How is the spark produced by a spark plug in an internal combustion engine?

Balasore, Orissa

A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.

Spark plugs have an insulated central electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.

Spark plugs don't produce high voltage they just make sparks with high voltage.

An ignition coil is an induction coil in an automobile's ignition system which transforms the battery's 12 volts to the thousands of volts (20 to 30 thousand volts or more) needed to spark the spark plugs. The wire which goes from the ignition coil to the distributor and the wires which go from the distributor to each of the spark plugs are called spark plug wires.

This specific form of the autotransformer, together with the contact breaker and a capacitor, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine.

When the contact breaker closes, it allows a current from the battery to build up in the primary winding of the ignition coil.

Once the current has built up to its full level, the contact breaker opens. Since it has a capacitor connected across it, the primary winding and the capacitor form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and the capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the secondary of the coil.

As the electrons flow from the coil, a voltage difference develops between the central electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes.

Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows electrons to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to 'fire' properly, although it can go up to 45,000 volts.

They supply higher current during the discharge process resulting in a hotter and longer-duration spark.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion.


(Member of American Society of Mechanical Engineers)

Mechanical Engineering

Andhra University


Andhra Pradesh