A spark plug is a device for delivering electric current from an ignition system to the cylinders of an engine to ignite a compressed fuel/air mixture. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil, coil pack or magneto. The plug has an electrode than is well insulated from heat and cross arcing. At the end of the plug is the gap section. The threaded part is a ground and grounds to the threads of the head. The electricity travels through the electrode and sends a spark across the gap from the electrode to the metal tip connected to the side threads. This spark ignites the fuel air mix and causes the explosion that we call ignition.
The plug is connected to the high voltage generated by an ignition coil, coil pack or magneto. As current flows from the coil, a voltage develops between the central and side electrodes. Initially, no current can flow because the fuel and air in the gap is an insulator, but as the voltage rises, 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 and allows current to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to "fire" properly, although it can go up to 45,000 volts. They supply higher current during the discharge process, resulting in a hotter and longer-duration spark.
As the current of electrons surges across the gap, it dramatically raises the temperature of the spark channel. The intense heat in the channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" heard when observing a spark, similar to lightning and thunder. The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark plug gap as the gases burn on their own. The size of this fireball, or kernel, depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded, and a large one as though the timing was advanced.
A spark plug is composed of a shell, insulator and the central conductor. It passes through the wall of the combustion chamber and therefore must also seal the combustion chamber against high pressures and temperatures without deteriorating over long periods of time and extended use.
Spark plugs are specified by size, either thread or nut, sealing type (taper or crush washer), and spark gap. Common thread (nut) sizes are 10mm (16mm), 12mm (14mm, 16mm or 17.5mm), 14mm (16mm, 20.63mm) and 18mm (20.63mm).
Dissected modern spark plug
Spark plug gap
Spark plugs are typically designed to have a spark gap which can be adjusted by bending the ground electrode slightly. The same plug may be specified for several different engines, requiring a different gap for each. Spark plugs in automobiles generally have a gap between 0.024 and 0.071 in. The gap is adjusted using a spark plug gap gauge.The gap adjustment can be crucial to proper engine operation.
Variations on the basic design
Over the years variations on the basic spark plug design have attempted to provide either better ignition, longer life, or both. Such variations include the use of two, three, or four equally spaced ground electrodes surrounding the central electrode. Other variations include using a recessed central electrode surrounded by the spark plug thread, which effectively becomes the ground electrode. Also, there is the use of a V-shaped notch in the tip of the ground electrode.
The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine, normally between 932 and 1,472 °F).
A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature.
The heat range of a spark plug is affected by the construction of the spark plug: the types of materials used, the length of the insulator and the surface area of the plug exposed within the combustion chamber. For normal use, the selection of a spark plug heat range is a balance between keeping the tip hot enough at idle to prevent fouling and cold enough at maximal power to prevent pre-ignition or engine knocking.
Spark plug manufacturers use different numbers to denote heat range of their spark plugs. Some manufacturers, such as Denso and NGK, have numbers that become higher as they get colder. By contrast, Champion, Bosch, BRISK, Beru, and ACDelco use a heat range system in which the numbers become bigger as the plugs get hotter.
Reading spark plugs
Tips of spark plugs can be "read" to indicate conditions within the running engine. A light brownish discoloration of the tip indicates proper operation; other conditions may indicate malfunction. See chart, below.
Indexing spark plugs
"Indexing" of plugs upon installation involves installing the spark plug so that the open area of its gap, not shrouded by the ground electrode, faces the center of the combustion chamber rather than one of its walls. The theory holds that this will maximize the exposure of the fuel-air mixture to the spark, also ensuring that every combustion chamber is even in layout and therefore resulting in better ignition.
Spark Plug Parts Reference
Here are some NGK spark plug references for typical Challenger engines:
2009 6.1L= NGK Iridium; LZTR5AIX-13; Gap .052
2018 3.6L= NGK Iridium; LKR7DIDX-11S; Gap .044
2018 5.7L= NGK Iridium; LFRJAIX-11; Gap .044
2018 6.4L= NGK Laser Platinum; LZTR6AP11EG; Gap .044