Physic of the Gap – Spark Plugs, That is! Part I

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with both at 300KV, the arc still completes at a distance of 54.7cm, while with the 100cm diamater spheres it will only complete at a gap of 11.5cm.

almost a factor of 5.


And considering the physics of the spark gap, why is the case?
 
I think you have to read the table a different way. The smaller the diameter of the point, the higher the localized field strength, and the lower the voltage required to strike a spark.

For instance, a voltage of 75kv will create a 10cm spark with a needle point, but it takes 275kv to create a spark of the same length with a 100cm sphere.


Originally Posted By: RobertISaar
here it is. took a while putting the right keywords into google.

http://www.kronjaeger.com/hv/hv/msr/spk/

extreme test point on there: needle vs 100cm diameter spheres.

with both at 300KV, the arc still completes at a distance of 54.7cm, while with the 100cm diamater spheres it will only complete at a gap of 11.5cm.

almost a factor of 5.
 
You got it jaj.

There are a number of ways of looking at the chart.

As the size of the center electrodes are made smaller, the electric field intensity is stronger, the initial arc diameter is smaller, and the result is a higher current density flowing through the arc.

Since Outdoor Equipment (OE) does not produce as high a spark voltage as automotive systems, recent "quick start" plugs have been introduced which have a tapered or smaller center electrode diameter to take advantage of this effect.

Take a look at the recent Bosch 2 or +2 series of spark plugs.

They have a very small center electrode (maybe 0.005 ") embedded in the ceramic insulator and two or more ground electrodes. The center electrode is almost flush with the insulator.
 
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seems like that with a thinner electrode, you'll have spark events happen more reliably when under poor conditions compared to larger electrodes.

as long as it sparks though.... wouldn't the end-result be the same, since less voltage needs to build up on the coil's secondary circuit before the arc happens?
 
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Quote:
as long as it sparks though.... wouldn't the end-result be the same


Taking the Bosch plugs as an example, the finer center electrode and multiple grounding tips gives rise to a more probable spark. That is, in case one side of the plug gets more fouling than another side, which I have seen happen, the other electrode provides a current path for the arc.

The smaller tips cause a higher temperature plasma arc with the net result being the heat of the arc is greater and the shock wave created is more intense (at a higher overpressure).

So the expanding sphere of plasma does a more thorough job of igniting the rest of the mixture, even under low temps, high cylinder pressures, and rich mixtures.

The downside is the increased heat of the electrode would lead to greater tip erosion if not for the platinum or iridium tip materials.

You have to admit, even the old 0.1" (2.5 mm) copper center electrodes have served us well through history.
 
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so, there is no disadvantage to using a thinner(sharper) electrode other than there being less material so all other factors remaining equal, would erode faster? the only other possibility that comes to my mind is if the electrode maintained too high of a temperature and caused pre-ignition/detonation. seems like that could be countered by dropping a heat range though.

i kind of want to take some copper plugs and "sharpen" the center of the electrodes and the ground strap as well to see if any noticable positive effect can be had.
 
Originally Posted By: RobertISaar
so, there is no disadvantage to using a thinner(sharper) electrode other than there being less material so all other factors remaining equal, would erode faster?

i kind of want to take some copper plugs and "sharpen" the center of the electrodes and the ground strap as well to see if any noticable positive effect can be had.


You might need some sensitive equipment to detect any changes. What I think you would see visually is that as you sharpen the electrodes, you can open the gap and still get a decent spark. I would try it with a lawnmower first!


Not quite. The thinner electrode has the main advantage in that it fires with a more intense arc, which should lead to improved combustion, all other things being equal.

Due to higher temps from the more intense arc, a copper electrode would erode faster, which is why we now have platinum and iridium tips.

It's the platinum and iridium tips that allow us to go 100,000 miles with the higher energy ignition systems.
 
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Also notice that with the advent of the modern high energy ignition systems, such as the coil over plug or the one coil per cylinder engines, the plug gaps have increased.

What once were the standard gaps of 0.030" or 0.035" have given way to the 0.040" to 0.050".
 
Funny, I've been pulling plugs out and scratching my head over their performance for over 40 years and I NEVER saw one with a copper center electrode.

There were also cars in the late 70's that we ran gaps of .060 in!
 
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Originally Posted By: MolaKule
Also notice that with the advent of the modern high energy ignition systems, such as the coil over plug or the one coil per cylinder engines, the plug gaps have increased.

What once were the standard gaps of 0.030" or 0.035" have given way to the 0.040" to 0.050".



oddly, some applications are closing up their gaps... the GM 3100 and 3400 engines run .060 gap. their predecessors ran .045(with lower compression), yet with the same ignition system. their replacements are down to .040 with an even more powerful ignition system than what was used on the 2.8/3.1 and then 3100/3400.

Originally Posted By: SteveSRT8
Funny, I've been pulling plugs out and scratching my head over their performance for over 40 years and I NEVER saw one with a copper center electrode.

There were also cars in the late 70's that we ran gaps of .060 in!


a friend had a 83ish oldsmobile 88, it ran a .080 gap, which surprised me when i saw that.
 
Originally Posted By: SteveSRT8
Funny, I've been pulling plugs out and scratching my head over their performance for over 40 years and I NEVER saw one with a copper center electrode.

There were also cars in the late 70's that we ran gaps of .060 in!



I had not intended for this paper to discuss park plug construction and history since I assumed most people had prior knowledge of plugs or could Google but here goes:

Here is a link to a picture of a modern spark plug and the materials used.

Modrn spark plug Construction


From the terminal to the tip
1. terminal to Glazed ceramic body
2. resistor to reduce EMI to vehicle electronics
3. spring between Resistor and terminal
4. copper center electrode to conduct heat from tip to body
5. insulator forward to tip. Length and thickness determines "heat range"
6. spark tip; a pellet of platinium-, iridium-, palladium-, ytrium-, alloys laser welded to center electrode (4.). Older plug tips were made of copper alloy or nickel alloy pellets, depending on manufacturer.
8. Lower Threaded shell of nickel-steel
9. Nickel-alloy ground electrode.

The copper core spark plug had been used in piston engine spark plugs since the 1930's.
 
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Originally Posted By: RobertISaar
Originally Posted By: MolaKule
Also notice that with the advent of the modern high energy ignition systems, such as the coil over plug or the one coil per cylinder engines, the plug gaps have increased.

What once were the standard gaps of 0.030" or 0.035" have given way to the 0.040" to 0.050".



oddly, some applications are closing up their gaps... the GM 3100 and 3400 engines run .060 gap. their predecessors ran .045(with lower compression), yet with the same ignition system. their replacements are down to .040 with an even more powerful ignition system than what was used on the 2.8/3.1 and then 3100/3400.

Originally Posted By: SteveSRT8
Funny, I've been pulling plugs out and scratching my head over their performance for over 40 years and I NEVER saw one with a copper center electrode.

There were also cars in the late 70's that we ran gaps of .060 in!


a friend had a 83ish oldsmobile 88, it ran a .080 gap, which surprised me when i saw that.


Spark plug gaps for Racing applications generally run wide for various reasons.

Spark gap and heat range depends on various engine parameters such as available spark energy, fuel delivery system, compression ratio, combustion chamber geometry, etc.
 
My misunderstanding occurs at the tip. The center of the plug is NOT the point at which the spark occurs. The tip of the plug that actually sparks is never just copper IME.
 
Originally Posted By: SteveSRT8
My misunderstanding occurs at the tip. The center of the plug is NOT the point at which the spark occurs. The tip of the plug that actually sparks is never just copper IME.


No problem Steve.

Historically, the firing tip at one time was pure copper, then they went to a nickel-copper alloy, then a nickel-steel alloy, then to the exotic noble metals you currently see.

The reason for the copper "core" center electrode is primarily for heat conduction, since it is right above the hottest spot in the plug. You want the tip's thermal energy to be conducted to the body as soon as possible to keep the tip temperature from rising too quickly and too high.

It's an amazing device in terms of electrical and thermal physics.
 
Been watching this thread, as it's been a favourite subject for me over the last 30 years, including smokey's Plasma ignitions, and one of our university lecturers who was quite passionate about shape effects, flame kernel propogation, misfire rate etc. The working in electrical switchyards and hearing the coronal discharges on a foggy morning.

Back in the day (late 80s, early 90s), the plugs had copper cores, as I was bending the side electrode out of the way, using a mall triangular file to create a "V" in the centre electrode, then trying to index the plugs so that the major axis of the "V" ran towards the exhaust valve...they were copper.

Intention was that the gap stayed close (for the points/capacitor) ignition systems. There were two sharp edges that were reasonably protected from erosion. There was a large volume available for the flame kernel, and propagating flame towards the exhaust valve reduces knock.

Nowadays, with the ignition systems as they are, they can fire enormous gaps and reduce the "need" for that type of Saturday afternoon activity.





One of my other experiments was to try to make a better moustrap than the "spark gap" plugs that were available briefly, and make a "capacitor with inbuilt spark gap" to fit in the coil line. Two concentric brass tubes about 0.5mm apart. Epoxy to seperate, then all wrapped in HV tapes and epoxy.

Seemed to have an effect on idle (when mucking around with EGR at idle), but while tuning one day was silly enough to grab the assembly. The induced (or leakage, but the engine still ran) shocks locked my hand over the assembly, had my elbow, shoulder and jaw spasmining painfully, and I couldn't let go.
 
Thank you for this lesson Molekule. I for one really appreciate what you more knowledgeable members share with us,and me.
I've learned so much in this forum. No where in my day to day life would I be able to rub shoulders with the the people who frequent bitog. And that guys like you,Shannow,Caterham,Jim iirc(I am apologizing right now to those I may have forgotten but find solace in that I follow your posts with that neat little feature)actually take the time to answer my questions and share your vast experience. This forum really is something.
I'm looking forward to the next volume.........
 
Originally Posted By: Shannow

Back in the day (late 80s, early 90s), the plugs had copper cores, as I was bending the side electrode out of the way, using a mall triangular file to create a "V" in the centre electrode, then trying to index the plugs so that the major axis of the "V" ran towards the exhaust valve...they were copper.


You could have just bought the NGK V power and call it a day.
 
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