Spark Plug Tips

MolaKule

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With the use of new exotic metals for spark plug electrodes, these tips last much longer with little tip erosion.


The following question concerns the spark tip geometry and a bit of electrical/physics theory.


Q: Why do many, if not most, modern plugs today have what is known as the "Fine Wire" center electrode tips?



This question is NOT open to any engineering discipline or physicists.





Fine Wire Spark Plugs.jpg
 
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I don't fully understand it. But from what i understand , the smaller tip increases the density of the spark. I don't really understand the plasma part of it though. I'm thinking the tip being smaller also stays hotter/ cleaner with less area, and that necessitates the use of metals like platinum and iridium to survive for long.
 
Fine wire also allows more fuel to ignite around the tip plus what others have stated above, something called the flame kernel.

My info is based on info from various Motorcycle mags and their interpretation may or may not match up to others interpretations but in principle is presented based on application .

Think about the size of a motorcycle engine and the power output, HPvs CI. My bike is 948 cc ( 60 ci ?) and produces 71 HP. These are standard without mods but as delivered. If a Car engine was made more HP than Cubic Inches and we have cars delivered today that meet that example, so the plug type and style were developed to meet that need.

We / You have access to Standard tip, Projected tip, U Grove , Fine wire, multiple side electros ,Capacitive discharge E3 style .

Gaps have been closed or expanded to meet the demand to basically meet the electrical demands of the combustion process.

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It's easier to get a spark from a pointed object as the electrical field is stronger at a sharp edge.
As the new materials used for the electrodes are more resistant to erosion they can be made smaller/thinner and still last.
 
Three reasons

1) The fine wire is made from expensive exotic metal (Iridium, Platinum) so reducing the diameter of it saves cost
2) The fine wire geometry makes it easier to generate a spark across a given gap size. Easier on the coils.
3) Fine wire allows better room around the spark for the explosion to expand and fill the combustion chamber (small performance benefit)
 
Thinner/sharper electrode has a lower voltage requirement to reliably produce a spark.
U-groove and/or larger gap make a bigger flame kernel. Bigger and uniform flame kernel
should ignite more of the charge evenly and efficiently. I think it should increase flame front speed, but I could be wrong.
Would be interested on the full info Mola
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All good answers folks.
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As the diameter of the center electrode is made smaller, the electric field intensity between the electrodes is stronger.

The arc diameter being smaller results in a higher current density of electrons flowing through the gas.

The higher density of electrons produce more intense collisions (imparts more kinetic energy) to the gas molecules in the gas/air mixture resulting in a spark gap temperature of 6000 Kelvin for more efficient combustion.

The exotic (refractory) inner electrode metals resist erosion in the glow discharge phase.

In the glow discharge phase, 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.


In the White Papers and Technical Discussions there were these articles which have additional background information:

Quote
Physic of the Gap I and II (combined)

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. Platinum, Iridium, yttrium and other refractory metals have replaced copper as the inner electrode. The ground electrode is usually composed of nickel-ferrous alloys.

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
~ 2,450.

The spark plug gap is essentially a capacitor with two plates, the center electrode and the "ground" electrode. The voltage across the gap is increased until the voltage exceeds the breakdown voltage of the gas. A current then flows which ignites the gas and a hot, plasma cylinder forms in the gap at T = 6000K (5727C, 10340F). Later in time, the plasma forms a sphere that propagates outward from the spark 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 1.2 mm. This results in a breakdown voltage of 3.600 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 D between the electrodes, Pressure of the gas, and Temperature T, or Vb = f(Pcycl, Dgap, Tgas). 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.

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 3,600 volts or greater than 18,000 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.

We now 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 < 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 for about 10 nanoseconds. The ionization channel is a cylinder of about 40 micrometers. The temperature in this ionization column gets to 6,000 degrees Kelvin (5727C, 10340F).

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 pulse creates a flash of heat that helps the fuel charge reach the required light-off temperature to ignite the air-fuel mixture.

The heat provided by the plasma gives the air-fuel mixture a head start to achieving the temperature required to ignite. In addition, this pulse ionizes the gaseous air/fuel mixture, breaking down air/components like H² and O² into their atomic state H and O where they are most volatile. These highly excited elements react to the spark by igniting instantly. The result is improved throttle response.

The high-intensity pulse breaks apart the long hydrocarbon chains found in the nearby air-fuel mixture into shorter chains that react quickly. This rapid burn creates higher peak pressure

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 3,600 degrees Kelvin. The arc increases in size due to heat conduction and mass diffusion.

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 1.25 miliseconds, the current and voltage have decayed to zero.

How much energy is delivered per phase is determined by the ionization physics of the plasma channel.

When the ECU commands a current pulse to the ignition coil's primary, you want as fast a change in coil current as possible in order to create a high voltage at the coil's secondary.

You want the highest voltage available in the shortest amount of time at the SP gap to ignite the plasma. The overall rise time of the voltage on the coil's secondary is a function of the coil's leakage inductance, resistance of wiring and plug, capacitance of external circuit, gap width and cylinder pressure, and gas species. That voltage rise time is designed into the ignition system.

Too much ignition system delay wrt piston position affects combustion efficiency, which in turn affects engine performance and mpg.

When I am speaking of the SP gap rise time, I am speaking of the initial current curve showing the leading edge of the SP gap current as the voltage rises to about 35,000 volts and just before current flows. Upon ignition of the plasma, the current rises to about 200 Amps but only sustains this level for about 10 (10^-9) seconds or 10 nanoseconds.

The total amount of Energy supplied to the spark is about 80 milliJoules and is divided into 3 phases:

During the breakdown phase, an energy of 20 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%.

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.

Arc Energy = I^2arc X Rarc X delta [time], so for the initial current rise of 200 Amps in 10 nanoseconds, Energy = 2.0 uJoule.

So, give thanks for this device that works so hard in such a high temperature, high pressure environment.
 
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Wow, that is so much interesting information.

Do you think the arc shape changes depending on which side of Paschen's law it falls on? For instance if the gap is small enough to increase the breakdown voltage significantly, do the arcs tend to bulge out and take meandering paths? If this is the case then it makes sense you would want a small gap. A smaller gap has less distance for a more energetic discharge as well as having a higher voltage potential at discharge, and perhaps a more meandering discharge path. But perhaps at some point as you reduce gap size the spark nucleation becomes chaotic and unpredictable, as Paschen's law is essentially a statistic of the available molecules for electron collisions.

I'm guessing the reason the plasma stage has 94% heat in the space between electrodes is that, the air being the dielectric, it is discharging energy already stored in it in the form of the electric field gradient.

My spark plugs measure 7.3k resistance, so to flow 200A would require 1.46 million volts. So this current must be coming from energy stored in the ceramic element after the resistor and air gap of the spark plug, not from the ignition coil.

It also begins to make more sense why spark plugs and spark plug wires are so resistive. 10nS is the wavelength of 100MHz, the FM radio band. If that energy weren't confined to the spark plug tips, the plug wires would act like antennas and the potential for a radio frequency interference nightmare is immense. The spark plug wires have to be super lossy carbon fibers because that is the only way to keep the wires from acting like a transmitter antenna.
 
All good questions although I would recommend you re-read the information given.

Originally Posted by keantoken
Wow, that is so much interesting information.

Do you think the arc shape changes depending on which side of Paschen's law it falls on? For instance if the gap is small enough to increase the breakdown voltage significantly, do the arcs tend to bulge out and take meandering paths? If this is the case then it makes sense you would want a small gap. A smaller gap has less distance for a more energetic discharge as well as having a higher voltage potential at discharge, and perhaps a more meandering discharge path. But perhaps at some point as you reduce gap size the spark nucleation becomes chaotic and unpredictable, as Paschen's law is essentially a statistic of the available molecules for electron collisions.


Paschen's basic law has been modified for spark ignition systems. Engine systems designers use the empirically derived equation Vspark = 4700*(d*CR)^0.718, where d is spark gap and CR is compression ratio.

There are two competing requirements: 1) large enough gap to still fire under deposit formation on both electrodes, 2) small enough gap to create a hot plasma for efficient combustion.

Originally Posted by keantoken
I'm guessing the reason the plasma stage has 94% heat in the space between electrodes is that, the air being the dielectric, it is discharging energy already stored in it in the form of the electric field gradient.


The total resulting energy is due to the externally supplied coil energy and the chemical potential energy in the fuel/air mixture.

The fuel/air mixture dielectric has a complex set of Dielectric Constant relationships as the Dielectric Constant K changes with pressure, temperature and gas constituents. The Dielectric Constant K is very complex because it involves Coulombic, Thermal, gap distance, and Pressure considerations.

Originally Posted by keantoken
IMy spark plugs measure 7.3k resistance, so to flow 200A would require 1.46 million volts. So this current must be coming from energy stored in the ceramic element after the resistor and air gap of the spark plug, not from the ignition coil.


You are forgetting that each phase represents "short-lived" transient phenomena and waveforms that occur for nanoseconds to milliseconds. For example pulsed radar may have output powers of 50 Megawatts but that power is delivered in microseconds.I.e, the Peak power and the Average powers are different.

Originally Posted by keantoken
IIt also begins to make more sense why spark plugs and spark plug wires are so resistive. 10nS is the wavelength of 100MHz, the FM radio band. If that energy weren't confined to the spark plug tips, the plug wires would act like antennas and the potential for a radio frequency interference nightmare is immense. The spark plug wires have to be super lossy carbon fibers because that is the only way to keep the wires from acting like a transmitter antenna.


You are forgetting that FM systems only respond to Frequency Modulated waves. The transient phenomena in spark systems represent Amplitude Modulated waves that generates harmonics. In addition FM systems strip off any AM in a circuit called a "limiter."

The shorter the lead from coil to spark plug the better since radiated RFI/EMI is reduced, hence the Coil Over Plug system is best.

References for further Information and Study:

1. W. Mitianiec, Factors Determining Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels,
INTECH, 2012.

2. N. Kawahara, et. al., Plasma temperature of spark discharge in a spark-ignition engine using a time series of spectra
measurements
, 18th symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Lisbon Portugal,
2016.

3. A. Joshi, Effect of spark advance and fuel on knocking tendency of spark ignited engine, Michigan Technological
University, 2017.

4. A. Franke, Diagnostics of Electrical Phenomena in Gases for the Monitoring of Spark-Ignited Combustion, Lund Institute
of Technology, Lund University, 2000.

5. I. Andersson, Cylinder Pressure and Ionization Current Modeling for Spark Ignited Engines, Division of Vehicular
Systems, Department of Electrical Engineering, Linkopings Universitet, Linkoping, Sweden, 2002.

6. Technical Information No. 7, All About Ignition Coils, BERU. Division of Federal Mogul.

7. P. Matthew, et. al., Experimental verification of modified Paschen's law in DC glow discharge argon plasma, AIP
Advances 9, 025215, 2019.

8. Y. Raizer, Gas Discharge Physics, Springer, 1997.

9. B. HNATIUC et. al., SPECTROSCOPIC DIAGNOSTIC OF TRANSIENT PLASMA PRODUCED BY A SPARK PLUG,
Technical University, Romania, 2011. Paper presented at the 15th International Conference on Plasma Physics and
Applications,1-4 July 2010, Iasi, Romania.
 
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Originally Posted by danez_yoda
Three reasons

1) The fine wire is made from expensive exotic metal (Iridium, Platinum) so reducing the diameter of it saves cost
2) The fine wire geometry makes it easier to generate a spark across a given gap size. Easier on the coils.
3) Fine wire allows better room around the spark for the explosion to expand and fill the combustion chamber (small performance benefit)



There is no explosion. This is a combustion process, not an explosion; there is a difference.

Let me explain: In an explosion the volume of gases is confined until the pressure in the vessel exceeds the material strength of the walls of the constraining vessel and fails and erupts into particles of solids (shrapnel).

In combustion, the expansion of gases is not confined as above but the piston moves downward to increase the volume preventing pressure spikes and subsequent grenading of the head and cylinder walls.

Now as for rails and nitrous powered vehicles using high compression engines, sometimes the gases will pre-ignite due to hot spots in the combustion chamber or the valves and will explode because the pressure spike comes at the wrong time (wrong crank angle).
 
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