by Jacques Gordon
The latest technology in sparkplugs is the iridium center electrode. Some plugs have an iridium button on the ground electrode too. The main advantage is durability. Iridium has an extremely high melting temperature of 4,435°F (2,446°C) and is virtually impervious to chemical corrosion; ideal qualities for living inside the combustion chamber of today’s high-specific-power engines.
Iridium is the second densest metal, right behind osmium. It’s a platinum-group metal extracted from platinum ore, and it’s extremely hard and brittle, almost impossible to machine. It’s commonly alloyed with osmium to make things like tiny bearings, and it’s also alloyed with platinum to make that soft metal harder. In addition to the ore, a thin layer of iridium also exists everywhere on Earth in a layer of sediment that dates back 65 million years, suggesting that it arrived on Earth with the meteor that may have wiped out the dinosaurs.
Electrodes made of traditional alloys of nickel, copper or chromium are fine for many engines, but a growing percentage of today’s automotive engines demand much more from a sparkplug. Combustion chamber temperatures can reach 5,400˚F (3,000˚C), and cylinder pressure oscillates between zero and 750 psi (50 BAR). A strong spark of up to 80,000 volts must jump the gap more than 60 times a second, every single time.
Sparkplugs have a center electrode shaped like a round bar, flat at the end with a sharp edge. The spark emanates from the sharp edge, and every time it fires, a few molecules of the electrode are vaporized. Gradually the sharp edge wears away and the gap grows wider, increasing the amount of voltage required to jump the gap. Even if the ignition coil can provide the increased voltage, it stresses every part of the secondary ignition system, shortening its life and compromising reliability.
Sparkplugs made with iridium and platinum are called “fine wire” plugs. First developed for aircraft in the 1960s, these hard-alloy center electrodes resist wear and absorb less heat, increasing reliability and service life. The smaller size of the fine wire requires less voltage to jump a wider gap, creating a more intense spark without stressing the rest of the ignition system. Recent techniques developed to weld iridium to the sparkplug’s copper core have made it affordable for automotive use. Iridium tipped sparkplugs may not be required in every application, but they’re indispensable in engines designed to use them.
Over the last two years, the price for one ounce of pure iridium hovered around $1,080. Next time you remove iridium spark plugs, think about recycling them through the same company that recycles catalytic converters.
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