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Spark plug life

Spark plug life — when to change spark plugs?

Spark plug life and change intervals depend on more than just the type of metals used in the electrodes. It also depends on the type of ignition system and engine design. For example, iridium spark plugs in a naturally aspirated port-injected engine with a coil-over-plug ignition system can sometimes last up to 120,000 miles. But those same iridium spark plugs in some lean-burn turbo engines with direct injection may last only 30,000 miles.

What determines spark plug life?

Spark plug electrode material

A spark always jumps from the sharpest edge of the center electrode to the sharpest edge of the side electrode. As a spark plug wears, the sharp edges of the center and side electrodes erode and round off. How fast that erosion happens depends on the type of metal used, the type of ignition system and the cylinder pressures and temperatures, the air/fuel mixture, and the amount of oxidation that occurs during combustion. Generally speaking, a spark plug electrode that’s made with a harder metal with a higher melting point outlasts a spark plug made with metal that has a lower melting point.

spark plug melting points
Spark plug electrode size and shape affect spark plug life

The amount of voltage needed to fire a spark plug depends on several factors like: gap size, electrode shape and size, gas density, gas and electrode temperature, and gas type.

Smaller electrodes reduce firing voltage

Many of the newer iridium spark plugs are made with a “thin wire” design. Studies show that the thinner the center electrode the lower the voltage needed to initiate the spark. A lower firing voltage reduces stress on the ignition coil and allows for a longer spark duration, which is critical in lean-burn engine designs. The lower firing voltage also increase spark plug life by reducing electrode erosion.

Smaller electrodes reduce heat absorption and reduce spark plug quenching

Since a thinner electrode has less surface area it absorbs less heat from the flame kernel, making the electrodes last longer. The lower heat absorption also reduces the quenching action created by the center and side electrode metals. In summary; smaller electrodes absorb less heat from the flame kernel and reduce quenching which increases the ignitability of the air/fuel mixture.The smaller footprint also increases flame propagation.

Spark plug life is affected by temperature and oxidation

Platinum tipped spark plugs show a high resistance to temperature and oxidation erosion. As engineers evaluated pure Iridium they found that even though is has a much higher melting point than platinum, Iridium is more susceptible to oxidation erosion during heavy engine operation.

To combat the oxidation erosion, engineers developed an iridium alloy by adding 10% Rhodium to the mix.

Iridium spark plug benefits

Laboratory testing proves that iridium spark plugs afford the following benefits

• Lower firing voltage
• 3% improvement in fuel economy during idling and 1.3% improvement during highway operation
• improved acceleration
• Faster starting in cold temperatures
(Development of New Iridium Alloy for Spark Plug Electrodes Hironori Osamuraand Nobuo Abe— DENSO CORP)

Why some carmakers recommend more frequent spark plug changes

As mentioned earlier, some iridium spark plugs can last as long as 120K miles. Yet some carmakers recommend iridium spark plug changes at 30,000, 45,000 or 60,000 mile intervals. If iridium spark plugs are so good, why don’t they all last 120K miles?

Engine design affects spark plug life

Even though iridium spark plugs have increased resistance to erosion and oxidation, their service life is affected by firing voltage, spark duration, combustion pressures and air/fuel mixtures.

Lean burn, direct injection and turbocharging reduces spark plug life

Lean air/fuel mixtures are much harder to ignite, so they require a longer spark duration. At the same time, direct injection and turbocharging increase cylinder pressures, making ignition even harder to accomplish. When you combine the factors of longer spark duration, leaner mixtures and higher pressures, you get reduced spark plug life.

Copper spark plugs have a much shorter life compared to platinum or iridium

A traditional copper spark plug contains a copper spark plugcopper-core center electrode with a fused nickel alloy tip. That design provides lower electrical resistance than precious metal plugs, so you lose less ignition coil energy. Lower electrical resistance gets you a hotter spark that results in more power. That’s a huge advantage in performance applications where spark plug life isn’t an issue. In addition, traditional copper spark plugs run cooler, further increasing performance.

Unfortunately, the nickel-alloy isn’t as hard as other metals and has a lower melting point. So it erodes faster, resulting a rounded edge and a faster wearing gap.  As the spark plug gap increases, misfires and fouling increase. This is why copper spark plugs must be replaced every 20,000 miles.

Platinum spark plugs

A platinum tip welded to a copper core provides longer spark platinum spark plugplug life simply because platinum is a harder than copper (4.5 versus 3) and has a higher melting point (4,435°F versus 1,984°F). Car makers switched to platinum spark plugs increase spark plug life in distributorless ignition systems (DIS). DIS ignition systems generate higher spark voltages and temperatures that are needed to ignite leaner fuel mixtures.

Single tipped platinum spark plugs use a copper-core center electrode with a platinum tip on the center electrode only. Car makers switched to platinum spark plugs in DIS systems due to the higher voltage and higher temperatures needed to ignite leaner fuel mixtures.

Double platinum-tipped spark plugs contain a small piece of platinum double platinum spark plugon the side electrode to reduce gap erosion in DIS systems where the waste spark jumps from the side electrode to the center electrode.

Yttrium spark plugs

Yttrium spark plugs were offered by Bosch in their earlier +4 spark plugs. Those plugs have four side electrodes. Four electrodes reduce the chance of misfire due to gap erosion. Yttirum is much cheaper than platinum but also has a lower melting point.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat



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