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  #11  
Old 08-16-2012, 12:43 PM
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Certainly lots of good info here.

Mike, can you talk about the use of multiple coils on modern engines? There must be a significant advantage in order to justify the additional expense, right?
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Old 08-16-2012, 12:55 PM
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I'd like to hear that too.

In a book I read about 20 years ago, it stated that Smokey Yunick back in the 60s theorized that he was sometimes blowing NASCAR engines at high rpm with crossfire and approached GM with the coil/cyl idea, but they didn't go for it. It said Mercedes was the first to implement it commercially.
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Old 08-16-2012, 01:58 PM
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What is sorta comical to me, is the concept of the waste spark....



This is all rather similar to the pulse field metal detectors we used to make some 30 years ago, from airport security looking for guns, to the units Mel Fisher used to find the Atocha treasures here in Florida region....
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Old 10-01-2012, 05:51 PM
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What is sorta comical to me, is the concept of the waste spark....

This is all rather similar to the pulse field metal detectors we used to make some 30 years ago, from airport security looking for guns, to the units Mel Fisher used to find the Atocha treasures here in Florida region....
Why?
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Old 10-01-2012, 07:37 PM
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What is sorta comical to me, is the concept of the waste spark....

This is all rather similar to the pulse field metal detectors we used to make some 30 years ago, from airport security looking for guns, to the units Mel Fisher used to find the Atocha treasures here in Florida region....
Why?
I have never tried it, but it's sorta like reversing the +- on a typical coil, spark goes rong way on the plug, I heard it makes things not run right, but forget why , been a while.....

so if you have a waste spark, the same coil gets fired the same way each time, say 1-6 out of those two, one or six has to have the spark going the rong way when it fires.....kinda hard to not do that with the primary winding being hard wired....NO?? then I would think that the use of one coil/cylinder on the LS would obviously get around that 'problem'.....

what say you???
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Old 10-01-2012, 09:04 PM
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What is sorta comical to me, is the concept of the waste spark....

This is all rather similar to the pulse field metal detectors we used to make some 30 years ago, from airport security looking for guns, to the units Mel Fisher used to find the Atocha treasures here in Florida region....
Why?
I have never tried it, but it's sorta like reversing the +- on a typical coil, spark goes rong way on the plug, I heard it makes things not run right, but forget why , been a while.....

so if you have a waste spark, the same coil gets fired the same way each time, say 1-6 out of those two, one or six has to have the spark going the rong way when it fires.....kinda hard to not do that with the primary winding being hard wired....NO?? then I would think that the use of one coil/cylinder on the LS would obviously get around that 'problem'.....

what say you???
You are correct that on an 8 cylinder (with four coils such as an LT5) that half the plugs fire with the center electrode negative polarity, and half the plugs fire with the center electrode positive polarity. I'm sure you recall that in a vacuum tube that a hot negative electrode boils of electrons easier than a positive electrode. The plug electrodes operate in a similar manner. It takes a touch more voltage to fire the center electrode positive gap than the other plug that has the center electrode at negative polarity (relatively speaking of course, comparing compression stroke arcs or exhaust stroke arcs). It's just a matter of storing a bit more energy in the coil to not only jump two gaps, but also adjusting for the penalty of a positive polarity gap on one of those plugs. (Remember, the exhaust stroke waste spark takes very little energy, and even a distributor setup has two gaps to contend with.)
Adding a little more current/energy to the coils is more cost effective than doubling up on the coil count. It also provides a smaller ignition mass contribution to the overall engine weight.

There are some advantages to a coil per plug (which I'll discuss in a future post), but as evidenced in the (7000 RPM) LT5 application, shared coils and waste spark performs quite acceptably.
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Old 10-01-2012, 10:27 PM
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What is sorta comical to me, is the concept of the waste spark....

This is all rather similar to the pulse field metal detectors we used to make some 30 years ago, from airport security looking for guns, to the units Mel Fisher used to find the Atocha treasures here in Florida region....
Why?
I have never tried it, but it's sorta like reversing the +- on a typical coil, spark goes rong way on the plug, I heard it makes things not run right, but forget why , been a while.....

so if you have a waste spark, the same coil gets fired the same way each time, say 1-6 out of those two, one or six has to have the spark going the rong way when it fires.....kinda hard to not do that with the primary winding being hard wired....NO?? then I would think that the use of one coil/cylinder on the LS would obviously get around that 'problem'.....

what say you???
You are correct that on an 8 cylinder (with four coils such as an LT5) that half the plugs fire with the center electrode negative polarity, and half the plugs fire with the center electrode positive polarity. I'm sure you recall that in a vacuum tube that a hot negative electrode boils of electrons easier than a positive electrode. The plug electrodes operate in a similar manner. It takes a touch more voltage to fire the center electrode positive gap than the other plug that has the center electrode at negative polarity (relatively speaking of course, comparing compression stroke arcs or exhaust stroke arcs). It's just a matter of storing a bit more energy in the coil to not only jump two gaps, but also adjusting for the penalty of a positive polarity gap on one of those plugs. (Remember, the exhaust stroke waste spark takes very little energy, and even a distributor setup has two gaps to contend with.)
Adding a little more current/energy to the coils is more cost effective than doubling up on the coil count. It also provides a smaller ignition mass contribution to the overall engine weight.

There are some advantages to a coil per plug (which I'll discuss in a future post), but as evidenced in the (7000 RPM) LT5 application, shared coils and waste spark performs quite acceptably.
Interesting that there could ever be any relationship/similarity between a spark plug and a old vacuum tube....VT cathodes were coated with something to make for better forward conduction,.....Phosphorus ????

damnit man, hate scratching my head over olde tyme details....

at any rate, I find it curious that there is any difference at all from polarity in a ICE....what with the presence of air and fuel, not a vacuum....

and yes, looking for your next post on this, me being a geek and all.....

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Old 05-07-2015, 03:04 AM
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Default Multiple ignition coil Pros and Cons

I apologize for the (damn long) delay in getting this updated. Just was never around a computer whenever I was thinking about this thread.

I'll start out with the Cons, as that's the shortest list in my head at the moment. Assume an eight cylinder engine.

Cost: Eight ignition coils are obviously more expensive than the old single coil design. The lack of a distributor is a cost saving, but probably not more than the cost of the additional seven coils.
ECM cost increases a bit as now there has to be eight high-power transistors to switch the coils (or the power transistors can be installed in the coil housing, moving the cost to the coil price). Additional support circuitry is needed to control/sequence the eight transistors.

Weight/mass: Eight coils weigh more than one, and probably more than one coil and one distributor.

Reliability?Generally, the more parts in an assembly the more opportunity for something to break/fail. IIRC, Porsche used to have a slogan to focus on not making things any more complicated than needed. It was something like: Parts not installed cost nothing, and seldom break. However, multiple coils allow each to work at a significantly smaller duty cycle, which should increase reliability.

Pros (the interesting part):

Shorter or no plug wires. This has multiple benefits. In the case of no plug wires, there's the reduced system cost and they won't break. The short wires in an LS engine are less expensive than longer wires in the old small blocks, and the shorter length provides fewer places for the wire to break open or short to the engine, resulting in greater reliability (and lower warranty costs).

More efficient energy delivery: A shorter plug wire (versus the old style long wires from the distributor) can more easily reduce the energy losses when the secondary current passes through the wire. Basic stuff. A less well known advantage to short (or no wires) is the greatly reduced capacitance in the secondary wiring. Recall that the coil secondary voltage rise (up to its eventual multiple kV required to jump the plug gap) is not an instantaneous event. It takes time for the magnetic field to collapse and generate the output voltage. While the coil secondary terminal is ramping up in voltage consider what the coil terminal is hooked to: a (long) metallic conductor, surrounded by an insulation (dielectric), which is then also in proximity to a lot of conductive material (the engine ). Electrical types will recognize what two conductors separated by a dielectric is: A capacitor. The plug wire and it's surrounding environment comprise a parasitic, unwanted capacitor. So, while the coil is in the process of ramping up the secondary voltage to fire the plug, this undesired capacitance is also charging up. That capacitor charging unfortunately slows the speed that the voltage rises prior to the plug gap arcing over. You say, so what, I'll just advance the timing a touch to make up for this delay(and that's done at a micro scale when we turn the distributor to set the timing to what we want). So where's the downside? We're all used to pulling the spark plugs and noticing the light to heavy carbon residue covering the spark plug insulator (obviously the newer FI engines have less carbon fouling than our older carbureted engines). We all know that carbon is a conductor. The result is, while the secondary voltage is ramping up (and the spark plug center electrode is hooked directly to the coil secondary terminal) the plug carbon fouling will provide an electrical path to drain off the coil energy while it is ramping up the voltage to jump the plug gap. This is an efficiency loss because the coil stored a measured amount of energy (produced by the alternator), but some of that stored energy was "flushed down the toilet" across the carbon path on the insulator. Eventually, the secondary voltage reaches a high enough point where the plug gap arcs over, and now the coil's energy is used for productive work (initiating combustion). Also fortunately, the energy stored in the parasitic capacitance of the plug wire and surrounding metal discharges into the plug gap, returning its energy to the intended purpose. Net effect: the shorter the plug wire the less parasitic capacitance, resulting in a faster secondary voltage rise, and reducing the loss of secondary energy across the plug insulator carbon tracks. (For an amusing example of all this, think about the parasitic capacitance of the old big block plug wires with the braided covering grounded to the valve covers.)

Dwell time/angle: A coil per cylinder opens up a couple options. Recall with the old points systems that the dwell was set at a constant distributor angle (30*, or 60* crank angle) out of a possible 45* distributor angle or 90* crank angle. Simple systems (like points and a ballast resistance) are usually a compromise, and with a points system the dwell (time) was always too long at low RPMs (and just heating things up), while at higher RPMs the dwell was always a bit shorter than desired (but thank goodness for the drop in VE at high RPMs in most engines, which requires less spark energy to light things off). The multiple coil setup only has to fire once every 720* (two crank revolutions). This means it is possible to use high inductance, low current coils (which require long dwell times) but don't heat up much, and can be made with smaller wire (which may help with packaging size) due to the modest primary current. However, a long dwell time at high engine speeds means that there may be two coils charging simultaneously. Outside of some extra software sophistication, it's not a big deal. Or, the coil can be made with a low inductance construction, which allows an extremely short dwell time. The down side of low inductance coils is the need to push additional primary current through it to compensate for the reduction in windings (inductance). High currents require bigger wires, but the fast dwell/charge time does reduce the duty cycle of the coil, allowing it more time to cool off between dwell periods. I expect that the peak RPM and coil cost numbers are the prime drivers on which coil design (higher or lower inductance) is chosen for production.

Reliability (again): Because these coils are turned off a majority of the time (the dwell time is short compared to the 720* between spark events for each cylinder) they will run at a lower average temperature than a single coil application. Electrical items generally show an inverse relationship between temperature and reliability (higher temps--> lower reliability, lower temps--> greater reliability). The cooler running coils experience smaller-magnitude thermal expansion and contraction cycles, which is conducive to better long term durability.

More to come.

Last edited by 69427; 05-08-2015 at 02:00 AM.. Reason: Multiple edits to prevent timing out.
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  #19  
Old 05-07-2015, 10:07 AM
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An amusing old thread, and surely you refreshed my memory there, no question....but my only comment is that the electronics of any spark coil are so far faster than any silly engine could ever rev, 600 rpm is only 5 sparks/cylinder/second.....6000 rpm is only 50 sparks......time for the electronics to roll over and grab another snore in bed......
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Old 02-14-2017, 01:32 AM
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An amusing old thread, and surely you refreshed my memory there, no question....but my only comment is that the electronics of any spark coil are so far faster than any silly engine could ever rev, 600 rpm is only 5 sparks/cylinder/second.....6000 rpm is only 50 sparks......time for the electronics to roll over and grab another snore in bed......
The electronics, yes. The coils, no. The formula/physics of putting energy into a coil/inductor hasn't changed in a century of usage. It still takes time (dwell) to charge up a coil. To speed up the process means lower inductance coils (requiring higher, more dangerous amperage, and more expensive switching transistors) and larger/more expensive wiring, or installing multiple coils on the engine to allow dwell overlaps, but more coils are more expense, and require more software and possibly faster/more expensive processors in the ECM.

There's no free lunch in the world.
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