There is nothing humble about the old spark plug

Spark plugs have always been precision components. Anything that has to function reliably in temperatures ranging from 500C to 850C, while withstanding rapid pressure variations and voltage spikes, necessarily requires precision manufacturing and rigid quality control.

Spark plugs have always been precision components. Anything that has to function reliably in temperatures ranging from 500C to 850C, while withstanding rapid pressure variations and voltage spikes, necessarily requires precision manufacturing and rigid quality control.

And yet they have a certain humility about them.

Take the electrodes for instance. For many years they were made from a nickel-based alloy, or more recently a copper core with a nickel jacket around it.

Nickel is resistant to corrosion. Copper is a good conductor of heat. So a copper core inside a nickel jacket will be a good choice for the material to make electrodes. And it was indeed.

But not good enough. You see, nickel is tough - it is a major ingredient in stainless steel - but sometimes it is just not tough enough.

Spark plugs with nickel and copper electrodes would only last about 25000km before the centre electrode became rounded at the tip through spark erosion and the spark gap began to widen appreciably.

The spark discharge always starts more easily from a sharp edge, so a rounded tip on the centre electrode was bad news, as was a gap that required higher voltage to jump than the ignition system was designed to deliver.

In the days before catalytic convertors, that was never a problem: the plugs would give you ample warning that they needed a service by symptoms of hard starting and the occasional engine misfire under load.

Once you had taken care of that, peace and joy would return. All that changed when catalytic convertors appeared on the scene.

"Cats" dislike unburnt fuel being blown into them with the exhaust gases. It has all sorts of dire effects on their innards and on their ability to do their job.

And every time an engine misfires or struggles to start, it is inevitable that unburnt fuel will be pushed down the exhaust pipe and into the cat.

So the hunt was on for materials to use in the plug electrodes that would make them last longer than nickel electrodes.

The hunt led metallurgists to two remarkable metals, platinum and iridium.

Platinum is highly resistant to chemical attack and does not oxidise in air, even at very high temperatures. It is also very heavy, 11percent heavier than gold.

It has outstanding catalytic properties, which explain its use in catalytic convertors .

It is extremely rare and very costly to mine.

The result of all these factors is that platinum is almost twice as expensive as gold.

A block of platinum the size of two bricks will have a mass of 78kg, the same as an average man.

Platinum is able to knock sparks off good old nickel, in a manner of speaking.

So when plug manufacturers decided to use platinum to make their plugs more durable, they only put a thin layer of platinum around the tip of the centre electrode and perhaps around the tip of the side electrode, where the spark would jump.

That was enough to extend the lifetime of a spark plug hugely.

But that is not where it ends. Wait until you hear the story of iridium.

Iridium is the most corrosion- resistant metal known.

It is extremely hard and, when combined with platinum, produces an alloy that outperforms pure platinum in spark plugs.

Plugs could now be produced that would function reliably for 100 000km.

Iridium is rare on earth, but it is relatively common in meteorites.

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