Topic: Ignition Theory: Plug Polarity

[Pinhead]

In preparation for my MPG project for my '79 CB650, I've come upon an ignition system that will bring me much closer to my mileage goals. However, I've got a few questions regarding our dual plug system.

Attached is a basic diagram of a Kettering ignition system. I fully understand how the system works. The question I have is about our coils, and more specifically about the voltage polarity at points A and B.

Are the voltages at points A and B in phase with eachother? Or are points A and B 180° out of phase (basically one plug being positive while the other plug is negative)?



This question is important because it would determine whether or not my Super Spark ignition system would be easily adaptable to our motorcycles' ignition system.


EDIT 6/08/09

FOR A WORKING DIAGRAM SEE REPLY #30

[TwoTired]

As I understand it, when the arc occurs at A and B, the secondary circuit is closed, and current flows through the secondary.  I would expect polarity to be consistent but opposite at each point A & B.

[Pinhead]

As I understand it, when the arc occurs at A and B, the secondary circuit is closed, and current flows through the secondary.  I would expect polarity to be consistent but opposite at each point A & B.

So the current would be flowing from one ground point, across gap A, through the coil secondary, across gap B, and then back to ground?

In other words, with respect to ground (and assuming a 30kv output), point A would be -15kv and point B would be +15kv?

[TwoTired]

No, the current flows from the B lead, across the B arc to "ground" (which really has no relevance here, as the head is simply a conductive path), Back up across the A plug arc to the A lead.
I'm not sure why your ignition coil shows two separate windings for the secondary.  The standard coil only has one secondary winding.  When current flows, it flows from one lead to the other.

[Pinhead]

No, the current flows from the B lead, across the B arc to "ground" (which really has no relevance here, as the head is simply a conductive path), Back up across the A plug arc to the A lead.
I'm not sure why your ignition coil shows two separate windings for the secondary.  The standard coil only has one secondary winding.  When current flows, it flows from one lead to the other.

Ok, I understand what you're saying. Now I have no idea how to adapt the hot spark system to our coils without burning them up.

The basic premise of the system is pretty simple. The typical ignition coil supplies the high voltage to ionize the mixture which causes the initial spark to jump the gap. Once the mixture is ionized enough to start the spark, the relatively low voltage power source supplies additional current to create a very powerful spark.

Here's a basic schematic of how it is typically wired.

EDIT: IMAGE REMOVED; DIAGRAM WAS INCORRECT DUE TO LACK OF SLEEP.

[martino1972]

[Pinhead]

OOPS!!! I left out a few components!!! You are completely correct on all accounts, Martino!! I guess I shouldn't be drawing schematics after being awake for 26 hours...

[martino1972]

good luck,don't touch the engine block.....

[Pinhead]

good luck,don't touch the engine block.....

This last one should fix it. . . At least for a single plug system.

[Pinhead]

How about this one for the dual plug system that our bikes have??

EDIT 6/08/09

THIS DIAGRAM WILL NOT WORK. SEE REPLY #30

[martino1972]

good luck,don't touch the engine block.....

This last one should fix it. . . At least for a single plug system.

this one is the only one that made sense if your goal is to put 120hz. 120vdc on your sparkplug....

still don't understand what that voltage is gonna help in running better though.....

BTW,our dual sparkplug system is nothing more then having 2 sparkplugs fire of the one coil,but one cylinder is on compression stroke,the other just finished exhaust stroke...

that very last scematic you posted don't work....

[Pinhead]

still don't understand what that voltage is gonna help in running better though.....

THIS is why the DC bias will help...

[martino1972]

i will have to look into that,got some questions about it,like does it throw off the timing,won't it burn a hole in the piston,won't it plasma cut the sparkplug tip.......

[Pinhead]

i will have to look into that,got some questions about it,like does it throw off the timing,won't it burn a hole in the piston,won't it plasma cut the sparkplug tip.......

If you don't put resistors in-line with the circuit you will most definitely melt the spark plug tip. In fact, running the circuit resistor-less at 10 Hz will melt the spark plugs in about 20 seconds.

The only way it would throw off the timing would be due to burning the fuel more quickly in the chamber; the original spark from the original ignition system initiates the spark just as before. This system only adds current and therefore power to the discharge.

[TwoTired]

The typical ignition coil supplies the high voltage to ionize the mixture which causes the initial spark to jump the gap. Once the mixture is ionized enough to start the spark, the relatively low voltage power source supplies additional current to create a very powerful spark.

Once the atmosphere between the electrode is ionized, it forms a plasma conduit that conducts electricity between the electrodes.  It does take less voltage to maintain the plasma conduit than to form it.

Larger spark gaps require more energy to start and to maintain.  But, the extra area of hot plasma initiates more of the A/F mixture which should continue and cause a more complete burn of available fuel.  However, once the flame front travels away from the plasma arc channel, any extra energy in the plasma channel is superfluous.

That's what I was taught, anyway.

[Pinhead]

The initial firing of the plug and resultant start of the flame front is extremely important in maintaining an even and consistent burn throughout the chamber and between each cylinder. The hotter the spark, the more consistent the burn will be; it will commence combustion across a much wider range of A/F ratios. This hot spark will allow me to tune leaner at idle and low throttle angles to maximize gas mileage.

I plan on leaning out the idle a bit and hopefully lowering the main needle jet to slightly lean out mid-throttle. Main jets will remain the same so I don't have to sacrifice horsepower. The hot spark should also help reduce or eliminate the transition stumble between the leaner idle/part throttle angles and the upper-end.

This isn't going to be the only modification to the engine I do, however. I've had a spare cylinder head worked over to increase combustion turbulence, fuel homogenization, and therefore burn rate. I'm hoping to be able to reduce ignition timing due to the combination of these modifications without sacrificing power or mileage.

Sorry for going a little off topic here.

So, TT, how do you think I could get the circuit to work with our dual plug coils?

[TwoTired]

Where did you learn that higher voltage creates a hotter plasma, or a "hotter spark"?  Any references to educate me?

Further, does the forest fire burn hotter, faster, or more completely if the starting source is a simple match match or an acetylene torch at 5000 degrees?

I'm wondering why you are not using known and proven techniques like capacitive discharge ignition.  Where the coil output voltage is increased by pulsing the coil primary with 200-400 V instead of the 12v it normally gets?

[Pinhead]

A "hotter" spark is made with more power which means either higher voltage with the same current or the same voltage with higher current. This system maintains the stock peak voltage but adds current for more power.

The more powerful spark will also last a bit longer and can "stretch" as windage blows through the gap. Windage isn't much of a problem with our relatively (key term) stagnant combustion chamber; it usually becomes an issue in forced induction engines. However, the combustion chambers that I'll be working with have had "surface turbulence" added to them.

I've read from multiple sources that CDI ignitions aren't very good at lighting lean A/F mixtures. While the "overall" A/F ratio might be right at what you're aiming for, random parts of the chamber will be richer, leaner, and every point in between. If a lean spot happens to be right around the plug when the quick CDI spark fires, you'll get a lean-out misfire. This is why multiple-strike CDI was produced. In this system, the longer duration of an inductive ignition's spark helps alleviate this problem while aAdding power to that spark will help even more, as the spark will be able to light off much leaner mixtures.

With the combination of more turbulence and leaner mixtures, you can see how the ignition system needs to be all it can be, to get optimum combustion out of the aging hemi chamber design.

[Pinhead]

From 34:1 AFR discussion at GoFastNews.com:

Every engine we have set up in the last 10 years for R&D has had the spark positioned in the area of maximum or near maximum flow. We never shroud the spark from the flow. If the engine needs more ignition energy to light the mixture (lean/rich/alternate fuel) then that's what it gets.

In this scenario a long and powerful spark is important. A short spark ignites what is in the gap then the kernel blows away and hopefully keeps combusting. A long spark (of sufficient power) ignites whatever mixture blows through the plug gap for as long as the spark continues i.e. the kernel becomes a streamer of some length.

Some time back I read this paper “Cyclic Variations of Initial Flame Kernel
Growth in Honda VTEC_E Lean-Burn SI Engines”. It describes the same
condition Clint is referring to. Here’s my interpretation of a small portion of
what the paper concludes. All of what is in the paper is focused on the
pent roof design:

Spark duration varied cycle to cycle, long spark duration is beneficial and
preferred over larger spark energy with shorter duration. It is believed the
long duration masks effects of variations in local A/F ratio.

Flame kernel shapes were far from spherical, size and shape differ significantly
from cycle to cycle. The flame develops in the direction of kernel stretch
caused by large scale flow. This is beneficial creating larger flame area as long
as the stretching is not too intense. It becomes a balance, too much flow
causes excessive flame stretch, too little causes poor kernel development;
either can lead to flame quench.

As for plug direction it was found that the preferred orientation is cross flow
meaning if the large scale in chamber flow is east to west the ground
electrode should be north to south.

The ability to burn lean also translates also to the ability to handle rich-to-lean transition, i.e. 13 to 18 to one AFR, without “lean surges” or “stumbling”.

I invite you to read that thread (though you'll have to devote some time, as it's 12 pages long). It explains many of the principles that I'm trying to incorporate into my motorcycle.

[TwoTired]

Do you know the inductance of the ignition coil?

[Pinhead]

Do you know the inductance of the ignition coil?

The specific inductance of the 650's coil, in mH? No, I don't... Why?

[TwoTired]

Do you know the inductance of the ignition coil?

The specific inductance of the 650's coil, in mH? No, I don't... Why?

It is needed to calculate circuit operation.

[Pinhead]

Isn't coil inductance frequency dependent, as with impedance?

Where would I find said information? None of the manuals that I've searched through give this value...

[Joel]

I believe coil inductance is independent of frequency.  It can be measured with a meter but I think they're rather expensive.

[martino1972]

don't think it's really important in this case....cause it's more or less just assisting/adding on to the stock system...the one diode even prevents it to get feedback into the coils.....