Requirements

1. It must be reliable at high voltage transmission i.e. up to
   40,000 V.
2. It must have good insulation capability even at temperatures
   of 1000 0C, and prevention of arcing and flashover.
3. It must possesses resistance to thermal shock (Hot exhaust gases-cold
   intake mixtures)
4. It must make pressure tight and gas-tight sealing with the
   combustion chamber.
5. It must be capable to resist oscillating pressures up to approx.
   100 bar.
6. It should have high mechanical strength for reliable
   installation.
7. It must have good thermal conduction by insulator tip and
   electrodes.
8. It must possess resistance to spark erosion, combustion
   gases and residues.
9. It must be capable of preventing of build-up of deposits on
   the insulator.
10. They are made with high quality materials to meet the above
    requirements.

1. Plug Terminal: It
   is the portion that is connected to the high tension cable coming from the
   distributor cap. It conducts the high voltage to the central electrode.
2. Ceramic Insulator:
   It is made up of Aluminum oxide ceramic and acts as an insulator. It separates
   the central electrode from earth at up to 40000 Volts. It can be manufactured
   in plain form or with profiles to prevent flashover.
3. Metal Body: it is
   steel shell manufactured with precision rolled threads for a secure fit, and
   easy installation and removal. It provides electrical ground to the cylinder
   head and helps to cool plug by transferring heat to the cylinder head.
4. Central Electrode:
   It is made nickel based alloys consists of a copper core enclosed in it. Depending
   upon the type, the central electrode can be in platinum or iridium. The high
   voltage is applied to the central electrode from the secondary winding through
   the distributor.
5. Ground Electrode:
   It is welded to the metal body of the SP. It makes spark path with the central
   electrode. It is made up of nickel based alloys ( or iridium or titanium
   reinforcement)
6. Sealing washer/ Gasket:
   It makes sealing with the cylinder head and helps in heat dissipation.
7. Insulator tip: It
   is extended into the combustion chamber. It has greater influence on the
   thermal rating of the spark plug
8. Electrode Gap: It
   is the distance in between the central electrode and ground electrode. The
   electrode has crucial role in the spark generation. If an appropriate gap is
   not provided to the plug than if cannot produce sufficient spark to ignite the
   fuel and may leads to misfire.

Types of Spark Plug

On the basis of the relative operating temperature range of
the tip of the high tension electrode, it is divided into two types.

1. Hot spark plug:
   It has long heat transfer path and a large area exposed to the combustion gases.
2. Cold spark plug:
   It has short heat transfer path and small area exposed to the combustion
   gasses.

Working

• When a high voltage current from the distributor is passes
  to the spark plug.
• The central electrode and ground electrode is applied to a
  very high voltage up to 40000 V.
• Due to this high voltage difference between the central
  electrode and ground electrode, the air in between the electrode gap gets
  ionized.
• The ionized gas becomes conductor and conducts current from
  central electrode to the ground electrode producing spark.
• The spark produced is used to ignite the air-fuel mixture in
  the engine cylinder.

Application

It is used in the petrol engines of scooters, motorcycles, cars, etc. where the petrol is burnt
with the help of spark