ignition coil

[RootesRacer]

If you reverse the polarity on such a system, which is designed with a negative HT polarity, you will degrade its performance as there will be insufficient voltage to initiate discharge.

Rootesracer discounts these as a myth, but on what basis?

Chris,

Based on your argument the waste spark arrangement will leave one of its cyl pairs with a weak spark (since in your opinion the polarity of current flow significanly impacts the ability to arc), this simply is not true, and is very easy to prove.

In my business, I have the pleasure to work with both exotic cars, as well as specialized tools to allow me to analyze things like (insert drum roll here.......) ignition energy.

Tools I have at my disposal include a spark chamber, which is a metal chamber with 8 spark plugs, thick glass windows at the plugs, a pressure regulator feeding CO2 to the chamber, and a variable bleed going out of the chamber. The premise is that at higher pressures, the CO2 acts like compressed air/fuel and becomes tougher to ignite (it really does).
Now the ionization voltage and primary and secondary current are instrumented into my storage scope, which when I fire the coil(s) I get to see the following:
1) Spark quality through a window on the two plugs firing.
2) Measured coil energy (integral of individual plug voltage times current) in mJ.
3) Effect of CO2 pressure (analogous to compression ratio or boost) on the spark quality as defined above.
4) Ability of the coil to induce arc (which at higher and higher loads becomes less probable to impossible).

What I "see" and can measure is that each plug in the waste spark chain has the same affinity to arc, and has measurably significant correlation to spark energy, ionization voltage and pressure at which the coil doesnt have the rise time (Dv/Dt) to light it off. Alternating the two waste spark plugs between high pressure and atmospheric yields no significant variation in spark parametrics between the two plugs on the high pressure test).

Additionally, while working with high boost performance engines (40+ PSIG), I have implemented pressure based dwell control, which allows crazy boost levels before the spark fails to light off, levels that most people say you need a magneto to fire off.

What I have learned with waste spark is that at the point where the coil no longer has the voltage or energy to light off a mixture, either cylinder (the positive or negative plug polarity) is just as likely to fall on its face, an NO ONE CYLINDER of the waste spark pair fails to fire while the other continues.

I could give compelling anecdotes that electrode material (doner vs. receptor) will have just as profound ability to initiate arc as the current polarity, and we could have a wonderful theoretical discussion on that. The information I have given you however is empirical.

Anyhow, the lesson is done, take what you want from it, or design an experiment to to show me otherwise.

 

[V_Mad]

Guys, I am glad no one is taking exception in this debate. It just shows how what a good bunch of guys the Alpine owners are! And I think this is becoming more like a physics forum than an Alpine forum

Mr Rootesracer, on the subject of 'sparks', I am not suggesting that the wasted spark system has a problem with a weak spark. As I said, I expect the designers have already ensured there is adequate voltage overhead to ensure this does not occur. But an Alpine sytem will not have been designed as such. And you must have seen plugs with the earth electrode badly eroded (due to positive polarity)?

If you dont mind me commenting on your spark chamber experiments, I would like to make the following points:

1. Do I understand that there is no asymmetry in the voltage at which arcing is initiated when comparing a positive with a negative polarity under the same conditions? (you really dont need to mention energy again). If this is the case, I would have expected some difference purely due to electrode shape (as you will know electrons will leave a sharp edge more readily than a rounded/smooth one).

2. There is apparently no heating of the spark plugs. As I said this is very significant in arc initiation.

3. A minor point, but using compressed CO2 may not be very representative of the fuel/air mixture.

Finally, no one has shot me down yet on the reversal of the LT connections. Is that almost an agreement?

Will we still be on talking terms when this is over?

 

[RootesRacer]

Mr Rootesracer, on the subject of 'sparks', I am not suggesting that the wasted spark system has a problem with a weak spark. As I said, I expect the designers have already ensured there is adequate voltage overhead to ensure this does not occur. But an Alpine sytem will not have been designed as such. And you must have seen plugs with the earth electrode badly eroded (due to positive polarity)?

If waste spark IYO doesnt have an issue with weak spark, then what exactly is your point? IIRC your argument relates to (and I quote) "This causes the earth electrode to erode, and the spark will be weaker (*).".

There is sufficient voltage available to initiate a spark, by this, I mean there is enough overhead in secondary voltage to have enough potential to induce arc at worst case scenario, but then again normal single ended coils usually have surplus too.

Absolutely no difference in wear rate on the plugs between paired plugs.
In fact Ive seen lots on OEM engines go 150,000 miles on the original set of plugs albeit with a rather opened gap. Pull the plugs and you cant tell any difference in wear.

If you dont mind me commenting on your spark chamber experiments, I would like to make the following points:

1. Do I understand that there is no asymmetry in the voltage at which arcing is initiated when comparing a positive with a negative polarity under the same conditions? (you really dont need to mention energy again). If this is the case, I would have expected some difference purely due to electrode shape (as you will know electrons will leave a sharp edge more readily than a rounded/smooth one).

There is symmetry between the two plug voltages once arc begins.
The arcing voltage is unimportant becuase there is sufficient coil voltage overhead. The only asymmetry of plug voltages are between the high and low pressure plug, which of course alternates.

2. There is apparently no heating of the spark plugs. As I said this is very significant in arc initiation.

So what, your argument relates to polarity, and the plugs go from room temp to several hundred degrees C in seconds when running the test.
Since each plug sees the same temperature, it is a fair apples for apples test regarding difference in polarity plug energy.

3. A minor point, but using compressed CO2 may not be very representative of the fuel/air mixture.

But actually it is, it correlates very well with measurable loss of ignition on real engines when boost pressure is high enough to dis-allow arcing across the plug, and in fact when this occurs, the coil will arc across the outside of the plug as it is easier to ionize an inch gap of air than a highly boosted mixture at some pressure.

Finally, no one has shot me down yet on the reversal of the LT connections. Is that almost an agreement?

I figured I would leave this one alone since it depends on if the primary and secondary are counter-wound, in which case, the collapsing magnetic fields will be partially negated by a common series current.

So yes, it can matter, just depends of if the coil is wound to invert the secondary or not.

If you have spice, its an easy circuit to simulate.

 

[V_Mad]

Before there was any discussion about wasted spark systems, my point was, simply that if you reverse the Alpine ignition connections (and hence the EHT polarity), this causes the earth electrode to erode, and the spark will be weaker. That is still my point. Nothing you have said since then really convinces me otherwise. But then that doesnt really matter does it.

 

[sammaw@bellsout]

Hi guys, I appreciate the thought going into the responses. You guys certainly know your stuff. I called Crane cams and posed the same question to one of their tech reps. He put me on hold a couple of minutes, and then came back with the following link

http://www.cranecams.com/pdf/90002000a.pdf.

Although this instuction sheet is for application of an electronic ignition system, (not what I am doing)on pg 18 there is a drawing with note in the lower left hand corner indicating that the "CB" correlates to the "+" terminal on older british cars. Does this help conclude the discussion, or just intensify it?

 

[V_Mad]

Hi guys, I appreciate the thought going into the responses. You guys certainly know your stuff. I called Crane cams and posed the same question to one of their tech reps. He put me on hold a couple of minutes, and then came back with the following link

http://www.cranecams.com/pdf/90002000a.pdf.

Although this instuction sheet is for application of an electronic ignition system, (not what I am doing)on pg 18 there is a drawing with note in the lower left hand corner indicating that the "CB" correlates to the "+" terminal on older british cars. Does this help conclude the discussion, or just intensify it?

Hi Sammaw
Sorry, we have been debating this in depth and this has not been too helpful to you.
I am afraid there are two schools of thought on this.

Going back to your original posting, you have a pos earth car, and you want to know how to connect your new coil marked (+) and (-). I would advise you to connect the (+) to the points, and the (-) to the switched ignition. This keeps the spark polarity the same as on your old coil.

If you connect the other way round, no damage will be done, but I dont think you will get reliable ignition under all conditions, and the earth electrode of your plugs will erode (wear away). But I am sure you realise there is another opinion on this.

 

[Tom H]

Sammaw, Let me repeat what I posted 9 hrs ago.

"Sammaw, if you're still with us, I think what I would do is try it one way and see how it runs- maybe test it going up a hill. Just see how well it runs or even use a stop watch going up a fairly steep hill. Then reverse the connection and try again. Then go with whatever seems to work best. I've always found an empirical solution, i.e. whatever seems to work best, works for me. "

I am very confident that this will resolve your issue. If it seems to make no difference that's fine, use either. If one makes the car run better, then how can you not go with that. But do test it with a bit of load like a hill but in a fairly high gear. Don't make the engine lug too much, but don't use a lower gear than really needed.

And no, the note on pg 18 of the document doesn't help. We already know that "CB" on a coil from an older Pos Grd British car correlates to + . What we don't agree on is what the + on the Crane coil correlates to. Ideally the + would be connected to the Points on your Pos Gnd car (as Rootes would have you do) and the spark out of the HT would be negative (as MadV would prefer) . But I think if you wire it like Rootes says you'll get a Pos spark, which Mad V says is bad.

So try them both and see which seems best. It's easy to do and either way will cause no harm to try. And tell us what you conclude!

Tom

 

[jumpinjan]

Because of how an automotive coil is wound internally, the "points" side ALWAYS goes to the points regardless of its a pos or neg ground.

That's true...

This is becuase the coil secondary circuit current returns through the primary into the battery.

I'm not too sure about that point. My Peterson published book on basic ignition (1969), says the secondary returns directly to the battery and not through the primary.
Basically, you are both saying the correct thing.
They also go on and say, "...the two wires leading to the coil have been reversed due to incorrect re-connection. When this happens the spark voltage has positive polarity. It should always be negative regardless of the way the battery is installed in the vehicle."
They speak of a polarity test using a lead pencil inserted into a gap between the plug wire & spark plug. (a lot of you may know this) The test is to see which side the spark jumps. If it jumps between the pencil lead & plug (or ground), you're good.
Jan
PS. Now I'm looking at another Peterson book and it has the secondary returning through the primary???? I'm confused now. Need to find a third book now, and see what they say

 

[Tom H]

Jan,

I think RootesRacer meant that when you wire it wrong "the coil secondary circuit current returns through the primary into the battery."

Because he also states:
"An automotive coil is simply an auto-transformer. There are two windings that connect at the one terminal, this terminal MUST be used as the connection to the points."

When you wire it this way, the return for the secondary is directly into the battery.

The whole disagreement is about which is more important:
A) Having the secondary return through the battery (i.e. the terminal that has a direct connection to the secondary goes to the points) regardless of the resulting spark polarity.
B) Having a Neg going spark, regardlless of which terminal gets connected to the points to achieve that Neg spark

If your coil is made such that both A and B can be met, there's no disagreement on how to wire it. But if you have a coil made for a Neg earth car and you install it on a Pos earth car, you need to make a choice of A or B. That's where the standoff arises.

Will be interesting to see what your third Peterson book says.

Tom

 

[V_Mad]

Jan,

I think RootesRacer meant that when you wire it wrong "the coil secondary circuit current returns through the primary into the battery."

Because he also states:
"An automotive coil is simply an auto-transformer. There are two windings that connect at the one terminal, this terminal MUST be used as the connection to the points."

When you wire it this way, the return for the secondary is directly into the battery.

The whole disagreement is about which is more important:
A) Having the secondary return through the battery (i.e. the terminal that has a direct connection to the secondary goes to the points) regardless of the resulting spark polarity.
B) Having a Neg going spark, regardlless of which terminal gets connected to the points to achieve that Neg spark

If your coil is made such that both A and B can be met, there's no disagreement on how to wire it. But if you have a coil made for a Neg earth car and you install it on a Pos earth car, you need to make a choice of A or B. That's where the standoff arises.

Will be interesting to see what your third Peterson book says.

Tom

Tom, if I may come back in here, the current in the secondary returns through the primary, not the battery as Rootesrace says. The reason is that (when the points open) the primary continues to draw current from the battery (until this decays to zero). Since the secondary current is much smaller (the sum of currents at the node is zero, Kirchoffs law) the nett current is out of not into the battery.

So my point is still that the circuit works perfectly well in this mode of connection.

(I am surprised that I even rembered Kirchoffs law, its been about 40 years since I learnt that!).

 

[Tom H]

Chris , sure you can come back on this. I find it extremely fascinating that something so common and supposedly well understood for decades is not actually so understood. And I am still a bit confused myself.

As to whether the "secondary returns to the battery" I guess we first have to agree as to which state we are discussing - contact closed or contact open. So I made myself three drawings, to try to understand:

Going back to square one, picture the "normal" Neg gnd system and coil designed for it. I draw the autotransformer as two parallel coils connected at the bottom, with their two free ends toward the top. I think we all agree, the battery + connects to the free terminal of the LT coil (at the top of the dwg) . That is called the SW or + terminal. The other LT terminal (at the bottom, connected also to the HT coil), labelled CB or -, connects to the points and goes to ground when the points are closed. The HT post goes to the spark plug. If we marked the polarity of the autotransformer on the drawing we would put a Dot near primary coil near the end connected to the + terminal and and a Dot near the end of the secondary near the CB end. This shows that the spark plug end of the seconday produces a Neg voltage if the primary has Pos at its Dot. Right??

Now when the points open there is only one path for all the current. All the current goes through both coils. Current goes from battery thru primary thru secondary, through the HT wire and thru the spark to Grd and back to the battery. Thus Kirkoff and all of us are happy. I think the small voltage across the primary actually subtracts a little from the total V applied to the plug, but losing 12 V out of 12 KV or so is not significant. (keeping in mind Kirkoff's other rule about voltage around a loop). I think everyone agrees this is how a Normal system works with a Neg Gnd and a coil designed for Neg Gnd. And the spark is from a Neg going voltage, as in most cars, and neg spark is better and easier due to hotter center electrode and has less wear..

Now take that same circuit dwg, but flip the battery over. We keep the common point of the two coils connected to the points. That keeps the current paths exactly the same as original. Current goes thru both coils when the points are open. Everything works fine - except we have a Pos voltage applied to the plug. Some think this is bad. Best solution to solve THAT issue is a new coil - one designed with the secondary wound in the reverse direction from the other coil. You would then change the dwg to show the secondary has its Dot at the Top of the secondary, near the HT terminal.
Changing coils would make everyone happy. The terminals and current paths would be the same as a "normal" car AND the spark would be from a Neg voltage .

BUT if we keep the same coil when we changed battery polarity but flipped the coil over, this is what we get: The end of the coil with both LT and HT coils connected to it goes to the battery. And the terminal for the LT, or primary coil only, goes to the points. Now when the points open, the current path for the spark goes thru only the secondary. I guess the current in the secondary goes into the capacitor across the points. I don't fully understand all that happens, but it is clear that the actions are different that in the "Normal" situation, but the spark polarity is the same.

So those are the choices if you use the a Neg Gnd coil in a Pos Gnd car. Keep the same current path and get a pos polarity spark. Or change the current paths and get the same Neg polarity spark.

Interestingly, the WSM Prefers reversing conections - presumably to maintain Neg spark. :
http://www.rootes1725cc.info/wsm145/wsm_b/b_40.htm


I still appreciate RootesRacer's contention that spark polarity is moot, and am impressed with the example of waste spark as proof. But I think I see that waste spark system designs have taken steps to eliminate or minimize the issue. First they up the HT voltage to overcome weak spark and I have read in several places that sometimes auto makers with Wasted spark systems use different spark plugs in different cylinders to minimize electrode wear in the Pos spark ones.

So right now I think, in an Alpine, wire the coil to provide Neg spark is preferable. But my thinking is split almost the same as the Hillary vs Obama margin. And if there is a re-vote in MI I may change my mind - about the coil.

Tom

 

[Bill Blue]

Tom, not to take sides (I don't understand this AT ALL), but the Ford Zetec engines have two different plugs. One and two are different than three and four. I've always wondered why, perhaps now I know. But when replacing, they specify a single plug.

Thought you might find that interesting.

Bill

 

[Tom H]

Bill,

Yes, this is an example of a car using wasted spark and thus using two different plugs, one type for the Pos spark coming from one end of the new type coil and a different type plug for the spark coming from the Neg end of the coil. I think they use platinum on the electrode (center or ground electrode) that would wear the most in that cylinder getting that polarity. The platinum wears out less quickly, apparently. And they can do that in a controlled production process. But in the field replacement they cannot count on mechanics putting the correct plug type in the proper cylinder. So they specify a plug that has platinum on both electrodes. A little more expensive, but not a budget killer for just 4 plugs. But in the factory, building 1000s of cars, the savings are worthwhile and the assembly controls are in place to allow more selective installation.

I didn't know hardly any of this until digging into this issue on this thread. I am an electrical engineer so I understand voltage, current, coils, etc, and I knew about waste spark systems, which impressed me when I first heard of them 10 yrs ago or so. But I had never even thought about spark polarity and its ramifications until this thread.

Wow, how fascinating these details and discusion on such an old old concept. Reminds me of the time I ran into a distant cousin in the airport. I was on my way to a high tech electronics convention. He was on his way to a "brick convention". I'd never thought there would be enough new stuff about bricks to warrant a convention!!! Hey, bricks have been around for 1000's of years old, what more can be done? But that shows there's always something new.

Tom

 

[V_Mad]

Tom, our analyses so far of the circuit I think helps to explain how it works in basic terms, whereas in fact the full operation of the circuit is far from simple.

As you say, it is a familiar circuit, and it has just a few components, but modelling its operation accurately is not easy even using tools such as Spice, as was mentioned by Rootesracer. Mathematical analysis is fine up to the point where the spark occurs, and then it becomes a complex non-linear problem. Up to the point of spark occurrence, the sequence is more or less as you outlined.

For either circuit connection, here is my expanded version:

Starting with the points closure, current will build up in the primary. This is approximately a linear ramp, the rate of increase being a function of battery volts, and coil inductance (L).

When the points open, the capacitor (C) is then in series with the coil, and the current decays due to loss of DC supply. The rate of decay depends on L and C values, and follows an approximate sine wave. This is in fact like a tuned circuit with a frequency ½(PI)(LC)^1/2. The rapid decay in current generates a ‘back emf’ voltage across the primary coil (e=Ldi/dt) of about 300 volts, which is transformed to a very high voltage (~20 kV) in the secondary due to the coupling between primary and secondary. When the secondary voltage reaches breakdown, the voltage reaches a plateau until the arc extinguishes.

If you follow the above sequence of events for either way of connection I think it still works the same. The only small difference is, as you say in the secondary voltage, which is about 300 volts less because the primary is connected in series.

These days, most vehicles seem to use the EDIS system, however, I don’t think they are all based on a wasted spark system are they? I have read that some of them use one coil per cylinder, but I admit I don’t know much about these.

Bill, interesting info on the Zetec ignition, I hadnt heard about that. I wonder how often they need to change plugs?

 

[sammaw@bellsout]

Hi Guys sorry to cause so much lost sleep. I was drinking a few beers with Kirchoff while watching Clemson BEAT duke in the ACC tourny, he told me that it really didn't matter which way them electrons went as long as there was fuel/air mixture and compression, then boom!! He also admitted he was drinking heavy when he made up them wacky laws

 

[Bill Blue]

Bill, interesting info on the Zetec ignition, I hadnt heard about that. I wonder how often they need to change plugs?

Don't know. We have two of them that are at 125,000 miles and going strong on original plugs.

Bill

 

[Tom H]

Bill, just to close on the different plugs concept here's a link that confirms my explanation:
http://www.miracerros.com/mustang/sparkplugs.htm

Tom