The plug spark is a Driving element of the with positive ignition. It is the device which causes the ignition of the gas mixture. It draws, obviously, its name of the ignition system of the old engines, where an incandescent wick caused the combustion of the mixture, pointing out the Bougie lighting.

A little history

In 1885, the Belgian Etienne Lenoir invented an electric spark plug very near to those which we use nowadays.

The lighting of a mixture Fuel/Air by a spark was recommended by Italian Volta in 1777, then by Isaac de Rivaz for the internal combustion engine commonly called “  engine with explosion  ” in 1807.

Operation

The spark plug is an element of the ignition system of the positive ignition spark-ignition engine.

It is the vector of the Electric arc, which occurs thanks to strong the tension provided by the winds, at the time when the system of interrupters (platinized Vis or electronic system) orders it, allowing the combustion of the gas mixture, which releases energy thus.

The tension which is necessary to start the electric arc is about 10  Kv for a Ratio compression close to 10. With the atmospheric pressure, this tension of breakdown would be 10 times weaker.

It is characterized by operating temperatures going from (700 to 1  000°C).

Constitution

The candle consists of a central electrode, isolated by Porcelaine, likely to start a arc with an electrode of mass.

The length of insulator determines the operating temperature of the candle.

The candle, usually, is supplied with electricity by a reel.

Number

Usually, there is a candle by cylinder. However, in order to optimize combustion, certain engines are provided with two candles per cylinder ( Twin Spark , dual ignition at Alfa Romeo).

Spacing of the electrodes

The spacing of the electrode S is fixed by the manufacturer of the engine according to the rate of Compression, of the fuel used, the power of the igniter coil, and, to finish, of the discounted energy. The spacing is generally fixed between 5 and 9 tenth of millimetre.

Breakdowns

• the perlage due to a " perle" of Calamine or carbon, which puts in short-circuit the electrodes. It is enough to clean the electrodes to give the candle in state. The perlage is more frequent on the two-stroke engines, the cause is the unburnt residues of the Huile contained in the fuel.
• the consumption of the electrode, which involves a maladjustment of lighting and thus of combustion.
• clogging, generally due to a bad combustion.
• Porcelain insulating material broken.
• Bad tightening involving a bad adjustment and a loss of compression.

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