Spark plug – what is it?

In short..

A spark plug is a device that uses the electricity to ignite the mixture of fuel and air in the cylinder of the petrol and jet engines.


Etienne Lenoir invented the first form of the spark plug back in 1860. He used it in the first internal combustion piston engine.

Nikola Tesla, Frederick Richard Simms and Robert Bosch are also credited for the early patents for spark plugs.

The development of the spark-ignition engine was only possible after the invention of the commercially viable high-voltage spark plug as  part of a magneto-based ignition system invented by Gottlob Honold in 1902.

The structure

Spark plug – the structure

The terminal (connector) makes the very top of the spark plug. The terminal connects a spark plug with the rest of the ignition system.

Insulator is the coat that wraps the interior parts of the spark plug. It is mostly made from sintered alumina which is a very hard ceramic material with high dielectric strength. The insulator is glazed so it can improve resistance to surface spark tracking.

The tip of the insulator that is protruding into the combustion chamber is made of the same material as the rest of the insulator with the main difference that it is unglazed. It can withstand the temperatures of more than 650 °C.

The spark plug seals ensure that there is no leakage from the combustion chamber. They are mostly made of compressed glass/metal powder.

The metal shell of the spark plug withstands the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the central electrode to the side electrode.

The central electrode is connected to the terminal through an internal wire. It is usually designed to eject the electrons because it is the hottest part of the plug. The side electrode is made from high nickel steel. It also runs very hot.

What does it actually do

The spark plug is located at the top of the cylinder. The piston first travels down the cylinder, drawing in a mixture of fuel and air. After that, it returns to the top of the cylinder compressing the mixture. When the piston reaches the top dead center, the spark plug sparks and ignites the mixture. The explosion forces the piston to travel down to create power for the vehicle and is pushed back up again to clear out the exhaust. After that, the process repeats itself.

A thing to note is that a four cylinder (standard) engine has four spark plugs, one for each of the cylinders. As the number of cylinders goes up, so does the number of spark plugs.

How does it work

The spark plug is connected to the ignition coil or magneto. As electric current flows from the coil, a voltage develops between the central and side electrodes. At first, no el. current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows current to flow across the gap. And so, they supply higher current during the discharge process, resulting in a hotter and longer-duration spark.

As the el. current surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion.

The heat and the pressure force the gases to react with each other, and at the end there should be a small fireball in the spark gap as the gases burn on their own. The size of this ball of fire, or kernel, depends on the exact composition of the mixture between the electrodes. A small fireball will make the engine run as though the ignition timing was retarded. And, similarly, a large fireball as though the timing was advanced.

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