Basic function of ignition spark plug is to ignite the air and fuel mixture inside the combustion chamber. The resulting flame front expansion forces the piston from Top Dead Center to Bottom Dead Center. This process happens within 3 to 20 milliseconds depending on engine RPM. This piston movement is converted trough the connecting rod and crankshaft into rotating kinetic energy. Speed and the expansion characteristic of the flame front directly effects the developed mean pressure in the combustion chamber, which acts on the piston and is converted into power. But ignition spark plug is also exposed to a very challenging environment. The temperature inside of the combustion chamber reaches 2500 degrees Fahrenheit and the pressures reaching hundreds of MPa.
Spark plugs carry electrical energy from the ignition coil and wire inside of the combustion chamber. Spark occurs between spark plug center and ground electrode- igniting the A/F mixture and turning fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system and reach the spark plug in order to generate spark across the spark plug's gap. This is called "Electrical Performance" of the spark plug.
THERMAL PERFORMANCE OF THE SPARK PLUGThe temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance" of the spark plug, and is determined by the spark plug heat range selected.
HEAT RANGE EXPLAINED - High Performance Brisk USA Spark Plugs
It is important to understand that the spark plugs does not create heat, they only remove heat from its tip to prevent it from getting too hot and glowing. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the tip of the spark plug, and transferring the heat to the engine's cylinder head and cooling system. The spark plug heat range is defined as a plug's ability to dissipate heat.
The rate of the spark plug heat transfer is determined by:A spark plug's heat range has no relationship to the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the spark plug center electrode and ceramic tip. The spark plug heat range measurement is determined by several factors; the length of the spark plug ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the spark plug insulator and center electrode material.
The spark plug ceramic insulator nose length is the distance from the firing tip of the ceramic insulator to the point where the spark plug ceramic insulator meets the metal spark plug shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is the primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a moped, boat, or a race car, the spark plug tip temperature must remain between 475-850°C in order to properly operate. If the spark plug tip temperature is lower than 475°C, the ceramic insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits which are conductive and “conduct away” some of the available voltage that is needed to generate the spark. These accumulated deposits can result in spark plug fouling leading to spark plug misfire due to insufficient voltage for spark to occur.
If the spark plug tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister or crack and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the spark plug center electrode and ceramic tip. A projected style spark plug firing tip temperature is increased by about 15°C to 25°C.
The spark plug firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The temperature borderline between the fouling and optimum operating regions is 500°C. This temperature is called the spark plug self-cleaning temperature and is where the accumulated carbon and combustion deposits are burned off.
It is very important to keep in mind that the spark plug ceramic insulator nose length is a key-determining factor in the heat range of a spark plug. The longer the ceramic insulator nose is, the more ceramic nose surface area is exposed to the hot combustion gasses, and less heat is dissipated by the spark plug as the heat from the tip must travel further before it reach the spark plug metal shell and is transferred into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot spark plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose, have less surface exposed to the hot combustion gases and dissipates more heat from the center electrode and ceramic tip as the heat from the tip does not need to travel as far to meet the metal shell and transfer into the cylinder head water jackets. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation. Failure to use a cooler heat range in a modified application can lead to spark plug failure and severe engine damage.
Below is a list of external influences on a spark plug's operating temperature. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur.
Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.
Rich air/fuel mixtures cause the spark plug tip temperature to drop, causing fouling and poor drivabilityCompression can be increased by performing any one of the following modifications:
a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, milling heads, etc.)As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug and adequate higher-octane fuel can lead to spark plug/engine damage
Advancing Ignition Timing – dramatically increases the temperature in the combustion chamberIncreases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed
Ambient Air Temperature