Spark Plug is a device which is used to ignite the air fuel mixture in the engine cylinder. They are generally used in petrol engine. For the combustion of the fuel, we need spark to initiate the combustion process in gasoline engine (Petrol engine)
Requirements of a Spark Plug
- It must be reliable at high voltage transmission i.e. up to 40,000 V.
- It must have good insulation capability even at temperatures of 1000 0C, and prevention of arcing and flashover.
- It must possess resistance to thermal shock (Hot exhaust gases-cold intake mixtures)
- It must make pressure-tight and gas-tight sealing with the combustion chamber.
- It must be capable to resist oscillating pressures up to approx. 100 bar.
- It should have high mechanical strength for reliable installation.
- It must have good thermal conduction by insulator tip and electrodes.
- It must possess resistance to spark erosion, combustion gases and residues.
- It must be capable of preventing of build-up of deposits on the insulator.
They are made with high quality materials to meet the above requirements.
The main parts of a spark plug are
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 of Spark Plug
- 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.
It is used in all petrol engines of scooters, motorcycles, cars, etc. where the petrol is burnt with the help of spark.