Thanks, this is a good explanation of why precious metals are required on both electrodes of a spark plug in a waste-spark system.The block type coils are waste spark. They fire two plugs from each internal coil. The circuit is connected in series so one pulse from the coil always fires both plugs simultaneously. The two plugs chosen are 180 degrees apart in the firing order so that the one which fires the wasted spark is at the top of the exhaust stroke and has no fuel in the cylinder.
The implication for maintenance is that spark plugs which have platinum / iridium etc. only on the center electrode should not be used. The "double" type which also has precious metal on the ground electrode are required. Half of the plugs fire with reversed polarity which will wear on the ground electrode.
I reread Tom's linked article. Because the Ford system has a completely separate secondary winding, there's no way for current to flow from a high-potential point in the middle of the circuit (the coil) out to both plugs.Thanks, this is a good explanation of why precious metals are required on both electrodes of a spark plug in a waste-spark system.
It's implied, though, that for each pair of plugs, current will flow from the centre electrode to the ground electrode on one plug, and opposite (ground electrode to centre electrode) on the other plug.
Furthermore, it's implied that the current always flows in the same direction, leading to one plug losing material off its center electrode, and the other losing material off its ground electrode.
Is this the case, or am I misunderstanding?
And if so, that doesn't jibe with Tom Stick's linked article, which says that the coil generates high voltage in the middle of the 2-plug circuit, and both plugs provide a path to ground.
What am I missing here? Thanks!
Thanks, excellent explanation! I was being very dense (which is not as infrequent as I'd like); I 'd not considered that the coil's secondary winding would be polarized in one direction only (I.e. DC, not alternating). That is, one side would always be +ve, and the other -ve. The plugs and plug wires are merely extensions of the coil. The circuit is open in two places (the gap at each plug) until the voltage is sufficient to cause a spark to jump the gap. Once the circuit is complete (when the voltage is high enough to overcome the resistance of the gap), current will flow from the "ground" of the one spark plug, across that plug's gap, through the plug wire, through the secondary winding, through the 2nd plug wire, across the 2nd plug's gap, to circuit ground.Unlike a COP or other single coil per plug system, there is no ground on the secondary winding of the coil. The positive side of the coil is connected to one plug and the negative side to the other one. Current flows from one end of the coil to the first plug, through the engine block to the other plug, and back to the coil through the other plug wire. Both plugs spark at the same time(*). The current flow is "backwards" in one plug. It doesn't matter which plug is the one actually firing a fuel charge. The electrical operation is the same for every spark.
Some people who are OCD about this would swap the plugs or the plug wire connections at the coil halfway through the plug life to make the other plug the one that fires backwards, but this isn't necessary with double precious metal plugs.
* well not exactly at the same time. Once a spark forms, the plug has a low resistance and only drops a few hundred volts out of the kilovolts available from the coil. The other plug then gets extra voltage which generally ensures that it will spark.
The heads are made at different facilities. Toyota still does different plugs on each head. My 15 Tacoma was that way.Further to this, I've read here on BITOG about some V6 engines, I think Toyota, coming from the factory with NGK plugs on one bank, and Denso on the other. Would this be in conjunction with a waste-spark system? If so, would they be using plugs with precious metal on the ground electrode only on the ground-electrode-to-centre-electrode current-flow side, and plugs with precious metal on the center electrode only on the other side?
It sounds a bit improbable, but if plugs with precious metal on both electrodes are unnecessary in a waste-spark system, and if the auto manufacturer can save, say $0.50 per plug, that's $3.00/vehicle, that would add up to big savings over millions of engines over the years.
I think it was Henry McNamara, then with Ford, who quantified the advantages of saving 10 cents per car, back in the 1950s.
Of course for us backyard mechanics, it makes more sense to just buy replacement plugs with platinum, iridium, or ruthenium on both electrodes.
Oh wait! My crazy theory is valid ... Yay!MK378 has it right. The Ford waste spark system that came in the OP's van actually came with different plugs on each bank due to reversed spark polarity on the driver's side. Just replace them with double platinums and you don't have to worry about what type goes on each side.
No. It seems the densos wear slightly better than the ngks. The 3.4L has double ground. Maybe because if the double firing. My 4.0L has single ground.Thanks, that's interesting. So no difference in the plugs functionally ... that is, they're not different due to the waste-spark ignition.
Great question! Let's look at the two extreme ways a plug could fail, and how that could affect the other plug in that pair. (This is just speculation on my part; I hope the real techs and mechanics jump in to correct me if I'm wrong.)Incredible information all. I really appreciate it. Didn't just get an answer - actually learned how and why. Brilliant. Now, one more question... can a bad/worn plug on one end of the system affect the cylinder it's paired with on the coil?
The compression firing plug takes what it needs as far as kV, Even though the kV waveforms are superimposed on top of each other on a Scope....You can still see the wasted firing plug uses 25-45% lower kV.Thought about it some more ... realized that the voltage at either end of the secondary winding would be referenced to chassis ground (i. e. the negative battery terminal). I (for no good reason) assumed 40 kV across the secondary winding, but realized it would be split, with one side 20 kV about ground, and one side 20 kV below. One side would be positive, and one would be negative, w.r.t. ground, and therefore current flow would be in the same direction, which means the two plugs would experience opposite current flow (one ground electrode to centre electrode, and the other opposite). Please see my rough sketch.
View attachment 38354
I realize that the voltage would likely not be a 50-50 split - the plug wires have a significant resistance, and the coil is typically close to one cylinder and farther from the other, so a voltage divider situation applies. Plus the plug on the compression side takes a fair bit more voltage to fire.
But regardless, I now understand why in a waste-spark system, the current flows oppositely through the two spark plugs.
I have one more question; I assume that the electron flow (i. e. the opposite of conventional current) carries material away from the one electrode toward the other. That is, if the electron current is from the centre electrode to the ground electrode, there will be wear on the centre electrode (and build-up on the ground electrode?), and vice versa. This makes me think of the peaks and valleys on old breaker points. Now I'm dating myself!