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

[MolaKule]

Physic of the Gap – Spark Plugs, That is! Part II (by permission by the author)

As we stated in 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


In Part II, we examine the nuances and the timing of events in the spark itself. We will not discuss the spark timing with relation to piston position, which is today determined by various sensors, such as crankshaft sensors and lookup tables in the ECU’s software.

In a typical spark discharge system, the electrical field is increased until the voltage across the electrode gap breaks down the cylinder’s gaseous mixture. The impedance of the gap decreases when a streamer reaches the opposite electrode. Ionizing streamers then give rise to a current which increases rapidly.

There are three stages to this process: The breakdown phase, the arc phase, and the glow discharge phase.

All of these phases happen within window of about 2.0 milliseconds.

In the breakdown phase, the voltage rises until current flows through the ionizing mixture, which can be as high as 200 Amps. But this high current only lasts about 10 nanoseconds. The ionization channel is a cylinder of about 40 micrometers. The temperature in this ionization column gets to 60,000 degrees Kelvin. A shock wave is also created with a pressure of about 250 atmospheres (3, 625 PSI). As the shock or blast wave propagates outward, the temperature and pressure of the ionization channel falls rapidly. The current then drops rapidly to the arc phase.

During the arc phase, the voltage drops to about 150 volts but the current is still up around 100 Amps, and the temperature of the ionization channel drops to about 6,000 degrees Kelvin. The arc increases in size due to heat conduction and mass diffusion. It is during this phase that much of electrode material is eroded away from the center electrode.

A glow discharge period occurs when the current reaches about 300 milliAmps and the temperature drops to 3,000 degrees Kelvin. It is during this phase that most electrode material is eroded away from the center electrode. The glow discharge phase lasts longer than do the previous two phases.

The material eroded away during each spark is very minute, but does add up over time, which is why replacement is necessary. As material is eroded away of course, the plug gap increases.


At about 2 miiliseconds, the current and voltage have decayed to zero.

The total amount of energy supplied to the spark is about 70 milliJoules.

During the breakdown phase, an energy of 10 milliJoules is distributed as follows: Plasma energy – 94%, Heat loss to electrodes – 6%.

During the arc phase, an energy of 10 millijoules is distributed as follows: Plasma energy – 55%, Heat loss to electrodes – 45%.

During the glow phase, an energy of 50 milliJoules is distributed as follows: Plasma energy – 30%, Heat loss to electrodes – 70%.


So, give thanks for this device that works so hard in such a high temperature, high pressure environment.

 

[michaelluscher]

To the spark plug, I say.....

Well done & well played.

 

[oilboy123]

I used to be pretty fascinated by the whole ignition concept. I had an MSD on my old Chevy and was always tinkering with the plug gaps, timing curve on the distributor.

I even did a report about ignitions systems when I took some college courses in auto tech. The speed that things occur in an engine are truly staggering.

Like the show It's really a modern marvel.

I'll add that I was once late for class at the college and had an MSD ignition system on my car. It would not turn off because of a key switch problem. I panicked and tried pulling the primary wire to the coil and got blasted with I believe was about 450 volts. I was not able to pull the wire off.
So without thinking I pulled the secondary wire off the coil and got blasted by about 40,000 volts. My hand pulsed with each zap of the spark until the engined quit turning. I'll be 52 this year and am convinced I would have had a heart attack if that happened today 28 years later.

 

[Clevy]

So Molekule.
As far as spark plugs go is there a design or brand that is superior or is a spark a spark.
I indexed the ones on my Harley and I could feel the improvement an indexed plug at low speed and low rpm. Exhaust smelled less like fuel as well.
I was firmly set in my belief that a spark was a spark,and if you indexed them you have the perfect conditions for the spark event.
Then I went last spring to get plugs for a mustang and all they had 8 of were the E3 brand so I bought them.
As I pulled the old ones I saw the previous owner had used a 4 electrode type,and looked in great condition however the vehicle was new to me and the plug change was a good excuse to investigate the engine and read the plugs as I also noted if the tips had oil or excessive carbon build up.
So all pulled. The rear passenger side had wet oil on the tip,so I now knew which cylinder that puff of smoke was coming from in the mornings from the passenger bank.
Anyways I got er all buttoned up with new wires,even though the previous ones were fine and installed an inverse oiler filled 50/50 mmo/seafoam and opened the valve fairly wide to do a top end clean while driving instead of the standard drowning.
The first start felt like it caught faster. Then I get moving and it feels like a new car. I puttered to the freeway at low rpm to get the engine hot and rowed the gears down an on ramp and even with that light smoke trail I'm leaving behind the engine is pulling far better than before the swap.
So what I'm asking is if these E3 plugs do something special or why they seem to add some pep to my cars and bikes step.
I just put them in the hemi. Which was a waste since the originals looked new,like less than 100 miles new however I figured what perfect conditions these are to see if E3 plugs helped an engine perform better than standard copper tips(my usual pick).
And again I could feel it right away. Much more responsive although I don't think any real power was realized. My Harley saw the largest jump in responsiveness which tells me alot about Harley plugs.

I read somewhere that a spark plug may not always spark enough to initiate burn and inevitably that translates into a less forceful explosion on those occasions.
Is there any truth to that?
And if so could that mean these E3 plugs just ignite that little bit better which translates to complete burn and more forceful explosion and give the end user a more responsive engine.
That's all I can think of that could account for the extra light pedal zoom I feel now.
At wot its much less perceptible,on all 3 vehicles but it is there.
Any insight you want to share will be well received.
I'm still leaning towards the spark is a spark idea however my own experience is now forcing me to open my mind to an idea I've dismissed all my life. I'm sure you understand my confusion and any info you can share would go a long way with me.

Thanks

 

[OVERKILL]

Clevy:

If a plug doesn't start the burn, you have a misfire.

A combustion event isn't an explosion but rather a forceful flame-front that propagates across the bore. The plug simply initiates this, so I can't see how some magic spark plug is somehow going to make this any different

 

[SteveSRT8]

IME there is little to be gained in a modern car in real world hp or efficiency due to the massive improvement in ignition systems. More and more cars are now using multi strike systems and high current super sparks abound!

I love indexing, still do it on the boat and my toy, but never even tried it on the Hemi with 16 plugs! That would take hours.

Not a believer...

 

[MolaKule]

Here is another interesting fact:

The resistance of the arc varies with time in a SP but is on the order of 0.005 ohms for the first few nanoseconds and then increases to about 1 ohm for the other two phases.

Energy = I^2arc Rarc X Delta [time], so for the initial current rise of 200 Amps in 5 nanoseconds, Energy = 1 microJoule.

So, with that information and knowing that heating Power is equal to I^2 X R, why is it that most center electrode erosion occurs in the second and third phases?

 

[NateDN10]

Originally Posted By: MolaKule
Here is another interesting fact:

The resistance of the arc varies with time in a SP but is on the order of 0.005 ohms for the first few nanoseconds and then increases to about 1 ohm for the other two phases.

Energy = I^2arc Rarc X Delta [time], so for the initial current rise of 200 Amps in 5 nanoseconds, Energy = 1 microJoule.

So, with that information and knowing that heating Power is equal to I^2 X R, why is it that most center electrode erosion occurs in the second and third phases?

If the energy released during the first phase is 1 uJ and the total energy released is 70 mJ, then 99.999% of the energy is released during the second and third phases. And I guess more energy means more erosion?

 

[MolaKule]

Quote:
And I guess more energy means more erosion?


You got it Nate. Well done.

Since Energy = I^2arc Rarc X Delta [time], more heating (I^2arc Rarc) X Longer time periods results in more erosion during the second and third phases.

 

[OVERKILL]

Originally Posted By: MolaKule


Quote:
And I guess more energy means more erosion?


You got it Nate. Well done.

Since Energy = I^2arc Rarc X Delta [time], more heating (I^2arc Rarc) X Longer time periods results in more erosion during the second and third phases.


What does this mean in terms of the use of precious metals (Platinum/Iridium) on modern spark plugs where the OEM runs a larger gap?

 

[MolaKule]

Originally Posted By: OVERKILL

What does this mean in terms of the use of precious metals (Platinum/Iridium) on modern spark plugs where the OEM runs a larger gap?


A wider gap simply means a slightly higher voltage is needed.

A wider gap gives rise to a longer plasma channel which evolves into a larger sphere of plasma as it propagates outward through the fuel/air mixture.

The spark plug's center electrode tip is capped with the precious metal of Platinum, or Iridium, or Ytrium which are more resistant to erosion caused by heating in the plasma channel.

 

[OVERKILL]

Originally Posted By: MolaKule
Originally Posted By: OVERKILL

What does this mean in terms of the use of precious metals (Platinum/Iridium) on modern spark plugs where the OEM runs a larger gap?


A wider gap simply means a slightly higher voltage is needed.

A wider gap gives rise to a longer plasma channel which evolves into a larger sphere of plasma as it propagates outward through the fuel/air mixture.

The spark plug's center electrode tip is capped with the precious metal of Platinum, or Iridium, or Ytrium which are more resistant to erosion caused by heating in the plasma channel.



The erosion was where I was going with that. Is the reason for the use of fine tip precious metal electrodes so that they can run a larger gap without the same amount of erosion as they would experience with the traditional steel electrode material?

 

[MolaKule]

Quote:
Is the reason for the use of fine tip precious metal electrodes so that they can run a larger gap without the same amount of erosion as they would experience with the traditional steel electrode material?


The reason for the use of the exotic metal tips is to increase the longevity of the spark plug, specifically the tip as higher energy ignition systems came into vogue.

When engine manf. needed larger gaps and higher energy ignition systems, the old nickel-steel tips could not go much beyond past 35,000 miles without regapping. With 100k warranties, etc., the manf. wanted spark plugs to go at least 110,00 miles, hence the exotic metal tips.

 

[OVERKILL]

Originally Posted By: MolaKule
Quote:
Is the reason for the use of fine tip precious metal electrodes so that they can run a larger gap without the same amount of erosion as they would experience with the traditional steel electrode material?


The reason for the use of the exotic metal tips is to increase the longevity of the spark plug, specifically the tip as higher energy ignition systems came into vogue.

When engine manf. needed larger gaps and higher energy ignition systems, the old nickel-steel tips could not go much beyond past 35,000 miles without regapping. With 100k warranties, etc., the manf. wanted spark plugs to go at least 110,00 miles, hence the exotic metal tips.


OK, that's about where I was with that, thanks for confirming

 

[martinq]

The special alloy tips also allow for a small/sharp electrode that will still far outlast a conventional tip. These fine-tip plugs have lower voltage requirements and, in my experience, offer superior starting/running/emission performance.

Plug of choice is the common NGK G-Power (single platinum).

 

[MolaKule]

A conventional 2.5mm tip (0.1") creates an initial arc current density of 80,0000 Amps/m.

The smaller tips, such as Bosch 2's increases the current density through the plasma arc. Assuming the ignition system supplies the same current and voltage, a tip diameter of 0.025mm (1/4 that of the above) would result in an initial current density of
8 million Amps/m.

As such, these tips show greater localized heating and erosion.

Irridium, platinum, and ytrium are transition metals which are very hard, brittle, and highly corrosion-resistant metals even at temperatures reaching 2000°C.