What is Spark Plug? Construction and Working.


The spark plug is a device in the ignition system of the vehicle that produces an electric spark to ignite the compressed air-fuel mixture inside the cylinder. The spark plug is screwed in the top of the cylinder so that its electrode projects in the combustion chamber.

Spark Plug
Spark Plug


A spark plug consists of mainly three parts:

  1. Centre electrode or insulated electrode.
  2. The ground electrode or outer elečrode.
  3. Insulation separating the two electrodes.

The upper end of the center electrode is connected to the spark plug terminal, where HT. cable from the ignition coil is connected. It is surrounded by a porcelain insulator. The lower half of the insulator is fastened with a metal shell. The lower portion of the shell has a short electrode attached to one side and bent in towards the center electrode so that there is a gap between the two electrodes.

The two electrodes are separated by the insulator. The sealing gaskets are provided between the insulator and the shell to prevent the escape of gases under various temperature and pressure conditions. The lower part of the shell has screw threads and the upper part is made in hexagonal shape, so that the spark plug may be screwed in or unscrewed from the cylinder head.

In some engines, a sealing gasket is also provided between the shell and the engine head. It provides a seal between the two parts as well as aids in the conduction of heat. In other designs, a tapered fit is used. Some spark plugs are provided with a built-in resistor, which forms part of the center electrode.

Spark Plug
Spark Plug

The resistor serves two purposes :

  1. It reduces radio and television interference from the ignition system.
  2. It reduces spark plug electrode erosion caused by excessively long sparking.


The materials used in the construction of different parts of a spark plug are as follows:

  1. Shell. Steel.
  2. Insulation. Porcelain, mica, sintered alumina. The porcelain has disadvantages of brittleness and low resistance to thermal shocks. Mica is somewhat attacked by fuels. Sintered alumina is now almost extensively used for insulation.
  3. Electrode. Nickel, an alloy of nickel and manganese, an alloy of nickel, manganese, and silicon. Platinum alloy, the addition of manganese (2.5- 3.5%) improves tensile strength and resistance to sulfur attack at high temperatures. Platinum alloys are better for electrodes, but their high costs limit their use.


  • A good spark plug must be able to function under all working conditions of temperature and pressure. It is designed to operate under 20,000 to 30,000 volts and withstand pressure as high as 420 N/cm2.
  • It must maintain the proper gap between two electrodes under all conditions.
  • The seals of the spark plug must be able to withstand the high pressure and temperature created in the combustion chamber during the power stroke.
  • The spark plug must offer very high resistance to current leakage.
  • It must be perfectly gas-tight. Any leakage of hot gases will upset the normal steady state of the plug causing such high temperature to be reached that the insulator and the electrode will disintegrate and fall into the cylinder.
  • It must be corrosion resistant.
  • It must have proper reach into the combustion chamber. Reach is the length of the threaded portion that enters the cylinder. If a long reach plug is fitted into a short reach hole, it will reduce the combustion space and increase the compression ratio. On the other hand, if a short reach plug is fitted into a long reach hole, it will increase the combustion space and reduce the compression ratio.
  • It should have reduced interference to radio and television from the ignition system.
  • It should also reduce the electrode erosion caused by excessively long sparking.

Watch the video below to know How does the Spark Plug works?


The function of the spark plug is to produce an electric spark that ignites the air-fuel mixture inside the cylinder. It must produce the spark at the correct movement at the end of the compression stroke. A proper gap is to be maintained between the two electrodes of the spark plug so that the sparking may take place.

When the spark plug is screwed in the cylinder head, the ground electrode is said to be connected with the ground. The terminal of the center electrode is directly connected with the H.T. lead of the ignition coil in case of the single-cylinder engine, or through the distributor in case of a multi-cylinder engine. The secondary circuit of the electrical system is to be completed through the gap between the electrodes. When the H.T. current passes through the circuit, it jumps the gap producing a spark, which ignites the compressed air-fuel mixture in the cylinder.

Spark Plug
Spark Plug

Spark Plug Gap

The gap between the center electrode and the ground electrode is called the spark plug gap. This gap is adjusted to the recommended specifications by bending the ground electrode. It varies from 0.4 mm to 1.0 mm. It is measured with a feeler gauge. The electrical resistance of the spark plug depends upon the nature and compression of the fuel mixture and also upon the gap. Too large or too small gap reduces the efficiency of the entire ignition system, which in turn causes losses in engine power and operating efficiency. It is, therefore, of great importance to maintain the proper spark plug gap.

No foreign materials. such as carbon, should be deposited on the electrodes, which will reduce the gap. Also, the electrodes should not corrode, which will increase the gap. The carbon deposited on the outside insulator between the plug terminal and the shell allows some of the high voltage currents to by-pass the gap.

This weakens the intensity of the spark causing losses in combustion efficiency and a decrease in engine power. If the electrical leaks become excessive, the plug will not produce a spark at all. Cracked insulators are usually caused by the careless installation of the plug or by careless adjustment of the plug gap.


Spark plugs are of two types according to the heat range:

  • Hot spark plug.
  • Cold spark plug.

Hot spark plug

Heat range is a means of designating how hot a plug will run in operation. It refers to the ability of the spark plug to heat transfer from the firing tip of the insulator to the cooling system of the engine.1 The temperature that a spark plug will attain depends upon this distance through which the heat is transferred. If the path of heat travel is long, the plug will run hotter than if the path is short. A hot spark plug has a longer path of heat travel and runs hotter than the cold spark plug which has a shorter path of heat travel and runs cooler.

Cold spark plug

Cold spark plugs are used in heavy-duty or continuous running high-speed engines to avoid overheating. When a plug runs too cold, the carbon deposits on the insulator around the center electrode. A hotter running plug will burn this carbon away and prevent its formation. A plug that runs hot will wear more rapidly since the high temperatures cause the electrodes to burn away more quickly.

If a plug runs too hot, the insulator may take on the white or greyish cast and may appear blistered. Low speed, medium-duty engines running at colder operating conditions require a hot spark plug. Different engines working in various conditions need a plug having a specific heat range; and manufacturers producing plugs in several heat ranges.

In between the hot and cold spark plugs, medium hot and medium cold spark plugs are also available to suit the specific condition of the engine. The projected core nose type of plug has the plug tip cooled by the incoming charge at high engine speeds, enabling the plug to run hotter at low speeds so extending the heat range.

Cleaning the spark plug

Due to the combustion of fuel in the cylinder, carbon particles deposit on and around the electrodes, which not only reduce the plug gap but also prevent the spark occurring. If the spark is still occurring, then it is too weak that it cannot ignite the fuel. Hence, the spark plug is to be cleaned. There is no specified period after which the spark plug is cleaned.

It depends upon the carbon deposits which can take place due to any reason like, nature of fuel mixture strength, lubricating oil, etc. The spark plug can be cleaned either by sandpaper or by sandblasting. For this purpose, remove it from the cylinder head, clean it, correct the gap, check carefully if any sand particle is sticking on it and then fit it again in the cylinder head properly.

If the insulator or sealing is broken, the spark plug should be replaced. The plug should be given an engine performance test after it is replaced in the cylinder. It should be checked carefully that the center electrode end is perfectly flat and not round. If it is round, it should be filed flat and the gap adjusted to specification.

Watch the video below for how to clean Spark Plug?

how to clean Spark Plug?

Spark plug failure

A spark plug will fail in its function due to the following reasons:

  • Plag fouled by engine oil entering the combustion chamber.
  • Plug fouled by the too rich mixture.
  • Plug badly covered with carbon from poor ignition.
  • Plug gap incorrect.
  • Burned electrodes or broken lower insulators caused by overheating.
  • Red, brown or yellow oxide deposits on plug interior that short the insulator.
  • Accumulation of dirt or moisture on the outside of the insulator that shortens the plug by grounding the high voltage.
  • Cracked or broken insulator sealing.

Spark plug Replacement Guide

Spark plug Replacement Guide

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Creative Blogger And Automobile Engineer.

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