The function of spark plug is to provide a gap in combustion chamber for discharge of a high voltage electric pulse that will ignite the air fuel mixture at the desired time. In order to do this task effectively a spark plug must fulfill following requirements:

1) It must provide a gap or spacing and be located in the combustion chamber such that it allows smooth engine operation over the entire operating range from idling to wide open throttle running with maximum fuel economy. The proper gap must be maintained under all conditions.

2) None of the spark plug parts must ever become hot enough to cause pre-ignition i.e. ignition of mixture before the occurrence of spark. Pre-ignition occurs if surface temperature exceeds 950°C.

3) The firing end of spark plug should be resistant to fouling. A long insulator tip and large clearance volume will minimize both carbon and lead fouling.

4) The size, temperature and composition of electrodes should be such that erosion from electric discharge between electrodes and that due to chemical action of the combustion products is minimal. The optimum temperature to avoid electrode erosion is 850°C.

5) Due to high pressures (as much as 50 bar) that the spark plug has to withstand, the insulator material must possess properties of high electrical resistance, good thermal conductivity and sufficient mechanical strength.

6) The spark plug when fitted must be gas tight. Its joint between cylinder head and plug body and between insulator and central electrode must be gas tight.

7) The spark plug also has to conduct heat to the cylinder head in order to keep the operating conditions under control.

8) Very high resistance to current leakage.

9) Resistance to corrosion.

10) Sufficient reach into the combustion chamber.

11) The construction of spark plug should be such that the heat from local heating which ultimately leads to explosion of mixture, does not escape too quickly.