Spark plugs - What are spark plugs for, what is their purpose?

In 1902, only 300 were produced, while today's circulation is over 350 million candles a year. In one hundred years of production, Bosch has developed over 20.000 different types and produced over 7,5 billion pieces.

Gasoline engines are external ignition timing motors that translate the chemical energy of the fuel into kinetic, ie. mechanical energy, to perform certain work. In these, the fuel feed system forms a mixture of air and fuel just before entering the combustion chamber.

The mixture is aspirated into the chamber by the subpressure created by the piston moving toward the lower dead point. A periodic, externally defined ignition timing (as opposed to self-ignition of the diesel) ignites the mixture over the spark plug for a moment before the piston enters the upper dead center (GMT) and thus begins the combustion process.

Candles

This process takes place during the third stroke, the only working stroke for four-stroke internal combustion engines. After a spark plug ignites a mixture of air and fuel, the temperature rises as a result of the mixture burning. The pressure in the cylinder increases and pushes the piston toward the lower dead point (DMT), transferring the work via the piston rod to the crankshaft (crankshaft). This work is available to the driver in the form of engine power, which increases with increasing engine speed.

So the ignition is done by an electric spark, more precisely by the energy it possesses. The ignition system generates high voltage (over 30.000 volts) at certain moments, which creates a spark between the spark plug electrodes in the combustion chamber. This high voltage is generated in the transformer coil. In current passenger cars, an ignition system with inductively accumulated energy is used.

Candles

If there is a high enough voltage, the spark jumps between the spark plug electrodes. At the moment of ignition, the voltage between the spark plug electrodes rises very rapidly until the moment of sparking (the so-called spark gap). breakdown voltage which is about 15.000 volts). After jumping the spark, the voltage between the electrodes drops to some value called spark voltage. The time that elapses from the moment the spark is created to its disappearance is called duration of the spark. Another question to answer here is, why is it necessary for the ignition system to generate a voltage of more than 30.000 volts if the breakdown voltage is less than 15.000 volts? The answer is basically very simple, over time, the spark gap between the electrodes increases due to spark plug wear, which inevitably leads to an increase in the breakdown voltage. The other elements of the ignition system should also be worn, as well as battery deterioration. The difference between the available and the breakdown voltage is called ignition voltage reserve. It is necessary that at the very end of the service life of the vehicle (engine), the system will be able to create all the necessary conditions for the formation of a spark, that is, for the ignition of the mixture.

Spark Plug Requirements

The spark plug requirements are extreme: it is also exposed to periodic process changes in the combustion chamber and outside the engine. It is subject to four types of requirements: electrical, mechanical, chemical and temperature.

Electrical requirements imply that no electric arc can be skipped or electricity cut through the insulator. The electrical resistance of the insulator must also be satisfactory at a temperature of 1000 degrees Fahrenheit and must not be changed during the spark plug life.

Mechanical requirements mean that the spark plug must withstand high pressures (up to 50 bar) that periodically appear in the combustion chamber and must therefore, for the entire period of its operation, remain gas-tight. In addition, high mechanical strength of all parts in the joint is required, especially the ceramic and the metal body of the spark plug.

The chemical requirements for the tip of the spark plug must be completely satisfied, because that part directly senses the combustion process that can cause it to become hot and, therefore, exposed to chemical processes at high temperatures. If the melting point is reached, the aggressive chemical components in the fuel are deposited on the spark plug and may change its characteristics.

Under the temperature requirements, it is considered that the spark plug, ie its ceramic part, must have high thermal "shock" resistance, because at one moment it will suddenly receive the heat of hot combustion gases, and at another it will be exposed to a cold mixture of fuel and air.

Spark Plug Construction

The spark plug consists of metal, ceramics and glass. Careful design of the spark plugs results in the full utilization of the positive properties of these materials. The most important parts of the spark plug are the connecting wire, insulator, body and electrodes.

Connecting conductor, is made of steel and gas tightly sealed to the insulator by pouring special glue into the gap between them. This electrically conductive adhesive is a joint with the central electrode. At its end emerging from the insulator, the connecting conductor has a connecting thread with a connecting nut for connection to the high voltage cable.

Izolator is made of special ceramics (aluminum with additive additive) and is intended to isolate the central electrode and the connecting conductor from the spark plug body. For better current loss prevention, there are five recesses on the insulator. They serve to increase the surface over which current losses pass. Due to these grooves, there is no unwanted power outage even under extreme driving conditions. Otherwise, the insulator is used as a support for both the central electrode and the connecting conductor.

Body  is made of steel and is designed to secure the spark plug to the cylinder head. The surface of the body is electrically coated with nickel for corrosion protection, to keep the thread free and to prevent it from clogging, especially in aluminum motor heads. Depending on the installation conditions, the spark plug may be supplied with a sealing ring or cone.

Electrode wear out under the action of erosion (caused by ignition sparks) and corrosion (chemical and thermal influences). Their wear leads to an increase in breakdown voltage, ie. to reduce the voltage reserve. The electrode of the mass is attached to the body of the spark plug and usually has a rectangular bent shape. It can be made as a front, or as a side, depending on its position in relation to the central electrode. By thinning the mass electrode profile as well as by placing multiple mass electrodes, the lifespan of the spark plug is significantly extended. The central electrode is gas-tightly fixed in an insulator with a special electrically conductive glue. It has a slightly smaller diameter than the opening of the insulator tip due to expansion due to heating. The most commonly used material for making the central electrode of ordinary spark plugs is copper (for making the core) and nickel alloy (for making the sheath around the core). The special materials used for this purpose are silver and platinum, but in that case the electrode is much smaller in diameter.

The gap between the electrodes is the smallest distance between the central electrode and the mass electrode, and as such is a measure of the length of the spark path. The smaller the gap, the smaller the breakdown voltage required. Typically, this value ranges from 0,7-1,1 mm for carburetor engines, while for electronic fuel injection and ignition systems this value ranges from 1,1-1,8 mm. The electrode position is determined by the gap type. There are two types: air spark gap, when the mass electrode is in front of the central electrode and superficially spark gap, when one or more electrodes of mass are located on the side of the central electrode. Thus, the spark position corresponds to the gap position between the spark plug electrodes.

The thermal characteristics of the spark plug

Thermal range candles is a measure of the thermal load of the spark plug (motor) and it must match the characteristics of the spark plug (motor). This term is crucial for choosing the right spark plug for a single engine.

Spark Plug Operating Temperature is the temperature that the spark plug reaches during engine operation and it represents the balance between the amount of heat received (from the inflamed mixture) and the amount of heat transferred (the head of the motor which is in contact with the threads). It ranges from 400 to 850 degrees Fahrenheit.

The lower limit is determined by the lowest self-cleaning temperature of the spark plug from carbon black and unburned carbon and the upper by the highest temperature at which the mixture will not self-ignite (due to its contact with heated parts of the spark plug). The main reason why the same type of spark plug cannot be used in all engines lies in the fact that the higher power motors release more heat and have a higher combustion chamber temperature than the lower power motors. That's why there are, roughly speaking, two basic types of spark plugs, "gather"I"cold".

It is important to underline that most of the heat received from the chamber (about 80%) conducts the spark plug from the top of the insulator through its inner sealing ring to the spark plug body, from where it is further transferred to the cylinder head via a threaded connection. In the case of a "hot" spark plug, the tip of the insulator is in maximum contact with the hot gases from the combustion chamber. It is used with lower power engines because (the top) needs a larger amount of heat to heat up and "enter" its operating temperature range.

With "cold" spark plugs, things are quite the opposite (Figure 4). Different lengths of the free tip of the insulator affect the different thermal range of the spark plug. To make it clearer which spark plug is in question, the heat range is marked with a number called the spark value of the spark plug. Lower heat numbers (eg 2… 4) mean “cold” spark plugs with a low heat intake through the short circuit of the insulator, while higher heat values ​​(eg 7… 10) mean “hot” spark plugs with a large heat intake through the long tip of the insulator.

Abnormal spark plug operating conditions

Abnormal operating conditions, which can occur during the combustion of the mixture, can irreparably damage the engine and spark plug. Here we mean the most basic anomalies such as: self-ignition of a mixture, that is, its detonative combustion.

The self-ignition of the mixture is due to the local overheating (at full load) of the spark plug tip, the exhaust valve, the bulging parts of the cylinder head as well as the separated sediment particles. This is an uncontrolled ignition process where the temperature in the combustion chamber can rise sharply to cause serious damage to spark plugs and other engine parts.

Note that self-ignition is NOT the same as detonation or "diesel". Detonation is uncontrolled combustion with a very steep increase in pressure. It is caused by the spontaneous ignition of parts of the mixture that have not yet been affected by the spatial expansion of the candle flame front.

Combustion is performed several (dozens) times faster than normal (gradual) combustion. Pressure oscillations with very large amplitudes and high frequency occur. As a consequence, overheating of the engine parts occurs.

The effects of detonation on the spark plug are visible in the form of pitting of the mass electrode. The root cause of this anomaly lies in the poor quality of the fuel (gasoline).

It is quite certain that the science and technology of making spark plugs over the past hundred years has made a huge step towards improving the internal combustion engine. Of course, there are many more temptations and challenges ahead for the constructors, but they already belong to some new time ahead and to a new chapter on spark plug development.

Prepared by: Dušan Ković
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