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Spark plug

A spark plug (in British English, a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated wire which is connected with a induction coil or magneto circuit on the outside, and forms, with another terminal on the base of the plug, a spark gap inside the cylinder. The spark plug was invented by Nikola Tesla, US patent 609,250,1898

Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.

Spark plugs are typically designed to have a variable spark gap. The spark gap may be adjusted by the technician installing the spark plug. A spark plug gap gauge or feeler gauge is used to set the gap. The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly.

Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use glow plugs to improve cold start characteristics.


Spark plug construction

A spark plug is composed of a shell, insulator and the conductor. It pierces the combustion chamber and must also seal the combustion chamber.

  • Insulator: The insulator is typically made from an aluminum oxide ceramic as is designed to withstand 1,200° F and 60,000 V. It extends from the metal case into the combustion chamber. The exact composition and length of the insulator partly determines the heat range of the plug.
  • Insulator tip: The tip of the insulator surrounding the center electrode is within the combustion chamber and direct affects the spark plug performance.
  • Ribs: The physical shape of the ribs function to improve the insulator and prevents electrical energy from leaking from the terminal to the metal case along the side of the insulator. The disrupted and longer path makes the electricity encounter more resistance all the surface of the spark plug.
  • Metal case: The metal case is the chassis of the spark plug and bears the torque of tightening the plug. It also serves to remove heat from the insulator and passes it on to the head. It acts as the ground for the sparks passing through the center electrode to the side electrode.
  • Center electrode: The center electrode can be made of copper, nickel-iron or precious metals. The center electrode is usually the one designed to eject the electrons because it is the hottest (normally) part of the plug; it is the cathode. It is much easier to pull electrons from a hot surface. It is also easier to pull electrons from a pointed electrode but a pointed electrode would melt after only a few seconds. The development of precious metal high temperature electrodes (using metals such as platinum) allows the use of a much smaller center wire, which is closer to a point but won't melt or corrode away. The smaller electrode also absorbs less heat from the spark and initial flame energy.
  • Side electrode: The side electrode is made from high nickel steel and is welded to the side of the metal case. The side electrode also runs very hot, especially on projected nose plugs. Some spark plug designs use multiple side electrodes that do not overlap the center electrode.
  • Seals: As the spark plug also seals the combustion chamber of the engine when installed, the seals ensure there is no leakage from the combustion chamber.
  • Terminal: The top of the spark plug contains a terminal to connect to the ignition system system. The exact terminal construction varies depending on the use of the spark plug.


The spark plug is connected to the high voltage generated by the coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. An ionized gas becomes a conductor and an ionized gas can pass electrons.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" you hear when watching a spark.

The heat and pressure force the gasses to react with each other and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded and a large one like the timing was advanced for that individual cycle.

Heat Range

The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine. This is determined by a number of factors, but primarily the actual temperature within the combustion chamber. There is no direct relationship between the actual operating temperature of the spark plug and spark voltage. However the level of torque currently being produced by the engine will strongly influence spark plug operating temperature because the maximum temperature and pressure occurs when the engine is operating near peak torque output (torque and RPM directly determine the horsepower output). The temperature of the insulator responds to the thermal conditions it is exposed to in the combustion chamber but not vice versa. If the tip of the spark plug is too hot it can cause pre-ignition leading to detonation/knocking and damage may occur. If it is too cold, electrically conductive deposits may form on the insulator causing a loss of spark energy or the actual shorting-out of the spark current.

A spark plug is said to be "hot" if it is a better insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with some manufacturers using ascending numbers for hotter plugs and some using descending numbers for hotter plugs.

Heat from the combustion chamber escapes through the exhaust gases, the side walls of the cylinder and the spark plug itself. The heat range of a spark plug has only a minute effect on combustion chamber and overall engine temperature. A cold plug will not materially cool down an engines running temperature. Too hot of a plug may indirectly lead to a runaway pre-ignition condition that can increase engine temperature. The primary affect of a "hot" or "cold" plug is to effect the temperature of the tip of the spark plug.

The heat range of a spark plug (i.e. in scientific terms its thermal conductivity characteristics) are affected by the construction of the spark plug: the types of materials used, the length of insulator and the surface area of the plug exposed within the combustion chamber. For normal use, the selection of a spark plug heat range is a balance between keeping the tip hot enough at idle to prevent fouling and cold enough at maximum power to prevent pre-ignition leading to engine knocking.

Reading spark plugs

The spark plug's firing end will be affected by the internal environment of the combustion chamber. As it can be removed for inspection, the effects of the combustion chamber on the spark plug can be examined. An examination, or "reading" of the characteristic markings on the firing end of the spark plug can indicate conditions within the running engine. The spark plug tip will bear the marks as evidence of what is happening inside the engine. Usually there is no other way to know what is going on inside an engine running at peak power. Engine and spark plug manufacturers will publish information about the characteristic markings in spark plug reading charts (e.g. a general spark plug reading chart)

For example, a sandblasted look to the tip of the spark plug means persistant, light detonation is occurring and it often unheard. The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder. Heavy detonation can cause outright breakage of the spark plug insulator and internal engine parts before appearing as sandblasted erosion but is easily heard.

As another example, if the plug is too cold, there will be deposits on the nose of your plug. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar. The material which seals the center electrode to the insulator will boil out.

An idling engine will have have a different impact on the spark plugs than one running at full throttle. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs.

Spark plug reading viewers, which are simply combined flashlight/magnifiers, are available to improve the reading of the spark plugs.

See Also

Last updated: 10-10-2005 05:57:27
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