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How spark plugs work

Understanding how spark plugs work to ignite fuel in your engine

We all know that a spark plug produces the spark that ignites the air/fuel mixture. But what actually happens when you apply high voltage to a spark plug. What has to happen before the spark can jump the gap and why is the gap size so important. It’s all here, and when you’re done, you’ll have a full understanding of how spark plugs work to produce power in your engine.

What is a spark?

It’s plasma that forms 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, power to the ignition coil is shut off, causing the magnetic field to collapse. The collapsing field enters the secondary ignition coil, which amplifies the voltage to close to 40,000 volts. That high voltage shoots down the center electrode where it hits; a gap.

Voltage has a had time jump across and air gap because air is an insulator. But it has an even harder time jumping across a gap that’s filled with an air/fuel mixture under pressure. The high voltage is ready to leap from the center electrode to a ground point, any ground point but it needs something conductive to complete its path.

That’s where ionization comes in. 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

©, 2021 Rick Muscoplat

 

 

 

 

Posted on by Rick Muscoplat



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