Thermal Performance

Thermal Performance

Spark plugs do not create heat, they only remove heat from its firing tip to prevent it from getting too hot and glowing thus preventing pre-ignition. However, the spark plug temperature also needs to be high enough to prevent fouling. Fouling is the accumulation of foreign substance such as fuel, oil or carbon on the firing centre of the spark plug ultimately causing misfiring due to inadequate voltage.

Thermal performance is determined by the spark plug heat range selected.

 

Heat Range

The heat range of a spark plug is determined by the spark plugs ability to transfer the heat from within the combustion chamber to the engine’s cooling system. The speed of which the heat is transferred is determined by the following spark plug design: –

  • The nose length of the ceramic insulator
  • Material used in manufacturing the centre electrode and ceramic insulator.
  • Amount of metallic surface area (spark plug) exposed within the combustion chamber

The distance between the end ceramic insulator (the firing tip), to the start of the ceramic insulator (where it meets the metal shell) is known as the ceramic nose length. The longer the ceramic nose length means the more insulation is provided to the spark plug. This would mean less heat is dissipated from the tip of the spark plug, making the tip hotter and operating at a higher temperature. This spark plug will be called a Hot plug.

The tip of the spark plug is the hottest part of the spark plug, the operating temperature is between 480o C and 855o C, as such the tip temperature is one of the cause of fouling that leads to misfiring. When the tip temperature is below 480o C, the tip is not enough to burn of the carbon deposits left behind during combustion thus causing fouling.

If the operating temperature of the spark plug is above 855o C, overheating occurs that will lead to cracking of the ceramic insulation and the electrodes burning off. This will lead to pre-ignition (misfiring). The optimum spark plug temperature is 500 o C, this will enable the spark plug to burn off the carbon deposits without compromising the spark plugs.

The spark plug firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The temperature borderline between the fouling and optimum operating regions is 500°C. This temperature is called the spark plug self-cleaning temperature and is where the accumulated carbon and combustion deposits are burned off.

It is very important to keep in mind that the spark plug ceramic insulator nose length is a key-determining factor in the heat range of a spark plug. The longer the ceramic insulator nose is, the more ceramic nose surface area is exposed to the hot combustion gasses, and less heat is dissipated by the spark plug as the heat from the tip must travel further before it reaches the spark plug metal shell and is transferred into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot spark plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.

Below is a cross reference of the Heat Ranges of Spark Plug of other makes.

Heat Range Cold Hot
Brisk 8 10 12 14 15 17
NGK 10 9 8 7 6 5
Denso 31 27 24 22 20 16
Bosch 2,  07 3,  08 4 5 6,  7 8
Champion 55,  57 4,  59 6,  61,  63 7,  8 9,  10 11,  12

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