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How Many Spark Plugs Does a Diesel Have: 90% Don’t Know?

Spark plugs are an integral part of combustion engine that is responsible for igniting the fuel mixture to produce engine power. But Diesel engine works on different mechanisms and use glow plug instead.”

According to statistics, the spark plug market will touch 5.01 billion by 2029. It may be small, but the spark plug is a significant part of cars that makes the combustion engine work.

One possible reason for this boost in demand for spark plugs can be related to the high demand for automobiles anticipated by 2029.

Getting back to the question – How many spark plugs does diesel have? – You will be amazed to know that No Spark plug is required for diesel engines. Sounds unrealistic? It is because the working mechanism of internal combustion in a diesel engine is entirely different than the gasoline engines, which need a spark plug for ignition. Diesel engines use glow plugs instead.

How is the Working of Gasoline Engines Different from Diesel Engine?

While the internal combustion engine is standard for the Gasoline and Diesel engine, its working mechanism is different.

The fuel is mixed inside the piston chamber with air in a gasoline engine. The piston chamber then compresses this fuel mixture, and the spark plug ignites this air/fuel mixture. The role of the spark plug is indispensable in any gasoline engine. If there is no spark plug, there will be no combustion in the engine.

The electric spark from the spark plug creates a small explosion to force the engine’s piston down and generate energy for the car to move. A faulty spark plug will not help the engine to start.

Do You Know? A spark plug can last up to 1,00,000 miles

The diesel engine first compresses the air to high temperatures and then adds the fuel into the compressed hot air. As the temperature of the gas molecule rises, the diesel combusts spontaneously with the help of a glow plug.

Every cylinder in the diesel engine has a glow plug. If there is a four-cylinder engine, there will be four glow plugs.

What is a Glow Plug?

As the spark plug, Glow plugs are used to create a spark in a diesel engine. Once you turn the key in a diesel truck or car, the compression process starts and moves to the chamber to compress as the piston lifts upward in the engine, the pressure in the chamber increases.

The pressure forces the fuel inside and then mixes in the compressed air. The glow plug heats up as the fuel evaporates due to mixing, and the fuel in the chamber ignites. With enough pressure in the chamber, the engine starts.

Why does Diesel Engine Have Glow Plug and Not Spark Plug?

Both diesel and gasoline engines have internal combustion engines, but both undergo different mechanisms to generate power to start the engine. This difference is due to the difference in the fuel nature. Gasoline can be ignited in any state. The diesel needs to be atomized first to ignite.

The heating element at the glow plug’s tip emits light when it gets heated. Once the diesel is sprayed on this tip and mixed with compressed air, and then evaporates, it starts the combustion process.

What are the different types of Glow Plugs?

As all the diesel engines are not the same, their glow plugs are not the same. While these plugs have the same functionality, they have a price and quality differential.

Here are broadly three types of Glow Plugs:

1. Pressure Sensor Glow Plug

Besides helping the engine start, these glow plugs can measure pressure readings in the cylinder. It gives you an excellent idea of the engine’s efficiency and indicates the vehicle computer when the pressure in the cylinder is low.

They are capable of working even under cold temperatures but not as good as ceramic glow plug does.

2. Steel Glow Plugs

if you are looking for the most affordable glow plug to replace the bad Glow Plug, steel glow plugs are the ideal choice. Bu their steel construction takes more time to heat up in winters. But once the engine starts, they can discharge their duties efficiently.

3. Ceramic Glow Plug

Known for their faster healing properties, ceramic glow plugs can produce consistent heat to make the diesel engine work great. They can reach higher temperatures quickly, so they are the ideal pick for low-temperature areas.

As they turn hot quickly and maintain the heat for a longer time, they are often tagged as efficient glow plugs. As the vehicle takes less fuel to ignite, it also improves vehicle efficiency. The only downside is their pricier tag.

Reasons Why Glow Plug replaces Spark Plug-in Diesel Engine:

Here are the significant reasons why Glow Plug works with Diesel engines and not the spark plug:

1. Difference in Volatility

Gasoline cannot generate the same compression ratio as diesel engines can—gasoline’s auto-ignite at the higher compression levels due to its high volatility compared to diesel. Hence the self-ignition temperature of Diesel is relatively low, which can be quickly burned in the heated air.

As we already know, the air compressed by piston results in heating. The high compression ratio lets the diesel spray overheat air as droplets. Being low in volatility, diesel does not need high temperatures to ignite.

2. Auto Ignition Temperature

Auto ignition temperature is the temperature at which fuel starts burning. Due to low volatility, diesel can burn itself at low temperatures, while gasoline has high volatility and needs a very high temperature for auto-ignition.

It is the reason why the raised temperature due to air compression is sufficient for the atomized diesel, but gasoline needs an even higher temperature.

Why Does Gasoline Engine Lead to High Risk of Engine Knock?

As the Gasoline reacts differently from the diesel at high temperatures, there are higher chances that the gasoline will pre-ignite easily, leading to engine knock. On the other hand, diesel has a low auto-ignition temperature and thus can burn quickly. Hence, the spark plug is not required for diesel engines.

But, it does not mean that no diesel engine ever has a spark plug. Many diesel trucks and cars work on multiple fuel types and thus come with the spark plug.

So, what happens if a diesel engine has a spark plug?

Spark plugs are only designed to work on gasoline engines. So, they will not work on a diesel combustion engine. One possible reason can be the differential in the combustion temperature. As the spark plugs are designed to ignite at a specific temperature much higher than the diesel combustion, the diesel will ignite before the spark plug comes into the picture.

Can I Start My Diesel Engine Without Glow Plugs?

No, the diesel engine will not start without Glow Plugs. The anatomy of the Diesel combustion engine is constructed in such a way that glow plugs are essential aids without which the engine will not start. But like bad spark plugs, bad glow plugs also help start the engine but can damage your engine in the long run.

What are the Various Functions of Glow Plug?

A glow plug-in diesel engine comes with three functions primarily as:

1. Pre-heat the engine

Glow plugs play a pivotal role in starting up the ignition process. Once your turn on the ignition key, glow plugs heat the cylinders.

2. Maintaining temperature

Turning too hot as they sit on the cylinder may melt it. Hence, it is evident and essential that the glow plugs are devised with an in-built temperature monitor and controller.

These plugs maintain the optimal temperature by making required changes.

Hence, Glow plugs are capable of monitoring the temperature of their own as well as cylinder temperature.

3. Post-heating Process

Once you turn off the engine, glow plugs are responsible for maintaining the heat so you can restart the car quickly.

Frequently Asked Questions (FAQs):

Q: What Does Glow Plugs Made up of?

A glowplug is a long, thin metal tube with a heating element at its end. Glow plugs are used when starting an engine cold because they provide instant heat to the combustion chamber. Glow plugs are made of special alloys that resist oxidation and high temperature.

Q: How does a glow plug work?

Glow plugs are used primarily to heat the engine at starting up. They are designed to be very reliable and durable and to last long enough to allow the engine to reach operating temperature. Glow plugs are not required after the engine reaches operating temperature. In warm weather, the diesel engine may not require a glow plug.

 Q: How Many Glow Plugs Are there in Diesel Engine?

Depending on the count of its cylinder, there can be four to ten glow plugs in a diesel engine. Once glow plug is required to install in each cylinder. So, a diesel truck with six cylinders will have six dedicated glow plugs.

Q: Is It Possible To Replace Glow Plug With Another Type Of Plug?

Yes, replacing the glow plug with another type of plug is possible. But, the replacement should be done by a certified technician or mechanic.


The spark plugs are meant for gasoline engines, not diesel engines. Due to high volatility between the two fuels – Diesel and gasoline, the internal combustion engine works differently. At the same time, the gasoline engine depends on a spark plug, while the diesel engine works with Glow Plug’s help. Hence, if someone asks how many sparks plug in a Diesel Engine, the answer is None!

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My Name is Christopher Angels, and I am a postgraduate in mechanical engineering. Cars have always excited me as a child, and soon I decided to dive into the world of cars by pursuing mechanical engineering. I also worked as a Mechanic for over 3 years to understand Cars' anatomy and how each part contributes to its working.

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