Coils Do Not Charge Up . . .

[tomm]

I was reading a thread earlier today and noticed a comment that coils charge up. I think it's the third or fourth time now in about 3 months that I've seen a similar comment. It's usually been made in reference to the ignition timing allowing more or less time for the coils to charge up, thereby improving or degrading the spark.

Please Note: Coils do not charge up!

Coils are inductive loads. An inductive load is a load that interacts with a magnetic field. The relationship between voltage and current in an inductive load is v = Ldi/dt, or voltage = inductance times the rate of change of current with respect to time. An inductive load looks like a short circuit to DC electricity, but a resistance to AC electricity. The higher the frequency of the AC electricity (the faster it waves), the higher the resistance (impedence). AC electricity in an inductor produces a changing magnetic field. This changing magnetic field will induce AC electricity in another inductor that is in the field.

The property of a coil that means something is inductance (L). It represents the voltage induced by a change in the current through the coil. The greater the inductance, the higher the voltage induced for the same change in current.

In an ignition, coils have current running through them, and then they don’t. It is the transition that induces spark. And since electricity essentially flows at the speed of light (well not quite because it is flowing through a conductor and some circuitry) and it takes light approximately 0.000000003335640952 seconds to travel a meter, getting from the battery through the coil and ignition and back to the battery again before the next power stroke, even at red line, doesn’t even cause it to work up a sweat. The point at which the current stops flowing is what is important. It doesn't need more time to start flowing.

With points, it is essentially a mechanical switch that opens up based on the points rubbing against a cam that stops the current flow. With electronic ignition, it is an electronic switch that opens up, based on an electrical pulse from a magnet moving past a small wire coil. But the affect is the same. The current through the coil is shut off.

Within an ignition coil, there are actually two windings of wire wound very close together, but not physically connected to each other. The relationship between two windings located in this manner is that power dissipated in each will be essentially the same, but that the ratio of the voltage between them will be proportional to the number of wire turns in each winding. As an example, if winding 1 has 10 turns of wire and winding 2 has 1000 turns of wire, the voltage across winding 2 will be approximately 100 times greater than the voltage across winding 1. However, since the power must be the same, and power = voltage times current (not quite right for AC, but close enough for discussion), the current through winding 2 will be 1/100th of the current through winding 1.

Bottom line: It is the moment that current stops flowing through the coil primary winding that induces spark. The voltage produced in the primary winding of the coil is proportional to the inductance of the primary winding. The voltage induced in the secondary winding of the coil is proportional to the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. The ignition controls the moment that the current stops flowing through the primary winding. The voltage induced in the secondary winding is high enough to jump across the spark plug air gap. That, and a little gas and compression, and Suzi's humming.

[SPARKSS]

Howdy Tom. It's obvious you've studied some electricity. You posted the equation v=Ldi/dt which explains a PART OF what's happening in an ignition system. The terms di (delta current or change in current) and dt (change in time) are further defined by the formulas for "LR time constant" which is a missing key to this post. (I won't delve into the meat of that here).

You also infer that we're dealing with DC but in fact, we're dealing with pulsating DC which follows the same rules for AC.....and indeed, AC of any waveform, follows basic DC laws at any instant in time.

The time required to cause a maximum current (and thus maximum magnetic field) in the coil (a minimum amount of dwell) is insignificant at idle BUT the heat dissipated by the coil if left connected IS a factor in it's durability. (A side note here is that many an ignition coil has been destroyed when ignition power was left on and the circuit was closed, without the engine running!) At high rpm the dwell must be sufficient to allow good coil saturation as the time required to build that magnetic field is fixed by the coil construction but the time available due to engine speed shortens.

Transformer action is a factor in the step up of battery voltage to ignition levels but it's not the only one. When the points (insert electronic ignition also) open, instantly the magnetic field tries to collapse across the primary side of the ignition coil......This would cause a voltage to appear across it IF current could flow......well it does (SIGNIFICANT) ....momentarily as seen by the spark across those mechanical points through the resistance of that spark (HIGH) and the rest of the circuitry (LOW). From ohms law E (voltage here) = IR it is calculated (and measured!) that this PRIMARY voltage is now high (several hundred volts - referred to as inductive kick for good reason!). It is THIS high primary voltage now which, with transformer action of the ignition coil, becomes 10-20KV or more, across the spark plug (which then sparks or completes it's half of the circuit).

Hopefully this hasn't bored you but shed light for others. The key part to this is that there is resistance which slows the process requiring time......often referred to as charging or discharging time (just like that which surrounds the battery except that we're talking about milliseconds instead of minutes or hours).

Now bring in the reason we do all this.........igniting a fuel air mixture. For a given fuel air mixture ratio, a definite amount of time (a few mS) is required to complete combustion to a good level, thus, at a given engine piston speed, the mixture had better be correct or it'll run out of time, AND that spark which starts the combustion had better happen early enough to allow enough time......which is why we start the spark BEFORE top dead center, allowing the combustion pressure to rise to a useful level at the point where the piston starts its downstroke. Speed the engine up AND/OR change the fuel/air mixture and the whole event requires a different amount of time, OR start of time to happen! This is why we time the ignition, cause advance to occur and why ignition dwell (circuit closed) and open time (spark happening with circuit collapsing) become a significant part of the total equation.

Spark plugs are given different gaps for different applications and engines and this confuses the understanding further. Why if we can SEE a relatively low voltage spark at a set of mechanical points, wouldn't this be enough to do away with the coil altogether?
Engine COMPRESSION of the fuel mixture and then the resulting higher COMBUSTION PRESSURE require a high voltage to enable a spark to occur and sustain under these conditions (which become more severe as the plug wears and the engine degrades). Because the reality is that a many fuel/air mixtures will not burn completely on their own, under a variety of conditions, a compromise TIME is required to sustain that ignition spark.......long enough at idle but not too long at high speed so as to have enough time to restore the ignition coil (recharge wouldn't be a poor term) for the next spark. The time needed to keep a plug continously arcing is defined by your V=Ldi/dt equation with the decay in voltage occuring because of the LR TIME CONSTANT, AND with an available very high voltage to start with which lowers over a couple of mS until the arc (spark) no longer sustains.

All of the above can be observed on a suitable oscilloscope (or other suitable graphing meter).

Hope this clarifys or opens discussion for some readers

[tomm]

Hi SparkSS,
WOW. I thought I was getting a little too complicated. But this stuff clearly isn't foreign to you either, and your elaboration is right on. About the only other thing I might add would be the role of the condensors in a points ignition to suppress that spark across the points. Agreed that diving into the meat of time constants in L-R-C circuits is probably a little out of scope here, but it does have relevance as the frequency increases.
I do confess that it's been a while since I studied this stuff, so I'm a bit rusty on some of the finer points. My first EE course included a section on vacuum tubes. Thanks for the clarifications.
Hopefully the thread helps to clear up some misconceptions about the ignition circuit in general though, or at least provokes questions / discussion. Even if the details don't make a lot of sense to some, I think the message is that things like comparing resistance measurements of two different coils to determine which one will provide the bigger spark is meaningless, as I saw stated in a thread some time back, or that converting from points to a Dyna ignition would provide a better spark simply because it gives more time for the coils to charge up, as I saw in another, more recent thread. The basic circuitry is pretty simple, and determining the reason for a 'no spark' condition is within the capability of pretty much anybody with a meter and a basic understanding of the circuit. The details of fine tuning that circuitry is not, as you have clearly pointed out, and troubleshooting a 'weak spark' condition or optimizing performance is a whole different story.

[SPARKSS]

Good points there as well Tom. The capacitor (errrrrr condenser) across the switching device also helps keep the spark going by sustaining a little oscillation with the coil primary which is why an ignition system may act a little poor when the condenser gets bad (or shuts down if shorted of course).

One error often made about circuits switching DC at a mere 100 Hz or less is that they're low frequency circuits and thus (with a sine wave in mind) should respond as such. Although true in that they do switch at a slow rate, the RATE OF CHANGE is often nearly square and it's THIS RATE OF CHANGE that is greatly influenced by the addition of inductors and capacitance.

Nothing quite like a good vacuum tube circuit to show, hear, smell and sometimes feel what just took place heheh.

[chef1366]

Thanks guys. This is what the tech section is for. Learning.
I've learned a bit about coils just by reading your posts.