How Spark Plugs Work
Broadly speaking, a spark plug is a small electrical device used in internal combustion engines. But its size can be misleading. The tip of this small plug is capable of firing a small electric discharge using anywhere from 4000 to 28,000 volts of power. Internal combustion engines use this electric discharge or arc to fire the air-fuel mixture inside the combustion chamber of a cylinder to power lawnmowers, boats, racing cars and passenger vehicles. To better understand what a spark plug does, though, it is a good idea to know what role it plays within the system it operates and the different components and characteristics that help it do its job.The ignition systemInternal combustion engines use an ignition system that provides the spark needed to produce engine rotation work. The ignition process in a passenger vehicle equipped with a gasoline engine, for example, starts with the 12 volts provided by the battery to theignition coilin the system. This coil works as astep-up transformer, turning this small voltage into an electric charge containing 40,000 or more volts of power. The coil then sends this powerful voltage through a special electrical wire to the spark plug. This supercharged voltage is strong enough to jump the gap it encounters at the tip of the plug, crossing towards the side electrode-the small metal hook-sitting in front of the tip. This electrical arc then ignites the air-fuel mixture, creating a small explosion that pushes down on the piston inside.The parts of a spark plugThrough the center of the spark plug runs a metal rod called the center electrode. It is in charge of transporting the electrical current sent by the ignition coil. However, to prevent this current from jumping around, the electrical plug uses a ceramic insulator, which covers most of the upper half part of the plug’s body. Beneath the ceramic, the spark plug has a steel shell, which provides supports to the plug and the threads to properly mount the electrical device on the engine’s cylinder head.The complete the path for the electrical current, the spark plug also comes equipped with a grounding side electrode. This is the small hook at the end of the spark plug that bends over the center electrode. This gap, created between the center and side electrodes, is what creates a path for the electric arc that ignites the air-fuel mixture. The side electrode provides a way for the electric current to travel back to the battery using the engine block and frame in a vehicle. Although all spark plugs share the same basic components, their final configuration can reveal subtle but important differences to suit the needs of different types of engines.Resistor and non-resistor spark plugResistor plugs are so called because of the small resistor that sits at the center of the plug, breaking the center electrode in two sections. This resistor provides about 10,000ohmsof resistance to help lower the voltage traveling through the spark plug, which in turn helps reduce static and noise associated with signal interference on car radios and television sets. This is the main reason resistor plugs are widely used in passenger and commercial vehicles equipped with internal combustion engines. Non-resistor plugs lack this small resistor and can be found in many racing vehicles says James E. Duffy in “Modern Automotive Technology.”Spark plug reachInternal-combustion engines come in different size and configurations. Thus, to accommodate different designs, spark plugs come at different lengths. Simply stated, reach refers to how far the tip of the spark plug will extend into the engine’s combustion chamber. Using a spark plug with a long reach in a small combustion chamber can pierce through the engine piston and render the engine useless. Vehicle manufacturers provide an alphanumeric code in their car owner’s manual for proper spark plug replacement.Spark plug heat rangeBesides resistance and reach, spark plugs differ in the amount of heat they can dissipate during operation as well. The high voltage produced by the ignition system can heat the tip of a spark plug up to 900 F (482 C) or more. Spark plugs are designed to dissipate this heat at different rates. This is determined by the length and diameter of the insulator surrounding the tip. A spark plug with a short insulator around the tip, for example, will dissipate heat at a higher rate. A spark plug with this characteristic is referred to as a cold plug and is found mostly on vehicles used for long distant travel at highway speeds to prevent engine overheating. On the other hand, a spark plug with a long insulator around the tip will dissipate heat at a lower rate. A spark plug with this characteristic is referred to as a hot plug and is found mostly on vehicles used in stop-and-go, city traffic conditions.Spark plugs are very efficient electrical devices. In spite of their small size, they are powerful enough to make possible the operation of the ignition system of an internal combustion engine. Even though spark plugs share many physical characteristics, they also have important differences to suit different engine configurations and temperature needs. Without them, any internal combustion engine would be no more than a piece of heavy, useless junk.
