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What Is a Glow Plug?

By Mary Lougee
Updated May 23, 2024
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A glow plug, sometimes also written “glowplug,” is a device used in car and some truck engines that helps with fuel combustion and engine starting. It’s used almost exclusively in engines that rely on diesel fuel. Older passenger cars relied on them almost exclusively to get started, but modern cars and trucks using diesel fuel need them as well, and they’re also common in many remote controlled engines in hobby cars, trucks, boats, and even airplanes. The plugs essentially work as mini-heaters that warm up the engine chamber enough to facilitate combustion of fuel, which is necessary for the engine to get started and ultimately power the vehicle. How it generates heat can be somewhat complicated, but put very simply it is built with a heating element with an electrical resistance component that responds to pressures in the ignition chamber. They’re similar to spark plugs, and only occur in cars that don’t use spark-based models.

Core Concept and Functionality

Diesel fuel, like regular octane fuel, requires some sort of heated combustion in order to become useful as a powering agent for cars and other vehicles. Most of the time, this combustion happens in an engine, where different chambers control and channel the reaction into useable energy. The glow plug is an important part of this process in many vehicles, and is essential to getting the engine started in these cases.

When the plug is "on," it warms the engine, much like the heating elements warm up the space inside a toaster. Once the engine block is sufficiently warm, the vehicle can be started. This process is the same on both diesel vehicles and remote control cars and only takes approximately two to three seconds.

Older model vehicles normally needed a multi-step process in order to start up. First, the operator needed to turn the key to the "on" position. This would illuminate an indicator on the dashboard showing that the plug was lit. When the dashboard indicator light turned off, this meant that the engine was ready to start. The operator would turn the ignition key to the "run" position. If the vehicle's engine was warm from recent use, the plug wasn't needed and the operator could immediately start the engine. The process has been streamlined in most modern vehicles, but the functionality of the plug has remained about the same.

Basic Identification

Identifying a glow plug is pretty simple. It normally looks a lot like a spark plug, so people familiar with standard octane engines will often notice it right away. The plug has a pencil-like shape and is made from a high heat-resistant metal, like platinum or iridium. On its end is a heating element with a temperature sensor.

Uses in Diesel Automobiles

Diesel fuel often seems a lot like octane fuel to the untrained observer. They aren’t normally interchangeable, though, and the engines designed to work with each sort of fuel are usually set up really differently. For one thing, they rely on compression to bring about combustion, and they their chambers are designed with air temperature and pressure as key goals. The plugs can be either in the pre-combustion chamber, where fumes accumulate, or in the actual combustion chambers; a lot of this depends on the style of the engine and the volume of fuel being injected at any given time.

Remote Control Engines

Remote control cars, trucks, boats and airplanes frequently also use these sorts of plugs to start their engines. These sorts of machines aren’t always driven by true diesel engines, as many are nitro methanol-powered or gasoline-powered. They work in many of the same ways, though, and, importantly, they don’t normally have spark plugs.

Smaller remote-controlled engines hold less heat and require a hotter plug for ignition. The reverse is true for larger remote-controlled cars — these often hold more heat and do not require as hot of a glow plug to operate. The higher concentrated fuels also require hotter plugs so that the engine can combust correctly and burn the excess fuel off as it operates.

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Discussion Comments

By Reminiscence — On Mar 07, 2014

When I was in college, I wanted to buy an inexpensive car so I could drive to a job or shop in another town. I found a used car dealership that offered cheap transportation cars for college students. As it turned out, the dealer said he sold the last $400 car on the lot two hours before I arrived, but for a little bit more, he could help me finance a better car.

He pointed me in the direction of a horrible green sedan in the back of the lot. This was at a time when diesel cars were rare in the United States, so the only place to buy fuel was at truck stops out of town. He asked me if I knew anything about diesel engines, and I said no. He explained that the car had a glow plug that ignited the diesel fuel, almost like a cigarette lighter heating the end of a cigarette. I almost bought the car, but didn't like the idea of looking everywhere for diesel fuel. In retrospect, I'm glad I didn't buy that car. Now I would consider buying a diesel car, simply because most gas stations sell diesel fuel.

By Phaedrus — On Mar 06, 2014

I guess this is why diesel-powered trucks will idle for hours at truck stops and parking lots. It would be easier to keep the engine running rather than allow the glow plug to cool down completely.

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