secondary cylinder is now on the intake stroke... With a long spark duration and higher RPMs, that means the plug is still firing for both cylinders, primary on power, and secondary on intake.
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JimK
Joined: 30 Aug 2005
Posts: 67
Location: Denver
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Pantera EFI
Joined: 12 Feb 2005
Posts: 1283
Location: So. California
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| Posted: Mon May 28, 2012 8:48 am Post subject: Cylinder Ignition at Valve Overlap |
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Most older motorcycle engines were "waste spark" firing.
At start-up some would miss-fire causing breakage, even a leg.
A running engine with waste spark CANNOT re-fire the cylinder at Valve/Port Overlap as the cylinder is IN-FIRE, the same as throwing a match on a bonfire.
The MAIN problem is the use of "twin post" coils on high output engines.
One cylinder (plug) will fire positive, the other cylinder (plug) will fire negative.
Many EMS can fire "waste spark" with "single post" coils, all with correct polarity.
Lance |
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Mon May 28, 2012 11:17 am Post subject: |
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Thanks!
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Mon May 28, 2012 11:18 am Post subject: Re: Cylinder Ignition at Valve Overlap |
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| Pantera EFI wrote: | Most older motorcycle engines were "waste spark" firing.
At start-up some would miss-fire causing breakage, even a leg.
A running engine with waste spark CANNOT re-fire the cylinder at Valve/Port Overlap as the cylinder is IN-FIRE, the same as throwing a match on a bonfire.
The MAIN problem is the use of "twin post" coils on high output engines.
One cylinder (plug) will fire positive, the other cylinder (plug) will fire negative.
Many EMS can fire "waste spark" with "single post" coils, all with correct polarity.
Lance |
Lance, thanks for your reply - but I'm not concerned about the primary cylinder. My concern is spark duration overlapping the waste cylinders exhaust to intake stroke...
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Thu May 31, 2012 1:42 am Post subject: |
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Ok, so I got the word directly from Electromotive:
| Quote: | | This question comes up now and again, but from my experience the only time it's an issue is with gaseous fuels (like propane and natural gas) and then, only during start up. I think that's because the wasted spark has very little energy in it and the gaseous fuel requires very little energy to light. The wasted spark has less than 10% of the energy of the spark in compression so it doesn't really cause an issue because there isn't enough energy to actually ignite the fuel during the intake stroke. |
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alexchevelle1
Joined: 03 Aug 2009
Posts: 27
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| Posted: Thu Aug 09, 2012 1:30 pm Post subject: |
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I never experienced troubles on my TT SBC running 25° under boost.
But we built an M5 with Link ECU running waste spark. To keep torque under control we retarded the ignition to 13° under boost and we had some troubles igniting the incoming charge around peak torque. We went back to 16° and had no more problems.
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RootesRacer
Joined: 04 Apr 2005
Posts: 464
Location: Arvada, Colorado
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| Posted: Thu Aug 09, 2012 3:25 pm Post subject: |
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| alexchevelle1 wrote: | I never experienced troubles on my TT SBC running 25° under boost.
But we built an M5 with Link ECU running waste spark. To keep torque under control we retarded the ignition to 13° under boost and we had some troubles igniting the incoming charge around peak torque. We went back to 16° and had no more problems. |
The spark duration is going to vary with cylinder pressure.
High cylinder pressures will require a higher ionization voltage to strike the arc and a higher static voltage as the plugs stay ionized.
What may yield several ms at idle chamber pressures may only be 100us or even less at high load pressure.
When you start getting to the point where the ionization voltage for that chamber pressure is near or greater than the coils output potential, the energy is dissipated very rapidly and it may not even light off the mixture which of course leads to a misfire event.
I suspect you needed to retard your timing beyond optimal in order to spark into a lower compression pressure and successfully light the mixture. I would also suspect that a higher energy ignition could have made a difference and taken up some of the power you left on the table due to lack of optimal advance. |
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John at J&S
Joined: 12 Nov 2005
Posts: 270
Location: GARDEN GROVE, CA
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| Posted: Sun Aug 12, 2012 7:34 am Post subject: |
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Also, half the plugs are "firing backwards".
Single fire coils apply negative high voltage to the center electrode of the plug. The center electrode is hotter than the ground electrode, and emits electrons more easily, due to thermionic emission.
With waste spark coils, positive high voltage is applied to the center electrode of half the plugs. It's harder for electrons to jump from the relatively colder ground electrode to the positively charged tip. |
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Thu Aug 16, 2012 6:44 pm Post subject: |
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| RootesRacer wrote: |
The spark duration is going to vary with cylinder pressure.
High cylinder pressures will require a higher ionization voltage to strike the arc and a higher static voltage as the plugs stay ionized.
What may yield several ms at idle chamber pressures may only be 100us or even less at high load pressure. |
Agreed.
| RootesRacer wrote: | | I suspect you needed to retard your timing beyond optimal in order to spark into a lower compression pressure and successfully light the mixture... |
Retarding timing (on the BTDC side) to closer to TDC would be a higher compression pressure, not lower.
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Thu Aug 16, 2012 6:45 pm Post subject: |
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| John at J&S wrote: | Also, half the plugs are "firing backwards".
Single fire coils apply negative high voltage to the center electrode of the plug. The center electrode is hotter than the ground electrode, and emits electrons more easily, due to thermionic emission.
With waste spark coils, positive high voltage is applied to the center electrode of half the plugs. It's harder for electrons to jump from the relatively colder ground electrode to the positively charged tip. |
John,
I'm sure there isn't an exact answer, but how much harder is it for the plugs jumping ground->center than the correct way?
Is it enough to warrant running different heat-range plugs?
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John at J&S
Joined: 12 Nov 2005
Posts: 270
Location: GARDEN GROVE, CA
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| Posted: Thu Aug 16, 2012 6:51 pm Post subject: |
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| I will ask Ulf to comment. |
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Pantera EFI
Joined: 12 Feb 2005
Posts: 1283
Location: So. California
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| Posted: Fri Aug 17, 2012 10:50 am Post subject: Twin-Plug with Waste Spark |
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AGAIN Ant, "spark firing at overlap is the same a throwing a match into a bonfire".
YES, the coil "rise time" will change with reverse polarity.
I WAS taught how to test coil polarity (spark electron flow direction) at an age of three years using a pencil.
My father had an inline four motor cycle engine with a week cylinder.
THE COIL WIRING WAS REVERSED ON THE POOR CYLINDER.
Today with much stronger coils this is an advantage on "twin-plug" engines.
The Porsche 12 plug six setup is to use the REVERSE sparkplug wires on the bottom plug to allow for a different spark instant.
This will produce a longer in-cylinder spark duration.
Lance |
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Rogue_Ant
Joined: 27 May 2012
Posts: 8
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| Posted: Fri Aug 17, 2012 1:08 pm Post subject: |
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...Lance, I'm not sure why you mis-understood my original question. But, I already talked with Electromotive, got the answer, and have since moved on.
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-Joshua |
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