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(Jim) Firing Voltage versus Compression

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John T

Well-known Member
ATTENTION SPARKIES AND WANNABEES:

This friendly discussion got lost below between Jim and myself and I would like to get more opinions regarding this question. It concerns the voltage necessary to arc jump current across the spark plug gap in an internal combustion engine.

I always thought as the compression increased inside the combustion chamber it took a higher voltage to fire the plug. I thought if the plug was outside the tractor at atmosphere the plug would fire at a lower voltage then if it were inside where the pressure may be say 100 PSI, and as the compression increased, the voltage necessary to fire the plug likewise increased. Thats the how n why I always thought why in extreme high compression racing engines they had to use the high voltage capable coils which rose maybe to 30,000 to 40,000 volts (versus maybe 10,000 in a stock engine, depends on compression and fuel) to insure the plugs fired.

NOTE the voltage of a coil is NOT instantly say 10,000 volts, it ramps up only high enough to the point where current arc jumps the plug gap and no higher. i.e. even if you use a so called high voltage 40,000 volt coil in a stock set up, it will still fire the plug at the same voltage the old stock coil did, its just that the HV coil has the CAPACITY to achieve higher voltage if necessary as in high compression engines (under my theory)

So I did a very quick search AND ALTHOUGH THIS IS NOT PERFECT OR CONCLUSIVE what I did find seems to me to confirm my belief that voltage has to increase (to fire the plug) as compression rises.

Please take a look and provide opinions/reasons/theories why or why not my thinking was correct or wrong as rain??????


http://www.stealth316.com/2-sparkplugtech.htm#j4c

Factors Affecting Required Voltage

The following operational factors affect the required voltage.
• Spark gap - the voltage needed to ionize the electrode gap is directly proportional to the gap width and the required voltage increases with a larger gap.
• NOTE Compression pressure - required voltage increases in direct proportion to cylinder pressure or as the air density increases.
• Engine load and speed - required voltage changes in direct proportion with engine torque and throttle opening because these change the cylinder air density.
• Air-fuel ratio - required voltage is at a minimum with a stoichiometric (14.7) air-fuel mixture because the air is just damp enough to improve electrical conductivity; rich mixtures increase required voltage because grounding across the insulator increases slightly, and lean mixtures increase required voltage because the air is drier.
• Electrode temperature - between 200º and 600ºC (~400º and ~1100ºF) required voltage decreases rapidly as temperature rises; above 600ºC required voltage falls at a much-slower rate.
• Electrode diameter - required voltage increases as the electrode diameter increases and as the roundness of the electrode edges increases.
• Electrode polarity - using the model that electrons flow from the negative electrode to the positive electrode and knowing that electrons are released more freely when metal is very hot (even though resistivity of metals generally increases with temperature), because the center electrode is hotter than the ground electrode, the required voltage can be lower when the center electrode has the negative polarity (true for half of our spark plugs).



http://holdenpaedia.oldholden.com/Timing_Light#The_Ignition_Voltage_vs_Compression_Phenomenon:

The Ignition Voltage vs Compression Phenomenon:

An important electrical phenomenon appears when a Xenon Timing Light is hooked up to a
Spark Plug Lead on a Petrol Engine.

The Timing Light will flash when the Compression Pressure inside the Cylinders is high, and not flash when the Compression Pressure inside the Cylinders is low. Essentially the Spark Plug can short out the Spark Plug Lead when the Compression Pressure is low and doesn't leave enough Voltage to trigger the Timing Lamp.

NOTE:::::::::::This is because it takes more Voltage to fire the Plugs when the
Compression Pressure is high and minimal Voltage when the Compression Pressure is low.

When the Engine is decelerating the lamp will stop flashing because
the Compression Pressure inside the Cylinders is very low. It's low at this time because the
Engine is compressing the high vacuum that was created in the inlet manifold with the throttle
closed and the Engine decelerating. This weak spark requirement is the reason why
Engines polluted heavily on the overrun prior to VK Commodores.

The Lamp will flash brightly under acceleration and when there is
enough Compression to allow the Engine to idle.

If you use a very expensive and sensitive Timing Light you may not see the symptom.

The observation explains why Engines with weak Ignition stall on take off. Take off is the time the Ignition needs are greatest and Idle the time when the Ignition requirements are least. Clearly the overrun case needs even less spark, but no spark at all is needed
for the overrun case.



Thanks gang

An ever curious Ol John T
 
(quoted from post at 22:50:44 12/12/10) ATTENTION SPARKIES AND WANNABEES:

This friendly discussion got lost below between Jim and myself and I would like to get more opinions regarding this question. It concerns the voltage necessary to arc jump current across the spark plug gap in an internal combustion engine.

I always thought as the compression increased inside the combustion chamber it took a higher voltage to fire the plug. I thought if the plug was outside the tractor at atmosphere the plug would fire at a lower voltage then if it were inside where the pressure may be say 100 PSI, and as the compression increased, the voltage necessary to fire the plug likewise increased. Thats the how n why I always thought why in extreme high compression racing engines they had to use the high voltage capable coils which rose maybe to 30,000 to 40,000 volts (versus maybe 10,000 in a stock engine, depends on compression and fuel) to insure the plugs fired.

NOTE the voltage of a coil is NOT instantly say 10,000 volts, it ramps up only high enough to the point where current arc jumps the plug gap and no higher. i.e. even if you use a so called high voltage 40,000 volt coil in a stock set up, it will still fire the plug at the same voltage the old stock coil did, its just that the HV coil has the CAPACITY to achieve higher voltage if necessary as in high compression engines (under my theory)

So I did a very quick search AND ALTHOUGH THIS IS NOT PERFECT OR CONCLUSIVE what I did find seems to me to confirm my belief that voltage has to increase (to fire the plug) as compression rises.

Please take a look and provide opinions/reasons/theories why or why not my thinking was correct or wrong as rain??????


http://www.stealth316.com/2-sparkplugtech.htm#j4c

Factors Affecting Required Voltage

The following operational factors affect the required voltage.
• Spark gap - the voltage needed to ionize the electrode gap is directly proportional to the gap width and the required voltage increases with a larger gap.
• NOTE Compression pressure - required voltage increases in direct proportion to cylinder pressure or as the air density increases.
• Engine load and speed - required voltage changes in direct proportion with engine torque and throttle opening because these change the cylinder air density.
• Air-fuel ratio - required voltage is at a minimum with a stoichiometric (14.7) air-fuel mixture because the air is just damp enough to improve electrical conductivity; rich mixtures increase required voltage because grounding across the insulator increases slightly, and lean mixtures increase required voltage because the air is drier.
• Electrode temperature - between 200º and 600ºC (~400º and ~1100ºF) required voltage decreases rapidly as temperature rises; above 600ºC required voltage falls at a much-slower rate.
• Electrode diameter - required voltage increases as the electrode diameter increases and as the roundness of the electrode edges increases.
• Electrode polarity - using the model that electrons flow from the negative electrode to the positive electrode and knowing that electrons are released more freely when metal is very hot (even though resistivity of metals generally increases with temperature), because the center electrode is hotter than the ground electrode, the required voltage can be lower when the center electrode has the negative polarity (true for half of our spark plugs).



http://holdenpaedia.oldholden.com/Timing_Light#The_Ignition_Voltage_vs_Compression_Phenomenon:

The Ignition Voltage vs Compression Phenomenon:

An important electrical phenomenon appears when a Xenon Timing Light is hooked up to a
Spark Plug Lead on a Petrol Engine.

The Timing Light will flash when the Compression Pressure inside the Cylinders is high, and not flash when the Compression Pressure inside the Cylinders is low. Essentially the Spark Plug can short out the Spark Plug Lead when the Compression Pressure is low and doesn't leave enough Voltage to trigger the Timing Lamp.

NOTE:::::::::::This is because it takes more Voltage to fire the Plugs when the
Compression Pressure is high and minimal Voltage when the Compression Pressure is low.

When the Engine is decelerating the lamp will stop flashing because
the Compression Pressure inside the Cylinders is very low. It's low at this time because the
Engine is compressing the high vacuum that was created in the inlet manifold with the throttle
closed and the Engine decelerating. This weak spark requirement is the reason why
Engines polluted heavily on the overrun prior to VK Commodores.

The Lamp will flash brightly under acceleration and when there is
enough Compression to allow the Engine to idle.

If you use a very expensive and sensitive Timing Light you may not see the symptom.

The observation explains why Engines with weak Ignition stall on take off. Take off is the time the Ignition needs are greatest and Idle the time when the Ignition requirements are least. Clearly the overrun case needs even less spark, but no spark at all is needed
for the overrun case.



Thanks gang

An ever curious Ol John T
Click to expand...
We used to have a combination sparkplug sand blaster and spark tester in the shop that was made by AC Sparkplug Company. It had a switch that enabled you to change between a new sparkplug and the plug being tested as you watched spark intensity through a glass window. There was also an air pressure regulator that simulated compression pressure. Customers in the 1950's thought it the ultimate in high tech science!
 
I have a method of assessing comparative firing voltage. Purchase an NE2 Neon lamp from radio shack or other. It is the size of a dash light bulb with two wire leads out the bottom. Attach a stranded 20ga insulated wire to both leads. Make one about 20" and the other long enough to go to a spark plug wire from the operator"s platform or driver"s seat. Wrap the spark plug wire with 15 turns of the 20gauge wire and tape in place with black electrical tape. Leave the end free!

Ground the other lead to the chassis.

At night, or in a dark building (with appropriate venting) run the engine. An put it through load variation. The neon light will dim and brighten as firing voltage changes.

Best of luck, and interesting (I am willing to be wrong) but not yet, I have seen too many spark lines on the scope. Jim
 
It takes 15,000 volts to fire a clean dry spark plug in open air gapped at .035" if ignition is coming from a distribor (with rotor and cap). If no rotor is involved. it takes 12,000 volts to fire the plug.

It takes only slighty more to do the same inside the engine - e.g. 1000-2000 volts more but the need can ramp up alot. Based on exactly what, I don't know. I've observed voltage increase, but cannot say I ever had any of knowing if it's due to higher compression, or a fuel-rich environment, etc.
 
Glad to hear from ya Jim, Yep what youre saying reads like the timing light discussed in the research I found. Like you, Im stickin with my assumption (firing voltage increases as compression rises, like the articles indicated) for now at least, if it keeps snowing here in Indiana I may have plenty time to research the matter further.

Keep warm n dry now yall

John T
 
NOPE Im still here on Rockport Road..... Oh well a couple weeks it may get better or worse grrrrrrrrr

John T
 
Hi John T: Your answer is that it takes more voltage to jump a gap when the air pressure goes up. OK? Don't ask for formula etc.. It has been about 46 or 47 years ago when I worked in a Research & Development Lab at Collins Radio in Cedar Rapids IA. One job I was on for a few months was to evaluate a new antenna coupler for the then front line F-4 Phantom fighter. I put it in a large pressure chamber and tested it at various pressure settings.. Sea level to 50,000 feet in 5 & 10K steps. Amazing how much RF energy was lost into space with less air pressure. I should have also run it in the dark and taken pictures of electric "sparks" all over the unit. Same concept as your question: Voltage needed with change in air pressure. More air pressure needs more voltage. Use the top fuel quarter mile engine voltage needed as a referance.. Duration(time) of the spark plays a big part too in amount of electrial power needed.
I quit Collins after just 6 years since I liked flying more and it payed better.. even though Collins R&D lab was a Great job. ag. ret.
 
Yep thats how I always understood it although I cant explain the physics of why. It seems somehow with more molecules packed in there in the space (higher PSI denser air) between the plugs electrode and ground strap theres more resistance to the striking of the arc across the crowed space, but if pressure decreases and especially if it were in a vacuum, it takes less voltage potential before the arc will occur.

The arc is ionizing the air molecules right??? Im still not sure how or why but Im confident it takes more voltage across the gap to arc current as the pressure increases.

Wheres a good physicist when we need him lol

John T
 
Sounds like the xenon timing light works about like the vacuum wipers on my old 50 chevy. Floor the thing in a driving rain storm and ya can't see cause the wipers stopped.

This isn't a part of your theory discussion here, but as compression increases doesn't the amount of spark have to increase to ignite the fuel/air mixture as well as in increasing the gap? i.e, with a stronger coil you can increase the gap and get more fire in your cylinder as well.
 
Voltage spikes on an oscilliscope certainly do go up under load. One of the tests with the old Sun Tester was the snap throttle test which would see the voltage spikes rise.

Secondary ignition missing occurs when voltage required exceeds voltage available. This is why a marginal ignition system will miss going up a hill or under acceleration.

If you were to follow the opposite view, as posted below, to its logical conclusion, then cars would miss at idle and then smooth out under load and acceleration. This is of course absurd.

I am somewhat skeptical that sparkplugs fire with the lowest voltage at the stoichiometric fuel ratio of 14.7 to 1. The stoichiometric mixture is the perfect mixture for controlling emissions of CO, HC, and Oxides of Nitrogen because it is in this window that the graphs of these three pollutants intersect. If the engine is run leaner, CO and HC will go down but NO will go up. Conversely, if the engine is run richer NO will go down, but CO and HC will go up. However if we are merely interested in smooth idle, such as a farm tractor with no emission requirements, then I believe 14.7 is a little too lean.
 
You ask:

"but as compression increases doesn't the amount of spark have to increase to ignite the fuel/air mixture as well as in increasing the gap?"

ANSWER: You would have to define "amount of spark"????? What is true is that as compression increases, the VOLTAGE across the spark plug gap has to rise higher BEFORE it fires its spark (which ignites the fuel)

Then you state: with a stronger coil you can increase the gap and get more fire in your cylinder as well.

ANSWER If the gap is increased (just like if compression is raised, read the articles in my post) that likewise increases the voltage necessary for the plug to fire. YES its true some after market so called "high voltage" coils have the capacity/ability to achieve higher voltage (as may required in high compression racing engines) buttttttttt regardless of HV or stock coil, the voltage only rises high enough to fire the plug so the HV coil wont fire at any higher voltage then the stock coil. ITS JUST THAT IF COMPRESSION IS HIGH OR GAP REAL WIDE (read my articles) THE HV COIL HAS THE ABILITY TO ACHIEVE THE NECESARY HIGHER VOLTAGE...

Great question, fun discussion yall

John T
 
Good info Tom, fun discussion for us sparkies lol

Yep under a load,,,,,,,,,or higher compression,,,,,,,,or a wider plug gap,,,,,,(read the attached articles) all increase the load on the ignition system and YES misfire occurs more under a load versus sitting there idling not working. I looked more this morning and CAN NOT find any literature that says the voltage required to fire a plug becomes LESS as compression rises or the plug gap increases,,,,,,,,,, but find only those that say the voltage must increase if comporession or the gap increases. If you find any let me know as Im always willing to learn something new

Thanks

John T
 
Good Morning John T: I didn't know how much you wanted to know but I should have guessed that you would also like to know WHY. My understanding, back then with that F-4 antenna coupler, was that the dielectric of whatever is in the path of the current will play a big role. (you were on the right path) The compressed air AND fuel molecules make it harder for the electric current to get through regarding a sparkplug.. I'll suggest reading the Wikipedia on Dielectric. I'll add the link below (without errors?) By the way, a few other things I learned back then were that any sharp corners vs a smooth surface played a role. Sharp corners bleed off electric current into space quicker then smooth surface. On sparkplugs I always use a small 3 sided file to round off all sharp corners on the ground strap. The study of the spark in a sparkplug could be used as the begining for a study of thunderstorm lightening. Many of the same laws of physics apply.. and there is still a lot that is unknown.. even to a physicist. I always enjoy watching a good science program on TV that deals with lightening.
By the way, that antenna coupler on the F-4 was an automatic tunning unit between any electronic item being used by the pilot and the antenn needed. Battle damage could be reduced with an antenna coupler to maintain electronics. Probably a $50k Black Box by now. Do you have a little better understanding of a "spark" now? Take care. ag. ret.
Dielectric
 
THANKS, yep it seems a bit crude in the analysis, but as compression rises the air is denser snd theres just more "stuff" lol which is dielectric in nature thats in the way of and hampers a current arc passing through it

IE AS COMPRESSION RISES IT TAKES MORE VOLTAGE BEFORE CURRENT WILL ARC JUMP THE GAP

Like I told Tom I find articles that say as the spark gap widens or compression rises it takes more voltage to fire the plug BUT CAN FIND NO ARTICLES THAT SAY IT TAKES LESS

So THATS MY STORY N IMA STICKIN TO IT LOL

thanks for the help and info ag, fun discussion

Ol John T long retired EE
 
Hello John T again: With a smile, your fighting the answer..
If a farmer sez that it takes more horses to pull a wagon up a hill.. you should assume that fewer horses are needed to go back down that hill.
In other words: Less voltage needed to fire a sparkplug if there is less air... accept it without finding it written in a science book.

By the way my link to Dielectric is a fairly good source of info... Did you read it all? It covers this topic..
ag..
 
In my job before retirement I worked with high voltage switch gear.

We had 4160 volt breakers that interrupted current in up to 5000 hp motors. As the breaker opened a piston in the switch gear blew a blast of air at the arc. The arc was extinguished and the switch gear wear was reduced.

IMHO the higher the density of the gas and air the easier to strike an arc as there are more molecules with loose electrons closer together. But as the engine is speeding up the turbulence in the engine cylinders is increasing at a high rate. To keep the arc from quenching from the turbulence more voltage is required.

also the spark plug gap has to be larger in high compression engines at high speeds to burn the gas at a much faster speed. This will increase voltage needed.

well if everyone agreed there would be no real discussions.

George
 
Filing off sharp edges of the sparkplug is nonsense. If that were true the plug would fire better as it wears into natural eroded roundness! Years ago when plugs were cleaned they were filed back to their original sharpness as electrons jump more easily from sharp surfaces than round ones. There is a reason lightning rods have points on them.
 
(quoted from post at 11:44:38 12/13/10) Hello John T again: With a smile, your fighting the answer..
If a farmer sez that it takes more horses to pull a wagon up a hill.. you should assume that fewer horses are needed to go back down that hill.
In other words: Less voltage needed to fire a sparkplug if there is less air... accept it without finding it written in a science book.

By the way my link to Dielectric is a fairly good source of info... Did you read it all? It covers this topic..
ag..
Click to expand...
t isn't linear or even close. The best dielectric (harder to strike an arc) is the vacuum, then it gets easier to strike the arc (poor dielectric) with a near-vacuum/some air present/about 0PSI, then as pressure increased to atmospheric (14.7PSI) it becomes a better dielectric (more difficult to arc), and this situation continues along the 'more difficult' trend as pressure further increases to say a cylinder pressure of 150PSI.
As can be seen from the graphs, general ratios are about 4X better dielectric at a 300,000 feet than at 150,000 feet and about 3X better at atmospheric than at 150,000 feet and 4X better dielectric at 150PSI than at atmospheric. It isn't good to become too focused on exact values of volts/inch, because the shape of the electrodes, the place up & down the whole voltage range (i.e., 8,000v or 70,000v) will all alter the specific arc over voltage. The text book atmospheric number it typically given as 76,000 volts per inch, but no HV circuit designer would use that number in determining spacing from ground...more typically 15,000v/in at high voltages and on a printed circuit board at lower voltages & close etch spacing, typically 8,000 volts per inch. Many variables. Whole books have been written on just dielectric properties of atmosphere. Part of the explanation of the behavior of these curves is in number of molecules present and the free travel path available in which to gain energy before striking another molecule. At vacuum extreme, no gas molecules present, no molecules to collide & result in ionization, thus infinite dielectric strength and in the middle pressures, some molecules, long free travel paths in which to gain energy, resulting in ion producing collisions & poor dielectric strength. Then at high pressures, the dense molecules have very short free travel paths in which to gain the energy required to disrupt another molecule & resulting ionization, thus high dielectric strength. Various contaminants can alter dielectric strength, some for the better such as sulfur hexafluoride, some for the worse such as water vapor. Hope you find these tid bits interesting.
dielectric_strength.jpg

dielectric_strength_2.jpg
 
WOW, Very interesting, its getting where I can half way understand it now. Its one of those things I learned somewhere maybe years ago but had no idea how to explain it (the pure physics of it). The air molecules and ionization discussion makes sense.

What surprised me was the vacuum part....

"The best dielectric (harder to strike an arc) is the vacuum, then it gets easier to strike the arc (poor dielectric) with a near-vacuum/some air present/about 0PSI, then as pressure increased to atmospheric (14.7PSI) it becomes a better dielectric (more difficult to arc), and this situation continues along the "more difficult" trend as pressure further increases to say a cylinder pressure of 150PSI."

I thought as pressure increased it was harder to arc so the converse was at less pressure it was easier, but you point out the differences at vacuum versus atmosphere etc.

THANKS TO EVERYONE HERE, it was fun learning experience, but very technical and likely boring to many.

God Bless n Merry Christmas to all

John T
 
Hi JMOR

I do not understand what your graphs are illustrating.

I understand the vacuum and one atmosphere(14.6abs) , but is the 150,000 feet means altitude or pressure?

If it illustrates altitude then the air pressure is lower than 14.6 pisa(lbs sq inch absolute).The same with 300,000 feet.

IMHO

Not sure why anyone would use feet as a pressure measurement. We would then would have to know feet of what ,water or what? Never heard of feet of air pressure.

anyway guys I do not understand why a extremely hot reactive mixture(gas air) would have high resistance to a spark. I do believe that a high compression engine will have extremely high squish or gas swirling past the plug tending to blow the arc out. The faster the engine turns over the more turbulence will be around the plug.

As I surf the web many site do mention high compression in engines cause more voltage needed , but looking at the physics I do not see it.

High pressure molecules may have a short electron travel but it would be extremely hot and close to others causing a electron flow quickly.


George
 
(quoted from post at 20:24:52 12/13/10) Hi JMOR

I do not understand what your graphs are illustrating.

I understand the vacuum and one atmosphere(14.6abs) , but is the 150,000 feet means altitude or pressure?

If it illustrates altitude then the air pressure is lower than 14.6 pisa(lbs sq inch absolute).The same with 300,000 feet.

IMHO

Not sure why anyone would use feet as a pressure measurement. We would then would have to know feet of what ,water or what? Never heard of feet of air pressure.

anyway guys I do not understand why a extremely hot reactive mixture(gas air) would have high resistance to a spark. I do believe that a high compression engine will have extremely high squish or gas swirling past the plug tending to blow the arc out. The faster the engine turns over the more turbulence will be around the plug.

As I surf the web many site do mention high compression in engines cause more voltage needed , but looking at the physics I do not see it.

High pressure molecules may have a short electron travel but it would be extremely hot and close to others causing a electron flow quickly.


George
Click to expand...
n those graphs, 1 atmosphere is 14.7PSIA
150,000 feet = 2PSIA = 1.1 TORR = 0.043 inch Hg (near the valley of graph)
300,000 feet = 1.5PSIA = 0.00076 TORR = 0.00003 inch Hg (approx left side of graph)

Why would I use altitude? Because it gives meaning/relative feel to the Ivory soap crowd (the 99.44% of readers to which "Torr" has no meaning). If I had a chart with a different pressure scale, I would not have used this one.

It is more difficult, requires higher voltage, for ionizing arc to occur with higher pressures.

As for arc blow out.........high velocity air can be used to blow out an arc, but until the arc is established in the first place, there is no arc to blow out.
 
Physical Conditions within the Spark Plug Gap affecting voltage needed for spark.

1. Gas pressure at the instant of ignition. This pressure is dependent upon ignition timing, engine load, compression ratio and engine RPM.

2. The gas velocity passing through the spark plug gap at the instant of ignition.

3. Engine fuel. The actual composition of the fuel being fed into the gap.

4. The temperature of the gas.

5. The fuel/ratio at the instant of ignition

lots of factors here with compression ratio just a small portion.

George
 
(quoted from post at 11:19:08 12/14/10) Physical Conditions within the Spark Plug Gap affecting voltage needed for spark.

1. Gas pressure at the instant of ignition. This pressure is dependent upon ignition timing, engine load, compression ratio and engine RPM.

2. The gas velocity passing through the spark plug gap at the instant of ignition.

3. Engine fuel. The actual composition of the fuel being fed into the gap.

4. The temperature of the gas.

5. The fuel/ratio at the instant of ignition

lots of factors here with compression ratio just a small portion.

George
Click to expand...
..and what did I say from the outset, George?
"Many variables. Various contaminants can alter dielectric strength, some for the better such as sulfur hexafluoride, some for the worse such as water vapor. Hope you find these tid bits interesting."

Good that you enumerated a few.
 
TOM 43

your comments are right on. The spark coming out of a sharp corner on the electrode is much hotter than one coming out of a rounded electrode. If dressing plugs going back to the original shape is the thing to do.

George
 
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