Spark Plug: Main parts, types, and how it works

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A spark plug is a device used to ignite the air/fuel mixture in an engine cylinder. It is generally used in petrol engines, as the combustion of fuel requires a spark to initiate the combustion process. This article describes the main parts of a spark plug, types of spark plugs and how it works.


  • PLUG TERMINAL: This is the portion that is connected to the high-tension cable coming from the distribution cap. It conducts the high voltage to the central electrode.
  • CERAMIC INSULATOR: This is made of aluminum oxide ceramic. It acts as an insulator, and it separates the central electrode from earth at up to 4000 volts. It can be manufactured in plain form or with profiles to prevent flashover.
  • METAL BODY: this is the steel shell, manufactured with precision rolled threads for a secure for easy installation and removal. It provides electrical ground to the cylinder head and also helps to cool the plug by transferring heat to the cylinder head.
  • CENTRAL ELECTRODE: This is made of nickel-based alloys, and it contains a copper core. Depending on the type, the central electrode can be made of platinum or iridium. The high voltage is applied to the central electrode from the secondary winding through the distributor.
  • SEALING WASHER/GASKET: This is made to seal with the cylinder head and it helps in heat dissipation.
  • INSULATOR TIP: This is extended into the combustion chamber. It has a greater influence on the rating of the spark plug.
  • ELECTRODE GAP: This is the distance between the central electrode and the ground electrode. The electrode plays a crucial part in the spark generation. If an appropriate gap is not provided to the plug, then it cannot provide sufficient spark to ignite the engine.Spark Plug


There are two types of spark plugs, based on the relative operating temperature of the tip of the high-tension electrode.

  • HOT SPARK PLUG: This type of spark plug has a long heat transfer path and a large area exposed to the combustion gases. It transfers heat at a much slower rate, which keeps the firing tip hotter.
  • COLD SPARK PLUG: This has a shorter heat transfer path and small area exposed to the combustion gases. It transfers heat rapidly from the firing tip to the engine head, thus keeping the firing tip cooler.


First, a high voltage is passed from the distributor to the spark plug, which is then applied to the central electrode and ground electrode. Due to the high voltage difference between the central electrode and the ground electrode, the air in the electrode gap gets ionized, and the ionized gas becomes a conductor and conducts current from the central electrode to the ground electrode, thus causing a spark. The spark produced is then used to ignite the fuel-air mixture in the engine cylinder.


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