How do spark plugs work?

The spark plug is a seemingly simple device, although it is tasked with a couple of different but critical jobs. First and foremost, it creates (literally) an artificial bolt of lightning within the combustion chamber (cylinder head) of the engine. The electrical energy (voltage) it transmits is extremely high in order to create a spark and to “light the fire” within the controlled chaos of the combustion chamber. Here, the voltage at the spark plug can be anywhere from 20,000 to more than 100,000 voltage.

Thermal performance spark plugs

The ability of a spark plug to dissipate heat from the combustion chamber is defined by the spark plug “heat range.” The temperature of the firing end of the spark plug must be maintained at a level high enough to prevent fouling, but low enough to prevent pre-ignition. Spark plug manufacturers refer to this as “thermal performance.”

Cold spark plugs versus hot spark plugs

“Cold” spark plugs normally have a short heat flow path. This results in a very quick rate of heat transfer. Additionally, the short insulator nose found on cold spark plugs has a small surface area, which does not allow for a massive amount of heat absorption.
On the other hand, “hot” spark plugs feature a longer insulator nose as well as a longer heat transfer path. This results in a much slower rate of heat transfer to the surrounding cylinder head (and consequently, the water jacket).

Spark plug construction

Ribs: 
Insulator ribs provide added protection against secondary voltage or spark flashover and also help to improve the grip of the rubber spark plug boot against the plug body.
The insulator body is molded from aluminum oxide ceramic. In order to manufacture this part of the spark plug, a high-pressure, dry molding system is utilized. After the insulator is molded, it is kiln-fired to a temperature that exceeds the melting point of steel. This process results in a component that features exceptional dielectric strength, high thermal conductivity and excellent resistance to shock.

Insulator: 

The insulator body is molded from aluminum oxide ceramic. In order to manufacture this part of the spark plug, a high-pressure, dry molding system is utilized. After the insulator is molded, it is kiln-fired to a temperature that exceeds the melting point of steel. This process results in a component that features exceptional dielectric strength, high thermal conductivity and excellent resistance to shock.

Hex: 
The hexagon provides the contact point for a socket wrench. The hex size is basically uniform in the industry and is generally related to the spark plug thread size.

Shell: 
The steel shell is fabricated to exact tolerances using a special cold extrusion process. Certain types of spark plugs make use of a steel billet (bar stock) for shell construction.

Plating: 
The shell is almost always plated. This enhances durability and provides for rust and corrosion resistance. The steel shell is fabricated to exact tolerances using a special cold extrusion process or, in other specialized cases, machined from steel billet. The hexagon machined onto the shell allows you to use a socket wrench to install or remove the plug.

Gasket: 
Certain spark plugs use gaskets while other examples are “gasketless.” The gasket used on spark plugs is a folded steel design that provides a smooth surface for sealing purposes. Gasketless spark plugs use a tapered seat shell that seals via a close tolerance incorporated into the spark plug.

Threads: 
Spark plug threads are normally rolled, not cut. This meets the specifications set forward by the SAE along with the International Standards Association.

Ground electrode: There are a number of different ground electrode shapes and configurations, but for the most part, they are manufactured from nickel alloy steel. The ground electrode must be resistant to both spark erosion and chemical erosion, both under massive temperature extremes.

Center electrode: 
Center electrodes must be manufactured from a special alloy that is resistant to both spark erosion and chemical corrosion. Keep in mind that combustion chamber temperatures vary (and sometimes radically). The center electrode must live under these parameters.

Spark park electrode gap
The area between the ground electrode and the center electrode is called the gap. Center electrodes must be manufactured from a special alloy that is resistant to both spark erosion and chemical corrosion.

Insulator nose: 
There are a large number of insulator nose shapes and sizes available, but in essence, the insulator nose must be capable of shedding carbon, oil and fuel deposits at low speeds. At higher engine speeds, the insulator nose is generally cooled so that temperatures and electrode corrosion are reduced.

Post a comment

0 Comments