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A spark plug (spark plug) is an integral part of the ignition system that generates a spark to ignite the air-fuel mixture in the combustion chamber. The mixture in the cylinder is ignited at a precisely specified moment from an electric spark that passes between the spark plug electrodes.

Spark plugs are installed on naturally aspirated and turbocharged gasoline internal combustion engines. There are no spark plugs in diesel engines. The spark plug design includes the following main elements:

  • Spark plug device
  • contact rod;
  • central electrode;
  • insulator;

Also, the spark plug device assumes a resistor, a sealing washer, a housing, a thread for screwing into the cylinder head and a side electrode.

Spark plugs are found in all types of systems: contact, contactless and electronic ignition systems. The contact rod is the junction of the spark plug with a high-voltage wire or ignition coil, which depends on the individual design features of implementing a particular ignition system on a particular engine.

The central electrode in the spark plug device is the cathode, which is alloy steel. For manufacturing, an alloy of chromium and nickel is often used. To increase the service life, the central electrode of a modern spark plug can also be made of alloys containing rare metals: platinum, iridium, tungsten, palladium, and others. Due to the presence of such metals, the spark plug is called platinum, iridium, etc. The resource of spark plugs can vary greatly, which directly depends on the quality, features, and manufacture materials. Ordinary candles serve about 25 thousand kilometres, while platinum or iridium counterparts can exceed this indicator in terms of service life by 2-3 times.

The connection between the centre electrode and the contact rod is made through a spark plug resistor. A resistor is a material that conducts current. The specified material fills the free space between the rod and the electrode. The use of a resistor is dictated by the need to protect electrical equipment from electrical noise that occurs during the formation of a spark.
The spark plug insulator is ceramic and heat resistant. Its inner part is called a heat cone. The specified cone serves to determine the thermal condition of the spark plug. The temperature regime of the candles has certain thresholds. The minimum threshold is the temperature at which deposits begin to burn out on the heating cone of the plug. The maximum threshold is considered to be such heating of the cone when the short-circuit ( glow ignition ) effect occurs. Such spontaneous ignition of the fuel mixture can cause engine detonation, which is an abnormal process of combustion of a fuel charge during operation of the power unit and leads to devastating consequences for the internal combustion engine.

Depending on the size of the thermal cone of the insulator, spark plugs have a different glow number and are divided into the so-called “hot” and “cold”. There are also intermediate values. Hot candles have a glowing number of 11-14; average candles are considered 17-19; cold candles have an indicator of 20 or more. Universal or unified spark plugs have a heat rating of 11-20.
Hot candles heat up faster, while heat dissipation is slower. For this reason, hot plugs are installed on engines that have a low compression ratio and run on low octane fuel. Cold candles are characterized by slow heating and fast heat dissipation. These plugs are installed on forced-aspirated and turbocharged engines with a high compression ratio, designed for gasoline with a high octane number. When selecting spark plugs, it is necessary to separately consider the thermal mode of the engine and the heat rating.

The spark plug body is made of metal alloys. It contains the basic structural elements of the candle. The outer part of the housing is threaded so that the spark plug is screwed into the cylinder head. For additional sealing, a special sealing washer is often present on the body.

Also, on the body of the candle, there is a hexagon for the candle wrench. It is necessary to tighten the spark plug with the recommended torque using a spark plug wrench. The insufficient tightening torque can lead to depressurization of the combustion chamber, exceeding often results in damage to the spark plug and threads in the cylinder head spark plug well.

At the bottom of the housing is one or more spark plug side electrodes. The specified electrode can be either nickel or made of rare metal alloys. Plugs with multiple electrodes have an increased service life since, after the destruction of one electrode, the spark plug continues the sparking process with the help of additional side electrodes. The centre and side electrodes have a defined spark gap between them, also known as the spark plug gap. The specified gap on different plugs is set at the factory and may differ, depending on the design features of the plug and the suitability of a particular engine model.

The increased clearance allows for better sparking and more efficient ignition of the mixture and requires more voltage. For this reason, too large a gap on the spark plugs can lead to misfiring, increased fuel consumption, etc. At the same time, a significant reduction in the gap leads to the fact that the process of ignition of the fuel charge in the combustion chamber becomes less efficient, the engine loses power. Because of the above, self-adjustment of the spark plug gaps by bending the side electrode without proper experience of such actions is strongly discouraged.

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