Spark plugs - what are they for and how they work


Spark plug

No gasoline internal combustion engine can be started without a spark plug. In our review, we will consider the device of this part, how it works and what you need to consider when choosing a new replacement kit.

What are spark plugs

A candle is a small element of the auto ignition system. It is installed above the motor cylinder. One end is screwed into the engine itself, a high-voltage wire is put on the other (or, in many engine modifications, a separate ignition coil).

Spark plugs - what are they for and how they work

Although these parts are directly involved in the movement of the piston group, it cannot be said that this is the most important element in the engine. The engine cannot be started without other components such as a gas pump, carburetor, ignition coil, etc. Rather, the spark plug is another link in the mechanism that contributes to the stable operation of the power unit.

What are candles in a car for?

They provide a spark to ignite gasoline in the engine's combustion chamber. A bit of history.

The first internal combustion engines were equipped with open-fire glow tubes. In 1902, Robert Bosch invited Karl Benz to install his design in his motors. The part had almost the same design and worked on the same principle as modern counterparts. Throughout history, they have undergone minor changes in materials for the conductor and dielectric.

Spark plug device

At first glance, it seems that the spark plug (SZ) has a simple design, but in fact, its design is much more complicated. This element of the engine ignition system consists of the following elements.

Spark plugs - what are they for and how they work
  • Contact tip (1). The upper part of the SZ, on which a high-voltage wire is put on, coming from the ignition coil or individual. Most often, this element is made with a bulge at the end, for fixing according to the latch principle. There are candles with a thread on the tip.
  • Insulator with external ribs (2, 4). The ribs on the insulator form a current barrier, preventing breakdown from the rod to the surface of the part. It is made of aluminum oxide ceramic. This unit must withstand temperature surges up to 2 degrees (formed during the combustion of gasoline) and at the same time maintain dielectric properties.
  • Case (5, 13). This is the metal part on which ribs are made for fixing with a wrench. A thread is cut on the lower part of the body, with which the candle is screwed into the spark plug well of the motor. The body material is high-alloy steel, the surface of which is chrome-plated to prevent the oxidation process.
  • Contact bar (3). The central element through which an electrical discharge flows. It is made from steel.
  • Resistor (6). Most modern SZ are equipped with glass sealant. It suppresses radio interference that occurs during the supply of electricity. It also serves as a seal for the contact rod and electrode.
  • Sealing washer (7). This part can be in the form of a cone or a regular washer. In the first case, this is one element, in the second, an additional gasket is used.
  • Heat dissipating washer (8). Provides fast cooling of the SZ, expanding the heating range. The amount of carbon deposits formed on the electrodes and the durability of the candle itself depend on this element.
  • Central electrode (9). Initially, this part was made of steel. Today, a bimetallic material with a conductive core covered with a heat-dissipating compound is used.
  • Insulator thermal cone (10). Serves for cooling the central electrode. The height of this cone affects the glow value of the candle (cold or warm).
  • Working chamber (11). Space between body and insulator cone. It facilitates the process of igniting gasoline. In "torch" candles, this chamber is expanded.
  • Side electrode (12). A discharge occurs between it and the core. This process is similar to earth arc discharge. There are SZs with several side electrodes.

The photo also shows the value of h. This is the spark gap. Sparking occurs more easily with a minimum distance between the electrodes. However, the spark plug must ignite the air / fuel mixture. And this requires a "fat" spark (at least one millimeter long) and, accordingly, a larger gap between the electrodes.

More about clearances is covered in the following video:

Iridium candles - is it worth it or not?

To save battery life, some manufacturers use an innovative technology for creating SZ. It consists in making the center electrode thinner (less energy is required to overcome the increased spark gap), but at the same time so that it does not burn out. For this, an alloy of inert metals (such as gold, silver, iridium, palladium, platinum) is used. An example of such a candle is shown in the photo.

Spark plugs - what are they for and how they work

How does a spark plug work?

All mechanisms of the ignition and fuel supply system must work synchronously. Only in this way the candle will give a spark at the right moment and ignite the air-fuel mixture.

The battery supplies 12 volts to the ignition coil. There, the voltage increases to 25-30 thousand V. Then, through the high-voltage wires, the current is supplied to the ignition spark.

Spark plugs - what are they for and how they work

At this moment, the piston in the cylinder has reached its "dead" (top position) point and compressed the fuel mixture in the combustion chamber. Here a spark is formed between the electrodes of the candle, an explosion occurs in the cylinder, which pushes the piston in the opposite direction.

The connecting rod, attached to the piston, turns crankshaft, on which the DPKV (crankshaft position sensor) is installed. When the marks of the crankshaft rod coincide with the corresponding notches of the sensor, the second sends a signal to the control unit about the need for a new pulse. The coil again generates the required voltage and feeds it through the wires to the candle.

Spark plugs - what are they for and how they work

The pulses are alternately fed to the individual cylinders. This sequence depends on the modification of the motor. For example, in one engine, the candle of the first cylinder first fires, then the second, then the fourth, and finally the third. The other motor runs in a 1-3-4-2 sequence. In the six-cylinder model, the clock order can be 1-5-3-6-2-4, and in the eight-cylinder model it can be 1-5-4-8-6-3-7-2.

Spark plugs - what are they for and how they work

This distribution of strokes is necessary for maximum smoothness of the crankshaft rotation. This ensures the durability of the bearings and the performance of the motor.

Most modern motors are equipped with several coils (one for each spark plug), which are controlled by an electronic control unit. But the principle remains unchanged - DPKV sends a signal to the ECU, the control unit - to the coil, the coil issues a discharge to the candle.

Types of spark plugs

The main parameters by which all SZ differ:

  1. number of electrodes;
  2. central electrode material;
  3. glow number;
  4. case size.

Firstly, candles can be single-electrode (classic with one electrode "to ground") and multi-electrode (there can be two, three or four side elements). The second option has a longer resource, because a spark stably appears between one of these elements and the core. Some are afraid to acquire such a modification, thinking that in this case the spark will be distributed among all the elements and therefore will be thin. In fact, the current always follows the path of least resistance. Therefore, the arc will be one and its thickness does not depend on the number of electrodes. Rather, the presence of several elements increases the reliability of sparking when one of the contacts burns out.

Spark plugs - what are they for and how they work

Secondly, as already noted, the thickness of the central electrode affects the quality of the spark. However, thin metal burns out quickly when heated. To eliminate this problem, manufacturers have developed a new type of plugs with a platinum or iridium core. Its thickness is about 0,5 millimeters. The spark in such candles is so powerful that carbon deposits are practically not formed in them.

Spark plugs - what are they for and how they work

Thirdly, the spark plug will work properly only with a certain heating of the electrodes (the optimal temperature range is from 400 to 900 degrees). If they are too cold, carbon deposits will form on their surface. Excessive temperature leads to cracking of the insulator, and in the worst case - to glow ignition (when the fuel mixture ignites from the temperature of the electrode, and then a spark appears). Both in the first and in the second case, this negatively affects the entire motor.

Spark plugs - what are they for and how they work

The higher the glow number, the less SZ will heat up. Such modifications are called "cold" candles, and with a lower indicator - "hot". In ordinary motors, models with an average indicator are installed. Industrial equipment often operates at reduced speed, so they are equipped with "hot" plugs that do not cool down so quickly. Sports car engines often run at high revs, so there is a risk of overheating of the electrodes. In this case, "cold" modifications are installed.

Fourthly, all SZ differ in the size of the faces for the key (16, 19, 22 and 24 millimeters), as well as in the length and diameter of the thread. What size of spark plug is suitable for a particular engine can be found in the owner's manual.

The main parameters of this part are discussed in the video:

What you need to know about spark plugs

Marking and service life

Each part is labeled with a ceramic insulator to determine if it will fit a given motor or not. Here's an example of one of the options:

A - U 17 D V R M 10

Position in markingSymbol meaningDescription
1Thread typeA - thread М14х1,25 М - thread М18х1,5 Т - thread М10х1
2Support surfaceK - conical washer - - flat washer with gasket
3DesignМ - small-sized spark plug У - reduced hexagon
4Heat number2 - the "hottest" 31 - the "coldest"
5Threaded length (mm)Н – 11 Д – 19 – – 12
6Heat cone featuresB - protrudes from the body - - recessed into the body
7Availability of glass sealantP - with resistor - - without resistor
8Core materialM - copper - - steel
9Upgrade serial number

Each manufacturer sets its own timing for replacing spark plugs. For example, a standard single-electrode spark plug must be changed when the mileage is no more than 30 km. This factor also depends on the indicator of engine hours (how they are calculated is described using an example car oil changes). More expensive ones (platinum and iridium) need to be changed at least every 90 km.

The service life of SZ depends on the characteristics of the material from which they are made, as well as on the operating conditions. For example, carbon deposits on the electrodes may indicate malfunctions in the fuel system (supply of an excessively rich mixture), and white bloom indicates a mismatch of the glow number of the spark plug or early ignition.

Spark plugs - what are they for and how they work

The need to check spark plugs can arise in the following cases:

  • when the accelerator pedal is pressed sharply, the motor reacts with a noticeable delay;
  • difficult starting of the engine (for example, for this you need to turn the starter for a long time);
  • decrease in motor power;
  • significant increase in fuel consumption;
  • lights up the check engine on the dashboard;
  • complicated starting of the engine in the cold;
  • unstable idling (motor "troit").

It is worth noting that these factors indicate not only a malfunction of the candles. Before proceeding with their replacement, you should look at their condition. The photo shows which unit in the engine requires attention in each case.

Spark plugs - what are they for and how they work

How to choose spark plugs?

In some cases, the answer to this question depends on the financial capabilities of the motorist. So, if the ignition and fuel supply systems are correctly configured, the standard plugs are changed only because the manufacturer so requires.

The best option is to buy the plugs recommended by the engine manufacturer. If this parameter is not specified, then in this case one should be guided by the size of the candle and the parameter of the glow number.

Spark plugs - what are they for and how they work

Some motorists have in stock two sets of candles at once (winter and summer). Driving for short distances and at low revs requires the installation of a "hot" modification (more often such conditions occur in winter). Long-distance trips at higher speeds, on the contrary, will require the installation of colder analogs.

An important factor when choosing a SZ is the manufacturer. Leading brands take money for more than just the name (as some motorists mistakenly believe). Candles from manufacturers such as Bosch, Champion, NGK, etc. have an increased resource, they use inert metal alloys and are more protected from oxidation.

Timely maintenance of the fuel supply and ignition systems will significantly extend the life of the spark plugs and ensure the stability of the internal combustion engine.

For more information on how spark plugs work and which modification is better, see the video:

Which spark plugs are better


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