Spark Plug

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What is a Spark Plug?

Spark Plug is a small device inside the cylinder that ignites the air mixture and fuel using electricity of high voltage and the resulting combustion powers the vehicle. Spark Plugs are located in the cylinders of the engine.


  • Spark Plug was invented by Edmund Berger in 1839, but failed to patent it.
  • Recorded documents show Sir Oliver Lodge as an inventor.
  • First electric spark plug was used for the first internal combustion piston engine by Jean Etienne Lenoir in 1860
  • First high-voltage spark plug that can also be commercially viable was invented by Gottlob Honold in 1902, an engineer for Robert Bosch, which also led to the development of spark-ignition system.

Albert Champion and his brothers, sons of Sir Oliver Lodge played a key role in manufacturing R & D which made Lodge Ignitors a profit-making company founded in 1903.

Important Parts


This is the very top of the spark plug which has snapped wired that electrically connects the ignition system either by clip or a threaded nut.

Ribs / Corrugations

Gives better insulation by preventing electricity from leaking from surface while it transmits from terminal to metal case. Provides seamless path for better electricity resistance even in the presence of dirt and moisture


It is made from a high tolerant and resistant material like sintered alumina, a hard ceramic component to withstand high voltage and temperatures. Provides support and insulation to center electrode, while providing path for extended spark. Provides good conduction to heat, thereby reducing the chances of insulator glowing in excessive temperatures

Insulator Tip

An unglazed tip made of same sintered aluminium oxide which extends into a combustion chamber and can withstand high temperatures upto 650 degree C. Sintered alumina was introduced by Siemens in1930s, replacing Mica which posed problems of lead deposits as a result of use of leaded petrol back then the heat range of the plug is determined by the dimensions of the insulator and metal conductor. The one with shorter dimensions are usually cooler ones.

Seal / Gasket

It ensures that the combustion chamber is sealed airtight by the Spark plug. Compressed glass or metal powder is used for internal seals whereas a metal washer is typically used for external seals.

Metal Shell

Metal casing and threads are made with cold rolled metal to prevent the weariness arising out of thermal cycle. It provides a protective casing to withstand the torque and acts as a ground for spark to travel through the central electrode to side electrodes. It also removes the heat from the insulator passing to the cylinders.

Center Electrode

The center electrode actually transmits the electrons that stream from the edges of flat electrodes creating spark and hence is the hottest part of the plug. The spark gets weakened as the edges of the electrodes erode, which in early days needed to be maintained through manual cleaning or sand-blasting. However the use of new high-resistant metals like iridium, palladium etc and high value metals like gold, platinum, and silver for small and sharp centre wires for electrodes in modern day plug ensures that they will not melt or corrode easily.

It is connected to the terminal through internal wire and insulation to reduce RF noise emission from the sparking.

Ground / Side Electrode

It is made from high strength nickel steel; side electrode is wielded directly to the side of the metal shell. It also runs hot as the electricity runs from centre electrode to side electrode which is grounded to the car battery Some plugs may have multiple side electrodes which do not overlap the center electrode.


The gap is critical for a good engine performance, as a narrow gap may produce weaker spark to ignite the air and fuel mixture, whereas too wide a gap would fail to produce a spark at all causing reduced fuel efficiency or power. Same plug can be used in different engines by adjusting the gap settings to better engine performance.



Type of Spark Plugs:

Distinct Spark plugs basis different metal alloys used for center electrodes.

Nickel Alloy Plugs:

Nickel provides good ignition and can last till 20,000 – 40,000 Kms. Both center and side electrode are flat edged and are made from Nickel for better conduction.

Copper Spark Plugs:

Copper increases the rate of heat conduction, hence used for high-performance or modified engines for greater power. These are most commonly used in vehicles as it can withstand higher temperatures, safe guarding against overheating issues and providing greater resistance. Race cars generally use resistor less copper plugs.

Iridium Spark Plugs

Iridium tip covers the center electrodes with either nickel or platinum used on tips of side electrodes. These are extremely durable with long life and can last upto 60,000 to 1, 00,000 Kms. Iridium plugs provide better ignitability due to sharp tipped centre electrodes, although they tend to overheat.

Platinum Spark Plugs

The plug incorporates platinum at the tip of the center electrode and the platinum pad for side electrode. It provides high ignitability and precision firing with reduced gap erosion lasting longer. However, platinum plugs too tend to overheat as it is less conducive to heat and can sometimes affect performance


Working of Spark Plug

Before the ignition, the fuel pump transfers the fuel to the chamber through fuel lines and the air is sucked into throttle body as well through cold air intake system. It gets mixed together and sent to cylinders for combustion.

  • The plug is connected by an ignition coil which releases a high voltage. The centre electrode is insulated to prevent leakage.
  • The flow of electrons from coil creates high voltage passing through the centre electrode to side electrode.
  • The fuel and air in the gap remains insulated till the time voltage rises.
  • The temperature needs to rise to a point where the fuel morphs to gases and becomes ionised. The voltage required by Spark plugs can be anywhere between 12000-25000 volts to fire properly.
  • Electrons flow though ionised gases to the gap in the plug resulting into combustion.

Once the heat increases, gases expand rapidly resulting in small explosions. It finally generates tremendous heat and pressure creating a ball of fire which eventually burns on its own to generate torque.


Mistri Tips

As most vehicles use copper spark plugs with life upto 40,000 Kms, as a rule, one should check the spark plug for wear and preferably change it once the vehicle crosses the mark. Certain vehicle conditions can also prompt a spark plug check and learning to read the spark plug effectively can help a great deal. Here a few things that can indicate a spark plug problem.

Rough Car-start or No ignition: A bad spark plug can result in a weak engine start. Also check the gap of the plug. If the gap is too wide, it would fail to generate enough spark to ignite. Faulty plug can also cause battery drain eventually leading to ignition problems.

Rough engine noise during idling: If the spark plug doesn’t fire normally during idling it may lead to rich fuel and air combustion. A quick way to know is check the plug which may look carbon fouled with dry soot deposits on electrodes. Excessive vibrations could have also lead to terminal nut wear.

Misfire and Late starts: Misfiring can lead to erratic running of engine indicating problems with firing cylinders. If this is true, you would find light brown ash deposits on the tip indicate the oil leakage in combustion chambers.

Low Power for Acceleration: Any sluggishness during acceleration suggest problem with firing cylinders. Check for the narrow gap on the tip which maybe producing weaker spark. If you find blisters on the Spark plug, it is caused by overheating which can occur if the gap is wide or the plug is fitted incorrectly.

Low Fuel Economy: Worn out spark plugs can reduce the fuel economy by as much as 30-35 %.

Engine Surging: This can occur with abnormal combustion in cylinder due to either faulty fuel line, faulty cooling system or spark occurring too early. This can result in overheated plug, broken ceramics, melted electrodes and deposits.

Overheated plugs: They look glossy and white as a result of long insulation tips that burn away the deposits. Too hot plugs can ignite fuel even before spark is created. Change to a cold plug with narrow insulation tip that will shorten the length the current travels and thereby control heating.

Other problems: It can include oil deposits on tips due to oil leakage in cylinders or broken and chipped or cracked insulator tips as a result of improper gap setting or general wear over longer duration. This can also lead to piston damage hence a wise decision to change them immediately.


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