Spark plugs are used to ignite fuel in gasoline/petrol engines; spark plugs are not required in diesel engines. Internal combustion engines can be split into two ignition categories, these are ‘spark ignition’ (gasoline/petrol engines) and ‘compression ignition’ (diesel engines).
Spark plugs are installed within the cylinder head and are attached by screwing the spark plug into the head. Careful attention must be paid when tightening the spark plug to the head otherwise the thread may become stripped and tightening will no longer be possible.
Spark Plug Cross Section
High voltage (20-40,000 Volts) is applied to the terminal connection at the top of the spark plug in order to raise the electrical potential of the centre electrode. Electrical current flows through the centre electrode (copper conductor usually) and through a resistor. The high voltage present in the centre electrode causes the gasses within the the electrode gap to become ionised. When the gasses are ionised they become a much better conductor and the electrons are then able to ‘jump’ the electrode gap in order to reach the ground electrode. The ‘jump’ can be seen as a spark and this is where the spark plug obtains its name; the proper technical name for this jump is ‘arc’.
The arc between the centre and ground electrode contains enough energy and heat to ignite the air/fuel mixture within the combustion space. Igniting of the fuel occurs at approximately 5 degrees before top dead centre (TDC) and signifies the start of the power stroke.
Not all spark plugs contain a resistor, but almost all automobile spark plugs do. The resistor reduces the amount of electrical noise created by the spark plug. Electrical noise is also referred to as electromagnetic interference (EMI) or radio frequency interference (RFI). Because the spark plug is effectively a conductor, it creates its own magnetic field and so does the electrical arc created by the spark plug. These electromagnetic fields can interrupt other nearby electronic circuits and cause adverse effects. In order to reduce the likelihood of EMI on items such as the engine control unit (ECU), car radio and other electronic circuits, a resistor is installed.
A spark plug’s heat range defines the spark plug’s thermal conductivity properties. A heat range scale of 1-10 is usually used, but the scale depends upon the manufacturer i.e. sometimes it can be 1-11. Spark plugs are graded anywhere within the spark plug heat range e.g. a heat range value of 4.
Spark plugs are often referred to as either hot or cold. A cold spark plug is one that rejects heat quickly and heats-up slowly. A hot spark plug is one that rejects heat slowly and heats-up quickly. Hot spark plugs have a long thin insulator nose whereas a cold spark plug has a short and thick insulator nose.
Hot and Cold Spark Plugs
Small low performance engines e.g. lawn mowers, hedge trimmers etc. use hot spark plugs because the operating temperatures and pressures within the combustion space are relatively low. High performance engines e.g. racing cars, use cold spark plugs because the operating temperatures and pressures within the combustion space are relatively high.
Low performance engine spark plugs may have a heat range rating between 1-4. High performance engines typically have a heat range rating between 8-10.
The reach of a spark plug is roughly equal to the length of its threaded section, this is measured from where the spark plug body rests on the cylinder head until the bottom end of the thread. It is important that the correct reach spark plug is always used. Spark plugs with too long a reach risk impacting upon the piston crown as the piston approaches TDC. Installing a spark plug with too short or too long a reach risks carbon deposit build-ups on the threads as well as incorrect firing.
Spark Plug May Impact Piston Crown If Reach Is Too Long
3D Model Components
This 3D model shows all major components associated with a typical spark plug, these include:
- Terminal Connection
- Centre Electrode
- Ground Electrode
- Electrode Gap
- Creepage Current Barrier
- Hex Nut
Hot and cold spark plug image attribution:
By User:Emmanuel.boutet (File:Dilatation-spark plug-bougie allumage-fr.svg) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons