Categories Car Mechanic

Spark Plugs: Definition and Use


 – What is a spark plug?

 – Spark plugs: technical characteristics

 – Maintenance of the spark plug

 The spark plug is a critical element in petrol and LPG engines and is non–existent in the diesel engine.

 It has an essential catalytic role: it creates the spark necessary for combustion.

 What is a spark plug?

 The spark plug consists of two electrodes:

 – A ground electrode connected to the body of the spark plug and, by contact, to the cylinder head.

 – A positive electrode is insulated from the spark plug body in a central position. It is connected to the coil via the spark plug end cap; an internal resistor is inserted in it, the purpose of which is to protect the vehicle’s electronic environment from interference.

 – The spark plug is screwed into the cylinder head so that the spark plug’s electrode side opens into the cylinder’s combustion chamber.

 – The electrical impermeability between the body and the central electrode is ensured by a ceramic. It is resistant to thermal shocks and ensures a good dissipation of engine heat.

 – The lower part consists of a thread and a seal.

 Role of the spark plug

Spark plugs

 The spark plug is an essential element in the initiation of combustion:

 – When the air–fuel mixture is in the compression phase, a few degrees before Top Dead Center, the voltage generated by the coil (10 to 20,000 volts) is directed into the spark plug.

 – The gap between the two electrodes creates an electric arc (ionization voltage), which ignites the mixture.

 Spark plugs: Technical data

 Spark plugs must dissipate the heat of combustion (400 to 900 °C) and function optimally within a certain temperature range. That is why each engine receives a specific spark plug reference, which is mainly defined by

 – Mechanical characteristics: the diameter and length of the thread, the sealing system, the length of the spark plug, and the size of the hexagonal form for tightening and loosening are all parameters that will vary depending on the configuration and type of engine.

 – Thermal characteristics: these are defined by the thermal index:

 ◦ Indeed, depending on the engine’s characteristics influencing the combustion pressure and temperature (volumetric ratio – combustion pressure – combustion temperature – settings – tourism use – competition, etc.), the spark plug will have to dissipate the combustion heat more or less. To do this, the central porcelain will be more or less in contact with the base for calibrated heat removal.

 ◦ The thermal index characterizes a range from a cold spark plug (intended for high–performance engines, for competitions, for example) to a hot spark plug with a low thermal index (less efficient engines). It is coded in the spark plug reference and is specific to each equipment manufacturer.

 Spark plug brand

 Cold glow plug/index

 Hot spark plug/index


 Minimum index 2  Max. index 10


 Min. index 11.5  Maximum index 2

 The same engine has different qualities of spark plugs, mainly determined by the type of electrodes (multi–electrode – iridium – platinum, etc.).

 The better the performance, the higher the cost.

 Maintenance of your spark plug

 The spark plug is an element that must be replaced periodically, as technical progress has spaced out maintenance operations over time (from 10 to 15,000 km 30 years ago – from 30,000 to 120,000 km today).

 Spark plugs: Symptoms

 A thorough examination of a spark plug allows for establishing a precise diagnosis (a good tuner will adjust his carburation to the color of the combustion deposit on the spark plug):

 – charred electrodes: oil consumption;

 – black color: too rich carburation;

 – grey color with small deposits like pearls: poor carburation (carburation dosage – high ignition advance);

 – broken porcelain of the positive electrode: bad assembly or abnormal combustion.

 Replacing the spark plug

Spark plugs

 The spark plugs must be replaced per the manufacturer’s specifications:

 – suitable reference;

 – Tightening torque must be respected (torque wrench);

 – It is strongly advised against buying generic brands (supermarkets…) which are very cheap and of poor quality.

 Note: The well–known original brands are Bosch – NGK – Champion – Beru, etc.

 What budget should I spend?

 The price of spark plugs varies depending on the vehicle and the range… Count on a range from $6 to $20.

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