Spark Plugs 101

You probably know what sparkplugs are. If not, here’s quick definition: “spark plugs are devices for delivering electric current from an ignition system to the cylinders of a spark-ignition engine.” Much clearer now, eh?

OK, seriously, if you have a gasoline-powered car, lawnmower, chainsaw or anything else with an internal combustion engine, chances are that it will have one or more sparkplugs in it. They are usually easy to spot because they are white porcelain rods attached to a long, black ignition wires. See the image below. Nothing else on an engine quite matches that description.

Spark plugs have been around for over 100 years and still perform the same function that they originally were designed for. In fact, a sparkplug from the early 1900s looks almost the same as the one they make today. In this article, we will take a deeper look at these devices.

Spark plug function

Spark plugs perform a relatively simple function: to fire the air/fuel mixture in an engine. Here are the details: A spark plug is connected to the high voltage electricity generated by an ignition coil. As current flows from the coil and down into the spark plug, a voltage develops between the central and side electrodes on the tip of the spark plug. Initially no current will flow but as the voltage rises further it begins to change the structure of the gases in the gap. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized and electricity jumps across the gap. This “spark” then ignites the air fuel mixture in the cylinder which drives a piston down.

We learned something interesting from our subject matter experts, Don Vance Chrysler of Marshfield, a local Dodge, Chrysler Ram, Jeep dealer in Marshfield, MO. First, in the old days spark plugs required voltages of 12,000–15,000 volts or so. Today they put out 25,000 – 30,000 Volts. This is so a powerful spark is developed that jumps across the electrode gaps. Now here’s the interesting part: as the current of electrons jumps the gap, it raises the temperature of the spark to 60,000 K. This is 6 times the surface temperature of the sun! This intense heat in the spark channel causes the ionized gas to expand very quickly which quickly ignites the air/fuel mixture in the cylinder.

Spark plug construction

A spark plug has a metal-threaded jacket which is electrically isolated from a central electrode by a porcelain insulator. The diagram below shows a cross-section of an older spark plug. The metal jacket is designed to withstand the torque of tightening the plug and also serves to remove heat from the plug assembly. Plus, it is the ground for the sparks passing through the central electrode to the side electrode.

The metal shell is screwed into the engine’s cylinder head. A central electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and the inner end of the grounded threaded shell.

The top of the spark plug contains a terminal to connect to an ignition wire. The exact terminal construction varies depending on the use of the spark plug. Most standard spark plug wires snap onto the terminal of the plug. The white section of a spark plug is an insulator typically made from sintered alumina, a very hard ceramic material. Its major function is to provide mechanical support and electrical insulation for the central electrode.

Because spark plugs thread into the cylinder head of an engine, seals are required to ensure there is no leakage from the combustion chamber. The external seal is usually a crush washer, but some manufacturers use the cheaper method of a taper interface and simple compression to attempt sealing.

The old days

At one time, it was common to remove the spark plugs from a car engine, clean any deposits off the ends, and file electrodes to restore the sharp edges. This was one of the maintenance items performed during a “tune-up.” And when the plugs accumulated some 30,000 miles of use or so, they were replaced.

The development of noble metal high temperature electrodes (such as yttrium, iridium, tungsten, or palladium) has eliminated the changing of spark plugs every 30,000 miles. This is because the center wires are smaller, and have sharper edges that will not melt or corrode away. The result is that today’s spark plugs last up to 100,000 miles before replacement.

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