Informational Guide

Do Diesel Engines Have Spark Plugs?

In this guide, we explore this question and discuss diesel engine components.

The automotive spark plug is used in gas-powered engines designed to ignite the air-fuel combination in the combustion chamber and can add horsepower. Do diesel engines have spark plugs?  

The answer is no.  

We will discuss why the diesel engine doesn't have spark plugs and answer can we use a spark plug in a diesel engine? 

Diesel engines operate similarly to gasoline engines because internal combustion occurs in both. However, diesel engines have a compression-ignited injection system rather than one that uses a spark-ignited system.

With the compression-ignited model, diesel fuel gets injected into the combustion chamber and is ignited by the high temperatures that occur when the fuel is compressed by the pistons.  

The diesel engine is made of several essential parts. 

  • Glow Plugs 
    Produce the heat that assists the engine in starting. 
  • Pistons 
    A metal cylinder that moves up and down within another cylinder inside the combustion engine.
  • Crankshaft 
    Pistons connect to the crankshaft, which is responsible for regulating their movement. 
  • Cylinders 
    Chambers containing the pistons. 
  • Cylinder Head 
    Makes up the top of the cylinder, finishing off the combustion chamber.
  • Combustion Chamber 
    Area where the fuel and air mix, compress, and ignite.
  • Valves 
    A long tube-shaped funnel that allows fuel and air to come in and out. Diesel motors contain both exhaust and intake valves. 
  • Fuel Injectors 
    Pressure sprayer that injects the fuel into the combustion chamber. Every cylinder contains an injector.
diesel engine

Why Don't Diesel Engines Have Spark Plugs? 

Do you know why diesel engines don't have a spark plug? There is no spark plug in a diesel engine because these ignition parts aren't needed. So, how does a diesel engine work without a spark plug?  

The diesel engine is very similar to a gas-powered motor, except there are no spark plugs. In fact, the diesel engine doesn't require the same level of ignition that gas-powered engines do. Here are a few reasons why. 

Diesel Engines Are Less Volatile  

Diesel motors contain high compression ratios, with the fuel and air infusion happening differently. The gas engine isn't able to generate an equal compression ratio, or it would auto-ignite. Because diesel is less volatile, it contains a much lower self-ignition temperature, causing it to burn if it gets sprayed on hot air.  

Self-Ignition Temperature Is Low 

The temperature at which the fuel begins burning is lower with diesel than gasoline. With a lower auto-ignition temperature, warm air is all that's needed to combust the diesel.  

Compressed Atmospheric Air Creates High Pressure 

The air inside the diesel engine is compressed at higher rates, causing the cylinder air temperature to become much hotter. This heat is what leads to combustion. 

spark plug

What Do Diesel Engines Have Instead of Spark Plugs?

The diesel glow plug is a device that creates heat. It takes the place of spark plugs in the combustion process. This part was made with a metal tip where heat can be emitted to warm the mixture in the combustion chamber. They are known as glow plugs because they can start to shine when they are hot. 

However, glow plugs aren’t necessarily needed to keep the diesel engine running properly. While they will help get the motor started, they have no function in keeping the engine going. You shouldn't notice any major performance issues with bad glow plugs once you have started it.  

How Many Glow Plugs Does A Diesel Engine Have? 

How many spark plugs are in a diesel engine? None. The amount of glow plugs in the engine depends on how many cylinders it has. You can’t determine how many spark plugs are needed in a diesel engine, only glow plugs. First, you want to figure out what type of engine your vehicle has.

For example, a six-cylinder diesel engine is going to have six glow plugs. That's because there is one glow plug per cylinder in the diesel motor. Unlike spark plugs, you won't find two glow plugs per cylinder in some models.  

glow plugs

What Happens If Diesel Engines Use Spark Plugs?

Do diesel engines use spark plugs? There's no location for the diesel engine to use a spark plug. However, some equipment does contain a diesel engine spark plug.

In fact, you can find this configuration on some diesel tractors that use a variety of fuel types, such as the Allis-Chalmers engine or the Farmall Super M-D tractor.  

People Also Ask (FAQs)

Will a diesel engine start without a glow plug? 

In warm weather, the glow plugs might not be needed to start the engine. However, you might notice some white smoke until the engine reaches its proper temperature. In cold weather, the lack of functioning glow plugs can keep the engine from starting.  

How often should you change your glow plugs? 

Glow plugs aren't considered a maintenance item. Therefore, you shouldn't need to replace them unless something goes wrong. If you are having trouble starting the engine or you notice white smoke once it is running, have the glow plugs checked.

How much will it cost to replace a glow plug? 

It should cost $50 to $100 for the glow plugs, depending on what type you use. If you can put the glow plugs in yourself, you won't need to pay any labor charges either.  

What happens if you don't change glow plugs? 

If the required temperature can't be reached because of defective glow plugs, it could become harder to start the motor. There might also be white smoke coming from the exhaust until the engine reaches the proper temperature.  

Do new glow plugs improve performance? 

Glow plugs aren't used for keeping the engine running. Because of this design, bad glow plugs shouldn't cause any engine performance issues, only starting trouble. If the engine won't start easily or white smoke is noticed, you might want to change the glow plugs.  


Diesel engines do not contain spark plugs. However, the motor uses glow plugs to warm the air/fuel mixture and create combustion. With this design, it is easier to start the diesel engine, especially in colder temperatures.