How Many Spark Plugs Does a Diesel Have?

Do Diesel engines have spark plugs. This is a question that many people may have wondered, but few have dared to ask. So, if you have ever asked yourself “how many spark plugs does a diesel have?” Then you may want to give this article a read.

The answer to this question is quite simple. No.

spark plugs are one of the most crucial components in an engine. They have the function of providing a spark to ignite the gas in the combustion chamber. Without spark plugs your car would not work.

We now know what spark plugs do. So, it makes perfect sense to assume diesel engines have one, right? Well yes. Although, it’s not the case.

In this article, we will cover all the reasons why diesel engines don’t use spark plugs, as well as having a FAQs section that answers the most commonly asked questions about diesel engines.

Diesel Engines Function in a Different Way

Spark Plugs

To get a better understanding of why diesel engines don’t use spark plugs, it’s essential to know the differences between a diesel and gas engine. 

While both the gas and diesel engine are both engines, that is where the similarities end. However, they do share some similar components.

Arguably the most integral part of an engine is the combustion system, and this is where they both differ from one and other. They work in different ways because the fuel they both use has different properties.

Petrol and gasoline can be ignited in any state. In contrast, diesel must be atomized in order for it to be utilized as fuel. 

Because of this, diesel engines use a compression-ignited injection system and gas engines use a spark-ignited system.

That still leaves us with the question, “well, what can diesel engines use for their ignition system if it’s not spark plugs?”

Instead of having spark plugs, diesel engines use a heating plug. This works because the tip of the glow plug allows the diesel to mix with compressed air. After that, combustion can take place.

What are the main differences?

  • Gas and diesel engines do not have the same combustion system.
  • Gas engines use spark plugs to ignite the fuel, while diesel engines use a heating plug.
  • Gas and petrol have different requirements in order to become utilized as fuel.

Diesel Engines Are Less Explosive

Diesel is much less reactive than petrol. Because of this, diesel does not need high temperatures in order to auto-ignite.

Diesel Engines

This is because diesel will burn when it is sprayed into heated air, whereas gas would immediately ignite. This leads to diesel engines having much higher compression ratios than gas engines.

In short, this allows diesel engines to get more bang for their buck.

Because of the combined factors of diesel being less volatile, burning easily, and having a low ignition temperature, this leads to diesel engines having no need for spark plugs – they would be rendered redundant.

The main takeaway;

  • Diesel is not as volatile as petrol.
  • Diesel has a lower ignition temperature than gas.
  • Diesel burns easily.


In this section, we will be answering the most commonly asked questions about spark plugs and diesel engines.

What are spark plugs made out of and what are their dimensions?

Spark plugs are usually made out of either copper, iridium, or platinum, with copper being the most common. In addition, the outer shell is typically made out of nickel alloy.

Most spark plugs measure between 5/16 inches or 13/16 inches. Although, there are other sizes out there.

How many spark plugs does a diesel have?

None. As discussed in the article, diesel engines do not use spark plugs.

Can spark plugs be used in cars with diesel engines?

Diesel engines are not designed with spark plugs in mind. Because of this, you would not be able to fit any spark plugs into any diesel engine. It is highly unlikely that diesel engines would ever be made to be able to fit spark plugs as it would be inefficient.

Could diesel engines start without a glow plug?

No. Glow plugs are what allows diesel engines to begin the ignition process. Without a glow plug, the fuel simply wouldn’t ignite.

Why are spark plugs unnecessary in diesel cars?

Spark plugs are not essential because of a multitude of reasons.

For one, the air is condensed which raises the temperature significantly. The high temperature will then create the conditions for the diesel to ignite.

In addition, diesel is much less volatile than petrol which makes it burn easily.

How does a glow plug work?

A glow plug performs a similar function to the spark plug. It is responsible for igniting the diesel by allowing it to mix with condensed air so it atomizes. From there, the diesel can then be utilized as fuel.


If you have ever considered asking a co-worker “how many spark plugs does a diesel have?” you are not alone. Everyone assumes that this is something they should know, but in reality, it’s not common knowledge.

To summarise, diesel engines do not use any spark plugs. Instead, they use heat plugs, which are much better suited to diesel engines due to the various reasons we mentioned throughout the article.

Leave a Comment