Spark plug is a device used for generating spark in spark ignition engine (SI engines like petrol, CNG etc) to burn the air fuel-mixture in order to have power stroke. It is connected to heavily insulated high voltage wire attached to the output terminal of the magneto or more general used term as induction coil (if more combustion chamber than one as in multi-cylinder engines then to the distributor). In this article we are going to get behind the working of spark plugs used in petrol engines.

Lets start by knowing the various parts of spark plug which are as follows:-

Different parts of a spark plug

• A metal shell having external thread so that it can be screwed into the cylinder head to have a perfect fitting (some spark plug come up with tapered or conical shoulder which incorporates leak proof fitting thus replacing the need for gasket).
• Central metal electrode with the high voltage current.
• Tip at the top of central electrode (acting as cathode) is made of un-reactive metal or alloys like titanium, iridium, tungsten, platinum etc because of their durability and resistant to corrosion.
• Porcelain coating, covering the central electrode, thereby acting as insulator separating and preventing leakage from the metal part.
• Terminal- The other end of the spark plug contains a terminal for having connection to the ignition system.
• Lateral or side electrode (anode) is welded or hot forged to the metal part. This is also made of inert metal or alloy such as platinum.
• Small gap between the electrodes is maintained so as to produce fairly good spark. Spark plugs in automobile can have a gap between the electrodes ranging from 0.5mm to 1.8mm.

It is crucial as length of spark depends on it. Greater the gap more the potential difference (voltage)required and vice-versa. The gap size increases or decreases because of the deposit from the burnt gases or erosion with time.

Working operation of Spark Plugs

It is quite interesting to know how a spark is produced in such a small device. Well, as discussed above, the central electrode has the high tension current from the distributor and other earthed electrode creates the potential difference across them such that it ionizes the gases in between (air fuel mixture) thus forming a path for the current to flow (gases are insulators).

This flow (jumping) of current between electrodes makes the spark. Lightning bolt in sky and electric arc welding has the same principle involved. During spark formation it acts as miniature radio transmitter and creates interference to nearby phone, radio and television etc. This is removed by using suppressor (resister) in high tension lead or connector.