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SPARK PLUG WORKING AND TYPES

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SPARK PLUG Construction and Materials

 

SPARK PLUG

Spark plug is a device to produce electric spark to ignite the compressed air fuel mixture inside the cylinder. The spark plug screwed in the top of the cylinder so that it’s the electrode projects in the combustion chamber.

Construction : A spark plug consist of three parts

1.      Centre electrode or insulated electrode.

2.      Ground electrode or outer electrode.

3.      Insulation separating the two electrodes.

The upper end of the centre electrode is connected to the spark plug terminal, where H.T cabel from the ignition coil is connected. It is surrounded by porcelain insulator. The lower 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 two electrode. The two electrode are separated by the insulator. The sealing gasket are provided between the insulator and the shell to prevent the escape of gases under various temperature and pressure condition. 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 part as well as aids in the conduction of heat. In other design tapered fit is used.  Some spark plug are provided with a built in resistor which part of the centre electrode. The resistor serves two purpose.

1.      It reduces radio and television interference from the ignition system.

2.      It reduces spark plug electrode erosion caused by excessively long sparking.



 SPARK PLUG

   

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

1.      Shell.  Steel

2.      Insulation. Porcelain, mica, sintered alumina. The porcelain has disadvantage of brittleness and low resistance of thermal shock. Mica is a somewhat attacked by fuel. Sintered alumina is now almost extensively used for insulation

3.      Electrode. Nickel, alloy of nickel and manganese, alloy of nickel, manganese and silicon. Platinum alloy, addition of manganese (2.5 -3.5%) improves tensile strength and resistance to sulphur attack at high temperature. Platinum alloy are better for electrodes, but their high costs limit their use.



Spark plug with naming


Requirements:

·        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 420N/cm^2 psi

·        It must maintain the proper gap between two electrode 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 y the electrode will  disintegrated and fall into the cylinder.

·        It must corrosion resistance.

·        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 ignition system.

·        It should also reduce the electrode erosion caused by excessively long sparking.

 

 

 

 

 

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