High voltage coil discharge polarity for vehicle spark plugs

[jerromyjon]

TL;DR Summary

Automotive gasoline engine ignition voltage polarity inversion

I've recently had a discussion with fellow technicians in the automotive industry about spark plugs and some systems having "opposite polarity" on the ignition coils. (specifically the secondary coil which sends the high voltage pulse to the plug which sparks the fuel)

Common sense tells me that since the base of the plug which contains the electrode is grounded through the engine to the negative terminal of the battery, the high voltage that comes from the coil to the tip of the plug down through the insulator to the other electrode has to be positive polarity.

The evidence they claim is that the "arm" (ground electrode) will wear on one plug and the "center" (spark electrode) will wear on the other, in such systems that use a reversed polarity spark. Also some referred to the "waste spark" coils which have 2 terminals for cylinders which spark on opposite cycles, claiming they also spark in opposite polarity, but they all use the same typical plugs which all ground though the base.

Can anyone make any sense of this?

 

[sophiecentaur]

Summary: Automotive gasoline engine ignition voltage polarity inversion

Common sense tells me that since the base of the plug which contains the electrode is grounded through the engine to the negative terminal of the battery, the high voltage that comes from the coil to the tip of the plug down through the insulator to the other electrode has to be positive polarity.

You can produce either a positive or negative pulse by choosing which end of the secondary coil is Earthed. However, the primary and secondary are connected in series so the approximately 400V pulse that is applied to the primary as the contact breaker operates can be used to augment the secondary volts - hence the polarity that is chosen.
Otoh, I had a Citroen 2CV which used a very crude ignition system with a circuit which went:

Engine block - plug 1 - one end of coil - other end of coil - second plug - engine block.

Both plugs got a spark every time but the volts were nearly all developed across the plug in the cylinder that was under compression at the time. However, the polarity of the two pulses was different for the two plugs, So it cannot make that much difference to the way the engine runs. Problem: the spark coil was situated right above the air-cooling fan and was open to wind and rain and used to need frequent drying out and plenty of WD40 water repellent.

 

[Baluncore]

However, the polarity of the two pulses was different for the two plugs, So it cannot make that much difference to the way the engine runs.

In waste systems the plug in the cylinder with lowest pressure breaks down at lowest voltage, while the greater voltage appears across the compressed cylinder plug.
https://en.wikipedia.org/wiki/Paschen's_law
The same current flows through both, so most of the energy is delivered to the plug where it is needed to ignite the compressed air fuel mix.

The positive electrode will suffer more wear than the negative electrode. If you regularly swap the two plugs in a waste system, they will wear evenly, and so more economically.

 

[Baluncore]

Can anyone make any sense of this?

Spark erosion of electrodes is caused when accelerated electrons from the negative (cathode) hit the positive (anode) and knock atoms out of the solid metal electrode. The electrons flow freely through the metals, so are not lost. The dislodged metal atoms do not fall back into their secure places in the crystal lattice, so they react chemically during combustion and are discharged through the exhaust.

Heavier metal atoms with higher melting points are less likely to be eroded, so more expensive spark electrodes are sometimes plated with platinum or iridium to reduce erosion.

 

[Tom.G]

As of 20 years ago, many V8 engined cars in the USA used the waste system with opposite polarity on the plugs (the Ford Police cars were one model). They came from the factory with 2 different types of spark plugs. Half the plugs had the Platinum catalyst plating on the center electrode and half had it on the side electrode. This was noted in the service manuals with the warning to make sure the correct plug was in the correct cylinder.

The retail automotive parts stores solved the problem; they sold only plugs with Platinum on both electrodes.

Cheers,
Tom

 

[jerromyjon]

This was noted in the service manuals with the warning to make sure the correct plug was in the correct cylinder.

With all the crazy spark plug designs I see these days, I guess we are lucky they didn't make them left-handed threads so you can't mix them up!

I don't why I didn't just google schematics in the first place, because I found this simple diagram, and it took me a few seconds... but it hit me like shock from a coil once I saw the arrow on the cylinder head!

In waste systems the plug in the cylinder with lowest pressure breaks down at lowest voltage, while the greater voltage appears across the compressed cylinder plug.
https://en.wikipedia.org/wiki/Paschen's_law

Thanks, that was an interesting read! I had the right idea that the cylinder with the compressed fuel/air mixture got most of the voltage, but I thought it for the wrong reasons...

 

[sophiecentaur]

but it hit me like shock from a coil once I saw the arrow on the cylinder head!

That's another occasion when showing the direction of electron flow along wires is not helpful. They found it important to relate the plug wear to electron flow so they should have indicated that on the diagram differently (Perhaps with insets of electrode details). As it stands, when the diagram is looked at without the text it will just 'baffle brains'.

 

[jerromyjon]

If you regularly swap the two plugs in a waste system, they will wear evenly, and so more economically.

I was thinking once halfway through their expected life, would be the ideal. Thanks for the insight!

But then that's the expected, non-rotated, life? Then they can be rotated more frequently, if need be, but I'd want new plugs after that much life...