Basic Theory of Spark Plug Operation

Basic Theory Of Spark Plug Operation

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 PLUG BASICS:
The spark plug has three primary functions:

Spark plug has to: Seals the combustion chamber
Spark plug has to: Ignite air/fuel mixture
Spark plug has to: Transfer heat from the combustion chamber


ELECTRICAL PERFOMANCE OF THE SPARK PLUG

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 PERFOMANCE OF THE SPARK PLUG

The 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:

The spark plug ceramic insulator nose length
The spark plug gas volume around the ceramic insulator nose
The spark plug materials/construction of the center electrode and ceramic insulator


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 drivability
Lean air/fuel mixtures cause spark plug tip and cylinder temperature to increase, which may lead to pre-ignition, detonation, and possibly serious spark plug and engine damage
It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

Compression 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.)

b) Adding forced induction (Nitrous, Turbocharging or Supercharging)

c) camshaft change


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 chamber

Advancing ignition timing by 5-10° causes spark plug tip temperature to increase by approx. 70°-100° C

Engine Speed and Load

Increases 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

As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased

Pre-ignition

Defined as: ignition of the air/fuel mixture before spark plug spark
Caused by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, low octane fuel, too hot a spark plug, lean air/fuel mixture, too high compression, or insufficient engine cooling

Pre-ignition spikes the combustion temperature dramatically and most often leads to detonation


A change to a higher-octane fuel, a colder plug, richer fuel mixture,
or lower compression may be in order

You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events
Combustion temperatures spike to over 3000°F during the combustion process (in a racing engine)

 

Detonation

The spark plug's worst enemy! (Besides fouling)
Can break spark plug ceramic insulators or break off ground electrodes. Most frequently caused by insufficient octane rating and hot spots in the combustion chamber.
Detonation occur as propagating flame front further squeezes the yet unburned mixture in the combustion chamber to the point of self ignition and detonate => the unburned mixture self ignite spontaneously generally from a side of the piston which tends to be the hottest part in the combustion chamber.  Sometimes the pre-ignition starts at the side of the piston and than the spark plug ignite secondary flame front. As the two flame fronts collide in the combustion chamber prior to the TDC maximum pressure and temperature occurs with the piston still trying to go up.
As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion and or detonation) and it can't go down (because of the upward motion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. Most of the damage than an engine sustains when "detonating" is from excessive heat and pressure.
The spark plug is damaged by both – the elevated temperatures and the accompanying shock wave, or concussion. Prolonged pre-ignition and/or detonation will burn a hole trough the piston; bend rod and can completely destroy the engine.

Misfires

A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect spark plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is being created)

Some people believe that the spark plug will either ignite the mixture or will not.  They believe there is nothing in between.  This belief is incorrect.  Most modern OBD-II engines can even calculate the % of misfire from the crankshaft position sensor by evaluating the crank angle speed million times per second.  On some vehicles the check engine will illuminate with the P0300 (random misfire) code or specific cylinder misfire code P0301- P0312) if just 10% cylinder misfire is detected which is sometimes far before average driver even notice that there is something not quite right with the engine.
Severe misfires will cause poor fuel economy, poor drivability, and if prolonged even catalytic convertor damage.


Fouling

Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
Wet-fouled spark plugs must be changed...spark plugs will not fire
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature or driving for few minutes in a low gear

Light media blasting can be also used, but only lightly on the ceramic insulator tip of the center electrode, end extreme caution should be paid to blow-off all the remaining media from the spark plug after the cleaning.  Liquid cleaners of any type should not be used for cleaning spark plugs, as they tend to imbed into the ceramic insulator and hamper it’s dielectric properties.

Before changing fouled spark plugs, be sure to eliminate root
cause of fouling