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

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MolaKule

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

Today’s spark plugs operate in a very hostile environment and can continue to operate to over 100,000 miles due to advanced gap materials.

An overview of ignition systems can be found at :

http://www.jetav8r.com/Vision/Ignition/CDI.html

The vehicle’s electronics supply current to the ignition coil’s primary and step up the voltage from 12 V to 30,000 volts or more on the secondary, a voltage step-up ratio of 2,920.

The spark plug gap is essentially a capacitor with two plates, the center electrode and the side or “ground” electrode. The voltage across the gap is increased until the electrical field exceeds the breakdown voltage of the gas. A current then flows which ignites the gas and a plasma forms in the gap.

In dry air at Standard Atmospheric Pressure (STP), this breakdown voltage is 3,000 volts per millimeter. For a gap of 0.030 inches, the gap is equal to 0.762 mm. This results in a breakdown voltage of 2,286 volts for this gap for dry air.

But there is some added complexities here.

1) There is a law of Physics called, “Paschen’s Law” which says the breakdown voltage (we call it ‘potential’ in Physics) is a function of the distance between the electrodes and pressure of the gas, or Vb = f(P, D). The greater the pressure and distance between the electrodes, the greater the voltage has to be developed between the electrodes for an arc to cross the gaps.

2) The temperature of the gas mixture also adds some complexity to the breakdown voltage required.


3) The constituency (makeup) of the gas (hydrocarbons and atmospheric gas molecules) in the cylinder also adds to the breakdown voltage requirement.

So we need at least 5 times the 2,286 volts or greater than 10,000 volts.

The ignition voltage of modern systems is about 3 to 4 times that in order to insure reliable combustion in cold temperatures, and to overcome resistances in the wiring and spark plugs.
 
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Thank MolaKule for another "White Paper" on automotive subjects. I'm sure that we all have a learning experience from these. FWIW--Oldtommy
 
The study of what happens across a plug gap mainly involves plasma physics. It's also why secondary waveform analysis can be so valuable.

I'll also say that, next to oil, "performance" ignition components are probably one of biggest scams perpetrated on an unsuspecting motoring public.
 
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Originally Posted By: skippy
The study of what happens across a plug gap mainly involves plasma physics. It's also why secondary waveform analysis can be so valuable.

I'll also say that, next to oil, "performance" ignition components are probably one of biggest scams perpetrated on an unsuspecting motoring public.

Really? Man, you all need to shut off the computers, close the scientific journals, and come up from the basement and get some fresh air. I've been a hot rodder my whole life. You sure don't need all the tech analysis. A higher compression performance engine is gonna benefit from higher voltage at the plug. A CDI multiple spark set up will give a more complete burn and higher combustion efficiency giving you easier starts and better throttle response. Nothing wrong with a well tuned point setup though as long as it sparks when it needs to. A plug with a heat range too cold will foul, too hot will cause pre- ignition. Sure ain't rocket science.
 
Nothing wrong with a little emprirical evidence to go along with the theory.
You're right, in that there was never anything wrong with ignition points.
They worked fine in my earlier cars, although they did require a level of PM unknown to modern car owners.
Imagine the typical owner of today confronted with a distributor containing ignition points and a condenser.
This is one of the reasons you used to see so many dead cars by the side of the road or sitting in parking lots.
Failure to maintain a very simple system combined with an utter incomprehension of how it worked.
 
"more complete burn and higher combustion efficiency giving easier starts and better throttle response."

That's some funny chit right there. Backyard mechanics always crack me up. They're so Dunning-Kruger.
 
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You may be demonstrating a little Dunning-Kruger effect yourself.
Guys who pretend superior knowledge crack me up.
They're so easy to make look foolish in front of an audience.
 
So, if you have a system 'capable' of providing 30.000 volts at the plug, will the system always provide 30k volts (or will the power just rise until an arc is made, up to 30k volts?)

I say this because I believe the a 'Hot' coil on a stock engine may not be an advantage.
 
Originally Posted By: expat
...(or will the power just rise until an arc is made, up to 30k volts?..


You're correct. The voltage will rise until ionization occurs at which point current begins to flow and voltage drops significantly. The point at which that happens involves not only gap size but cylinder pressure and the air fuel mixture within the gap. It's why this statement...

"A higher compression performance engine is gonna benefit from higher voltage at the plug."

...is mostly nonsense.
 
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I don't claim to have any knowledge but i have some experience to share. My Subaru Forester called for a plug change at 30k. I just changed them at 148k. They were nubs, but there was never any degradation in perfomrance or efficiency, and i can't even tell i changed them. Although i feel better. I think morern ignition systems can overcom most any flaw at the plug level.
 
This paper was written because spark plug questions popped up on the Lawn Mower and Small engine Forum.

Even welders can use a review of electrical concepts especially as it relates to arcs.

Quote:
My Subaru Forester called for a plug change at 30k. I just changed them at 148k. They were nubs, but there was never any degradation in perfomrance or efficiency, and i can't even tell i changed them. Although i feel better. I think morern ignition systems can overcom most any flaw at the plug level.


Modern ignition systems can actually measure the current at each firing in each individual cylinder and adjust the current. So when electrodes erode and the gap widens, the computer can send more current through the coil and increase voltage to compensate.
 
Originally Posted By: skippy
Originally Posted By: expat
...(or will the power just rise until an arc is made, up to 30k volts?..


You're correct. The voltage will rise until ionization occurs at which point current begins to flow and voltage drops significantly. The point at which that happens involves not only gap size but cylinder pressure and the air fuel mixture within the gap...


Quote:
1) There is a law of Physics called, “Paschen’s Law” which says the breakdown voltage (we call it ‘potential’ in Physics) is a function of the distance between the electrodes and pressure of the gas, or Vb = f(P, D). The greater the pressure and distance between the electrodes, the greater the voltage has to be between the electrodes for an arc to cross the gaps.
 
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Modern ignitions have become extraordinary in the last ten years or so. There was a time when a spark box (CDI), coil, etc. could help you but usually not now unless you are at ridiculous power levels. Horribly over sold without any benefit to newer cars at all.
 
I've read, don't know if factual, that the wider the gap the more voltage is needed. If running a way out of spec plug gap you could strain the coil leading to possible failure
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Whimsey
 
The design of modern ignition systems (high voltage capabilities) is such that they will fire the gap for a wide range of gaps.

A spark plug gap short or near short due to deposits is probably tougher on the coil since more secondary coil current has to flow through that resistance.

With today's ECU fault detection algorithms, the CEL will probably come on with an ignition fault code for either a short or open.
 
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Most cars are extremely sensitive to the misfire codes. Obviously it is a serious problem, as it affects many downstream components.

While some particular models have coil issues, most are extremely hard to kill and never require any service during the life of the platform.
 
has nobody mentioned electrode shape yet?

i can't find the link for it, but it definitely has a measurable effect on voltage needed to overcome the spark gap.
 
Originally Posted By: RobertISaar
has nobody mentioned electrode shape yet?

i can't find the link for it, but it definitely has a measurable effect on voltage needed to overcome the spark gap.


Actually a great point, get it?

Seriously, a sharp point encourages a spark where a dull rounded shape makes it harder to jump the gap.

Good one.
 
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