SPARK PLUG

Spark plug with single-ground electrode.

In 1860 Étienne Lenoir used an electric spark plug in his first internal combustion engine and is generally credited with the invention of the spark plug.

Early patents for spark plugs included those by Nikola Tesla (in U.S. Patent 609,250 for an ignition timing system, 1898), Frederick Richard Simms (GB 24859/1898, 1898) and Robert Bosch (GB 26907/1898). But only the invention of the first commercially viable high-voltage spark plug as part of a magneto-based ignition system by Robert Bosch's engineer Gottlob Honold in 1902 made possible the development of the internal combustion engine. Subsequent manufacturing improvements can also be credited to Albert Champion, the Lodge brothers, sons of Sir Oliver Joseph Lodge, who developed and manufactured their father's idea and also Kenelm Lee Guinness, of the Guinness brewing family, who developed the KLG brand.

A spark plug (very rarely in British English: a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.

Spark plugs have an insulated central electrode which is connected by a heavily insulated wire to an ignition coilmagneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder. or

Reciprocating internal combustion engines can be divided into spark-ignition engines, which require spark plugs to initiate combustion, and compression-ignition engines (diesel engines), which compress the air and then inject diesel fuel into the heated compressed air mixture where it autoignites.

Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.

Components of a typical, four stroke cycle, DOHC piston engine.

(E) Exhaust camshaft, (I) Intake camshaft,

(S) Spark plug, (V) Valves, (P) Piston,

 (R) Connecting rod, (C) Crankshaft,

(W) Water jacket for coolant flow.

Operation

The plug is connected to the high voltage generated by an ignition coil or magneto. 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.
As the current of electrons 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. This is the "click" heard when observing a spark, similar to lightning and thunder.
The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded, and a large one as though the timing was advanced.

Parts of the Plug

Diagram of single-ground spark plug.

• Terminal : The top of the spark plug contains a terminal to connect to the ignition system.
• Insulator : Its major function is to provide mechanical support for the central electrode, while insulating  the high voltage.
• Ribs : to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case.
• Insulator Tip : must resist high temperatures while retaining electrical insulation.
• Seals :  the seals ensure there is no leakage from the combustion chamber.
• Metal Case :  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.
• Central Electrode : The central electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug.
• Side Elektrode : Some designs have provided a copper core to this electrode, so as to increase heat conduction.