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Spark plugs
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Principles
The igniting spark
The spark plug plays a vital role in the petrol engine. It is responsible for igniting the air-fuel mixture. The quality of this ignition directly affects several factors which are of great importance for both the quality of the driving experience and the environment. This includes starting, smooth running, general engine performance and efficiency as well as the reduction of harmful emissions.
When one considers that a spark plug must ignite a fresh charge of air and fuel between 500 and 3500 times per minute, it becomes clear how difficult the spark plug’s job is and how important the contribution of spark plug technology is for adherence to current emissions standards and to the reduction of fuel consumption.
Function of a petrol engine
Ambient air is drawn into the cylinder in the petrol engine. Fuel is also supplied by means of the fuel injection or carburettor. A combustible mixture is therefore formed and is then ignited by the ignition spark. Here you can see the function of a current four-stroke engine schematically:
1. Intake: The piston moves downwards creating a low pressure area. Air and fuel are therefore drawn in through the intake valves.
2. Compression: The pistons move upwards again greatly compressing the mixture. Ignition of the gasses takes place as the piston approaches top dead centre.
3. Power: The temperature of the burning gasses in the combustion chamber rapidly increases up to 2600°C and the pressure increases to up to 120 bar. The piston is pushed down towards the crankshaft at speeds of up to 20 metres per second.
4. Exhaust: The burnt gasses leave through the exhaust valve on the next upward movement of the piston in the cylinder.
Installation point of the spark plug
The spark plug is screwed into the cylinder head.
The upper part of the plug, visible from the outside of the engine is the electrical connection point. Depending on the applied technology, either the spark plug cover/connector of the high tension ignition cable is pushed onto this connector or – with more modern technical concepts – it may have its own individual ‘pencil’ ignition coil.
The other end of the spark plug protrudes into the combustion chamber and contains the spark electrodes. During engine operation the high voltage applied to the spark plug creates an electric arc between the electrodes. This arc creates an area of intense heat which ignites the air-fuel mixture.
- Electrical terminal
- Intake valve
- Exhaust valve
- Spark electrodes
- Combustion chamber
- Piston
Ignition sequence
Speed
In older systems a central ignition coil generates the high voltage necessary for the ignition of the air-fuel mixture.
A mechanical ignition distributor ensures that this voltage is sent to the correct cylinder’s spark plug at the right time.
The ignition high voltage is fed through a distributor cap and the ignition HT lead which is connected to the spark plug terminal. From here it reaches the tip of the centre electrode and when sufficient voltage is available to overcome the resistance of the air gap between the centre and ground electrode electrical energy discharges to earth in the form of a spark.
In more modern systems an individual ‘pencil coil’ is often fitted directly to each spark plug. With these fully-electronic ignition systems neither mechanical ignition distributors or high-voltage ignition cables are required.
Construction
Connection terminal
The connection terminal is usually an SAE ‘barrel’ shaped connection or a 4mm thread. The high tension ignition lead or a pencil coil is plugged onto the terminal. This connection allows the high voltage to be transferred to the firing end of the spark plug.
Requirements
The optimal temperature window
In order to maintain efficient operation the firing end of the spark plug must be kept within a certain temperature range.
The lower threshold of this window is a firing end temperature of 450°C, the so-called self-cleaning temperature. Above this temperature threshold, the carbon particles which have collected on the insulator tip are burned off.
Carbon is a by-product of the combustion process and is deposited on surfaces within the combustion chamber and if the operating temperature at the firing end is continuously below 450°C, electrically conductive carbon particles can collect, forming deposits until the ignition voltage flows over the carbon layer to the vehicle earth instead of forming a spark at the gap.
Firing end temperatures above 800°C can heat the electrodes to the point where they start to glow, which can ignite the incoming charge before the spark occurs. This results in uncontrolled ignition leading to abnormal combustion and possible engine damage.
The meaning of the heat rating
The generation of heat varies greatly from engine to engine. For example, a lawn mower will generate less heat than a racing car or a turbocharged engine can run significantly hotter than a naturally aspirated engine.
Therefore a spark plug must be selected that can conduct a precisely defined amount of heat energy to the cylinder head and ensure that the optimal operating temperature range is maintained.
The heat rating of a spark plug provides information about its thermal performance. The following applies for NGK spark plugs: The higher the heat rating number, the higher the thermal performance.
The heat rating number is to be found within the technical part number designation which is marked on the body of NGK’s spark plugs (e.g. BKR6ES).
Heat conduction and heat flow
The vast majority of the heat energy transferred away from the centre electrode takes place via the thread and the sealing gasket. A small amount flows away though the external section of the insulator and up the centre electrode.
The insulator absorbs the heat in the combustion chamber and conducts it into the interior of the spark plug. Where the insulator comes into contact with the metal shell, heat is conducted.
Therefore by increasing or decreasing the contact surface area between the insulator and the metal shell the plug’s ability to dissipate more or less heat to the surrounding cylinder head can be selected.
The insulator surface area in the combustion chamber is smaller for spark plugs with higher thermal performance (red shaded area) and for spark plugs with lower thermal performance it is larger.
Special requirements
Wear
The replacement intervals for spark plugs as recommended by automobile manufacturers can vary between 20,000 and 120,000 kilometres (12,500 to 75,000 miles.)
This represents a challenge because each time a spark occurs it wears away a microscopic amount of electrode material. Through this "spark erosion" the gap grows and changes shape, increasing the ignition voltage requirement by approximately 500 volts per 20,000 kilometres (12,500 miles).
These effects can be counteracted by the use of multiple ground electrodes and/or the use of wear-resistant electrode materials such as platinum and iridium. Iridium alloys are currently the most modern and erosion-resistant variant.
Efficient combustion and emissions
Modern engines must adhere to strict environmental guidelines. Therefore a great challenge lies in improving engine efficiency and the reduction of fuel consumption as well as the reduction of harmful emissions.
Significant reductions in harmful emissions have already been achieved in recent years. In addition carbon dioxide emissions have also been minimised – despite the fact that vehicles have often become heavier.
The reliability of the ignition over the entire service life of the spark plug plays a vital role in the process.
Downsizing
The physical size and cubic capacity of petrol engines is generally becoming smaller whilst the power output is maintained or even increased.
This trend is referred to as "downsizing“ and this strategy offers reductions in both fuel consumption and environmental pollution.
In order to provide the necessary power output such engines are often used in conjunction with some form of pressure charger such as a turbocharger and/or a supercharger (compressor).
In these engines the intake and exhaust valves tend to be larger, as do the engine coolant passageways. This leaves less room available for spark plug installation.
Due to this restriction of space, spark plugs used in these engines typically have a reduced thread diameter of 12mm instead of the more conventional 14mm. The more compact the spark plug, the smaller the wall thickness of the insulator. This presents a challenge for spark plug design as a high dielectric strength must be maintained to cope with the voltage peaks.
- Large spark plug bore
- Insulator ceramic
- Small valve diameter
- Hexagon
- Thread
- Smaller coolant passages
Gas as fuel
Ever more engines are being converted to "bivalent operation". That means: They are being adapted so that they can run on conventional petroleum or a gas (LPG /CNG).
This can have a profound effect on the spark plugs. The combustion of gas is similar to that of normal petrol-fuelling operation however there are some important factors to be considered. The combustion of LPG increases the thermal load on the spark plug increasing firing end temperatures by approx. 80 degrees Celsius or higher.
The air-gas mixture is more difficult to ignite. The ignition voltage requirement is up to 5000 volts higher than with petrol. The entire ignition system, and especially the ignition coils, is placed under considerably increased stress by this increase in voltage requirement.
Spark plug types
Spark plugs vary in shape, diameter, thread length, the number and shape of electrodes as well as the electrode materials which are used. Here you can see some of the more important spark plug types currently in use.
Spark plugs in action
Spark plugs vary in shape, diameter, thread length, the number and shape of electrodes as well as the electrode materials which are used. Here you can see some of the more important spark plug types currently in use.
Installation
The right torque
A torque wrench is required for the correct installation of a spark plug. Even for a professional technician the estimation of the tightening torque is virtually impossible.
This is because torque is calculated by the multiplication of two factors: the force which is applied and the distance at which that force is applied relative to the centre of rotation.
Most spark plug failures can be traced back to an incorrect tightening torque. If it is set too low, there is a risk of compression loss and plug overheating. It is also possible that the insulator or centre electrode may also be damaged as a result of excess vibration.
If the tightening torque is set too high, the spark plug metal shell will be over stressed and possibly yield. The shell can also expand or distort. The distortion causes the heat dissipation zones to be disrupted, overheating and melting of the electrodes or even engine damage can occur.
Illustration of how tightening torque values applied to an M14 spark plug in an aluminum cylinder head affect the spark plug
Overview of torque settings
The torques depend on the cylinder head material and the thread diameter.
Installation video
Take a look over the shoulder of an NGK SPARK PLUG professional.
You will see that by paying attention to all the important steps in the procedure, the installation of spark plugs can be performed quickly and safely.
Diagnosis
The firing end appearance
