What is the Purpose of Spark Plugs in Cars?

A spark plug is an electrical device that fits into the cylinder head of most of the internal combustion engine and ignites the compressed fuel by means of an electric spark. It has a metal threaded shell that is electrically isolated from a central electrode by a porcelain insulator.

On the basis of the relative operating temperature range of the tip of the high tension electrode, is divided into two types. One is a hot spark plug which has a long heat transfer path and more area is exposed to the combustion gases. Another type is the cold spark plug which has a short heat transfer path and a very small area gets exposed to the combustion gases.

Working principle

The spark plug is connected to the high voltage generated by an ignition coil. As the electrons flow from the coil, a voltage difference develops between the center electrode and side electrode. As a result, no current can flow because of the fuel and air in the gap act as 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 a voltage in excess of 20,000 volts to fire properly.

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. And the spark is generated for the ignition process.

Different components of a spark plug

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.

Ceramic Insulator

It is made up of an 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.

Metal Body

It is steel shell manufactured with precision rolled threads for a secure fit, and easy installation and removal. It provides an electrical ground to the cylinder head and helps to cool plug by transferring heat to the cylinder head.

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.

Ground Electrode

It is welded to the metal body of the SP. It makes a spark path with the central electrode. It is made up of nickel-based alloys (or iridium or titanium reinforcement).

Sealing Gasket

It makes sealing with the cylinder head and helps in heat dissipation.

Insulator tip

It is extended into the combustion chamber. It has a greater influence on the thermal rating of the spark plug

Electrode Gap

It is the distance between the central electrode and the ground electrode. The electrode has a crucial role in the spark generation. If an appropriate gap is not provided to the plug then it cannot produce sufficient spark to ignite the fuel and may lead to a misfire.

Functions of the spark plug

Basically, the spark plug has two main functions which can be illustrated as:

To ignite the air-fuel mixture

Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end when the plug receives a sufficient amount of voltage. This electrical spark ignites the fuel-air mixture in the combustion chamber.

To remove heat from the combustion chamber

Spark plugs cannot create heat, they can only remove heat. The temperature of the end of the plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engines cooling system. Always check the spark plug & when getting any problem then immediately do the preventive maintenance. The heat range of a spark plug is defined as its ability to dissipate heat from the tip.

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