The capacitor also gives an extra ~400V to the primary of the coil, when the engine is running. This ~400V comes from the voltage spike stored in it. Due to the preceding cycle of the system.

The ~400V that is momentarily present on the cap during the secondary arc doesn't remain on the cap after the arc is over to help during the next cycle. After the arc is extinguished, the voltage on the cap quickly becomes equal to the battery voltage, assuming we're talking about the circuit shown in post #4.

That seems like a stretch as far as being why they call it a condenser.

If sized properly, it prevents a spark from forming at the points at all -- upon opening anyway.

Since the voltage can't change across the capacitor (and if you buy a replacement in an auto parts store they are almost always labeled as condensers) instantaneously, the voltage across the points has to change continuously, starting from zero, as the points open. As the voltage builds in the capacitor, so does the voltage across the point gap. But the point gap is widening while, at the same time, the energy is being expended in the spark at the plug. If the capacitor is big enough, the voltage across the point gap never reaches the value needed to create a spark. If the capacitor is too big, it will rob too much of the energy and reduce the spark strength. As the points close, they are closing on a charged capacitor and that is when any spark happens and also when you get the greatest current through the points as it rapidly discharges that cap. The smaller the cap, the less energy has to be dumped, but the higher the voltage it will have charged to and, hence, the earlier in the closing motion the arc will strike. Its primarily this arc that pits the points in normal operation, as indicated by the much smaller pits compared to those seen when the condenser fails or is removed.

Again assuming we're talking about the circuit arrangement shown in post #4, the voltage on the cap each time the points close is just equal to the battery voltage. The relatively high voltage the cap "charges to" during the secondary arc doesn't remain on the cap for long. It's gone as soon as the arc is over and certainly doesn't last until the next closure of the points.

 

Did I mis-read that? The ignition spark happens when the points OPEN.

The points cap has little effect on spark intensity at all. It's primary function is to reduce points arcing when the points open, to increase points contact life and reduce RFI.

The other principle use is to quench the arc, if the arc is sustained, then the primary of the coil remains conducting while this arc is in effect.

A single coil 8cyl engine revving at 4000rpm has just over 260 firing cycles/sec.

It is very important to quench the arc immediately at this frequency, otherwise timing is drastically affected.
Any one who doubts the conduction capability of ionized air only has to watch a plasma torch in operation.
The arc occurs at points opening.
Max.

 

Again assuming we're talking about the circuit arrangement shown in post #4, the voltage on the cap each time the points close is just equal to the battery voltage.

That depends on the time constants involved. If they are given enough time to die out, then yes, the cap voltage will just be the battery voltage. But if the points close before the transients die, they may not be.

 

How would the cap "get too big"? With any normal points cap the cap timeconstant is very small compared to the period of the points cycle or dwell. So the job of the cap is done at the instant the points open or close, not for the duration of dwell or pause.

The same goes for the time constant of the coil, the magnetic field completely collapses and makes the spark in the first couple of percent of the pause (period of points open). For 95% of the pause time (the points are open) the cap and coil both do absolutely nothing.

 

How would the cap "get too big"?

Putting too big a cap there?

Keep in mind, I'm discussing the topological circuit since the discussion started with a very generic setup that didn't involve points or a condenser at all and the discussion moved to the effect of a capacitor across the opening switch.

With any normal points cap the cap timeconstant is very small compared to the period of the points cycle or dwell. So the job of the cap is done at the instant the points open or close, not for the duration of dwell or pause.

The same goes for the time constant of the coil, the magnetic field completely collapses and makes the spark in the first couple of percent of the pause (period of points open). For 95% of the pause time (the points are open) the cap and coil both do absolutely nothing.

But these are because the components have been sized to make this happen. It is NOT a consequence of the circuit topology.

 

That depends on the time constants involved. If they are given enough time to die out, then yes, the cap voltage will just be the battery voltage. But if the points close before the transients die, they may not be.

Keep in mind, I'm discussing the topological circuit since the discussion started with a very generic setup that didn't involve points or a condenser at all and the discussion moved to the effect of a capacitor across the opening switch

In post #37, you said "But the point gap is widening while, at the same time, the energy is being expended in the spark at the plug." I got the definite impression that you were talking about a vehicle ignition system, spark plugs, points and all.

Then you said "The smaller the cap, the less energy has to be dumped, but the higher the voltage it will have charged to and, hence, the earlier in the closing motion the arc will strike."

I didn't realize that you had made a transition in the middle of the paragraph from talking about vehicle ignition systems in particular to talking about a generic coil, condenser and points topology. But, we could consider two cases.

Case #1: In a vehicle the time constants are such that the voltage across the capacitor will be the battery voltage when the points close. The statement in red is not true in this case.

For the sake of generality we can consider the case of a very much larger time constant.

Case #2: A possibly non-vehicle situation where the cap is much larger than in Case #1. Consider that the time from opening the points until the next closure remains fixed (constant engine RPM if it were in a vehicle). The cap is large enough that the cap voltage is greater than the battery voltage when the points next close. Now decrease the cap size. The peak voltage the cap rings up to is higher, but the time constant is shorter so the voltage across the cap decreases more rapidly, sooner, and in fact the voltage across the points when they next close after the peak (presumably we don't want to close them before the peak) will be lower, not higher, as shown by analysis.

The statement in red isn't true in this case either. One must take into account not only the higher peak voltage across the smaller cap, but also the shorter period of the oscillation.

 

In post #37, you said "But the point gap is widening while, at the same time, the energy is being expended in the spark at the plug." I got the definite impression that you were talking about a vehicle ignition system, spark plugs, points and all.

Then you said "The smaller the cap, the less energy has to be dumped, but the higher the voltage it will have charged to and, hence, the earlier in the closing motion the arc will strike."

I didn't realize that you had made a transition in the middle of the paragraph from talking about vehicle ignition systems in particular to talking about a generic coil, condenser and points topology. But, we could consider two cases.

Case #1: In a vehicle the time constants are such that the voltage across the capacitor will be the battery voltage when the points close. The statement in red is not true in this case.

For the sake of generality we can consider the case of a very much larger time constant.

Case #2: A possibly non-vehicle situation where the cap is much larger than in Case #1. Consider that the time from opening the points until the next closure remains fixed (constant engine RPM if it were in a vehicle). The cap is large enough that the cap voltage is greater than the battery voltage when the points next close. Now decrease the cap size. The peak voltage the cap rings up to is higher, but the time constant is shorter so the voltage across the cap decreases more rapidly, sooner, and in fact the voltage across the points when they next close after the peak (presumably we don't want to close them before the peak) will be lower, not higher, as shown by analysis.

The statement in red isn't true in this case either. One must take into account not only the higher peak voltage across the smaller cap, but also the shorter period of the oscillation.

So basically you are claiming that it is physically impossible to size the cap (and/or coil), either intentionally or accidentally, such that the points can close on a cap that is at a higher voltage than the battery voltage?

Perhaps this is the better way to view things. You are in the process of designing the first ever ignition system using this topology. What are the constraints and concerns and consequences of your component selections?

 

All interesting nits to pick.
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All interesting nits to pick.
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They are. And, as is commonly the case, there are several somewhat out-of-sync perspectives that people are viewing the discussion from. While that can be frustrating, it is also informative in its own right.

 

Delco Ignition System

Delco Ignition:
Function of the Capacitor

There is a capacitor (sometimes called a condenser) connected across the points. If the capacitor were missing or defective then the primary current would arc across the opening points allowing the current and therefore the magnetic field to collapse slowly causing a weak or no spark. As the points begin to open the primary current is diverted to charge the capacitor. By the time the capacitor is charged the points are open wide enough to prevent the arc and the current and magnetic field collapse quickly and cleanly providing a strong pulse. When the points close the capacitor discharges through the points helping to quickly re-establish primary current.

Max.

 

Perfect Max, that's what I was trying to show in post 12. No capacitor, very little to no spark.