Spark Plugs 101
Spark plugs are the ignition source for the air/fuel mixture in engines. The spark that’s delivered by the spark plug must overcome the intense combustion chamber pressures and temperatures. They require between 12,000 – 25,000 volts to fire correctly, but can be as high as 45,000 volts. The actual electricity (spark) does not ignite the air/fuel mixture, but rather the heat generated by the electricity (spark) ignites the mixture. Spark plug gap (air gap between the center electrode and side electrode) ranges from .035 – .070 inch. A spark plug can fire up to 35 times per second in flame temperatures of over 3000°F (1649°C). The spark created at the electrode tip ionizes the air/fuel mixture causing it to slit and form a conductive path. The gas ionization initiates a flame kernel that grows into a flame front and will travel thru the combustion chamber. Voltage at the spark plug gap is determined by several factors:
⇒Air/ fuel mixture
⇒Spark plug gap
⇒Spark plug electrode material and design
⇒Combustion chamber pressure
⇒Combustion chamber temperature
⇒Improper installation (not torqued)
Available Voltage – The voltage the ignition system provides the spark plugs.
Bridging – When deposits accumulate on the spark plug tip and connect the center electrode and side electrode creating an improper spark path.
Carbon Tracking – Unwanted ionization (spark) outside the spark plug wire or coil that can lead to misfires.
Center Wire – Carries secondary ignition voltage, high voltage spark.
Central Electrode – Carries secondary voltage across the gap to the side terminal (cathode).
Compression Pressure – The required voltage increases in direct proportion to cylinder pressure or as density increases.
Core Nose Tracking – Happens when the spark does not jump the gap from the center electrode, but runs from the center electrode along the ceramic insulator up inside the plug where it meets the metal casing and grounds out.
Corona Discharge (Corona Effect) – It is a rare phenomenon that causes a bluish white glow on spark plug wires or around spark plug. The cause for corona discharge is the ionization of the surrounding air. The ionization releases electrons from the oxygen molecule that is resultant energy release creates photons (light) to be produced. Certain contaminants enhance the appearance
Corona Stain – A discoloration around the ceramic insulator surface between wire boot and metal case of plug. It is caused by charged oil particles in the air around the ceramic insulator surface that are attracted by the corona discharge. These particles will stick to the insulator surface. The staining should not be confused with combustion gases leaking thru the spark plug. The staining will not affect function or performance of the spark plug.
Crush Washer – This the sealing washer between the spark plug and the cylinder head. When torqued to the proper specifications, the washer will collapse to form a leak proof seal.
Deposits – Are accumulation of deposits on the spark plugs on the center electrode, side electrode, shell, and center electrode insulator. The deposits can bridge for an improper spark path. Can be caused by:
⇒Excessive oil leaking into combustion chamber
⇒Engine’s operating temp or duration
Detonation – Caused by extensive pre-ignition (pinging), incorrect air/fuel mixture, or too advanced ignition timing. The center electrode ceramic insulator may appear cracked or broken away.
Dielectric – An insulator prevents current flow.
Dielectric Strength – The maximum non-conductivity of a material before it breaks down and becomes conductive.
Erosion (Spark Erosion) – Each time the spark plug fires, minute metal particles from the electrodes are worn away. Some of the affects of too much erosion (wear) are:
⇒Higher fuel consumption
⇒Added strain on the ignition system, the expanded gap requires higher voltage to fire.
Exotic Metals – See Noble Metals.
Flame Front – After the flame kernel, the first part of the air/fuel mixture that starts to burn is called the flame front as it travels igniting the rest of the mixture.
Flame Kernel – The initial shape of a freshly ignited air/fuel mixture during the first few milli seconds of combustion that happen at the spark plug electrode.
Flashover – This is when voltage sent to the spark plug does not fire between the center and side electrode. Instead it shorts between the exterior terminal nut and the metal casing of the spark plug causing a misfire.
Fouling – Occurs when the spark plugs firing tip becomes coated with excessive oil, fuel, or combustion deposits so that is unable to produce a spark. Can be caused:
⇒Too rich air/fuel mixture
⇒Excessive oil leaking into combustion chamber
⇒Too “cold” heat range spark plug
⇒Low speed engine operation
⇒Retarded engine timing
⇒Cold running engine
Gap – The voltage that is needed to ionize the electrode gap. The larger the spark gap the higher the required voltage is needed to ionize the electrode gap.
Heat Range – Refers to the operating temperature of the spark plug. It is determined by the length of the electrode insulator (ceramic) nose to the metal casing.
Heat Transfer – The function of a spark plug to remove heat from the combustion chamber.
Insulator – The ceramic portion of the spark plug that is made of sintered alumina. It is very hard and has a high dielectric strength.
Ionizing – Becoming electrically conductive, as in the air/fuel mixture becomes ionized, it creates as path for the spark to travel from the center electrode to the side electrode.
Kv – Kilo-volt.
Noble Metals – Metals used in the electrode tips that can be yttrium, iridium, tungsten, palladium, platinum, silver and gold.
Overheated – An overheated spark plug will have deposits that have melted on the insulator tip and give the insulator tip a glazed or glossy appearance.
Precious Metals – See Noble Metals.
Pre-Ignition (pinging or knocking) – It is the ignition of the air/fuel mixture before the spark from the spark plug ignites it. Can be caused:
⇒Lean air/fuel mixture
⇒Hot spots in combustion chamber (carbon deposits)
⇒Too “high” heat range spark plug
Quenching – Is the heat generated by the spark and is absorbed by the center or side electrode instead of the air/fuel mixture. This is unwanted, because the desired effect is for the air/fuel mixture to absorb all the heat. Manufacturers are creating different designs to reduce the quenching effects.
Required Voltage – The voltage required to produce a spark.
Resistor Type Spark Plugs – A ceramic resister of 4.5K-5K Ohms built into the center wire of a spark plug to suppress spark generated electromagnetic noise that can interfere with a vehicle’s on board electronics and radio signals.
Ribs – Protrusions on the ceramic insulator to help seal the spark plug boot.
Shadowing – The design effect of the ground electrode blocking the spark from the air/fuel mixture. Spark plug designs made to minimize this effect for better combustion.
Spark Corona – See Corona Discharge (Corona Effect)
Terminal – Top of plug that connects to ignition system coil or spark plug wire.
Combustion chamber temperatures can range from 3600-4500 degrees F (2000-2500 degrees C) and the heat transfer into the spark plug firing end can range from 750-930 degrees F (400-500 degrees C) at light load situations. At heavy loads the spark plug firing end temperature can range from 1560-1830 degrees F (850-1000 degrees C).
Spark Plug Heat Range:
Spark plugs insulator tip and center electrode are designed to work within the ideal heat range of about 930-1560 degrees F (500-850 degrees C). If the spark plug firing tip exceeds over 1900 degrees F (1040 degrees C) the air/fuel mixture in the combustion chamber tends to pre-ignite. If the temperature is less than 750 degrees F (400 degrees C) then the insulator tip and center electrode tend to foul with carbon and oil deposits. This can lead to misfires, poor drivability and higher emissions. The spark plug heat range is measured how fast heat is transferred away from the insulator tip and electrode through the case threads into the cylinder head and cooling system. The length, projection of the insulator nose, surface area, the center core electrode material, and thermal conductivity of the insulator material determine the control of heat range.
In a hotter range spark plug, the longer insulator nose creates a longer path for heat to dissipate to the threads and into the head and cooling system. This allows a higher heat build up in the tip of the spark plug.
In a colder range spark plug, the shorter insulator nose creates a shorter path for heat to dissipate to threads and into the head and cooling system. This allows heat to be transferred easier, and not so much heat is builds up at the spark plug tip.
Factors affecting spark plug temperature:
Ignition timing – Ignition timing too advanced.
Compression pressure – The higher the compression ratio, the higher the spark plug temperature.
Engine speed and load – The higher the engine load and speed the higher the temperature.
Air/fuel ratio – A lean air/fuel mixture will cause higher spark plug temperatures as opposed to a rich air/fuel that will cause a cooler spark plug temperature, but a very rich air/fuel mixture can cause fouling.
Tightening Torque – Improperly tightened (loose) spark plugs cannot transfer heat properly and spark plug temperature will increase. Over tightened spark plugs will decrease temperature very slightly.
Low octane gasoline – Has a faster burn rate and is more susceptible to knock, increasing temperature.
Cooling system problems – A cooling system that is not cooling properly will cause all the components on the engine to run at higher temperatures.
Excessive engine deposits – Too many deposits in the combustion chamber will increase temperatures.
Hot range spark plugs – Spark plugs that are in the hot range will retain more heat.
Spark Plug Material Types:
Exotic Material (Noble Metal) Spark Plugs:
Spark plug design is the same basic one for all. The main difference is in the electrode design and materials. Manufacturers are using noble metals to increase efficiency and longer service intervals. The use of these metals offers high resistance to spark erosion and the corrosive environment of the combustion chamber. Spark plugs can go 100,000 miles on vehicles these days with these types of spark plugs. Some of the noble metals used on the electrode tips are:
The use of these noble metals allow the use of a smaller center wire, which has sharper edges but will not melt or corrode away. Some of the advantages of fine wire electrodes are:
⇒Less voltage required to fire
⇒More consistent spark
⇒Better air/fuel mixture around electrode for more efficient burn
⇒Less quench effect
⇒Longer durability with the use of noble metals
The smaller electrode absorbs less heat from the spark and initial flame energy. The only drawbacks to noble metal plugs are price because of the metals used, but is outweighed by their long life and may not be available for older vehicle applications.
Conventional Spark Plugs:
Conventional spark plugs differ from noble metal plugs in the spark plug electrode tip. The electrode center tip can be:
⇒Combination of Nickel-iron, copper, or chromium
The side electrode is:
⇒High nickel steel
The drawbacks of conventional spark plugs to noble spark plugs are that they are not as efficient as noble metal spark plugs and have a much shorter life. The real plus of conventional spark plugs is price.
Manufacturers are consistently making refinements to the electrode design to provide a more efficient spark to ignite the air/fuel mixture and provide longer life. Some designs incorporate channels in the center electrode and also on the side electrode. Other designs use notches and bridges across and on the electrodes. The use of noble metals has made the electrodes even more efficient. One design is to incorporate multiple side electrodes of 2 to 4 electrodes. Only one electrode will work at a time, the reason for this is to give the spark more than one route to travel. Electricity is lazy and will travel to the path with least resistance, and with this style of plug it will travel to the electrode with least resistance. As electrodes wear down the spark will go to the next of lower resistance.
Reading Spark Plug:
A spark plug that has been running on an engine can tell what combustion characteristics are happening by the color of the electrode and insulator tip. This is called reading a spark plug and can determined many variables that can be happening during combustion. When spark plugs are removed, keep track of the corresponding cylinder they came out of. This way if a potential problem with a particular cylinder is occurring, it can be identified much easier. The following pictures show some of the characteristics in the combustion chamber.
Spark Plug High Performance Tuning:
There are high performance engine tuning tricks with spark plugs. Some of these are:
⇒Switching over to noble metal material spark plugs that have a thin electrode design or the multiple electrode design. They require less voltage to fire and have better air/fuel flow around electrode.
⇒Finding the correct heat range spark plug. Engines that have been modified tend to have different combustion characteristics than stock, so finding the correct heat range is vital for proper performance. Reading the spark plugs is a way to determine correct heat range. Caution should exercised when trying different heat ranges of spark plugs. Running an engine with a very high heat range spark plug can cause damage to the engine.
⇒Indexing spark plugs is another tuning trick. This requires indexing washers that come in different thicknesses. The idea behind this is to have the open end of the electrodes facing the incoming air/fuel mixture for better combustion. The different size washers allow for this adjustment.
There are some special tools for spark plug servicing and repair:
Spark plug gap tool – This tool sets the air gap between the center electrode and the side electrode (follow manufacturer’s specifications and procedures).
Spark Plug Gap Pliers – These pliers sets the air gap between the center electrode and the side electrode (follow manufacturer’s specifications and procedures).
Spark Plug Boot Pliers – These pliers help remove the ignition wire boot without damaging the boot.
Spark Plug Socket – Spark plug sockets comes in magnetic, clip style or rubber boot to prevent the spark plug from falling out.
Spark Plug Hole Repair Tools – Damaged spark plug holes whether stripped or cross threaded can be repaired by cleaning the threads, or inserting a thread adapter. Many kit variations are available for these types of repairs.
Broken Spark Plug Remover – Removes broken spark plugs without dropping porcelain into cylinder.
Spark Plug Anti-Fouler Insert – These inserts pull the spark plug away from the combustion chamber. They are used in engines that are passing too much oil into the combustion chamber and oil fouling the plugs.
Spark Thread Start Tool – Helps in starting to thread spark plugs into cylinder head without the risk of cross threading. A piece of hose can also be used to install.
Spark plug replacement is an important service maintenance procedure to maintain good vehicle performance, fuel economy and good emissions. Some simple procedures will make this a trouble free procedure. Keep in mind that some vehicles will require special extension and adapters to reach them. Here are some tips on spark plug replacement, but follow manufacturers specifications and procedures.
⇒When removing spark plug wires, use spark plug boot pliers not to damage wires.
⇒Use compressed air to blow out spark hole before removing plug. This will prevent any dirt or debris from falling into cylinder when removing spark plug.
⇒It is a good idea to have two spark plug sockets. One socket to remove and one to install. The removal socket can get dirty and oily, so the install socket will always be clean and not contaminate the new spark plugs. Contaminating the new spark plugs can cause carbon tracking and misfires.
⇒If recommended by manufacturer check spark gap. Some plugs can’t be gaped or the manufacturer ships them pre-gaped. Again refer to manufacturer’s specifications on installation.
⇒Uses a piece of hose inserted into porcelain insulator or use an install tool to help start the threads of the spark plug into the cylinder head. This will help prevent cross threading into cylinder head.
⇒Torque spark plugs to manufacturers specified torque. Improperly torqued spark plugs can cause misfires, make spark plugs run hotter, and can damage spark plug coils.
⇒Use dielectric grease on spark plug boot or coils if recommended by manufacturer.
****UPDATE: In the original article it was stated to use anti-seize on the spark plugs threads prior to installation. Which was the statement below:
⇒Use a small amount of anti-seize on the spark plug thread to prevent galling of the threads of the cylinder head.
Anti-seize should not be used, unless the manufacturer specifies the use of it. Most spark plugs manufacturers and vehicle manufacturers do not recommend the use of anti-seize. Reasons for not using anti-seize:
⇒Anti-seize will reduce conductivity between spark plug and cylinder head, which can result in cylinder misfire and spark plug failure.
⇒If anti-seize gets on the tip of the spark plug the metals in the anti-seize will stick and bond to the electrode tip causing a misfire.
⇒Use of anti-seize will reduce the tightening torque friction, this can lead to over tightening of spark plugs. This can strip out cylinder heads or damage the spark plug.
⇒Most good quality spark plugs are coated or plated with metals that act like anti-seize. This is why manufacturers don’t recommend the use of it.
⇒The bottom line is to follow the vehicle manufacturer’s recommended installation procedures for spark plug replacement.
*Special thanks to Professor John Frala, Alternate Fuels Technology at Rio Hondo College in Whittier CA for your training and input.