How Does A Glow Plug Work?

April 23, 2022

A set of glow plugs for diesel engine.

Have you ever tried to dismantle a diesel engine? If you had, you might have seen a glow plug up close. Glow plug, of course, is less popular than the sparkplug. In fact, many would mistake it for the sparkplug. Yet, it is different from the sparkplug, for you will only find it in diesel engines. Yet, the glow plug has almost the same function as the sparkplug, helping the engine start. 

To understand why diesel engine has glow plugs, it will be helpful to understand the diesel fuel. Diesel, as a fuel, has a higher boiling point than gasoline. It also has a different hydrocarbon compound mixture than gasoline. Hence, diesel requires a different igniting mechanism to create internal combustion.

High-speed diesel engines, for example, will have a hard time starting during cold weather compared to gas engines because cold weather negatively influences the diesel engine’s ignition process. Moreover, the diesel engine’s cylinder block and cylinder head readily absorb the compression heat. They are heat hogs, absorbing more heat. For this reason, designers of diesel engines positioned glow plugs inside the combustion chamber to draw more heat to the engine. 

How Do Diesel Engines and Glow Plug Work?

The diesel engine is a perfect example of an internal combustion engine, in which the ignition happens inside the engine. This ignition is caused by high air temperature within the cylinder due to mechanical compression. The diesel engine also needs oxygen, fuel, and an ignition point to complete this internal combustion. 

The ignition point in a diesel engine is the glow plug, while the ignition point in the gasoline engine is the sparkplug. A 1.5V battery in the glow ignitor makes possible this ignition in the diesel engine. However, some more recent cars come with onboard batteries that engender the ignition of the glow plugs. 

These onboard batteries are part of the electric starting system of these new cars. Hence, there’s no more need for power in these cars to keep the glow plug hot to provide continuous ignition to the diesel engine.

Additionally, the glow plug is usually wrought in various materials, alloyed together for additional strength to handle vibration and heat. It has a specific element—platinum—which catalyzes ignition. This catalytic reaction involving platinum engenders the ignition of methanol of the diesel fuel.

Applications and Uses of Glow Plugs

How the glow plugs operate depends on the type of diesel engine you have and the ambient temperature. Hence, if the ambient temperature is already hot, more often, there is no longer any need for glow plugs. But to give you a good idea of how glow plugs work, you can check out the following steps on how glow plugs operate:

1) Preheating

If you’re using an older generation diesel engine, you will need to activate the glow plugs for a specific time. You don’t set the key to start position and expect the engine to start quickly. Moreover, early diesel engines featured a Thermostart-like glow plug in the inlet manifold. These glow plugs usually require 20 seconds to get to a working temperature. So, you need to time the twenty seconds manually. 

Over time, the glow plugs transformed into in-cylinder glow plugs. These in-cylinder glow plugs come with a warning light on the dash that indicates how long the preheating process should be. This preheating phase also automatically activates when you turn the key on for a certain period. 

Besides, the glow plug relay turns on the glow plugs, and you’ll see the light illuminates on the instrument cluster, and this operation is called glowing or preheating. Most modern diesel cars also come with automatic activation of the glow plugs when you unlock the vehicle or open its door.

2) Start‌ing

If your diesel engine has in-cylinder glow plugs, there is a pre-set time, and when it elapses, the glow plug relay turns off the wait-to-start light. The preheating cycle might last from two to five seconds. Then, you can turn to the “start” position the key while the relay turns off the glow plug once the engine is running. 

Some diesel cars, however, to comply with emission regulations, have glow plugs that you can operate quickly after starting the engine or when the engine is in extended idling when engine temperature decreases a bit. The Thermostart plug is in the inlet manifold. 

After several seconds, diesel vapors will start to imbue the inlet manifold. Once the glow plug goes on heating up, it opens a particular valve that lets diesel from a reservoir above the Thermostart. The onrushing diesel goes into the Thermostart. The plug vaporizes this new diesel and fills the inlet manifold likewise. 

The air gets infused into the chamber within twenty seconds, and the diesel adjacent to the plug ignites. The engine cranks, and the burning diesel move into the combustion chambers, adding more diesel. The additional diesel ignites, making the engine start with ease. 

Cars with Thermostart glow plugs don’t activate more often through the standard ignition switch. You’ll find that they have a button to start the motor. If the Thermostart plug gets activated by the switch as the ignition, you can activate the Thermostart plug by turning its switch a notch counterclockwise. 

Once the twenty-second has elapsed, you can achieve cranking if you turn the ignition switch further counterclockwise. The ignition switch is released once the engine has been fired up and running, springing back to its OFF setting. Then, you can turn the ignition switch back to the ON position.

3) Warm Engine Start

If your car had just run recently or if the temperature around is hot, you might not see the wait-to-start light. In such a case, you may go on turning the key to its start position. Then, start the engine sans waiting for the light to come on.

If your engine has a Thermostart, you can activate the glow plug or not. If an ignition key controls the Thermostart, you can turn the key two notches clockwise to start unheated cranking.

Frequently Asked Questions

Aside from knowing how the glow plugs work, it will also help to be familiar with the FAQs about glow plugs, for they might also be the questions playing on in your mind:

How Are the Glow Plugs of Model Engines Used?

Glow plugs in full-sized diesel engines work differently from those of the model engines. The glow plug works for starting the engine only with the full-sized engines. However, glow plugs on model engines get integrated with the ignition system because of the platinum’s wire catalytic effect on the methanol of the fuel. Moreover, the glow plugs of the model engine get also utilized in theatrical pyrotechnics as re-usable igniters. They also get used in the special effects industry.

If the Element is Hot, What Will Determine the Ignition Point?

The catalytic reaction between platinum and methanol usually depends on two factors: pressure and heat. So, it depends more on the temperature of the element. The thing is, the hotter the element, the faster it will ignite. On the other hand, the higher the pressure within the combustion chamber, the quicker will the fuel ignite likewise. 

What is the Temperature Range of Glow Plugs?

Glow plugs get controlled by various heat range plugs, and there are varying heat ranges. But most heat ranges fall under these heat ranges: cold, medium, and hot. If you’re a bit confused as to which heat range to use, you can consult the manufacturer of your engine to know the recommended heat range for your engine. If you use, for example, a hotter than what is recommended, you will inadvertently raise the ignition point. If you use a colder one, you will likewise slow down the ignition point.


If it’s your first time tinkering with your diesel engine, you might get confused with the seemingly mysterious function of the glow plugs. Yet, there is no mystery shrouding its operations. It is also not hard to understand the science behind the working of the glow plugs. 

If ever you would replace your diesel engine’s glow plugs, you only need to check the manufacturer’s manual and see what the manual recommends. Once you’ve figured it out, you can then appropriately adjust the temperature of the glow plug and the nitro-content as needed.