Compression Misfire Again

[Steve@Advance]

A few posts down, Flying Belgian asked:

"Why would a plug fail with more compression? It's just a gap between a positive and ground electrode."

The question got danced around, Pete 23 gave some answers, but no one ever got down to the real "why" this happens.

There is no doubt it happens. A weak spark will idle, but fail under increased throttle, which means more cylinder pressure somehow cancels the spark.

The old spark plug testers proved the same, with a known good spark, put in a good plug, it would continue to arc as the regulated air pressure was increased. Put in a worn plug and the arc would fail under pressure.

So, lets try to figure out WHY this happens!

It has to be strictly pressure related, because it happens with the plug tester, which only introduces air, no fuel, no heat.

I tried searching for an answer, really didn't find any good answers, just verification that it happens.

So I went the other way, "Can an electric arc occur in a vacuum?"

Most of the answers went over my head, but the general response was it is more difficult to establish an arc in a vacuum. My reasoning, the presence of "nothing" has the highest possible resistance.

So, wouldn't the opposite apply, increasing the pressure would reduce the resistance making the arc easier to establish? Wouldn't the various gasses in air, along with some moisture, have "just a little" conductivity? Cram a lot in the gap, shouldn't it be easier to jump the same distance?

Remember, we are only dealing with air pressure, just as in the tester. Yes, there is fuel present, and heat, no fire yet...

Some of what I observed with the plug tester, a dirty partially fouled plug would begin arcing down the inside of the insulator instead of jumping the gap. I blame that on increased voltage looking for a place to go. But again, why? Why did increasing pressure make the gap more difficult to jump to the point the charge bled off through impurities stuck to the insulator?

So... Why???

John T, you want some of this!

 

[dsebens]

Its because arc resistance increases when you increase atmospheres. Combustion pressure is way higher than compression pressure, it is also load dependent even in NA. When you increase load you increase combustion pressure and you blow out the spark. In reality its not blowing out its creating an insulating atmosphere between the electrode and strap. Its why turbo engines have a tighter gap since they increase combustion pressure even more.

 

[olddog]

Excellent question!!!
For which I certainly don't have a proper answer.
But I wonder.... Does lightening need a properly ionized path
to get to earth?

 

[MarkB_MI]

They're called vacuum tubes for a reason. They wouldn't work otherwise.

 

[showcrop]

Pete's explanation is perhaps actually an observation. Here is what is happening: Combustion is a reaction which consists of the ionization of the fuel, causing dramatic expansion which is what generates the energy. The first stage of combustion is the spark. in order for the spark to propagate across from one electrode to the other the molecules of fuel in the path have to ionize which takes high voltage. The cylinder pressure is a dampening force on the expansion that ionization requires. The spark is, so to speak, being delayed by the squeezing of the molecules under pressure. Smaller center electrodes will propagate a spark better under high pressure, but they erode away faster unless they are made of iridium.

 

[dpendzic]

I believe that its the increased density of the compressed air which in turn has a higher resistance than atmospheric pressure

 

[High Octane]

We called it the birthday candle effect. Weak plugs n high cyl psi blows spark out like a candle.

 

[rustyfarmall]

(quoted from post at 21:43:24 10/27/20) Its because arc resistance increases when you increase atmospheres. Combustion pressure is way higher than compression pressure, it is also load dependent even in NA. When you increase load you increase combustion pressure and you blow out the spark. In reality its not blowing out its creating an insulating atmosphere between the electrode and strap. Its why turbo engines have a tighter gap since they increase combustion pressure even more.

So in theory then, if the increased pressure in the spark plug tester causes the spark to "blowout", narrowing the spark plug gap SHOULD cause the plug to begin firing again?

 

[showcrop]

(quoted from post at 05:52:29 10/28/20) We called it the birthday candle effect. Weak plugs n high cyl psi blows spark out like a candle.

Flames will blow out-spark won't because they are electric.

 

[dsebens]

Y es but when you narrow the gap the kV of the spark drops and it makes a weaker spark intensity. Which is why modern ignition is a high energy to m make a strong spark and small gaps. Also a truly worn out plug will have a huge gap. In a pinch you could smash it back down to run it for a bit. Its not ideal because the strap wont be perpendicular to the electrode but it could get you done. Assuming the porcelin isnt cracked

 

[John T]

Good Morning Steve "John T, you want some of this! " Sorry I don't have an answer grrrrrrrrrr my bad Im too old for this lol but will offer the following non technical faded memory best guess:

Like you Ive heard and know this to be true, ultra high compression racing engines typically require higher energy higher voltage capable ignition systems, but WHY I cant explain.....Old radio vacuum tubes created a vacuum inside to allow for electron emission off a heated cathode up to the relatively cooler plate. OPPOSITE TO A VACUUM, as compression increases the density of fuel/air molecules in the compression chamber increases and that SOMEHOW?? makes it more difficult to cause electrons to arc jump across the plug gap ?? Also perhaps it has something to do with the ionization of air in the gap becoming more difficult if the density (higher compression) increases ?? DARN IF I KNOW

Professor Jim is more current in this area maybe he knows????????????

Fun sparky chatting with you

John T

 

[sotxbill]

in a vacuum, it will arc at lower voltage. In the military, we have to put a special rtv over the high voltage leads on the aircraft radar systems. As they would NOt arc on the ground, but would arc as the aircraft went higher. My understanding is.....

that the air itself acts as a higher resistance to current flow but as it thins, this resistance barrier of molecues is reduced, so that the same voltage will now start arcing to nearby objects.


And yes, I have 3 or 4 spark plug testers in my garage I have have bought over the years. ALL plugs will FAIL if your turn the pressure up high enough. So that would indicate the air molecules have a higher resistance, the denser that they are packed. I have old honda motorcycles from the 60s that are very finicky on sparkplugs. So I clean and then test them. I also clean and test my mower and tractor plugs. You will be very surprised at how many plugs fail out of the box. On american brand plugs, it one in four that dont pass the required pressure test. I end up putting one of the used plugs back in the tractor is its actually better than a new one in many cases. Japanese plugs seem to always pass to the required pressure level. So for an engine running high/higher compression, the plug brand can be pretty critical. And yes, the bad plugs are not making it to 120 lbs in most cases, so that means they fial in a lot of medium compression engines as well.


Another question to ponder... When they fail, its because you no longer see an arc through the little glass window. Could it still be passing electricity, but since there is no air you can no longer see it???? Or there is no longer heat as the oxygen is removed? Curious minds want to know?? As the radar continues to work perfectly when you see NO arc, I'm assuming there is NO longer transfer of electricity....

 

[pete 23]

I did say my comments were not science, just that we know it happens.

 

[DoubleO7]

"Another question to ponder... When they fail, its because you no longer see an arc through the little glass window. Could it still be passing electricity, but since there is no air you can no longer see it???? Or there is no longer heat as the oxygen is removed? Curious minds want to know?? As the radar continues to work perfectly when you see NO arc, I'm assuming there is NO longer transfer of electricity...."

i would agree, the arc is visible because of certain elements and particles in the atmosphere.
The same creates resistance which creates heat.
Extract all elements and particles in the space and you loose the ability to see the arc and you eliminate the heat?

And as far as in a vacuum or vacuum tube, there can't be "nothing"
There is still "something" in there. Suchas an introduced inert and purified gas, etc. There is still something in the vacuum.
Otherwise in a perfect vacuum, the container would collapse to the point of zero space.

 

[Determined]

Paschen's law

Basically the dielectric strength of air increases when under pressure.

 

[buickanddeere]

Air is an insulator . The more air between the electrodes , the more insulation value there is .

 

[Janicholson]

A perfect vacuum is not happening on earth, but so close that there can be just a few atoms per CC. All the container needs to do is hold out 14.7 psi to be
successful. There is no physical reason for creating a micro black hole. Jim

 

[Janicholson]

The article link discusses the use of dry compressed air as the working gas in a high voltage switch. This switch was originally designed to operate in an insulating gas made with Sulfur Hexafluoride compound HF6 that are nasty to work with and hard on the actual atmosphere we breathe. Air is universally addictive, and rather OK to use as a non conductive gas to quench are ionization and provide heat dissipation in the switch. It turns out that it is effective and works. As voltages were increased, the only test modification needed was the increase in air pressure inside the switch. Nitrogen is 78 % of air, and a good insulator. It is also more insulating as it is compressed. The dielectric value of air is high at 1.0003XXX Permittivity (not ohms) Jim
Science stuff

more

 

[FarmingEngineer]

Like others have said, the dielectric strength of air goes up with pressure. But you can actually blow out an arc with air. It is a common way to suppress arcs in high voltage circuit breakers. In air, the arc travels along a path of ionized gas and the air flow can disrupt this path. The moving air also helps remove heat from the area which makes it harder to ionize the air. The turbulence in the cylinder can make it harder to establish and maintain an arc.

 

[Oregon Tractor Jack]

A friend of mine builds high power ignition systems for nitro powered dragsters. He built a custom distributor machine to test magnetos used on these engines. This tester accounts for rpm and air fuel density. It is amazing the amount of spark energy that is need to over come the super high compression in a fuel dragster when it is running at speed.

OTJ

 

[Steve@Advance]

Bill, the pressurized testers you have, is the test chamber simply an adjustable pressure vessel, or does it simulate turbulence, air blowing through as the pressure increases?

I haven't had my hands on one of those since I was a teen. But seems I remember it blew air through the chamber, but I could be imagining things!

 

[Steve@Advance]

That should be the ultimate test!

 

[Steve@Advance]

I agree, the combination of dielectric strength, and turbulence.

 

[Steve@Advance]

Thanks John!

I never thought of air being an insulator, but appears it is!

I think the turbulence has an effect also.

Always gotta be something! Life can't just be simple can it! LOL

Thanks!

 

[Steve@Advance]

Interesting stuff!

I think I understand some of it...

Thanks!

 

[Steve@Advance]

I remember back in the 80's I think, GM started using really wide gap plugs with HEI distributors.

The plugs had an X suffix, gaped at .060 if I remember. It was a short lived experiment, sold a lot of rotors and coils!

I think they were trying a last gasp effort at extreme lean carb jetting.

 

[ffreeb]

I worked for 30 years at a Westinghouse, later Eaton plant in Horseheads, NY. They make Vacuum Interrupters there. They are high voltage, high current switches. The contacts are in a vacuum because vacuum is an insulator. I can’t answer your question more than that.