What does a spark plug have to withstand?

Spark plug
Spark plug

Without the spark plugs, the gasoline engine would not be able to operate. A relatively unobtrusive part of the engine that has to withstand significant temperatures and pressures. There are different spark plugs, what are their most important characteristics?

Although the spark plug is a small part at first glance, but the conditions in which it is forced to work, it deserves recognition. As it seeks to increase engine power and at the same time seeks to extend the life of the candle, more and more demanding ones are placed before the spark plug.

By lighting a spark plug in the engine combustion chamber, the spark plug must be able to withstand rapid temperature changes in the range of about 2000 to 2500 degrees and a pressure of up to 6 MPa. At the same time when sucking the engine there is a negative pressure in the cylinder and lowering the temperature to about 80 degrees. But that's not all. Interesting - for a six-cylinder engine at an engine speed of 5000 rpm. one minute takes 15.000 sparks. In one minute, one spark plug is lit 2500 times, which is more than 40 times per second! Furthermore, the spark plug has unfavorable chemical conditions in the combustion chamber, not to mention the different operating conditions of the engine. And also voltage surges from 25 to 30 kV (kilovolts).

Spark plug
Spark plug

Like a spark plug

Ignition of the mixture with a spark plug is caused by a spark between the electrodes. We are talking about the so-called discharge between the electrodes. The spark itself occurs when a jump voltage occurs between the middle and side electrodes (there can be more side electrodes). This converts energy from the coil into an electric spark. The voltage depends on the distance of the electrode, the geometry of the electrode, the pressure in the combustion chamber, as well as the air / fuel ratio at the time of ignition - the richness of the mixture. During the operation of the motor, the spark plugs wear out, which leads to an increase in the distance of their electrodes, which leads to a gradual increase in the jump voltage.

Especially well insulated

What is a spark plug made of? The body of the candle is made of insulator, ceramic. At the top is a connector for connecting the ignition cable or induction coil. On the underside is a metal housing that includes a thread to screw the candle into the cylinder head. An external (sometimes called lateral) electrode is connected to the metal housing. The middle of the candle is guided by a medium positive electrode connected to the end of the ignition cable or coil (rod coil that goes directly to the spark plug). The outer electrode is connected to the body of the vehicle by a metal housing and is therefore the negative pole of the electrical system.


There are many types of candles. At first glance, they can vary in thread diameter in the dimensions M18, M14, M12 and M10. Together, the thread width is also different. 1,5 to 1,25 to 1,0 mm. Further, the shape of the sealing surface of the candles in the cylinder head varies, may be conical or straight. They have short and long threaded spark plugs.

The next division is in accordance with the layout (construction) of the middle and outer electrodes, which can be up to four electrodes. In addition, spark plugs may be different from the material used to manufacture the electrodes or the shape of the housing.

In order to maintain the required and increasing requirements for a spark plug, it is important to choose the right electrode material. Spark plugs are usually made to indicate a trade-off between longevity and price in the daytime. Tungsten, platinum and iridium alloys are used. Alternatives can be iron and chromium alloys. Even better, silver, which has great thermal properties, excels in durability and extends spark plug life up to 70.000 km. The downside, of course, is the price. Platinum is also used. Even it is dear, with good corrosion resistance. Medium electrodes often consist of two different materials.

Spark plug
Spark plug

Hot, cold and more… more

When evaluating spark plugs, three important values ​​are evaluated, among other things, from which other traits are derived. The first is the electrode distance already mentioned. This is the minimum distance between the middle and side electrodes. The smaller this distance, the lower the punching voltage required to create a spark.

However, with a short electrode distance, the length of the spark is short. This releases little energy, which reduces the ignition of the mixture. There is no spark, the engine is noisier, and flue gas emissions are deteriorating. Instead, high ignition voltage is required for greater distance, and ignition may be delayed at high speeds motorboat.

Another feature is the location of the spark. It is the distance between the end of the middle electrode and the front of the spark plug.

The third characteristic is the heat value of the spark plug. It is a measure of the heat capacity of a spark plug, which must be adapted to the characteristics of the engine. The spark plug must not exceed a certain temperature range during operation. In practice, a particular spark plug may overheat in one engine without having a low operating temperature in the other engine. Therefore, it is distinguished by warm candles with high heat value and cold candles, which have lower heat value. The temperature of the spark plugs is determined mainly by the surface of the lower part of the insulator. If the insulator tip is long, the spark plug will have a high heat value. On the other hand, a cold spark plug (low heat value) is connected to the short tip of the insulator.

How to Know the Right One

The properties described above and the resulting differences between the spark plugs on them are interesting, but for practice, to be precise, to know which spark plugs your car needs, the knowledge doesn't matter. When buying spark plugs, it is important that you have the correct spark plug markings to make sure they are designed for a particular engine.

Unfortunately, some spark plug manufacturers use a different labeling methodology. Fortunately, there are catalogs available to every auto parts dealer. For interest, the Bosch W7D spark plug at Champion bears the N9I or BPM7 mark NGK . It is the same spark plug in terms of properties and characteristics.

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