Times Have Changed: Cold Starting

Using an air gap to measure voltage isn't the same thing at all as claiming that an air gap will CREATE a higher voltage. I agree completely that an adjustable air gap can be used to measure the potential available between two points, in this case the secondary coil and ground. That is pretty basic physics. What I don't believe is that making a gap larger will INCREASE the voltage available from the coil. I maintain that the coil has a certain voltage capability and the size of the gap you put in series with the secondary doesn't change that one whit. If the gap is small enough so that the voltage available will ionize the air in the gap, then a spark will occur. If the gap is larger than what the available voltage can span, then no spark will occur. But making the gap wider doesn't make the voltage higher. At last I've seen no theory or evidence that this is the case.

Matt

If the secondary voltage is raised by a gap upstream of the plug gap, it's higher *through the whole secondary path*, including at the plug gap.

It didn't do anything useful on engines in anything approaching proper repair. It only propped-up neglected or worn-out systems.

DS

Yep. It's a neat trick for getting a recalcitrant, neglected engine started, or wringing every last possible tenth of a mile out of a thoroughly whipped engine, but that's about it.

Joe Pfeiffer wrote in news: snipped-for-privacy@cs.nmsu.edu:

As I understand it, once the field in the coil has collapsed and the HT voltage has been generated, that voltage is present until grounded.

The bigger the gap to jump, the longer the coil windings have to saturate, and thus the bigger the jolt generated once the field does collapse.

Again, as I understand it, the more more volts, the more likely it will be that some of the current will manage to jump the gap on a sub-optimal spark plug.

As always, corrections welcome.

It should be the voltage on the primary (probably 12-14 volts) times the turn ratio between the primary and secondary. That is basic transformer theory.

This link explains it quite simply. Notice that the voltage on the secondary is a function of the voltage used to "charge" the coil and the turns ratio, and not a function of any air gap on the secondary circuit.

*-http%3A//

Matt

It won't increase the total voltage available from the coil, if the coil has a maximum capability of 40 KV, then that is the most you will get. But looking at the spark tester, if a smaller gap represented where the number 20 is equals a coil output of 20KV and a larger gap where the number 40 is equals a coil output of 40KV, what other conclusion could be reached other than a larger gap will cause the circuit voltage to increase?

I'm not talking about the total voltage capability, ignition coils do not normally run nor would you want them to run at there maximum total capability. If making the gap wider doesn't make the voltage higher, then please explain what value does need to change in a case such as a worn spark plug where the gap has increased because it sure as hell shows up as an increased voltage every time I've ever encountered it.

You haven't answered the question. Either you know it and have seen (measured) it or you haven't. I'm patient, I'll wait. Take your time. But I'll tell you this, a little practical application will go a whole lot farther than what you learned in college. Take your volt meter, set it on min-max and connect it to the coil of your minivan. Aww hell, let's quit screwing around, go here:

They give some great examples of exactly what I've been saying. Compare the distributor ignition waveforms to the DI waveforms, notice on the conventional waveforms that the firing voltage AND the spark line voltage are measurably higher than those on a DI system. Look at the waveform labeled DIS waste parade, it shows a very high firing voltage due to there being a gap greater than the spark plug gap (an open in a wire). Of course this can all be dismissed I suppose by simply claiming that Interro is some bogus fly by night outfit because you've never heard of them before.

That link is about as basic as it gets and in NO way delves into what we're talking about here, and just out of curiosity, what are the authors qualifications?

Not quite clear what higher "through the whole secondary path" would mean...

However - first, I misremembered. The coil does indeed have an induced voltage, not the induced current I was remembering.

And, it turns out (isn't the web wonderful?) that "The Subsidiary Gap as a Means for Improving Ignition", NACA report 57 (1920), is available at

The weird thing is that it turns out that the capacitances in the circuit are crucial. Basically, when the voltage is building up to fire across the series gap, a charge is also building up. When it gets to breakdown, this discharges, and a charge builds up across the fouled plug. The voltage across the plug is determined by the charge and the capacitance, and will be much higher than you'd expect from the current and resistance.

That is my premise, but that isn't what has been claimed by a couple of others here. I'm not ruling out that this may even be possible, I just don't see how and haven't seen any theory that is supported by well-known laws of physics.

No, the larger gap just shows that more voltage was available. It didn't change it. That is the same as looking at your outdoor thermometer when it is 80 out and then looking again and seeing that it is now down to 40 and claiming that your thermometer made it get colder out! :-) Measuring something and changing something are two different concepts.

How does a coil change its voltage capability? This is determined by the coil ratio and the voltage on the primary side. Are you saying that old cars with points ignition systems had a way to adaptively change the voltage being applied to the primary side of the coil? I don't think so. And you can't change the coil ratio so what else are you changing to affect the voltage at the secondary?

Matt

Excellent find! I scanned this report briefly, but will have to study it a little later. Looks like that the key issue is controlling the rate of discharge of the coil so that the energy peak will be higher than would be the case discharging through a fouled plug where the leakage would allow broadening of the energy pulse and thus lower the peak. Very interesting and finally some good theory and practice discussed.

How did you come across this?

I wonder if this still applies to ignitions where the coil is switched via a transistor rather than a contact opening...

Matt

Certainly not by me.

You either missed or are ignoring my previous comments where I stated that I had used this device while connected to my secondary ignition scope, when set to the 20KV gap, the coil output was 20KV, when set to the 40KV gap, the coil output was

40KV , that amounts to changing something and measuring the results. That is the result when you open the gap.

I never said it could. But they certainly do not output the same voltage under all operating conditions. Just like an engine does not produce the same horsepower under all conditions even though it may have a horsepower capability of 350 H.P.

That would be one of the things one might observe a ballast resistor doing under various operating conditions although I'd maintain that it's actual purpose is to control current.

The gap. It doesn't change the maximum voltage that an ignition coil can output but it (the gap) will change the voltage output up to the point where maximum voltage is achieved. Open the gap, it takes more voltage to arc across, cram more oxygen molecules in between the gap, and it will take more voltage to arc across the gap. Observe firing voltage at idle, let's say it's 12 KV, snap the throttle, the firing voltage increases to 22 KV, the reason it increased is because opening the throttle allowed more air molecules into the combustion chamber/in between the plug gap. When the engine decelerates, the firing voltage may drop to 4 KV. Secondary ignition voltage does not remain constant under various operating conditions. Observable fact, easily observable.

The problem arises not when the plug resistance is too high, but when its too LOW (fouling providing an alternate conductive path around the gap). The slight conductivity of the fouled plug can prevent the coil from fully saturating, even though its still a realtively high resistance path- ignitiion coils don't generate much current at all, so it doesn't take much "leakage" to prevent it from saturating. Inserting another gap "upstream" of the plug will prevent current from "leaking" through the plug, allow the coil to saturate, and then when the upstream gap breaks over the full voltage suddenly appears at the plug gap and jumps it despite the "leakage" path around the gap.