Spark Plug


Spark plugs are used to ignite fuel in gasoline/petrol internal combustion (IC) 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 in both two stroke and four stroke engines.

Spark Plug

Spark Plug


Spark Plug Components

Spark plug designs vary, but the main components of a spark plug do not.

Spark Plug With Labelled Parts

Spark Plug With Labelled Parts

The main components of a spark plug include:

  • Terminal Connection - where electrical current is supplied to the spark plug.
  • Centre Electrode - a straight conductor passing through the centre of the spark plug.
  • Ground Electrode - a curved shaped conductor that connects the spark plug to ground.
  • Resistor - used to reduce electrical noise.
  • Electrode Gap (spark gap) - the space between the ground and central electrode; this is where the spark (electrical arc) occurs.

Electrode Gap

Electrode Gap

  • Creepage Current Barrier - prevents current leakage from the conductors to ground.
  • Seat - where the spark plug presses against the cylinder head.
  • Hex Nut - used to install and remove the spark plug.


How do spark plugs work?

The below video is an extract from our Internal Combustion Engine Basics Online Video Course.


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 center electrode. Electrical current flows through the center electrode (copper conductor usually) and through a resistor. The high voltage present in the center electrode causes the gasses within the electrode gap to become ionized. When the gasses are ionized 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 center 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 center (TDC) and signifies the start of the power stroke.


What is the purpose of the resistor in a spark plug?

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.


What is a spark plugs 'heat range'?

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

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.


What is a spark plug's 'reach'?

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

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

Spark Plug Cross Section


Additional Resources


Image Attribution

Hot and cold spark plug image attribution: By User:Emmanuel.boutet (File:Dilatation-spark plug-bougie allumage-fr.svg) [GFDL (, CC BY-SA 3.0 ( or CC BY-SA 2.5 (], via Wikimedia Commons