Tip Temperature and Firing End Appearance

Tip Temperature and Firing End

The appearance of the firing end of a spark plug very much depends on the tip tmperatur of the spark plug. Spark plugs diagnosis depends on three criteria:

• Good
• Fouled
• Overheated

The spark plug self cleaning temperature is defined as the borderline between the optimum and fouling operating regions (500 degree Celsius). At this temperature, the accumulated carbon and combustion deposits are burned off.

Heat range of a spark plug is highly dependant on the insulator nose length of the spark plug. The longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water journals. This implies that the internal temperature of the spark plug is higher, and such spark plug is said to be hot plug. The function of a hot spark plug is to maintain a higher operating temperature so that it will burn off the oil and carbon deposits, and is not related to spark quality or intensity.

But, a cold spark plug consists of a shorter insulator nose which helps absorbing more combustion chamber heat. This helps the heat to travel a shorter distance, and allow the plug to operate at a lower internal temperature. It is necessary to keep a colder heat range when performance based engine modifications are done, subjected to heavy loads, or when the engine is run at high rpms. The cold spark plug removes heat faster, thus by reducing the possibility of pre-ignition/detonation and damage or melting of the firing end of the spark plug. (Temperature of the engine can affect the operating temperature of the spark plug, but no the spark plug’s heat range)

The spark plug’s operating temperature depends on certain potential external influences. The actual temperature of spark plug is affected by the following conditions or symptoms. These conditions cannot be created by a spark plug, but the spark plug should be capable to grapple with the levels of heat, or else it will affect the performance and may damage the engine.

Spark Plug operating temperature and engine performance highly depends on the Air/Fuel mixture.

• Rich Air/Fuel mixture drops the tip temperature which causes fouling and poor drivability.
• Lean Air/Fuel mixture increases the cylinder temperature and the plug tip temperature, causes pre-ignition, detonation, or seriously damage the engine or spark plug.
• Spark plug needs to be read many times during the tuning process in order to achieve the optimum air/fuel mixture.

Spark plugs tip temperature and in cylinder temperature will be elevated by Higher compression ratios/Forced Induction.

• Any of the below mentioned modifications can increase the compression of the engine:

1. Reduce the volume of the combustion chamber (ie, smaller chamber heads, domes pistons, milling heads, etc)
2. Add forced induction (Turbo charging, supercharging or Nitrous)
3. Change camshaft

• It is necessary to have a careful attention to ignition timing, air/fuel ratios, higher fuel octane and colder heat range plug, while the compression increases. It may damage the engine or the spark plug if a colder spark plug is not selected.

Advance the ignition timing

• Ten degrees of advance in ignition timing results in an increase of approximately 70 to 100 degree Centigrade of the tip temperature.

Engine load and speed

• Increase of firing end temperature is proportional to engine load and engine speed. A colder heat range spark plug should be installed when a vehicle is designed to travel at consistent high speeds or carrying/pushing heavy load.

Ambient Air Temperature

• Since air density is directly proportional to air temperature, the air/fuel mixture becomes linear as temperature increases.
• This increases the cylinder temperature / pressure, resulting in the temperature decrease of the spark plug and hence the fuel delivery should be increased.
• Air density decreases with increase of air temperature, as does the intake volume. So the fuel delivery should be decreased.

Humidity

• Air intake volume decreases with humidity
• This result in lowering the combustion pressure and temperature, thus by increasing the temperature of the spark plug tip and reduce the available power.
• Depending on the ambient temperature, air / fuel mixture should be linear.

Barometric Pressure and Altitude

• Altitude also affects the temperature of the tip of the spark plug.
• The cylinder pressure lowers with the increase in altitude. The plug’s tip temperature also decreases with the decrease in the cylinder temperature.
• Several car mechanics try to “chase” tuning of the engine by changing the heat ranges of the spark plug.
• The real answer is to adjust air/fuel mixtures by re-jetting in an effort to put more air back into the engine.

Related posts:

1. Spark Plugs: Heat Rating and heat Flow Path
2. Spark Plug Basics
3. Detonation

Heat rating and heat flow path of spark plugs

The distance from the point where the metal shell and the insulator meets to the firing tip of the insulator is termed as Insulator Nose Length. The tip temperature is a primary and significant factor in pre-ignition and fouling, since the insulator tip is considered to be the hottest part of the spark plug. The temperature of the spark plug tip should range between 500°C and 850°C independent of the vehicle it is used, let the spark plug be used in a street car, a race car, a speed boat or even a lawnmower. The temperature of the insulator area around the center electrode will not be hot enough to burn off the combustion chamber deposits and the carbon deposits, if the temperature of the tip of the spark plug is lower than 500°C. The deposits of carbon and the combustion chamber deposits can make the car misfire due to spark plug fouling. Conversely, if the tip temperature of the spark plug is greater than 850°C, it will overheat the spark plug and the ceramic around the center electrode might blister thus by melting the electrodes. This may cause detonation / pre-ignition and may lead to severe engine damage which is expensive. In identical types of spark plugs, the difference from one heat range to the next is the ability to remove around 70°C to 100°C from the combustion chamber. While in a projected style spark plug, the temperature of the firing tip is increased by 10°C to 20°C.

Related posts:

1. Spark Plug Basics
2. Fouling
3. Tip Temperature

There are two primary functions for spark plug:

1. Ignite the air/fuel mixture
2. Remove heat from the combustion chamber

In short, spark plugs transmit electrical energy that turns fuel into working energy. The ignition system should supply a sufficient amount of voltage to cause it to spark across the spark plug’s gap. This is called “Electrical Performance”.

The temperature at the firing end of the spark plug must be kept low enough to prevent preingnition, but the temperature should be high enough to prevent fouling. This is called “Thermal Performance”. The Thermal Performance is determined by the heat range selected.

Spark plugs do not create heat, they are designed only to remove heat.  By pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine’s cooling system, the spark plug works as a heat exchanger. The spark plug’s ability to dissipate heat is termed as heat range.

There are certain factors that determine the rate of heat transfer:

1. The insulator nose length
2. Gas volume around the insulator nose
3. The materials/construction of the center electrode and porcelain insulator

The heat range of a spark plug has no relationship to the actual voltage transferred through the spark plug. The measure of spark plug’s ability to remove heat from the combustion chamber is termed as heat range. There are several factors that determine the heat range measurement. The factors that determine the heat range measurement are:

• The length of the ceramic center insulator nose.
• The spark plug’s ability to absorb and transfer combustion heat.
• The material composition of the insulator.
• The central electrode material.

Related posts:

1. Spark Plugs: Heat Rating and heat Flow Path
2. Tip Temperature
3. Spark Plugs

Overview of Spark Plug

Spark plugs are categorized as one of the most misunderstood components in an engine. Many people are confused and numerous questions have surfaced over the years.

The discussion here aims to assist the technician, hobbyist, or race mechanic to understand, use and troubleshoot spark plugs. The information in this guide applies to all sorts of internal combustion engines: 2 stroke engines, rotary engines, high performance racing engines and street cars.

Spark plugs can be said as the “window” of you engine and is considered as a valuable diagnostic tool. Just like a thermometer for a patient, it is the spark plug that displays symptoms and conditions of the performance of an engine. An experience car tuning personal can verify the spark plug behavior and analyze the symptoms to track down the root cause of many problems or determine the air / fuel ratios.

Related posts:

1. Spark Plugs: Heat Rating and heat Flow Path
2. Spark Plug Basics
3. Fouling