How Many Spark Plugs Does a Diesel Have?

A four-cylinder gas fuel vehicle will have four spark plugs; a six-cylinder gas fuel vehicle will have six, and so on. But what about diesel vehicles? How many spark plugs does diesel have?

Diesel cars and trucks have no spark plugs. Unlike gasoline-powered engines, diesel engines do not require spark plugs to ignite their fuel. Instead, these engines rely on compression to raise the air temperature in the combustion chamber to a point where the diesel fuel will combust immediately once introduced.

Read on to learn more about how many spark plugs are in a diesel.

How Many Spark Plugs Does a Diesel Have?

how many spark plugs does a diesel have

Or a more appropriate question would be, “Do diesel engines have spark plugs?”

Diesel engine vehicles do not have any spark plugs. The reason is that gasoline ignites differently compared to diesel fuel. Gasoline is the more volatile of the two. It does not take much to make it catch fire, which is why gasoline engines use spark plugs.

The spark plug supplies an arc of electricity (a spark, hence the name). This is to ignite the fuel inside the combustion chamber of the engine. It will then cause the pistons to move, and so forth.

Diesel fuel is not that easy to ignite. If you throw a lit match into a puddle of diesel fuel, the flame will go out. In other words, a tiny spark from a spark plug will not be enough to ignite it. However, if you atomize diesel fuel, like when you spray it out from a spray bottle, an open flame will cause it to ignite and create a fireball. It is essentially what happens in a diesel engine.

Can a Diesel Engine Run Without a Glow Plug?

As mentioned earlier, a glow plug’s function is to heat the diesel fuel. This can help ensure that the engine will be easier to start. If it is already warm outside, the diesel fuel will be flowing easily. You can then expect it to be ignited easily by the superheated air inside the combustion chamber.

Older diesel engines can start even if all the glow plugs in the engine are faulty. However, the weather needs to be nice and warm to do so. Will a diesel engine still start in cold weather even without a glow plug? Yes, but it will take quite a few cranks for it to turn over and run properly.

How Do Diesel Engines Work With No Spark Plugs?

The combustion cycle of a diesel engine is much different compared to a gasoline engine. In a gasoline engine, both air and fuel are injected into the chamber. A spark from the spark plug will then ignite the fuel-air mixture. In a diesel engine, the air is introduced into the chamber. The piston will then compress it.

Compressed air will increase temperature. When it reaches a certain volume, the fuel injector will spray diesel fuel directly into the superheated air. This will cause the mixture to combust and the piston to move up and out of the chamber, and so on.

If the diesel does not have spark plugs, what are those things that look like them? One reason why you may be asking about diesel engines and spark plugs is that you discovered something oddly similar to it while cleaning your engine bay. That is not a spark plug. It is actually a glow plug.

Unlike spark plugs, you can’t use glow plugs to ignite the diesel fuel. What they do, though, is heat the fuel before injecting it into the combustion chamber. As a result, it makes it easier for the diesel fuel to ignite.

Do Glow Plugs Continuously Work Like Spark Plugs?

Spark plugs are integral parts of the internal combustion process in gasoline engines. The reason is that they are the ones that ignite the fuel. Glow plugs, on the other hand, do not work that way.

The main use of the glow plugs is to get diesel engines started. If the weather outside is warm, a diesel engine will not even need the glow plug to get started. It will be fine on its own. However, when it is cold outside, the air in the engine may not get hot enough to ignite the diesel fuel.

This will result in a misfire, causing the vehicle to belch out much black smoke from the exhaust. If it is freezing outside and the glow plug malfunctions, it is almost impossible to get the engine started.

How Do Glow Plugs in a Diesel Work?

do diesel engines have spark plugs

Glow plugs are essentially heating coils encased in a metal tube. An electrically insulating ceramic powder then fills the tube. Sealing comes after at one end. When electricity from the vehicle’s battery is sent through the glow plug, it can get as hot as 1000°C in a couple of seconds.

There are also ceramic glow plugs that are more expensive but more efficient than regular metal-encased ones. The ceramic allows the glow plug to heat up much faster. It is a lot more durable than metal glow plugs too.

The glow plugs do not just heat the diesel fuel. It also helps heat the entire engine so that it can function properly right from the get-go.

If you want to get a bit more technical about how a glow plug functions, it typically goes through three phases:

1. Pre-heating

This is the phase where the glow plugs quickly heat up to help start the engine. First, the glow plug will glow red hot, and then the fuel injector will blow atomized diesel fuel over it. After that, it will get heated up considerably so that the pressurized air in the chamber can ignite it easily.

2. Temperature Maintenance

The glow plugs maintain their temperature to ensure that the diesel fuel ignites properly inside the combustion chamber. They will also stay energized for a dozen or so engine cycles to ensure that it has started properly.

3. Post-heating

The glow plugs remain energized for a couple more minutes to maintain the combustion process after the engine starts. After that, they will continue heating the fuel a bit longer to minimize the chances of the engine dying before the engine gets up to its normal running temperature.

Symptoms of Bad Glow Plugs in a Diesel

how many spark plugs are in a diesel

Although older diesel-powered vehicles could start even when all glow plugs are not working, you can’t say the same about modern vehicles. Here are some symptoms that indicate that your vehicle may have a faulty glow plug or two:

1. Hard Starting

If your vehicle usually starts with one turn of the key but now takes longer to start, then the glow plugs may be problematic. Properly functioning glow plugs aid by heating the air and diesel fuel just enough to ignite much easier in the combustion chamber.

Faulty glow plugs may not reach hot enough temperatures to prime the diesel fuel before injecting it into the combustion chamber. This will result in hard starts, especially during the colder months of the year.

2. Engine Misfire

If the glow plugs could not heat the diesel enough, it might cause the engine to misfire. It means it will not ignite the fuel injected into the chamber. The combustion chamber may also get flooded with fuel. This can result in reduced power output, at least until the remaining fuel is burned. Engine misfires can also cause issues with the engine, like engine knocking and clogged fuel injectors.

3. Engine Does Not Start at All

If the weather is cold and your engine will not start, you may have several malfunctioning glow plugs. You can tell if the glow plugs are at fault when you hear the engine turning over without successfully starting. If the battery were the issue, you would only be hearing clicking sounds. Instead, it indicates that there is not enough power going into the starter motor.

4. Decreased Fuel Efficiency

You are consuming fuel when you start the engine. If your glow plugs malfunction, your engine continuously injects diesel into the combustion chamber without getting ignited. As a result, the unburned fuel goes to waste instead of getting used to power the vehicle forward.

Also, hard starting puts a lot of stress on the engine’s components. This can lead to a lot of unnecessary wear and tear. Eventually, the cylinders will lose a bit of compression. This means that the engines will not be as efficient as they were before.

5. Black Smoke

Black smoke coming from a diesel engine means that the fuel is not burned efficiently. For example, if your tailpipe blows a huge cloud of black smoke after you spent a while trying to get the engine started, one possible cause was that your glow plug did not heat the diesel before entering the combustion chamber. As a result, it was not also burned completely, resulting in black smoke.

However, if your vehicle is still belching black smoke after a couple of minutes, or it does so when you step on the accelerator, there might be another mechanical issue that you need to look into.

6. Check Engine Light Goes On

If there’s one thing that sends chills down the spine of vehicle owners, it’s when the check engine light turns on. There are dozens of reasons why the check engine light would turn on. In diesel-powered engines, one of them is a fault in the glow plugs.

Although your vehicle would still run, having that check engine light constantly on can be quite nerve-wracking. If you have an automotive diagnostic tool, you can check if the problem truly is with the glow plugs. If you get an error code “P0380”, then the issue is a malfunctioning glow plug or two.

Again, how many spark plugs does diesel have? Do diesel engines have spark plugs? Diesel engines do not have spark plugs. Unlike gasoline-powered engines, diesel engines do not need spark plugs to ignite their fuel. Instead, diesel engines rely on compression to raise the air temperature in the combustion chamber to a point where the diesel fuel will combust immediately once introduced.

How Many Spark Plugs Does a Diesel Have? FAQs

How Many Glow Plugs Are in a Diesel Engine?

The answer to this will depend on the type of vehicle you are driving and its engine size. The reason is that the number of glow plugs will depend on the cylinders the engine has. Diesel cars and pickups can have anywhere from four to ten cylinders.

On the other hand, a diesel locomotive can have more than twelve huge cylinders. This is because diesel engines typically have one glow plug per cylinder. This means that if you have a six-cylinder engine, then it will have six glow plugs.

How Often Do You Need to Replace Glow Plugs?

If your vehicle has a four-cylinder diesel engine, just one faulty glow plug will make it difficult to start. Two faulty plugs will make starting the engine a real pain. If there are three, then starting the car would be next to impossible, especially during cold mornings.

As mentioned earlier, you need to replace damaged glow plugs immediately as they can trigger serious mechanical problems later. Preferably, you should replace glow plugs as close to the end of their service life as possible. So now the question is, how long before you need to replace glow plugs?

Since you only use them for starting the engine, Glow plugs have quite a long service lifespan. They can last up to 100,000 miles – after which, they will deteriorate gradually.

Aside from being long-lasting, glow plugs are also not that expensive. They just cost a little more than a regular spark plug. So you can get a performance glow plug for just a couple of bucks. This means that replacing the whole set should not break the bank at all.

Conclusion – How Many Spark Plugs Are In a Diesel?

How many spark plugs are in a diesel engine? None. Diesel engines do not have any spark plugs. Unlike gasoline-powered engines, diesel uses the heat from the compressed air in the cylinder to ignite the atomized fuel. Instead of spark plugs, diesel uses glow plugs. And although they look somewhat similar, their functions are different.

Glow plugs only heat the atomized diesel fuel in the cylinder. They do not ignite it. Also, glow plugs are only energized when starting the engine. This makes them different from spark plugs that are continuously used while the engine is running.