Short Answer:

A spark plug is an important device used in spark ignition (SI) engines to ignite the air-fuel mixture inside the combustion chamber. It produces an electric spark between its electrodes at the right moment, which causes the compressed mixture to burn rapidly and release energy. This energy pushes the piston down, generating useful mechanical power.

The spark plug is fitted on the engine cylinder head and connected to the ignition system. It must withstand high pressure and temperature inside the engine. Its correct functioning ensures smooth engine starting, complete combustion, higher efficiency, and reduced fuel consumption.

Detailed Explanation :

Spark Plug

A spark plug is an electrical device fitted into the cylinder head of a spark ignition (SI) engine. Its main function is to ignite the compressed air-fuel mixture by producing a spark at the right time. This ignition initiates combustion, which increases pressure and temperature rapidly and pushes the piston down during the power stroke. The spark plug, therefore, plays a vital role in converting chemical energy into mechanical work in petrol or gas engines.

The spark plug works with the ignition system, which supplies a high voltage (usually between 10,000 to 30,000 volts). This voltage jumps the gap between the central and ground electrodes of the spark plug, producing a spark. This spark ignites the air-fuel mixture inside the combustion chamber. The spark must occur at a precise moment to achieve maximum power and efficiency — this timing is known as ignition timing.

Construction of Spark Plug

A spark plug has several important parts that allow it to perform effectively under harsh engine conditions. The main parts include:

Insulator:
It is made of porcelain or ceramic material and provides electrical insulation between the central electrode and the metal shell. It also protects the spark plug from high temperatures.
Central Electrode:
This is the main conducting rod connected to the ignition coil. It carries high voltage current to the electrode gap. It is usually made of nickel, copper, or precious metals like platinum or iridium to resist wear and heat.
Ground Electrode:
The ground electrode is attached to the outer metal shell and positioned near the central electrode to form a small air gap. The electric spark jumps this gap to ignite the mixture.
Metal Shell (Body):
This part provides mechanical support to the spark plug and helps in transferring heat from the insulator to the engine cylinder head. It also has threads that allow it to be screwed tightly into the engine head.
Gasket or Sealing Washer:
It provides a gas-tight seal between the spark plug and cylinder head to prevent combustion gases from escaping.
Terminal Nut:
The top end of the spark plug has a terminal nut that connects the ignition cable or wire carrying high voltage from the ignition coil.

Working of Spark Plug

When the piston compresses the air-fuel mixture inside the combustion chamber, the ignition coil delivers a high voltage current to the spark plug. This voltage passes through the central electrode, jumps across the small air gap to the ground electrode, and produces a spark. The spark is hot enough (around 6000°C) to ignite the compressed mixture. The combustion releases heat and pressure, forcing the piston downward to perform useful work.

This process occurs repeatedly in every cycle of the engine. In a four-stroke engine, it happens once in every two revolutions of the crankshaft, while in a two-stroke engine, it happens once every revolution. The spark plug must operate reliably thousands of times per minute under high pressure, high temperature, and continuous vibration.

Types of Spark Plugs

Spark plugs are classified based on various factors such as their heat range, material, and design.

Hot Spark Plug:
It has a longer insulator nose and operates at higher temperatures. It is used in low-speed engines to prevent fouling.
Cold Spark Plug:
It has a shorter insulator nose and runs cooler. It is suitable for high-speed or high-temperature engines to avoid pre-ignition.
Standard Type:
This is the most commonly used plug in ordinary engines with normal operating conditions.
Platinum and Iridium Spark Plugs:
These are advanced plugs with longer life, better spark quality, and improved performance due to the use of precious metal electrodes.

Functions of Spark Plug

To produce a spark at the correct moment for ignition of the air-fuel mixture.
To ensure complete combustion of the mixture, leading to higher engine efficiency.
To operate under extreme conditions of temperature and pressure without damage.
To transfer heat from the combustion chamber to the engine cooling system.
To help maintain stable engine performance, smooth idling, and easy starting.

Maintenance and Importance

A spark plug requires regular inspection and cleaning to ensure proper engine performance. Deposits of carbon, oil, or fuel residues on the electrodes can reduce spark strength or cause misfiring. Proper gap setting between electrodes is also essential for effective ignition. The average spark plug lifespan depends on the engine type and driving conditions, but it typically ranges from 15,000 to 50,000 kilometers.

A faulty spark plug can cause problems such as poor acceleration, hard starting, high fuel consumption, and engine vibration. Therefore, timely replacement is important to maintain fuel efficiency and reduce emissions.

Modern vehicles often use multi-electrode or iridium spark plugs, which provide a more reliable spark and longer service life. They are also better suited for engines using electronic ignition systems.

Conclusion:

A spark plug is an essential component of spark ignition engines that provides the necessary spark to ignite the air-fuel mixture. It ensures smooth engine operation, efficient combustion, and better performance. By maintaining correct spark timing and gap, the spark plug plays a vital role in fuel economy, reduced emissions, and reliable power generation. Its proper care and maintenance are necessary for the overall health and efficiency of an engine.