How does a coil fire?

[GregH]

While I was helping a fellow tech with a misfire concern, the question of how a coil fires came up.. Turns out we have a variety of opinions in the shop.

 

We were talking about the coil on plug setup. The primary wiring has KOEO power and the PCM provides a pulsed ground in time with the desired cylinder firing. This erects and collapses a magnetic field around the primary wiring in the coil assembly. As the magnetic field collapses, it sweeps through the secondary wiring, inducing a current flow. The ratio of the number of windings in the primary and secondary indicate the voltage multiplication.

 

The secondary current flows from the coil down the spring into the spark plug. It then jumps the gap, igniting the mixture in the cylinder, and returns via the spark plug casing into the cylinder head.

 

Here's where the difference of opinion is. How does the electricity complete the loop? What route does it take to get back to the secondary winding?

 

One coworker says the loop is not closed. There is no return path for the electricity.

 

Another says the return path is through the PCM - within the coil the primary negative and secondary negative are connected together. Current flows through the plug casing, into the cylinder head and ground circuit of the vehicle, into the ground for the PCM, through the circuit board and back through the coil break wire to the coil.

 

A third coworker (me) says the return path is through the vehicle's battery. Within the coil, the primary positive and the secondary negative are connected together. Return power goes through the plug casing into the cylinder head to the negative battery terminal, through the battery, BJB, coil power circuit and back to the coil secondary.

 

A beer is riding on this. Who is right?

[Adam_Dodson]

I think it would depend on the vehicle. It should take the shortest path back to ground and all ground points go back to the battery. Once it hits ground it's dead and nothing goes back to the coil.

[Jim Warman]

Limiting our discussion to modern, e-core type ignition coils....

 

First and foremost, we need to remember that these coils are positive ground devices. (occasionally, you will run across a coil that has isolated windings - that is to say that the primary winding has two poles and the secondary winding has two distinctly separate poles - I don't know of any modern ignition coils wound this way).

 

Here is an extremely simplified diagram....

 

 

You already know that the secondary and primary windings are joined, so let's concentrate on the path(s) our electrons can follow. FWIW, this breaker point diagram works no different than a system using electronic "switches" (transistors, triacs or any devices I will surely miss) along with any current limiting circuitry (ignition coils always used to be 9 volt devices - in a 14 volt system - and I don't think that has changed).

 

Long story made short. if you follow the path of power through our ignition coil, you see that the "spark" finds it's ground through the positive side circuitry - ignition switch, fuse panel, battery. FWIW, modern e-core type ignition coils are wound in this same manner.

 

The tech that opined that the loop is not closed should take his resume to MacDonalds.... current will not flow if there is no complete circuit (In the case of a spark plug circuit, the circuit is considered "complete" as long as the spark plug gap is ionized).

 

The tech that opined that the return path is through the PCM will, one day if he uses a spark tester grounded to the wrong place on a plastic intake manifold, find that the PCM can handle that kind of abuse once and only once (some hapless bugger did it at the Edmonton training facility... Darryl loves recounting that story).

 

Greg... that beer should taste mighty good for you.

[Adam_Dodson]

I'm confused as hell. Isn't the ground the side electrode on the spark plug unless it's a waste spark?

[Brad Clayton]

All electricity has to get back to the source. In this case the battery is not the source, the ignition coil secondary windings are the source...so the elctrical flow has to make its way back the coil.

 

Same as alternator rectification. If you look at the diode schematic of any alternator, it is very confusing to see how full voltage could be rectified. But you have to remember that the electrical source of an alternator is not the battery but the alternator itself thus looking at the diodes with this in mind clears all confusion.

 

Either that or it makes it as clear as mud.

[Jim Warman]

Well, it can be confusing... until we put everything into it's perspective.

 

At the moment we have spark, there are a lot of things happening.

 

As for the spark plug (not shown in my simple and very stolen illustration)... we will use current flow theory. Current flows from the spark plug cable terminal, through the spark plug. The gap ionizes and the spark leaps from the centre electrode to the side electrode. Our current then flows back through the common (negative ground) circuit, through the battery and back through the positive terminal of the coil (making the positive terminal the coils "ground").

 

The "waste spark" system is given it's name because one spark plug in a running pair is sparked during it's exhaust stroke making that spark event "wasted". If we deal with the 6.2, (in the abscence of anything to tell me I'm mistaken) I think we have 8 positive spark plugs and 8 negative spark plugs. However, no spark occurs on an exhaust strke so no spark is "wasted".

 

All the while that this is happening, there is some nifty shit going on inside the coil. The magnetic field is collapsing across both the primary and secondary windings of the coil. This induces current flow which induces a magnetic field which forms and collapses which induces current flow which induces a magnetic field.... At this point, what does collapse is my understanding of the circuitry required. I am unsure if the semi-conductors we have now can withstand the PIV (peak inverse volts)induced in the switched side of the coil primary without the buffering effect of a capacitor (condensor). There will probably have to be some sort of "clamping" device built into the circuit - but our "need to know" stopped before we got this far

 

Anyway... the circuit HAS to be complete. Even though there is a spark plug gap - what we might normally consider as an "open" circuit - this circuit is completed when the spark plug gap is ionized and spark is occuring.

 

Long story short... if we consider current flow theory.... current will flow from a point that is more positive to a point that is less positive. What matters is VOLTAGE DROP across a device not total potential of the source voltage.

[Adam_Dodson]

Ah I get it...I was thinking of the battery as the source. I know I learned this shit in school And still mix up dual spark and waste spark...

[Jim Warman]

Could be worse.... SnapOff can sell you a $500 timing light (c'mon guys, once upon a time we NEEDED them ) and forget to tell you it can't deal with multi-strike ignitions

[lmorris]

And that right there is the one reason I stick with fixing diesels.