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Spark Plug: Types, Parts, Working Principle, Requirements and More

In this post, you learn what is a spark plug, types, parts, working principle, how to clean the spark plug, spark plug gap and more.

Spark Plug and types

The spark plug is a device it produces an electric spark to ignite the compressed air-fuel mixture insides the engine cylinder. The spark plug is screwed in the top of the cylinder so that its electrodes project in the combustion chamber.

Spark Plug

Main Parts of a Spark Plug

Following are the main parts of a spark plug:

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

Description of Parts

The upper end of the centre electrode is connected to the spark plug terminal, where H.T. cable from the ignition coil is connected it is surrounded by a porcelain insulator.

The below half portion 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 centre electrode so that there is a gap between the two electrodes.

Main parts of a Spark plug

The twp electrodes are thus 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 like a nut 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 design, a tapered fir in used. Some spark plugs are provided with a built-in resistor, which forms part of the centre electrode. 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.

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Material Used In Spark Plug

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

  1. Shell and 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, the alloy of nickel, manganese and silicon. Platinum alloy, the addition of manganese (2*5-3*5%) improves tensile strength and resistance to sulphur attack at high temperatures. Platinum alloys are better for electrodes, but their high cost limits their use.

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Working of Spark Plug

The use of the spark-plug is to produce an electric spark to ignite the compressed air-fuel mixture inside the engine cylinder. It must produce the spark at the correct movement at the end of the compression stroke.

Working of spark plug

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 so screwed in the cylinder head, the ground electrode is said to be connected with ground.

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The terminal of the centre 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.

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Types of Spark Plug

There are two different types of spark plug listed below:

  1. Hot spark plug.
  2. 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.

Hot spark plug

The temperature that a spark plug will attain depends upon this distance through which the heat is transferred. If the oath of heat travel is long, the plug will run hotter than if the path is short.

This types of spark plug have a longer path of the heat travel and run hotter than the cold spark plug which has a shorter path of heat travel and runs cooler.

Cold spark plug

This types of the spark plug are used in the heavy-duty of the continuous running high-speed engine to avoid overheating. When a plug runs tool cold, the carbon deposits on the insulator around the centre 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 temperature causes the electrode to burn away more quickly.

Cold spark plug

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 condition require a hot spark plug.

Different engine working in various condition need a plug having a specific heat range, and manufacture 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.

How To Clean The Spark Plug

Due to the combustion of fuel in the cylinder, carbon particles deposited on and around the electrodes, which not only reduce the plug gap but also prevent the spark to occur. If the spark is still occurring.

It is too weak that it cannot ignite the fuel. Hence the spark plug is to be cleaned. There is no specific period after the plug is cleaned. It depends upon the carbon deposits which can take place due to any reason like, nature of the 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 a broker, 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 centre electrode end is perfectly flat and not round. If it is round it should be filed flat and the gap adjusted to specification.

Spark Plug Gap

The gap between the centre 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 between 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.

Why Does a Spark Plug Fail To Function

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

  1. Spark fouled by engine oil entering the combustion chamber.
  2. Plug fouled by the too rich mixture.
  3. Spark plug badly covered with carbon from poor ignition.
  4. Plug gap incorrect.
  5. Burned electrodes or broken lower insulator caused by overheating.
  6. Red, brown or yellow oxide deposits on plug interior.
  7. Accumulation of dirt or moisture on the outside of the insulator that short the plug by grounding the high voltage.
  8. cracked or broken insulator sealing.

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Requirements of a Good Spark Plug.

  1. 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 600 psi.
  2. It must maintain the proper gap between two electrodes under all conditions.
  3. The seals of the spark plug must be able to withstand the high pressure and temperature created in the combustion chamber during the power strokes.
  4. The spark plug must offer very high resistance to current leakage.
  5. 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.
  6. It must be corrosion resistant.
  7. 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 sort reach plug is fitted into a long reach hole, it will increase the combustion space and reduce the compression ratio.
  8. It should have reduced interference to ratio and television form ignition system.
  9. It should also reduce the electrode erosion caused by excessively long sparking.

That is it. Thanks for reading. If you have any questions about “types of spark plug” leave a comment. If you found this post helpful share with your friends.

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Saif M
Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at theengineerspost.com