Spark plugs are devices that deliver an electrical current, from the ignition system to the combustion chamber, where it ignites a compressed fuel and air mixture, with an electrical spark. It is important that the spark plug goes through this process, without allowing pressure from the combustion chamber to escape, because that would steal power from the engine. The main body of a spark plug is composed of a shell, insulator, and central conductor. As the spark plug's metal shell is screwed into the cylinder head, it becomes electrically grounded.
A spark plug works by connecting to the high voltage ignition coil or magneto. As the electrons flow from the coil, a voltage difference develops between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor which means the electrons can now flow across the gap. The average spark plug will typically need 20,000 volts for this process to fully function properly, however that amount may vary depending on the richness of the gas/air mixture. As the current of electrons surges across the gap, it raises the spark plug channels temperature. The intense heat in the spark channel causes the ionized gas to expand very quickly and create a small explosion. The heat and pressure created force the gases to react with each other which creates a small fire ball. The size of this fireball is determined by the composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark.
There are some main components that play a key role in the functionality of a spark plug. The first component is the terminal which is located at the top of the plug so that the ignition system can be easily connected. The construction of the terminal can vary, however they are typically designed so that the ignition wires can be snapped on or off so that the plug can be changed easily. Another important part of the spark plug is the insulator which is a hard ceramic material that provides mechanical support and electrical insulation for the central electrode. The insulator also provides an extended spark path for flashover protection and easier access to the terminal. Since the surface between the high voltage terminal and the grounded metal case of the spark plug is lengthened by the insulator, ribs are used to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case. Some modern spark plugs don’t have ribs because improvements have been made to strengthen the insulator, in order to protect against leaks. Seals are also used on the spark plug to seal the combustion chamber from any leaks. The spark plugs internal seals are typically glass/metal powder, or braze on older internal seals and a crush washer as the external seal. Since the spark plug is an inlet into the combustion chamber, it is important that the plug is completely secured. To do this, a metal case is used on the plug so that it doesn’t deform from excessive torque while it’s being tightened. This metal jacket also helps disperse heat that is generated in the plug by passing it to the metal of the cylinder head.
Getting the proper spark plug for a specific engine typically refers to the spark plugs gap. Most automobile engines have a gap that is between 0.035"–0.070" depending on the size of the engine and if it is fuel injected or carbureted. Many vehicles today that are fuel injected use a much larger gap size than that of a carbureted engine. The gap size can be slightly adjusted with the use of a gap gauge which is a circular device that measures the size of the gap. As a spark plug ages and the metal on the tip and hook erode, the gap will begin to widen. Because of this, manufacturers often set the gap on a new spark plug to the minimum recommended requirement.
Over the years, there have been many variations of spark plugs that can produce better ignition, longer life, or both. Such variations include the use of two, three, or four ground electrodes that are equally spaced and surround the central electrode. Other variations include using a recessed central electrode surrounded by the spark plug thread, which effectively becomes the ground electrode. Multiple ground electrodes generally provide longer life because when the spark gap widens due to electric discharge wear, the spark moves to another closer ground electrode. The disadvantage of multiple ground electrodes is that a shielding effect can occur in the engine combustion chamber inhibiting the flame face as the fuel air mixture burns, which can result in a less efficient burn and increased fuel consumption.
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