Spark plugs are required in some engines in order to ignite the fuel mixture within an engine cylinder prior to combustion. There are two main types of engine, these are ‘compression ignition engines’ and ‘spark ignition engines’.
Compression ignition engines: The piston travels up the cylinder compressing the injected fuel and air mixture (compression stroke), the temperature rises as the mixture is compressed and this causes the fuel to ignite. The rapid expansion due to combustion is called the ‘power’ or ‘combustion’ stroke (see our engine introduction page). Diesel fired engines are an example of compression ignition engines.
Spark ignition engines – The piston travels up the cylinder compressing the fuel and air mixture; the temperature rises due to compression. However, the temperature may not be sufficient to cause the mixture to ignite, or, may cause ignition at the incorrect time. In order to avoid this, spark plugs are used to ignite the combustible mixture. Petrol fired engines are an example of spark ignition engines.
Spark Plug Operational Theory
Electrical current is generated by an engine ignition coil and is conducted by a terminal located on the top of the spark plug. Current flows through a centre electrode (conductor) in the centre of the spark plug until it reaches the base of the centre electrode from where it has no path to ground. By increasing the voltage to typically between 10,000 to 50,000 volts, the air/fuel mixture between the centre electrode tip and the ground electrode becomes ionised. This ionisation allows current to flow from the centre electrode to the ground electrode. Essentially, the high voltage allows the electricity to ‘jump’ the gap between the centre electrode tip and the ground electrode. The heat created by this electrical arc ‘jump’ is enough to ignite the fuel/air mixture. The gap between the tip of the centre electrode and the ground electrode is called the ‘electrode gap’.
Spark Plug Construction (see our Spark Plug Cross Section Model)
Tapered Ground Electrode
The tapered shape improves ignition performance by reducing the quenching effect. This electrode forms the negative part of the circuit.
The centre electrode is usually manufactured from iridium or nickel based alloy as this can withstand the high temperatures created by the ignition spark and combustion. The centre electrode forms the positive part of the circuit. The centre electrode usually has a copper core to carry electrical current and improve thermal efficiency.
The insulator nose insulates the centre electrode from the housing.
The screw thread allows the spark plug to be easily installed and removed. It also allows a defined torque (tightness) to be obtained when tightening the spark plug to the cylinder head.
The gasket ensures the combustion chamber is sealed from outside ambient air. It is important the gasket is tightened to the correct toque in order to prevent blow-by (leakage from the combustion chamber).
The shield protects internal spark plug components from foreign object damage.
The insulator prevents current flow from the centre electrode to the shield (leakage current).
A hexagonal nut used to join the upper and lower parts of the spark plug.
The insulator is used to prevent current flow from the centre electrode to the housing. The ribbed external shape increases the creepage path from the terminal to the housing.
Current flows to the terminal, through the centre shaft and then to the centre electrode. Typical voltage ranges are between 10,000 to 50,000 volts.