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How spark plugs work

Understanding how spark plugs work

Before you start tuning up your engine, you should understand how spark plugs work. What is a spark? Why is the spark plug gap so important? Are some spark plugs better than others? I’ll answer all those questions is this post: How spark plugs work.

What is a spark?

It’s plasma between the center and side electrodes. There are five phases of matter: Solids, Liquids, Gases, and Plasma.

What generates plasma?

During the compression stroke, the piston rises and compresses the air/fuel mixture, forcing the molecules closer together. Slightly before the piston reaches the top of the compression stroke, the ignition coil fires, shooting upwards of 40,000 volts down the center electrode towards the spark plug gap. The voltage is “ready to leap” from the center electrode to a ground point, any ground point, to complete the circuit. It just needs something to “conduct” it. However, the compressed air/fuel mixture isn’t a good conductor. In other words, it has high resistance.

The extremely high voltage sitting at the tip of the center electrode causes the compressed air/fuel molecules in the spark plug gap to ionize. In the simplest of terms, ionization is the “lining up” of gaseous ions so they can conduct electricity. Once the ionization process is complete, the aligned ions create a path to the side electrode that has lower resistance than the air/fuel molecules away from the gap. In other words, the ionized particles offer the voltage a “path of least resistance.”  Once the ionization formation completes, the high voltage travels across the path of ions and the gas becomes plasma, allowing the voltage to travel to the side electrode while creating intense heat.

What part does the spark plug gap play?

Think of the gap as a big resistor. The larger the gap, the higher the resistance. So, the voltage needed to create plasma is directly proportional to the gap size; the larger the gap, the more voltage is needed to jump the gap. The correct gap generates the hottest plasma and exposes that plasma to the largest possible area of the air/fuel mixture, allowing it to propagate across the cylinder.

Spark plug gap size also plays a part in spark duration. Typical spark duration is in the 6-10-millisecond range. A larger gap, along with the proper voltage for that gap creates a longer burn time, and a smaller gap and less voltage gap results in a shorter burn time.

So firing voltage and gap size go hand-in-hand.

What happens if the spark plug gap is too large?

If the gap is too large too much ignition energy is required to ionize the air/fuel mixture and create the spark. The spark will be cooler and shorter in duration, resulting in poor or incomplete combustion or no combustion at all (misfires). Even if the air/fuel mixture ignites, it is too weak, resulting in poor flame front propagation.

If the spark plug gap is too small (low resistance),

If the spark plug gap is too small, less energy is needed to ionize the air/fuel mixture and spark event begins too early before optimum compression is achieved. The result is a weaker spark, poor flame front propagation and less power. A small spark plug gap ignites more consistently than a gap that’s too large, but the smaller gap results in a less efficient burn.

What part does the electrode shape play in generating a spark?

Ionization and plasma always occur at the sharpest

iridium tipped spark plug

An iridium tipped center electrode and a platinum disc fused to the side electrode.

edge of the center electrode and complete at the sharpest edge of the side electrode. As the electrodes wear, the sharp edges round off, and the gap increases. The result is shorter spark duration, less heat, and poorer flame propagation.

Older spark plug electrode designs were made from nickel-plated copper. Copper afforded extremely low resistance, but shorter life spans; sometimes as little as 25,000 miles. To meet stricter emissions guidelines, carmakers began using leaner mixtures and those mixtures require a hotter spark to ignite.

So carmakers switched to electrode materials like platinum, yttrium, iridium. Those precious metals have higher melting points, so they resist edge and gap erosion. Some current engine designs using iridium tipped electrodes can go up to 125,000 miles between spark plug changes.

double platinum spark plug

Some side electrodes are just nickel-plated copper, while others are nickel-plated copper with a fused disc of platinum or iridium.

Most are a single “J” shaped. But some side electrodes have

denso U groove electrode

Denso U-groove “J” shaped side electrode provides more sharp surfaces for the spark

“U-shaped” channel cut into them. The purpose of the channel is to provide at least four sharp surfaces.

What about multiple side electrodes?

The spark only jumps from the center electrode to ONE side electrode. Spark plug manufacturers use multiple electrodes with less costly precious metals (or none at all) ONLY to ensure that as one side electrode wears, another side electrode can take over. When new, multiple side electrode spark plugs perform exactly the same as single, “J” electrode spark plugs. In other words, using multiple side electrodes is a way for the plug manufacturer to save money on precious metals, while still maintain proper spark operation over the life of the plug.

Do some spark plugs make more power than others?

No. If you compare one new spark plug with sharp electrode edges to a different new spark plug with sharp electrode edges, and both spark plugs are gapped the same, have the same internal resistance, and are installed on the same engine they will both produce the same spark energy, no matter what the spark plug manufacturer claims. Why? Because spark energy is determined by the ignition coil, spark plug gap, internal resistance, and electrode edge sharpness. Period

What about E3 spark plugs?

Let’s take a look at the official documentation from the E3 spark plug site. The jist of the E3 spark plug story seems to be that they create a larger flame “kernel” by starting the flame kernel earlier than a normal spark plug and they maintain “edge-t-edge” discharge.

E3 spark plugs start the flame process earlier

“This means that just a slight increase in flame kernel strength can cause a cascading improvement in the entire combustion process. By getting the flame process started earlier, the mass fraction burned at any given crank angle position away from TDC is increased. Since the exhaust valve opening occurs at a fixed point in the crankshaft’s position, we understood how important it is to get as much of the fuel burned before it is vented off during the exhaust cycle. To increase power and reduce emissions, we created an electrode design that burns more of the existing air-fuel mixture in the combustion chamber. E3 Spark Plugs are “Born to Burn”.” — https://e3sparkplugs.com

E3 spark plugs use “edge-to-edge” discharge

“Lastly, the strongest part of the E3 electrode design is our forced Edge-to-Edge spark discharge, which was proven to be the best way to direct a spark as it leaves the electrode’s surface. Our design improved upon the phenomena that race car drivers used for years. They would “cut back” ordinary spark plug electrodes to improve the overall spark discharge. Since the spark itself occurs only when an avalanche of electrons migrates from the two electrodes (cathode to anode), sharp edges are better at initiating migrations, and accelerated electrons collide inside the spark zone.” — https://e3sparkplugs.com/diamondfire-electrode/

DUH! Frankly, I’m surprised they would even claim this since ALL spark plugs operate using edge-to-edge discharge.

Now let’s look at E3’s power degradation test

The E3 Standard Spark Plug Power Degradation Test chart shown below portends to show how quickly a normal spark plug’s horsepower degrades based on combustion cycles. Both the “standard” spark plug and E3 spark plug start at the same horsepower. In other words, E3 spark plugs with their “earlier flame start on the flame process” and their “edge-to-edge discharge” doesn’t claim to produce any more horsepower than a “standard” spark plug when both are new.

E3 spark plugs

The test chart implies that the E3 spark plugs maintain the engine’s horsepower longer than a “standard” spark plug. But they don’t disclose what type of spark plug E3 is testing against. If the “standard spark plug” used in the test is a copper plug, then the results of E3’s test are entirely predictable since the sharp edges on copper spark plugs are known to degrade faster than a spark plug made with platinum, yttrium, and iridium tipped center/side electrodes.

©, 2021 Rick Muscoplat

 

 

 

 

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