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Why Are Spark Plugs Important?

Spark plugs are (in our opinion) one of the coolest little inventions in the history of cars. An electronic device that provides a ignition source to burn fuel in the engine would have seemed like wizardry to most people in the 1860's, especially considering that the Edison electric illumination company wouldn't be founded until 1880.

So what is a spark plug?

The simplest way to describe a spark plug is as a high voltage arcing device. Simply put, the spark plug has a high voltage (12,000-25,000+V) positive probe, surrounded by a porcelain insulator and a threaded earthing strap that screws into the engine. When high voltage electricity flows through the centre of the plug to the tip of the positive probe, it jumps the gap to the earthing strap and creates an arc, kind of like a taser.

But don't most cars work on 12V batteries?

Well, yes. However, this is where one of the most interesting parts of the ignition system comes in, the coil pack. We'll go into this in greater detail in a future blog post but, the coil pack in a car is a simple transformer that changes low voltage and high current battery power into high voltage low current ignition power. Truly amazing stuff for something invented in the late 1800's.

While this is an absolutely awesome system, it isn't without it's failings. For example, if the gap between the positive probe and the earthing strap is too big, the electrical arc can't jump the gap. Additionally, if there is too much air and fuel in the combustion chamber (such as in modified turbocharged cars), the spark can be blown out. Finally and possibly most annoyingly, poor mixtures of fuel and air or oil can stick to spark plugs and not burn properly, a situation called fouling.

Most importantly, spark plugs suffer from electrical erosion over time. This is exactly the same as the kind of river bed forming, beach altering, Grand Canyon creating erosion taught in school but much faster. With each arcing/sparking taking a little piece of metal of each end of the plug causing the gap to widen.

Over time these weak points have been somewhat eradicated with the use of slightly higher quality metals and by improving the fuel mixing strategies and a modern spark plug might go 100,000km before changing compared to 20,000km in some old cars.

Realistically, the spark plug is one of the most crucial parts of engineering needed in creating the modern automobile and it's an incredibly simple device that is much the same today as it was 120 years ago.


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