How Many Spark Plugs Does a Diesel Have?

Diesel engines have a complex combustion system that requires a number of moving parts in order to start. Compared to regular gasoline, diesel engines have a slightly different mode of operation due to the fuel they use.

As a first-time user of a diesel engine, you may be curious about how often you’ll need to replace the spark plugs and how many you should stock up on. To find out how many spark plugs a diesel has and how to change them, read on.

How Many Spark Plugs Does A Diesel Have?

How Many Spark Plugs Does A Diesel Have?

There are no spark plugs in a diesel engine. As the combustion mechanism is different, spark plugs are unnecessary. When a spark plug is ignited, it causes a little explosion within the combustion chamber of a gasoline engine. That explosion is what ignites the gasoline and kick-starts the engine.

While operating a diesel engine, a glow plug is used in place of a spark plug. Every cylinder in a diesel engine needs its own glow plug, much like a spark plug is needed in a gasoline engine.

A diesel engine’s glow plug count would then be equal to its cylinder count. As an illustration, if your diesel engine has six cylinders, it also requires six glow plugs.

How Many Spark Plugs Does A Diesel Have?

How Many Spark Plugs Does A Diesel Have?

Engines that run on diesel do not have spark plugs. Gasoline burns differently than diesel because it is more volatile and has a different ignition point. Because it’s so easy to start a fire, gasoline engines need spark plugs.

An arc of electricity is made when a spark plug is turned on. This is done to start the engine’s combustion process by setting the gasoline in the chamber on fire. This will cause the pistons to move, and so on.

Diesel fuel isn’t that easy to light. If you put a lit match in diesel fuel, it will go out as soon as it touches it. In other words, it won’t start with just a tiny spark from a spark plug. But if diesel fuel is atomized, like when it is sprayed from a spray bottle, an open flame will light it.

Glow plugs are used in diesel engines

One glow plug is installed in each cylinder of a diesel engine to initiate combustion. If you have a diesel engine with six cylinders, for instance, you’ll need six glow plugs.

The glow plug is designed to preheat the diesel fuel. To put it simply, this will facilitate engine starting. When it’s cold outside, the glow plug helps diesel fuel to flow more smoothly.

Older diesel engines can be started even if their glow plugs are completely drained. That said, pleasant, warm weather is a prerequisite for any activity.

Does anyone know if a diesel engine, without a glow plug, can be started in subfreezing temperatures? Sure, but it needs a few good twists to get going.

How Many Spark Plugs Does A Diesel Have?

How Do Glow Plugs Work in a Diesel Engine?

A glow plug is made by wrapping heating coils in a metal tube. The tube is then filled with a powder made of ceramic that stops electricity from going through it. When powered by the vehicle’s battery, glow plugs can heat up to 1000°C in just a few seconds.

There are both metal-cased glow plugs and ceramic ones, which are more expensive but work better. Because it is made of ceramic, the glow plug heats up a lot faster. Compared to glow plugs made of metal, it is also more durable.

The glow plugs heat more than just the diesel fuel. It also helps heat up the whole engine so that it works right from the start.

You can learn more about how a Glow Plug works if you look at its three phases:

1 – Engine Start

In order to help start the engine, glow plugs quickly warm up during this time. The diesel fuel is atomized and blown across the glow plug by the fuel injector while it is hot. Following that, the temperature will increase to a point where the compressed air in the chamber is easily ignited.

2 – Continuous Heat

Controlling glow plug temperature ensures diesel fuel ignites properly in the combustion chamber. They will also stay powered for a dozen or more engine cycles to verify engine start.

3 – After Start

After the engine is started, the glow plugs stay on for a few more minutes to keep the combustion process going. After that, they’ll keep heating the fuel for a while longer to make it less likely that the engine will die before it gets to its normal operating temperature.

When Should You Replace Your Glow Plugs?

It’s crucial to keep your engine in good working order as a car owner. By keeping up with routine maintenance, you can extend the life of your vehicle’s engine.

Glow plug replacement has to be a regular part of your maintenance schedule. To be expected, given that their purpose is to produce heat, they eventually wear out. The more frequently you use your car, the quicker its parts will wear out.

The typical lifespan of a glow plug is 100,000 miles before it loses efficiency or breaks. As 12,000 miles is the sweet spot for vehicle maintenance in general, most mechanics will advise you to check your glow plugs at that interval.

Nonetheless, all glow plugs should be replaced whenever your vehicle reaches 100,000 miles. Changing them all at once can keep your engine running smoothly in every cylinder. Keep in mind that if even one glow plug fails, it might reduce your engine’s efficiency.

It is more convenient to change all of the spark plugs or glow plugs at once.

When Should You Replace Your Glow Plugs?


Diesel engines’ combustion method does not require spark plugs because it uses pressure instead.

Glow plugs are used to start a diesel engine. For each cylinder in a diesel engine, there is a glow plug, which performs a similar function to spark plugs.

In order to keep your car running smoothly, it’s a good idea to replace the glow plugs every 100,000 miles.

Frequently Asked Questions

Do any diesels have spark plugs?

Spark plugs are only ever present in petrol engines, so diesel cars do not have spark plugs. Diesel engines use something called a glow plug instead. This is because of the way the combustion system in a diesel engine works. Diesel engines ignite the fuel using compression, whereas petrol engines use the spark plugs.

How many spark plugs does a 8 cylinder diesel have?

Most diesel engines run on one glow plug per engine cylinder. This means a four cylinder engine will have four glow plugs, a six cylinder diesel engine will have six plugs, and a V8 engine will have eight glow plugs.

How Is A Spark Plug Different From Glow Plug?

A spark plug is a critical part of a gasoline or gas engine. It generates a spark through the spark plug gap to ignite the air fuel mixture inside the combustion chamber.  On the other hand, a glow plug heats up the compression ignition system so that combustion can take place easily. 

Can a diesel run without glow plugs?

To start, a diesel engine requires the help of glow plugs. A glow plug produces the heat that is necessary for a diesel engine to start, run, and function optimally- especially in colder weather conditions.

Andrea Barbuti
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