A spark plug is an essential component in internal combustion engines, located in the cylinder head of the engine. Its main function is to initiate combustion of the air-fuel mixture inside the cylinder by means of an electric spark generated by a current arc.
It acts as an electrical device that converts energy from the ignition coil into a spark that ignites the compressed mixture, allowing the explosion that powers the engine.
Spark plugs are, in simple terms, electrical conductors that receive a potential difference from the ignition coil, transmitting it to the interior of the cylinder. In addition to initiating combustion, they also perform the function of dissipating the heat generated during this process, helping to maintain the optimal temperature inside the engine.
The design of a spark plug includes an insulated center electrode, which is connected by a heavily insulated wire to an outer circuit of the ignition coil. This electrode is positioned opposite a grounded terminal at the base of the spark plug, forming a spark gap within the cylinder, where ignition occurs.
How the spark plug works
The operation of the spark plug is based on Lorentz's law, where the electric current and the magnetic field interact to generate a spark. When electrons flow from the ignition coil, a voltage difference is created between the central and side electrodes of the spark plug.
Under normal conditions, the air-fuel mixture in the cylinder acts as an insulator, preventing the flow of current. However, as the voltage increases, the structure of the gases between the electrodes changes, and when the voltage exceeds the dielectric strength of the gases, they become ionized.
The ionized gas becomes conductive, allowing electrons to flow through the gap between the electrodes, creating a spark that ignites the fuel-air mixture. For this spark to occur properly, spark plugs require a voltage in excess of 20,000 volts, which ensures efficient ignition. Because of their location in the engine, spark plugs also serve as an indicator of engine health and can signal performance problems or the need for maintenance.
Heat rating of a spark plug
The heat rating of a spark plug is a parameter that determines its ability to dissipate the heat generated during combustion. This heat rating depends on the insulating material that covers the central electrode, usually porcelain, and plays an important role in the efficiency of the spark plug's cooling system.
The heat rating of the spark plug directly influences its performance. A "cold rating" spark plug has a greater capacity to dissipate heat, making it ideal for engines that operate at high speeds or with high thermal output. A "hot rating" spark plug, on the other hand, dissipates less energy and is more suitable for low-power engines, where temperatures are lower.
Spark plug gap
To ensure proper engine operation, it is essential that the spark plugs are correctly calibrated. Proper spark plug calibration improves combustion, reduces fuel consumption and optimizes gas expulsion. One of the key aspects in calibration is the distance between the electrodes, which must be as specified by the manufacturer. If this distance is not respected, the current flow will be poor, which could result in inefficient ignition or even engine failure.
One way to keep spark plugs in good condition is to clean them and file the electrodes periodically to ensure there is no buildup of debris that could interfere with operation. It is also important to check the electrode gap before reinstalling the plug in the engine to ensure proper spark generation.
Types of spark plugs
Spark plugs are classified according to several parameters, such as resistance, current arc size, or material type. Common types include:
- Spark plugs with naked resistor : The resistor is visible and exposed.
- Spark plugs with protected resistance : The resistance is covered with a sleeve to improve heat transmission.
- Spark plugs with anti-interference resistance : Similar to the protected ones, but with an additional resistance to eliminate electromagnetic interference.
- Normal gap spark plugs : The current arc jumps up to 0.9 mm.
- Large gap spark plugs : The current arc can reach up to 2.05 mm, which requires high voltage coils to produce the spark.
- Platinum or iridium spark plugs : More resistant and with lower electrical conductivity, but with a more efficient central electrode to generate the spark.
- Copper spark plugs : These are the most common and economical.
- Spark plugs for gasoline engines : They have similar characteristics to copper spark plugs and are the most commonly used in gasoline vehicles.
- Spark plugs for diesel engines : They are designed to withstand high temperatures and transfer heat directly to the combustion chamber.
- Spark plugs for gas engines : Used in Compressed Natural Gas (CNG) and Liquefied Petroleum Gas (LPG) engines. They are nickel-plated to resist oxidation and withstand higher ignition voltage.
Each type of spark plug is designed to meet the specific needs of the engine and ensure optimal performance under different operating conditions.
Spark plugs for diesel engines
In diesel engines, spark plugs are not used to ignite the fuel mixture, as in gasoline engines, but serve a different function. These plugs, known as glow plugs , are tasked with heating the air inside the cylinders before the engine starts, especially in low-temperature conditions.
Because diesel engines operate on compression, where the air is heated by high pressure before the fuel is injected, glow plugs facilitate this process by reducing the amount of effort needed for the engine to start, helping the correct atomization of the fuel and its combustion in the initial moments.
Unlike gasoline engines, which rely on a spark generated by the spark plug to ignite the air-fuel mixture, diesel engines operate by autoignition due to the high temperature reached by the compression of the air in the cylinder.
Glow plugs are particularly useful in cold conditions, where the internal temperature of the engine may be insufficient for fuel self-ignition. In these cases, the glow plugs are activated to raise the temperature of the air in the cylinders and ensure a smooth and efficient start of the diesel engine.