What Is a Glow Plug?

Even if you’re not a DIY-er concerning your car, it’s a good idea to have some basic knowledge of your car’s more important parts. Without knowing the difference between parts like a glow plug and a spark plug, it’s hard to have a good understanding of whether your car is suffering a major or a minor repair, what the cost of maintenance and repairs might be, and whether your mechanic is charging you an appropriate amount or too much.

What exactly is a glow plug?

Mechanic, potentially checking glow plugs in a truck.
Car being worked on | Getty Images

If the mention of a glow plug has you scratching your head, you’re not alone. The term spark plug is far more common to car owners.

So, what is a glow plug? You’d be wrong if you think it might be a tricked-out, neon version of a spark plug. As per Champion Auto Parts, a glow plug is a heating device specifically designed to help diesel engines start in cold weather.

In the winter, drivers might encounter difficulty starting their diesel vehicles because the engine cylinder block and head absorb the heat of the ignition process. But glow plugs, which can be found in pre-chambers in pre-chambered engines and inside the combustion chamber in direct injected engines, ignite the fuel when the engine is too cold.

Glow plugs are necessary because diesel engines don’t rely on spark plugs to ignite the diesel fuel. Instead, they rely on compression to elevate the temperature high enough to induce spontaneous combustion. But when temperatures drop precipitously, it’s hard to get the temperature high enough. That’s where glow plugs come in.

What do glow plugs look like?

Unlike their name might suggest, glow plugs aren’t beautiful pieces of machinery. They look rather dull. They’re shaped like a pencil and made of metal. If you happen upon one at an auto parts store, you’ll see that its cylindrical shape varies in thickness up and down the length of the glow plug.

One end of the glow plug has a heating filament on its tip that will light up when activated. The filament is typically made of a material that can handle high temperatures and resist oxidation. Typically the filament is composed of platinum or iridium. When the glow plug is inactive, it’s typically a dull silver or dark gray.

When they’re activated, you’ll see a yellow coil-like indicator appear on your instrument panel. And despite being fairly pedestrian in appearance themselves, glow plugs serve a critical purpose. Without them, starting your full-size diesel truck, souped-up SUV, or sedan would be impossible when the seasons change.

What is the difference?

So what’s the difference between a glow plug and a spark plug? Well, first, you need to know what a spark plug is. If you don’t know, again, you’re not alone. A spark plug fires off the spark of electricity your gas-powered car needs to start, igniting the air and fuel that gets your car moving.

When you turn the key in your ignition, voltage is generated by a magneto or ignition coil in the spark plug. As the voltage rises, the fuel and air become ionized, allowing current to flow. As the current flows, the gas expands very quickly, resulting in the combustion that fires up your engine.

Hence, the key difference between a spark plug and a glow plug is that the former is required to fire up gas-powered engines every time you start a car. In contrast, glow plugs facilitate ignition when outside temperatures might make starting a diesel engine difficult.

Spark plugs look a bit different too. They resemble fasteners, like nuts, more so than pencils, and are typically composed of a terminal, insulator, metal case, and central electrode. There are several common designs in use, which vary depending on the type of engine and vehicle size. But no matter which one is in your vehicle, if your spark plug is shot, good luck trying to start your car.

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