Diesel engines can be highly effective, and due to this, many people opt for them. However, with this effectiveness, diesel engines have some problems as well, and the most common issue is a misfire. This issue can start due to weak fuel efficiency and also rough idling, and the only way to fix it is to change the glow plugs. Now you may be wondering what glow plugs are? Are they the same as spark plugs? Are both of them present in diesel engines? How many are there? etc. So if such questions are clouding up your mind, then worry no more; simply read the article below and learn everything there is about diesel engines, spark plugs and glow plugs. Read related article:  Best Spark Plug Gap Tool

Do Diesel Trucks Have Spark Plugs?

To find out if spark plugs in a diesel engine exist, you need to understand what these spark plugs are. A spark plug is simply a car component that relays current through the ignition. This current then goes towards the chamber of combustion present and causes a spark mechanism. This allows the compressed gas or fuel mixture to spark up, which begins the fuel combustion and leads to your vehicle moving. Due to this function, spark plugs are present in gasoline engines but are useless for diesel engines. So the question arises, why are diesel spark plugs not present and how are they useless? Read on below to find out.

Why Do Diesel Engines Lack Spark Plugs?

To figure out why diesel engines lack spark plugs, you need to understand how the engine works. A diesel engine is similar to a gasoline engine. They both have a combustion chamber present in them. However, just like their name suggests, diesel engines run on diesel, and gasoline engines run on gasoline. Both of these engines use different fuels, which differentiates them. Furthermore, diesel engines have a compression-ignited injection system, whereas gasoline has a spark-ignited system. Due to this compression injecting system, diesel is inserted into the engine, and this leads to ignition when there is an increase in temperature. This temp gets high via gas compression by the engine’s piston. With the difference in the combustion process of gasoline and diesel, diesel engines do not use spark plugs and instead rely on glow plugs instead. However, the question remains, why spark plugs in a diesel engine are not used; well, here is why:
  • Low Volatility

Diesel engines are less volatile as compared to gasoline engine, they also have a high compression ratio due to the air and fuel infusion in diesel engines. However, gasoline engines tend to ignite automatically and so they need spark plugs. But since diesel is less volatile with low self-ignition temperature, they do not need spark plugs. Furthermore, this high compression ratio allows the diesel to be sprinkled as droplets over hot air. Due to the low volatility, it does not need a high temperature to auto-ignite, rendering these plugs useless.
  • Auto-Ignition Temperature

Auto-ignition temp refers to the temperature when fuel gets hot and heats up. Now gasoline engines do not ignite easily whereas diesel engines can. The reason behind the easy ignition of diesel engines is the low auto-ignition temperature. So the temperature that is produced by compressing air is perfect for igniting diesel and it doesn’t need any spark plugs to help with this process.

Do Glow Plugs And Spark Plugs Have The Same Working Mechanism?

If you want to learn about the working mechanism of these two plugs and whether it is similar, then you need to first know that spark plugs cannot fit into a diesel engine where glow plugs can. Spark plugs are not important for diesel but they are a very crucial part of the gasoline engine and its combustion process. Spark plugs have the job of igniting the fuel in this engine and is solely responsible for it.  On the other hand, glow plugs are different from spark plugs in their working mechanism but they are also responsible for combustion in diesel engines. 

How Do Glow Plugs Work in a Diesel Engine?

As mentioned above, diesel engines cannot use a spark plug but they do have glow plugs instead. So the question arises, how do these plugs work and what are they?  Well, a glow plug is simply a device that is heated electrically and can be found in the cylinder of a diesel engine. The tip of these glow ups is integrated with a heating element and when the plug gets heated up, the tip tends to emit light and this is how they get their name, glow plugs.  When diesel is sprinkled onto the tip of the glow plug, it starts to evaporate since the tip is very hot and this starts the process of combustion in the engine. In order to understand the step by step working mechanism of these plugs, read on below. 
  • Pre-heating

Pre-heating is the first step of these glow plugs and this means that the tip of the plugs get extremely hot. When the plugs get really hot they get all red in color and start to glow. This is when the fuel injector sprays atomized diesel over these tips and once that is done, the plugs get more heated up and allows the pressured chamber air to ignite quickly.
  • Temperature Maintenance

The next step of glow plugs is maintaining their temperature. Since the plugs get very heated up, they need to maintain this hot temperature to allows the diesel to ignite properly in the combustion chamber. Glow plugs stay energized for quite a while and keep the engine cycle incredibly healthy.
  • Post-heating

The last phase is post-heating. This phase allows the plugs to stay energized for a few minutes in order to keep the combustion process going. After that, the plugs continue to stay heated so that the engine does not die and stays at its normal running temperature.

Signs of A Faulty Glow Plugs in a Diesel Engine

The older diesel-powered vehicles can start even when there is an issue with the glow plugs; however, this is not the same with modern vehicles. Even though glow plugs help in starting diesel engines and they work perfectly fine but during the colder months, glow plugs can be difficult to handle.  If the glow plug doesn’t work in the freezing months, it can be impossible to start your engine. It is crucial that you know beforehand if your plugs are working efficiently or not and so look out for the following signs that will let you know if the glow plug needs replacement:
  • Engine Misfire

When the glow plugs are unable to heat the diesel properly, it can lead to an engine misfire. This means that the fuel induced in the combustion chamber will not ignite, leading to the chamber drowning in excess fuel. Due to this, there can be a reduced power output until the leftover fuel is burned.
  • Difficulty To Start Your Engine

If your vehicle starts up easily but recently it takes some time, this indicates an issue with the glow plugs. Proper working glow plugs help in heating the diesel and fuel right away and don’t delay ignition in the combustion chamber.
  • Engine Is Refusing To Start

Apart from the difficulty in starting the engine with a fault glow plug, sometimes the engine will not start at all. However, sometimes this engine issue can be due to a faulty battery and so focus on the sound of the engine; if there is a clicking sound, then it is a battery issue; otherwise, it is due to faulty glow plugs.
  • Giving Out Black Colored Smoke

If you see your engine is giving out black colored smoke, then this implies that the fuel is not heated up properly, and this is because of the glow plugs. If these plugs are unable to heat the diesel  fuel properly before it enters the chamber of combustion, then it can lead to black smoke. However, sometimes black smoke can occur due to a mechanical issue, so make sure you get your car checked if the glow plugs are working fine.
  • Check Engine Light Goes On

Every vehicle owner knows the horror of having the check engine light on, and there are many reasons why this happens. One of the reasons for this light is faulty glow plugs. If you own an automated diagnostic tool, you can check the reason for this engine light. The code P0380 refers to a malfunctioning glow plug, and so if the tool shows this code, then the glow plugs should be replaced.

Frequently Asked Questions (FAQs):

What Can Happen If Somebody Uses A Spark Plug-In A Diesel Engine?

If you think installing a spark-plug in diesel engine is easy then you are wrong. Spark plugs aren’t designed for these engines and so they cannot accommodate them either.

What Do Diesel Engines Use If They Do Not Rely On Spark Plugs?

Rather than using spark plugs, these diesel engines rely entirely on another component called the glow plugs. These plugs help with kick-starting your engine via fuel combustion.

What Is The Number Of Spark Plugs In Diesel Engine?

As mentioned above, diesel engines work through fuel combustion and so they do not need any spark. This makes the number of spark plugs used in diesel engines “0”.

Why Is A Spark Plug Rendered Useless In Diesel Engines?

The reason why spark plugs are entirely useless for a diesel engine is because diesel engines work through fuel combustion. They do not need any spark for ignition and so spark plugs are not needed for its functioning.


Diesel spark plug jokes can be very confusing, and they make the reader think that diesel spark plugs do exist, but that's not true. Do diesel use spark plugs? The answer is no. Spark plugs are only present in gasoline-powered engines, whereas diesel engines only use glow plugs. So enjoy reading this article and understand all about these plugs and how they work!