Spark Plugs

Spark plug is a device used commonly in Spark Ignition (S.I) engines to ignite the air-fuel mixture in the combustion chamber. It produces electric spark when current is passed by the ignition system.

Gottlob Honold of Bosch developed a spark plug that helped in the development of spark ignition engines in 1902. Later in 1903, Oliver Lodge developed a more reliable version of spark plug to fit in S.I engines.

When does a spark plug fire?

We all are aware of the four strokes of S.I engines. At the end of compression stroke, the pressure build up in the combustion chamber becomes intense. The fuel breaks into finer particles due to the compression of charge. This is the easiest time to ignite the fuel and hence spark plug is excited by the ignition system to generate an electric spark that generates enough temperature to ignite the air-fuel mixture.

Spark plug is installed at the top of the cylinder head and ignites when the piston reaches top dead centre (TDC) in the compression stroke.

Functions of a Spark Plug:

A Spark plug basically has two main functions:

·         To ignite the air-fuel mixture. The electric energy passes through the central electrode and jumps the gap at the tip to generate spark, which then ignites the air-fuel mixture.

·         To dissipate the heat from combustion chamber. We might be at a misconception that spark plug creates heat, but no. Spark plugs act as a heat exchanger remove the heat from combustion chamber and transferring it to the cooling system. The temperature at the tip should be maintained such that it should not be high enough to lead to pre-ignition and it should not be low enough to result in fouling.

CONSTRUCTION OF A SPARK PLUG:

Insulator Body:

Insulator body is made of aluminium oxide ceramic. The shape of the body is moulded in a dry moulding system. After it is dry moulded, it is heated to a temperature higher than the melting point of steel. This provides high thermal conductivity and shock resistance. The surface is ribbed for better grip and protection from secondary voltage.

Terminal:

It is also known as connector. This is where the spark plug wire from the ignition coil is connected to the central electrode.

Hex Head:

This part is to fit the socket wrench in order to tighten and loosen the spark plug in its hole in the cylinder head.

Shell:

Shell is made of steel and is fabricated with the help of cold extrusion process. The shell is plated for corrosion resistance.

Gasket:

Gasket is used for sealing purpose. Threads are provided to screw the spark plug in the cylinder head.

Central Electrode:

The central electrode is connected to the outer terminal. It is an internal wire that conducts current from outer terminal to its tip. The tip is usually made of copper to carry away the heat of combustion.

Ground or Side Electrode:

They are manufactured from nickel alloy steel. They have an edge shaped area at the bottom to create the gap between central and side electrode. The high voltage jumps from the central electrode to the side electrode to create the required spark.

Spark Plug Working:

The high voltage required to generate a spark is supplied by an ignition coil. As the electrical energy passes through the central electrode, a voltage difference is created between the central electrode and the ground electrode. We all know that spark plug is fitted at the top of the cylinder head. Hence, air-fuel mixture will be present in the gap between the two electrodes.

Air-fuel mixture acts as an insulator, therefore no current can flow between the electrodes. Once the voltage is high enough to exceed the dielectric strength of the air-fuel mixture, the mixture is ionized. Ionized gases act as a conductor and allow the electrons to flow through them, thereby creating a spark. Different spark plugs require different voltages to generate the spark. It usually ranges between 20,000 V to 1,00,000 V.

A small flame is created at the tip of the spark plug which then flows as a flame front at the top of the piston depending on the composition of air-fuel mixture.

Types of Spark Plugs Based on Temperature:

Spark plugs can be divided into two types: Cold plug and Hot plug.

·         Cold Plug: Cold plugs are used in engines that generate high horse power and high compression pressure and temperature. It has less insulation; therefore it can transfer more heat from combustion chamber to the cooling system. It is of prime importance to transfer the excess heat from the combustion chamber to the outside in order to prevent pre-ignition and engine damage.

·         Hot Plug: Hot plugs have more insulation compared to cold plugs. These plugs are available in most standard engines. These spark plugs retain more heat to burn off the excess carbon deposits.

What is Spark Plug Fouling?

Fouling is known as coating of the insulator tip with foreign particles such as oil, carbon or fuel. This allows the high voltage to leach back down to the metal shell and to the ground instead of jumping the electrodes gap and creating spark.