Heat engines

What is an engine spark plug? Operation, calibration and types

What is an engine spark plug? Operation, calibration and types

A spark plug is an electrical device located in the cylinder head of some internal combustion engines. The function of the spark plug is to ignite the compressed fuel-air mixture by means of an arc of electrical current that generates a spark.

Basically they are electrical conductors that receive a difference in electrical potential from the ignition coil to introduce it into the engine cylinder.

In addition to starting fuel combustion, they also double as a heat sink.

Spark plugs have an insulated center electrode that is connected by a tightly insulated wire to an external ignition coil circuit, forming, with a grounded terminal at the base of the plug, a spark gap inside the cylinder.

How does a spark plug work?

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

Once the voltage exceeds the dielectric strength of the gases, the gases ionize. The ionized gas becomes a conductor and allows electrons to flow through space. This flow of current generates an electrical spark igniting the fuel mixture.

Spark plugs generally require a voltage greater than 20,000 volts to properly 'fire'.

Because the spark plug is inside the engine and is the only easily removable part, it can be used as an indicator of the engine's tuning and condition. The spark plug also tells if you need to change something in the mechanics.

What is the heat rating of a spark plug?

The thermal degree of a spark plug determines the ability to dissipate heat energy generated during fuel combustion. The efficiency of the spark plug cooling system depends on the insulating material that covers the center electrode (usually porcelain).

The heat dissipation capacity is dependent on the spark plug operating temperature. This ability to dissipate heat increases inversely with temperature. A hot plug dissipates less energy and is a good choice for low horsepower engines. For engines that work at high speeds, a cold plug that has a high thermal degree is better.

Spark plug calibration

To ensure proper engine operation, the spark plugs must be properly calibrated. A good calibration reduces fuel consumption and optimizes the expulsion of gases.

One way to improve its performance is to file and clean the electrodes and recalibrate the spark plug before replacing it.

The potential difference between the electrodes and the electrical spark it generates depends on the spark plug setting.

The distance between the electrodes is determined by the manufacturer and is expressed in millimeters. If this distance is not respected, the electric current will not pass correctly.

Spark plug types

Spark plugs can be classified in many ways depending on different parameters such as resistance, current arc size or type of material.

The different types are:

  • Naked resistance in which resistance is in sight.

  • Protected resistance in which the electrical resistance is covered with a cover to better transmit heat.

  • Antiparasitic resistance is like the previous one but it has a resistance to eliminate interference with the rest of the vehicle's electrical system.

  • Normal opening. The arc of current jumps up to 0.9 millimeters.

  • Large opening. In them the arc of current can be up to 2.05 millimeters. These types of spark plugs need coils with a high voltage in the ignition system to produce the spark.

  • Platinum or iridium. They are the strongest and have the lowest electrical conductivity. To compensate, they have a central electrode with a tip that facilitates the jump of current.

  • Coppermade; they are the most common.

  • For gasoline engines. They have the same characteristics as the naked ones; they are the most common and known.

  • For diesel engines. They are located in the cylinder head to send the heat directly to the combustion chamber. During the ignition of diesel it can reach 1000 degrees Celsius.

  • For gas engines. They are used in Compressed Natural Gas (CNG) and Liquefied Petroleum Gas (LPG) engines. The main drawback of this type of spark plug is that the electrodes suffer high wear due to oxidation. To compensate for this problem, they are used to covering with nickel. The ignition voltage is higher to facilitate the combustion of the fuel.

Publication Date: December 1, 2017
Last Revision: November 11, 2021