How Many Spark Plugs Does a Diesel Engine Have?

Four spark plugs go into a 4-cylinder gasoline engine; six spark plugs into a 6-cylinder gasoline engine; and so on. But what about diesel engines?

How many spark plugs does a diesel engine have?

Diesel engines have zero spark plugs. Unlike gasoline engines, diesel engines do not use spark plugs to burn their fuel. Instead, the air temperature in the combustion chamber is raised by compression in these engines, causing the diesel fuel to burn upon introduction rapidly.

But how do diesel engines operate without spark plugs? And what is the spark plug alternative that diesel have?

Why do diesel engines don’t have spark plugs?

Vehicles powered by diesel engines do not use spark plugs. Gasoline burns differently than diesel because it has a different ignition point and is more volatile than diesel.

Source: Testing Autos

Spark plugs in gasoline engines are necessary because of the ease of catching fire. There is an arc of electricity produced by a spark plug. The purpose of this is to start the engine’s combustion process by igniting the gasoline inside the chamber. As a result, the pistons will start moving, and so on.

Ignition of diesel fuel isn’t that simple. Lighted matches in diesel fuel will extinguish themselves as soon as they touch it. In other words, a tiny spark from a spark plug will not be sufficient to start it.

Source: Gido van Tienhoven / Youtube

However, if you atomize diesel fuel, such as when spraying it from a spray bottle, an open flame will cause it to ignite.

Diesel engines have glow plugs

Source: Amazon

In a diesel engine, each cylinder has one glow plug, and the glow plugs are used to start the combustion process. For example, in a 6-cylinder diesel engine, there would be six glow plugs.

Source: Bosch Auto Parts / Youtube

The purpose of the glow plug is to heat up the diesel fuel. This will make it easier to start the engine. Diesel fuel will flow more efficiently with the help of the glow plug if the weather is cold.

Even if the engine’s glow plugs are entirely dead, older diesel engines can still start. However, this can only be done if the weather is good and warm.

But is it possible to start a diesel engine in the cold weather without a glow plug? Yes, but you’ll have to give it a couple of turns before it starts working properly.

How Do Glow Plugs Work in a Diesel Engine?

Heating coils are wrapped in a metal tube to create a glow plug. The tube is then filled with a ceramic powder that acts as an electrical insulator. Glow plugs may reach temperatures of up to 1000°C in just a few seconds when powered by the vehicle’s battery.

Take a look at this video showing the glow plugs in action:

In addition to metal-encased glow plugs, ceramic ones are also available, which are both more costly and more efficient. The glow plug heats up significantly more quickly, thanks to the ceramic. Compared to metal glow plugs, it is also more robust.

The diesel fuel is not the only thing heated by the glow plugs. It also aids in the heating of the entire engine to ensure appropriate operation from the start.

You may become more technical about the workings of a Glow Plug by looking at its three phases:

Source: BoschSource: Bosch Auto Parts / YoutubeSource: Bosch

1- Engine Start:

Glow plugs quickly heat up during this period to aid in starting the engine. When the glow plug is hot, the fuel injector blows atomized diesel fuel across it. After then, the temperature will rise sufficiently enough that the pressured air in the chamber may easily ignite.

2- Continuous Heat:

Maintaining a constant temperature in the glow plugs ensures that the diesel fuel ignites correctly within the combustion chamber. They will also remain powered for a dozen or more engine cycles to check that the engine has started correctly.

3- After Start:

After the engine is started, the glow plugs stay active for a few more minutes to keep the combustion process going. Afterwards, they’ll keep heating the fuel for a little while longer to reduce the likelihood of the engine dying before it reaches its regular operating temperature.


Many people know that diesel engines work differently from gasoline and don’t need any spark plug to operate. But not many know that they actually have an alternative to the spark plug, the glow plug.

Have you ever heard of the glow plug? And do you think it is important for the diesel engine to function properly?

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