Do diesel engines have spark plugs?

Diesel Car Advice
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Spark plugs are an integral part of a petrol engine.
Marcus Craft
Contributing Journalist
24 Jul 2025
4 min read

Q: Do diesel engines have spark plugs?

A: No, diesel engines do not have spark plugs.

What are spark plugs? What do spark plugs do?

Spark plugs are located at the top of the cylinder head in petrol-fuelled internal-combustion engines.Ā 

As the name implies, these engines rely on the combustion process to power the engine.Ā 

And a spark plug – as the name implies – is an electrical component which produces a spark that ignites the petrol-air mixture, causing the combustion process to kick off in order to power the engine. That spark is created between two electrodes at the tip of the plug as a result of a high-voltage charge received via the ignition coil at the other end of the spark plug.

2025 Toyota LandCruiser 70 Series
2025 Toyota LandCruiser 70 Series

What kinds of cars have spark plugs?

Spark plugs are found in cars with petrol engines.

Do diesels have spark plugs?

No, they don’t.

How many spark plugs are in a diesel?

None, but they do have glow plugs which we’ll get to in a couple of sentences below this. Most diesel engines run on one glow plug per engine cylinder.

Why don't diesel engines have spark plugs?

Because diesel engines use high compression, rather than an actual spark, to ignite the fuel. Air is drawn into the engine cylinder and compressed to an extremely high pressure and consequent temperature.

2025 GWM Tank 300 diesel
2025 GWM Tank 300 diesel

What do diesels have instead of spark plugs?

Glow plugs.

What do glow plugs do in a diesel engine?

As mentioned above, diesel engines use high compression, rather than an actual spark, to ignite the fuel.Ā 

So, instead of spark plugs, diesel engines use glow plugs, electrical heating devices located in each cylinder of the diesel engine, which preheat the air in the engine’s cylinders to expedite the combustion process necessary to start the engine.

A glow plug in a diesel-fuelled vehicle is only needed during the ignition process, whereas a spark plug in a petrol-fuelled vehicle works continuously while you’re driving.

2025 Toyota LandCruiser 70 Series
2025 Toyota LandCruiser 70 Series

Do all diesels have glow plugs?

No, not all diesel engines have glow plugs.

When the diesel-air mixture is compressed quickly in a diesel engine, it ignites without any need for an actual spark and that produces heat enough to kick-start the combustion process.

But smaller diesel engines have difficulty maintaining the temperature necessary for combustion and the purpose of glow plugs is to warm the diesel-air mixture in order for it to combust of its own accord.

2025 Mazda CX-70 GT D50e
2025 Mazda CX-70 GT D50e

Many larger diesel engines don’t need any glow plugs because their diesel-air mixture ignites without any need for assistance. . The diesel-air mix has sufficient heat coming from the air, which becomes increasingly hotter as it is quickly compressed.

How does a diesel engine work?

This is best explained by a panel of diesel experts over at Cummins who came up with this nifty explanation … and it reads like poetry to me:

ā€œIn general, a diesel engine works by using a piston to compress air to increase the temperature in the cylinder and then injecting atomised diesel fuel into this cylinder.

2024 Kia Sorento GT Line diesel
2024 Kia Sorento GT Line diesel

ā€œWhen the fuel comes into contact with the high temperature, it ignites, creating energy that drives the piston down transferring energy to the crankshaft and through the powertrain.

ā€œThis process is repeated over and over again at a high speed, making a diesel engine a powerful piece of technology.Ā 

ā€œDifferent types of diesel engines will have varying compression ratios. The compression ratio of the diesel engine impacts how much power the engine puts out. The higher the ratio, the more power is generated.ā€

Couldn’t have said it better myself.

Marcus Craft
Contributing Journalist
Raised by dingoes and, later, nuns, Marcus (aka ā€˜Crafty’) had his first taste of adventure as a cheeky toddler on family 4WD trips to secret fishing spots near Bundaberg, Queensland. He has since worked as a journalist for more than 20 years in Australia, London and Cape Town and has been an automotive journalist for 18 years. This bloke has driven and camped throughout much of Australia – for work and play – and has written yarns for pretty much every mag you can think of. The former editor of 4X4 Australia magazine, Marcus is one of the country’s most respected vehicle reviewers and off-road adventure travel writers.
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