Spark plugs

Spark plug is a device used for the ignition of compressed air-fuel mixture in an internal combustion engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded.

Spark plug construction:

A spark plug is composed of a shell, insulator and the central conductor.

Parts of plug:

• Terminal: The top of the spark plug contains a terminal to connect to the ignition system.
• Insulator:  The main part of the insulator is typically made from sintered alumina, a very hard ceramic material with high dielectric strength.
• Ribs: The physical shape of the ribs functions to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case.
• Central electrode: The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of RF noise from the sparking. The tip can be made of a combination of copper, nickel-iron, chromium, or noble metals. The central electrode is usually the one designed to eject the electrons because it is the hottest part of the plug.

 Operation:

The plug is connected to the high voltage generated by an ignition coil. As the electrons flow from the coil, a voltage difference develops between the central electrode and side electrode. No 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 electrons to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to 'fire' properly, although it can go up to 45,000 volts. They supply higher current during the discharge process resulting in a hotter and longer-duration spark.